Hidehiro Matsuoka

Comparison of pre-FFT and post-FFT processing adaptive arrays for OFDM systems in the presence of co-channel interference

For future high-speed wireless communications using orthogonal frequency division multiplexing (OFDM), two major system requirements emerge: throughput improvement and elimination of rich interference. Adaptive array antennas can potentially suppress interference by using plural antennas and spatial coefficient adaptation. In addition, in the case of no interference, it is predicted that the adaptive array can improve either data throughput or potential coverage. In general, the pre-FFT (fast Fourier transform) domain diversity or beamforming in an OFDM system could be a reasonable solution for easy implementation with low complexity. However, in rich multipath environments for both desired signal and interference, the performance will deteriorate drastically. In contrast, it is possible for the post-FFT array processing to provide high performance even in such severe environments, at the cost of complexity. This paper presents a comparison of antenna array architectures for OFDM system using pre-FFT and post-FFT domain array processing, and clarifies the dependency of the channel property for each array processing. It is found that the performance of the pre-FFT array processing is strongly dependent on the total number of multipaths and interferences. Also, it is shown that it is possible for the post-FFT array processing to reduce the complexity by decreasing the number of antennas and the corresponding analog components because its performance depends on the number of signal sources, furthermore, since the proposed post-FFT array processing can achieve high performance with much lower complexity by using subcarrier clustering, it is a promising candidate for real implementation.

Antenna-selective transmit diversity technique for OFDM-based WLANs with dual-band printed antennas

An antenna-selective transmit diversity (ASTD) technique, applied to a PC card for an orthogonal frequency division multiplexing (OFDM)-based wireless local area network (WLAN) system, is presented. The PC card, which is based on IEEE 802.11 a/g, is equipped with dual-band printed antennas. The ASTD processing is implemented in the developed WLAN system, which has two antennas for transmit and receive modes. The effects of the ASTD technique on the TCP (transmission control protocol) throughput performance are experimentally evaluated. Measurement results with the PC card in an imitated office show that the TCP throughput is improved on both the 2.4 and 5.2 GHz bands under the conditions of an indoor radio propagation channel due to the ASTD technique. In particular, application of the ASTD technique to an access point (AP) achieves four times the average TCP throughput on the 5.2 GHz band.

Path diversity using an adaptive array with directional antennas for high bit rate mobile communication systems

This paper proposes a new path diversity method using an adaptive array with directional antennas for high bit rate mobile communications. The proposed scheme selects some optimum elements from plural directional antennas based on the results of all delay profile estimation, and combines some outputs of the adaptive array for each delayed wave. Therefore, interference waves can be suppressed by this space-time processing approach with less processing power. Computer simulations show that the performance of the proposed scheme can be improved drastically by the path diversity combining method which minimizes mean square errors, even if delayed waves are incident from the same directions as a direct wave.

A smart antenna receiver testbed with directional antenna elements

This paper reports the configuration of a smart antenna receiver testbed for land mobile communication base stations and the experimental results obtained for it. The antenna pattern and bit error rate (BER) are measured in a radio anechoic chamber. These results show the interference can be suppressed effectively by beamforming using an adaptive algorithm, e.g., LMS or CMA.

A smart antenna with pre- and post-FFT hybrid domain beamforming for broadband OFDM system

This paper proposes a smart antenna with pre-FFT beamforming based on eigenanalysis and post-FFT subcarrier diversity for broadband OFDM systems. This method can achieve signal-to-noise ratio (SNR) enhancement due to two independent eigenbeams in the pre-FFT domain and maximum ratio combining in individual subcarriers in the post-FFT domain. Two pre-FFT eigenbeams focus on the efficient combining of desired signal energy as diversity branches while they achieve lower computational load compared with the optimum post-FFT diversity which consists of the same number of FFT circuits as antennas. Computer simulation results show the complexity is reduced by 50% while keeping the similar bit error rate (BER) performance to the optimum diversity in typical cases of multipath environments

Development of a PC card using planar antennas for wireless LAN on 2.4/5 GHz bands

We have developed a wireless LAN PC card for 2.4 GHz and 5 GHz dual-band operation using printed planar antennas, and evaluated its throughput performance. The planar antennas, which combine an inverted-F antenna with an inverted-L antenna, can realize a dual-band at low cost in a limited integrated area on the PC card. The radio propagation performance was evaluated in a shielded room which is modeled after an office. In the 2.4 GHz band, operation is stable and independent of the deployed position and the direction of a laptop PC with the PC card. On the other hand, in the 5 GHz band, although the performance is affected by surrounding objects and antenna directivity, the PC card with the printed antennas can realize as high a throughput as one with external omnidirectional antennas.

Novel reduced complexity algorithm based on group detection in multiple antenna systems

Group detection (GD), the combination of maximum likelihood (ML) decoding and linear processing for multiple antenna systems, has been shown to attain reasonable performance and practical complexity. However, the diversity gain of conventional GD degrades since the degrees of freedom at the receive antennas are exploited for the linear processing before the local ML search. This paper proposes a novel GD with overlapped grouping. The proposed GD employs selection diversity within the overlapped signals by using the likelihood value, after the local ML search. Therefore, the proposed GD can achieve higher diversity gain compared with the conventional GD.

Reduced Complexity Max-Log-MAP Sphere Decoder Using Group Detection in MIMO-OFDM Systems

This paper proposes a Group Detection (GD) algorithm with Max-Log-MAP Sphere Decoder (MLM-SD) in order to reduce the complexity of signal detection in a Multiple-Input Multiple-Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system. The proposed algorithm divides spatial streams into multiple partial spatial streams by using Minimum Mean Square Error (MMSE) detector, followed by multiple MLM-SDs with reduced number of spatial streams. Although the spatial diversity gain in the MLM-SD degrades because of the lack of the degrees of freedom exploited by the MMSE detector, its diversity gain is recovered by combining the metrics obtained by the multiple MLM-SDs. In a MIMO wireless LAN multipath fading environment, the complexity of the proposed algorithm is 10% of that of the original MLM-SD and the performance degradation in terms of SNR is slightly less than that of the original MLM-SD in 4-by-4 MIMO architecture with 64QAM achieving 216 Mbps. It is also found that the proposed algorithm is robust against the limitation of the number of searches in sphere decoder.