JP2002539666A - Unsupervised adaptive chip separation filter for CDMA terminals - Google Patents

Abstract

(57) Abstract A receiver for use in a CDMA telecommunications system is disclosed. The receiver includes at least one antenna for receiving a signal from a CDMA channel, where the received signal includes a desired user signal. The receiver also includes a combining circuit for performing chip waveform filtering and maximal ratio combining to produce a correlated chip estimate of the received signal. The receiver includes an adaptive separator for adaptively separating the correlated chip estimates, and despreads the output of the adaptive separator to provide an estimate for a desired user signal data symbol. And a correlator for obtaining. The receiver further includes an estimating circuit coupled to the combining circuit for estimating a channel response, the combining circuit using the channel response estimate as a reference.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] This is described in "Unsupervised A" by Petri Komulainen and Markku J. Heikkila.
US Patent Application No. 09 / entitled "daptive Chip Separation Filter for CDMA Terminal" (Attorney Docket No. 872.8547 USU, filed March 3, 2000, Ex
press Mail No .: EL 470231259US), which is Petri Komula.
"Unspervised Adaptive Separation Filt" by inen and Markku J. Heikkila
er "claims priority under 35 U.S.C. 119 (e) from provisional patent application Ser. No. 60 / 123,603, filed Mar. 10, 1999. The entire disclosure of this provisional patent application is incorporated by reference.

FIELD OF THE INVENTION [0002] The present invention relates generally to communication systems, and more particularly to a receiver that performs adaptive channel equalization.

BACKGROUND OF THE INVENTION CDMA systems are based on digital wideband spread spectrum technology.
It transmits a number of independent user signals over an allocated portion of the radio spectrum. In CDMA, each user signal includes a different orthogonal code and a pseudo-random binary sequence, which modulates a carrier to spread the spectrum of the waveform and allow multiple user signals to share the same frequency spectrum. . The user signals are separated at the receiver by a correlator capable of despreading only signals having the selected orthogonal code. Other user signals whose codes do not match are not despread and directly cause system noise. The signal-to-noise ratio of the system is determined by the ratio of the desired signal power to the sum of all interfering signals, and is enhanced by the system processing gain and the ratio of spreading bandwidth to baseband data rate.

In the cellular direct sequence code division multiple access (DS-CDMA) downlink (from base station to mobile station), various users are typically multiplexed onto the channel by orthogonal spreading codes. Third-generation wideband CDM currently proposed
This is also the case with the A (WCDMA) standard, where different spreading factors and variable user data rates can be supported simultaneously. However, due to multipath propagation and frequency selective fading, orthogonality between the various user waveforms is degraded and multiple access interference degrades the performance of the receiver. For uplink (mobile terminal to base station) receivers, a multi-user detection scheme has been proposed to mitigate multiple access interference. However, the mobile terminal cannot take into account the same level of computational complexity as the base station.

[0005] As a means of low complexity interference suppression for CDMA receivers, several adaptive algorithms based on stochastic gradient methods and least mean square error (MMSE) criteria have been proposed. When a known training data sequence exists,
A least mean square (LMS) algorithm can be used.

In this regard, “An Adaptive Direct-Sequence Code-Division Multiple Acc
ess Receiver for Multiuser Interference Rejection "(SL Miller, IEEE
Transactions on Communications, vol. 43, pp. 1746-1755, February-April 1995).

[0007] "Advanced receivers for wideband CDMA, Doctoral thesis" (M. Latva-aho
Sufficient training for LMS can also be provided by a conventional rake receiver, as disclosed in the Department of Electrical Engineering, University of Oulu, Finland, 1998).

To avoid the need for training, “Blind adaptive multiuser detection
(ML Honig, U. Madhow and S. Verdu, IEEE Transactions on Informati
on Theory, vol. 41, pp. 944-960, July 1995), and also `` Blind adaptation algorithms for direct-sequence spread-spectrum CDMA.
single-user detection "(N. Zecevic and JH Reed, IEEE International
As described in Vehicular Technology Conference, VTC'97, May 1997, pp. 2133-2137), a blind adaptive scheme has been proposed.

