Wideband Fading Channels Research Articles

Cache-Aided Communications With Multiple Antennas at Finite SNR

We study the problem of cache-aided communication for cellular networks with multi-user and multiple antennas at finite signal-to-noise ratio. Users are assumed to have non-symmetric links, modeled by wideband fading channels. We show that the problem can be formulated as a linear program, whose solution provides a joint cache allocation along with pre-fetching and fetching schemes that minimize the duration of the communication in the delivery phase. The suggested scheme uses zero-forcing and cached interference subtraction, and hence, allows each user to be served at the rate of its own channel. Thus, this scheme is better than the previously published schemes that are compromised by the poorest user in the communication group. We also consider a special case of the parameters for which we can derive a closed form solution and formulate the optimal power, rate, and cache optimization. This special case shows that the gain of MIMO coded caching goes beyond the throughput. In particular, it is shown that in this case, the cache is used to balance the users such that fairness and throughput are no longer contradicting. More specifically, in this case, strict fairness is achieved jointly with maximizing the network throughput.

Deterministic and stochastic simulators for non-isotropic V2V-MIMO wideband channels

In this paper, we consider a novel two-dimensional (2D) geometry-based stochastic model (GBSM) for multiple-input multiple-output (MIMO) vehicle-to-vehicle (V2V) wideband fading channels. The proposed model employs the combination of a two-ring model and a multiple confocal ellipses model, where the signal is sum of the line-of-sight (LoS) component, single-bounced (SB) rays, and double-bounced (DB) rays. Based on the reference model, we derive some expressions of channel statistical properties, including space-time correlation function (STCF), Doppler spectral power density (DPSD), envelope level crossing rate (LCR) and average fade duration (AFD). In addition, corresponding deterministic and stochastic simulation models are developed based on the reference model. Moreover, we compare the statistical properties of the reference model and the two simulation models in different scenarios and investigate the impact of different vehicular traffic densities (VTDs) on the channel statistical properties of the proposed model. Finally, the great agreement between simulation models and the reference model demonstrates not only the utility of simulation models, but also the correctness of theoretical derivations and simulations.

Unified Capacity Limit of Non-Coherent Wideband Fading Channels

In non-coherent wideband fading channels where energy rather than spectrum is the limiting resource, peaky and non-peaky signaling schemes have long been considered species apart, as the first approaches asymptotically the capacity of a wideband AWGN channel with the same average SNR, whereas the second reaches a peak rate at some finite critical bandwidth and then falls to zero as bandwidth grows to infinity. In this paper it is shown that this distinction is in fact an artifact of the limited attention paid in the past to the product between the bandwidth and the fraction of time it is in use. This fundamental quantity, called bandwidth occupancy, measures average bandwidth usage over time. For all signaling schemes with the same bandwidth occupancy, achievable rates approach to the wideband AWGN capacity within the same gap as the bandwidth occupancy approaches its critical value, and decrease to zero as the occupancy goes to infinity. This unified analysis produces quantitative closed-form expressions for the ideal bandwidth occupancy, recovers the existing capacity results for (non-)peaky signaling schemes, and unveils a trade-off between the accuracy of approximating capacity with a generalized Taylor polynomial and the accuracy with which the optimal bandwidth occupancy can be bounded.

Non-Peaky Signals in Wideband Fading Channels: Achievable Bit Rates and Optimal Bandwidth

In the context of fading channels it is well established that, with a constrained transmit power, the bit rates achievable by signals that are not peaky vanish as the bandwidth grows without bound. Stepping back from the limit, we characterize the highest bit rate achievable by such non-peaky signals and the approximate bandwidth where that apex occurs. As it turns out, the gap between the highest rate achievable without peakedness and the infinite-bandwidth capacity (with unconstrained peakedness) is small for virtually all settings of interest to wireless communications. Thus, although strictly achieving capacity in wideband fading channels does require signal peakedness, bit rates not far from capacity can be achieved with conventional signaling formats that do not exhibit the serious practical drawbacks associated with peakedness. In addition, we show that the asymptotic decay of bit rate in the absence of peakedness usually takes hold at bandwidths so large that wideband fading models are called into question. Rather, ultrawideband models ought to be used.

