Jaume Ramis

Cross-Layer Design of Adaptive Multirate Wireless Networks Using Truncated HARQ

To provide heterogeneous quality-of-service (QoS) guarantees to applications, most wireless communications standards combine the error-correcting capability of hybrid automatic repeat request (HARQ) protocols at the data link control layer with the adaptation ability of adaptive modulation and coding (AMC) strategies at the physical layer. In this paper, a novel cross-layer multidimensional discrete-time Markov chain (DTMC)-based queuing model is developed to jointly exploit the capabilities of HARQ and AMC. The analytical DTMC-based model, which generalizes and extends previous results on this topic, stems from a comprehensive consideration of packet multimedia traffic sources modeled as discrete-batch Markovian arrival processes, finite queue-length systems, truncated HARQ protocols, AMC strategies, and a wireless channel first-order 2-D Markov model that relies on the amplitude and rate of change of the fading envelope. Based on the stationary state probability distribution of this multidimensional DTMC, closed-form analytical expressions for performance metrics such as throughput, average packet delay, and packet loss rate, which were caused either by buffer overflow or by exceeding the maximum number of allowed retransmissions, are derived. Furthermore, the proposed analytical framework is used to formulate multidimensional and simplified 2-D constrained optimization problems aiming at maximizing the system throughput under prescribed QoS constraints. Computer simulation results are carried out to verify the validity of the proposed analytical model and to quantify the performance gain due to cross-layer optimization and the use of truncated HARQ protocols.

Using Two-Dimensional Markov Models and the Effective-Capacity Approach for Cross-Layer Design in AMC/ARQ-Based Wireless Networks

This paper proposes a novel framework for the cross-layer analysis and design of wireless networks combining adaptive modulation and coding (AMC) at the physical layer with an automatic repeat request (ARQ) protocol at the data-link layer. Most previous works rely on first-order amplitude-based finite-state Markov chains (AFSMCs) to model the physical layer. It is shown that these models present several deficiencies that could compromise the design of higher layer protocols. Thus, a physical-layer first-order 2-D Markov model using both the amplitude and the rate of change of the fading envelope is presented. Based on this multidimensional physical-layer Markov model, the quality-of-service (QoS) performance at the data-link layer is investigated through the use of two different approaches. The first one relies on an analytical framework based on a discrete-time Markov chain (DTMC) that jointly describes the statistical behavior of the arrival process, the queueing system, and the physical layer. The second one is based on the effective-bandwidth and effective-capacity theories. Both the DTMC-based and the effective-bandwidth/capacity-based approaches are analyzed and compared in combination with our proposed physical-layer first-order 2-D Markov model in a cross-layer design aiming to satisfy the required average packet loss probability constraint by maximizing the average throughput of the system. Numerical results show that our proposed framework represents a significant improvement over previous models.

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 QoS-Constrained Optimization of Adaptive Multi-Rate Wireless Systems using Infrastructure-Based Cooperative ARQ

This research work aims at analyzing the effects of using cooperative-ARQ (CARQ) in wireless relay systems employing AMC at the physical layer and an infinitely persistent type-I Hybrid FEC/CARQ protocol at the DLC layer. To that end, a cross-layer multidimensional discrete-time Markov chain (DTMC)-based queuing model is developed to jointly exploit the capabilities of CARQ and AMC. Based on the stationary state probability distribution of the proposed multidimensional DTMC, closed-form analytical expressions for performance metrics such as average throughput, queue length, packet delay and packet loss rate are derived, and the impact of cooperative relaying on these performance parameters is analyzed. Numerical results confirm the validity of the presented AMC/CARQ cross-layer analytical framework and reveal that the proposed CARQ protocol consistently outperforms the classical Type-I Hybrid FEC/ARQ schemes and provides a noticeable improvement when facing correlated fading channel scenarios.

Cross-Layer Optimization of Adaptive Multi-Rate Wireless Networks Using Truncated Chase Combining HARQ

A cross-layer performance analysis of a wireless network using adaptive modulation and coding at the physical layer and a truncated Chase combining hybrid automatic repeat request (HARQ-CC) scheme for error control at the data link layer is developed. Based on a Markov chain queueing model, analytical expressions for performance metrics such as throughput, average packet delay and packet loss rate are derived and then used to formulate a constrained optimization problem to maximize the system throughput under the prescribed Quality-of-Service constraints. Numerical results reveal that HARQ-CC consistently outperforms the classical Type-I Hybrid forward error correction/automatic repeat request schemes.