“Adaptive bootstrap CDMA multi-user detection” (Y. Bar-Ness and J. B.
Punt, Wireless Personal Communications, Kluwer Academic Publishers, vo
l. 3, no. 1, pp. 55-71, 1996) and "Simplified bootstrap adaptive"
decorrelator for CDMA downlink "(P. Komulainen, Y. Bar-Ness and J. Li
lleberg, IEEE International Conference on Communications, ICC'98, Atlant
a, USA, June 1998, pp. 380-384) both disclose algorithms based on blind signal separation, which have been shown to have comparable performance with linear MMSE receivers. However, it should be noted that in multipath channels, the blind adaptation scheme requires some form of channel response estimation. A common pilot channel or dedicated pilot symbols can be used for channel estimation.

[0010] Previously proposed adaptive approaches have focused on the detection of data symbols, so that the symbols are cyclized at the symbol level.
ostationary). Unfortunately, this is due to the long pseudo noise (PN
3.) Prevents application to systems employing scrambling codes, but changes the correlation properties of the signal from one symbol interval to another.

In most digital wireless communication systems, inter-symbol interference (ISI) results from multipath propagation in the channel. This problem is particularly pronounced at high data rates, which can be mitigated by channel equalization.

[0012] In this regard, "Wireless channel equalisation" (DP Taylor, GM Vit
etta, BD Hart and A. Mammela, European Transactions on Telecommunication
tions, vol. 9, no. 2, pages 117-143, 1998).

In a CDMA downlink, multiple access interference is inherently caused by the channel because all user signals propagate through the same frequency selective multipath channel to the receiver of interest. Therefore, "Downlink channel decorrelation in
CDMA systems with long codes "(S. Werner and J. Lilleberg, IEEE Interna
national Vehicular Technology Conference, VTC'99, Houston, Texas, 1999 5
By compensating for channel effects as described in (Mon), orthogonality between users can be restored and interference can be suppressed.

OBJECTS AND ADVANTAGES It is an object and advantage of the present invention to provide an improved receiver that utilizes an adaptive channel equalization function to restore orthogonality between different user waveforms.

It is another object of the present invention to utilize adaptive channel equalization in a receiver that performs cancellation of linear inter-chip interference chips by adaptive chip separation by decorrelating successively transmitted chips. And benefits.

It is another object and advantage of the present invention to provide a receiver having improved performance suitable for systems using long scrambling codes, such as third generation WCDMA systems.

SUMMARY OF THE INVENTION The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus of embodiments of the invention.

A receiver for use in a CDMA telecommunications system is disclosed. This receiver is C
The system includes at least one antenna for receiving a signal from the DMA channel, and the received signal includes a desired user signal. The receiver also includes a combining circuit that performs chip waveform filtering and maximum ratio combining to produce a cross-correlated chip estimate of the received signal. The receiver further comprises:
An adaptive separator for adaptively separating the mutually correlated chip estimates, and a correlator for despreading the output of the adaptive separator to obtain an estimate for the data symbol of the desired user signal. Contains.

Further, the receiver includes an estimator coupled to the combiner for estimating the response of the channel, the combiner utilizing the channel response estimate as a reference.

The above and other features of the present invention will become more apparent when the following detailed description of the invention is read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION As mentioned above, in a synchronous CDMA downlink using orthogonal codes, multiple access interference (MAI) is inherently due to multipath channels. Therefore, MAI can be suppressed by linear channel equalization. A receiver using a channel equalization algorithm is disclosed, which performs linear inter-chip interference cancellation by adaptive chip separation. The method is based on the proposed third generation broadband C
It is suitable for a system with long code scrambling, such as a DMA system. The results that provide significant performance gains when compared to conventional rake receivers are shown below.

In one aspect of the present invention, a CDMA terminal space-time adaptive receiver for the downlink of a CDMA system employing long code scrambling is disclosed. The receiver's ability to suppress MAI is based on equalizing the effect of the multipath channel, which essentially restores orthogonality between different users. The adaptation rule is derived from the bootstrap principle, whose applicability derives here from the finding that the downlink signal is a sequence of uncorrelated, rather strong signal elements, ie multi-user chips. The purpose of the adaptive separation is to remove the correlation between chips due to the channel. The receiver performs linear channel equalization by adaptively decorrelating successively transmitted chips. The signal transmitted by the synchronous base station transmitter is an uncorrelated multi-user
This approach is particularly applied to CDMA downlink, as it is formed by a sequence of chips and has a suitable signal-to-noise ratio for this application. The power of the entire multiple access signal can be used for adaptation.

