Toshiaki Koike

Hybrid ARQ scheme suitable for coded MIMO transmission

An automatic-repeat-request (ARQ) scheme for improving the system throughput efficiency is investigated in coded multiple-input multiple-output (MIMO) transmissions. The improved ARQ scheme employs two strategies, referred to as trellis-coded modulation (TCM) reassignment and antenna permutation. TCM reassignment increases the free distance after code combining through the use of different TCM codes for retransmissions. Antenna permutation exploits spatial dimension for preventing successive frame errors by changing the connections between transmitters and transmit antennas upon retransmission. It is shown through computer simulations that this ARQ scheme achieves high throughput even in severe conditions of low signal-to-noise ratio and high Rician factor over spatially and temporally correlated Nakagami-Rice fading MIMO channels.

Capacity Improvement of Multihop Inter-Vehicle Communication Networks by STBC Cooperative Relaying

In this paper, we evaluate the effect of space-time coded cooperative relaying technique in multihop inter-vehicle communication (IVC) networks. The IVC systems have an issue that communication links are often blocked by obstacles such as heavy vehicles. The breakage of a radio link in multihop connections may significantly decrease the system throughput in multihop IVC networks. It is demonstrated through system-level evaluations that the cooperative relaying can offer remarkable capacity enhancement by exploiting multi-route diversity and overcoming accidental link breakage resulting from frequent topological changes.

Prototype Implementation of Real-Time ML Detectors for Spatial Multiplexing Transmission

SUMMARY We developed two types of practical maximum-likelihooddetectors (MLD) for multiple-input multiple-output (MIMO) systems, us-ing a field programmable gate array (FPGA) device. For implementations,we introduced two simplified metrics called a Manhattan metric and a cor-relation metric. Using the Manhattan metric, the detector needs no multi-plication operations, at the cost of a slight performance degradation within1dB. Using the correlation metric, the MIMO-MLD can significantly re-duce the complexity in both multiplications and additions without any per-formance degradation. This paper demonstrates the bit-error-rate perfor-mance of these MLD prototypes at a 1Gbps-order real-time processingspeed, through the use of an all-digital baseband 4 ×4 MIMO testbed inte-grated on the same FPGA chip. key words: multiple-input multiple-output (MIMO), spatial multiplex-ing, maximum-likelihood detection (MLD), field programmable gate array(FPGA), real-time MIMO detector 1. Introduction Recently, wireless communications systems that exploitmultiple transmitting and receiving antennas have receiveda significant amount of attention. These multiple-inputmultiple-output (MIMO) systems are expected to achievea dramatic increase in spectral efficiency because of theantenna diversity in scattering-rich wireless environments.Therefore, the MIMO system has been one of currentlypromising techniques to realizeGbps-class high-speed wire-less transmission for future communications systems [1].Many researchers have extensively investigated severaltechniques for multiple-antenna systems, such as the BellLaboratories layered space-time architecture (BLAST) [2]and space-time coding (STC) schemes [3].In MIMO spatial multiplexing systems, maximum-likelihood detection (MLD) schemes offer excellent perfor-mance [4]. Since the receiver estimates the most-likely sig-nals out of all the possible transmitting signals, the compu-tational complexity of distance metric calculations becomesextremely high in general. Consequently, many researchershave focused on developing more computationally efficientschemes. As a kind of low-complexity detectors, we canuse spatial filters based on the minimum mean-square error

Genetic designing of near-optimal training sequences for spatial multiplexing transmissions

This paper describes an efficient design method of training sequences for spatial multiplexing systems, where multiple antenna elements are utilized for highly spectrum-efficient communications. Recently, these wireless communication systems have been greatly interested in and investigated intensively, e.g. space-time coding (STC) scheme and Bell laboratories layered space-time architecture (BLAST). In these systems, the error-rate performance is seriously dependent on the residual channel estimation error. Therefore, good training sequences for channel estimations, that are short sequences and minimizing the estimation error, are desirable. However, it is not straightforward to construct the optimal sequences due to the large search space for multiple antenna systems. In this paper, we propose the design method using the genetic algorithms (GA) for optimizing training sequences, and demonstrate that the near-optimal sequences regarding least-squares (LS) criteria can be given efficiently. In addition, it is shown that optimized 16PSK sequences offer better performance than optimized BPSK sequences for short lengths.

