Optimum near-far resistance for dual-rate DS/CDMA signals: random signature sequence analysis

Optimum near-far resistance is studied for synchronous dual-rate DS/CDMA systems. Three multirate access schemes are considered: multicode (MC) access where high-rate users multiplex their data bits onto multiple codes and form a single-rate system; variable spreading length (VSL) access where the spreading lengths of signature sequences are inversely proportional to users' data rates; and variable chipping rate (VCR) access where the chipping rates of the signature sequences are proportional to users' data rates. In order to remove the influence of signature sequences in the comparison of the three schemes, random signature sequences are assumed. Optimum mar-far resistance is then averaged over all possible realizations. Two types of code sets are considered for the VSL system: general random codes and random repetition codes. Bounds and approximations are provided for the average optimum near-far resistance. Analytical results show that the performance depends on the access schemes and the data rate of the users. The results for the VSL scheme with general random codes are extended for performance evaluation of systems with signature sequences which span many symbol intervals.