Space-Time Receiver for Multi-Rate CDMA Systems: Multi-Code and Variable Spreading Length Access Methods Comparison

This work proposed the linear minimum mean square error (LMMSE) space-time receivers for synchronous multi-rate direct sequence code devision multiple access (DS/CDMA) systems. The performance of the proposed receivers is analyzed. The high-rate (HR) LMMSE space-time receiver with combining technique can provide essentially the same performance for low-rate (LR) users as LR LMMSE space-time receiver while eliminate the time delay for the HR users. The performance of the proposed multi-rate LMMSE space-time receivers are studied in the context of two multi-rate access methods: multi-code (MC) access where high data rate users multiplex their information streams onto multiple codes, and variable spreading length (VSL) access where signature sequences of different lengths are assigned to users with different data rates. Through the simulation results, we show that: (a) the proposed multi-rate LMMSE space-time receivers are optimum near-far resistant; (b) the proposed multi-rate LMMSE space-time receivers have a clear performance improvement compared to the time only LMMSE dual rate receiver; (c) there is a performance gain of the proposed multi-rate LMMSE space-time receivers compared to the multi-rate decorrelating space-time receiver when near-far ratio (NFR) is low; (d) The performance difference between MMSE combining and MRC in multi-rate system is much more pronounced than in single-rate system. Simulations also show that the VSL access method is more robust than MC access method considering changing rate-ratio in multi-rate systems.