Experimental Evaluation of Spatial Interleaving in Trellis-Coded MIMO Transmission

This letter reports laboratory experiments of trellis-coded multiple-antenna systems. We evaluate the effect of spatial interleaving for the vector Viterbi algorithm under several conditions such as line-of-sight (LOS), non-LOS propagation, frequency-flat and frequency-selective channels.

FPGA implementation of 1 Gbps real-time 4/spl times/4 MIMO-MLD

We developed two types of practical maximum-likelihood detectors (MLD) for multiple-input multiple-output (MIMO) systems with 1 Gbps-order real-time processing speed, using an FPGA device. We introduce two simplified metrics for implementations; referred to as a Manhattan metric and a correlation metric. In using the Manhattan metric, the detector needs no multiplication operations, at the cost of slight performance degradation within 1 dB. By using the correlation metric, the MIMO-MLD can significantly reduce the complexity in both multiplications and additions without any performance degradation. This paper demonstrates the BER performance of these MLD prototypes through the use of an all-digital baseband 4/spl times/4 MIMO testbed integrated on the same FPGA chip.

Iterative MLD equalizer preceded by MIMO-FDE for wideband spatial multiplexing systems

We propose an iterative maximum-likelihood detection (MLD) equalizer using frequency-domain equalization (FDE) at the former stages for wideband transmissions in multiple-input multiple-output (MIMO) systems. The proposed equalization scheme extends the MIMO-MLD for frequency-selective channels, instead of the use of OFDM transmissions. It achieves nearly the ML sequence estimation (MLSE) performance by means of the iterative ISI cancellations. Moreover, we derive an approximated MLD for complexity reduction from a viewpoint of the optimization problem. It is demonstrated that the iterative equalization scheme employing the approximated MLD can offer excellent performance under extremely heavily-dispersive wideband channels, with no requirement of exponential complexity order.

Adaptive MLSE Equalizer with Per-Survivor QR Decomposition for Trellis-Coded MIMO Transmission

A trellis-coded multiple-input multiple-output (MIMO) transmission technique, which exploits multiple-antenna elements at both transmitter and receiver sides and employs trellis-coded modulations (TCMs), has potential to significantly increase spectral efficiency in wireless communications. At the receiver, an adaptive equalizer based on maximum-likelihood sequence estimation (MLSE) deals with intersymbol interference (ISI) incurred in wideband transmissions and jointly decodes multiplexed TCM signals. Recently, a sphere-constrained maximum-likelihood detection, so-called sphere decoding, has drawn much attention for reducing the computational burden in MIMO transmission systems. This paper describes the super-trellis structured Viterbi algorithm applying per-survivor sphere decoding, and evaluates the effect of the complexity reduction in branch metric computations.

Spatial interleaving for vector Viterbi equalizer with super-trellis decoding in trellis-coded space-time transmission

This paper evaluates the effect of spatial interleaving in the Viterbi algorithm with combined intersymbol interference (ISI) equalization, trellis-coded modulation (TCM) decoding and multi-signal detection. Through computer simulations, it is confirmed that spatial interleaving provides substantial diversity gains in both frequency-flat and frequency-selective Rayleigh fading channels. In Nakagami-Rice fading channels, performance improvement is significant at low Rician factor because of overcoming the burst errors incurred by signal fading. Spatial interleaving does not require additional computational complexity, memory buffer and decoding delay, differently from time interleaving. Moreover, the optimum combined TCM and ISI maximum likelihood sequence estimation (MLSE) can employ interleaving with almost no extra complexity, and iteration between equalization and decoding is not needed through the use of spatial interleaving.

Metric-segmented low-complexity ML detection for spectrum-efficient multiple-antenna systems

In this paper, we reveal that a maximum-likelihood detection (MLD) for multiple-input multiple-output (MIMO) systems requires not exponential complexity order in multiplication operations but cubic order against the number of antennas, by introducing the so-called correlation metric. However, the MIMO-MLD requires still exponential order in arithmetic additions. In order to further reduce the complexity, we propose effective techniques referred to as a metric-segmentation and a norm-constraint approach. We show that the proposed MLD using these techniques can significantly reduce the computational complexity without any performance degradation. Moreover, the proposal does not need additional high-complexity operations such as QR decompositions and matrix inversions.