1. Downlink signal model A. Signals Transmitted In the CDMA downlink, the signals of the various users are symbol synchronized,
They are identified by orthogonal (Walsh) codes. A typical structure of a single base station transmitter with K simultaneous users is depicted in FIG. The symbols b ₁ ,..., B _k are
Complex quadrature shift keying (complex quadraphase
shift keying (QPSK)) data symbol. The Walsh codes for each user are represented by s ₁ (n),..., S _k (n) and are applied at the junctions 100 _{1 to} 100 _k . Power control amplitude a ₁ which depends on the user, ..., have been shown as a _k, is applied at the junction 110 ₁ to 100 _k. The resulting signals are combined by summation function 120. A common complex scrambling code c (n) is applied to the combined signal at junction 130. The resulting signal is shown as d (n).

The entire multi-user chip sequence is:

(Equation 1) Where a _k is the real positive amplitude resulting from power control, b _k (i) is the i th QPSK data symbol, and s _k for the k th user.
Is a Walsh code. Here, for n = 0, 1,..., N−1, s _k (
n) = ± 1, otherwise 0. The period of the common complex scrambling code c (n) can be spread over the entire frame of symbols. As a result of long PN code scrambling, {d (n)} _n is a sequence of uncorrelated complex signal elements whose amplitude distribution is not known to the receiver as a result of user dependent power control. However, in terms of adaptation, d (n) represents the signal to be estimated.

The chip is provided with a transmission waveform p (t) having a limited bandwidth. Thus, a continuous time model for a multiple access baseband signal is

(Equation 2) Where Tc is the chip interval duration.

B. Received signal As a result of multipath propagation, the chip waveform received at the mth receiver antenna is

(Equation 3) Where γ _ml is the complex gain and τ _l is the relative delay of the l-th path of the multipath channel. Since the channel parameters change slowly over time compared to the chip rate, they may be considered constant over the time interval of interest. Therefore, the signal received by the mth antenna is

(Equation 4) , And the where η _m (t) is the process of white Gaussian background noise with power spectral density of the dihedral N _0/2 (AWGN). For matrix display, the continuous time waveform h _m a (t-NTC) can be discretized into a vector h _m (n). Stack vector of received infinite M-dimensional signal

(Equation 5) Which is the determinant

(Equation 6) Where the vector d is the transmitted multi-user chip

(Equation 7) And the matrix H is the received waveform

(Equation 8) And each of the column vectors of the matrix H for n =... -1, 0, 1,.

(Equation 9) Represents the received waveform that conveys the information of the multi-user chip d (n) to be transmitted.

II. Adaptive receiver A. Overall Receiver Structure A block diagram of the structure of the receiver 10 is depicted in FIG. Signals r _l (t) to r _M (t
) Is received through at least one or more antennas 140 _l ··· 140 _M, chip matched filter 0.99 _l · · · 15 each of the corresponding
_Tied to 0M. Each chip matched filter 150 _l・ ・ ・ 150 _M
Produces at least one sample per chip at its output. Tip ・
The sample sequence passes through delays 160 ₁ ... 160 _M to compensate for any possible channel estimation delay.

To determine the correlated chip estimates, the coherent rake receiver 170 uses a multipath component and a chip maximum ratio combining (m
It includes a unit called chip MRC175 for performing aximal ratio combining (MRC). The operation corresponds to channel matched filtering, as such, the number of signal dimensions (e.g.
dimentionality).

Regarding that point, “Exploiting multipath activity using low complexity e
qualification techniques for high speed wireless LANs ”(I. Kaya, AR Ni
x and R. Benjamin, IEEE International Vehicular Technology Conference,
VTC '98, Ottawa, Canada, May 1998, pp.1593-1597).

