TWI442748B - Method of transmitting control signals in wireless communication system - Google Patents

Description

Method for transmitting control signals in a wireless communication system

The present invention relates to wireless communications; more specifically, to methods of transmitting control signals over a control channel.

The Institute of Electrical and Electronics Engineer (IEEE) 802.16 standard proposes a technology and communication protocol to support broadband wireless access. The above standards were originally proposed in 1999 and did not pass the IEEE 802.16-2001 standard until 2001. The IEEE 802.16-2001 is based on a single carrier (SC) physical layer, and the above physical layer is called "WirelessMAN-SC". In 2003, the IEEE 802.16a standard was adopted. In addition to the above WirelessMAN-SC, the IEEE 802.16a standard further adds "WirelessMAN-OFDM" and "WirelessMAN-OFDMA" to the physical layer. After completing the IEEE 802.16a standard, the IEEE 802.16-2004 standard was revised in 2004. In order to correct the errors and defects of the IEEE 802.16-2004 standard, in 2005, IEEE 802.16-2004/Corl (hereinafter referred to as IEEE 802.16e) was proposed in the form of an errata.

In recent years, the standardization of IEEE 802.16m has gradually become a new technology standard based on IEEE 802.16e. IEEE 802.16m is a newly developed technical standard that must be designed to support previous Out of IEEE 802.16e. That is to say, the technology used by the newly designed system (i.e., IEEE 802.16m) must be configured to effectively operate using an existing technology (i.e., IEEE 802.16e).

Orthogonal frequency division multiplexing (OFDM) systems can reduce inter-symbol interference in less complex situations and are therefore considered as one of the next generation wireless communication systems. In OFDM, successively input data symbols are converted into N parallel data symbols, and then the independent N subcarriers are used to carry and transmit the data symbols, respectively. The subcarriers can maintain orthogonality in a frequency dimension. Each of the orthogonal channels can undergo mutually independent frequency selective attenuation, and the interval of one of the transmitted symbols is increased, thereby minimizing intersymbol interference. In a system using OFDM as a modulation architecture, orthogonal frequency division multiple access (OFDMA) is a multiple access architecture, which provides multiple use of some of the available subcarriers independently. To achieve multiple access. In OFDMA, multiple frequency resources (ie, subcarriers) may be provided to individual users, and in general, individual frequency resources do not overlap each other because these frequency resources are independently provided to multiple uses. By. Therefore, frequency resources can be configured to individual users in a mutually exclusive manner.

In an OFDMA system, frequency selective scheduling can be utilized to obtain frequency diversity provided to multiple users, and the subcarriers can be separately configured according to an arrangement rule of subcarriers. In addition, a spatial multiplex architecture using multiple antennas can be utilized to increase the efficiency of a spatial domain. For support The above various architectures must transmit control signals between a mobile station (MS) and a base station (BS). The embodiment of the control signal includes a channel quality indicator (CQI), which is used when the MS reports the channel status to the BS; an acknowledge/unacknowledged (ACK/NACK) signal, which is a response to the data transmission; Used for precoding information, antenna information, or the like in a multi-antenna system.

The variety of system functions has led to an ever-increasing variety of control signals to be transmitted. When more control signals have to be transmitted using limited radio resources, the number of radio resources available to a user is also reduced.

Correspondingly, there is a need in the related art to provide a method for efficiently transmitting various control signals in a efficient manner using limited radio resources.

The present invention proposes a method of efficiently transmitting control signals.

According to an aspect of the present invention, a method for transmitting a control signal in a wireless communication system includes at least configuring at least one control signal in a control channel region, wherein the control channel region includes a plurality of tiled tiles, and each of the parallel blocks Composed of a plurality of consecutive subcarriers in a frequency domain of a plurality of orthogonal frequency division multiplexing (OFDM) symbols in a time domain; and transmitting at least one control signal, wherein the number of available sequences according to the control channel region is Determining at least one control signal with the number of bits that each control signal can carry The number of numbers.

The number of available sequences may be equal to the number of subcarriers in a side by side block.

The sequences can be orthogonal to each other.

The control signal can be an ACK/NACK (acknowledgement/unacknowledged) signal.

If each control signal carries the number of bits m, each of the above control signals can be mapped to one of the 2 ^m sequences.

