VLSI Design and Investigation of an Area Efficient and Low Power MOD-R2MDC FFT for MOMO-OFDM

Abstract

INTRODUCTIONMulti input multi output-orthogonal frequency division multiplexing (MIMO-OFDM) is an efficient solution for transmitting and receiving the data over a long distance. The sub-carrier frequency has been chosen in our proposed MIMO-OFDM transceivers so that cross-talk between the subchannels are eliminated, hence, the inter-carrier guard bands are not required [1]. This greatly simplifies the design of both the transmitter and the receiver; unlike conventional frequency division multiplexing, a separate filter for each sub-channel is not required [2]. The orthogonally allows for the efficient modulator and demodulator implementation using the fast Fourier transform (FFT) algorithm [3]. OFDM transceiver is popular for wideband communications today by way of lowcost MIMO-OFDM in wireless telecommunication system. It requires very accurate frequency synchronization between the receiver and they have reduced the complexity [4]. In the transmitter; with frequency deviation, the sub-carriers shall no longer be orthogonal, causing inter-symbol interference [5]. The 5/6 coding rate would be not effective for error correcting by a viterbi decoder [6]. This paper describes the very large scale integration (VLSI) implementation of the proposed modified radix-2 multipath delay commutation (MOD-R2MDC) for MIMO-OFDM systems, that is, MOD-R2MDC pipeline FFT based MIMO-OFDM system. The R2 algorithm with multi delay commutation architecture is to support 4 channel 8, 16, 32, 64, 128, 512, 1024 and 2048 point FFT operations [7, 8]. We compare this proposed architecture with existing 8 point R2, R4 FFT and existing R2MDC FFT and also give the design and implementation results of the proposed MOD-R2MDC FFT processor. OVERVIEW OF MIMO OFDMThe general transceiver structure of MIMO-OFDM is presented in Figure 1. The system consists of N transmitter antennas and M receiver antennas. According to [9] and [10], the cyclic prefix is assumed to be a longer than the channel delay spread. The OFDM signal for each antenna is obtained by using IFFT and can be detected by FFT. There are two methods widely used for transmitting MIMO data. If the channel has a negligible error rate, we can send several data simultaneously over multiple antennas. This is known as spatial multiplexing, which utilizes the spectrum very efficiently.In contrast, if the environment has high error rate, we transmit the same data over multiple antennas. This is called as space-time coding. The purpose of this approach is to increase the diversity of MIMO to combat signal fading. The essential purpose of an MIMO system is to determine, which antenna is corresponding to which data on the receiver side. As shown in Figure 1, R×1 receives data from all the transmitter antennas, T×1, T×2, T×3 and T×4. Thus, we must have a special decoding algorithm to identify which antenna has transmitted which data to R×1. N×M MIMO-OFDM: N indicates the number of transmitter antennas and M indicates the number of receiver antennas, respectively. For example, 4×4 MIMO-OFDM has four...