A sliding window PDA for asynchronous CDMA, and a proposal for deliberate asynchronicity

The probabilistic data association (PDA) method is extended to multiuser detection over symbol-asynchronous code-division multiple access (CDMA) communication channels. A direct extension as well as a sliding window processing method are introduced. While achieving near-optimal performance with O(K³) computational complexity in synchronous CDMA, K being the number of users, it is shown that, in asynchronous CDMA, the probability of group detection error of the proposed PDA method is very close to the performance lower bound provided by an ideal clairvoyant optimal detector, and the computational complexity is only marginally increased to O([h/s]K³) per symbol, where h and s are the width and the sliding rate of the processing window, respectively. Due to the outstanding performance of the PDA detector in heavily overloaded asynchronous systems, it is observed that an optimally designed synchronous system can be easily outperformed by an arbitrarily designed asynchronous system. Hence, it is proposed to use asynchronous transmission deliberately, even when synchronous transmission is possible - asynchronous is better than synchronous!.