If the number of bits carried by each control signal is m, the number of at least one control signal may be equal to the number of available sequences divided by 2 ^m .

The side by side blocks can be dispersed in the time domain.

The side-by-side blocks can be dispersed in the frequency domain.

The at least one control signal may be repeatedly configured in a plurality of parallel blocks.

The number of parallel blocks in the control channel can be three.

Each side-by-side block may include 2 consecutive subcarriers in a frequency domain on 2 OFDM symbols in a time domain.

According to an aspect of the present invention, a method for transmitting a control signal in a wireless communication system includes multiplexing the control signal into a parallel block, the parallel block comprising a plurality of orthogonal frequency division multiplexing (OFDM) in a time domain a plurality of consecutive subcarriers in a frequency domain on the symbol; and transmitting the control signal, wherein when the control signal is multiplexed, the control signal can be used in the parallel spreading code in the parallel block Expanding, and determining the number of bits of the spreading code based on the ratio of the number of control signals transmitted on the side-by-side block to the total number of all subcarriers constituting the side-by-side block.

The number of bits of the spreading code can be the same as the number of spreading codes.

The number of control signals to be processed by multiplexing may depend on the number of bits of the spreading code.

The above spread spectrum code is derived from one of the following: Hadamard code, discrete Fourier transform (DFT) sequence, Walsh code, Zadoff-Chu (ZC for short) The sequence is associated with a fixed amplitude zero auto-correlation (CAZAC) sequence.

Since a control channel area can be configured in different ways, multiple control signals can be adaptively transmitted. In addition, control signals can be configured without channel estimation.

Figure 1 illustrates a wireless communication system. There are several ways to configure a wireless communication system to provide a variety of communication services, such as voice, packet data, and the like.

Referring to FIG. 1, the wireless communication system includes at least one mobile station (MS) 10 and a base station (BS) 20. The MS 10 may be fixed or removable, and may be used to refer to any other technology, such as user equipment (UE), user terminal (UT), subscriber station (SS). , wireless devices, etc. BS 20 is usually a fixed base station that can communicate with MS 10 Communication is performed and can be used to refer to other technologies, such as Node B, base transceiver system (BTS), access point, and the like. One or more cell units may be encompassed within the communication range of the BS 20.

A downlink (DL) represents a communication link from the BS 20 to the MS 10; and an uplink (UL) represents a communication link from the MS 10 to the BS 20. In the DL, the transmitter can be part of the BS 20 and the receiver can be part of the MS 10. In the UL, the transmitter can be part of the MS 10 and the receiver can be part of the BS 20.

The wireless communication system may be an Orthogonal Frequency Division Multiplexing (OFDM) / Orthogonal Frequency Multiple Access (OFDMA) based system. OFDM utilizes multiple orthogonal subcarriers. In addition, OFDM uses the orthogonality between inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT). The transmitter can transmit data by performing IFFT. The receiver then replies to the original data by performing an FFT on the received signal. The transmitter uses IFFT to combine the subcarriers, and the receiver uses the FFT to split the subcarriers.

Figure 2 illustrates an embodiment of a hierarchical frame structure. Frame refers to a sequence of data that is used within a fixed time interval based on physical specifications.

Referring to FIG. 2, a frame-level architecture includes a superframe, a radio frame (or frame), and a subframe. The Hyperframe contains one or more radio frames. no The line telecommunications box contains one or more sub-frames. The Hyperframe contains one or more control areas based on the Superframe. In the following, the superframe-based control area is called the hyperframe header. The Super Frame header can be assigned to the first frame of the frames that make up the Hyperframe. A common control channel can be configured to the hyperframe header. The common control channel can be used to transmit information related to the radio frame of the hyperframe or control information (such as system information) that can be used by all MSs. System information is a necessary information. You must know the above system information to communicate between the MS and the BS. The BS can periodically transmit system information. For example, system information can be transmitted periodically every 20 to 40 milliseconds (ms). The size of the hyperframe can be determined by considering the transmission period of the system information. Although the size of each of the hyperframes shown in FIG. 2 is 20 milliseconds, this is only an example of the embodiment of the present invention, and thus the present invention is not limited thereto.

When using the OFDMA parameters exemplified in Table 1 below, the radio frame consists of 8 sub frames. A subframe can be configured for UL or DL transmission. There are three types of subframes: 1) type-1 subframe, which contains 6 OFDM symbols; 2) type-2 subframe, which contains 5 OFDM symbols; and 3) type-3 subframe A box containing 7 OFDM symbols.

