Analysis of the RAKE receiver performance in low spreading gain DS/SS systems

In wideband DS-SS systems the Rake receiver is commonly used to utilize the multipath diversity of the mobile radio channel. In the case of good spreading code correlation properties the multipath components of the received signal can be separated with low interpath interference (IPI). At low spreading gains the good correlation properties of the spreading codes are lost and the Rake receiver performance is degraded by IPI. The IPI consists of symbol self interference (SSI) and intersymbol interference (ISI). In this paper the impact of the IPI on the BER performance of the Rake receiver is analyzed. It is shown that in the presence of SSI the noise at the Rake combiner output is correlated with the IPI, which has a significant influence on the receiver performance. Both BPSK and QPSK data and spreading modulation are considered. Numerical results are given for a system employing random spreading.