This paper proposes approximated log likelihood ratios (LLRs) for single carrier millimeter-wave (mmW) transmission systems in the presence of phase noise. In mmW systems, phase noise on carrier wave signals in very high frequency bands causes severe performance degradation. In order to mitigate the impairments of phase noise, forward error correction (FEC) techniques, such as low density parity check (LDPC) code, are effective. However, if the probabilistic model does not capture the exact behavior of the random process present in the received signal, FEC performance is severely degraded, especially in higher order modulation or high coding rate cases. To address this issue, we carefully examine the probabilistic model of minimum mean square error (MMSE) equalizer output including phase noise component. Based on the derived probabilistic model, approximated LLR computation methods with low computational burden are proposed. Computer simulations confirm that the approximated LLR computations on the basis of the derived probabilistic model are capable of improving bit error rate (BER) performance without sacrificing computational simplicity in the presence of phase noise.

This paper proposes a novel iterative multiple-input multiple-output (MIMO) receiver for orthogonal frequency division multiplexing (OFDM) systems, named as an “iterative MIMO receiver employing virtual channels with a Turbo decoder.” The proposed MIMO receiver comprises a MIMO detector with virtual channel detection and a Turbo decoder, between which signals are exchanged iteratively. This paper proposes a semi hard input soft output (SHISO) iterative decoding for the iterative MIMO receiver that achieves better performance than a soft input soft output (SISO) iterative decoding. Moreover, this paper proposes a new criterion for the MIMO detector to select the most likely virtual channel. The performance of the proposed receiver is verified in a 6×2 MIMO-OFDM system by computer simulation. The proposed receiver achieves better performance than the SISO MAP iterative receiver by 1.5dB at the bit error rate (BER) of 10-4, by optimizing the number of the Turbo iteration per the SHISO iteration. Moreover, the proposed detection criterion enables the proposed receiver to achieve a gain of 3.0dB at the BER of 10-5, compared with the SISO MAP iterative receiver with the Turbo decoder.

In this paper, we propose demodulators for the Golden and Alamouti codes in amplify-and-forward (AF) cooperative communication with one relay. The proposed demodulators output exact log likelihood ratios (LLRs) with recursion based on the Jacobian logarithm. The cooperative system with the proposed demodulator for the Golden code has the benefit of efficient data transmission, while the system for the Alamouti code has low demodulation complexity. Quantitative analyses of computational complexity of the proposed demodulators are conducted. The transmission performance for various relay location and power settings is evaluated on cooperative orthogonal frequency division multiplexing (OFDM)-based wireless local area network (LAN) systems. In evaluations, the optimal relay location and power settings are found. The cooperative system with the proposed demodulators for the Golden and Alamouti codes offers 1.5 and 1.9 times larger areas where 10.8 and 5.4Mbit/s can be obtained than a non-cooperative (direct) system in a typical office environment, respectively.

This paper proposes a multiple-input multiple-output (MIMO) precoding scheme for the down link of single user (SU) cooperative base station (BS) systems. The proposed precoding scheme mitigates the performance degradation caused by large inter-BS path-loss imbalance and large intra-BS antenna correlation by controlling two parameters. The proposed precoding scheme multiplexes the multiple layers by adjusting the amplitude of each layer, and then decreases the occurrence probability of the small absolute value of the log likelihood ratio (LLR), and so reduces the bit error rate (BER). Link level simulation results show that the proposed precoding scheme decreases the required signal-to-noise ratio (SNR) of BER = 0.001 by 5.5 dB, 2.2 dB, and 0.7 dB in the case of QPSK and coding rate 1/1, 3/4, and 1/2 respectively. The proposed precoding scheme is also evaluated in terms of spectrum efficiency using rank adaptation and adaptive modulation, showing that it improves the spectrum efficiency when the SNR per a receiver antenna is higher than 4 dB.

In this paper we focus on the decoding error of the Log-Likelihood Ratio Belief Propagation (LLR-BP) decoding algorithm caused by oscillation. The decoding error caused by the oscillation is dominant in high Eb/N0 region. Oscillation of the LLR of the extrinsic value in the bit node process (ex-LLR) is propagated to the other bits and affects the whole decoding. The Ordered Statistic Decoding (OSD) algorithm is known to improve the error rate performance of the LLR-BP decoding algorithm. The OSD algorithm is performed by deciding the reliability of each bit based on a posteriori probability. In this paper we propose two decoding algorithms based on two types of oscillations of LLR for LDPC codes. One is the oscillation-based OSD algorithm with deciding the reliability of each bit based on oscillation. The other is the oscillation-based LLR-BP decoding algorithm that modifies ex-LLR based on oscillation. In the oscillation-based LLR-BP decoding algorithm, when ex-LLR oscillates, then we reduce the magnitude of this ex-LLR to reduce the effects on the other bits. Both algorithms improve the decoding errors caused by oscillation. From the computer simulations, we show that paying attention to the oscillation, we can improve the error rate performance of the LLR-BP decoding algorithm.