Performance Analysis of Multiuser Detectors for Uplink Quasi-synchronous MC-CDMA Systems

This paper analyses and compares the average bit error rate (BER) of different multiuser detectors (MUD) in the uplink of a multicarrier code- division multiple access (MC-CDMA) system. In particular, maximum likelihood, zero-forcing, minimum mean-square error and interference cancellation-based multiuser detectors have been analysed for the special case of uncorrelated subcarriers. The derived BER expressions are based upon previous results on diversity combining and also on recent findings on multiple input multiple output (MIMO) architectures. The subcarrier correlation is considered in the context of physical parameters currently under discussion for future wireless systems to give an indication up to what extent the assumption of uncorrelated subcarrier fading is plausible.

Cross-layer modeling of AMC/ARQ-based wireless networks with outdated CSI

State-of-the-art wireless communications standards combine (hybrid) automatic repeat request (HARQ) protocols at the data link control (DLC) layer with adaptive modulation and coding (AMC) strategies at the physical layer. Most previous work on cross-layer modeling of these networks assumes the availability of ideal channel state information (CSI). However, in practical systems the CSI used to perform link adaptation may be outdated. This paper presents a generalized cross-layer multidimensional discrete-time Markov chain (DTMC)-based queuing model that jointly exploits the capabilities of ARQ and AMC, by explicitly accounting for imperfections in the AMC scheme caused by the delay in the CSI feedback channel. Based on the stationary state probability distribution of the proposed multidimensional DTMC, closed-form analytical expressions for performance metrics such as average throughput, queue length, packet delay and packet loss rate are derived, and an analysis of the impact of using outdated CSI on these performance parameters is presented.

A Two-Dimensional Markov Model for Cross-Layer Design in AMC/ARQ-Based Wireless Networks

Nowadays, there are lots of research efforts focusing on cross-layer designs combining adaptive modulation and coding (AMC) with an automatic repeat request (ARQ) protocol. One of the main drawbacks of these research works is that they rely on first-order amplitude-based finite-state Markov chains (AFSMC) to model the wireless fading channel. Furthermore, most of the analytical models used in these works present several deficiencies that could compromise the design of cross-layer protocols. To overcome the aforementioned problems, in this paper we propose a novel cross-layer analytical framework that is based on both the use of a first-order two-dimensional Markov model and a judicious implementation of the AMC threshold searching algorithm used in the transmission mode (TM) selection. This threshold search is designed independently from the channel model and is able to discriminate between useful and useless TMs. To exploit the joint impact on QoS performance measures of both AMC and ARQ we present a formulation of an AMC/ARQ cross-layer design as a constrained optimization problem.

Performance of downlink group-orthogonal multicarrier systems

Group-orthogonal multi-carrier code division multiple access (GO-MC-CDMA) has recently been proposed as a promising technique for the uplink segment of wireless systems. In this paper we propose and analyze a related scheme, group-orthogonal multi-carrier code division multiplexing (GO-MC-CDM), suitable for the downlink segment. The proposed receiver is shown to offer a similar bit error rate (BER) performance as the downlink version of GO-MC-CDMA at a fraction of its computational complexity. An analytical expression for the BER when using maximum likelihood (ML) detection is derived providing valuable insight into the parameters affecting the system performance and providing a basis for its optimization. Simulation results using parameters and channel models aiming at the next generation of wireless systems are provided confirming the analytically derived results.

On the Design of Group Orthogonal MC-CDMA Systems

This paper presents some insights into the design of the recently proposed group-orthogonal multi-carrier code-division multiple-access (GO-MC-CDMA) technique. A new theoretical derivation for the bit error rate (BER) performance when using maximum likelihood (ML) detection in correlated channels is presented and validated. This expression reveals important aspects to be taken into account when partitioning the available subcarriers into groups. It also shows that for large signal-to-noise ratios, the spreading codes do not affect the performance of the system. A design example is given using parameters aiming at the next generation of wireless systems