David Morales-Jiménez

Spectral Efficiency of Dynamic Coordinated Beamforming: A Stochastic Geometry Approach

This paper characterizes the performance of coordinated beamforming with dynamic clustering. A downlink model based on stochastic geometry is put forth to analyze the performance of such a base station (BS) coordination strategy. Analytical expressions for the complementary cumulative distribution function (CCDF) of the instantaneous signal-to-interference ratio (SIR) are derived in terms of relevant system parameters, chiefly the number of BSs forming the coordination clusters, the number of antennas per BS, and the pathloss exponent. Utilizing this CCDF, with pilot overheads further incorporated into the analysis, we formulate the optimization of the BS coordination clusters for a given fading coherence. Our results indicate that: 1) coordinated beamforming is most beneficial to users that are in the outer part of their cells yet in the inner part of their coordination cluster and that 2) the optimal cluster cardinality for the typical user is small and it scales with the fading coherence. Simulation results verify the exactness of the SIR distributions derived for stochastic geometries, which are further compared with the corresponding distributions for deterministic grid networks.

Physical Layer Performance of Long Term Evolution Cellular Technology

Despite commercial 3G networks are starting to be fully operational and high speed data packet access (HSDPA) is on its way to be deployed, operators and manufacturers are already in a race towards 4G technologies. The road to 4G has a mandatory milestone in long term evolution (LTE) as it is a promising technology which will allow backwards compatibility besides a higher performance. Thus operators, manufacturers and research institutes are cooperating in the definition of LTE specifications. This paper presents a performance evaluation of LTE downlink physical layer according to the latest 3GPP specifications. Particularly, the main features at the LTE physical layer (like spatial multiplexing or adaptive modulation and coding) are described and analyzed.

Outage probability analysis for η-μ fading channels

In this Letter we derive exact closed-form expressions for the outage probability (OP) in η-μ fading channels. First, a general expression in terms of the confluent Lauricella function is derived for arbitrary values of μ. Next, we restrict the analysis to physical η-μ channel models, i.e. to integer values of 2μ, and obtain exact closed-form expressions for the OP in terms of Marcum Q, Bessel and elementary functions. The results in this Letter are applicable to the OP analysis of maximal ratio combining (MRC) over i.i.d. η-μ or Hoyt fading channels.

On the Bivariate Nakagami-m Cumulative Distribution Function: Closed-Form Expression and Applications

In this paper, we derive exact closed-form expressions for the bivariate Nakagami-m cumulative distribution function (CDF) with positive integer fading severity index m in terms of a class of hypergeometric functions. Particularly, we show that the bivariate Nakagami-m CDF can be expressed as a finite sum of elementary functions and bivariate confluent hypergeometric Φ3 functions. Direct applications which arise from the proposed closed-form expression are the outage probability (OP) analysis of a dual-branch selection combiner in correlated Nakagami-m fading, or the calculation of the level crossing rate (LCR) and average fade duration (AFD) of a sampled Nakagami-m fading envelope.

Outage probability analysis for Nakagami-q (Hoyt) fading channels under rayleigh interference

Exact closed-form expressions are obtained for the outage probability of Nakagami-q (Hoyt) fading channels under co-channel interference (CCI). The scenario considered in this work assumes the joint presence of background white Gaussian noise and independent Rayleigh interferers with arbitrary powers.

Outage Probability Analysis for MRC in η-μ Fading Channels with Co-Channel Interference

Exact closed-form expressions are obtained for the outage probability of maximal ratio combining in η-μ fading channels with antenna correlation and co-channel interference. The scenario considered in this work assumes the joint presence of background white Gaussian noise and independent Rayleigh-faded interferers with arbitrary powers. Outage probability results are obtained through an appropriate generalization of the moment-generating function of the η-μ fading distribution, for which new closed-form expressions are provided.

Base station cooperation with dynamic clustering in super-dense cloud-RAN

In this paper, a downlink cellular model based on stochastic geometry is put forth in order to analyze the performance of base station (BS) cooperation with dynamic clustering in highly dense BS deployments. Utilizing the proposed model, a closed-form expression for the complementary cumulative distribution function of the instantaneous signal-to-interference ratio (SIR) is derived in terms of relevant system parameters, chiefly the number of cooperating base stations and the path-loss exponent. From the derived SIR distribution, some of the benefits of cooperation can be characterized. Through simulation, the derived SIR distribution is verified and further compared with the corresponding distribution for a deterministic grid model.

Power Allocation Schemes for Multicell Massive MIMO Systems

This paper investigates the sum-rate gains brought by power allocation strategies in multicell massive multiple-input- multiple-output systems, assuming time-division duplex transmission. For both uplink and downlink, we derive tractable expressions for the achievable rate with zero-forcing receivers and precoders, respectively. To avoid high-complexity joint optimization across the network, we propose a scheduling mechanism for power allocation, where, in a single time slot, only cells that do not interfere with each other adjust their transmit powers. Based on this, corresponding transmit power allocation strategies are derived, aimed at maximizing the sum rate per cell. These schemes are shown to bring considerable gains over equal power allocation for practical antenna configurations (e.g., up to a few hundred). However, with fixed number of users N, these gains diminish as M → ∞, and equal power allocation becomes optimal. A different conclusion is drawn for the case where both M and N grow large together, in which case improved rates are achieved as M grows with fixed M/N ratio, and the relative gains over the equal power allocation diminish as M/N grows. Moreover, we also provide applicable values of M/N under an acceptable power allocation gain threshold, which can be used to determine when the proposed power allocation schemes yield appreciable gains and when they do not. From the network point of view, the proposed scheduling approach can achieve almost the same performance as the joint power allocation after one scheduling round, with much reduced complexity.

System-Level Performance of Interference Alignment

Capitalizing on the analytical potency of stochastic geometry and on some new ideas to model intercell interference, this paper presents analytical expressions that enable quantifying the spectral efficiency of interference alignment (IA) in cellular networks without the need for simulation. From these expressions, the benefits of IA are characterized. Even under favorable assumptions, IA is found to be beneficial only in very specific and relatively infrequent network situations, and a blanket utilization of IA is found to be altogether detrimental. Applied only in the appropriate situations, IA does bring about benefits that are significant for the users involved but relatively small in terms of average spectral efficiency for the entire system.

Connections Between the Generalized Marcum

This paper presents a new connection between the generalized Marcum-Q function and the confluent hypergeometric function of two variables, Φ3. This result is then applied to the closed-form characterization of the bivariate Nakagami-m distribution and of the distribution of the minimum eigenvalue of correlated noncentral Wishart matrices, both important in communication theory. New expressions for the corresponding cumulative distributions are obtained and a number of communication-theoretic problems involving them are pointed out.