Detectors for CDMA systems

Details Detectors for CDMA systems Detectors for CDMA systems

1998-08-04

2001-10-09

Chin, Stephen (Department: 2634)

Pulse or digital communications

Repeaters

Testing

C375S142000, C375S342000

Reexamination Certificate

active

06301293

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to direct sequence (DS) code division multiplex access (CDMA) systems subjected to multipath fading and multiple access interference (MAI), and more particularly to signal detection techniques for use in such systems.
BACKGROUND OF THE INVENTION
In wireless CDMA systems such as those proposed for next generation mobile wireless standards, the major impediments at the physical layer are multipath fading and multiple access interference caused by co-channel users which are not orthogonal to the desired user. The rake receiver, described in R. Price and P. E. Green, “A Communication Technique for Multipath Channels,” Proceedings of the IRE, Vol. 46, pp. 555-570, March 1958, attempts to combat multipath fading by coherently combining resolvable multipath replicas of the desired signal. Multiuser detection described in S. Verdú, “
Multiuser Detection
,” Cambridge University Press, New York, 1998, addresses the problem of MAI by actively accounting for its presence when detecting the desired user.
More recently, there has been growing interest in using array processing for further improving receiver performance. These techniques have focused on using multiple antennas at the base station receiver to provide antenna gain and/or diversity gain and allow the possibility of spatial processing. By combining these space-domain techniques with time domain techniques like rake detection and multiuser detection, the resulting space-time detectors show promise of improving the capacity of CDMA systems as compared to traditional time-domain-only detectors. See, for example, A. Paulraj and C. Papadias, “Space-Time Processing for Wireless Communications,” IEEE Signal Processing Magazine, Vol. 14, No. 6, pp. 49-83, November 1997. The first generation of space-time CDMA detectors used array processing with either rake detection or multi-user detection. See, respectively, A. Naguib and A. Paulraj, “Performance of Wireless CDMA with M-ary Orthogonal Modulation and Cell Site Antenna Arrays,” IEEE Journal on Selected Areas in Communications, Vol. 14, No. 9, pp. 1770-1783, December 1996 or S. Miller and S. Schwartz, “Integrated Spatial-Temporal Detectors for Asynchronous Gaussian Multiple-Access Channels,” IEEE Transactions on Communications, Vol. 43, No. 2/3/4, pp. 396-411, February/March/April 1995. Later space-time CDMA detectors combined all three processing techniques. See: H. Huang; S. Schwartz, S. Verdú, “Combined Multipath and Spatial Resolution for Multiuser Detection: Potentials and Problems,” Proceedings of the IEEE International Symposium on Information Theory, p. 380, 1995; or M. Nagatsuka and R. Kohno, “A Spatially and Temporally Optimal Multi-User Receiver Using an Array Antenna for DS/CDMA,” IEICE Transactions on Communications, Vol. E78-B, No. 11, pp. 1489-1497, November 1995.
While the foregoing systems operate satisfactorily, improvements can be made, in particular to the space-time detectors which combine all three processing techniques. While the detector in the Nagatsuka and Kohno paper is optimum in the maximum likelihood sense, its computational complexity is exponential with respect to the number of users. Hence it is too complex to implement for practical systems. In the paper by Huang, Schwartz and Verdú, a tradeoff between performance and complexity is made, but this detector was not implemented adaptively since it used a zero-forcing criteria. Adaptive implementations allow receivers to account for unknown sources of interference thus improving the detector performance and increasing the system capacity. For example, a base station receiver could account for interference from adjacent cells or from an embedded microcell, while a handset receiver could account for interference from signals it is not explicitly demodulating.
SUMMARY OF THE INVENTION
This invention detects DS-CDMA signals utilizing a rake receiver, array processing, and multiuser detection. When combined with array processing, the rake receiver is often called a space-time rake receiver. The invention uses a minimum mean-squared error (MMSE) criterion in the multiuser detector. This criterion allows for relatively simple implementation in the form of a linear detector and also allows for adaptive implementation. Adaptive implementations are useful in practical situations where there is limited knowledge of the various received signals. As discussed above, both uplink and downlink capacity can be improved using adaptive detectors which account for unknown interference.
For pulse amplitude modulated (PAM) data signals, two embodiments of the invention provide options for trading off between performance and adaptive implementation complexity. The first embodiment performs better if there is perfect knowledge of the users' signal parameters. On the other hand, it requires more explicit channel information for adaptive implementation. Suppose the receiver consists of P≧1 antennas and the received signal consists of K DS-CDMA data signals, each with L delayed/weighted multipath replicas. At each antenna, a bank of filters is matched to the KL spreading codes with their multipath timing delays. The filter outputs are weighted according to the complex conjugate of the estimated channel (multipath and array) parameters and combined to form a K-vector where each component corresponds to one of the K codes. The real part of each component is taken. Estimates of the multipath delays and channel parameters may be obtained, for example, from a training or pilot signal. The conventional space-time rake receiver would pass each vector component to a decision device for estimating the corresponding user's PAM data symbol. However, unless the users' signals are orthogonal in the space-time domain, the vector components are contaminated with multiaccess interference from the other users. This invention proposes the use of a linear combiner prior to the decision devices to suppress the multiaccess interference. This linear combiner is represented by a real K-by-K matrix W
A
which multiplies the real K-vector. The matrix minimizes the mean squared error between its product with the real K-vector and the K-vector of data symbols. Because the matrix uses the minimum mean-squared error (MMSE) criterion, well-known adaptive algorithms can be used to adaptively obtain it. Each component of the final K-vector output corresponds to one of the K codes and is passed to either a decision slicer or a decoder for further processing.
The second embodiment has marginally inferior performance when the channel parameters are known exactly. However, under practical conditions of channel mismatch, the second embodiment often performs better. In terms of adaptive implementation, this embodiment requires less information (it does not require explicit channel estimates) but may be slower to adapt. As with the first embodiment, the front end of the second embodiment consists again of a bank of KL matched filters for each of the P antennas followed by weighting and combining. However, unlike the first embodiment, the order of the real operator and linear combiner (K-by-K matrix multiplication) are exchanged. In doing so, the weighting, combining, and linear combiner can be represented by a single complex K-by-KLP matrix W
B
. The real parts of the components in the resulting K-vector are passed to either a decision device or a decoder for further processing. In the adaptive implementation, adaptive algorithms can be used to obtain the matrix W
B
; hence channel estimates are not explicitly required.
For quadrature PAM (QAM) data signals, real operators are not required in the detector since the signal constellation is two-dimensional. In this case, a variation on the second embodiment would be used which does not use the real operators but follows the linear combiner directly with the appropriate decision device or decoder.
While the MMSE detection techniques are powerful in and of themselves, their performance can be further enhanced by using them with other multiuser detection

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