The mutually correlated chip estimates are adaptively separated by an adaptive separation filter 180. The output of the adaptive separation filter 180 is coupled to a correlator 190, which despreads the signal, ie, multiplies it by the conjugate length scrambling code provided by the code generator 200 and the user-specific Walsh code, and then To obtain an estimate for the desired user data symbol. The output of correlator 190 is coupled to deinterleaver 210, which is coupled to decoder 220. Deinterleaver 210 and decoder 22
The configuration and operation of 0 may be conventional.

Assuming complete knowledge of the channel, the n th output element of chip MRC 175 is

(Equation 10) Where () ^* means take the complex conjugate and () ^H means take the Hermitian transpose. In the matrix form, the total output sequence x = [..., x (-
1), x (0), x (1), ...] ^T is

[Equation 11] Where the correlation matrix P = H ^H H is shown.

[0032]

(Equation 12)

From equation (11), it can easily be seen that chip correlation cancellation or zero-forcing equalization is performed by multiplying the vector x by P ⁻¹ from the left. However, such operations have little advantage as a result of considerable noise enhancement.

B. Adaptive Separation Due to multipath propagation, the synthesized chips are correlated. The purpose of the adaptive separation filter 180 is to remove this correlation. Since the correlation matrix P is of the Toeplitz type, the separation is reduced to a filtering problem. As shown in FIG. 3, a symmetric filter v is used. This filter is ideally infinitely long, but in practice it is truncated to a manageable length. A suitable filter length can be determined primarily by the channel delay spread.

(Equation 13) In this case, 2F + 1 is the filter length, but the separation filter 1
80 output,

[Equation 14] Can be expressed as

As with the bootstrap algorithm described above by Y. Bar-Ness and JB Punt, the adaptation is based on blind linear decorrelation. f = 1,2, ...
.., The weight adaptation step for F can be expressed as:

(Equation 15) Where μ (n) is the (possibly normalized) step size parameter. Note that initially, for f = 1, 2,..., F and v _f = 0, the entire receiver operates as a conventional rake receiver.

From equation (13), when the adaptation reaches a steady state (on average), the following condition:

(Equation 16) Can be proved to be satisfied. When the noise level is insignificant, this condition is also satisfied by the zero forced equalizer.

It should be noted that the weights of the steady state filters cannot be solved from Equation (14) without ambiguity, and that this algorithm does not provide global convergence. However, simulations have shown that the adaptation is stable if the step size is kept reasonably small.

III. Performance Evaluation This section presents the results of a computer simulation that determines the bit error rate (BER) performance of the adaptive chip separator and compares it with that of a conventional rake receiver. Consider a WCDMA downlink signal with K active same power users, QPSK data modulation, a real orthogonal Walsh code of length N = 4 or 32, and a long complex Gold sequence for scrambling. A fixed step size parameter is used for adaptation, and the separation filter length is set to 17 (F = 8). Error correction coding is excluded from this simulation.

The receiver is tested in a time-varying Rayleigh channel, with a delay width of 1 μs, with three independently resolvable paths of the same average power that fade according to the classical Doppler spectrum. 4 MHz chip rate, root raised cosine pulse shape filtering,
A carrier frequency of 2 GHz and a vehicle speed of 5 km / h are used in this simulation. Both single and two antenna receivers are simulated. In the case of two antennas, full diversity is assumed as a result of independent fading between the elements. A complete estimate of the channel impulse response is provided to the receiver.

FIG. 4 depicts the BER performance curves of a single antenna receiver for a spreading factor N = 32 and various numbers of users. The performance of the two-antenna receiver under the corresponding channel load conditions is shown in FIG. Here, the x axis is one antenna
It shows E _b / N ₀ per element. Correspondingly, the low diffusion coefficient N
The results for a single antenna receiver and a two antenna receiver for the case of = 4 are given in FIGS.

As can be seen, the chip separator offers significant gain over rakes, especially in heavily loaded channels. This separator seems to benefit more from the extra dimensions provided by the diversity antenna. However, for a single user with a low data rate and a large spreading factor N = 32, the detection performance is slightly reduced due to adaptive jitter. However, for high data rate users, where N = 4 is clearly single user, it is quite worthless to benefit from separation.

This simulation shows that the performance of the adaptive chip separator outperforms conventional rake receivers, especially at high data rates and low spreading factors, or when the system is heavily loaded by many users. Is shown.