Artificial Wideband Multi User Channels for Rural High Speed Vehicle to Vehicle Links

Long haul transport can be considered an obvious deployment scenario for communication between vehicles travelling in convoy, where environments will be comparable to that of a rural highway. Therefore this paper addresses antenna array techniques applicable to such circumstances, which provide little scattering to the propagation channel and little support for diversity. Only beamforming is realistically suited to increase the capacity in the vehicle to vehicle link. However, the possibility to enable multi user channels is limited in such environments since the propagation channels are highly correlated when more than two vehicle to vehicle links are in close proximity. The deliberate introduction of delay spread through wide spaced and combined multiple antenna topology thus creating an artificial wideband fading channel is proposed in this paper, which results in de-correlated vehicle to vehicle links. Measurements and experimental results are presented that demonstrate the means by which uncorrelated wideband channels between separate vehicle to vehicle links can be formed to enable multi user capabilities specific to vehicles on a rural highway.

Wideband Fading Channels With Feedback

The Rayleigh flat fading channel at low SNR is considered. With full channel state information (CSI) at the transmitter and receiver, its capacity is shown to be essentially SNR log(1SNR) nats/symbol, as SNR goes to zero. In fact, this rate can be achieved with a just one bit of CSI at the transmitter (per fading realization) and with no receiver CSI. The capacity for the case of noisy transmitter CSI is also found. Then a Rayleigh block fading channel of coherence interval T ≤ 1/ SNR is considered which has causal feedback and no a priori CSI. A training based scheme is proposed for such channels, which achieves a rate of SNR logT nats/symbol in the limit of small SNR and large T. Thus, when coherence interval T is of the order 1/SNR, without any a priori CSI at either end, the capacity with full CSI at both ends is achievable. For smaller values of T, a rate of SNR logT nats/symbol is shown to be achievable.

Wideband Fading Channel Capacity With Training and Partial Feedback

We consider the capacity of a wideband fading channel with partial feedback, subject to an average power constraint. The channel is modeled as a set of parallel independent block Rayleigh fading subchannels with finite coherence time (L channel uses). The transmitter probes a subset of subchannels during each coherence time by transmitting pilot sequences for channel estimation. For each subchannel probed, one bit of feedback indicates whether or not the channel gain exceeds a threshold allowing transmission. Our problem is to optimize jointly the training (both length and power), number of subchannels probed (probing bandwidth), and feedback threshold to maximize the achievable rate (lower bound on ergodic capacity) taking into account the subchannel estimation error. Optimizing the probing bandwidth balances diversity against the quality of the subchannel estimate. We show that the achievable rate increases as S log L, where S is the signal-to-noise ratio, and exceeds the capacity with impulsive signaling (given by S) when L exceeds a (positive) threshold value. Moreover, the optimal probing bandwidth scales as SL/log <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> L. In contrast, without feedback the optimal probing bandwidth for the probing scheme scales as SL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/3</sup> and the achievable rate converges to S, where the gap diminishes as SL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/3</sup> .

Nonparametric mobile speed estimation in fading channels: Performance analysis and experimental results

In this paper we propose a new speed estimation technique, applicable to both mobile and base stations, based on the characteristics in the power spectrum of mobile fading channels. Our analytic performance analysis, verified by Monte Carlo simulations, shows that our low-complexity estimator is not only robust to both Gaussian and non-Gaussian noises, but also insensitive to nonisotropic scattering observed at the mobile. The estimator performs very well in both two- and three-dimensional propagation environments. The robustness against both nonisotropic scattering and line of sight can be further increased, by taking advantage of resolvable paths in wideband fading channels, due to the differences among the Doppler spectra observed at different paths. We also extend this technique to base stations with antenna arrays. By exploiting the spatial information, the proposed space-time estimator exhibits excellent performance over a wide range of noise power, nonisotropic scattering, and line-of-sight component, verified by simulation. The utility of the new method is further demonstrated by applying it to the measured data.

