HK1086401B - Pilot transmission schemes for wireless multi-carrier communication systems - Google Patents

Description

Pilot transmission scheme for wireless multi-carrier communication systems

This application claims priority to provisional U.S. application serial No. 60/438,601 entitled "PilotTransmission Schemes for Wireless Multi-Carrier communication systems," filed on 7/1/2003, assigned to the assignee of the present invention and incorporated herein by reference in its entirety.

Technical Field

The present invention relates generally to communication, and more particularly to a pilot transmission scheme for a wireless multi-carrier communication system.

Background

Multi-carrier communication systems use multiple carriers for transmitting data to a single endpoint. These multiple carriers may be used, for example, in Orthogonal Frequency Division Multiplexing (OFDM) or some other multi-carrier modulation technique. OFDM effectively partitions the overall system bandwidth into multiple (N) orthogonal subbands, which are also referred to as tones, frequency bins, and frequency subchannels. In OFDM, each subband is associated with a respective carrier on which data is modulated.

In a wireless communication system, data to be transmitted is processed (e.g., encoded and modulated) at a transmitter and upconverted to a Radio Frequency (RF) carrier signal to generate an RF modulated signal. The RF modulated signal is then transmitted over a wireless channel and may reach a receiver through multiple propagation paths. The characteristics of the propagation path typically change over time due to a number of factors, such as, for example, attenuation, multipath, and external interference. Thus, the transmitted RF modulated signal may experience different channel conditions (e.g., different fading and multipath effects) and may be associated in time with different complex gains and signal-to-noise ratios (SNRs).

In a wireless communication system, a pilot is often transmitted from a transmitter (e.g., a base station) to a receiver (e.g., a terminal) to assist the receiver in performing a number of functions. The pilot is typically generated based on known symbols and processed in a known manner. The pilot may be used by the receiver for channel estimation, timing and frequency acquisition, coherent data demodulation, received signal strength measurements, and so on.

Various challenges are faced in the design of pilot transmission schemes for multicarrier communication systems. One consideration is that since pilot transmission represents intra-system overhead, it is desirable to minimize pilot transmission as much as possible while still providing the desired performance. Another consideration is that the pilots are transmitted in a manner that enables a receiver in the system to detect and distinguish between pilots transmitted by various transmitters in the system. Furthermore, the pilot transmission scheme needs to address the additional dimensionality of the multi-carrier system established by the multiple carriers.

There is therefore a need in the art for a pilot transmission scheme in a multi-carrier communication system.

Disclosure of Invention

A pilot transmission scheme suitable for use within a wireless multi-carrier communication system (e.g., an OFDM system) is provided herein. These pilot transmission schemes may utilize frequency orthogonality, time orthogonality, or both to achieve orthogonality among the pilots transmitted by multiple base stations on the downlink. Frequency orthogonality may be achieved by transmitting pilots from different base stations on disjoint sets of subbands. Time orthogonality may be achieved by transmitting pilots using different orthogonal codes (e.g., Walsh codes). The pilots may also be scrambled with different scrambling codes that randomize pilot interference and enable identification of the transmitter identity of the pilots.

The pilot transmission scheme described herein effectively facilitates channel estimation and pilot monitoring. These schemes allow terminals in the system to obtain high quality wideband channel estimates as well as pilot strength estimates for base stations in the system. These estimates may be used to implement coherent data demodulation, soft handoff, and hard handoff, as described below.

Techniques for pilot interference estimation and cancellation are also provided. Pilot interference cancellation may be implemented to improve performance because subbands used for data or pilot transmission by one transmitter may also be used for pilot transmission by another transmitter (i.e., an "interfering" transmitter). The estimation of the pilot interference can be achieved by: obtaining an estimate of the channel to the interference source; generating pilots in the same manner as implemented by the interfering transmitter; and multiplying the generated pilot by the channel estimate. The pilot interference is then subtracted from the received symbols to obtain quality-improved pilot-canceled symbols.

Various aspects and embodiments of the invention are described in detail below.

The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

brief description of the drawings

Fig. 1 illustrates a wireless multiple-access multi-carrier communication system;

FIG. 2A illustrates an OFDM subband structure;

FIG. 2B illustrates T disjoint sets of subbands based on the OFDM subband structure illustrated in FIG. 2;

FIGS. 3A and 3B illustrate example subband assignments for a 9-sector 3-cell cluster and a 21-sector 7-cell cluster, respectively, to achieve frequency orthogonality;

FIGS. 4A and 4B respectively illustrate example assignments of orthogonal codes to achieve time orthogonality for a 3-sector 1-cell cluster with one antenna and two antennas per sector;

FIGS. 4C and 4D illustrate exemplary subband and orthogonal code assignments for a 9-sector 3-cell cluster and a 21-sector 7-cell cluster, respectively, to achieve frequency and time orthogonality;

FIG. 5 shows an example system layout in which a different scrambling code is allocated to each 7-cell cluster;

FIGS. 6A and 6B illustrate pilot transmission from multiple sectors for a synchronous burst pilot transmission scheme and a synchronous continuous pilot transmission scheme, respectively;

fig. 7 shows a block diagram of a base station and a terminal;

fig. 8 shows a block diagram of a modulator in a base station;

FIGS. 9A and 9B show block diagrams of two embodiments of a demodulator within a terminal; and

fig. 10 shows a block diagram of a pilot interference canceller within a demodulator.

Detailed Description

The term "exemplary" is used herein to mean "serving as an example, instance, or illustration. Any embodiment or design described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments or designs.

Fig. 1 illustrates a wireless multiple-access multi-carrier communication system 100 that supports multiple users and is capable of implementing the pilot transmission scheme described herein. System 100 includes a plurality of base stations 110 that support communication for a plurality of terminals 120. A base station is a fixed station used for communicating with the terminals and may also be referred to as an access point, a node B, or some other terminology.

As shown in fig. 1, various terminals 120 can be dispersed throughout the system, and each terminal can be fixed (i.e., stationary) or mobile. A terminal may also be called a mobile station, a remote station, a User Equipment (UE), a wireless communication device, an access terminal, or some other terminology. Each terminal may communicate with multiple base stations on the downlink and/or uplink at any given moment. The downlink (i.e., forward link) refers to the communication link from the base stations to the terminals, and the uplink (i.e., reverse link) refers to the communication link from the terminals to the base stations. In fig. 1, terminals 120a through 120o receive pilot, signaling, and possibly user-specific data transmissions from base stations 110a through 110 g.

A system controller (not shown in fig. 1) is typically coupled to base stations 110 and may be designed to perform a number of functions, such as (1) coordination and control for the base stations coupled to it, (2) routing data among the base stations, and (3) access and control of the terminals served by the base stations.

System 100 may be a cellular system or some other type of wireless system. System 100 may also be designed to implement any of a variety of standards and designs, such as Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), and so forth. CDMA standards include IS-95, CDMA2000, IS-856, W-CDMA, and TS-CDMA, while TDMA standards include GSM. These criteria are well known in the art.

Each base station 110 within the system provides coverage for a particular geographic area 102. The coverage area of each base station may be defined as, for example, an area where a terminal may obtain a particular grade of service (GoS). The size and shape of the coverage area of each base station generally depends on various factors such as terrain, obstructions, and the like. For simplicity, the coverage area of each base station is typically represented by an ideal hexagon. A base station and/or its coverage area are typically referred to as a "cell," depending on the context in which the term is used.

In a typical system deployment, to increase capacity, the coverage area of each base station may be divided into multiple sectors. If each cell is divided into three sectors, each sector of a sectorized cell is generally represented by an ideal 120 degree wedge, which is 1/3 for the cell. In actual deployment, the coverage area of each base station typically has a shape that is different from an ideal hexagonal shape, and the shape of each sector often differs from an ideal 120-wedge shape. Also, the sectors of a sectorized cell typically overlap at the edges. Each sector may be served by a corresponding Base Transceiver Subsystem (BTS). For a sectorized cell, the base station for that cell typically includes all of the BTSs serving the sector of the cell. The term "sector" is also often used to refer to a BTS and/or its coverage area, depending on the context in which the term is used.

For simplicity, the following description assumes that each cell is divided into three sectors and its BTSs are located within the base station of that cell. The base station is located in the center of the cell. Also for the sake of brevity, in the following description, the term "base station" is used generically for a fixed station that serves a cell as well as a fixed station that serves a sector.

For a CDMA system, the pilot transmitted by each base station is spread across the entire system bandwidth on a frequency channel before being transmitted on a wireless channel. At the terminal, the pilot transmitted by each base station may be received with a low signal-to-noise ratio (SNR). However, the complementary despreading operation performed by the terminal provides processing gain, which relies on recovering the pilot in the presence of a significant amount of noise and interference. Direct sequence spread spectrum processing of the pilot as used in CDMA is generally not possible for multi-carrier systems. Other means must be used to transmit the pilot from each base station so that it can be easily detected by the terminals in the system.

