Power Allocation for Goodput Optimization in BICM-OFDM systems I. Stupia†, L. Vandendorpe‡, J. Louveaux‡, F. Giannetti†, V. Lottici†, N.A. D’Andrea† †Department of Information Engineering, University of Pisa, Via G. Caruso, 16 - I-56122 Pisa, Italy ‡Communications and Remote Sensing Lab, Universit´ e Catholique de Louvain, Place du Levant, 2 - B-1348 Louvain La Neuve, Belgium {ivan.stupia, f ilippo.giannetti, vincenzo.lottici, aldo.dandrea }@iet.unipi.it, {luc.vandendorpe, jerome.louveaux}@uclouvain.be Abstract—This paper deals with the power allocation problem for coded multicarrier transmission. Specifically, we focus on a bit interleaved coded modulation (BICM) packet transmission implemented with Orthogonal Frequency Division Multiplexing (OFDM) and in the presence of automatic repeat request (ARQ) protocol. Capitalizing on the binary-input output-symmetric (BIOS) nature of the BICM channel it is provided a simple upper-bound of the rate of information bits received without any error, the so called goodput. Based on this theoretical characterization, we develop a power allocation strategy among the different subcarriers so that the system goodput performance metric is maximized. The effectiveness of the proposed method is numerically testified for BICM-OFDM transmission in the context of the typical WLAN scenario. I. I NTRODUCTION At the high data rates typically required to support mul- timedia services, harsh multipath propagation conditions are typically experienced in both urban outdoor and indoor terres- trial scenarios, thereby making the design of an efficient and reliable transmission scheme a particularly demanding task. A viable answer to this need consists in a cross-layer design approach where a mix of up-to-date efficient techniques for modulation, channel coding and link adaptation are properly combined. In the context of high data rate transmissions over wireless frequency selective channels, one of the most efficient modulation formats is represented by multicarrier (MC) techniques. In the form of orthogonal frequency division multiplexing (OFDM), MC schemes have been embedded in several standards such as Wi-Fi WLAN IEEE 802.11 a/g/n, Wi-Max broadband wireless access IEEE 802.16, and digital audio and video broadcasting (DAB and DVB). In order to fur- ther increase the system robustness against the troubles arising from the wireless propagation channels, an efficient modula- tion scheme has to be combined, however, with an as much as powerful channel coding technique. This is the case of BICM, which was proposed in 1992 by Zehavi as a pragmatic coding scheme for bandwidth-efficient communications [1]. This is based on the insertion of a bit-interleaver between the channel encoder and the modulator in order to increase the diversity order. Later, a theoretical foundation for BICM was given by Caire, Taricco and Biglieri [2]. Different methods to evaluate the performances of BICM systems have been proposed. Most of them are based on the union bound and the expurgation technique proposed in [2]. Recently, in [3], thanks to the binary-input output-symmetric (BIOS) nature of the channel, a simple yet accurate computation of the Pair-Wise Error Probability (PEP) based on the saddlepoint approximation has been provided. In [10] this approach has been extended to the MIMO-BICM systems. The superior code diversity and the design flexibility of BICM have motivated many common wireless applications. For instance IEEE 802.11 WLAN is an application of BICM which is implemented with OFDM. The goal of a cross-layer design is fully achieved on condition that efficient link adaptation schemes be properly employed as well. The idea is to optimize the overall system performance under the constraint of fixed radio resources. To this end, the water-filling policy [11] has gained a considerable interest to power and bit resource allocation across the subcar- riers of a OFDM-based system. In [4], through the extension of the conventional BICM-OFDM Pair-Wise Error Probability (PEP) analysis, a bit and power allocation algorithm has been proposed to improve the total bit-rate. However, in some WLAN-based applications, only error-free packets are kept by the receiver, while the others are retransmitted through an automatic repeat request (ARQ) retransmission mechanism. Therefore, an optimized allocation strategy has necessarily to maximize over the available resources the number of transmitted bits in the error-free packet by unit of time, or goodput for short. Based on the above baseline and differently from the works published so far in the literature, the aim of this paper is to present a power allocation strategy that aims at improving the goodput achievable in a packet-based BICM- OFDM sytems. The theoretical foundation of the allocation algorithms consists in a simplified PEP analysis. After de- lineating in Sect. II the BICM-OFDM model, Sect. III will be devoted to the evaluation of a simple upper bound of the goodput metric. Simulation results will be provided in Sect. IV for a typical WLAN scenario, followed some concluding remarks in Sect. V. Notations: Matrices are in upper case bold while column vectors are in lower case bold with an underscore. (·) T is used to denote the transpose and D(·) is the diagonalization operator which converts an N -dimensional vector into an N × This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the ICC 2008 proceedings. 978-1-4244-2075-9/08/$25.00 ©2008 IEEE