Although the size of the radio frame is 5 ms in the embodiment illustrated in Fig. 2, the present invention is not limited thereto. The radio frame can be applied to a time division duplexing (TDD) architecture and a frequency division duplexing (FDD) architecture. In the TDD architecture, when UL transmission and DL transmission are distinguished in a time domain, the complete frequency band can be utilized for UL transmission and DL transmission. In the FDD architecture, UL transmission and DL transmission can be performed simultaneously, but the two occupy different frequency bands. The radio frame may include a preamble, a frame control header (FCH), a DL-MAP, a UL-MAP, a burst region, and the like. The BS and MS can use the preamble signal to perform the initial synchronization process, cell unit search and frequency offset and channel estimation between the two. The FCH contains information related to the DL-MAP message length and the DL-MAP coding architecture. The DL-MAP is an area for transmitting DL-MAP messages. The DL-MAP message defines access to the DL channel. It can be seen that the DL-MAP message defines DL channel indication and/or control information. The UL-MAP is an area for transmitting UL-MAP messages. The UL-MAP message defines access to the UL channel. It can be seen that the UL-MAP message defines UL channel indication and/or control information.

Radio resources are limited, and because of this, if a large amount of radio resources are used to transmit control signals, the radio resources used to transmit data may be insufficient. Correspondingly, there is a need to propose a method for transmitting control signals more efficiently.

Figure 3 illustrates an embodiment of a method of multiplexing multiple control signals into a control channel region.

Referring to Figure 3, the control channel area can be located in a radio frame for UL transmission. The control channel area contains at least one tile. A plurality of side-by-side blocks may be arranged consecutively or separately along a time axis, a frequency axis, or a time/frequency axis in a control channel region.

Although the format of the collective block shown in FIG. 3 is, for example, "time * frequency = 6 * 6", and three parallel blocks constitute a control channel region, the present invention is not limited thereto. That is to say, the side-by-side blocks can have various different formats, such as "time*frequency=3*4, 3*3, 4*3, 6*4, 3*6, 6*2, 2*6" and the like.

In addition, although FIG. 3 shows that a plurality of control signals are multiplexed into the control channel region by using a code division multiplexing (CDM) architecture, in the above architecture, each control signal is utilized. Multiply the orthogonal spread spectrum code; or use the time division multiplexing (TDM) architecture or the frequency division multiplexing (FDM) architecture. Multiplex processing control signals.

In this way, a plurality of control signals can be configured in a control channel area to avoid waste of radio resources. Here, the control signals mean a plurality of control signals to be transmitted by one MS or a plurality of control signals to be transmitted by a plurality of MSs. In other words, a control channel area can be configured to multiple MSs, so that the control signals of the MSs can be multiplexed into a control channel area; or a control channel area can be configured to an MS. Therefore, the plurality of control signals of the MS can be multiplexed into a control channel area.

A method of multiplexing a plurality of control signals into a control channel region according to a CDM deployment architecture will be described below. In the CDM deployment architecture, orthogonal control codes are used to develop multiple control signals, and then these control signals are carried by a parallel block. The above spreading code can expand the control signal in a time domain or a frequency domain. The above spreading code may be orthogonal coding such as Hadamard code, discrete Fourier transform (DFT) sequence, huaxue code, Zadoff-Chu (ZC) sequence and fixed amplitude zero autocorrelation (CAZAC) sequence, and the like.

The ZC sequence is one of the CAZAC sequences and can be expressed by Equation 1 below.

Where c(k) represents the kth element in the ZC sequence with the M index; and N represents the length of the ZC sequence. The index M is a natural number less than or equal to N, where M and N are mutually prime. Each MS can be identified using ZC sequences with different cyclic shift values. That is, different cyclic shift values can be utilized to identify multiple control signals. The number of available cyclic shift values can vary with channel delay spread. The above description is for illustrative purposes, and thus other sequences with good correlation may also be utilized.

