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 sy stems. Three multirate access schemes are considered: multicode (MC) access where high-rate users multiplex their data bits onto multiple codes and fo rm a single-rate system; variable spreading length (VSL) access where the s preading lengths of signature sequences are inversely proportional to users ' data rates; and-variable chipping rate (VCR) access where the chipping ra tes of the signature sequences are proportional to users' data rates. In or der to remove the influence of signature sequences in the comparison of the three schemes, random signature sequences are assumed. Optimum near-far re sistance is then averaged over all possible realizations. Two types of code sets are considered for the VSL system: general random codes and random re petition codes. Bounds and approximations are provided for the average opti mum near-far resistance. Analytical results show that the performance depen ds on the access schemes and the data rate of the users. The results for th e VSL scheme with general random codes are extended for performance evaluat ion of systems with signature sequences which span many symbol intervals.