Performance of Multicode DS/CDMA With Noncoherent $M$ -ary Orthogonal Modulation in the Presence of Timing Errors

This paper derives an accurate approximation to the bit error rate (BER) of multicode direct-sequence code-division multiple access (DS/CDMA) with noncoherent <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary modulation in wideband fading channels when timing errors are made at the receiver employing equal-gain combining (EGC). This reflects the practical scenario where the path delays are estimated imperfectly, leading to synchronization errors between the correlation receivers and the received signals. The analysis can be applied to any type of a fading distribution for both independent and correlated diversity branches. It is shown that the derived analytical expressions are in close agreement with the Monte Carlo system simulations, particularly in the case of small timing errors.

On training with feedback in wideband channels

Transmitter knowledge of channel state has a great impact on wideband fading channel capacity. However, in the low SNR regime, power per dimension does not suffice to provide an accurate measurement of the channel over the entire spectrum. In the presence of feedback, we may collect information at the transmitter about some aspects of the channel quality over a certain portion of the spectrum. In this work, we investigate the effect of such information. We consider channel testing with a finite amount of energy over a block-fading channel in both time and frequency. We consider a transmission scheme in which the wideband channel is decomposed into many parallel narrowband subchannels, each used with a binary modulation scheme. The quality of each subchannel corresponds to the crossover probability of a binary symmetric channel. We use a multi-armed bandit approach to consider the relative costs and benefits of allotting energy for testing versus transmission, and for repeated testing a single subchannel versus testing different subchannels. We give both upper and lower bounds on the number of subchannels that should be probed for throughput maximization under the scheme we have chosen. Our bounds are in terms of available transmission energy, available bandwidth and fading characteristics of the channel. Moreover, in our numerical results, the two bounds are close.

An Improved Deterministic SoS Channel Simulator for Multiple Uncorrelated Rayleigh Fading Channels

The generation of multiple uncorrelated Rayleigh fading waveforms is often demanded for simulating wideband fading channels, multiple-input multiple-output (MIMO) channels, and diversity-combined fading channels. In this letter, an improved deterministic sum-of-sinusoids (SoS) channel simulator with a new parameter computation method is proposed to simulate a large number of uncorrelated Rayleigh fading processes. Compared with the existing SoS channel simulators, the proposed deterministic SoS model yields a much better simulation efficiency while still preserving satisfactory approximations to the desired statistical properties of the reference model.

Stochastic Modeling and Simulation of Frequency-Correlated Wideband Fading Channels

For the simulation of practical frequency-diversity wireless communication systems, such as frequency-hopping systems, multicarrier code-division multiple-access systems, and orthogonal frequency-division multiplexing systems, it is often desirable to produce multiple Rayleigh fading processes with given frequency correlation properties. In this paper, a novel stochastic wide-sense stationary sum-of-sinusoids channel simulator is proposed to emulate frequency-correlated wideband fading channels, where the frequency correlation properties are controlled by only adjusting the constant phases. Closed-form expressions are provided for all the parameters of the simulation model. This enables us to investigate analytically the overall correlation properties (not only the correlation coefficients) of the simulated processes with respect to both time separation and frequency separation. It is shown that the wideband channel simulator will be reduced to a narrowband Rayleigh fading-channel simulator by removing the frequency selectivity. Furthermore, the COST 207 typical-urban and rural-area channels are applied to evaluate the performance of the resulting wideband and narrowband channel simulators, respectively. The correlation properties of the simulation models approach the desired ones of the underlying reference models as the number of exponential functions tends to infinity, while very good approximations are achieved with the chosen limited number of exponential functions

Accurate and efficient simulation of multiple uncorrelated Rayleigh fading waveforms

