Throughput and PN codephase acquisition for packet CDMA without preamble

In CDMA packet transmission, it is customary to start each packet with a preamble consisting of pseudo-random (PN) code with no data modulation. The preamble facilitates receiver acquisition of the spreading code alignment and this is essential for decoding the spread spectrum signal. Since packets are short, matched filters are employed to provide rapid acquisition and, in this case, we use a segmented matched filter that can provide codephase alignment even when there is data modulation. We thus eliminate the need for a packet preamble and improve the system throughput. However, if the matched filter fails to provide the correct codephase, a packet is lost. The probability of correct codephase detection (Pd) is increased by accumulating matched filter samples over several code cycles prior to making a decision. Using accumulated code cycles as a parameter, we present the probability of correctly detecting packet codephase as a function of the number of active co- users. Correct detection probabilities exceeding 99% are indicated from simulations with 25 co-users and 10 kHz Doppler shift or carrier frequency offset by accumulating five or more PN code cycles, using maximum selection detection criterion. Analysis and simulation also show that cyclic accumulation can improve packet throughput by 50% and by as much as 100% under conditions of high offered traffic and carrier frequency offset for both fixed capacity and infinite capacity packet systems.