Yusuke Asai

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.

Performance evaluation of distributed multi-cell beamforming for MU-MIMO systems

Inter-cell interference (ICI) from nearby cells can substantially degrade the sum-rate in WLAN systems. To improve the sum-rate in actual WLAN systems, some papers have investigated distributed multi-cell beamforming to mitigate ICI by zero forcing beamforming. All most of these approaches consider single-user multiple input multiple output (SU-MIMO) systems. In this paper, we present three techniques for distributed multi-cell beamforming for multi-user MIMO (MU-MIMO) systems with the same total number of transmit and receive antennas. The first technique is distributed multi-cell beamforming using receiver selection. The second one is antenna selection at the receiver. The last one is an iterative technique in which transmitter weight is updated at the transmitter to correspond to receiver weight. The sum-rate of these techniques with imperfect channel state information at transmitter (CSIT) is evaluated. We clarify the effectiveness of these techniques for MU-MIMO systems.

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.

Efficient Detection Algorithms for Spatial Multiplexing Systems

Recent studies indicate that spatial multiplexing (SM) systems have enormous potential for increasing the capacity of multiple input multiple output (MIMO) channels in rich scattering environments. In this paper, we present two efficient decision feedback equalization (DFE) algorithms for signal detection of SM scheme. The basic idea behind the new proposals is to reduce the complexity by separating the order search and filter generation. Compared with the conventional DFE detectors with optimal and suboptimal ordering rules, the new algorithms are less complex and achieve error performance that closely approximates that of the standard Bell Labs layered space time (BLAST) algorithm

A new synchronization scheme for packet mode OFDM-SDM signals in wireless LAN

This paper proposes a new synchronization scheme for packet mode orthogonal frequency division multiplexing (OFDM) signals employing space division multiplexing (SDM); it targets IEEE 802.11 TGn wireless LANs that offer over 100 Mbit/s. In addition, this paper shows the packet format of OFDM-SDM signals that ensures backward compatibility with IEEE 802.11a. The proposed synchronization scheme has simple open-loop construction and consists of automatic frequency control (AFC), symbol timing detection, channel estimation and phase tracking. The AFC and symbol timing detection are carried out in the time-domain. After OFDM demodulation, the proposed scheme processes channel estimation and phase tracking in the frequency-domain. Considering all the above synchronization tasks, we evaluate the packet error rate (PER) performance using the TGn-defined multi-input multi-output (MIMO) channel model. The proposed scheme shows superior performance; it suppress the required Eb/N0 degradation to within 0.4 dB (0.3 dB) with 64 byte (1000 byte) packets compared to the performance achieved if only channel estimation is considered; the RMS delay spread = 50ns.

Network controlled frequency channel and bandwidth allocation scheme for IEEE 802.11a/n/ac wireless LANs: RATOP

The first giga-bit class wireless local area network (wireless LAN) below 6 GHz frequency (the IEEE 802.11ac standard) is attracting attention due to its capability of handling much more of the rapidly growing data traffic. The increased density of wireless LAN access points (APs), however, has induced inter-cell interference that severely degrades system performance. This problem seems to be getting worse with the increase of 802.11n- and 802.11ac-based APs, which establish basic service sets (BSSs) with wider channel bandwidths. In order to mitigate this problem, we propose an efficient radio resource allocation scheme called RATOP that can be applied to a managed wireless LAN system with a central coordinator. On the basis of channel monitoring results, capability, and data traffic information obtained from APs, the central coordinator computes the quasioptimal frequency channel and channel bandwidth of each AP in such a way that the given utility function is maximized. Numerous simulations with UDP traffic flows on the coexistence scenarios of 802.11a/n/ac show that the RATOP works well and reduces the number of overlapping BSSs by 65%-89%. As a result, the RATOP improves the minimum throughput and fairness among throughput of APs even when there exist foreign APs of other network domains that cannot be controlled by the coordinator.

A Packet Combining Demodulation Scheme for Multi-Hop Wireless Systems using Network Coding

Network coding has been referred to as the way to improve the network throughput of multi-hop communication network systems. Therefore a lot of researches have focused on IP and MAC layers. However, this report focuses on the demodulation scheme for the physical layer network coding and propose a packet combining demodulation scheme for network coded multi-hop wireless systems. With this scheme, a system can receive the benefits of both cooperative transmission and network coding, and the communication quality is drastically improved. Numerical results show improvements in the packet error probability of the network coded system based on the IEEE 802.11a standard.

Tree Search based Approximate Maximum Likelihood Detector for Spatial Multiplexing Systems

In a spatial multiplexing system with multiple antennas on both sides of a wireless link, it is preferable to adopt the maximum likelihood (ML) detection to fully extract both multiplexing and diversity gain. In this paper, we present a new tree search based near ML detector that applies a new ordering in conjunction with efficient node exploration strategy. The error performance of the new detector closely approaches that of the ML detector. However, it is far less complex than conventional suboptimal tree detectors and maintains a fixed detection delay.

Development and experimental validation of downlink multiuser MIMO-OFDM in gigabit wireless LAN systems

Multiuser multiple-input multiple-output (MU-MIMO) has been proposed as a means to improve spectrum efficiency for various future wireless communication systems. This paper reports indoor experimental results obtained for a newly developed and implemented downlink (DL) MU-MIMO orthogonal frequency division multiplexing (OFDM) transceiver for gigabit wireless local area network systems in the microwave band. In the transceiver, the channel state information (CSI) is estimated at each user and fed back to an access point (AP) on a real-time basis. At the AP, the estimated CSI is used to calculate the transmit beamforming weight for DL MU-MIMO transmission. This paper also proposes a recursive inverse matrix computation scheme for computing the transmit weight in real time. Experiments with the developed transceiver demonstrate its feasibility in a number of indoor scenarios. The experimental results clarify that DL MU-MIMO-OFDM transmission can achieve a 972-Mbit/s transmission data rate with simple digital signal processing of single-antenna users in an indoor environment.