Simulating wideband fading channels, multiple-input multiple-output (MIMO) channels, and diversity-combined fading channels often demands the generation of multiple uncorrelated Rayleigh fading waveforms. In this letter, two appropriate parameter computation methods, namely the method of exact Doppler spread (MEDS) and L <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p </sub> -norm method (LPNM), for deterministic sum-of-sinusoids (SoS) channel simulators are investigated to guarantee the uncorrelatedness between different simulated Rayleigh fading processes. Numerical and simulation results show that the resulting deterministic SoS channel simulator can accurately and efficiently reproduce all the desired statistical properties of the reference model.

Burst-by-burst adaptive decision feedback equalized TCM, TTCM, and BICM for H.263-assisted wireless video telephony

Decision feedback equalizer-aided wide-band burst-by-burst adaptive trellis-coded modulation, turbo trellis-coded modulation (TTCM), and bit-interleaved-coded modulation-assisted H.263-based video transceivers are proposed and characterized in performance terms when communicating over the COST 207 typical urban wide-band fading channel. Specifically, four different modulation modes, namely 4QAM, 8PSK, 16QAM, and 64QAM are invoked and protected by the above-mentioned coded modulation schemes. The TTCM assisted scheme was found to provide the best video performance, although at the cost of the highest complexity. A range of lower-complexity arrangements will also be characterized. Finally, in order to confirm these findings in an important practical environment, we have also investigated the adaptive TTCM scheme in the CDMA-based universal mobile telecommunications systems terrestrial radio access (UTRA) scenario and the good performance of adaptive TTCM scheme recorded when communicating over the COST 207 channels was retained in the UTRA environment.

On-Off Frequency-Shift Keying for Wideband Fading Channels

-ary on-off frequency-shift keying (OOFSK) is a digital modulation format in which-ary FSK signaling is overlaid on on/off keying. This paper investigates the potential of this modulation format in the context of wideband fading channels. First, it is assumed that the receiver uses energy detection for the reception of OOFSK signals. Capacity expressions are obtained for the cases in which the receiver has perfect and imperfect fading side information. Power efficiency is investigated when the transmitter is subject to a peak-to-average power ratio (PAR) limitation or a peak power limitation. It is shown that under a PAR limitation, it is extremely power inefficient to operate in the very-low-SNR regime. On the other hand, if there is only a peak power limitation, it is demonstrated that power efficiency improves as one operates with smaller SNR and vanishing duty factor. Also studied are the capacity improvements that accrue when the receiver can track phase shifts in the channel or if the received signal has a specular component. To take advantage of those features, the phase of the modulation is also allowed to carry information.

A Generalized Performance Study of DS-CDMA Uplink/Downlink Receivers in NakagamiWideband Fading Channel

SUMMARY In this paper, we consider a mobile system consisting of a single isolated circular cell with K independent users simultaneously sharing the channel using binary DS-CDMA to establish a full duplex channel with the base station. Both coherent and differential detection RAKE receivers with Maximal Ratio Combining (MRC) techniques are considered. The performance of two uplink/downlink receivers in Nakagami wideband fading channel is studied. Our approach relies on the use of total instantaneous interference power calculations instead of the use of average power approximations. We analyzed and derived new exact formulae for bit error probabilities for the considered system, and presented a set of numerical results both for the exact formulae and Gaussian approximation. The performance comparisons suggest that the exact formulae provide superior performance to Gaussian approximation especially at low number of users and either high fading parameters of the desired user or low fading param

Turbo-Coded Adaptive Modulation Versus Space–Time Trellis Codes for Transmission Over Dispersive Channels

Decision feedback equalizer (DFE)-aided turbo-coded wideband adaptive quadrature amplitude modulation (AQAM) is proposed, which is capable of combating the temporal channel quality variation of fading channels. A procedure is suggested for determining the AQAM switching thresholds and the specific turbo-coding rates capable of maintaining the target bit-error rate while aiming for achieving a highly effective bits per symbol throughput. As a design alternative, we also employ multiple-input/multiple-output DFE-aided space-time trellis codes, which benefit from transmit diversity and hence reduce the temporal channel quality fluctuations. The performance of both systems is characterized and compared when communicating over the COST 207 typical urban wideband fading channel. It was found that the turbo-coded AQAM scheme outperforms the two-transmitter space-time trellis coded system employing two receivers; although, its performance is inferior to the space-time trellis coded arrangement employing three receivers.

