Combined Phase Compensation and Power Allocation Scheme for OFDM Systems Wladimir Bocquet France Telecom R&D Tokyo 3-1-13 Shinjuku, 160-0022 Tokyo, Japan Email: bocquet@francetelecom.co.jp Kazunori Hayashi and Hideaki Sakai Graduate School of Informatics, Kyoto University Yoshida-Honmachi, 606-8501 Kyoto, Japan Email: {kazunori, hsakai}@i.kyoto-u.ac.jp Abstract— This paper proposes a novel transmit power allocation combined with the phase compensation for OFDM system. This technique consists of adapting the power allocation and the phase compensation in the frequency domain depending on the channel variations. Optimization process is based on the optimality of the global Bit Error Rate (BER). Simulation results show significant performance gains can be obtained by the proposed scheme regardless of baseband modulation schemes. Keywords- OFDM, phase compensation, Lagrangian method, global BER optimization. I. I NTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) has received considerable interest over the last decade for its advantages in high-bit rate transmissions over frequency slective fading channels. In OFDM systems, the input high- rate data stream is divided into many low-rate streams [1], so called subcarriers, that are transmitted in parallel. Robustness to multi-path delay is obtained when appropriate guard interval is inserted in the transmitted frame. In frequency selective fading environment, fading conditions strongly affect the channel gains of each subcarrier. In this paper, we propose to combine the phase compensation at the transmitting part and the adaptation of the transmit power in the frequency domain in terms of channel condition of each load subcarrier. The proposed method consists of grouping a certain number of subcarriers and local power allocation in each subcarrier group. In addition, the power allocation value is combined with a phase, which compensates the channel selectivity in the frequency domain. The rest of the paper is organized as follows. In Section II we review the system model. In Section III, we introduce the estimate value of BER for channel encoded sequence and we describe in detail the proposed power adaptation scheme in the frequency domain. In addition, we highlight the combination of the phase compensation with the power adaption. Section IV gives the experimental results over QPSK and QAM modulations. Finally, conclusions are drawn in Section V. II. SYSTEM DESCRIPTION The principle of OFDM transmission scheme is to reduce bit rate of each sub-carrier [1] and also to provide high bit rate transmission by using a number of those low bit rate sub- carriers. Frequency bandwidth is divided into small ranges and each of them is handled by these low rate sub-carriers. The subcarriers are orthogonal to each other. To obtain this prop- erty, the subcarrier frequencies must be spaced by a multiple of the inverse of symbol duration. Multi-carrier modulation system can provide immunity against frequency selective fad- ing because each carrier goes through non-frequency selective fading. However, the channel must be estimated and corrected for each sub-carrier. Figure 1 shows the transceiver of the conventioanl transmis- Fig. 1. Conventional transceiver with phase compensation sion with phase compensation at the transmitting part. A. OFDM transmitter The High-speed binary data are first encoded (convolutional coding) and modulated (QPSK, 16-QAM or 64-QAM). Then data are converted to parallel low speed modulated data streams and fed to several subcarrier channels. The modulated signals are frequency-division multiplexed by an N-point in- verse discrete Fourier transform (IDFT). The resulting OFDM signal is then converted into an analog signal by a digital- analog (D/A) converter, up-converted (U/C) to the RF band and transmitted. For reliable detection, it is typically necessary that the receiver knows the wireless communication channel and keeps track of phase and amplitude variations. To enable the estimation of the wireless communication channel, the transmitter occasionally sends known training symbols. For in- stance, a preamble, which includes channel training sequences,