Emad Alsusa

Dynamic linear precoding for the exploitation of known interference in MIMO broadcast systems

This paper introduces a novel channel inversion (CI) precoding scheme for the downlink of phase shift keying (PSK)-based multiple input multiple output (MIMO) systems. In contrast to common practice where knowledge of the interference is used to eliminate it, the main idea proposed here is to use this knowledge to glean benefit from the interference. It will be shown that the system performance can be enhanced by exploiting some of the existent inter-channel interference (ICI). This is achieved by applying partial channel inversion such that the constructive part of ICI is preserved and exploited while the destructive part is eliminated by means of CI precoding. By doing so, the effective signal to interference-plus-noise ratio (SINR) delivered to the mobile unit (MU) receivers is enhanced without the need to invest additional transmitted signal power at the MIMO base station (BS). It is shown that the trade-off to this benefit is a minor increase in the complexity of the BS processing. The presented theoretical analysis and simulations demonstrate that due to the SINR enhancement, significant performance and throughput gains are offered by the proposed MIMO precoding technique compared to its conventional counterparts.

Resource Efficiency: A New Paradigm on Energy Efficiency and Spectral Efficiency Tradeoff

Spectral efficiency (SE) and energy efficiency (EE) are the main metrics for designing wireless networks. Rather than focusing on either SE or EE separately, recent works have focused on the relationship between EE and SE and provided good insight into the joint EE-SE tradeoff. However, such works have assumed that the bandwidth was fully occupied regardless of the transmission requirements and therefore are only valid for this type of scenario. In this paper, we propose a new paradigm for EE-SE tradeoff, namely the resource efficiency (RE) for orthogonal frequency division multiple access (OFDMA) cellular network in which we take into consideration different transmission-bandwidth requirements. We analyse the properties of the proposed RE and prove that it is capable of exploiting the tradeoff between EE and SE by balancing consumption power and occupied bandwidth; hence simultaneously optimizing both EE and SE. We then formulate the generalized RE optimization problem with guaranteed quality of service (QoS) and provide a gradient based optimal power adaptation scheme to solve it. We also provide an upper bound near optimal method to jointly solve the optimization problem. Furthermore, a low-complexity suboptimal algorithm based on a uniform power allocation scheme is proposed to reduce the complexity. Numerical results confirm the analytical findings and demonstrate the effectiveness of the proposed resource allocation schemes for efficient resource usage.

Resource Allocation for Energy Efficiency Optimization in Heterogeneous Networks

Heterogeneous network (HetNet) deployment is considered a de facto solution for meeting the ever increasing mobile traffic demand. However, excessive power usage in such networks is a critical issue, particularly for mobile operators. Characterizing the fundamental energy efficiency (EE) performance of HetNets is therefore important for the design of green wireless systems. In this paper, we address the EE optimization problem for downlink two-tier HetNets comprised of a single macro-cell and multiple pico-cells. Considering a heterogeneous real-time and non-real-time traffic, transmit beamforming design and power allocation policies are jointly considered in order to optimize the system energy efficiency. The EE resource allocation problem under consideration is a mixed combinatorial and non-convex optimization problem, which is extremely difficult to solve. In order to reduce the computational complexity, we decompose the original problem with multiple inequality constraints into multiple optimization problems with single inequality constraint. For the latter problem, a two-layer resource allocation algorithm is proposed based on the quasiconcavity property of EE. Simulation results confirm the theoretical findings and demonstrate that the proposed resource allocation algorithm can efficiently approach the optimal EE.

Dynamic Peak-Based Threshold Estimation Method for Mitigating Impulsive Noise in Power-Line Communication Systems

Impulsive noise (IN) is one of the most dominant factors responsible for degrading the performance of power-line communication systems. One of the common techniques for mitigating IN is blanking which is applied at the front end of the receiver to zero the incoming signal when it exceeds a certain threshold. Determining the optimal blanking threshold (OBT) is, however, key for achieving the best performance. Most reported work to find the OBT is based on the availability of the long-term characteristics of IN at the receiver. In this paper, we consider orthogonal frequency-division multiplexing (OFDM)-based power-line communications and propose a method for finding the OBT without requiring any knowledge about the IN. We show that there is a direct relationship between the OBT and the peak-to-average power value of the OFDM symbol and utilize it to identify the OBT. The results reveal that the proposed technique not only eliminates the need to prior knowledge about the characteristics of IN but also achieves a gain between 0.5-2.5 dB depending on the accuracy of the signal peak-to-average estimate. It will also be shown how the performance of the proposed method can be further enhanced by employing some basic signal per processing at the transmitter.

Performance Analysis of the Periodogram-Based Energy Detector in Fading Channels

In this paper, we provide a performance analysis of cognitive radio systems employing energy detection based on power spectral density estimation. Specifically, we derive mathematical expressions for the probabilities of false alarm and missed detection when the periodogram estimate is used as a decision statistic. First, using the characteristic function of the quadratic form representation of the periodogram, we study the characteristics of the product of the sample covariance matrix of the observed vector and the symmetric matrix of the quadratic form. We have found that this product is Rank-1 with one non-zero eigenvalue and therefore a simplified expression for the cumulative density function (CDF) is obtained. Second, the CDF is exploited to derive accurate mathematical expressions for the probabilities of false alarm and missed detection in both Rayleigh and Rician fading channels. Also, for each case, we have derived the probability distribution function (PDF) of the corresponding eigenvalues. The results demonstrate that the probabilities of false alarm and missed detection are independent of the length of the observations when all the other parameters remain fixed. The results also reveal that the considered frequency-domain based energy detector is superior to its conventional time-domain counterpart as it provides fewer false alarms.

