The performance of a direct-sequence/code-division multiple-access (DS CDMA) spreading ALOHA access protocol, which can be applied to wireless data network and personal communications services, is investigated. A Markov model is used to derive upper and lower bounds of the throughput of the underlying system. Time-driven simulation is also performed to investigate the tightness of the bounds. The results indicate that the upper bound is very tight. Numerical results also show that, unlike the conventional ALOHA system, the DS/CDMA spreading mainslotted ALOHA system provides better delay and throughput performance than the DS/CDMA spreading-slotted ALOHA system.>

Issues encountered in the design of reliable narrowband time-division multiple access (TDMA) digital cellular mobile communication systems are considered. In particular, the problem of compensating for the harsh multipath fading environment in systems whose transmission bandwidth is commensurate with the coherence bandwidth of the fading channel is considered. A TDMA channel characterization parameter, the slot-normalized fade rate, is introduced, and an adaptive bidirectional equalization technique, which estimates the location of a deep fade within a time slot, is proposed. The simulation results show that the carrier-to-noise ratio requirement is only 15.5 dB when this equalization technique is used. This is achieved without diversity, and with low complexity. An equivalent equalized land mobile radio channel model and the analytical solution for the optimal bit likelihood calculation for pi /4-shift quadrature differential phase-shift keying (QDPSK) modulation are also derived under certain channel conditions. The results are used as soft decisions for the convolutional decoder.>

The commenters offer a counterexample to the results presented by W. Yue (see ibid., vol.39, no.6, p.818-22, June 1991) concerning the effect of capture on multichannel slotted ALOHA. A simplified deterministic capture is assumed by Yue. Numerical results show that the actual average throughput is much higher than that obtained by Yue for the same parameters. >

The author investigates the performance of a bit-interleaved convolutionally encoded code division multiple access (CDMA) communication system with feedback power control over mobile radio channels. The analytical formulas for both optimal and suboptimal bit likelihood ratios, to be used as soft decision metrics for the decoder, are derived for noncoherent detection, with and without diversity reception. The resulting optimal bit soft decisions provide a decoded BER of 0.1% with 1.0-1.2 dB less power than that required for hard decisions in the proposed North American CDMA digital cellular system.>

The computation of upper and lower bounds to error probability in digital transmission over nonlinear channels with a finite memory is considered. By using orthogonal Volterra series, the authors derive a canonical representation for discrete nonlinear systems, based on a linear convolutional code and a memoryless mapper. This representation shows that finite-memory, discrete nonlinear systems can be analyzed in much the same way as TCM (trellis-coded modulation) schemes. In particular, TCM over nonlinear channels can be analyzed. A technique is derived that expresses an upper bound to error probability based on the computation of the transfer of a state diagram with N branches, and whose branch labels are matrices rather than scalars. Some examples of its application are given. In particular, error bounds are derived for nonlinear TCM schemes and for TCM schemes operating on nonlinear channels.>