Overloaded Multiple-input Multiple-output Systems Research Articles

Virtual Channel Optimization for Overloaded MIMO Systems

In this brief, we present a virtual channel (VC) optimization approach for overloaded multiple-input multiple-output (MIMO) systems. With this approach, a VC vector is generated at the transmitter, which is combined with the actual wireless channel matrix to form the overall transmission channel matrix. Through VC optimization, the overall transmission channels can be properly adjusted in favor of the signal detection, leading to an improved performance. Moreover, we investigate the overloaded MIMO system in which the number of receive antennas equals the number of transmit antennas. By using superposition VC method, this overloaded MIMO system allows more number of data streams to be transmitted than the number of transmit antennas. The methods for obtaining the optimal VC vector are presented. Simulation results illustrate the effectiveness of the proposed approaches.

Correlation Reduction Precoding for Joint Decoding in Overloaded MIMO-OFDM System on Rician Channel

This paper presents a correlation reduction precoding scheme for joint decoding in an overloaded multiple input multiple output (MIMO)-orthogonal frequency division multiplexing system. In the overloaded MIMO system, the number of receive antenna elements is less than that of transmit antenna elements. Through joint maximum likelihood (ML) decoding a frequency diversity suppresses the BER degradation owing to the spatial multiplexing of signal streams. On a Rician channel, the correlation among coded symbols deteriorates the performance of the joint ML decoding. Thus, this paper proposes the precoding scheme that reduces the correlation on the Rician channel. Numerical results obtained through computer simulation show that the proposed scheme improves the BER by a factor of 1/10 on the Rician channel with a Rician factor of 10.

Low Complexity Metric for Joint MLD in Overloaded MIMO System

This paper presents a low complexity metric for joint maximum-likelihood detection (MLD) in overloaded multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) systems. In overloaded MIMO systems, a nonlinear detection scheme such as MLD combined with error correction coding achieves superior performance than that of a single signal stream with higher order modulation. However, MLD demands large computational complexity because of multiplications in the selection of candidate signal points. Thus, a Manhattan metric has been used to reduce the complexity. Nevertheless, it is not accurate and causes performance degradation in overloaded MIMO systems. Thus, this paper proposes a new metric that is calculated with summations and bit shifts. New numerical results obtained through computer simulation show that the proposed metric improves bit error rate (BER) performance more than 0.2dB at the BER of 10^(-4) in comparison with a Manhattan metric.

Joint Turbo Decoding for Overloaded MIMO-OFDM Systems

To achieve an excellent error-correcting capability, the Third-Generation Partnership Project (3GPP) Long-Term Evolution (LTE) employs turbo codes as its forward error correction (FEC) standard. In addition, to achieve higher throughput, the LTE also implements multiple-input–multiple-output (MIMO) systems. Although a conventional turbo decoding scheme gives satisfactory performance in ideal MIMO systems, significant performance degradation occurs in an overloaded MIMO system when the number of transmit antennas is larger than that of receive antennas. In this paper, a joint decoding scheme for turbo codes, i.e., so-called joint turbo decoding, is proposed. In joint turbo decoding, calculations of soft information are conducted for each combination of bits from all streams instead of separately between each stream. In addition, to combine the trellis diagrams from all the streams, a super-trellis diagram is employed. The numerical results obtained through computer simulation show that the proposed scheme performs better than the conventional scheme, particularly in the case of an overloaded MIMO system. In the MIMO system with four transmit and two receive antennas, up to 2-dB performance enhancement can be attained at a bit error rate (BER) of 10−5. The proposed scheme also obtains improved throughput with a throughput/(SNR/stream) ratio close to the theoretical capacity of overloaded MIMO systems.

Open-Loop Correlation Reduction Precoding in Overloaded MIMO-OFDM Systems

This paper proposes an open-loop correlation reduction precoding scheme for overloaded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. In overloaded MIMO-OFDM systems, frequency diversity through joint maximum likelihood (ML) decoding suppresses performance degradation owing to spatial signal multiplexing. However, on a line-of-sight (LOS) channel, a channel matrix may have a large correlation between coded symbols transmitted on separate subcarriers. The correlation reduces the frequency diversity gain and deteriorates the signal separation capability. Thus, in the proposed scheme, open-loop precoding in order to reduce the correlation between codewords transmitted on different signal streams is employed at the transmitter of an overloaded MIMO system. The proposed precoding scheme changes the amplitude as well as the phase of the coded symbols transmitted on different subcarriers. Numerical results obtained through computer simulation show that the proposed scheme improves the bit error rate performance on Rician channels.

Codeword Metric Calculation Scheme for Outer Code in Overloaded MIMO-OFDM System

This paper presents a codeword metric calculation scheme in two step joint decoding of block coded signals in an overloaded multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) system. In the previous literature, the two step joint decoding scheme has been proposed for the complexity reduction of joint maximum likelihood decoding in the overloaded MIMO system. However, the two step decoding scheme has not been combined with an outer code. The selection of the candidate codewords in the inner block code may not always be able to provide the metric of a binary coded symbol for the outer code. The metric of the binary coded symbol calculated in the first stage is used in the proposed scheme. It is shown that the proposed two step decoding scheme shows better performance than a bit flipping based scheme and reduces the complexity by about 0.013 for 4 signal streams with the cost of bit error rate degradation within 0.5dB.

Experimental Investigation of Joint Decoding in Overloaded MIMO-OFDM System

This paper presents a joint decoding scheme for an overloaded multiple input multiple output (MIMO) -orthogonal frequency division multiplexing (OFDM) system. In the overloaded MIMO system the number of receive antenna elements is less than that of transmit antenna elements. It has been shown that under the overloaded condition the performance of joint detection deteriorates while diversity reduces the amount of performance degradation caused by signal multiplexing. Thus, this paper proposes a maximum likelihood joint decoding scheme of block coded signals in the overloaded MIMO-OFDM system. The performance of joint decoding over Rayleigh fading channels is evaluated through the experiments. It is shown that the diversity through block coding realizes a limited amount of performance degradation in joint decoding of 2 Hamming coded signal streams.