Here, the control signal is a signal transmitted by an MS to a BS, and the above signal facilitates the BS to schedule. Embodiments of the control signal include a channel quality indicator (CQI) for when the MS reports channel status to the BS; an acknowledge/unacknowledged (ACK/NACK) signal, which is a response to data transmission; a precoding matrix indication (precoding) A matrix indicator (PMI), which is used for precoding information in a multi-antenna system; or a rank indicator (RI) or the like. However, the invention is not limited thereto. Control information containing one or more bits is encoded and modulated to produce control signals S _k (k = 0, 1, ..., K).

FIG. 4 is a schematic diagram showing a method of multiplexing a plurality of control signals into a control channel region according to an embodiment of the present invention.

Referring to Figure 4, it is assumed that a control signal is placed in a parallel block. Although the format of the side-by-side block is "time*frequency=6*6", the present invention is not limited to this. For example, the format of the above-mentioned side-by-side block may be "time*frequency=3*6, 6*1 or 1*6".

If multiple control signals are multiplexed to one control according to the CDM architecture In the channel area, the orthogonal pilot codes can be used to expand the control signals, and then the control signals are carried in a side by side block.

If the number of control signals arranged in the parallel block is S, and the number of subcarriers constituting the parallel block is N, each control signal may be expanded into N/S subcarriers. That is, the maximum number of bits of the spreading code is N/S, and thus N/S control signals can be processed multiplexed. In FIG. 4, the number of control signals arranged in the parallel block is 1, and the number of subcarriers constituting the parallel block is 36. Thus, the control signal can be spread over 36 subcarriers. As shown in FIG. 4, the control signals can be sequentially expanded in a subcarrier direction and then expanded in a subcarrier direction on an adjacent time axis; vice versa. Alternatively, 36 pilot codes (ie, 36 bits) can be utilized to multiplex 36 control signals.

In Figure 4, coherent detection is not performed because a reference signal (or pilot) is not transmitted and channel estimation is not possible. However, non-coherent detection can be performed because the individual control signals utilize orthogonal spreading codes. Coherent detection is a method for detecting signals after channel estimation using pilot or reference signals. Non-coherent detection is a method of detecting a signal without channel estimation. When the control signal is processed multiplex as shown in Fig. 4, the control signal can be efficiently transmitted using limited radio resources. The same control signal can be repeatedly transmitted by arranging one or more side-by-side blocks as shown in FIG. 4, thereby increasing its diversity.

Figure 5 is a schematic diagram showing the multiplexing of a plurality of control signals into a control channel area according to another embodiment of the present invention. law.

Referring to Figure 5, the two control signals are arranged in a parallel block. Although the format of the side-by-side block is "time * frequency = 6 * 6", the present invention is not limited thereto. For example, the format of the side-by-side block can be "time * frequency = 3 * 6, 2 * 6 or 6 * 2".

In FIG. 5, the number of control signals arranged in a side by side block is 2, and the number of subcarriers constituting the side by side block is 36. Thus, each control signal can be spread over 18 (i.e., 36/2) subcarriers. As shown in FIG. 5, each control signal can be sequentially expanded in a subcarrier direction and then expanded in the subcarrier direction on the adjacent time axis. In addition, if the number of control signals arranged in the parallel block is 2, when the expansion of a control signal ends at a subcarrier, another control signal can be started at the next subcarrier of the subcarrier. Expand. 18 pilot codes (ie, 18 bits) can be utilized to process 18 control signals. Here, the same spreading code is used for the above two control signals.

In Figure 5, non-coherent detection can be performed because the orthogonal spreading code is utilized to multiplex the individual control signals. The diversity of control signals can be increased by configuring one or more of the blocks shown in Figure 5.

Here, multiple (ie, three or more) control signals can be transmitted in a side by side block. Even when more control signals are transmitted, the control signals can be multiplexed using the methods described in FIGS. 4 and 5. For example, if four kinds of control signals are transmitted in a parallel block, each control signal can be expanded into 9 (ie, 36/4) subcarriers. In addition, profitable 9 control signals are processed by 9 spread codes (ie, 9 bits). When more control signals are configured in a side-by-side block, the expansion interval is narrowed.

FIG. 6 is a schematic diagram showing a method of multiplexing a plurality of control signals into a control channel region according to another embodiment of the present invention.

Referring to Fig. 6, three parallel blocks of the format "time * frequency = 2 * 6" are successively or separately arranged along a time axis or a frequency axis to form a control channel region. However, the invention is not limited thereto. That is, the collective blocks may have various formats such as "time*frequency=3*6, 6*2, 6*6" and the like, and may be arranged consecutively or separately along the time axis or the frequency axis. Parallel blocks to form a control channel area.

