Atsushi Ohta

PRIME ARQ: a novel ARQ scheme for high-speed wireless ATM-design, implementation and performance evaluation

Error control for high-speed wireless ATM (WATM) operating at 20 Mbps or higher is an important research topic. Automatic repeat request (ARQ) for WATM is required to achieve high throughput as well as high transmission quality, i.e., low CLR (cell loss ratio). Selective repeat (SR) and go-back-N (GBN) are typical ARQ schemes. Though SR-ARQ is superior to GBN-ARQ in terms of throughput performance, the implementation complexity of SR-ARQs control procedures prohibits its application to high-speed wireless systems. GBN-ARQ, on the other hand, cannot avoid serious throughput degradation due to fairly high BER caused by multipath fading and shadowing. To solve this problem, this paper proposes a novel ARQ scheme named PRIME-ARQ (partial selective repeat superimposed on GBN-ARQ) for TDMA-based WATM. PRIME-ARQ achieves high throughput performance, almost equal to selective repeat ARQ, with a simple algorithm resulting in reduced implementation complexity for high speed operation. This paper describes the design, implementation, and performance of the proposed PRIME-ARQ. In addition, it shows the experimental results using a PRIME-ARQ processor.

Performance Evaluation of 8×8 Multi-User MIMO-OFDM Testbed in an Actual Indoor Environment

Multiple input multiple output (MIMO) systems have attracted much attention as they can provide increased channel capacity within a limited frequency band. In some important applications, such as wireless LAN and cellular systems, MIMO systems will likely operate in environments for which a single base station should simultaneously communicate with many users. Such applications require development of multiuser(MU)-MIMO techniques. The prior work on MU-MIMO systems have resulted in derivation of the ideal channel capacity as well as various proposed schemes for transmission control and scheduling. In order to incorporate MU-MIMO technologies into commercial systems, extensive evaluations are needed, since the amount of system parameters in MU-MIMO is much larger as compared to the Signleuser(SU)-MIMO. Due to such a volume of system parameters, the performance of MU-MIMO in actual systems is still not well understood. This paper describes MU-MIMO testbed which can deal with 8times8 MIMO-OFDM transmission based on the IEEE 802.11a signal format. We have conducted experiments in an actual indoor environment in order to evaluate the frequency utilization for downlink scheme implemented in the MU-MIMO testbed. Finally, we discuss the effectiveness of MU-MIMO for SU-MIMO when the number of receiving antennas is changed

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.

Proposal of compact MIMO terminal antenna employing Yagi-Uda array with common director elements

In this paper, a compact MIMO terminal antenna employing Yagi-Uda arrays with common director elements is proposed. The Yagi-Uda array provides a high gain in the horizontal direction even when the array is arranged horizontally. The size of the proposed antenna is fairly small because the director elements are shared between two Yagi-Uda arrays that face each other and the length of the array is approximately half of two individual Yagi-Uda arrays.

A novel application of Massive MIMO: Massive Antenna Systems for Wireless Entrance (MAS-WE)

This paper proposes a practical application of Massive MIMO technology, Massive Antenna Systems for Wireless Entrance (MAS-WE), and its related inter-user interference (IUI) cancellation and scheduling techniques. MAS-WE, on which the entrance base station (EBS) employs a large number of antennas, can effectively provide high capacity wireless entrance links to a large number of Wi-Fi access points (APs) distributed in wide coverage area. The proposed techniques have been ultimately simplified to be less impact for their practical implementation in order to spatially multiplex more than 16 signal streams with around 100 antenna elements at EBS side. SIR performance was evaluated by system level simulation considering imperfect channel state information (CSI) and the results showed that the proposed MAS-WE with simplified techniques can achieve high spectral efficiency with high level space division multiplexing.

User Selection Method for Block Diagonalization in Multiuser MIMO Systems

Multiuser (MU) - multiple input multiple output (MIMO) techniques were proposed to increase the spectrum efficiency because only a few antenna branches are allowed at a simple mobile terminal (MT). Among the MU-MIMO techniques, the block diagonalization (BD) algorithm is well known as a practical linear precoding technique for a downlink. In this paper, we analyze an achievable bit rate in MU-MIMO using BD algorithm and derive the simplified expression using the signal spaces of users. Based on the derived expression, two kinds of user selection methods for the BD algorithm (USBD) in MU-MIMO systems are proposed. The proposed methods estimate the improvement in the achievable bit rate compared to that in a single user (SU)-MIMO system using the signal spaces of the supported users. The first proposed method is USBD with a full search (USBD-FS) and it selects the combination of users to achieve the highest total throughput by considering all the combinations. The second proposed method is USBD with a tree structure search (USBD-TS), which drastically reduces the calculation complexity. Although all combinations of users are not considered in USBD-TS, a sufficient achievable bit rate is obtained while the calculation load is much less than that for the conventional method. Computer simulations confirm that USBD- FS and USBD-TS obtain the achievable bit rate of 100.0% and 91.1% compared to that for the best combination of users when the angle spread at the transmitter is 75deg.

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.

New Robust Beamforming Method for Frequency Offsets in Uplink Multiuser OFDM-MIMO

Multiuser multiple input multiple output (MU-MIMO) systems are attracting attention due to their frequency efficiency. However, in uplink MU-MIMO systems, different frequency offsets among multiple mobile stations (MSs) significantly degrade the transmission quality, especially when orthogonal frequency division multiplexing (OFDM) is used. In this paper, the influence of these frequency offsets is first analyzed in a frequency selective fading environment. Numerical analysis shows that an error floor occurs in the bit error rate and the influence of the frequency offset becomes larger in short delay spread environments. To overcome this problem, a new beamforming method is proposed to compensate for the frequency offset by introducing an auto frequency controller after frequency-space equalization in each data stream. The effect of the proposed method is evaluated in a frequency selective fading environment by computer simulations and measured results.

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.

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.