MMSE DFE for MIMO DFT-spread OFDMA

In this work, we propose a Decision feedback equalizer (DFE) for Multiple Input Multiple Output (MIMO) DFT-spread OFDMA. This DFE can handle Multiple users, multiple antennas at the transmitter and receiver and the inter- symbol interference in the received symbols of the DFT-spread OFDMA. The conventional method used in DFT-spread OFDMA is linear Frequency domain equalization (FDE). The DFE, with a feed forward (FF) filter operating in frequency domain and a feed back (FB) filter operating in the time domain can perform better than FDE, in channels with severe ISI. The block-wise circular convolution of the channel impulse response makes it difficult to initialize the DFE. We use an LE to get a temporary set of decisions and use them in initializing the DFE. I. INTRODUCTION DFT spread-Orthogonal Frequency Division Multiple Ac- cess(DFT spread OFDMA)/Single carrier Frequency Domain Multiple Access (SC-FDMA) has drawn great attention as an attractive alternative to Orthogonal Frequency Division Mul- tiple Access (OFDMA) , especially in the uplink communica- tions where lower PAPR greatly benefits the mobile terminal in terms of transmit power efficiency. An added advantage of DFT spread-OFDMA is that coding, while desirable, is not necessary for combating frequency selectivity, as it is in nonadaptive OFDM. It is currently a working assumption for uplink multiple access scheme in 3GPP Long Term Evolution (LTE) and it is proposed for the IEEE 802.16m Wireless MAN standard. The performance of conventional FDE is not enough for channels with severe ISI. An obvious alternative is the DFE, which makes use of previous decisions in attempting to estimate the current symbol with a symbol-by-symbol detector. Any tailing ISI caused by a previous symbol is reconstructed and then subtracted. A DFE for SISO single carrier modulation schemes is proposed by Benvenuto et al (4), where the initialization of DFE is achieved by a PN extension. In this paper, we propose a DFE for MIMO DFT-spread OFDMA, where the data from a user occupies a localized/distributed subset of subcarriers in the frequency band. We need to operate on this block which, in time domain, is circularly convolved with the channel. The basic MIMO DFE structure is formulated in Naofal et al (1). The multiple inputs can be transmitted from multiple users, where each is equipped with a single antenna or a single user (e.g., a base station) equipped with multiple antennas or the combinations of both. (1) deals with the equalization where the channel convolution is linear. The MIMO DFE operates to remove both the intersymbol and inter-antenna interference. The Widely-linear MIMO DFE is discussed in Mattera et al (2). In our work, we reformulate the DFE for the DFT spread- OFDMA scenario, where the channel convolution is circular. The major difficulty here, is that the ISI in the initial symbols are from the last symbols of the data block, and they are not detected yet. To overcome this, A Linear equaliser is first run and then use the decisions from it, to initialize the DFE. The Feed forward section is operating in the frequency domain, which reduces the complexity by avoiding the circular convolution. A Frequency domain block iterative DFE for single carrier modulation, which has a similar performance to the time- domain DFE is proppsed in Benvenuto et al (3).