A pilot transmission scheme suitable for use in a multi-carrier communication system, such as the one shown in fig. 1, is provided herein. As described above, pilots are transmitted to support various functions that may be required for normal system operation, such as timing and frequency acquisition, channel estimation, coherent data demodulation, and so on. Multiple carriers may also be provided by OFDM and some other multi-carrier modulation techniques. The pilot transmission scheme described herein is well suited for the downlink and may also be used for the uplink.

For clarity, the pilot transmission scheme is specifically described for the downlink of an OFDM system. The OFDM system has N orthogonal subbands. Each base station may transmit one OFDM symbol in each OFDM symbol period, as described below.

I. Pilot transmission structure

Table 1 lists three "configurations" that may be used for the pilot transmission scheme.

Table 1

Structure of the device	Description of the invention
		Frequency orthogonality	Transmitting pilots on different disjoint sets of subbands by different base stations to obtain frequency-domain orthogonality of pilot transmission
Time orthogonality	Use of different orthogonal codes (e.g., Walsh codes) for pilots by different base stations to obtain time domain orthogonality of pilot transmissions
		Scrambling code	Using different scrambling codes for pilots by different base stations for pilot interference randomization and base station identity

The orthogonal and scrambling "codes" are also referred to as "sequences" in the following description. Each of the constructs listed in table 1 are described in detail below. The processing of these constructs at the base station and terminal is also described below.

Various pilot transmission schemes may also be designed based on any one or any combination of these constructs. For example, the pilot transmission scheme may use (1) frequency and time orthogonality, (2) frequency orthogonality and scrambling code, (3) frequency orthogonality, time orthogonality, and scrambling code, or (4) some other combination.

1. Frequency orthogonality

Frequency orthogonality may be used to avoid interference from multiple base stations transmitting pilots simultaneously. For frequency orthogonality, pilots are transmitted by multiple base stations on different sets of subbands that are "disjoint" (disjoint as described below) to avoid interference. The frequency orthogonality performance is achieved in various ways, some of which are described below.

Fig. 2A illustrates an OFDM subband structure 200 that may be used for multicarrier system 100. The system has an overall system bandwidth of W MHz, which is divided into N orthogonal subbands using OFDM. In a typical OFDM system, only M of the N total subbands are used for pilot and data transmission, where M < N. The remaining N-M subbands are not used for pilot/data transmission but act as guard subbands to allow the system to meet spectral mask requirements. The M usable subbands include subbands F through F + M-1, where F is an integer generally selected such that the M usable subbands are centered in the operating band.

Fig. 2A also shows an embodiment that divides the M usable subbands for pilot transmission. In this embodiment, the M usable subbands are initially divided into K groups, each group including T consecutive subbands. In general, K, T and M can each be an integer greater than one, and K.T.ltoreq.M. The T subbands in each group are then assigned to the T sets such that the ith subband in each group is assigned to the ith set.

FIG. 2B illustrates the T subband sets generated based on the partitioning illustrated in FIG. 2A. The K subbands in each T set are shown with shaded boxes. For this embodiment, the K subbands in each set are uniformly distributed across the M usable subbands, and consecutive subbands in the set are spaced apart by T subbands. The T subband sets may be assigned to T cells or T sectors for pilot transmission. Each cell or sector can only transmit pilots on subbands in the set assigned to that cell/sector.

As a specific example, a multi-carrier system may have 512 sub-bands that are assigned indices of 1 through 512. Of these 512 subbands, 50 subbands may also be allocated for pilot transmission in each sector. Thus, these 512 subbands may also be used to form a set of 9 50 subbands (i.e., T-9 and K-50), as shown in table 2.

Table 2

These 9 subband sets may then be assigned to 9 different sectors for pilot transmission.

In general, the M usable subbands may be allocated to the T sets in various ways, and this is within the scope of the present invention. The T sets may include the same or different number of subbands. Also, the subbands in each set may be uniformly or non-uniformly distributed across the M usable subbands. The T subband sets are "disjoint" from each other to avoid interference. The subband sets are disjoint from each other such that each of the M usable subbands is assigned to at most one set. Each set also includes a sufficient number of subbands to enable the terminal to describe the channel based on pilot transmission on only those subbands. In general, the number of sets formed and the number of subbands included in each set (i.e., the particular values of T and K) may depend on various factors, such as:

● number of usable subbands in the system;

● delay spread and coherence bandwidth of the system, which determines the maximum separation between successive pilot subbands in each set to avoid performance degradation;

● size of the cluster to obtain frequency orthogonality; and

● whether time orthogonality is also used for pilot transmission.

The cyclic prefix of an OFDM symbol (described below) may be defined to include C_pA sample of C_pSuitably chosen based on the delay spread of the system such that the cyclic prefix includes a significant portion of all multipath energy. To avoid performance degradation, the number of subbands in each set (K) may be chosen such that K ≧ C_pAnd these subbands may be uniformly distributed across the system operating bandwidth. In this case, the maximum number of disjoint sets that can be formed is N/C_p. For example, if N is 256, and C_pUp to 16 subband sets may be formed. A fewer number of disjoint sets may also be formed, where each set includes more than C_pA subset of (a). In this case, including more than the minimum required number of subbands may result in higher signal quality for pilot reception, so that improved channel estimation as well as pilot strength estimation may be obtained. Alternatively, more disjoint sets may be formed, each set comprising less than C_pSub-bands of. In this case, including less than the minimum required number of subbands may result in insufficient characteristics of frequency selectivity of the operating band, and some occurrence may occurDeterioration of these properties.

For simplicity, the following description assumes that each of the T subband sets includes K subbands, that the subbands in each set are uniformly distributed and spaced by T subbands (as shown in fig. 2B), and that K · T ═ M. The number of sets formed depends on the desired frequency orthogonality and the size of the clusters, as described below.

Fig. 3A illustrates an example subband allocation to obtain frequency orthogonality for a cluster with 3 cells, where each cell includes 3 sectors (i.e., a 9-sector 3-cell cluster). Each of the 9 sectors within a cluster is assigned one of 9 subband sets (which may be formed, for example, as shown in table 2). The subband sets assigned to each sector are represented by numerical references adjacent to the arrows of fig. 3A. Each sector then transmits its pilot only on the subbands in its assigned set. The 9 sectors in a cluster may transmit their pilots on 9 disjoint sets of subbands at the same time while achieving frequency-domain orthogonality and avoiding interference.

Fig. 3B illustrates an example subband assignment to achieve frequency orthogonality for a cluster with 7 cells, where each cell includes 3 sectors (i.e., a 21-sector 7-cell cluster). Each of the 21 sectors within a cluster is assigned one of 21 subband sets. The 21 sectors in the cluster can simultaneously transmit their pilots on 21 disjoint sets of subbands while achieving orthogonality in the frequency domain and avoiding interference.

In general, a cluster may be defined to include any number of cells, and each cell may include any number of sectors. As an example, a cluster may be defined to include 1, 2, 3, 7, or 19 cells. The size of the clusters may depend on various factors, such as those listed above.

Frequency orthogonality may also be achieved for systems using multiple antennas at each sector for pilot and data transmission to achieve spatial dispersion and improve reliability. For example, each sector may transmit data from two antennas using a Space Time Transmit Diversity (STTD) scheme or an Almouti scheme. The STTD scheme is described in 3G TS25.211 and in provisional U.S. patent application serial No. 60/421,309, entitled "mimo lan system," filed on year 2002, month 10 and 25, assigned to the assignee of the present invention and incorporated herein by reference. The Alamouti scheme is described by S.M. Alamouti in entitled "A Simple Transmit diversity Technique for Wireless Communications," published in IEEE JSAC, Oct.1998, also incorporated herein by reference. For a system with sectors of multiple antennas, each antenna may be assigned a different set of subsets.

2. Time orthogonality

Time orthogonality may be achieved by "covering" the pilots of each cell or sector with different orthogonal codes. At the terminal, the pilots from each cell/sector may be recovered by "decovering" the received signal with the same orthogonal code used by that cell/sector. The cover process is that a given pilot or data symbol to be transmitted (i.e., a set of Q pilot/data symbols with known values) is multiplied by all Q chips of a Q-chip orthogonal sequence to obtain Q covered symbols, which are further processed and then transmitted. The decovering process multiplies the received symbols by (a) Q chips of an orthogonal sequence of the same Q chips and (b) the complex conjugate of the pilot or data symbol (or the complex conjugate of Q pilot/data symbols) to obtain Q decovering symbols, which are then accumulated to obtain an estimate of the transmitted pilot or data symbol. Covering and uncovering are known in the art and are described below. Decovering removes or cancels pilots transmitted by other cells/sectors that use different orthogonal codes for their pilots. In this way, orthogonality among pilot transmissions from multiple cells/sectors may be achieved.

The effectiveness of pilot orthogonality by coverage may depend on the information with base station timing. Time orthogonality may be achieved for sectors of the same cell because the sectors operate simultaneously. The cells within each cluster or all cells within the system may also operate simultaneously to allow time orthogonality to be achieved for the pilots transmitted by these cells.

