Single-Carrier Communication using Decision-Feedback Equalization for Multiple Antennas

tion III. We assess the multi-user DFE performance for 2 scenarios in section IV. In section V, we illustrate the equivalence between the multi-user DFE and the Generalized DFE as described in [11]. Finally, in section VI, we summarize the conclusions. II. SC-FD-DFE A. Scheme As shown in figure 1 the SC-FD-DFE consists of a scalar feedforward section in the frequency domain and a feedback part of Ldf e taps in the time domain. Matrices are denoted by bold capital letters, vectors have a bold normal font and scalars have a normal font. Frequency domain signals are indicated by a tilde. a represents the modulated data, H is the channel (a vector for a SISO channel, a matrix if multiple antennas are used), n is the noise, r is the received time domain signal, while ˜ r the received signal in the frequency domain. z is the equalized signal just before the slicer, ¯ a are the decisions and f the feedback coefficients.
Single-Carrier Communication using Decision-Feedback Equalization for Multiple Antennas
Jan Tubbax† , Liesbet Van der Perre, St´ ephane Donnay, Marc Engels∗ IMEC - Kapeldreef 75, 3001 Heverlee, Belgium
H a n r + FFT
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I. I NTRODUCTION There is an ongoing quest for the appropriate modulation scheme for broadband wireless. Orthogonal Frequency Division Multiplexing (OFDM) is a popular, standardized technique for such systems: it is used for Wireless LAN [1], [2], Fixed Broadband Access [3], DAB [4], DVB [5], ... Several authors [6] have shown that OFDM can reach full channel capacity at low equalization complexity. In recent years, Single-Carrier with Frequency Domain Equalization (SC-FDE) has received a lot of attention as another technique for broadband wireless communications [7]. SC-FDE allows for a more power efficient transmitter [8], which is a very important aspect for battery operated mobile terminals. Recently, a new SC scheme with Decision Feedback Equalization (DFE) has been introduced by Falconer [9] and Benvenuto [10]. In fact, this scheme has a structure very similar to the MMSE-DFE as described by Cioffi [6]. However, it explicitly assumes the feedforward part in the frequency domain, which forces a low complexity solution. With good coding and transmit optimization MMSE-DFE can reach the channel capacity. For mobile terminals in a WLAN scenario, transmit optimization is often not considered for practical reasons. In that case, the performance of MMSE-DFE is stillroperties make SC-FD-DFE a great scheme for a broadband wireless up-link: its performance comes close maximum capacity, it implies a simple and efficient transmitter, which is a real plus for mobile terminals with limited battery and processing power. Moreover, it puts all the processing complexity at the receiver which is a Base Station where typically more processing power is available. The structure of the paper is as follows. In section II, we briefly introduce Decision-Feedback Equalization. Next, we expand DFE for a multi-user, multi-antenna scenario in sec† Jan ∗ Marc
−Nts for SC-CP and Nsc for SC-TS ; using Nsc = 64 and Nsc Ncp = Nts = 16, this leads to a 20% overhead for SC-CP and 25% for SC-TS. However, SC-TS is able to exploit the full multi-path diversity, while this is not guaranteed for SC-CP [12]. Therefore, SC-TS has a performance advantage. The performance advantage and the overhead disadvantage are both small and approximately cancel each other out. However, the Training Sequence offers some additional advantages: it can be used for equalizer training and synchronization [13]. Moreover, SC-TS suits the DFE structure better since the Training Sequence is a known sequence, which can start up the feedback process. For these reasons, we choose to work with a Training Sequence. In this paper, we exploit the DFE advantage. However, since perfect channel knowledge is assumed,
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z IFFT f* +
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Fig. 1. Single Carrier scheme with FD-DFE.
Tubbax is also a Ph.D Student at the KULeuven. Engels is also a professor at the KULeuven.
Abstract— There is an ongoing discussion in the broadband wireless world between OFDM and Single-Carrier. Single-Carrier allows for more relaxed front-end requirements. OFDM, on the other hand, can yield better performance at low complexity. Recently, a new Single-Carrier scheme with frequency domain equalization and decision-feedback has been proposed for a SISO channel. This Single-Carrier scheme can reach the OFDM performance. We extend this new scheme for multiple antennas and multiple users. We show that this multi-user scheme with decision-feedback has a 3 dB gain over conventional linear Single Carrier multi-user schemes and still maintains a low complexity. We illustrate that our scheme is equivalent with the Generalized Decision-Feedback Equalizer, but has a lower complexity.