Bandwidth-E cient Multiple-Access via Signal Design for Decision-Feedback Receivers: Towards an Optimal Spreading-Coding Trade-O

| The new Bandwidth E cient Multiple Access (BEMA) scheme is introduced. It o ers a higher spectral e ciency than the dynamic versions of Time and Code Division Multiple-Access (TDMA, CDMA) as well as Orthogonal and Identical Waveform Multiple Access (OWMA, IWMA). This idea must be understood from the viewpoint that BEMA results from optimizing over a class of Correlated Waveform Multiple Access (CWMA) strategies that includes TDMA, CDMA, OWMA and IWMA as special cases. At the heart of this new BEMA technique are signal processing algorithms that realize the functions of multiuser equalization and signal design, and which are developed by taking a new and combined approach to those two problems. I. Bandwidth-Efficient Multiple-Access B EMA is an adaptive multiaccess strategy for mobile-tobase communications. Transmitter pulses or signature waveforms are continuously computed at the base station as a function of the multiuser receiver employed and the tra c conditions such as received power levels, number of active transmitters, etc. They are then dynamically allocated, multiple times within the duration of a call if necessary, from the base station to the mobiles at a frequency that is dictated by the rate of signi cant change of those conditions. We assume that this change takes place on a time scale that is su ciently slower than the data rate so as to allow e ective adaptation. Dynamic TDMA is a very particular case where \signal assignment" reduces to the allocation of non-overlapping time-slots and even that takes place exactly once at the time of setting up a call. Practical TDMA systems, such as the European digital cellular mobile radio (GSM) system [1], even impose the further restriction that the time-slots be of equal (burst) durations for each user, thereby surrendering the possibility of using the often disparate power levels of the incoming signals to their advantage. CDMA on the other hand is a multiple-access technique where the signature waveforms are thought of as being pre-assigned and quasi-orthogonal. This is a static multiaccess strategy that would in general be inferior to dynamic TDMA in bursty applications. Following the lead of dynamic TDMA however, it is easy to conceive of a dynamic version of CDMA where quasi-orthogonal signals (instead of time-slots) are dynamically allocated exactly once at the time of setting up a call from a base station drawing from a pre-designed set of such signals. However, the pre-design of the signature waveforms implies that the system is designed for some nominal operating point, and does not have the capability of adapting to current tra c conditions. FurThis work was supported in part by NSF grants NCR-9406069 and NCR-9706591. By CDMA, we mean the multiple-access technique that is referred to in the now considerable literature on multiuser detection (cf. [2] [3]). thermore, the condition of quasi-orthogonality forces a large fraction of bandwidth expansion to be allocated to spreading rather than to coding. This therefore is preemptive and without technical justi cation. So also is dynamic TDMA which relies on spreading to an even greater degree because it requires orthogonal non-overlapping waveforms. It is also possible to postulate a multiple-access technique which lies somewhere inbetween TDMA and CDMA where the waveforms are required to be orthogonal but they are allowed to overlap in time. We will refer to this as Orthogonal Waveform Multiple Access (OWMA). On the other extreme, we have IWMA which uses identical waveforms for all users and relies entirely on coding, and not at all on spreading. In contrast to all of the above mentioned multiple-access schemes, BEMA lets the tra c conditions dictate the optimum relative allocation of bandwidth expansion between spreading and coding. A design methodology is developed to solve the spreading-coding problem via a joint approach to signal design and multiuser equalization. In this paper, we demonstrate that by choosing BEMA as a CWMA strategy that o ers as high a spectral e ciency as possible while meeting given performance specications for each user, it is possible to obtain marked improvements in bandwidth e ciency over the other multiaccess strategies. We focus on the simple context of an additive white Gaussian noise channel with PAM transmitters operating in symbol synchronism. It is customary for practical PAM digital communication systems to employ transmitter pulses that are strictly time-limited to a symbol duration. In this case, they cannot be strictly bandlimited and so we instead consider the root-mean-squared, or RMS, bandwidth. In particular, we are interested in the minimum bandwidth required that will allow a decision feedback receiver to achieve a given level of performance measured in terms of signal-to-interference ratio for each user. The decision feedback receivers at the base station in turn perform multiuser equalization together with decoding with the objective of achieving high performance. They allow each user to use independent single-user codes and involve the implementation of only single-user, and hence o -the-shelf, codecs. In particular, we focus on a decision feedback receiver which we call the capacity maximizing decision feedback receiver (CM-DFR) which achieves the total capacity of the CWMA system at a vertex point of the CWMA channel capacity region [4]. This rate-tuple was hitherto known to be achievable only through random multiuser coding together with an extremely complex optimum joint decoder [5] [6] [7]. Based on the relationship between By spreading we mean the extra bandwidth incurred by using nonidentical waveforms.