Felip Riera-Palou

On the design of uplink and downlink group-orthogonal multicarrier wireless systems

Group-orthogonal multicarrier code-division multiple access (GO-MC-CDMA) has been proposed as an attractive multiplexing technique for the uplink segment of wireless systems. More recently, a variant of this scheme has also been proposed for the downlink. This paper presents a unified bit error rate (BER) performance analysis of group-orthogonal wireless systems when using maximum likelihood (ML) multiuser/multisymbol detection covering both link directions. Valuable design rules regarding the number of subcarriers per group and the selection of spreading codes are derived. Simulations results using realistic system parameters and ETSI BRAN channel models are also presented which serve to validate the analytical results.

Cross-Layer Fast Link Adaptation for MIMO-OFDM Based WLANs

This paper considers the use of cross-layer fast link adaptation (FLA) for WLANs employing a MIMO-OFDM physical layer. A packet error rate (PER)-based FLA technique that, without loss of generality, makes use of the exponential effective SNR mapping (EESM) is proposed. Additionally, an FLA scheme relying on bit error rate (BER) metrics is introduced that simplifies the link adaptation procedure without any significant performance degradation. Results show that both PER- and BER-based FLA techniques optimize the data throughput while satisfying prescribed quality of service constraints. Channel estimation errors have also been considered, revealing the importance of good channel estimators in order for FLA strategies to work satisfactorily.

Unified approach to cross-layer scheduling and resource allocation in OFDMA wireless networks

Orthogonal frequency division multiple access (OFDMA) has been selected as the core physical layer access scheme for state-of-the-art and next-generation wireless communications standards. In these systems, scheduling and resource allocation algorithms, jointly assigning transmission data rates, bandwidth and power, become crucial to optimize the resource utilization while providing support to multimedia applications with heterogeneous quality of service (QoS) requirements. In this article, a unified framework for channeland queue-aware QoS-guaranteed cross-layer scheduling and resource allocation algorithms for heterogeneous multiservice OFDMA wireless networks is presented. The framework encompasses different types of traffic, uniform and continuous power allocation, discrete and continuous rate allocation, and protocols with different amounts of channel- and queue-awareness. System parameters and QoS requirements are projected into utility functions and the optimization problem is then formulated as a constrained utility maximization problem. Optimal solutions for this problem are obtained for the uniform power allocation schemes, and novel quasioptimal algorithms are proposed for the adaptive power allocation strategies. Remarkably, these techniques exhibit complexities that are linear in the number of resource units and users. Simulation results demonstrate the validity and merits of the proposed cross-layer unified approach.

A Unified Approach to Dynamic Length Algorithms for Adaptive Linear Equalizers

The number of taps is an important factor which affects nearly all the performance and complexity measures of adaptive equalizers. Recently, dynamic length algorithms for the adaptive finite impulse response (FIR) filters have attracted considerable attention and several methods have been proposed. Nevertheless, the relationship among these techniques is still unclear which makes the performance comparison remaining difficult. In this paper, a unified explanation of these techniques is presented which summarizes all proposed algorithms and provides a platform for their performance comparison. Moreover, a new type of dynamic length algorithms, the varying threshold dynamic length algorithm (VT-DLA) is proposed. Simulation results show that the performance of the new algorithm outperforms that of existing algorithms

A fair MU-MIMO scheme for IEEE 802.11ac

The next generation of wireless local area networks (WLANs), currently standardized within IEEE 802.11ac, will surely adopt multiuser multiple-input multiple-output (MU-MIMO) technology. This paper addresses the design of a multiuser proportional fair scheduling scheme, based on MU-MIMO orthogonal frequency division multiplexing (OFDM), that is able to schedule simultaneous transmissions to various users while providing a high degree of fairness. The proposed scheme is complemented by a fast link adaptation (FLA) strategy that, in light of the available channel state information (CSI), determines the most appropriate transmission parameters for each selected user. Simulation results generated using specifications envisaged for IEEE 802.11ac demonstrate the gains offered by the proposed technique with practical modulation and coding schemes.