It should be understood that the functions described herein may be implemented in discrete circuit elements or as software routines executed by a suitable data processing device. A combination of circuit elements and software routines can also be used.

Although the present invention has been particularly shown and described with respect to preferred embodiments thereof, those skilled in the art will recognize that changes may be made in form and detail without departing from the scope and spirit of the invention.

[Brief description of the drawings]

FIG. 1 shows a block diagram of a CDMA base station transmitter.

FIG. 2 shows a block diagram of a receiver according to one aspect of the present invention.

FIG. 3 shows a block diagram of the structure of an adaptive separation filter according to the teachings of the present invention.

FIG. 4 shows a bit error rate (BER) performance curve for a receiver using a spreading factor N = 32 and a single receiver antenna.

FIG. 5 shows a BER performance curve using a spreading factor N = 32 and two antennas.

FIG. 6 shows a BER performance curve when using a spreading factor N = 4 and a single receiver antenna.

FIG. 7 shows a BER performance curve when a spreading factor N = 4 is used with a two antenna receiver.

────────────────────────────────────────────────── ─── Continued on front page (31) Priority claim number 09 / 521,439 (32) Priority date March 7, 2000 (2000.3.7) (33) Priority claim country United States (US) ( 81) Designated countries EP (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE), OA (BF, BJ, CF, CG, CI, CM, GA, GN, GW, ML, MR, NE, SN, TD, TG), AP (GH, GM, KE, LS, MW, SD, SL, SZ, TZ, UG, ZW), EA (AM, AZ, BY, KG, KZ, MD, RU, TJ, TM), AE, AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, CA, CH, CN, R, CU, CZ, DE, DK, DM, DZ, EE, ES, FI, GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, MN, MW, MX, NO, NZ, PL, PT, RO, RU, SD, SE, SG , SI, SK, SL, TJ, TM, TR, TT, TZ, UA, UG, UZ, VN, YU, ZA, ZW

Claims (7)

[Claims] 1. A receiver for use in a CDMA telecommunications system, said receiver comprising at least one antenna for receiving a signal from a CDMA channel, said signal comprising a desired user signal. An antenna, coupled to the antenna and performing chip waveform filtering and maximum ratio combining on multipath components and antenna elements to produce a correlated chip estimate of the received signal; An adaptive separator coupled to the combining circuit for adaptively separating the correlated chip estimates; and an adaptive separator coupled to the adaptive separator for despreading the output of the adaptive separator to generate the desired user signal. A correlator for obtaining an estimate for the data symbol. 2. The receiver of claim 1, further comprising an estimating circuit coupled to the combining circuit for estimating a response of the channel, wherein the combining circuit uses the channel response estimate as a reference. 3. The receiver of claim 1, wherein the despreader further comprises a circuit that multiplies the output of the adaptive separator by a conjugate length scrambling code and a Walsh code and integrates the result over a symbol period. . 4. A method for receiving a signal in a CDMA telecommunications system, the method comprising the steps of: receiving a signal from a CDMA channel, the signal comprising a desired user signal; Performing chip waveform filtering and maximal ratio combining on the antenna elements, and generating a correlated chip estimate of the received signal; and adaptively applying the correlated chip estimate to the received signal. And despreading the separated correlated chip estimates to obtain an estimate for the data symbol of the desired signal. 5. The method of claim 4, further comprising estimating the channel, wherein performing the chip waveform filtering and maximum ratio combining further comprises using the channel response estimate as a reference. Method. 6. The despreading step further comprises the step of multiplying the adaptively separated correlated chip estimates by a conjugate length scrambling code and a Walsh code, and then integrating the result over a symbol period. Claim 4 comprising
The method described in. 7. A coherent rake receiver for use with a received CDMA waveform comprising a desired user signal, the receiver comprising a multipath component and an antenna responsive to the received CDMA waveform. A combining circuit for performing chip waveform filtering and maximum ratio combining on the elements to produce a correlated chip estimate of the received CDMA waveform; and a combining circuit coupled to the combining circuit for combining the correlated chip estimate. An adaptive separator for adaptively separating; and a correlator coupled to the adaptive separator and despreading the output of the adaptive separator to obtain an estimate for a data symbol of the desired user signal. A coherent rake receiver.