Mahmoud H. Ismail

Exploiting interference alignment for sum rate enhancement in D2D-enabled cellular networks

Spectrally-efficient and low-latency support of local media services is expected to be provided by enabling underlay direct Device-to-Device (D2D) communication mode in future cellular networks. Interference Alignment (IA) can enhance the capacity of a wireless network by providing more degrees of freedom. In this paper, we propose using IA techniques in a D2D underlay network to enhance spectral efficiency. We compare IA transmission and traditional point-to-point (P2P) transmission from the Bit-Error-Rate (BER) and sum-rate points of view. Furthermore, we propose three grouping schemes for the D2D users into groups of 3-pairs such that IA can be applied using a limited number of signal extensions. Results demonstrate that although traditional P2P transmission can achieve better BER performance; IA transmission is still able to achieve gains in the sum rate. Also, system simulations show that the cell total D2D sum rate can be improved using IA. A gain of up to 31.8% is shown to be attainable at a reasonable transmit signal power level.

Performance of dual maximal ratio combining diversity in nonidentical correlated Weibull fading channels using Pade/spl acute/ approximation

In this letter, we evaluate the performance of the dual-branch maximal ratio combining (MRC) diversity scheme in nonidentical correlated Weibull fading channels with arbitrary parameters. We first use the Pade/spl acute/ approximation (PA) to find closed-form rational expressions for the moment generating function (MGF) of the output signal-to-noise ratio (SNR) of the MRC receiver. Different performance measures, such as the outage probability and the average symbol-error rate for different linear modulations, are then presented using the well-known MGF approach. Furthermore, the effect of the input SNRs unbalancing, the severity of fading, and the degree of correlation on the system performance are also studied. Our results are validated by comparing them with computer simulations, and we show that the PA technique is indeed a convenient tool for such performance evaluation studies.

On the use of padé approximation for performance evaluation of maximal ratio combining diversity over Weibull fading channels

We use the Padé approximation (PA) technique to obtain closed-form approximate expressions for the moment-generating function (MGF) of the Weibull random variable. Unlike previously obtained closed-form exact expressions for the MGF, which are relatively complicated as being given in terms of the Meijer G-function, PA can be used to obtain simple rational expressions for the MGF, which can be easily used in further computations. We illustrate the accuracy of the PA technique by comparing its results to either the existing exact MGF or to that obtained via Monte Carlo simulations. Using the approximate expressions, we analyze the performance of digital modulation schemes over the single channel and the multichannels employing maximal ratio combining (MRC) under the Weibull fading assumption. Our results show excellent agreement with previously published results as well as with simulations.

Outage probability in multiple access systems with weibull-faded lognormal-shadowed communication links

We consider the problem of determining the outage probability in multiple access systems where the desired signal as well as each co-channel interferer is lognormal shadowed and Weibull faded. We provide three expressions for the outage probability, which vary in their accuracy as well as their degree of complexity. The first is an exact expression, which is valid only for the case of even integer fading parameter values. The complexity of this expression grows with the increase in the number of interferers in the system. The second is a lower bound, which requires the computation of a single-fold integral, irrespective of the number of interferers in the system. Finally, we present an approximate expression, which provides better accuracy than the lower bound but requires the evaluation of a double-fold integral. We verify the accuracy of each of the exact, lower bound and approximate expressions by comparing them to the values of the outage probability obtained via Monte Carlo simulations.

On the relay placement problem in a multi-cell LTE-Advanced system with co-channel interference

One of the main problems in LTE-A networks is the small rates achieved by cell-edge users. This is due to the adoption of a frequency reuse factor of 1, which aims at increasing the overall capacity. This occurs, however, at the expense of increasing the interference level for cell-edge users. Cooperative communication through the use of relays is an efficient technique to solve this problem. However, careful placement of the relays is a crucial factor in determining the expected capacity gain. In this paper, the relay placement problem in an LTE-A network is studied, taking into consideration the effect of co-channel interference. An optimization framework is proposed to maximize either the total cell capacity or the total cell-edge capacity. Simulation results show a capacity gain factor of 8.027 for cell-edge users due to relay deployment, under 100% cell load in center cell and adjacent cells.