Capacity and reliability function for small peak signal constraints

The capacity and the reliability function as the peak constraint tends to zero are considered for a discrete-time memoryless channel with peak constrained inputs. Prelov and van der Meulen (1993) showed that under mild conditions the ratio of the capacity to the squared peak constraint converges to one-half the maximum eigenvalue of the Fisher information matrix and if the Fisher information matrix is nonzero, the asymptotically optimal input distribution is symmetric antipodal signaling. Under similar conditions, it is shown in the first part of the paper that the reliability function has the same asymptotic shape as the reliability function for the power-constrained infinite bandwidth white Gaussian noise channel. The second part of the paper deals with Rayleigh-fading channels. For such channels, the Fisher information matrix is zero, indicating the difficulty of transmission over such channels with small peak constrained signals. Asymptotics for the Rayleigh channel are derived and applied to obtain the asymptotics of the capacity of the Marzetta and Hochwald (1999) fading channel model for small peak constraints, and to obtain a result of the type of Medard and Gallager for wide-band fading channels.

A comparison of slow-frequency-hop and direct-sequence spread-spectrum communications over frequency-selective fading channels

Spread-spectrum modulation can provide protection from the selective fading that is typically encountered in mobile radio networks. Because the methods of combating frequency-selective fading are quite different for slow-frequency-hop (SFH) and direct-sequence (DS) spread spectrum systems, these two types of modulation perform very differently. The purpose of this paper is to compare the performance of SFH and DS systems under identical conditions for several models of the wide-band fading channel. Each system has the same bandwidth, transmits over the same frequency-selective Gaussian wide-sense-stationary uncorrelated-scattering channel, and uses error-correction coding to combat thermal noise and fading. The probability of bit error at the output of the decoder is determined for each system by a combination of analysis and simulation. Results are presented for systems with a single transmitter-receiver pair and for networks with multiple simultaneous transmissions (i.e., multiple-access communications). The multiple-access network is distributed, so that control of power by a central terminal, such as a base station, is not possible. The results illustrate the tradeoffs in performance between SFW and DS spread-spectrum systems as a function of the parameters of the signals and the channel model. The performance of the SFH system is shown to be less sensitive to the exact characterization of the channel delay spectrum than the DS system. For most of the channels considered in this paper, SFH spread spectrum gives better performance than DS spread spectrum.

Eigen analysis of wide-band fading channel impulse responses

An investigation of the complex path gain correlation properties of several types of wide-band channel impulse responses is presented. The correlation coefficients of the complex path gains of the channel's impulse responses exhibit a large range of values even for large path delay differences. This indicates that the autocorrelation matrix of the channel impulse responses is rarely diagonal, as often assumed in the study of frequency-selective fading channels. Also, using an eigendecomposition and subspace-based statistical signal processing on the autocorrelation matrix of the channel impulse responses, it is then shown that the propagation process is dominated by only a few orthogonal modes in many instances of channel propagation. The eigenvalues of the modes can be used as a measure of the actual diversity gain obtainable over the channels. From the receiver point of view, the dominant modes can be identified and used in the detection process to reduce the number of parameters to track. Among the channels studied, the wide-band conventional cellular and microcellular radio channels at 910 MHz for mobile applications and the wide-band indoor channels with fixed transmitter-receiver configurations at 950 MHz and 40 GHz are particularly important for today's and future applications. Several channel impulse response measurements have been analyzed to demonstrate these assertions.