Abstract—Transmission of information over a discrete-time memoryless Rician fading channel is considered, where neither the receiver nor the transmitter knows the fading coefficients. The spectral-efficiency/bit-energy tradeoff in the low-power regime is examined when the input has limited peakedness. It is shown that if a fourth-moment input constraint is imposed, or the input peakto-average power ratio is limited, then in contrast to the behavior observed in average-power-limited channels, the minimum bit energy is not always achieved at zero spectral efficiency. The lowpower performance is also characterized when there is a fixed peak limit that does not vary with the average power. A new signaling scheme that overlays phase-shift keying on ON–OFF keying (OOK) is proposed and shown to be optimally efficient in the low-power regime. Index Terms—Fading channels, low-power regime, memoryless fading, peak constraints, Rician fading, spectral efficiency. I.

Abstract — Transmission of information over a discrete-time memoryless Rician fading channel is considered. The noncoherent scenario, where neither the receiver nor the transmitter knows the fading coefficients, is assumed. If the input is subject to a peak-power constraint, it is shown that uniformly distributed phase is optimal and the capacity-achieving amplitude distribution is discrete with a finite number of mass points. It is also proven that if the input has only an average power constraint, the optimal input amplitude distribution has bounded support. Then, the Rician fading channel with uniform phase noise in the specular component is analyzed. It is shown that under an average power limitation, the optimal input amplitude distribution is discrete with a finite number of mass points. I.

Differential Unitary Space-Time Modulation (DUSTM) and its earlier nondifferential counterpart, USTM, permit high-throughput MIMO communication entirely without the possession of channel state information (CSI) by either the transmitter or the receiver. For an isotropically random unitary input we obtain the exact closed-form expression for the probability density of the DUSTM received signal, which permits the straightforward Monte Carlo evaluation of its mutual information. We compare the performance of DUSTM and USTM through both numerical computations of mutual information and through the analysis of low- and high-SNR asymptotic expressions. In our comparisons the symbol durations of the equivalent unitary space-time signals are both equal to T, as are the number of receive antennas N. For DUSTM the number of transmit antennas is constrained by the scheme to be M = T/2, while USTM has no such constraint. If DUSTM and USTM utilize the same number of transmit antennas at high SNR’s the normalized mutual information of the differential and the nondifferential schemes expressed in bits/sec/Hz are asymptotically equal, with the differential scheme performing somewhat better, while at low SNR’s the normalized mutual information of DUSTM is asymptotically twice the normalized mutual information of USTM. If, instead, USTM utilizes the optimum number of transmit antennas then USTM can outperform DUSTM at sufficiently low SNR’s.

Discrete-time Rayleigh fading single-input single-output (SISO) and multiple-input multipleoutput (MIMO) channels are considered, with no channel state information at the transmitter or the receiver. The fading is assumed to be stationary and correlated in time, but independent from antenna to antenna. Peak-power and average-power constraints are imposed on the transmit antennas. For MIMO channels, these constraints are either imposed on the sum over antennas, or on each individual antenna. For SISO channels and MIMO channels with sum power constraints, the asymptotic capacity as the peak signal-to-noise ratio tends to zero is identified; for MIMO channels with individual power constraints, this asymptotic capacity is obtained for a class of channels called transmit separable channels. The results for MIMO channels with individual power constraints are carried over to SISO channels with delay spread (i.e. frequency selective fading).

Transmission of information over a discrete-time memoryless Rician fading channel is considered where neither the receiver nor the transmitter knows the fading coefficients and the input amplitude is subject to second and fourth moment constraints. The main focus is on the power-limited (i.e., wideband, low-power) regime. First the structure of the capacity-achieving input signals is investigated. It is shown that uniform phase is optimal if there is a line of sight component and that the capacity-achieving input amplitude distribution is discrete in the low-power regime. The spectralefficiency/bit-energy tradeoff in the power-limited regime is examined and it is shown that the minimum bit energy is not always achieved at zero spectral efficiency. In this case, it is argued that one should avoid operating in the region where the spectral efficiency is lower than the spectral efficiency of the minimum bit energy point.

The wideband regime of bit-interleaved coded modulation (BICM) in Gaussian channels is studied. The Taylor expansion of the coded modulation capacity for generic signal constellations at low signal-tonoise ratio (SNR) is derived and used to determine the corresponding expansion for the BICM capacity. Simple formulas for the minimum energy per bit and the wideband slope are given. BICM is found to be suboptimal in the sense that its minimum energy per bit can be larger than the corresponding value for coded modulation schemes. The minimum energy per bit using standard Gray mapping on M-PAM or M 2-QAM is given by a simple formula and shown to approach-0.34 dB as M increases. Using the low SNR expansion, a general trade-off between power and bandwidth in the wideband regime is used to show how a power loss can be traded off against a bandwidth gain.

Abstract not found