On interpath interference suppression by MLSE detection in DS/SS systems

In wideband DS/SS systems the Rake receiver is commonly used to attain a diversity gain by taking advantage of the good correlation properties of the spreading codes in order to separate the multipath components of the received signal. However, at low spreading gains the good correlation properties of the spreading codes are lost and the Rake receiver performance is severely degraded by interpath interference (IPI), which in general consists of intersymbol interference (ISI) and symbol self interference (SSI). Hence, means have to be found to suppress this interferences. The optimum DS/SS receiver derived from the MLSE criterion can be separated into a Rake demodulator followed by an MLSE detector based on a metric similar to that of the Ungerboeck MLSE receiver for unspreaded systems. In this paper it is shown that the MLSE receiver combats the ISI but not the SSI. Thus, in channels with SSI the performance bound for DS/SS systems depends on the actual SSI distortion and the diversity bound can not be approached. Furthermore it is shown that in the case of a chip synchronous system the optimum receiver can be implemented as a modified MLSE detector working directly on the chip matched filter output sampled at chip rate.