Space-Time-Frequency Spreading and Coding for Multi-User MIMO-OFDM Systems Haysam Dahman, Y. Shayan, and X. Wang Department of Electrical Engineering, Concordia University, Montreal, Quebec Email: {h dahman,yshayan,xfwang}@ece.concordia.ca Abstract—In this paper, we propose a multiple-input-multiple- output, orthogonal frequency division multiplexing, code-division multiple-access (MIMO OFDM-CDMA) scheme. The main ob- jective is to provide extra flexibility in user multiplexing, and offer better diversity/multi-user-multiplexing tradeoff. This is done by spreading on all the signal domains; i.e, space-time- frequency spreading is employed to transmit users’ signals. Our study and simulation results show that MIMO OFDM-CDMA is capable of flexible data rates and easy user multiplexing, while simultaneously suppressing the multi-user interfering signals. In addition, our scheme achieve a diversity gain significantly larger than that of the conventional 2-D CDMA OFDM scheme. I. I NTRODUCTION Modern broadband wireless systems must support multime- dia services of a wide range of data rates with reasonable complexity, flexible multi-rate adaptation, and efficient multi- user multiplexing and detection. Channel fading is one of the major issues in wireless communications that degrades the performance, if not appropriately addressed [1]. An ef- ficient technique of combating fading effects is the use of OFDM-CDMA [2]. Various OFDM-CDMA schemes have been proposed and can be mainly categorized into two groups according to code spreading direction [3], [4], [2], [5]. One is to spread the original data stream in the frequency domain; and the other is to spread in the time domain. The key issue in designing an efficient system, is to combine the benefits of both spreading in time and frequency domains to develop a scheme that has the potential of maximizing achievable diversity in a multiple-rate, multiple-access envi- ronment. In [6], it has been proposed a novel joint time- frequency 2-dimensional (2D) spreading method for OFDM- CDMA systems, which can offer not only time diversity but also frequency diversity at the receiver efficiently. Each user will be allocated one orthogonal code and spread its infor- mation data over the frequency and time domain uniformly. In this work, it was not mentioned how this approach will perform in a MIMO environment, specially in a downlink transmission. On the other hand, in [7], it was proposed a technique, called space-time spreading (STS), that improves the downlink performance, however they do not consider the multi-user interference problem at all. It was assumed that orthogonality between users can somehow be achieved, but as we will show in this paper, this is a condition that is not trivially realized in general, as it was not mentioned what is the maximum number of users that can be accommodated by this spreading technique. Also, in [8], multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using STS was proposed. This scheme shows good BER performance with small number of users and it was not clear how it will perform with larger MUI. Recently, in [9], they adopted [8] scheme, it has shown better results for larger number of users, but both transmitter and receiver design were complicated. In this paper, we propose a MIMO OFDM-CDMA system using Space, Time, and Frequency (STF) spreading. The main goal is to achieve higher diversity in Space, Time, and Frequency for improved performance, with reasonable com- plexity. In addition, the system allows flexible data rates and easy user multiplexing which are required for next generation wireless communications systems. An important advantage of using STF-domain spreading in MIMO OFDM-CDMA is that the maximum number of users supported is determined by the product of the S-domain spreading factor, T-domain spread- ing factor and the F-domain spreading factor. Therefore, the MIMO OFDM-CDMA system using STF-domain spreading is capable of supporting a significantly higher number of users than schemes using solely T-domain spreading. Furthermore, spreading on all the signal domains provides extra flexibility in user multiplexing, and offers better diversity/multi-user- multiplexing tradeoff. The performance of MIMO OFDM- CDMA scheme using STF-domain spreading is investigated with the Matched Filter detector [10], [11]. It is shown that larger diversity gains can be achieved for a given number of users compared to other schemes. II. SYSTEM MODEL In this section, joint space-time-frequency spreading is proposed for the downlink of a multi-access system which employs multiple antennas at both the transmitter and the receiver. A. MIMO-OFDM Channel Model Consider a wireless OFDM link with N f subcarriers or tones. The number of transmit and receive antennas are N t and N r respectively. We assume that the channel has L ′ taps and the frequency-domain channel matrix of the q-th tone is related to the channel impulse response as [12] 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 4537 Authorized licensed use limited to: CONCORDIA UNIVERSITY LIBRARIES. Downloaded on February 15,2010 at 11:05:54 EST from IEEE Xplore. Restrictions apply.