Time orthogonality may be achieved with various types of orthogonal codes, such as Walsh codes and Orthogonal Variable Spreading Factor (OVSF) codes. The length of the orthogonal codes used for pilot coverage depends on the number of orthogonal codes required, which in turn depends on the size of the cluster that achieves time orthogonality. For example, if time orthogonality is desired for a cell with 3 sectors, 3 orthogonal codes are required (i.e., one code per sector), and each orthogonal code would have a length of 4 chips.

Table 3 lists 4 chip Walsh codes that can be assigned to up to four different sectors, cells, or antennas.

Table 3

Walsh code	Value of
		W(n)	1 1 1 1
W(n)	1 1 -1 -1
		W(n)	1 -1 1 -1

Walsh code	Value of
		W(n)	1 -1 -1 1

A particular Walsh code can be assigned to each sector or each antenna of a given cell. A value of "-1" of the Walsh code may indicate the inverse of the pilot symbol (i.e., the inverse of the pilot symbol)And a value of "1" may indicate no inversion. The same Walsh code may be applied to each subband used for pilot transmission. For each pilot subband, the four chips of the Walsh code are applied to four pilot symbols to be transmitted in four consecutive OFDM symbol periods. The length of the Walsh code is thus T_w＝4·T_symWherein T is_symRepresenting one OFDM symbol period. If the pilot transmission is longer than four OFDM symbol periods, the same Walsh code may be repeated as many times as necessary. Walsh codes are also known as Walsh sequences or Walsh symbols, and T_wRepresenting one Walsh symbol period.

Fig. 4A shows an example orthogonal code allocation to obtain time orthogonality for a cell with three sectors (i.e., a 3-sector 1-cell cluster). Each of the three sectors within a cell is assigned a different orthogonal code. The three orthogonal codes assigned to the 3 sectors are labeled a, B, and C. As shown in fig. 4A, the same subband set may be used by all three sectors in a cell. Time domain orthogonality is then achieved for the pilot transmissions sent from the three sectors by using different orthogonal codes.

Fig. 4B illustrates an example orthogonal code allocation to achieve time orthogonality for a cell with three sectors, each sector using two antennas for pilot and data transmission. Each of the three sectors in the cell is assigned two orthogonal codes, one for each antenna. The three pairs of orthogonal codes assigned to the three sectors are labeled a/B, C/D and E/F. The 3 sector cell then requires a total of six orthogonal codes, each of 8 chips in length.

The time orthogonality characteristic may be deteriorated due to temporal variations in the propagation path between the base station and the terminal. Therefore, it is desirable to use short orthogonal codes so that the propagation path is substantially constant for the duration of the orthogonal code.

3. Combined frequency and time orthogonality

A combination of frequency and time orthogonality may be used for pilot transmission. In one embodiment, frequency orthogonality is achieved for a plurality of cells within a cluster and time orthogonality is achieved for a plurality of sectors within each cell.

Fig. 4C illustrates an example subband and code allocation to achieve frequency and time orthogonality for a 9-sector 3 cell cluster. Each of the three cells within the cluster is assigned a different set of subbands to achieve frequency orthogonality among the three cells. The three sectors of each cell are also assigned three different orthogonal codes to obtain time orthogonality among the three sectors. Each sector of each cell would then transmit its pilot using its assigned orthogonal code and only on the subbands in the set assigned to its cell. Orthogonality is then achieved for the pilot transmissions from the nine sectors in the cluster and interference is avoided.

Fig. 4D illustrates an example subband and code allocation to achieve frequency and time orthogonality for a 21-sector 7-cell cluster. . Each of the seven cells within a cluster is assigned a different set of subbands. The three sectors of each cell are also assigned different orthogonal codes. Each sector of each cell would then transmit its pilot using its assigned orthogonal code and only on the assigned subbands.

Frequency and time orthogonality may also be achieved in some other way and this is within the scope of the invention. For example, multiple cells may be assigned the same subband set but different orthogonal codes. As another example, multiple subband sets may be assigned to multiple sectors of the same cell, and different cells within a cluster may be assigned different orthogonal codes.

For a system with sectors having multiple antennas, orthogonality can be achieved for pilot transmissions from the multiple antennas in a number of ways. In one embodiment, each cell is assigned a subband set and each antenna within a cell is assigned a different orthogonal code. If each sector includes two antennas, each sector may be assigned a pair of orthogonal codes, as shown in fig. 4B. In another embodiment, multiple sectors of a cell are assigned different orthogonal codes and multiple antennas of each sector are assigned different sets of subbands. The same subband set may be used for all sectors of the same cell, and antennas assigned the same subband set are assigned different orthogonal codes. For example, for a 3-sector cell, each sector includes two antennas, two subband sets (e.g., sets 1 and 2) may be assigned to the two antennas of each cell, and three sectors may be assigned orthogonal codes A, B and C. One sector of a cell may be assigned subband set/orthogonal code pairs 1-a and 2-a, a second sector may be assigned 1-B and 2-B, and a third sector may be assigned 1-C and 2-C.

4. Scrambling code

Scrambling codes can be used to randomize pilot interference and can identify base stations. A different scrambling code may be assigned to each sector, each cell, or each cluster. The scrambling code may be a pseudo-random number (PN) sequence or some other unique sequence. The scrambling code may be applied to the pilot in the frequency domain (e.g., before orthogonal code coverage), as described below. The scrambling code may also be applied in the time domain (e.g., after OFDM processing), in which case the scrambling code rate should not be greater than the OFDM code rate to preserve frequency orthogonality. Complementary processing is then performed by the terminal to recover the pilot. The scrambling and descrambling processes at the base station and the terminal are described as follows.

Fig. 5 shows an example system layout 500 in which a different scrambling code is assigned to each 7-cell cluster. Each cluster within the layout is drawn with a thick solid line. An example cluster is shown with seven shaded cells. For this embodiment, seven cells within each cluster are assigned different subband sets (labeled 1 through 7), and three sectors within each cell are assigned different orthogonal codes (labeled A, B and C). The pilot transmission from each sector within the layout can be identified as (1) the subband set assigned to the cell to which the sector belongs, (2) the orthogonal code assigned to the sector, and (3) the scrambling code assigned to the cluster to which the sector belongs. Other system layouts with different sub-bands, orthogonal codes, and scrambling code assignments may also be developed and are within the scope of the invention.

As shown in fig. 5, a terminal in a given sector will only receive pilot interference from other sectors assigned the same subband set and the same orthogonal code. For example, terminals in the sector labeled 1-A will only receive pilot interference from other sectors labeled 1-A in the arrangement.

Each scrambling code S_i(n) is a sequence of unique code chips, where n is the chip index of the sequence. In one embodiment, each scrambling code chip is of the formComplex value of (a), where s_i(n) andeach value can be 1 or-1. In other embodiments, the scrambling code may be defined in some other way, and the code chip values are either real or complex.

The scrambling code may be implemented in a number of ways, depending on the characteristics of the wireless channel. In general, the channel should be substantially constant over the entire duration of time that each scrambling code chip is applied. The time interval during which the channel is substantially constant is called the coherence time and can be denoted by τ. The length of the orthogonal code is denoted T_wWherein for the 4 chip Walsh sequence T shown in Table 3_w＝4·T_sym。

For the first scrambling scheme, if the coherence time of the channel is much greater than the orthogonal code length (i.e., τ > T_w) The scrambling code may be applied over a plurality of orthogonal sequences. In particular, each scrambling code chip may be applied to a length of T_wOn an orthogonal sequence. The same scrambling code chips may be applied to each of the K subbands used for pilot transmission. For the example Walsh sequences shown in Table 3, each scrambling code chip is applied to four Walsh code chips, which are applied to four pilot symbols transmitted in four consecutive OFDM symbol periods.

For the first scrambling scheme, to recover the pilot from a particular sector, the terminal may perform orthogonal code decovering and then scrambling code descrambling using the orthogonal code and the scrambling code assigned to that sector. The terminal may also perform coherent integration over all or part of the scrambling code sequence to recover the pilot and distinguish "co-channel" sectors (i.e., sectors assigned the same set of subbands but different orthogonal codes and/or scrambling codes). The coherent integration process refers to a process in which a plurality of complex-valued symbols are combined in a manner that takes their phase information into account.

For the second scrambling scheme, each scrambling code chip may be applied to one orthogonal chip if the coherence time of the channel is short, such that the terminal can perform coherent integration over a single orthogonal sequence (or one Walsh symbol). The same or different scrambling code chips may be used for the K pilot subbands. For example, for the exemplary 4 chip Walsh sequence shown in Table 3, the scrambling code may be defined by a length of 4-K. The first K scrambling code chips may be used for the K pilot subbands for the first Walsh code chip, the next K scrambling code chips may be used for the K pilot subbands for the second Walsh code chip, then K scrambling code chips may be used for the K pilot subbands for the third Walsh code chip, and the last K scrambling code chips may be used for the K pilot subbands for the fourth and last Walsh code chips.

