Takeshi Onizawa

A maximal ratio combining frequency diversity ARQ scheme for OFDM signals

This paper proposes a maximal ratio combining (MRC) frequency diversity automatic-repeat-request (ARQ) scheme which is suitable for high data rate orthogonal frequency division multiplexing (OFDM) signals. The proposed scheme regularly changes the subcarrier assignment pattern at every retransmission. This scheme sharply reduces the number of ARQ retransmissions in slow fading environments by virtue of the frequency diversity effect. Computer simulation results show that the proposed scheme reduces the number of retransmissions to about 3 at the accumulative correct packet reception rate (ACPRR) of 0.99 while the conventional schemes lead to more than 10.

A simple adaptive channel estimation scheme for OFDM systems

This paper proposes a simple adaptive channel estimation scheme for orthogonal frequency division multiplexing (OFDM) in order to realize high-rate wireless local area networks (LANs). The proposed estimator consists of simple frequency-domain FIR filters, which are adaptively selected according to the difference vector between adjacent subcarriers and channel amplitude of the subcarrier. Computer simulations show that the proposed scheme offers superior packet error rate (PER) performance to the least-squares scheme, by 1.1 dB in terms of required E/sub b//N/sub 0/ at a PER=0.1 in AWGN channels. They also show, for the same criterion, a 0.7 dB improvement for a frequency selective fading channel with a delay spread value of 100 ns.

A new simple adaptive phase tracking scheme employing phase noise estimation for OFDM signals

An adaptive phase tracking scheme for orthogonal frequency division multiplexing (OFDM) signals can provide superior PER performance in channels that exhibit a variety of phase noise power. It is an effective technique with which to achieve high-rate and high quality wireless transmission. This paper proposes a new simple adaptive phase tracking scheme for OFDM in order to realize high-rate wireless local area networks (LANs). The proposed scheme measures the integrated phase rotation in order to set appropriately the properties of the FIR filter in the phase tracking circuits. This scheme is based on the fact that the integrated phase rotation is correlated to the phase noise power. Assuming an RMS delay spread of 100 ns, computer simulations show that the proposed scheme offers superior required E/sub b//N/sub 0/ performance (versus the phase noise power) compared to the conventional fixed-tap scheme, where the phase-noise-to-signal power ratios are lower than -18 dB. It also offers excellent PER performance at the packet length of 1000 bytes, unlike the conventional schemes which suffer degraded PER performance.

A new signal detection scheme combining ZF and K-best algorithms for OFDM/SDM

This paper proposes a new signal detection scheme that combines zero-forcing (ZF) and K-best algorithms for orthogonal frequency division multiplexing with space division multiplexing (OFDM/SDM) systems. Among various signal detection algorithms for SDM signals, maximum likelihood detection (MLD) is one of applicable approaches, which achieves the optimal performance. However, it suffers from exponential computational complexity against the number of transmit antennas and modulation order. Thus, we consider the K-best algorithm for the reduction of the computational complexity. We proposed the modified K-best algorithm, which exploits the ZF algorithm for initial symbol estimation. The initial symbol estimation improves the decoding accuracy of the original K-best algorithm. Therefore, the proposed scheme achieves both the reduction of the MLD algorithms computational complexity and the improvement of the original K-best algorithms decoding accuracy. Computer simulation results show that the performance degradation from the MLD algorithm is suppressed to just 1 dB or so in the required E/sub b//N/sub o/ for packet error rate (PER) = 10/sup -2/, when either 16 quadrature amplitude modulation (16QAM) or 64QAM is applied with three transmit and three receive antennas. In these cases, about 87% and 99% fewer metric computations are required than the MLD algorithm, respectively. It is presented that the proposed algorithm offers significant reduction of the computational complexity compared to the MLD algorithm.

A fast burst synchronization scheme for OFDM

This paper proposes a burst synchronization scheme with a short preamble signal for high data rate wireless LAN systems that use orthogonal frequency division multiplexing (OFDM). The proposed scheme, which consists of automatic frequency control (AFC) and symbol timing detection, enables us to shorten the preamble length to 1/4 that of a conventional one. Computer simulation results show that the degradation in required E/sub b//N/sub 0/ due to the synchronization scheme is less than 1 dB in a selective Rayleigh fading channel.

