Time Domain Replica Signal Based Interference Compensation for SP-MIMO/OFDM with Large Delay Spread Channel

Time Domain Replica Signal Based Interference Compensation for SP-MIMO/OFDM with Large Delay Spread Channel

Taichi Sakaue (Graduate School of Engineering, Chiba University, Chiba, Japan), Chang-Jun Ahn (Graduate School of Engineering, Chiba University, Chiba, Japan), Tatsuya Omori (Graduate School of Engineering, Chiba University, Chiba, Japan) and Ken-Ya Hashimoto (Graduate School of Engineering, Chiba University, Chiba, Japan)
Copyright: © 2014 |Pages: 17
DOI: 10.4018/ijdst.2014100101

Abstract

In mobile communications, MIMO-OFDM transmission performance suffers severe degradation caused by the large delay spread channel greater than guard interval (GI). This is because the excess delay results in considerable inter-symbol interference (ISI) between adjacent symbols and inter-carrier-interference (ICI) among subcarrier in the same symbol. In case of scattered pilot (SP), the interference of pilot signals causes the deterioration of channel estimation. To mitigate this problem, in this paper, we propose the interference compensation scheme using the time domain replica signals. We make the time domain replica signals from detected signals and the excess channel impulse responses over GI. After compensation of the time domain replica signals and the received signals, we recalculate the channel state information (CSI) and the CSI is updated. Finally, we carry out the channel compensation with updated CSI for obtaining accurate compensated signals.
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Introduction

Orthogonal frequency division multiplexing (OFDM) is commonly used for broadband wireless transmission like IEEE 802.11a/g because of its ability to combat the multipath channel problems (Cimini, 1985) - (Bingham, 1990). The multipath channel causes the nonlinear interference among each symbol and each subcarrier. In a radio link, the RF signal from the transmitter may be reflected from objects such as hills, building, or vehicles. This gives rise to multiple transmission paths at the receiver. The relative phase of multiple reflected signals can cause constructive or destructive interference at the receiver. Delay spread is the time spread between the arrived of the first and last significant multipath signal seen by the receiver. In a digital system, the delay spread can lead to inter-symbol interference. This is due to the delayed multipath signal overlapping with the following symbol. In order to avoid inter-symbol interference (ISI) and inter-carrier interference (ICI) caused by multipath, a guard interval (GI) is appended to the OFDM symbol. In addition, multiple-input multiple-output (MIMO) system has attracted attention and has been innovated since MIMO system achieves high data rate transmission as 1Gbps without increasing the frequency bandwidth (Chiani 2010) - (Hu 2011). MIMO-OFDM using scattered pilots (SP) is applied to long term evolution (LTE) to identify the channel state information (CSI) (Abeta, 2010 & Comas, 2012). Scattered pilots are arranged between data subcarriers for time and frequency directions at regular interval and interpolated to identify the CSI (Won, 2009)-(Shin, 2009). Since scattered pilot can identify the CSI with small number of pilot, it can achieve high throughput performance. In addition, it is unable to identify the CSI accurately under fast fading channel due to pilot signals are arranged in time direction. However, when the maximum delay spread exceed the length of GI, unacceptable degradation of error performance is caused by ISI and ICI in the same symbol. In addition, in case of using the scattered pilots, we cannot identify the CSI accurately and the signal identification in MIMO system is poorly operated since the pilot signals are interfered by large delay spread channel. Until this time, several schemes have been proposed to mitigate the ISI due to the large delay spread channel (Gejoh, 2007)-(Ida, 2012). Gejoh proposed ISI reduction method by increasing the GI to overcome the above problems (Gejoh, 2007). However, transmission efficiency is deteriorated, and transmit power is increased. Suyama proposed several frequency domain ISI canceling schemes such as a smoothed fast Fourier transform (FFT) window and decision feedback equalizer (DFE) (Suyama, 2003). However the complexity is a serious problem. Ida proposed interference cancellation using the replica signals for high time resolution carrier interferometry (HTRCI)-MIMO/OFDM (Ida, 2012). However, this method did not consider for scattered pilot-MIMO/OFDM. Moreover this scheme has to repeat the cancellation along the symbols. To mitigate these problems, in this paper we propose the interference compensation scheme using the time domain replica signals for SP-MIMO/OFDM. In the proposed scheme, we make the time domain replica signals from detected signals and the excess channel impulse responses over GI. We can compensate the interference by subtracting the time domain replica signals from the detected signals without any equalization. After compensation of time domain replica signals and received signals, we recalculate the CSI and the CSI is updated. Finally, we carry out the channel compensation with updated CSI for obtaining accurate compensated signals (Sakaue, 2013).

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