A modified multi-stage co-channel interference cancellation in asynchronous CDMA systems

There is increasing interest in direct sequence code division multiple access (DS-CDMA) communication techniques. A modified multi-stage co-channel interference cancellation technique is proposed for asynchronous CDMA. In asynchronous CDMA a desired bit of any user is affected by co-channel interference from two bits from all other users. In the first stage of interference cancellation, by the end of the integration period of a desired bit, each interfering user would have already obtained an estimate for the earliest of its two interfering bits. Hence, the interference contributed by this bit can be rebuilt and subtracted from the conventional matched filter decision statistic of the desired bit. In the second stage, after waiting a period equals to a bit duration, the remaining bits would have been estimated using a first stage cancellation. Their interference can then be rebuilt and canceled. Also the interference that has been canceled in the first stage can be re-canceled using the estimates of the second stage of other users. This system is shown to provide a good performance. It is assumed that the received powers, delays and phases of all users are perfectly attained