For both second scrambling schemes, the same scrambling sequence may be used by all base stations for which time orthogonality is to be achieved. The scrambling code provides pilot interference randomization. Since the same scrambling sequence is used by multiple base stations, each base station may be identified by its assigned orthogonal code, possibly the scrambling code and its assigned set of pilot subbands.

For the scrambling scheme, to recover the pilot, the terminal may derive a pilot estimate for each pilot subband, as described below. The receiver may then (1) obtain a channel response estimate for each of the plurality of pilot and data subbands based on the pilot estimates for all K pilot subbands and (2) obtain a received pilot power estimate, which is the sum of the squares of the pilot estimate amplitudes for all K pilot subbands. The processing performed by the terminal for the pilot is described in detail below.

Pilot transmission scheme

The pilot may be transmitted by the base station on the downlink in various manners to facilitate pilot detection and channel estimation. Pilot detection may be used to facilitate system synchronization (frequency and timing acquisition), hard handoff, and soft handoff. Channel estimation may be used to facilitate coherent data demodulation. Table 4 lists four example pilot transmission schemes for a multi-carrier communication system

Table 4

For a burst pilot structure, each sector transmits its pilot in bursts at specified time intervals or time slots (rather than continuously). Each sector may transmit pilot and data in a Time Division Multiplexed (TDM) manner. For a continuous pilot structure, each sector transmits its pilot continuously on its assigned set of pilot subbands. Each sector may transmit data on the remaining available subbands that are not designated for pilot transmission.

For a synchronous system, the timing of all sectors of all cells within the system are synchronized (e.g., based on GPS time or some other common timing resource). For an asynchronous system, all sector timing of each cell may be synchronized, but the timing of different cells within the system is not synchronized.

For a synchronous burst pilot transmission scheme, sectors and cells in the system are synchronized and their pilots are transmitted in bursts in the same designated time slots. For this scheme, all sectors transmit their pilots simultaneously, but the pilots are orthogonalized using disjoint sets of pilot subbands and/or orthogonal codes. Data is not transmitted during the pilot transmission period. The terminal can obtain a higher quality channel estimate for the different sectors because no interference is received from the data transmission. Moreover, the channel estimate for a given sector can be further improved by canceling interference from pilots transmitted by other sectors on the same set of pilot subbands, using interference cancellation techniques as described below.

Fig. 6A illustrates transmission of pilots from multiple sectors for a sync burst pilot transmission scheme. For this scheme, the pulses T of a sector at a particular duration_pilotTransmitting its pilot on inherently disjoint sets of subbands with a specific time interval T between bursts_int. As indicated in fig. 6A, the timing of the sectors is synchronized so that the pilot bursts are aligned to them approximately at their transmission times. Each sector may transmit data in all available subbands in the time interval between pilot bursts. (for simplicity, frequency and time are not drawn to scale within FIGS. 6A and 6B).

For a synchronous continuous pilot transmission scheme, the sectors and cells in the system are synchronous and each sector continuously transmits its pilot on a designated set of pilot subbands. For this scheme, pilots from different sectors may be further orthogonal by using different orthogonal codes. For each sector, data is not sent on the subband set designated for pilot transmission.

Fig. 6B illustrates pilot transmission from multiple sectors in a synchronous continuous pilot transmission scheme. For this scheme, a sector transmits its pilot on disjoint sets of subbands consecutively. Each sector may transmit data on subbands that are not designated for pilot transmission. As shown in fig. 6B, the timing of the sectors is synchronized.

For an asynchronous burst pilot transmission scheme, sectors in the system send their pilots in bursts in designated time slots, and use disjoint sets of pilot subbands. The sectors within each cell may further orthogonalize their pilots by using different orthogonal codes. However, since the cells are not synchronized, pilots from different cells may arrive at the terminal at different times, and the terminal needs to perform a search for these pilot bursts. Moreover, since the cells are not synchronized, data transmissions from sectors within one cell may interfere with pilot transmissions from sectors of other cells, and vice versa.

For an asynchronous continuous pilot transmission scheme, the sectors and cells in the system are not synchronized, and each sector continuously transmits its pilot on a designated set of pilot subbands. Also, the sectors within each cell may orthogonalize their pilots by using different orthogonal codes. Since the cells are not synchronized, the terminal may need to determine the timing of each sector recovered.

For the sync burst pilot transmission scheme, the pilot for each sector experiences minimal degradation from co-channel interference, i.e., interference from other sectors assigned the same pilot subbands and orthogonal codes. For a synchronous continuous pilot transmission scheme, the pilot for each sector experiences degradation due to co-channel interference due to data transmission on that pilot subband by neighboring sectors. For the asynchronous burst/continual pilot transmission scheme, the pilot for each sector experiences degradation from co-channel interference due to data transmission plus inter-carrier interference caused by asynchronous OFDM symbol timing, where inter-carrier interference is not present without multipath.

Receiver processing at the terminal for each of these pilot transmission schemes is described in detail below.

Regardless of the pilot transmission scheme selected for use, the pilot subbands may be allocated to the sectors in various manners. In one embodiment, the set of subbands assigned to pilot transmission for each sector is fixed. In another embodiment, each sector transmits its pilot on a different set of subbands at a different time slot. This embodiment may allow the terminal to obtain a better channel estimate for the sector.

System III

Fig. 7 shows a block diagram of an embodiment of a base station 110x and a terminal 120x within the multi-carrier communication system 100. For simplicity, base station 110x performs processing for one sector and includes one antenna.

On the downlink, at base station 110x, a Transmit (TX) data processor 714 receives traffic data from a data source 712 and signaling and other data from a controller 730. TX data processor 714 formats, codes, interleaves, and modulates (i.e., symbol maps) the data to provide data modulation symbols, or data symbols only. A Modulator (MOD)720 receives and multiplexes the data symbols with pilot symbols, performs the required processing, and provides a stream of OFDM symbols. The processing of modulator 720 is described as follows. A transmitter unit (TMTR)722 then processes the stream of OFDM symbols to provide a downlink signal, which is then transmitted from an antenna 724 to the terminals.

At terminal 120x, the downlink signals transmitted by the multiple base stations for the multiple sectors are received by antenna 752. The received signal is processed (e.g., amplified, filtered, frequency downconverted, and digitized) by a receiver unit (RCVR)754 to provide samples. A demodulator (DEMOD)760 then processes the samples in a manner complementary to that implemented by modulator 720 to provide pilot strength estimates and data symbol estimates for the recovered sector. A Receive (RX) data processor 762 further processes (e.g., symbol demaps, deinterleaves, and decodes) the data symbol estimates to provide decoded data, which may then be provided to a data sink 764 for storage and/or a controller 770 for further processing.

The processing for the uplink may be the same as or different from the processing for the downlink. Data and signaling are processed (e.g., coded, interleaved, and modulated) by a TX data processor 784 to provide data symbols, which are multiplexed with pilot symbols and further processed by a modulator 790 to provide transmit symbols. Modulator 790 may implement OFDM processing, CDMA processing, etc., depending on the particular modulation technique used for the uplink. A transmitter unit 792 further processes the transmit symbols to generate an uplink signal, which is then transmitted from the antenna 752.

At base station 110x, the uplink signal from the terminals is received by antennas 724 and the received signal is processed by receiver units 738 to provide samples. The samples are further processed by a demodulator 740 to provide data symbol estimates, which are further processed by an RX data processor 742 to provide decoded data for each terminal that is recovered. The decoded data may be provided to a data sink 744 for storage and/or to controller 730 for further processing.

Controllers 730 and 770 control the operation of various processing units at the base station and terminal, respectively. Memory units 732 and 772 store data and program codes used by controllers 730 and 770, respectively.

1. Base station pilot processing

Fig. 8 shows a block diagram of an embodiment of a modulator 720. In this embodiment, pilot transmission occurs on the K pilot subband sets assigned to the ith sector. N for pilot symbols_WWalsh code of one chip W_i(n) covering and using scrambling code S assigned to i-th sector_i(n) scrambling code.

In general, the same pilot symbols may be used for all pilot subbands or different pilot symbols may be used for different pilot subbands. The pilot symbol is a modulation symbol derived based on a particular modulation scheme (such as BPSK, QPSK, or M-QAM), i.e., a complex value corresponding to a point in the signal constellation for the modulation scheme. Also, the same pilot symbols may be used by all sectors, or different pilot symbols may be used by different sectors. In one embodiment, a particular set of M pilot symbols is defined for the M usable subbands in the system. The pilot symbols used by each sector depends on the set of pilot subbands assigned to that sector. In any event, terminals in the system have a priori knowledge of the pilot symbols used by the sectors in the system.

In modulator 720, pilot symbols p transmitted by the i-th sector_i(n) are provided to a demultiplexer (Demux)812 and demultiplexed into K pilot symbol substreams for the K pilot subbands. For each OFDM symbol period, the same pilot symbols may be transmitted on all K pilot subbands, or a set of K pilot symbols may be sent on K pilot subbands. In any case, each of the K pilot symbol substreams is provided to a respective TX pilot subband processor 820 that processes pilot symbols for its assigned pilot subband.