Optimized spectrum sensing algorithms for cognitive LTE femtocells

In this article, we investigate to perform spectrum sensing in two stages for a target long-term evolution (LTE) signal where the main objective is enabling co-existence of LTE femtocells with other LTE femto and macrocells. In the first stage, it is required to perform the sensing as fast as possible and with an acceptable performance under different channel conditions. Toward that end, we first propose sensing the whole LTE signal bandwidth using the fast wavelet transform (FWT) algorithm and compare it to the fast Fourier transform-based algorithm in terms of complexity and performance. Then, we use FWT to go even deeper in the LTE signal band to sense at multiples of a resource block resolution. A new algorithm is proposed that provides an intelligent stopping criterion for the FWT sensing to further reduce its complexity. In the second stage, it is required to perform a finer sensing on the vacant channels to reduce the probability of collision with the primary user. Two algorithms have been proposed for this task; one of them uses the OFDM cyclic prefix for LTE signal detection while the other one uses the primary synchronization signal. The two algorithms were compared in terms of both performance and complexity.

Shrinking the reuse distance: Spectrally-efficient radio resource management in D2D-enabled cellular networks with Interference Alignment

Enabling underlay direct Device-to-Device (D2D) communication mode in future cellular networks has good potential for spectrally-efficient and low-latency support of local media services. Recently, it has become evident that shrinking the reuse distance over which wireless resources are reused is a key enabler for achieving high spectral efficiency. Moreover, Interference Alignment (IA) based transmission can enhance the capacity of a wireless network by providing more degrees of freedom. In this work, we exploit clustering of D2D users, frequency reuse over clusters and then using IA to enhance the sum rate. Specifically, we show that in a D2D environment, it is possible to achieve significant gains in attainable rates by constructing clusters of D2D pairs and reuse the available radio resources over the clusters. Moreover, within a cluster, it is possible to further enhance the spectral efficiency by constructing small-sized groups of D2D pairs over which IA is applied to offer additional degrees of freedom. We show that resource reuse over the clusters offer overall rate increase proportional to the number of formed clusters. In addition, interference alignment offers up to 33% increase in the overall rates in the high transmission power regimes compared to the normal Point-to-Point (P2P) communication.

Capacity of the alpha-mu fading channel with SC diversity under adaptive transmission techniques

Motivated by the generalization of the α-μ distribution in describing the small-scale fading over wireless channels, in this paper, closed-form expressions for the Shannon capacity of the α-μ fading channel operating under three adaptive transmission strategies are derived. These expressions are derived for the case of no diversity as well as for selection combining diversity with independent and identically distributed branches. The obtained expressions reduce to those previously derived in the literature for the Weibull as well as the Rayleigh fading cases, which are both special cases of the α-μ channel. Numerical results are presented for the capacity under the three adaptive transmission strategies and the effect of the fading parameter as well as the number of diversity branches is studied.

Performance of selection combining diversity in Weibull fading with cochannel interference

We evaluate the performance of selection combining (SC) diversity in cellular systems where binary phase-shift keying (BPSK) is employed and the desired signal as well as the cochannel interferers (CCIs) is subject to Weibull fading. A characteristic function-(CF-) based approach is followed to evaluate the performance in terms of the outage probability. Two selection criteria are adopted at the diversity receiver: maximum desired signal power and maximum output signal-to-interference ratio (SIR). We study the effect of the fading parameters of the desired and interfering signals, the number of diversity branches, as well as the number of interferers on the performance. Numerical results are presented and the validity of our expressions is verified via Monte Carlo simulations.

Outage probability analysis in cellular systems with noisy Weibull-faded lognormal-shadowed links

We consider the problem of determining the outage probability in cellular systems where each of the links is lognormal-shadowed, Weibull-faded and corrupted by additive noise. We propose two solutions for this problem; a simple lower bound on the outage probability based on the concavity of the logarithmic function and an approximate expression based on the use of the central limit theorem. The approximate expression is slightly more complicated than the lower bound but provides a better accuracy for most of the parameters values. We verify the accuracy of the proposed expressions by comparing them to the outage probability obtained by means of Monte Carlo simulations.