相河 聡

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.

This paper proposes a simplified decision-feedback channel tracking scheme for multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems designed for wireless LANs. In the proposed scheme, the channel matrix for each subcarrier is tentatively estimated from a replica matrix of the transmitted signal. A number of the estimated channel matrices, each determined for a different timing, are combined, and the previously estimated channel matrices are replaced by the latest ones. The packet error rate (PER) performances for the proposed scheme are evaluated by computer simulations. Assuming 5GHz band operation, the proposed scheme keeps the PER degradation at PER=10(-2) to less than 1dB when the velocity of transmitter is less than 11.8km/h. The result represents the proposed channel tracking scheme provides sufficient mobility for wireless LAN systems. We also evaluate the computation complexity of the proposed scheme. The proposed scheme requires less than 1% of the complex multipliers needed by the conventional channel tracking scheme based on Kalman filters.

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. © 2006 IEEE.

This paper describes the performance of transmit antenna selection for an eigenbeam multiple-input multiple-output with orthogonal frequency division multiplexing (MIMO-OFDM) system as measured in a testbed implemented with field programmable gate array (FPGA) it also targets wireless LANs. For this verification, we employ the determinant-based transmit antenna selection approach based on the estimated instantaneous MIMO channel matrix to reduce the computational complexity. The measured packet error rate (PER) performance confirms that the eigenbeam scheme with three-select-two antenna selection scheme provides a slight degradation in required carrier-to-noise power ratio (C/N), approximately 0.2 dB from the eigenbeam-only scheme, but with significantly lower computational complexity. Furthermore, with regard to the impact of Doppler frequency, we focus on the transmission interval between MIMO channel estimation and data transmission. To suppress the degradation in required C/N to within 1.5 dB, a transmission interval of approximately 15.0 ms is permitted without using the re-estimation procedure fd = 5 Hz, for eigenbeam 16QAM/MIMO-OFDM with three-select-two antenna selection. ©2006 IEEE.

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.

Fairness in the number of effective opportunities for wireless access between the uplink and downlink and that between stations cannot be achieved in wireless packet communications such as wireless LANs due to the packet error caused by packet collisions or received power degradation. In this paper, we propose Fair Wireless Access Control with Error Compensation (FWAC/EC) to achieve fairness over an unstable link. The key features of the proposal are release delay control and packet compensation control. The point of the release delay control is to control the waiting time for attempting a subsequent transmission after an ongoing transmission. The point of the packet compensation control is continuous transmission based on the number of retries. Simulation results show that FWAC/EC achieves fairness in the number of effective opportunities for wireless access between the uplink and downlink and that between stations over an unstable link.

IEEE 802.11無線LANが急速に普及し, オフィス, 家庭や公衆無線LAN等様々な環境で使用されている.このような環境では, 一般的に, 複数の無線端末が一つの基地局に接続し, DCF手順と呼ばれるチャネルアクセス手順を用いて一つの無線チャネルを共有している.このとき, 無線端末の台数が増加するとともに, 下りリンクのデータフレームの遅延時間が増大するという問題がある.また, 一部の無線端末の下りリンクの負荷が著しく大きい場合に, その他の無線端末の下りリンクのデータフレームの遅延時間が増大するという問題がある.そこで, 本稿ではこれらの問題を解決するためにDCF手順を拡張したチャネルアクセス制御方式であるフレームバースティング方式と当該方式に適用できるデータフレーム数の制御方式およびデータフレームの選択方式を組み合わせたデータフレーム送信制御方式を提案する.また, 計算機シミュレーションを用いた提案方式の特性評価の結果を報告する.

MIMO(Multiple-Input Multiple-Output)-OFDM(Orthogonal Frequency Divison Multiplexing)方式は、現在策定中の次世代高速無線LANの標準規格であるIEEE802.11nにおいて物理レイヤの有力な伝送方式として注目を集めている。IEEE802.11nでは、MACレイヤにおける高効率化を図るために一つのPHYフレーム内に複数のMSDU(MAC Service Data Unit)を収容するフレームアグリゲーションが提案されているが、フレームアグリゲーションによるPHYフレーム長の増加に伴い、伝搬路変動によるPER(Packet Error Rate)特性の劣化が問題となる。本稿では、MIMO-OFDMのための簡略化されたブラインド伝搬路トラッキング方式を提案する。提案する伝搬路トラッキング方式では、受信器において作成された送信信号のレプリカを用いて伝達係数行列を推定し、サブキャリア毎の複数の伝達係数推定行列を列ベクトル毎に重み付け合成し、伝達係数推定行列を逐次更新する。計算機シミュレーションによる提案手法の評価を行い、16個のMSDU(MAC Data Service Unit)から構成されるPHYフレームを送信した場合において評価した結果、PER=1e-2を達成するCNRで比較して、従来のトラッキングを行わない場合にはPHYフレームの後方に位置するMSDUのPERが大幅に劣化するのに対し、提案方式では後方のMSDUであっても先頭のMSDUとほぼ同等のPER特性を実現することを確認した。