Within each TX pilot subband processor 820, pilot symbol p for the assigned k-th pilot subband_i，k(n) scrambling code segment S provided to complex multiplier 822 and multiplied by the kth pilot subband_i，k(n) of (a). Scrambling can be achieved in various ways. For example, the scrambling code can be such that each scrambling code chip is applied to the entire Walsh sequence W for each of (1) K pilot subbands_i(n) (for the first scrambling scheme described above), (2) one Walsh code chip in one pilot subband (for the second scrambling scheme described above), (3) one Walsh code chip for all K pilot subbands, or (4) some other combination of Walsh code chips and pilot subbands.

The K segments of scrambling code chips used by the K TX pilot subband processors 820a through 820K may be the same or different, depending on the particular scrambling scheme implemented. For the first scrambling scheme, the same scrambling sequence is used for each of the K pilot subbands, and each scrambling code chip is applied to N_WA number of consecutive pilot symbols, which is determined by counting at N_WMaintaining the scrambling code chips unchanged for each successive OFDM symbol period. For the second scrambling scheme, scrambling sequence S_i(n) is divided into K scrambling code segments (e.g., as described above for the second scrambling scheme), one segment for each of the K pilot subbands. Each scrambling code chip is then applied to one pilot symbol for one pilot subband.

The scrambled pilot symbols from multiplier 822 are then provided to multiplier 824 and are provided with Walsh code W_i(n) covering. The covering is by being at N_WAre connected with each otherN transmitted in continuous OFDM code element time interval_WMultiplication of scrambled pilot symbols by Walsh code W_iN of (N)_WImplemented by chip multiplication, where N is the exemplary Walsh code shown in Table 3_W4. The covered pilot symbols are then multiplied by multiplier 826 with gain G_pilotScaling is performed, which determines the amount of transmit power used for pilot transmission. In general, the total transmit power P per sector or per antenna_totalSubject to constraints such as regulatory requirements and/or power amplifier limitations. The total transmitting power P_totalIs allocated for pilot transmission and the remaining power may be used for data transmission. Amount of power P for pilot transmission_pilotMay be selected such that pilot detection/acquisition for terminals in the sector is expedited while minimizing pilot interference to data transmissions by other sectors. Pilot power P_pilotMay be fixed or variable, and gain G_pilotBased on pilot power P_pilotAnd (4) determining. The processed pilot symbols from the K TX pilot subband processors 820a through 820K are then provided to an MxN switch 848.

Data symbol d to be transmitted by the ith sector for up to (M-K) subbands to be used for data transmission_i(n) are provided to demultiplexer 832 and demultiplexed into up to (M-K) data symbol substreams. Each data symbol is also a modulation symbol derived based on a particular modulation scheme (such as BPSK, QPSK, or M-QAM). The same or different modulation schemes may be used for pilot or data symbols. Each data symbol substream is provided to a respective TX data subband processor 840, which processes the data symbols for the assigned data subband. Each processor 840 may perform Walsh covering, scrambling, scaling, some other processing, or nothing. The processed data symbols from the (M-K) data subband processors 840a through 840q are also provided to switch 848.

Switch 848 orders the processed pilot symbols from K TX pilot subband processors 820 and the processed data symbols from (M-K) TX data subband processors 840 such that the symbols are provided to their assigned pilot and data subbands. Switch 848 also provides a signal value of zero for each unused subband. For each OFDM symbol period, switch 848 provides N sets of output symbols (including processed pilot and data symbols and zeros) for N total subbands by Inverse Fast Fourier Transform (IFFT) unit 850.

Within IFFT unit 850, the N symbols of each OFDM symbol period are converted to the time domain using an inverse fast fourier transform to obtain a "transformed" symbol that includes N time-domain samples. To remove inter-symbol interference (ISI) due to frequency selective fading, portions of each transformed symbol are repeated by cyclic prefix generator 852 to form corresponding OFDM symbols, which comprise N + C_pA sample of C_pIs the number of repeated samples. The repeated portion is commonly referred to as a cyclic prefix. The OFDM symbol period corresponds to the duration of one OFDM symbol. Cyclic prefix generator 852 provides a stream of OFDM symbols for transmission over one antenna.

If a sector is equipped with multiple antennas, the same pilot processing as shown in fig. 8 may be implemented for each antenna. In particular, the pilot symbols for each antenna are covered with a Walsh code, scrambled with a scrambling code, and multiplexed onto the set of K pilot subbands assigned to that antenna. Depending on the particular pilot transmission scheme implemented, the same or different Walsh codes may be assigned to multiple antennas, the same or different scrambling codes may be used for the antennas, and the same or different sets of subbands may be used for the antennas. The data symbols may be processed according to a STTD or Alamouti scheme for transmission over multiple antennas, as described in the aforementioned U.S. provisional patent application serial No. 60/421309.

2. Terminal pilot processing

Fig. 9A shows a block diagram of an embodiment of a demodulator 760a that may be used for the sync burst pilot transmission scheme described above, where a sector transmits its pilot in bursts in designated time slots. For each pilot burst, demodulator 760a may perform processing to recover the pilots transmitted from multiple sectors.

Within demodulator 760a, the received OFDM symbols are provided to a cyclic prefix removal unit 912, which removes the cyclic prefix appended to each OFDM symbol to obtain a corresponding received transformed symbol. FFT unit 914 then converts each received converted symbol to the frequency domain to obtain N received symbols for the N total subbands. An NxM switch 916 provides received symbols for each set of K pilot subbands, one processor 920 for each pilot subband in the set, to a respective set 918 of K RX pilot subband processors 920aa through 920 ak. For a synchronization burst pilot transmission scheme, pilots are received from multiple sectors on disjoint sets of subbands. The set of RX pilot subband processors may then be used to perform pilot processing for each sector to be recovered. Multiple sets of RX pilot subband processors may also be used to process a given set of pilot subbands because multiple sectors (e.g., from different cells or clusters) may transmit with different orthogonal codes on the same set of subbands. For simplicity, only one set of RX pilot subband processors is shown in FIG. 9A for each set of pilot subbands.

The pilot processing for a terminal is complementary to that performed by the sector and further depends on the characteristics of the channel. To improve pilot detection performance and better distinguish between pilots transmitted by different sectors, it is desirable to perform coherent integration over as many OFDM symbol periods as possible and as many pilot subbands as possible. However, the amount of coherent integration achieved in the time and frequency domains depends on the coherence time and coherence bandwidth of the channel, respectively. In particular, the duration of the coherent integration (i.e., the number of OFDM symbols over which coherent integration can be achieved) should be less than the coherence time of the channel (i.e., the duration over which the channel is substantially constant). Also, the frequency range including subbands that can be coherently added should be smaller than the coherence bandwidth of the channel. The coherence bandwidth is the frequency band within which the channel is substantially constant and is related to the delay spread of the channel.

The pilot processing shown in fig. 9A achieves coherent integration over a single Walsh symbol period and a single pilot subband. For simplicity, the pilot processing for a given sector i is described as follows. Received symbols r for the assigned k-th pilot subband in each RX pilot subband processor 920 for sector i_k(n) is provided to multiplier 922 and multiplied by the Walsh code W for sector i_i(n) of (a). The decovered symbols are then provided to a complex multiplier 924 and multiplied by the complex conjugate S of the scrambling code chip_i，k ^*(n) which is used by sector i for the k subband in the nth OFDM symbol period. The descrambling is performed in a manner complementary to the scrambling performed by sector i. For the first scrambling scheme, each scrambling code chip is determined by maintaining the scrambling code chip at N_WIs constant over a period of one continuous OFDM symbol and is applied to N_WA number of consecutive decovered symbols. For the second scrambling scheme, one scrambling code segment is used for each of the K pilot subbands, and each scrambling code chip is applied to one decovered symbol from multiplier 922 in one pilot subband. The descrambled symbols from multiplier 924 are then provided to complex multiplier 926 and multiplied by the complex conjugate p of the pilot symbol_i，k ^*(n) the pilot symbol is transmitted on the kth subband by sector I in the nth OFDM symbol period. The output from multiplier 926 is then accumulated over each Walsh symbol period by Accumulator (ACC)928 to provide pilot estimates for that Walsh symbol period

Multipliers 922, 924, and 926 operate at the OFDM symbol rate (i.e., 1/T)_sym). Accumulator 928 performs accumulation at the OFDM symbol rate, but provides pilot estimates for each Walsh symbol period and is cleared at the beginning of each Walsh symbol period. Filter 930 and unit 932 operate at the Walsh symbol rate (i.e., 1/T)_wOr 1/4T for a 4 chip Walsh sequence_sym)。

Pilot estimates from accumulator 928And may be further filtered by filter 930 to provide an estimate of the channel for the k-th pilot subband to sector iFilter 930 may be implemented with an accumulator, a Finite Impulse Response (FIR) filter, an Infinite Impulse Response (IIR) filter, or some other type of filter. Unit 932 computes pilot estimates from accumulator 928Provides a pilot strength estimate for the k subband assigned to sector i

Summer 934 receives pilot strength estimates for all K pilot subbands for sector i for each OFDM symbol period of pilot transmissionAnd summed to obtain a pilot strength estimate for the OFDM symbol periodAccumulator 938 then accumulates the pilot strength estimates over some or all of the pilot transmission slotsProviding a final pilot strength estimate for sector iFor example, accumulator 938 may perform accumulation over the entire pilot burst. Accumulator 928 performs coherent integration and accumulator 938 performs non-coherent integration.

