The information theoretic approach and the collision resolution approach to multiaccess channels are reviewed in terms of the underlying communication problems that both are modeling. Some perspective on the strengths and weakness of these approaches is given, and the need of a more combined approach focused on coding and decoding techniques is argued.

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A perspective on multiaccess channels

A new approach to asynchronous multiple access communications is presented. The technique, employing orthogonal convolutional coding and Viterbi decoding, is described and its performance characteristics are derived for the case in which other-user interference is the only source of noise. Results indicate that the technique is quite efficient in terms of the number of users supportable at a specified bit error rate in a given system bandwidth. Furthermore, the results of a design study are described, showing that the technique is a practical one to implement.

A New Coding Technique for Asynchronous Multiple Access Communication

A Mock code for the noiseless multiple access OR channel is introduced. An exponential error bound is proven if the sum of the equal code rates of the asynchronous T users is less than \ln 2 .

A block code for noiseless asynchronous multiple-access OR channel (Corresp.)

The performance of the periodic Hamming correlation function of the sequences of states of m -sequence simple linear feedback shift register generators is evaluated. We call these sequences state m sequences. Theoretical lower and upper bounds on the peaks of the Hamming cross-correlation functions of those sequences, which are often used in frequency-hopped spread-spectrum systems, are derived. Numerical results in support of the theory are presented, and it is shown that the worst-case shift of the cross-correlation for relatively short sequences is of burst nature. The state position mapped (SPM) sequences of the state m -sequences are introduced. Their suitability for OR-channel code division multiplexing is numerically examined. The behavior of the periodic crosscorrelation function of these SPM sequences is used as the basis for judging their performance. It is explained also that the lower bound on the peak of the periodic Hamming cross-correlation function of the state m -sequences is naturally a lower bound on the peak of the periodic crosscorrelation function of SPM state m -sequences. This is due to the general relationship (a decimation relationship) between the two types of correlation functions. This relationship is explained and illustrated by an example. Tables of the peaks of both cross-correlation functions are presented for all sets of sequences for sequence generators of 5, 6, 7, and 8 stages.

Bounds on the Cross-Correlation Functions of State m-Sequences

This paper describes a signal processing repeater which enhances the performance that can be obtained with random access discrete address (RADA) digital transmission links which operate in a multiple access mode. In general, RADA modulation techniques employ groups of pulses to transmit information. The pulses within a group normally have different carrier frequencies and are separated from each other by one or more time slots. The communication performance capability of a system which uses these techniques is a function of interference between the pulse groups of different users and background noise. A number of RADA techniques were proposed for the U. S. Army to provide communications within field army and divisions. For army application these had the advantages of rapid synchronization and ease of implementation, accompanied by the disadvantage of requiring large amounts of bandwidth. Significant bandwidth reductions can be obtained through the use of efficient data encoding-decoding techniques. Also, the downlink power requirements can be minimized through the use of an antipodal downlink signal. The processing repeater discussed in this paper can convert any number of randomly phased uplink signals to a single antipodal downlink signal which is amenable to phase coherent detection. The required downlink power is thereby minimized. Thus, the use of RADA techniques with efficient encodingdecoding and a processing repeater can provide a multiple access capability which efficiently uses bandwidth and downlink power.

R. Sommer

Papers

High Efficiency Multiple Access Communications Through a Signal Processing Repeater