Space-Time-Frequency Transmit Diversity in Broadband Wireless OFDM Systems

— We consider space-time-frequency transmit diversity with orthogonal designs in a broadband OFDM system. Based on capacity considerations for Gaussian as well as real world signal constellations, we show that the concatenation of an outer error control code, a bit interleaver and orthogonal spatial transmit diversity provides an attractive MIMO scheme which is simple and fairly robust to channel imperfections such as spatial correlation or keyhole effects. Particularly, we discuss orthogonal transmit diversity versus spatial multiplexing and space-time-frequency codes which have been discussed in literature recently. Furthermore, we propose a simple and pragmatic application of orthogonal designs for space-time-frequency transmit diversity and give design criteria which allow to use a simple combiner at the receiver also in time-varying and severly frequency-selective channels. I. INTRODUCTION A broadband OFDM 4G candidate system offers different sources of diversity: Temporal diversity, frequency diversity and spatial diversity. The optimum solution to exploit all sources of diversity would be a joint design of a big space-time-frequency code which codes across subcarriers, multiple OFDM symbols and antennas. Such a space-time-frequency coding has been proposed e.g. in [1]. However, this approach has high complexity and low flexibility. In this paper we deal with the question wether we loose significantly if simple space-time-frequency techniques are applied instead of theoretically optimum solutions. We have in mind that most wireless user equipment will remain small also in the future since it should be possible to carry it all the time. Therefore, in our considerations the number of antennas is limited to 2 or at most 4. Furthermore, we restrict ourselves to the realistic assumption that the transmitter has no channel state information which would allow beamforming or switching between different MIMO techniques. One of the simplest MIMO techniques is transmit diversity from orthogonal designs [2], [3]. The main advantage is that the full spatial diversity is obtained using a simple combiner at the receiver. In OFDM, orthogonal designs can be applied as space-time or space-frequency block codes [4]. The principle will be reviewed in Section III after a description of the channel and system model in Section II. Transmit diversity is the optimum solution in a MISO system with multiple transmit and a single receive antenna. However, in spatially uncorrelated MIMO systems with multiple transmit and receive antennas, spatial multiplexing, where independent data streams are transmitted simultaneously from multiple antennas, is theoretically optimum. Therefore, in Section IV-A we discuss the degradation …