Corrections to and Comments on “Throughput and Optimal Threshold for FFR Schemes in OFDMA Cellular Networks”

In a recent paper published in these Transactions, Xu et al. (see ibid., vol. 11, no. 8, pp. 2776-2785, Aug. 2012) proposed an analytical framework to calculate the throughput and optimal threshold for fractional frequency reuse (FFR) schemes in orthogonal frequency division multiple access (OFDMA) cellular networks. This correspondence points out a problem in the original derivation of Xu et al. and how this affects the claimed performance of the maximum signal-to-interference-plus-noise ratio (MSINR) scheduling rule. A fix to this problem is presented in this work, which turns out to extend the applicability of the analysis beyond the case studied by Xu et al. and at the same time shows how the performance of the round robin (RR) scheduling rule can be obtained as a special case of the MSINR scheduler. Monte Carlo simulation results demonstrate the validity and merits of our proposed approach.

Combining fractional frequency reuse with coordinated multipoint transmission in MIMO-OFDMA networks

This paper presents a systematic comparative study involving two fundamental mechanisms envisaged to play a crucial role in 4G networks, namely, fractional frequency reuse (FFR) and coordinated multipoint transmission (CoMP). A simulation framework is developed to assess the performance of the most relevant coordination schemes under different user scheduling policies while also revealing what are the corresponding optimum settings for the FFR component. Moreover, for one particularly interesting CoMP technique, namely, zero-forcing (ZF) precoding, a novel power allocation strategy is proposed that is shown to significantly enhance its performance. Simulation results serve to confirm the benefits of combining both techniques, FFR and CoMP, in comparison to non-coordinated architectures.

Improving STBC Performance in IEEE 802.11n Using Group-Orthogonal Frequency Diversity

This paper proposes the use of group-orthogonal frequency diversity combined with space-time block coding (STBC) to improve the performance of IEEE 802.11n wireless networks (WLANs). The extra diversity is achieved by using code-division multiplexing whereby each symbol is transmitted on more than one subcarrier. At reception, following the STBC decoding, a maximum likelihood-based turbo receiver is used to take full advantage of the available diversity. The performance of the proposed scheme is investigated over the two different operating bandwidths specified in 802.11n, (20 or 40 MHz) and with different number of transmit antennas. Simulation results using the current IEEE 802.11n specifications serve to illustrate the benefits of the proposed scheme under realistic conditions.

Analytical Performance Evaluation of OFDMA-Based Heterogeneous Cellular Networks Using FFR

Two-tier wireless architectures where the macro- cellular network is complemented with a deployment of femto-cells are becoming an attractive solution to increase the global throughput in the context of OFDMA-based 4G/5G networks. This paper presents an analytical framework allowing the performance evaluation of FFR-aided OFDMA-based two-tier HetNets. A worst-case scenario in terms of inter- tier interference is evaluated in which macrocell and femtocell tiers are assumed to be uncoordinated and co-channel deployed. The proposed framework allows the evaluation of the impact produced by both inter- and co-tier interferences on either the macro-user-equipments (MUEs) or the femto-user-equipments (FUEs). Analytical performance results are used to optimise the FFR parameters as a function of, for instance, the resource block scheduling policy used at both the MBSs and the FBSs, the number of MUEs per cell, the density of FBSs per area unit or the degree of isolation provided by wall penetration losses.

Opportunistic multiuser MIMO for OFDM networks

This paper considers the application of opportunistic multiuser multiple-input multiple-output (MU-MIMO) techniques to multicarrier wireless networks based on orthogonal frequency division multiplexing (OFDM). Unlike related work where the design of the MU-MIMO component is jointly performed with the subcarrier allocation among the users (e.g. some form of OFDMA is present), here it is assumed that the selected users will have to share the utilization of all the subcarriers in the system. This scenario would be typical of deployments based on time multiplexing policies (e.g., WLANs) where the proposed MU-MIMO add-on would serve to increase the system capacity. A new algorithm is introduced to select the users taking into account their channel response over all subcarriers and performing a joint frequency-space power allocation. Numerical results demonstrate very significant improvements in terms of sum-rate capacity in comparison to a conventional (opportunistic) TDMA scheme.