Coherent integration may also be performed over multiple Walsh symbol periods if allowed by the coherence time of the wireless channel. In this case, the pilot estimates for multiple Walsh symbol periods may be accumulated (e.g., by accumulator 928) and the resulting estimates may be provided to filters 930 and 932. Coherent integration may also be performed on multiple pilot subbands (e.g., some or all of the K pilot subbands), if allowed by the coherence bandwidth of the wireless channel. In this case, pilot estimates from accumulator 928 may be accumulated, magnitude squared (e.g., by another accumulator not shown in FIG. 9), for multiple pilot subbands and provided to summer 934. The frequency range that includes the pilot subbands over which coherent integration is performed should be less than the coherence bandwidth of the wireless channel. Coherent integration in the frequency domain may be performed to obtain improved pilot strength estimates, but channel estimates are typically obtained for a single subband. Coherent integration may also be performed over multiple Walsh symbol periods and multiple pilot subbands, if allowed by the coherence time and coherence bandwidth of the wireless channel, to provide an improved pilot strength estimate.

The pilot processing described above provides channel estimates for the K pilot subbands for sector i. The channel estimates for the remaining M-K subbands for sector i may be obtained based on the channel estimates for the K pilot subbands for sector i (e.g., by interpolation). Techniques for Channel Estimation for all M subbands based on Channel Estimation for K subbands are described in U.S. patent application serial nos. 60/422362 and 60/427896, entitled "Channel Estimation for OFDM Communication Systems", filed 2002, No. 10/29, and entitled "Reduced complex Channel Estimation for wireless Communication Systems", filed 2002, No. 11/19, both assigned to the assignee of the present invention and incorporated herein by reference. The signal estimates may be used for data demodulation and other purposes.

Channel estimates for all or a subset of the M usable subbands for sector i may be used to achieve coherent data demodulation for the data transmission received from sector i. Channel estimates may also be obtained for multiple sectors within the system. For each sector, pilot processing uses the Walsh sequence W used by that sector_i(n), scrambling code S_i(n) and a pilot symbol p_i(n) implementation.

The pilot processing described above also provides an estimate of the pilot strength for sector i. Pilot strength estimates may be obtained for multiple sectors within the system. The pilot strength estimates for multiple sectors may be used to determine the best sector to receive a data transmission, to handoff from one sector to another (e.g., for a mobile terminal), and possibly for other purposes.

For the sync burst pilot transmission scheme, demodulator 760a performs pilot processing only in the time slots in which pilots are transmitted by the sectors. The channel estimates for one or more sectors may be used to achieve coherent data demodulation of data transmissions received from the one or more sectors during the time period between pilot bursts.

Fig. 9A illustrates an example pilot processing technique that may be implemented by a terminal. Other pilot processing techniques may also be used and are within the scope of the invention. For simplicity, only the pilot processing for demodulator 760a is shown in FIG. 9A. Data processing may be performed by demodulator 760a in a manner described below.

Fig. 9B shows a block diagram of an embodiment of a demodulator 760B, which may be used for the synchronous continuous pilot transmission scheme described above, where each sector continuously transmits pilot on its designated set of pilot subbands and data on the remaining subbands. In the following description, demodulator 760b recovers the pilot and data transmitted by a given sector i.

Within demodulator 760b, the received OFDM symbols are processed by cyclic prefix removal unit 912 and FFT unit 914 in the manner described above. Switch 916 then provides the received symbols for the K pilot subbands to K RX pilot subband processors 920a through 920K and the M-K received symbols for the remaining subbands to M-K pilot interference cancellers 940a through 940 q.

Each RX pilot subband processor 920 performs pilot processing for one pilot subband for sector i in the manner described in fig. 9A. However, since the pilot subbands for sector i may be used as data subbands by other sectors, coherent integration may be performed over longer time intervals (e.g., multiple Walsh symbol periods) to cancel interference due to data symbols for other sectors to obtain more accurate pilot estimatesCoherent integration time slot byThe channel coherence time is determined and should be less than the coherence time.

Within each RX pilot subband processor 920, multipliers 922, 924, and 926 operate at the OFDM symbol rate (i.e., 1/T)_sym) And implements Walsh sequence W with sector i_i(n) scrambling code S_i(n) and pilot symbol p_iMultiplication of (n). Accumulator 928 operates at the OFDM symbol rate and accumulates the output from multiplier 926 over one or (preferably) multiple Walsh symbol periods to provide a pilot estimate for each accumulation gapFilter 930 operates at the accumulation rate and filters the pilot estimatesTo provide channel estimates for k subbands for sector iThe channel estimates for all K pilot subbands may be further processed (e.g., interpolated) to obtain channel estimates for the data subbands, as described above. Unit 932, adder 934, and accumulator 938 operate at the accumulation rate and provide a pilot strength estimate for sector i

For a continuous pilot transmission scheme, demodulator 760b may perform pilot processing at all times during the communication session. The channel estimate for sector i may be used to achieve coherent data demodulation of the data transmission received on the data subbands from sector i. The data processing may be implemented as follows.

For simplicity, only one set of K RX pilot subband processors 920 and one set of M-K pilot interference cancellers 940 are shown in FIG. 9B. Demodulator 760b may also be implemented with multiple sets of RX pilot subband processors and multiple sets of M-K pilot interference cancellers to concurrently process pilot and data transmissions from multiple sectors.

Demodulators 760a and 760b may also be used to implement pilot processing for the asynchronous burst/continuous pilot transmission scheme described above. If the sectors are asynchronous, the terminal may need to determine the timing of each sector to recover. This can be achieved by using sliding correlators, similar to those used in CDMA systems. The processing for each sector may then be implemented according to the timing of that sector. In particular FFT operation, Walsh sequence W_i(n) decovering and using scrambling codes S_iThe descrambling of (n) is performed based on the timing of the sector to be recovered. Moreover, coherent integration can be performed over longer time periods (e.g., multiple Walsh symbol periods) to cancel interference due to data symbols transmitted by other sectors, such that more accurate pilot estimates are obtained for the sector to be recovered

For an asynchronous burst pilot transmission scheme, pilot processing for each sector may be implemented (1) within a time gap when pilots are transmitted by the sector, and (2) based on the timing of the sector. For an asynchronous continuous pilot transmission scheme, pilot processing for each sector may be performed at all times based on the timing of the sector.

3. Terminal pilot frequency interference cancellation

As described above, pilot transmission from sectors in the system may be such that the subbands used for pilot transmission for a given sector i may also be used for pilot transmission for other sectors. For that sector i, pilot transmissions from other sectors on its pilot subbands represent an interference that, if effectively canceled, can improve the channel estimate and pilot strength estimate for that sector i. Moreover, the subbands used for data transmission by sector i may also be used for pilot transmission by other sectors (e.g., for a continuous pilot transmission scheme). For this sector i, pilot transmissions from other sectors on their data subbands represent an interference that, if effectively cancelled, improves data performance.

For example, a terminal may receive a data transmission from sector 1, with sector 1's pilot transmitted on subband set 1 (e.g., subbands 10, 20, 30,. 500 for the example OFDM system shown in table 2 and fig. 2B). The terminal also knows the pilots transmitted by other sectors. Some of these pilots are not transmitted on subband set 1, e.g., neighbor sector 2 may transmit pilots on subband set 2 (e.g., subbands 11, 21, 31.. 501). Sector 1 typically transmits data to terminals within its coverage area using nearly all of the available subbands that are not in subband set 1. Thus, subbands in set 2 (which may be used for pilot transmission by sector 2) may be used as data subbands by sector 1. Pilot transmission on subbands in set 2 used by sector 2 may act as interference to data transmission on those same subbands by sector 1.

The terminal is generally aware of the pilot transmission by sector 2 on subband set 2. Thus, the terminal may estimate pilot interference from sector 2 from subbands in set 2. The pilot interference estimate may be obtained by (1) estimating the channel from sector 2 to the terminal for each subband in set 2, (2) generating processed (i.e., scrambled and covered) pilot symbols for each subband in set 2 in the same manner as implemented by sector 2, and (3) scaling the processed pilot symbols with the channel estimate. The pilot interference estimate for each subband in set 2 for sector 2 is then subtracted from the received symbols for the same subband to obtain pilot-canceled symbols for that subband.

