In this paper, the effect of transmitter diversity on asynchronous
co-channel interference suppression and equalization is studied in
multi-user multiple input multiple output (MIMO) orthogonal
frequency division multiplexing (OFDM) systems. In an asynchronous
mode of data transmission, independent decoding without user
cooperation is a more appropriate approach. Thus, various
suboptimal transmitter diversity techniques such as space-time
block coding and beamforming are employed, and their performances
are evaluated. First, we develop space-time minimum mean squared
error (MMSE) receivers for both space-time block coded OFDM and
beamforming based MIMO OFDM. The average signal-to-interference
and noise ratio (SINR) resulting from MMSE combining indicates
that the bit-error rate (BER) performance of space-time block
coded (STBC) multi-user OFDM is highly dependent on the amount of
timing offset. Overall, simulation results demonstrate that
closed-loop MIMO systems need to be employed to obtain substantial
diversity gains in asynchronous multi-user OFDM channels with
random timing offset. Moreover, receiver diversity is an important
factor to the performance of a large system.
KEYWORDS: Orthogonal frequency division multiplexing, Receivers, Antennas, Multiplexing, Signal to noise ratio, Modulation, Telecommunications, Signal detection, Demodulation, Convolution
This paper proposes an iterative multi-user detection scheme for coded asynchronous orthogonal frequency division multiplexing (OFDM) systems. Although a spatially multiplexed multi-user communication system can achieve high spectral efficiency, multiple access interference deteriorates the quality of signal detection. A linear minimum mean squared error (MMSE) based multi-user receiver for asynchronous OFDM channels has been proposed, and hard interference cancellation schemes were developed for an uncoded system. In this paper, we develop a low-complexity iterative soft interference cancellation technique exploiting the circular convolution property of cyclic prefix based OFDM systems. The proposed algorithm mitigates the residual interference and improves the performance of a channel decoder in a coded system through iterative demodulation and decoding. Simulation results demonstrate that spatial multiplexing of two users with two receiving antennas can outperform a system serving a single user with one receiving antenna by employing the proposed turbo receiver.
KEYWORDS: Orthogonal frequency division multiplexing, Signal to noise ratio, Antennas, Error analysis, Data modeling, Algorithm development, Computer simulations, Receivers, Computing systems, Frequency division multiplexing
In this paper, a subspace-based semi-blind channel identification
scheme is developed for a zero padded-orthogonal frequency division multiplexing (ZP-OFDM) system with asynchronous interferers. In our model of multiuser OFDM signals, an asynchronous interferer has a guard interval within the transmitted data block when the observation window is synchronized with respect to the desired user. As a result, the interfering user's channel vector lies in a different null space relative to the desired user's subspace, allowing the desired user's channel vector to be separated and determined uniquely up to a multiplicative scalar. However, in practice, a noisy sample correlation matrix produces a number of small eigenvalues, which can cause poor performance when conventional subspace techniques are employed. This problem is
overcome by using several eigenvectors corresponding to the
smallest eigenvalues; the channel estimates are obtained by linear
combinations of these vectors. In order to find the linear
combining coefficients, known pilot symbols are required. However,
the pilot-only estimation method is not feasible when the channel
order and the number of active users are large. Therefore, we use
the preceding method to develop a subspace-based semi-blind algorithm that guarantees estimation of every user's channel vector. Simulation results demonstrate that the semi-blind channel estimator offers more than a 2dB gain over pilot-only channel estimation in the bit error rate (BER) performance.
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