Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Desmond P. Taylor
Channel sharing by orthogonal separation is fundamental to the designs of an extremely wide variety of multiple-user communications systems. Popular examples in digital communications include TDMA, FDMA, CDMA, and the temporary channel that exists during successful transmission in random-access channels. These engineering solutions have been so successful in practice that it may not be recognized that they were limited by the existing technology. Orthogonality is not, in general, a criterion of optimality. For example, in the additive multiple-user AWGN channel, performance is largely determined by the minimum Euclidean distance between signals at the receiver. Yet, there appears to be no signal design work based on maximizing the minimum Euclidean distance in the overall summed set of signals. The symbol-synchronous linearly-additive multi-user channel is considered in the first part of this work. Rules are presented to realize multi-user coded signals that do not lose minimum Euclidean distance in the receive space. In particular, superposition onto BPSK streams with unit symbol-energy is considered. A trellis code based on 4-dimensional unit-energy signals is constructed which allows a second user to transmit. 3 bits every 4 BPSK intervals without reducing the minimum Euclidean distance in the superposed signal. This design obtains a greater summed rate than previous two-user codes. Furthermore. less energy per bit is required for a given minimum distance. The collision event of the random-access channel is also reconsidered. It is shown that overlapping collisions are often separable by the proper processing algorithm. A generalization of ALOHA is given where 'slot'-collisions are replaced by 'subslot'-collisions, where, in general, there is more than one subslot per slot, with one subslot being equivalent to slotted-ALOHA. Throughputs obtained depend on the complexity available. For example, relative to a single user continuously transmitting, 91% throughput is achieved with 4 subslots, which requires an 8-state Verdu-Viterbi trellis detection algorithm, while 159% throughput is achieved with 10 subslots. requiring 512 states. Potentially, systems with K-subslots obtain K carrier-sense channels when carrier-sensing is available for each subslot. Although it was not the goal of this work, some results may have application to single-user communications. For example, the trellis-coded component of the multi-user constructions may be useful for low-rate, low-SNR signalling. Less anticipated is the result that, for certain values of K, a single-user may send K binary symbols separated in time by T/K, using raised-cosine time-domain pulses limited to duration T, with higher spectral-efficiency than M = 2ᵏ level PAM signalling with raised-cosine frequency-domain pulses at rate 1/T. This particular result appears to be unknown, nevertheless it is less relevant today in light of the more substantial gains that have been achieved with (single-user) trellis-coded modulations. Future investigations and other possible directions of research stemming from this work are discussed throughout and in the concluding chapter.
Ross, John Anderson Fergus, "Multiple-user communications with nonorthogonal signalling" (1994). Open Access Dissertations and Theses. Paper 3881.