In general, pilot interference cancellation can be implemented when any pilot is used for downlink pilot or data transmission in one sector and also for downlink pilot transmission in another sector, where the pilots are known to the terminal. Typically, the terminal will know the pilot transmitted by another sector because this information is used to facilitate terminal-assisted handoff between sectors. The terminal typically measures the pilot power received from its current serving sector as well as pilots received from other neighboring sectors that are candidates for handoff. The pilot power measurements may then be used by the terminal to request a handoff to a better serving sector.

Pilot interference cancellation may be performed for the pilot subbands to obtain higher quality pilot estimates since the interference from other sectors on the pilot has been removed. For example, for a sync burst pilot transmission scheme, all sectors transmit their pilots simultaneously, in which case pilot interference cancellation may be implemented to obtain an improved channel estimate for the selected sector. Pilot interference cancellation may also be implemented for the data subbands to obtain higher quality data symbol estimates where interference from other sector pilots to the pilots has been removed. For simplicity, pilot interference cancellation is described below for the data subbands.

Returning to fig. 9B, the received symbols for each of the (M-K) data subbands are provided to a corresponding pilot interference canceller 940. Each canceller 940 estimates the pilot estimates received by the terminal from each interfering sector, the interference designated to be cancelled. Each canceller 940 then (1) obtains a total pilot interference estimate for all designated interfering sectors, and (2) cancels the total pilot interference estimate from the received symbols to provide pilot-canceled symbols for the assigned data subbands.

Fig. 10 shows a block diagram of an embodiment of a pilot interference canceller 940x, which may be used for each of the pilot interference cancellers 940a through 940q of fig. 9. Pilot interference cancellation is achieved after a fast fourier transform in the frequency domain. Canceller 940x performs pilot interference cancellation for one data subband.

Within pilot interference canceller 940x, the received symbols for the assigned subbands are provided to L pilot interference estimators 1020a through 1020L, where L may be any zero or greater integer. Each estimator 1020 estimates pilot interference from the assigned jth interfering sector on the assigned kth subband and provides the pilot interference estimates with its assigned subbands and sectors

Reception of assigned kth sub-band within each estimator 1020Code element r_k(n) is provided to multiplier 1022 and multiplied by the Walsh code W used by the jth interfering sector_j(n) of (a). The output of multiplier 1022 is then multiplied by the complex conjugate S of the scrambling code chip by complex multiplier 1024_i，k ^*(n) the scrambling code is used for the kth subband by the jth interfering sector. The descrambled symbols from multiplier 1024 are then multiplied by the complex conjugate of the pilot symbol, p, by multiplier 1026_i，k ^*(n) the symbol is transmitted on the k subband by the j interfering sector.

The output from multiplier 1026 is then accumulated by accumulator 1028 over each Walsh symbol period to provide a pilot estimate for the kth subband in that Walsh symbol periodThe pilot estimate from accumulator 1028 is further filtered by filter 1030 to provide a channel estimate for the jth interfering sector for the kth subbandFilter 1030 may be implemented with an accumulator, FIR filter, or IIR filter. The response (e.g., accumulation duration) of filter 1030 may depend on the rate at which the channel is fading. To derive a pilot interference estimate for the jth interfering sectorPilot symbol p used by jth sector_i，k(n) (1) the scrambling code chips S are multiplied by multiplier 1034_i，k(n), (2) Walsh code W is used by multiplier 1036_j(n) covering, and (3) multiplying by a multiplier 1038 the channel estimateThe pilot interference estimates from all assigned estimators 1020 are then summed by summer 1042 to obtain a total pilot interference estimate for the k-th subbandTotal pilot interference estimationAnd then from the received symbol r by summer 1044_k(n) to obtain a pilot-canceled symbol for the k-th subband.

Returning to FIG. 9, the pilot-canceled symbols from pilot interference cancellers 940a through 940q for the M-K data subbands are provided to M-K RX data subband processors 980a through 980q, respectively. Each processor 980 processes the pilot-canceled symbols for the assigned data subbands in a manner complementary to that implemented by processor 840 in fig. 8. Each processor 980 can also perform coherent data demodulation by computing a dot product of the pilot-canceled symbols and the channel estimates for the assigned data subbands to provide data symbol estimatesWhich are estimates of the data symbols transmitted on the subband. The channel estimates for the recovered data subbands for the ith sector may be derived (e.g., using interpolation) based on the channel estimates obtained for the pilot subbands.

A multiplexer 990 then receives and multiplexes the data symbols from RX data subband processors 980a through 980q to provide recovered i-th sector data symbol estimates

The above-described pilot interference cancellation techniques may be directly extended to multiple receive antenna scenarios at the terminal. In this case, the same pilot processing may be implemented for the received signals obtained from each terminal antenna. The pilot-canceled symbols for each antenna may be further coherently demodulated with the channel estimate to provide data symbol estimates for that antenna. The data symbol estimates from all antennas are then weighted and combined to provide the final data symbol estimates, which are then decoded.

The pilot transmission and pilot interference cancellation techniques described above may be implemented in various ways. For example, the pilot transmission processing at the access point and the pilot detection and pilot interference cancellation processing at the terminal can be implemented in hardware, software, or a combination thereof. For a hardware implementation, the elements for processing pilots for transmission/reception and for pilot interference cancellation may be implemented within the following elements: one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, and other electronic units designed to perform the functions described herein, or a combination thereof.

For a software implementation, the pilot transmission/reception and pilot interference cancellation processing may be implemented within modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in memory unit(s) (e.g., memory units 732 and 772 in fig. 7) and executed by processors (e.g., controllers 730 and 770). The memory unit may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various means as is known in the art.

Headings are included herein for reference and to aid in locating certain sections. These headings are not intended to limit the concepts described under the headings and these concepts may apply to other sections throughout the specification.

Those of skill would further appreciate that the various illustrative logical blocks, modules, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. Various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. The skilled person will recognize the interactivity of the hardware and software in these cases and how best to implement the described functionality for each particular application.

The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (47)

1. A method for transmitting pilot on a downlink in a wireless multi-carrier communication system, comprising:

generating pilot symbols for transmission on a first set of subbands, wherein the first set of subbands is used for pilot transmission by a first transmitting entity and is disjoint from a second set of subbands used for pilot transmission by a second transmitting entity, and wherein each pair of adjacent subbands in the first set of subbands includes a range of frequencies that is less than or equal to a coherence bandwidth of a wireless channel in the system;

processing pilot symbols to obtain pilot signals with pilot symbols, the pilot symbols included in a first set of subbands; and

a pilot signal is transmitted from a first transmitting entity.

2. The method of claim 1, further comprising:

the pilot symbols are processed with an orthogonal code to obtain covered pilot symbols, and wherein the covered pilot symbols are further processed to obtain pilot signals.

3. The method of claim 1, further comprising:

the pilot symbols are processed with a scrambling code to obtain scrambled pilot symbols, and wherein the scrambled pilot symbols are further processed to obtain pilot signals.

4. The method of claim 1, wherein subbands in the first set of subbands are uniformly distributed across a plurality of subbands usable by the first transmitting entity for pilot and data transmission.

5. The method of claim 1, wherein the pilot signals are transmitted in bursts from the first and second transmitting entities.

6. The method of claim 1, wherein the pilot signals are transmitted from the first and second transmitting entities at synchronized timing.

7. The method of claim 1, wherein the multi-carrier communication system implements Orthogonal Frequency Division Multiplexing (OFDM).

8. An apparatus in a wireless multi-carrier communication system, comprising:

means for generating pilot symbols for transmission on a first set of subbands, wherein the first set of subbands is used for pilot transmission by a first transmitting entity and is disjoint from a second set of subbands used for pilot transmission by a second transmitting entity, and wherein each pair of adjacent subbands in the first set of subbands includes a range of frequencies that is less than or equal to a coherence bandwidth for a wireless channel in a system;

means for processing pilot symbols to obtain pilot signals with pilot symbols, the pilot symbols included in a first set of subbands; and

means for transmitting a pilot signal from a first transmitting entity.

9. The apparatus of claim 8, further comprising:

means for processing the pilot symbols with an orthogonal code to obtain covered pilot symbols, and wherein the covered pilot symbols are further processed to obtain pilot signals.

10. The apparatus of claim 8, further comprising:

means for processing pilot symbols with a scrambling code to obtain scrambled pilot symbols, and wherein the scrambled pilot symbols are further processed to obtain pilot signals.

11. A base station in a wireless multi-carrier communication system, comprising:

at least one pilot processor configured to receive and process pilot symbols designated for transmission on a first set of subbands, wherein the first set of subbands is used for pilot transmission by a base station and is disjoint from a second set of subbands used for pilot transmission by another base station in the system, and wherein each pair of adjacent subbands in the first set of subbands includes a range of frequencies that is less than or equal to a coherence bandwidth for a wireless channel in the system;

a conversion unit configured to derive time-domain samples with covered pilot symbols included in the first set of subbands; and

a transmitter unit to process the time-domain samples to obtain a pilot signal for transmission on a downlink.