In FIG. 6, the number of control signals arranged in a side by side block is 1, and the number of subcarriers constituting the side by side block is 12. Thus, the control signal can be spread over 12 (i.e., 12/1) subcarriers using orthogonal spreading codes and repeated in three side-by-side blocks (i.e., i, j, and k). As shown in FIG. 6, each control signal can be sequentially expanded in a subcarrier direction and then spread out in the subcarrier direction on the adjacent time axis. In addition, each control signal can be sequentially expanded in the time axis direction and then expanded in the adjacent subcarrier direction. 12 control signals can be multiplexed into a single block using 12 orthogonal spreading codes (ie, 12 bits).

In Fig. 6, non-coherent detection can be performed because individual control signals are processed by orthogonal spreading code multiplexing. Because the control signal is repeatedly Configured in multiple side-by-side blocks to increase the diversity of control signals.

Hereinafter, a method of multiplexing a plurality of control signals into a control channel region using a CDM sequence architecture will be described in accordance with an embodiment of the present invention.

First, L orthogonal sequences are generated, wherein the number of bits of the orthogonal sequence is equal to the total number L of subcarriers constituting all of the side-by-side blocks included in a control channel region. For example, if a control channel area contains three side-by-side blocks, and the format of each side-by-side block is "time*frequency=6*6", the total number of subcarriers constituting the control channel area is 108. Thus, 108 orthogonal sequences can be generated. Next, an orthogonal sequence (hereinafter simply referred to as "sequence") is assigned to the control signal. A control signal can be configured to each sequence.

Table 2 presents an embodiment of configuring control signals to a sequence.

In the above Table 2, it is assumed that MS 1 transmits two control signals, MS 2 transmits a control signal, and MS 3 transmits two control signals. Therefore, the control signals transmitted by the MS 1 can be configured to the sequence 1 and the sequence 2; the control signals transmitted by the MS 2 are configured to the sequence 3; and the control signals transmitted by the MS 3 are configured to the sequence 4 and the sequence 5.

Table 3 below presents another embodiment of configuring control signals to a sequence.

In Table 3 above, it is assumed that two ACK/NACK control signals are transmitted on the control channel area according to the CDM sequence architecture. Two sequences are required to distinguish between ACK and NACK from an ACK/NACK control signal. Thus, the first control signal can utilize sequence 1 for ACK transmission and sequence 2 for NACK transmission. In addition, the second control signal can use sequence 3 for ACK transmission, and sequence 4 can be used for NACK transmission. As described above, if L sequences are generated in a control channel region, L/2 ACK/NACK control signals can be multiplexed and transmitted on the control channel region.

The method for configuring the ACK/NACK control signal shown in Table 3 can also be applied to the transmission of other control signals. That is, since the length of the ACK/NACK control signal is 1 bit, 2 ¹ sequences are required. In this way, if the length of a control signal is m bits, 2 ^m sequences are needed. Therefore, if three control signals need to be transmitted on a control channel area and their length is m bits, 3*2 ^m sequences are needed.

The sequence for configuring the control signals is not limited to the orthogonal sequence. That is, a pseudo-othogonal sequence can be generated and used to configure the control signal. For example, a virtual orthogonal sequence is characterized in that the sequences as a whole are not orthogonal to each other, but the sequences in each side-by-side block have orthogonality.

Although in a specific embodiment of the present invention, the control signal is configured in a control channel region by using a CDM sequence architecture, the channel estimation is not The necessary steps. That is, since the pilot or reference signal is not transmitted, coherent detection between the BS and the MS, such as channel estimation, cannot be performed. However, since the control signal utilizes an orthogonal sequence, non-coherent detection can be performed.

The invention may be practiced using hardware, software or a combination thereof. When implemented in hardware, the hardware may utilize an application specific integrated circuit (ASIC), digital signal processing (DSP), programmable logic device (programmable logic device, Referred to as PLD), field programmable gate array (FPGA), processor, controller, microprocessor, other electronic units designed to perform the above functions, or a combination thereof. When implemented in a software, the software may be a module that performs the above functions. The above software can be stored in the memory unit and can be executed by the processor. Regarding the memory unit and or processor, various units well known to those of ordinary skill in the art to which the invention pertains may be utilized.

