The performance of a spread-spectrum CDMA (code-division multiple access) system in a mobile satellite environment is analyzed. Comparisons to single-channel-per-carrier FDMA systems are presented which show that the CDMA approach provides greater capacity. Results from computer simulations, laboratory tests, and field tests of a prototype modem are also presented. The tests results show excellent performance of the modem in the mobile environment and also the feasibility of the spread-spectrum approach to satellite mobile communications.>

A new contention-based demand assignment protocol (CPODA) is described. The protocol is designed to handle packetized data and voice traffic in a multiple access satellite broadcast channel, where the channel is typically shared by hundreds of earth stations. A sophisticated traffic environment is assumed, including multiple priority and delay class distinctions, variable message lengths, and an arbitrary load distribution among the stations. In addition, stations may have different receiving rate capabilities. The CPODA protocol is part of a broader research project concerned with developing protocols for a one-hop satellite network (SATNET). The CPODA and other protocols are currently undergoing experimental evaluation using an INTELSAT IV-A satellite and four earth stations in the Atlantic region.

Coding applications have grown rapidly in the past several years with cost-effective performance demonstrated on several channels. Convolutional coding with soft-decision Viterbi decoding has emerged as a standard technique and is particularly well adapted to the commnnication satellite channel. Decoder implementations are discussed and examples are cited. Robustness of code performance is emphasized and instances of actual coding gain surpassing theoretical or basic coding gain are given. Some promising future directions are noted.

This special issue on the use of coding and error-control techniques comprises four papers and two correspondences. Four of these articles are concerned with communication over burst-noise channels, one reports on sequential decoding over an essentially additive Gaussian noise channel, and one examines the source encoding of analog waveforms for transmission over a noisy binary channel. Coding can provide significant if not dramatic improvements in communication over certain channels in certain instances. Several of these instances are detailed in the papers in this issue. One is struck in reading these papers, however, that, with but one exception (diffuse coding), most of the applications are still experimental. The widespread use of coding is not yet here.

In sequential decoding, the number of computations which the decoder must perform to decode the received digits is a random variable. In this paper, we derive a Paretian lower bound to the distribution of this random variable. We show thatP [C > L]L^{-\rho}, whereCis the number of computations which the sequential decoder must perform to decode a block of\Lambdatransmitted bits, and is a parameter which depends on the channel and the rate of the code. Our bound is valid for all sequential decoding schemes and all discrete memoryless channels. In Section II we give an example of a special channel for which a Paretian bound can be easily derived. In Sections III and IV we treat the general channel. In Section V we relate this bound to the memory buffer requirements of real-time sequential decoders. In Section VI, we show that this bound implies that certain moments of the distribution of the computation per digit are infinite, and we determine lower bounds to the rates above which these moments diverge. In most cases, our bounds coincide with previously known upper bounds to rates above which the moments converge. We conclude that the performance of systems using sequential decoding is limited by the computational and buffer capabilities of the decoder, not by the probability of making a decoding error. We further note that our bound applies only to sequential decoding, and that, in certain special cases (Section II), algebraic decoding methods prove superior.

Chernoff bounds and tilted distribution arguments are applied to obtain error probability bounds for binary signaling on the slowly-fading Rician channel with L diversity. For the maximum likelihood receiver, the CB-optimum [optimum in the sense of minimizing the Chernoff (upper) bound on error probability] signal correlation is determined and plotted; it is found that antipodal signals should be used ifa > b^{2}(1 + b), where a is the signal-to-noise ratio of the specular components andbis that of the fading components. The CB-optimum number of diversity paths is then obtained. Ifa/b > 0.2, antipodal signaling with unlimited diversity is CB-optimum; whereas, ifa/b < 0.2, orthogonal signaling with properly chosen diversity is very nearly CB-optimum. If restricted to orthogonal signaling, unlimited diversity is CB-optimum whenevera/b > 1.0. Similar results are obtained for the generally nonoptimum square-law-combining receiver. In this case, orthogonal signaling with finite diversity is always CB-optimum.

Iterative error-detection codes are applied to a decision-feedback communication system employing a noisy binary-symmetric forward channel and a noise-free feedback channel. The optimum (maximum information rate) code is shown to be one in which only a single parity check is made at each stage of the iteration. This code is referred to as the SPC code. For this code, the probability of an undetected errorP_Mtends to zero algebraically, rather than exponentially, with block length. However, the complexity of the necessary decoder grows only as log\log 1/P_M, a drastic improvement over the\log 1/P_Mgrowth encountered in error-correction systems. The behavior of the SPC code under a memory (blocklength) constraint is also investigated, and the optimum length of each stage of the iteration is determined. The results have considerable practical import--modest memory size can be used with little adverse effect on information rate and with very small probability of undetected error. The over-all performance of the SPC code and decision feedback system is considered when the binary symmetric forward channel is imbedded in an additive Gaussian channel. The Gaussian channel bandwidth that maximizes information rate in bits per second is determined, under the constraint of iow error probability, and the efficiency of the code (ratio of information rate to infinite-bandwidth Gaussian channel capacity) is found to be 27 per cent, two or three times the efficiency obtainable with single-stage error-detection-only codes. Plots are provided of the variation in rate of specific SPC codes when the Gaussian channel is subject to slow fades.