12. A method of processing pilot received over a downlink in a wireless multi-carrier communication system, comprising:

receiving a first pilot signal from a first transmitting entity on a first set of subbands, wherein the first set of subbands is used for pilot transmission by the first transmitting entity and the first set of subbands is disjoint from a second set of subbands used for pilot transmission by a second transmitting entity, and wherein each pair of adjacent subbands in the first set of subbands includes a range of frequencies that is less than or equal to a coherence bandwidth of a wireless channel in a system; and

the first pilot signal is processed to obtain a pilot estimate for each subband in the first set of subbands.

13. The method of claim 12, further comprising:

a channel estimate is derived for each subband in the first set of subbands based on pilot estimates for the subband.

14. The method of claim 13, further comprising:

channel estimates are derived for at least one subband not in a first set of subbands based on channel estimates for subbands in the first set of subbands.

15. The method of claim 12, further comprising:

a signal strength estimate is derived for the first pilot signal based on pilot estimates for the first set of subbands.

16. The method of claim 12, further comprising:

receiving a second pilot signal from the second transmitting entity on the second set of subbands; and

processing the second pilot signal to obtain a pilot estimate for each subband in the second set of subbands.

17. The method of claim 16, further comprising:

a subband-based pilot estimate derives a signal estimate for each subband in the second set of subbands.

18. An apparatus in a wireless multi-carrier communication system, comprising:

means for receiving a first pilot signal from a first transmitting entity on a first set of subbands, wherein the first set of subbands is used for pilot transmission by the first transmitting entity and the first set of subbands is disjoint from a second set of subbands used for pilot transmission by a second transmitting entity, and wherein each pair of adjacent subbands in the first set of subbands includes a range of frequencies that is less than or equal to a coherence bandwidth for a wireless channel in a system; and

means for processing a first pilot signal to obtain a pilot estimate for each subband in the first set of subbands.

19. The apparatus of claim 18, further comprising:

means for deriving a channel estimate for each subband in the first set of subbands based on pilot estimates for the subband.

20. The apparatus of claim 18, further comprising:

means for receiving a second pilot signal from the second transmitting entity on the second set of subbands; and

means for processing the second pilot signal to obtain a pilot estimate for each subband in the second set of subbands.

21. A terminal in a wireless multi-carrier communication system, comprising:

a receiver unit configured to process pilot signals received on a first set of subbands, wherein the first set of subbands is used for pilot transmission by a first transmitting entity and the first set of subbands is disjoint from a second set of subbands used for pilot transmission by a second transmitting entity, and wherein each pair of adjacent subbands in the first set of subbands includes a frequency range that is less than or equal to a coherence bandwidth of a wireless channel in a system; and

at least one pilot processor configured to process the pilot signals to obtain pilot estimates for each subband in the first set of subbands.

22. A method of transmitting pilots in a wireless multi-carrier communication system, comprising:

generating pilot symbols for transmission on a first set of subbands used for pilot transmission by a first transmitting entity;

processing pilot symbols with a first code to obtain covered pilot symbols, wherein the first code is orthogonal to a second code used by a second transmitting entity for pilot transmission;

processing the covered pilot symbols to obtain a pilot signal with pilot symbols included in the first set of subbands;

processing the covered pilot symbols with a scrambling code to obtain scrambled pilot symbols, and wherein the scrambled pilot symbols are further processed to obtain a pilot signal, wherein the first code comprises chips, and wherein each chip of the scrambling code is applied to a pilot symbol; and

transmitting the pilot signal from the first transmitting entity.

23. The method of claim 22, wherein the first code is a Walsh code.

24. The method of claim 22, wherein the first code is applied to each subband in the first set of subbands.

25. The method of claim 22, wherein the scrambling code is a pseudo-random (PN) code.

26. The method of claim 22, wherein the first code comprises N_WA number of chips, and wherein each chip of said scrambling code is applied to N_WAnd a pilot symbol.

27. The method of claim 22, wherein the scrambling code is divided into a plurality of scrambling code segments, one scrambling code segment for each subband in the first set of subbands, and wherein pilot symbols for each subband in the first set of subbands are multiplied by the scrambling code segment for that subband.

28. The method of claim 22, wherein each chip of the scrambling code is applied to one pilot symbol.

29. The method of claim 22, wherein subbands in the first set of subbands are uniformly distributed across a plurality of subbands used by the first transmitting entity for pilot and data transmission.

30. The method of claim 22, wherein each pair of adjacent subbands in the first set of subbands includes a range of frequencies that is less than or equal to a coherence bandwidth of a wireless channel in the system.

31. The method of claim 22, wherein pilot symbols sent from the first transmitting entity are different from pilot symbols sent from the second transmitting entity.

32. The method of claim 22, wherein the first set of subbands is disjoint from a second set of subbands used for pilot transmission by a third transmitting entity.

33. The method of claim 32, wherein pilot signals are sent in bursts from the first and second transmitting entities.

34. The method of claim 32, wherein pilot signals are sent continuously from the first and second transmitting entities.

35. The method of claim 22, wherein pilot signals are sent from the first and second transmitting entities at synchronized timing.

36. The method of claim 22, wherein pilot signals are transmitted from the first and second transmitting entities at asynchronous timing.

37. The method of claim 22, wherein the multi-carrier communication system implements Orthogonal Frequency Division Multiplexing (OFDM).

38. An apparatus in a wireless multi-carrier communication system, comprising:

means for generating pilot symbols for transmission on a set of subbands used for pilot transmission by a first transmitting entity;

means for processing pilot symbols with a first code to obtain covered pilot symbols, wherein the first code is orthogonal to a second code used for pilot transmission by a second transmitting entity;

means for processing the covered pilot symbols to obtain pilot signals with pilot symbols included in a set of subbands;

means for processing the covered pilot symbols with a scrambling code to obtain scrambled pilot symbols, and wherein the scrambled pilot symbols are further processed to obtain a pilot signal, wherein the first code comprises chips, and wherein each chip of the scrambling code is applied to a pilot symbol; and

means for transmitting a pilot signal from the first transmitting entity.

39. A base station in a wireless multi-carrier communication system, comprising:

at least one pilot processor configured to receive and process pilot symbols with a first code to obtain covered pilot symbols, wherein the pilot symbols are designated for transmission on a set of subbands used for pilot transmission by a base station, and wherein the first code is orthogonal to a second code used for pilot transmission by another base station in the system; the covered pilot symbols are processed with a scrambling code to obtain scrambled pilot symbols, and wherein the scrambled pilot symbols are further processed to obtain a pilot signal, wherein the first code comprises chips, and wherein each chip of the scrambling code is applied to a pilot symbol;

a conversion unit configured to derive time-domain samples with covered pilot symbols included in the set of subbands; and

a transmitter unit to process the time domain samples to obtain a pilot signal for transmission over a wireless link.

40. A method of processing pilot received over a wireless link in a wireless multi-carrier communication system, comprising:

receiving a first pilot signal on a first set of subbands used for pilot transmission by a first transmitting entity;

processing a first pilot signal with a first code to obtain decovered symbols for a first transmitting entity, wherein the first code is used for pilot transmission by the first transmitting entity and is orthogonal to a second code used for pilot transmission by a second transmitting entity; and

the decovered symbols are processed for a first transmitting entity to obtain pilot estimates for each subband in the first set of subbands.

41. The method of claim 40, further comprising:

a channel estimate is derived for each subband in the first set of subbands based on pilot estimates for the subband.

42. The method of claim 41, further comprising:

a channel estimate is derived for at least one subband not in the first set of subbands based on channel estimates for subbands in the first set of subbands.

43. The method of claim 40, further comprising:

deriving a signal strength estimate for the first pilot signal based on pilot estimates for subbands in the first set of subbands.

44. The method of claim 40, further comprising:

processing the decovered symbols of the first transmission entity with a descrambling code of the first transmission entity.

45. The method of claim 40, further comprising:

receiving a second pilot signal on a second set of subbands used for pilot transmission by the second transmitting entity;

processing a second pilot signal with the second code to obtain decovered symbols for the second transmitting entity; and

the decovered symbols are processed for the second transmitting entity to obtain pilot estimates for each subband in the second set of subbands.

46. An apparatus in a wireless multi-carrier communication system, comprising:

means for receiving a pilot signal on a set of subbands used for pilot transmission by a first transmitting entity;

means for processing a pilot signal with a first code to obtain decovered symbols, wherein the first code is used for pilot transmission by the first transmitting entity and is orthogonal to a second code used for pilot transmission by a second transmitting entity; and

means for processing the decovered symbols to obtain pilot estimates for each subband in the set.

47. A terminal in a wireless multi-carrier communication system, comprising:

a receiver unit for processing a pilot signal received on a set of subbands used for pilot transmission by a first transmitting entity; and

at least one pilot processor configured to process pilot signals with a first code used for pilot transmission by a first transmitting entity and orthogonal to a second code used for pilot transmission by a second transmitting entity to obtain decovered symbols, and to process the decovered symbols to obtain pilot estimates for each subband in the set.