No abstract

No abstract

A series of experiments that were conducted to evaluate the relative merits of three types of diversities in the indoor portable radio channels at 900 MHz is described. Unmodulated tones were used to probe the channel in a typical university work environment. The statistics of the signal envelopes were analyzed to determine the correlation of the field strengths when the signals are separated in space, frequency or polarization. According to the results presented here, the perforOf a narrow-band system ( < 2oo kHz) at 900 MHz greatly benefit from a frequency diversity scheme provided that the separation between the two frequencies is greater than approximately 5 MHz. Similarly, the advantage of space diversity is found significant when the two antennas are spaced by 3/4 wavelength or more. It is also reported that co-polarized and cross-polarized signals are nearly uncorrelated and their relative field strengths differ by less than 2 dB in most locations.specifically at obtaining parameters pertaining to different diversity schemes. ~~f i~~~ and cox [71 and cox, M~~~~~, and Norris 181 used continuous wave (CW) signals to investigate the radio coverage inside a building when the transmitter was placed outside. Alexander [9], [IO] was one of the first to report on truly indoor propagation measurements. He used CW signals to probe the channel on one floor, where both the transmitter and receiver were placed in locations inside the buildi%. A h a n d e r and I1 l1 used a two-branch space diversity receiver with selection combining to see the possible improvements in the signal envelope. Patsiokas et al. 1121 performed indoor attenuation measurements at several frequencies using cw There have also been several reports on experimental data in papers devoted primarily to system studies. In all these studies, however, there has been little results that directly address the question of parameters from the impulse response measurements, but that the results and the In Section II a description of our experiment is given, followed by a presentation of the results in Section 111, and conclusions in Section Iv. THE EXPERIMENT

Abstract-This paper proposes a near-field broadband adaptive beamforming scheme for intelligent computer telephony and teleconferencing applications, namely the nested microphone array with adaptive noise canceller (NMA-ANC). The NMA-ANC scheme incorporates an harmonically nested array with a nonuniformly subbanded multirate filter bank. Each subband array employs several near-field delay-filter-and-sum beamformers and an adaptive noise canceller (ANC). The proposed NMA-ANC is evaluated via a noise rejection experiment and dereverberation experiment performed in an anechoic chamber and a real conference room, respectively. The experiment data are recorded by a multichannel digital recording system developed using commercial off-the-shelf (COTS) equipments. A perceptual analysis/measurement system (PAMS) test is also carried out using a COTS digital speech level analyzer. The results of the experimental evaluation and PAMS test show that the proposed NMA-ANC scheme is able to improve the sound quality by adaptively rejecting multiple interfering signals and attenuating the reverberant noises and avoiding the desired signal cancellation.Index Terms-Adaptive noise canceller, broadband beamforming, dereverberation, interference rejection, multichannel digital recording, near-field beamforming, nested microphone array (NMA), subband multirate systems.

Abstract-This paper proposes a robust near-field adaptive beamformer for microphone array applications in small rooms. Robustness against location errors is crucial for near-field adaptive beamforming due to the difficulty in estimating near-field signal locations especially the radial distances. A near-field regionally constrained adaptive beamformer is proposed to design a set of linear constraints by filtering on a low rank subspace of the near-field signal over a spatial region and frequency band such that the beamformer response over the designed spatial-temporal region can be accurately controlled by a small number of linear constraint vectors. The proposed constraint design method is a systematic approach which guarantees real arithmetic implementation and direct time domain algorithms for broadband beamforming. It improves the robustness against large errors in distance and directions of arrival, and achieves good distance discrimination simultaneously. We show with a nine-element uniform linear array that the proposed near-field adaptive beamformer is robust against distance errors as large as 32% of the presumed radial distance and angle errors up to 20 . It can suppress a far field interfering signal with the same angle of incidence as a near-field target by more than 20 dB with no loss of the array gain at the near-field target. The significant distance discrimination of the proposed near-field beamformer also helps to improve the dereverberation gain and reduce the desired signal cancellation in reverberant environments.Index Terms-Dereverberation, distance discrimination, interference suppression, microphone array, near-field beamforming, regionally constrained beamforming, robust adaptive beamforming, robustness against location errors.