Real-time Implementation of the Decorrelator Detector

1) Introduction One of the most serious problems that limits the capacity of CDMA systems is the multi-access interference (MAI), caused by the cross-correlation between spreading sequences of active users. The undesirable impact of the MAI becomes more remarkable and sometimes disastrous when the number of active users becomes large or when the power level of certain users is significantly higher than that of others. In this case, weak users may lose communication because of the overwhelming MAI. This phenomenon is known as the near-far effect. The conventional CDMA detector or the matched filter detector is a filter matched to the signature waveform of the desired user, which is optimum under white Gaussian noise conditions. Due to the MAI from other users in the same cell (intra-cell interference) and the MAI from adjacent cells (inter-cell interference), the conventional matched filter detector is not optimal. Moreover, its performance degrades greatly when the received powers of the interfering users are much greater than that of the desired users. Therefore, the conventional matched filter detector is not near-far resistant. Power control is suggested to mitigate the near-far effect. Although ideal power control is very difficult to implement, stringent power control is necessary and ensures that users have almost identical signal power when they arrive at the base station. The conventional detector treats MAI as additive white Gaussian noise (AWGN). This is a reasonable approach due to the fact that CDMA interference is comprised of contributions from independent interferers. However the MAI is not true AWGN since its structure is well defined through the cross-correlation matrix, and therefore could be exploited to achieve better performance results. Verdú's showed that the near-far problem is not inherent to the CDMA system [1]. In fact, it is due to the detection schemes. In addition, he showed that this problem could be