Iterative Detection and Decoding with PIC Algorithm for MIMO-OFDM Systems ()
1. Introduction
In multipath environments, multiple input multiple output wireless communication system can increase spectral efficiency. Furthermore, to achieve high data rate wireless communications, space-time communication systems need to be in wideband frequency selective channels. Orthogonal frequency division multiple (OFDM) has become a popular technique for transmission of signals over wireless channels. OFDM are robust to frequency selective fading channels for OFDM systems use the discrete Fourier transform (DFT) to modulate the data on orthogonal frequency carriers and effectively divide the wideband channel into a number of narrowband flat channels. One important advantage of the OFDM transmission technique is that the intersymbol interference (ISI) can be removed if the channel delay spread is less than he inserted guard interval.
Clearly, conventional MIMO detection algorithms can be applied for MIMO-OFDM system [1-4]. The MIMO maximum likelihood detection detector is the optimal receiver, but its complexity is best high. A number of sub-optimum receivers of low to moderate complexity have been devised, yet all suffer from rather limited performance. The conventional VBLAST algorithm exhibits the best tradeoff between performance and complexity. However, it involves an intensive computation and hence it may be difficult to implement it for high data rate communications. Linear ZF and MMSE have the best low complexity but the performances are the worse. The QR [3] detection receiver avoids the matrix inversion, but the performance is not good as ZF-VBLAST. The QR detection based on ordering MMSE criterion is proposed [5]. Compared to zero-forcing criterion the detection method based on MMSE criterion needs estimating the signalnoise ratio or variance of noise.
Now considerable research interests have been focused on techniques and algorithms which realize various benefits of turbo principle for MIMO systems [6-8]. The method based on turbo principle, is regarded as an essential technique to furthermore improve system performance with soft iterative detection and decoding through an exchange of information. One of major drawbacks of such turbo-MIMO concepts is that its complexity increases exponentially with the number of transmit/receive antennas, the number of bits per symbol and /or the code constraint length.
However, the research work about iterative detection and decoding based on hard information is very litter. The hard iterative detection method has markedly advantage to soft iterative method in complexity. In this paper, we consider MIMO-OFDM systems with hard iterative detection process. In [9], joint processing of zero-forcing detection and MAP decoding for MIMO-OFDM system. Inspired by [9,10], we introduce a new iterative detection receiver. This approach first utilizes parallel interference cancellation to detect signals of all layers, while the detected signals are regarded as input of channel decoder. For improving the performance of allover system, the output of decoder is regarded as input of PIC detection to do PIC again. By exchanging information between the MIMO detection and decoder, the performance of receiver may greatly be enhanced. Computer simulation result states the performance of proposed detection scheme is better than conventional coded MIMO-OFDM system.
The rest of this paper is organized as follows. In Section 2, we describe MIMO-OFDM systems model. In Section 3, a joint iterative detection and decoding scheme is proposed for MIMO-OFDM systems based on parallel interference cancel (PIC). The simulation results and performance analysis are presented in Section 4 and 5. Concludes follow in Section 6.
2. MIMO-OFDM Systems Model
Before introducing the signal detection, we briefly describe a MIMO-OFDM system. The combination of OFDM and VBLAST can overcome intersymbol interference in frequency selective fading channels. A multicarrier system can be efficiently implemented in discrete time using an inverse FFT (IFFT) to act as a modulator and an FFT to act as a demodulator. The VBLAST architecture is based on a single carrier signal processing algorithm. Therefore, to combine it with OFDM, the VBLAST detection process has to be performed on every subcarrier at the receiver. The detailed system configuration of the VBLAST-OFDM is shown in Figure 1-2.
2.1. MIMO-OFDM Systems
In this section, we consider a coded MIMO-OFDM communication system with 
transmit antennas and 
receive antennas, denoted by (
,
). Figure 1 is diagram of MIMO-OFDM transmitter. At the transmitter the input bit stream is de-multiplexed and coded to generate 
symbol streams. The encoded data stream is then interleaved and launched into the IFFT modulators and added cyclic prefix (CP). Finally, the OFDM signals are transmitted over every transmit antenna.
Figure 2 shows the block diagram of a VBLASTOFDM receiver. Each receiver antenna receivers signals sent from all transmit antennas. After the cyclic prefix is removed, each received signal passes through a FFT block for demodulation.