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A 3-D reference model for wideband multiple-input multiple-output (MIMO) mobile-to-mobile (M-to-M) channels is reviewed along with its corresponding first- and second-order channel statistics. To validate the reference model, an experimental MIMO M-to-M channel-sounding campaign was conducted for M-to-M vehicular communication with vehicles that travel along surface streets and expressways in a metropolitan area. To compare the first- and second-order channel statistics that were obtained from the reference model with those obtained from the empirical measurements, a new maximum-likelihood-based stochastic estimator is derived to extract the relevant model parameters from the measured data. The measured data is processed and compared with the analytical results. The close agreement between the analytically and empirically obtained channel statistics confirms the utility of the proposed reference model and the method for estimating the model parameters. Index Terms--Channel sounding, mobile-to-mobile (M-to-M) channels, parameter estimation, wideband channel model.

The performance of multiple-input-multiple-output systems with optimum combining (OC) is studied in a Rayleigh fading environment with arbitrary-power cochannel interference and thermal noise. Based on the joint eigenvalue distributions of quadratic functions of complex Gaussian matrices, a closed-form expression for the exact distribution of the output signal-to-interference-plus-noise ratio (SINR) is derived. A closed-form expression for the exact moment-generating function (MGF) of the output SINR of single-input-multiple-output (SIMO) systems is also derived. From the exact MGF, the moments of the output SINR and the symbol error rate of various M-ary modulation schemes are obtained. We verify the accuracy of our analytical results with numerical examples. The new analytical framework provides a simple and accurate way to assess the effects of equal-and unequal-power cochannel interferers and thermal noise on the performance of OC. Index Terms--Adaptive antennas, cochannel interference, fading channels, multiple-input-multiple-output (MIMO) systems.

The performance of multiantenna receivers in cochannel interference is investigated for narrowband links in a non-line-of-sight mobile-to-mobile channel. Channel impairments include Doppler spread and angular spread originating from scatterers in the vicinity of both the transmitter and the receiver. The analysis employs a two-ring scattering model and compares the performance of a sectored system employing selection combining (SC) to the performance of adaptive arrays implemented with both omnidirectional antennas and sectored antennas. Performance is estimated for environments with cochannel interference and is measured in terms of output signal-to-interference-plus-noise ratio (SINR). An ensemble of random scattering realizations is utilized in the analysis to estimate the average output SINR. The results demonstrate the improved performance offered by adaptive arrays over sectored systems with SC. Configurations involving both SC and adaptive array processing were also evaluated and found to yield improvements over conventional adaptive array architectures. Index Terms--Adaptive array, cochannel interference, mobile communications.

Use of companding for peak-to-average-power ratio (PAPR) control is explored for a link involving a nonlinear transmit power amplifier with orthogonal frequency division multiplexing (OFDM). Specifically, the objective of the study was to determine if companding using u-law compression/expansion at the transmitter/receiver, respectively, provides end-to-end performance gains relative to a system without companding. We consider the use of companding to ameliorate the impact of nonlinearities in the transmit amplifier. In the absence of companding, transmitter operation near saturation raises the signal level at the receiver but, because of the nonlinearities in the amplifier response, also results in distortion that impacts overall link performance. As the transmit power is backed-off from saturation, amplifier distortion is reduced, but error components due to lower SNR at the receiver become more significant. When companding is introduced in the system, the system is able to operate closer to saturation without substantial transmit distortion. However, requisite expansion of the compressed signal at the receiver yields noise amplification which can counteract any of the performance gains that would otherwise accrue from the increased SNR at the receiver. At issue is whether or not operating conditions exist (e.g., backoff, SNR, amplifier linearity model, etc) for which companding enhances the end-to-end performance relative to the link performance without companding. System simulation models were employed using Rapp's nonlinear power amplification models, where average symbol distance errors were used as performance metrics. We found that companding can provide very modest performance gains in comparison to systems that do not employ companding. Performance trends were corroborated in a hardware testbed with an amplifier chain, where average bit error rates were experimentally determined. Index Terms--Amplifier distortion, communication system performance, companding.

Cooperation diversity schemes have been proposed for cellular networks that permit mobile stations to relay signals to a final destination, thereby increasing the network capacity and coverage. The mobile relays either decode and retransmit the received signal or simply amplify and forward (A & F) the signal. The overall channel from the source to the destination via the relay in A & F systems is 'double' Gaussian with properties quite different from a typical cellular channel. Since very little is known about A & F relay fading channels, this paper considers their statistical properties such as the envelope probability density function, autocorrelation, level crossing rate, and system performance characteristics like frequency of outages and average outage durations. We briefly discuss the simulation of these channels and verify our analysis by simulations. Index Terms--Ad hoc cellular networks, channel statistics, cooperation diversity, Rayleigh fading, relay channels.

Mobile-to-mobile channels find increasing applications in futuristic intelligent transport systems, ad hoc mobile wireless networks, and relay-based cellular networks. Their statistical properties are quite different from typical cellular radio channels, thereby requiring new methods for their simulation. This paper proposes a "double-ring" model to simulate the mobile-to-mobile local scattering environment, and develops sum-of-sinusoids (SoS)-based models for simulating such channels. The proposed models produce waveforms having desired statistical properties with good accuracy, and also remove some drawbacks of an existing model derived by using the discrete line spectrum simulation method. Index Terms--Fading channel simulator, mobile-to-mobile channels, Rayleigh fading, sum-of-sinusoids (SOS).

Several new "sum-of-sinusoids" models have recently been introduced for the simulation of Rayleigh fading channels. These models are statistical in nature implying that their simulation parameters such as the Doppler frequencies are random. They have been shown to accurately reproduce some of the desired statistical properties of the faded envelope such as the time autocorrelation and the level crossing rate. However, a comparative analysis of these models, hitherto scattered throughout the literature, is not available. This paper compares these models in terms of their complexity and performance. The performance assessment is based upon the variance of the time average statistical properties from their ideal ensemble averages. Index Terms--Channel models, fading channel simulator, Rayleigh fading, sum-of-sinusoids (SOS).