The mesh topology based on the standard IEEE 802.11 for WLAN appears to be a very promising architecture for achieving a ubiquitous wireless access in the future. However, the IEEE 802.11 protocol is aimed at single-AP (Access Point) environments and many problems related to the wireless meshed interconnection of APs and Mobile Terminals (MTs) remain to be solved. Some proposed solutions to build mesh architectures are based on single-channel ad-hoc oriented schemes in which IEEE 802.11 protocol has been modified. The problem with this type of schemes, however, lies in the very low performance of the single-channel architecture itself. The task group S of IEEE 802.11 is currently working out standards for IEEE 802.11 WLAN-based mesh architectures in a number of usage scenarios such as residential, outdoor or hotspot but a lot of work remains to be done since the group was recently established. In this paper we propose a new multiple-radio multiple-channel mesh architecture for hot zones which works using a simple technique for interconnecting APs based on the Distributed Coordination Function (DCF) and a modified shortest path routing algorithm through which we use the new concept of radio routing. A major advantage of our proposed scheme is that, putting routing issues aside, it does not introduce any change into the MAC protocol of IEEE 802.11. Our simulations results obtained in OPNET show a lot of interesting results related to the mesh architectures itself which could lead to further research in the future. Moreover, it is shown through simulations that the proposed architecture yields a very good performance in terms of throughput and transmission delays

The mesh topology based on the standard IEEE 902.11 for wireless LANs appears to be a very promising architecture for achieving a ubiquitous wireless Internet access in the future. However, the current IEEE 802.11 protocol is aimed at single-AP (Access Point) environments and many problems related to the wireless meshed interconnection of APs and Mobile Terminals (MTs) remain to be solved. Some proposed solutions to build such mesh architectures are based on single-channel ad-hoc oriented schemes in which IEEE 802.11 protocol has been modified. The main problem with this type of schemes, however, lies in the very low performance of the sin-le-channel architecture itself The task group S of IEEE 802.11 is currently working out standards for IEEE 802.11-compliant mesh architectures in a number of usage scenarios including residential, office and campus/community/public access network but a lot of work remains to be done since the group was established just on last year. In this paper we propose a new multi-radio multichannel mesh architecture for hot zones which works using a Distributed Coordination Function (DCF)-based technique for interconnecting APs and a simple but effective radio routing mechanism. A major advantage of our proposed scheme is that putting routing issues aside, it introduces no change into the MAC protocol of IEEE 802.11. Our simulations results obtained in OPNET show the great effectiveness of radio routing for balancing the traffic load within the network as well as interesting results related with the size of packets and the mesh architecture itself that could lead to further research in the future. Moreover, the results show the proposed architecture yields a very good performance in terms of total throughput and transmission delay.

This paper proposes a new data frame transmission scheme for IEEE 802.11 wireless LANs [1]. IEEE 802.11 wireless LANs employ the Binary Exponential Backoff algorithm to lower the possibility of collisions without reducing efficiency. Unfortunately, it has been shown that the algorithm is, in the short term, highly unfair. To improve the data transmission efficiency, several schemes allow stations to transmit successive frames. Employing these schemes worsens the short-term unfairness. The proposed scheme ties successive transmission to the number of elapsed contention windows to improve the throughput performance without increasing the unfairness. Simulation results show that the proposed scheme achieves high throughput performance while maintaining fairness.

無線LANを自動情報取得システムに適用した際の圏内外管理方法について検討した。自動情報取得システムとはモバイルデータ通信で問題となるサービスエリアの不連続性をユーザが意識することなく、サービスエリア内を通適中に自動的に無線接続を確立し所望のデータのダウンロードを実現するサービスである。受信電界強度に応じて自動情報取得サービスの起動タイミングを変更する圏内外管理方法を提案しその有効性を示した。

無線LANの規格であるIEEE802.11規格では,マルチレートをサポートしており,伝搬環境に応じて伝送レートを切り替えることで,高い通信品質を保ちつつ,かつ,高いスループットを実現するシステムの構築が可能である.上記の規格では,アクセス方式として定期的な制御信号の交換を必要としないCSMA/CA方式を用いており,この方式を用いるシステムに適する動的レート切替方式として,過去の送信成否情報を用いる切替方式が提案されている.しかしながら,従来方式では,伝送レートを切り替えた後のパケット誤りの確率を推測することが困難であるため,伝送レートを切り替える際に不要なパケット誤りを生じ,スループットが低下するという問題がある.本稿では,上記の問題を解決する方式として,送信成否情報と伝搬路情報を用いた動的レート切替方式を提案する.また,計算機シミュレーションを用いて,従来方式とのスループット特性の比較を行った結果を報告する.

効率的な伝送と信頼性を両立するため、5GHz帯を用いた無線LANシステムでは、伝送速度が異なる複数の伝送モードを適応的に切り替えて用いるリンクアダプテーション技術が用いられる。このリンクアダプテーションを実現する上で鍵となる技術は、時間的に激しく変動する伝送路の状態に追従して、素早く正確に最適な伝送モードを選択する方法である。このモード選択の方法としては、送信の成功/失敗の履歴情報をもとにMAC層で最適伝送モードを推定する方法と、プリアンブル等の既知の信号を用いてPHY層で推定する2種類の方法がある。 PHY層で推定を行う方法は、MAC層で推定する方法よりもスループツト特性に優れるが、一方で精度の高いPER特性推定が要求される。本稿では、リンクアダプテーションにおける最適伝送モードを推定する指標である新たな評価関数を提案する。さらに、マルチパス環境でのHiSWANa標準準拠の無線LANシステムを想定したシミュレーションにより、提案方式の有効性を示した。

In this paper, how the new service of non-authenticated multicasting can be implemented on the AWA system is described. The AP is forced to send IGMP packets periodically in order to keep transmitting IP multicast data grams. The program tables of the non-authenticated multicast are transmitted as IP multicast data to allow the all clients to receive the material. Furthermore, the list of program tables which shows the pairs of the ID of the AP and the IP address of the program table are used so that different APs can offer different program tables and programs. Experiments on non-authenticated multicasting were successfully conducted and we confirmed that the proposed service works well.