Recently proposed opportunistic beamforming exploits the multiuser diversity to reduce the feedback by not requiring the precoding information used for closed-loop schemes to be known at the transmitter. Opportunism could also be beneficially employed for other multiple-antenna transmission techniques like cophasing and antenna selection. For opportunistic beamforming and antenna selection, we give closed-form expressions for throughput that closely approximate the performance of these schemes with a Proportionally Fair scheduler (PFS) at low signal-to-noise ratios (SNRs). For large number of transmit antennas, opportunistic cophasing has similar performance as opportunistic beamforming. Asymptotic dependence of the required number of users to achieve the gains of opportunism on the number of transmit antennas is exponential for opportunistic beamforming (and cophasing for large numbers of transmit antennas), and at best linear for opportunistic antenna selection. For multiple-antenna receivers, we additionally examine an opportunistic multiple-input multiple-output (MIMO) scheme that transmits multiple data streams simultaneously to the same user.

Communication through multiple base station antennas with downlink time division multiplexing, rate control and scheduling is considered for high-speed packet data delivery. In contrast to transmit diversity transmission, a new strategy is proposed. The proposed scheme, named distributed-multi-antenna-scheduling (DMAS), is a generalized form of parallel transmission from each and every available co-located antenna with joint scheduling, that results to simultaneous transmission of more than one packets to more than one intended users. The resulting transmission scheme is analyzed in a CDMA framework for single and dual antenna terminals and is shown, for the case of two base station antennas in downlink transmission, to achieve substantial increase in average data throughput compared to known transmit diversity schemes, in the presence of moderate multi-user diversity. The scheme increases the system throughput through the simultaneous transmission of multiple packets. It thus multiplies the number of simultaneously served users, which is also a desirable feature in the current high-speed data wireless solutions.

The authors consider the problem of bounding the information capacity of saturation recording. The superposition channel with additive Gaussian noise is used as a model for recording. This model says that for a saturation input signal, x(t) (i.e., one that can assume only one of two levels), the output can be expressed as y(t)=x(t)+z(t) where x(t) is a filtered version of the input x(t) and z(t) is additive Gaussian noise. The channel is described by the impulse response of the channel filter, h(t), and by the autocorrelation function of the noise. A specific example of such a channel is the differentiated Lorentz channel. Certain autocorrelation and spectrum expressions for a general Lorentz channel are derived. Upper and lower bounds on the capacity of saturation recording channels are described. The bounds are explicitly computed for the differentiated Lorentz channel model. Finally, it is indicated how the derived bounds can be applied in practice using physical measurements from a recording channel.>

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 channel's input waveform.>

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.>

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.>

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.>

Suppose N people are in a circle and each has a K-bit integer on his forehead and can see the others' integers but does not know his own. The problem is to determine how many bits of information must be exchanged among the players so that each can determine his own number after the exchange. The authors provide an algorithm for which the number of bits that suffices equals K plus the least integer >or=K/(N-1). In particular, if N>K, the total of NK bits can be resolved in K+1 transmissions. Since K bits are required to resolve any of the numbers, this is quite efficient, and is shown to be optimal. In addition, the optimal strategy is obtained when the numbers are of different lengths.>

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.>

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.>

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.

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.

Capacity limits are obtained when binary codes are used for communications over channels in the presence of high levels of interference. The ideal additive Gaussian noise channel and the Rayleigh fading channel are considered. For each channel, the performance of four types of decoders are considered. Both binary and channel measurement (soft decision) decoding are considered; for each of these cases, performance results are obtained when interference can and cannot be detected before decoding. Thus, capacity limits for the minimum achievableE_{b}/N_{o}as a function of the code rateRare obtained for eight different channel models of interest.

This paper gives an extensive analysis of a Packet satellite (PACSAT) communications system that uses a digital processing satellite in a packet data network. It is shown that multiple uplinks coupled with on-board switching and storage can provide significant gains in throughput, efficiency, and margin against instability. Design tradeoffs are presented and the theoretical results are verified by extensive computer simulations.