A delay independent decorrelating detector for quasi-synchronous CDMA

The decorrelating detector is a near-far resistant linear joint detector for a code-division multiple-access (CDMA) system. It consists of a bank of matched filters followed by a decorrelating matrix. For proper operation, both the matched filter bank and the decorrelating matrix require knowledge of the delays of all users. The delays are due to the different propagation times from each user to the base station. Delay estimation is a weak link in this system since it is complex and prone to error. The proposed decorrelating detector does not require exact knowledge of the user delays, but instead requires that the delays be bounded to a fraction of a symbol interval. The delays are naturally bounded in this way in many microcell and picocell systems where the round trip propagation time is small compared to the symbol interval. The new delay independent decorrelating detector is shown to be near-far resistant and, through appropriate spreading code selection, to experience a modest 3 dB signal-to-noise ratio (SNR) loss relative to orthogonal access schemes. It is also shown to limit capacity to a maximum of 50% of the spreading gain when the delays are bounded by a single chip interval. The complexity is similar to the conventional correlating receiver which is far less complex than the joint detection schemes proposed to date.