Transmit Diversity Vs Beamforming for Multi-User OFDM Systems Daniel V.P. Figueiredo § , Muhammad Imadur Rahman § , Nicola Marchetti § , Frank H.P. Fitzek § , Marcos D. Katz ‡ , Youngkwon Cho ‡ , Ramjee Prasad § § Center for TeleInFrastruktur (CTiF), Aalborg University, Denmark; e-mail: {dvpf,imr,nm,ff,prasad}@kom.aau.dk ‡ 4G Research Lab, Samsung Electronics Co. Ltd., Korea; e-mail: {marcos.katz,youngkn}@samsung.com Abstract— In an Orthogonal Frequency Division Multiplexing (OFDM) system, downlink beamforming can be implemented either before IDFT (frequency domain) or after IDFT (time domain) module in a base station transmitter. We denote the former scheme as Pre- IDFT downlink beamforming, and the latter as Post-IDFT downlink beamforming. In this work, we have compared Pre-IDFT downlink beamforming with Space-Time Block coded and Space-Frequency block coded transmit diversity schemes for 4 × 1 downlink multi-user OFDM (OFDM-TDMA and OFDMA) systems. The study is performed for indoor micro and pico cells and urban macro cells. Regardless of the multiple access scheme, it is found that beamforming always performs better in outdoor environment, where angular spread is lower, thus spatial correlation is higher. Similarly, indoor environment (high angular spread and low spatial correlation) suggests that transmit diversity schemes performs better than beamforming strategies. Keywords— Multi-user OFDM, Beamforming, Transmit Diversity I. I NTRODUCTION Wireless systems which operate at high data rate, providing higher multi-user capabilities, are always impaired by harsh wire- less channel. Multi-antenna techniques can be used to overcome these unwanted situations, e.g. diversity techniques can be used to obtain reliable transmission systems or beamforming can be used to increase the signal strength towards a particular user, thus reducing interference to others. Traditionally spatial diversity is exploited involving multiple antennas in transmitter (Transmit Diversity) and/or receiver (Receive Diversity). Transmit diversity is a lucrative and reasonable choice for downlink (DL), i.e. BS- to-MS, especially for portable receivers where current drain and physical size are important constraints. Space-Time Block Coding (STBC) [1] is an open loop transmit diversity scheme where the diversity is achieved at the receiver without the knowledge of the channel at the transmitter. A comple- mentary to this kind of transmit diversity scheme is beamforming (BF) [2]. When the wireless channels between transmit and receive antennas are correlated to each other, then transmit diversity scheme is not expected to perform well, i.e. if independent fading among the antenna signals cannot be achieved, BF is preferred over transmit diversity. In BF we exploit the fact that the antenna elements are close together so that appreciable coherence between the antenna signals is present. Orthogonal Frequency Division Multiplexing (OFDM) itself does not pose any built-in diversity, thus it is necessary to install some forms of diversity in an OFDM system for the purpose of achieving higher link quality and link availability without using any extra bandwidth. For example, channel coding and interleaving are used in IEEE 802.11a to obtain frequency diversity. Contrary to this, BF techniques can be used to achieve similar performance. In this work, we compare the usability of transmit diversity and BF for DL of OFDM based TDD (Time Division Duplex) cellular systems at indoor (micro and pico cells) and outdoor urban macrocell scenarios. In both approaches the configuration used is MISO (Multiple-Input-Single-Output), since it is supposed that multiple antennas are employed at the BS transmitter and a single antenna at the MS receiver. The target is to define the conditions in which one of the two techniques is preferred to the other for DL of MU- OFDM based. We compare the schemes based on Bit-Error-Rate (BER) performance, on various Angular Spread (AS) values and corresponding channel correlation status. In order to get enough array gain for BF, we should use at least 4 transmit antennas, so for reasonable comparison between the schemes, we have used 4 transmit antennas in all cases. This paper consists of seven main sections. Following a brief overview of the multi-antenna techniques considered in this work, to be found in Section II, the way to employ DL-BF in multi-user OFDM systems is explained in Section III. Section IV explains how transmit diversity and beamforming can be compared in multi-user OFDM systems. Simulation related issues are presented in Section V. Section VI presents the results obtained from simulations, and final conclusions are drawn in Section VII. II. MULTI -ANTENNA TECHNIQUES UNDER CONSIDERATION A. Beamforming In a BF system, the weights to be multiplied with the signals have to be carefully chosen. From antenna theory, an array with P antenna elements has different excitation currents according to the angle of direction of the waves arriving or departing from each element of the array. Considering linear phase progression, the weights have a phase that increases the same amount from one element to the next. Defining as δ this increment, then for Uniform Spaced Linear Arrays (USLA), presumed to be used throughout this paper, the array factor becomes F (θ)= P −1 X p=0 Ape jp(kd sin θ+δ) (1) Usually, BF is treated in literature as a method of increasing gain when receiving signals from a specific direction. Direction of Arrival (DoA) is the angle of the wave arriving to the antenna. However, in this study it is intended to analyse the DL case, placing the beamformer at the transmitter side, i.e. at the BS, since it is more feasible to have multiple antennas at the BS than at the MS. This will bring additional complications if it is assumed that there is complete channel knowledge at the receiver, but not at the transmitter. In order to solve this problem, the DL-BF technique to be employed in this analysis will extract the weights to be used in DL from the weights calculated in uplink (UL). This method can be used in TDD systems without losing performance. However, in Frequency Division Duplex (FDD) systems, as a consequence of the frequency dependent steering array response and uncorrelated fading, the UL weight reuse in DL degrades system’s performance, since the frequency for UL is different from the one used to DL. In this work TDD systems will be considered for analysis. A block diagram showing the implementation of DL-BF in an OFDM transmitter is presented in Fig. 1(a). In order to cancel interference and increase gain from DoA, Minimum Mean Square Error (MMSE) criterion is used. A number of four antennas at the transmitter side was chosen from a trade-off between the fact that