Multistage detector for adaptive separation of QAM-modulated multiuser CDMA signals

Code division multiple access (CDMA) proves to be an attractive method for multiuser communication due to its potential capacity increase and anti-multipath fading capability. However, its performance is limited by the existence of multiple access interference energy (MAI), which results in the so called near-far problem. To reduce the MAI effect, different structures for multiuser separators have been proposed. Among these are different decorrelating detectors, adaptive and non-adaptive. One of these adaptive decorrelators is the so called "bootstrap" decorrelator. To further improve performance, multi-stage receivers has also been proposed using as a second stage a fixed canceler that depends on knowing the signals amplitude or an adaptive canceler that needs no such knowledge. All these different detection methods assumed BPSK modulation. For high bit/cycle modulation such as QAM, one can extend the BPSK based detectors by modifying them to PAM and handle the I and Q rails separately. However, considering the QAM as complex modulation signals, we propose in this paper the generalized complex bootstrap, and implement it in a multi-stage receiver, which can handle multiuser M-QAM data. Simulation results showed the adequacy of the scheme in improving the performance of the detection by reducing the MAI.