Lawrence H. Ozarow

Information theoretic considerations for cellular mobile radio

We present some information-theoretic considerations used to determine upper bounds on the information rates that can be reliably transmitted over a two-ray propagation path mobile radio channel model, operating in a time division multiplex access (TDMA) regime, under given decoding delay constraints. The sense in which reliability is measured is addressed, and in the interesting eases where the decoding delay constraint plays a significant role, the maximal achievable rate (capacity), is specified in terms of capacity versus outage. In this case, no coding capacity in the strict Shannon sense exists. Simple schemes for time and space diversity are examined, and their potential benefits are illuminated from an information-theoretic stand point. In our presentation, we chose to specialize to the TDMA protocol for the sake of clarity and convenience. Our main arguments and results extend directly to certain variants of other multiple access protocols such as code division multiple access (CDMA) and frequency division multiple access (FDMA), provided that no fast feedback from the receiver to the transmitter is available. >

The capacity of the white Gaussian multiple access channel with feedback

Since the appearance of [10] by Gaarder and Wolf, it has been well known that feedback can enlarge the capacity region of the multiple access channel. In this paper a deterministic feedback code is presented for the two-user Gaussian multiple access channel, which is shown to allow reliable communication at all points inside a region larger than any previously obtained. An outer bound is given which is shown to coincide with the achievable region, thus yielding the capacity region of this channel exactly.

Nonequiprobable signaling on the Gaussian channel

Signaling schemes for the Gaussian channel based on finite-dimensional lattices are considered. The signal constellation consists of all lattice points within a region R, and the shape of this region determines the average signal power. Spherical signal constellations minimize average signal power, and in the limit as N to infinity , the shape gain of the N-sphere over the N-cube approaches pi e/6 approximately=1.53 dB. A nonequiprobable signaling scheme is described that approaches this full asymptotic shape gain in any fixed dimension. A signal constellation, Omega is partitioned into T subconstellations Omega /sub 0/, . . ., Omega /sub tau -1/ of equal size by scaling a basic region R. Signal points in the same subconstellation are used equiprobably, and a shaping code selects the subconstellation Omega /sub i/ with frequency f/sub i/. Shaping codes make it possible to achieve any desired fractional bit rate. The schemes presented are compared with equiprobable signaling schemes based on Voronoi regions of multidimensional lattices. For comparable shape gain and constellation expansion ratio, the peak to average power ratio of the schemes presented is superior. Furthermore, a simple table lookup is all that is required to address points in the constellations. It is also shown that it is possible to integrate coding and nonequiprobable signaling within a common multilevel framework. >

An achievable region and outer bound for the Gaussian broadcast channel with feedback (Corresp.)

A deterministic coding scheme is presented for additive white Gaussian broadcast channels with two receivers and feedback from the receivers to the transmitter. The region of data rates at which reliable communication is possible is larger than that of the corresponding channel without feedback. This is the first example showing that the capacity region of degraded broadcast channels can be enlarged with feedback.

Information rates for a discrete-time Gaussian channel with intersymbol interference and stationary inputs

Bounds are presented on I/sub i.i.d./-the achievable information rate for a discrete Gaussian Channel with intersymbol interference (ISI) present and i.i.d. channel input symbols governed by an arbitrary predetermined distribution p/sub x/(x). The lower and upper bounds on I/sub i.i.d./ and I are formulated. The bounds on I/sub i.i.d./ are calculated for independent equiprobably binary channel symbols and for causal channels with ISI memory of degree one and two. The bounds on I/sub i.i.d./ are compared to the approximated (by Monte Carlo methods) known value of I/sub i.i.d./ and their tightness is considered. An application of the new lower bound on I/sub i.i.d./ yields an improvement on previously reported lower bounds for the capacity of the continuous-time strictly bandlimited (or bandpass) Gaussian channel with either peak or simultaneously peak power and bandlimiting constraints imposed on the channels input waveform. >

Performance of multi-code CDMA wireless personal communications networks

Multi-code CDMA (MC-CDMA) was proposed as a technique to provide variable rate services with different quality requirements by varying the number of codes assigned to a user. When all other users are idle, a single user is able to use the full bit rate available in that cell. The system retains the CDMA advantage in combating multipath and does not require significant modifications of the radio frequency (RF) circuitry. It can evolve from commercial digital cellular systems based on IS-95. The paper presents preliminary results on the performance of MC-CDMA. The authors consider both voice and data users. Voice users are delay sensitive whereas the authors assume that data users can tolerate delay but require a lower bit error rate. They evaluate the instantaneous capacity as well as the average capacity which accounts for user activity factors and the control channel overhead needed for inactive users to maintain synchronization. Unrestricted access to the channel is given to voice bursts. Data bursts are required to wait until they are permitted to transmit. The authors show that the multi-code capability significantly improves the delay throughput performance of data users.

On the capacity of the Gaussian channel with a finite number of input levels

An analytic confirmation of an observation made by G. Ungerboeck (1965) that approximately optimal performance on a Gaussian channel with capacity C can be obtained with about 2/sup C+1/ levels is given. In particular, the results imply that by using about 2/sup C-1/ levels, a rate of nearly C-1 bit can be achieved, and that by using 2/sup C+1/ levels a rate of about C-0.4 bits can be achieved. >

Achievable rates for a constrained Gaussian channel

The authors obtained lower bounds to the capacity of the continuous-time filtered additive Gaussian noise channel with two-valued inputs. Essentially, the problem reduces to the continuous-time peak-limited case, which remains an unsolved problem of primary importance in communication theory. >

Upper bounds on the capacity of Gaussian channels with feedback

The capacity of the discrete-time additive Gaussian channel without feedback is known. A class of upper bounds on the capacity with noiseless feedback that are quite good for some exemplary channels is obtained. >

Random coding for additive Gaussian channels with feedback

A random coding strategy for discrete-time additive Gaussian noise channels with feedback is analyzed. It has long been known that feedback may increase the capacity of such channels as long as the additive noise process is not white. Here, it is proved that a strictly positive gain is always achieved and that, as the signal power goes to zero, the ratio of feedback capacity to capacity without feedback may be strictly greater than one if the noise spectrum has a null. This is not the case when the noise spectrum is bounded away from zero. It is also demonstrated that random coding, where the codewords are chosen from an ensemble of stationary Gaussian sequences, does not achieve capacity. >