The present invention has been described with reference to the accompanying drawings. Therefore, any further modifications to the specific embodiments of the invention are intended to be within the scope defined by the appended claims.

10‧‧‧Mobile Station (MS)

20‧‧‧Base Station (BS)

Figure 1 illustrates a wireless communication system.

Figure 2 illustrates an embodiment of a hierarchical frame structure.

Figure 3 illustrates an embodiment of a method of multiplexing multiple control signals into a control channel region.

4 is a diagram showing a method of multiplexing a plurality of control signals into a control channel region in accordance with an embodiment of the present invention.

FIG. 5 is a diagram showing a method of multiplexing a plurality of control signals into a control channel region according to another embodiment of the present invention.

Figure 6 is a diagram showing a method of multiplexing a plurality of control signals into a control channel region in accordance with another embodiment of the present invention.

10‧‧‧Mobile Station (MS)

20‧‧‧Base Station (BS)

Claims (20)

A method for transmitting a plurality of control signals in a wireless communication system, the method comprising the steps of: configuring the control signals in a control channel region, the control channel region comprising: at least one tile, each side by side The block includes a plurality of consecutive subcarriers in a frequency domain above a plurality of orthogonal frequency division multiplex symbols in a time domain; and transmitting the control signals, wherein the control signals are disposed in a parallel block, Each of the control signals is extended in a parallel block, and one of the control signals for each of the control signals is selected from the orthogonal spread spectrum code, and the control signal is ACK/ NACK (acknowledgement/non-confirmation) signal. The method of claim 1, wherein the maximum number of bits of the spread code is N/S, S represents the number of control signals arranged in a side by side block, and N represents a side by side block. The number of subcarriers. The method of claim 1, wherein the spread code for each of the control signals is selected from a Walsh code. The method of claim 1, wherein each of the control signals is sequentially extended on a frequency axis or a time axis. The method of claim 1, wherein the extended control signal begins with a subcarrier adjacent to the subcarrier at the end of the other extended control signal. The method of claim 1, wherein the side-by-side blocks are dispersed in the time domain. The method of claim 1, wherein the side-by-side blocks are dispersed in the frequency domain. The method of claim 1, wherein the control signals are repeatedly disposed in the side-by-side blocks. The method of claim 1, wherein the number of the side-by-side blocks is three in the control channel region. The method of claim 1, wherein each side-by-side block comprises: 2 consecutive subcarriers in a frequency domain above 6 orthogonal frequency division multiplex symbols in a time domain. An endpoint for transmitting a plurality of control signals in a wireless communication system, the endpoint comprising: a processor configured to: configure the control signals in a control channel region, the control channel The area includes: at least one tile, each side block comprising a plurality of consecutive subcarriers in a frequency domain above a plurality of orthogonal frequency division multiplex symbols in a time domain; and a transmitter The transmitter is configured to transmit the control signals, wherein the control signals are disposed in a parallel block, each of the control signals being extended in a parallel block for each of the control signals One of the spread spectrum codes is selected from the orthogonal spread spectrum codes, and the control signal is an ACK/NACK (acknowledgement/non-acknowledgement) signal. For example, in the end point described in claim 11, wherein the maximum number of bits of the spread code is N/S, S represents the number of control signals arranged in one side by side block, and N represents a side by side block. The number of subcarriers. The endpoint of claim 11, wherein the spreading code for each of the control signals is selected from a Walsh code. The endpoint of claim 11, wherein each of the control signals is sequentially extended on a frequency axis or a time axis. As for the endpoints mentioned in item 11 of the patent application, one of the extension controls The signal is started from the subcarrier adjacent to the subcarrier at the end of the other extended control signal. The endpoints of claim 11, wherein the side-by-side blocks are dispersed in the time domain. The endpoints of claim 11, wherein the side-by-side blocks are dispersed in the frequency domain. The endpoints of claim 11, wherein the control signals are repeatedly configured in the side-by-side blocks. The endpoint of claim 11, wherein the number of the side-by-side blocks is three in the control channel region. The endpoint of claim 11, wherein each side-by-side block comprises: 2 consecutive subcarriers in a frequency domain above 6 orthogonal frequency division multiplex symbols in a time domain.