Soft Linear Precoding for the Downlink of DS/CDMA Communication Systems

In this paper, we propose a transmitter-based linear-precoding scheme that outperforms conventional precoding by making use of a portion of the interference between the users in a code-division multiple-access (CDMA) system downlink. The utilization of part of the interference is achieved by selectively orthogonalizing the desired symbols to destructive interference by means of precoding while allowing interference that constructively contributes to the useful signals energy. The existence and exploitation of constructive interference effectively spreads the signal constellation and enhances the signal-to-interference-plus-noise ratio (SINR) at the receiver. SINR improvement is attained with no need for additional power-per-user investment at the transmitter since energy that is inherent in the CDMA system is utilized. The scheme introduced in this paper applies to the downlink of cellular phase-shift keying (PSK)-based CDMA systems. Theoretical analysis and comparative simulations show that significant performance improvement can be attained with the proposed technique.

On the Performance of Energy Detection Using Bartlett's Estimate for Spectrum Sensing in Cognitive Radio Systems

Energy detection can be used for spectrum sensing in cognitive radios when no prior knowledge about the primary signals is available. The performance of this technique, however, is strongly influenced by the available decision estimate. This paper presents accurate performance analysis of energy detection when using Bartletts estimate as a test statistic. Both independent and identically distributed Rayleigh and Rician fading channels are investigated for unknown signals with complex envelopes. The novel contribution here is threefold. First, the quadratic form representation of Bartletts estimate is formulated. Then, starting from the characteristic function, the cumulative distribution function is derived for each type of channel and accurate expressions are developed for the probabilities of false alarm and missed detection. Finally, a performance comparison with the raw periodogram is presented. The accuracy of the proposed analysis is confirmed using Monte Carlo trials. The results provide valuable insight into the performance of Bartletts based energy detection. It will be seen that the additional degrees of freedom associated with Bartletts method empower it to offer excellent performance. When compared with the raw periodogram, Bartletts consistently leads to lower probability of miss, but its probability of false alarm is only superior at relatively higher sensing thresholds.

Preprocessing-Based Impulsive Noise Reduction for Power-Line Communications

Signal blanking is a common technique for mitigating impulsive noise (IN) in power-line communications. When signal samples unaffected by IN are erroneously blanked, part of the useful signal will be lost and performance will degrade. In this paper, we show that the performance of this technique is sensitive not only to the blanking threshold but also to the signals peak-to-average power ratio (PAPR). We thus propose enhancing the capability of the conventional blanking technique by preprocessing the signal at the transmitter. With this in mind, a closed-form analytical expression for the probability of blanking error is then derived and the problem of blanking threshold optimization is addressed. The results reveal that the proposed technique is able to minimize the probability of blanking error dramatically and can provide up to 3.5-dB signal-to-noise ratio (SNR) improvement relative to the conventional technique. Furthermore, it will be shown that if the transmitted signals PAPR is maintained below a certain threshold, then not only can a considerable SNR enhancement be achieved but it is also possible to completely alleviate the need for any prior knowledge about the IN characteristics in order to optimally blank it.

Exact SINR Statistics in the Presence of Heterogeneous Interferers

We derive new results for the higher order moments of signal-to-interference-plus-noise ratio (SINR) in the presence of an arbitrary Poisson point process (PPP)-based heterogeneous interference field. The analysis leverages on a moment-generating-function (MGF) methodology, which only requires the statistics of intended signal and aggregate interference, thus eliminating the need for the exact distribution of SINR. We extend the existing results on interference statistics by deriving a generalized closed-form expression of the interference MGF considering Nakagami-m fading channels with exclusion region. In certain special cases, explicit expressions for the averages of different functions of SINR are found, which also lead to closed-form solutions for the probability distributions of aggregate interference reciprocal and signal-to-interference ratio. We prove that in such cases the effect of total PPP-based interference power on useful transmission is mathematically equivalent to the severe fluctuations from a one-sided Gaussian fading channel. As an application example, the proposed methodology is used together with stochastic geometry theory to characterize the average SINR and rate in heterogeneous cellular networks. The validity of our analytical derivations is confirmed via Monte Carlo simulations for various system settings. We show that with cellular network densification there exists a tradeoff between the average SINR and rate performance.

Adaptive code allocation for interference management on the downlink of DS-CDMA systems

A new technique based on adaptive code-to-user allocation for interference management on the downlink of BPSK based TDD DS-CDMA systems is presented. The principle of the proposed technique is to exploit the dependency of multiple access interference on the instantaneous symbol values of the active users. The objective is to adaptively allocate the available spreading sequences to users on a symbol-by-symbol basis to optimize the decision variables at the downlink receivers. The presented simulations show an overall system BER performance improvement of more than an order of a magnitude with the proposed technique while the adaptation overhead is kept less than 10% of the available bandwidth.