Joint Detection CDMA Techniques for Third-generation Transceivers
E‐L. Kuan, Lajos Hanzo · ePrints Soton (University of Southampton) · 1998
Multiple access communications using Direct Sequence Code Division Multiple Access (DS-CDMA) poses its own set of problems.The rst is multiple access interference (MAI) due to multiple users transmitting within the same bandwidth simultaneously.The signals from the users are separated by means of signature sequences that are unique to each user.Asynchronous transmission or the time-varying nature of the mobile radio channel destroys any orthogonality among users and results in co-channel interference.The channel also gives rise to intersymbol interference (ISI) due to multipath propagation, exacerbated by the fact that the mobile radio channel is time-varying.Conventional detectors, as exempli ed by the matched lter and RAKE combiner, are inecient, because they treat MAI and ISI as noise and do not exploit any knowledge about the signature sequences or the mobile channel.The success of these detectors is dependent on the cross-correlation between the spreading codes of all the users.Using conventional detectors to detect a signal corrupted by MAI and by a hostile channel typically results in an irreducible bit error rate (BER), even if the signal to noise ratio (SNR) is increased.Another weakness of conventional CDMA detectors is caused by the so-called ear-far eect".For conventional detectors to work satisfactorily, the signals from all the users have to arrive at the receiver with approximately the same power in order not to swamp the other signals.Therefore, very stringent p o wer control algorithms are needed to ensure that the signals arrive with similar powers at the receiver in order to achieve similar qualities of service among dierent users.Multiuser detection exploits our apriori knowledge about the spreading sequences and invokes channel estimation, in order to remove MAI.The optimum multiuser detector was rst proposed and analyzed by Verd u [ 1 ] for an asynchronous Gaussian channel.This optimum multiuser detector consisted of a bank of matched lters followed by the Viterbi algorithm to detect the most likely sequence transmitted by K users.The complexity of this algorithm was of the order of 2 K which is too daunting for real-time implementation.Following this seminal work, numerous sub-optimum multiuser coherent detectors have been derived.Excellent overviews of CDMA research can be found in the monographs by Prasad [2], Glisic and Vucetic [3].