Fast Algorithm for Decision Feedback Equalization in Multiple Input Multiple Output Channel

In this paper, we propose a new algorithm for zero-forcing decision feedback equalization (ZF-DFE) in multiple input multiple output flat Rayleigh fading channel. The basic idea is to separates the layer ordering and the filter construction operations, thus making both operations tractable and efficient. The new layer ordering strategy approximates the optimal ordering rule (VBLAST) from a geometrical perspective, and the filters in proposed ZF-DFE are constructed using a matrix Cholesky decomposition aided QR decomposition. Compared to the conventional ZF-DFE detectors utilizing optimal and suboptimal ordering rules (SQRD), the proposal is complexity efficient. Additionally, the bit error rate performance of the new algorithm significantly approximates that of the optimal ordering ZF-DFE. Thus a practical signal detection scheme for the vector channel is found

Experiments on FPGA-Implemented Eigenbeam MIMO-OFDM With Transmit Antenna Selection

This paper describes the performance of eigenbeam multiple-input multiple-output (MIMO) with transmit antenna selection using orthogonal frequency-division multiplexing (OFDM), as measured in a testbed implemented using field-programmable gate arrays (FPGAs); it also targets the downlink performance improvement of wireless local area networks (LANs). For this verification, we employ a determinant-based simple transmit antenna selection approach based on the estimated instantaneous MIMO channel matrix. We show extensive experiments on the testbed to confirm the performance of eigenbeam MIMO-OFDM with transmit antenna selection in the three-select-two antenna case. First, the measured packet-error-rate (PER) performance confirms that the eigenbeam scheme with the three-select-two antenna-selection scheme provides a slight degradation in the required carrier-to-noise power ratio (CNR) that is approximately 0.2 dB from the eigenbeam-only scheme with three transmit antennas but with significantly lower computational complexity. Second, to determine the impact of Doppler frequency, both 5 Hz and 20 Hz, we focus on the required CNR performance degradation under various transmission intervals between the channel sounding packet and the data packet. It is experimentally confirmed that the eigenbeam scheme with transmit antenna selection offers improved robustness to MIMO channel fluctuation compared with the eigenbeam-only scheme.

A Simple and Feasible Decision-Feedback Channel Tracking Scheme for MIMO-OFDM Systems

This paper proposes a simple and feasible decision-feedback channel tracking scheme for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems designed for wireless local area networks (LANs). In the proposed scheme, the channel state matrix for each subcarrier is tentatively estimated from a replica matrix of the transmitted signals. The estimated channel matrices, each derived at a different timing, are combined, and the previously estimated channel matrices are replaced with the latest ones. Unlike conventional channel tracking schemes based on a Kalman filter, the proposed scheme needs no statistical information about a MIMO channel, which makes the receiver structure quite simple. The packet error rate (PER) performances for the proposed scheme are evaluated on computer simulations. When there are three transmit and receive antennas, the subcarrier modulation scheme is 64 QAM, and the coding rate is 3/4, the proposed scheme keeps the SNR degradation at PER of le-2 less than 0.1 dB when the velocity of receiver is 3 km/h in an indoor office environment at 5 GHz band. In addition, compared to the conventional channel tracking scheme based on known pilot symbols, the proposed scheme improves throughput performance by 13.8% because it does not need pilot symbols. These results demonstrate that the proposed channel tracking scheme is simple and feasible for implementation in MIMO-OFDM systems based on wireless LANs.

MMSE Criterion Fast Decision Feedback Equalization Algorithm for Spatial Multiplexing Systems

Recent studies indicate that spatial multiplexing (SM) scheme has enormous potential for increasing the capacity of MIMO channels in rich scattering environments. In this paper, we present a new MMSE decision feedback equalization algorithm for signal detection in SM systems. The new algorithm separates the order search and the filter generation to avoid recursive ordering operations. Compared to the earlier MMSE-DFE detectors with optimal and suboptimal ordering rules, the new algorithm is less complex and achieves a bit error rate performance that closely approximates that of the standard BLAST algorithm.

A Reduced-Complexity Signal Detection Scheme Employing ZF and K-Best Algorithms for OFDM/SDM

This paper proposes a reduced-complexity signal detection scheme for Orthogonal Frequency Division Multiplexing with Space Division Multiplexing (OFDM/SDM) systems that utilize Zero-Forcing (ZF) and K-best algorithms. It is known that Maximum Likelihood Detection (MLD) with exhaustive search achieves mathematically optimal performance for SDM signal detection. However, it also suffers from exponential computational complexity against the number of transmit antennas and modulation order. In order to reduce the computational complexity of MLD, we apply the K-best algorithm for signal detection. It is known that the K-best algorithm itself inherently reduces the computational complexity of MLD because it avoids exhaustive search. In this paper, we propose the modified K-best algorithm, which exploits the ZF algorithm for initial symbol estimation. This initial symbol estimation improves the decoding accuracy of the original K-best algorithm. We evaluate the performance of the proposed scheme through computer simulations. The computer simulation results show that the performance degradation from the MLD algorithm is suppressed to just 1 dB or so in terms of the required E b /N 0 for packet error rate (PER) = 10 -2 , When either 16 Quadrature Amplitude Modulation (16QAM) or 64QAM is applied with three transmit and three receive antennas. In these cases, 87% and 99% fewer metric computations are required than the MLD algorithm. It is confirmed that the proposed MLD algorithm offers a significant reduction in the computational complexity from the MLD algorithm while suppressing the performance degradation.