Adaptive interference suppression in multiuser wireless OFDM systems using antenna arrays

Abstract

Abstract-This paper considers the problem of mitigating fading and interference in wireless orthogonal frequency division multiplexing (OFDM) multiple access communication systems. Applications include cellular mobile radio, wireless local loop, and wireless local area networks. The effect of interchannel interference (ICI) arising from time-selective fading and frequency offsets and co-channel interference (CCI) is analyzed. A loop-timing method that enables a synchronous uplink between multiple mobile transceivers and a base-station is described. Adaptive antenna arrays are utilized at the base for uplink reception, and optimum array combining based on the maximum SINR criterion is used for each subchannel over slowly timevarying channels. For operation over fast time-varying channels, a novel two-stage adaptive array architecture that incorporates combined spatial diversity and constraint-based beamforming is presented. While ICI alone is most effectively overcome by spatial diversity, combined beamforming and diversity are most effective to combat CCI in the presence of fading. The overall method is suitable for real-time implementation and can be used in conjunction with traditional coding schemes to increase the link-margin.Index Terms-Adaptive arrays, interference suppression, OFDM.

Low-Complexity Orthogonal Spectral Signal Construction for Generalized OFDMA Uplink With Frequency Synchronization Errors

“…The precision requirement of time synchronization can be relaxed by designing the length of CP to accommodate both maximum channel delay spread and maximum time offset among all users [30]. By this design, the time offsets between users and UR only introduce circular shifts of signals within the OFDMA block.…”

A Pre-FFT OFDM Adaptive Array Antenna with Eigenvector Combining

“…Although a post-fast Fourier transform (FFT) subcarrier-by-subcarrier combining OFDM adaptive array antenna is optimum in terms of maximizing signal-to-interference-and-noise power ratio (SINR), it requires the increased number of FFT processors and heavy computations which increase with the number of antennas and subcarriers, and furthermore it requires quite long training symbols [5]. Related to the post-FFT configuration, a direction of arrival (DoA)-based technique is proposed for co-channel interference suppression in [6], and a minimum mean square error (MMSE)-based technique is proposed for space diversity in [7]. On the other hand, a pre-FFT type OFDM adaptive array antenna, which requires only one FFT processor, can drastically reduce the computational complexity by tolerating some performance degradation [8], [9].…”

Deterministic Multiuser Carrier-Frequency Offset Estimation for Interleaved OFDMA Uplink

“…A common way to compensate for the time offsets among users is to select the length of the CP to accommodate both the maximum channel delay spread and the maximum time offset [23]. This is shown in Fig.…”

“…In the second method, we let and extend the length of CP to , where is a positive integer. The first samples are the original CP and are used to accommodate both channel delay spreads and timing offsets among multiple users [23]. The following samples are not contaminated by IBI from the previous OFDMA block and used by the structurebased algorithm with the remaining samples for estimation, such that .…”