Special Issue on Personal, Indoor and Mobile Radio Communica

+ The authors are with NTT Network Innovation Laboratories,

1678

IEICE TRANS. COMMUN., VOL. E83-B, NO. 8 AUGUST 2000

Special Issue on Personal, Indoor and Mobile Radio Communications

Adaptive Array Employing Eigenvector Beam of MaximumEigenvalue and Fractionally-Spaced TDL with Real Tap

Yasushi TAKATORI , Keizo CHO , Kentaro NISHIMORI , and Toshikazu HORI , Regular Members

SUMMARYThis paper proposes a new digital beamformingadaptive array antenna (DBFAAA) that is effective in severe multipathenvironments in which timing and carrier synchronization circuits can-not function ideally resulting in the DBFAAA losing control. The pro-posed DBFAAA has two stages. In the first, the DBFAAA captures thedesired signal and establishes synchronization. In the second, theDBFAAA optimizes the beam pattern of the signal. The proposed con-figuration employs an eigenvector beam of the maximum eigenvaluein the first stage beam-forming. In addition, a fractionally-spaced-tapped-delay-line (FS-TDL) with real tap weights, which is placed af-ter the beam-former, is applied to achieve timing synchronization. Thebehavior of the proposed DBFAAA for asynchronous sampling data isinvestigated and the results indicate that the proposed configurationenables asynchronous sampling at the A/D converter. A prototype ofthe proposed DBFAAA achieving 38-Mbps real-time data communi-cation is introduced and the transmission performance is shown.key words: adaptive array, digital beamforming, eigenvectorbeam, high-speed wireless access, fractionally-spaced-tapped-de-lay-line

1.Introduction

With the increasing proliferation of mobile computing, high-speed wireless access systems have gained popularity as freeaccess systems. However, in high-speed wireless access sys-tems, since the delay of multipath waves normalized by thedata rate becomes long, transmission performance is degradedby inter-symbol interference. Thus, anti-multipath techniquesare necessary [1]. One well-known and effective techniqueemploys sector antennas at both the base and terminal station[1]. Portability requirements push for compact terminal sta-tions; however, this must be weighed against the transmis-sion performance. Small terminal stations require an exces-sively large base station antenna to achieve narrow beam andadequate transmission performance. We previously proposedemploying the digital beamforming adaptive array antenna(DBFAAA) at base stations and confirmed by computer simu-lation that high transmission performance can be achievedeven with a small terminal station [2]. In the DBFAAA, thereceived signals at antenna branches are sampled in thebaseband and the antenna pattern is formed by digital signalprocessing. The data sampling rate is generally synchronizedwith the transmission data symbol rate. Optimum processingof the DBFAAA can be achieved by using the Wiener solu-tion that minimizes the mean squared error for reference sig-Manuscript received December 10, 1999.Manuscript revised March 18, 2000.

The authors are with NTT Network Innovation Laborato-ries, Yokosuka-shi, 239-0847 Japan.

nals [3]. Even in a severe multipath environment, DBFAAAeliminates all multipath waves when the maximum delayedsymbols of the incoming multipath waves are less than thenumber of antenna branches [2].

However, in an actual environment, the carrier and thetiming recovery cannot work ideally and stability degradesor control is lost. Thus, a robust beam forming method forthe carrier and timing synchronization error is needed torealize ideal operation in severe environments.

Several techniques have been proposed to improve theperformance of the DBFAAA in such environments. Someof the effective techniques introduce pre-beamforming, whichgenerates a beam pattern before the synchronization, usingthe fast-Fourier-transformation (FFT) algorithm [4] or theconstant modulus algorithm (CMA) [5], [6] and improvesthe SINR before the synchronization. In severe multipathenvironments such as that indoors, many reflected wavesarrive from all directions, however, the FFT algorithm cannotsynthesize waves from different directions and sufficientSINR improvement cannot be obtained to establish thesynchronization [2]. On the other hand, CMA can synthesizemany reflected waves and has achieved remarkableimprovement in the transmission quality. CMA, however, hasslow convergence so that it takes a long time to establish thesynchronization. This degrades throughput of the wirelesssystems.

Eigenvector beams are often used as a first stagebeamforming of the beam-space DBFAAA [7]–[9]. Theeigenvectors of the correlation matrix for the signals receivedby the antenna array are orthogonal to each other. Therefore,the DBFAAA employing the eigenvector beam achieves afast convergence speed. However, the computationalcomplexity is very high when calculating all eigenvectors asthey are applied to actual high-speed wireless access systems.

In this paper, we propose a new DBFAAA configurationthat can work using asynchronous sampled data. The proposedDBFAAA uses only the eigenvector beam of the maximumeigenvalue as pre-beamforming, but does not use othereigenvector beams to capture the signal of the highest power.Then, a fractionally spaced tapped delay line (FS-TDL) withreal tap weights is connected to the pattern generation part.The timing recovery is established by the FS-TDL. Thisconfiguration allows asynchronous sampling at the analog-to-digital converter (ADC). This paper also clarifies thebehavior of the eigenvector beam and the suitability to thepre-beamforming. Finally, we present a prototype that

+ The authors are with NTT Network Innovation Laboratories,

TAKATORI et al.: ADAPTIVE ARRAY EMPLOYING EIGENVECTOR BEAM OF MAXIMUM EIGENVALUE

achieves real-time operation for 38-Mbps signals and evaluatethe performance in a non-line-of-sight (NLOS) scenario.

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