arXiv subjects

Mansoor Shafi

Publications and source records attributed to Mansoor Shafi.

28 records · Page 2Linked to original sources

arXiv2015

Coordinated Two-Tier Heterogeneous Cellular Networks with Leakage Based Beamforming

In this paper we demonstrate the rate gains achieved by two-tier heterogeneous cellular networks (HetNets) with varying degrees of coordination between macrocell and microcell base stations (BSs). We show that without the presence of coordination, network densification does not provide any gain in the sum rate and rapidly decreases the mean per-user signal-to-interference-plus-noise-ratio (SINR). Our results show that coordination reduces the rate of SINR decay with increasing numbers of microcell BSs in the system. Validity of the analytically approximated mean per-user SINR over a wide range of signal-to-noise-ratio (SNR) is demonstrated via comparison with the simulated results.

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arXiv2015

On the Impact of Antenna Topologies for Massive MIMO Systems

Approximate expressions for the spatial correlation of cylindrical and uniform rectangular arrays (URA) are derived using measured distributions of angles of departure (AOD) for both the azimuth and zenith domains. We examine massive multiple-input-multiple-output (MIMO) convergence properties of the correlated channels by considering a number of convergence metrics. The per-user matched filter (MF) signal-to-interference-plus-noise ratio (SINR) performance and convergence rate, to respective limiting values, of the two antenna topologies is also explored.

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arXiv2015

Deployment Issues for Massive MIMO Systems

In this paper we examine a number of deployment issues which arise from practical considerations in massive multiple-input-multiple-output (MIMO) systems. We show both spatial correlation and line-of-sight (LOS) introduce an interference component to the system which causes non-orthogonality between user channels. Distributing the antennas into multiple clusters is shown to reduce spatial correlation and improve performance. Furthermore, due to its ability to minimize interference, zero forcing (ZF) precoding performs well in massive MIMO systems compared to matched filter (MF) precoding which suffers large penalties. However, the noise component in the ZF signal-to-noise-ratio (SNR) increases significantly in the case of imperfect transmit channel state information (CSI).

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arXiv2015

On the Convergence and Performance of MF Precoding in Distributed Massive MU-MIMO Systems

In this paper, we analyze both the rate of convergence and the performance of a matched-filter (MF) precoder in a massive multi-user (MU) multiple-input-multiple-output (MIMO) system, with the aim of determining the impact of distributing the transmit antennas into multiple clusters. We consider cases of transmit spatial correlation, unequal link gains and imperfect channel state information (CSI). Furthermore, we derive a MF signal-to-interference-plus-noise-ratio (SINR) limit as both the number of transmit antennas and the number of users tend to infinity. In our results, we show that both the rate of convergence and performance is strongly dependent on spatial correlation. In the presence of spatial correlation, distributing the antennas into multiple clusters renders significant gains over a co-located antenna array scenario. In uncorrelated scenarios, a co-located antenna cluster has a marginally better mean per-user SINR performance due to its superior single-user signal-to-noise-ratio (SNR) regime, i.e., when a user is close to the base station (BS), the links between the user and all transmit antennas becomes strong.

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arXiv2015

On the Convergence of Massive MIMO Systems

In this paper we examine convergence properties of massive MIMO systems with the aim of determining the number of antennas required for massive MIMO gains. We consider three characteristics of a channel matrix and study their asymptotic behaviour. Furthermore, we derive ZF SNR and MF SINR for a scenario of unequal receive powers. In our results we include the effects of spatial correlation. We show that the rate of convergence of channel metrics is much slower than that of the ZF/MF precoder properties.

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arXiv2012

The Effects of Limited Channel Knowledge on Cognitive Radio System Capacity

We examine the impact of limited channel knowledge on the secondary user (SU) in a cognitive radio system. Under a minimum signal-to-interference-and-noise ratio (SINR) constraint for the primary user (PU) receiver, we determine the SU capacity under five channel knowledge scenarios. We derive analytical expressions for the capacity cumulative distribution functions and the probability of SU blocking as a function of allowable interference. We show that imperfect knowledge of the PU-PU channel gain by the SU-Tx often prohibits SU transmission or necessitates a high interference level at the PU. We also show that errored knowledge of the PU-PU channel is more beneficial than statistical channel knowledge and imperfect knowledge of the SU-Tx to PU-Rx link has a limited impact on SU capacity.

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arXiv2009

On the Statistics of Cognitive Radio Capacity in Shadowing and Fast Fading Environments

In this paper we consider the capacity of the cognitive radio channel in a fading environment under a "low interference regime". This capacity depends critically on a power loss parameter, $α$, which governs how much transmit power the cognitive radio dedicates to relaying the primary message. We derive a simple, accurate approximation to $α$ which gives considerable insight into system capacity. We also investigate the effects of system parameters and propagation environment on $α$ and the cognitive radio capacity. In all cases, the use of the approximation is shown to be extremely accurate. Finally, we derive the probability that the "low interference regime" holds and demonstrate that this is the dominant case, especially in practical cognitive radio deployment scenarios.

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arXiv2009

Performance of Cognitive Radio Systems with Imperfect Radio Environment Map Information

In this paper we describe the effect of imperfections in the radio environment map (REM) information on the performance of cognitive radio (CR) systems. Via simulations we explore the relationship between the required precision of the REM and various channel/system properties. For example, the degree of spatial correlation in the shadow fading is a key factor as is the interference constraint employed by the primary user. Based on the CR interferers obtained from the simulations, we characterize the temporal behavior of such systems by computing the level crossing rates (LCRs) of the cumulative interference represented by these CRs. This evaluates the effect of short term fluctuations above acceptable interference levels due to the fast fading. We derive analytical formulae for the LCRs in Rayleigh and Rician fast fading conditions. The analytical results are verified by Monte Carlo simulations.

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arXiv2009

Interference and Deployment Issues for Cognitive Radio Systems in Shadowing Environments

In this paper we describe a model for calculating the aggregate interference encountered by primary receivers in the presence of randomly placed cognitive radios (CRs). We show that incorporating the impact of distance attenuation and lognormal fading on each constituent interferer in the aggregate, leads to a composite interference that cannot be satisfactorily modeled by a lognormal. Using the interference statistics we determine a number of key parameters needed for the deployment of CRs. Examples of these are the exclusion zone radius, needed to protect the primary receiver under different types of fading environments and acceptable interference levels, and the numbers of CRs that can be deployed. We further show that if the CRs have apriori knowledge of the radio environment map (REM), then a much larger number of CRs can be deployed especially in a high density environment. Given REM information, we also look at the CR numbers achieved by two different types of techniques to process the scheduling information.

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arXiv2008

Channel Capacity Limits of Cognitive Radio in Asymmetric Fading Environments

Cognitive radio technology is an innovative radio design concept which aims to increase spectrum utilization by exploiting unused spectrum in dynamically changing environments. By extending previous results, we investigate the capacity gains achievable with this dynamic spectrum approach in asymmetric fading channels. More specifically, we allow the secondary-to-primary and secondary-to-secondary user channels to undergo Rayleigh or Rician fading, with arbitrary link power. In order to compute the capacity, we derive the distributions of ratios of Rayleigh and Rician variables. Compared to the symmetric fading scenario, our results indicate several interesting features of the capacity behaviour under both average and peak received power constraints. Finally, the impact of multiple primary users on the capacity under asymmetric fading has also been studied.

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