Alex Oprea

Antenna systems for base station diversity in urban small and micro cells

The authors describe cross-correlation properties for compact urban base station antenna configurations, nearly all of which result in very low envelope cross-correlation coefficients of about 0.1 to 0.3. Specifically, polarization diversity systems are examined for their potential in improving link quality when hand-held terminals are involved. An expression is given for the correlation function of compound space and polarization diversity systems. Dispersion and envelope dynamic statistics are presented for the measured environments. For microcell applications, it is found that systems such as GSM having a bandwidth of 200 kHz or less can use narrowband cross-correlation analysis directly. >

Time reversal techniques for wireless communications

The application of the time reversal (TR) technique in a multiple input-single output (MISO) configuration can reduce the delay spread of the channel impulse response by a factor of 2-3, depending on the power allocation scheme. Moreover, this technique provides a gain of about 20 dB in peak signal power. These effects are demonstrated with measurements of fixed wireless access channels in the 5 GHz band. The measurements were performed at 3 different locations in downtown Toronto, and the channel has been measured with 8 element uniform linear antenna arrays at both ends, thus providing an 8/spl times/8 configuration.

Time Reversal and Zero-Forcing Equalization for Fixed Wireless Access Channels

We consider a wideband system with an M -element transmit array. In this paper we combine conventional zeroforcing pre-equalization and time reversal: we present a beamformer that perfectly equalizes the channel and preserves the spatial focusing properties of time reversal. We compare it to a pure zero-forcing beamformer and a pure time reversal system and demonstrate its superior bit error rate performance and low probability of intercept properties using actual measurements of a fixed wireless access system in an urban environment.

Dual domain coherence measures for FWA channels in the 5-6 GHz band

Fixed wireless access (FWA) channels in the 5 GHz band have been measured with 8 element uniform linear antenna arrays at both ends, thus providing an 8/spl times/8 MIMO configuration. The measurements were performed at 3 different locations in downtown Toronto. The channel stability is expressed via correlation functions in time and frequency. Eigen analysis reveals support for 2 parallel channels on average with the target SNR of 20 dB (using single polarized uniform linear arrays).

Assessment of GSM-link quality dependence on radio dispersion in rural environments

The results of two separate measurement setups to investigate GSM-link quality dependence on radio dispersion in rural Faroese and Danish environments, a repetitive GSM-data burst sequence system, and a correlation sounder are presented. Both operated in the 900 MHz band. It is shown that the GSM system performance seems to be sensitive to Q16 both below and above the specified value of 9 dB, while the delay spread is of minor influence when Q16 is infinite. The burst error distribution seems quite unsensitive to interleaving for BER <10% and vehicle speeds of 40 km/h.<<ETX>>

Measurement based performance evaluation of finite MMSE equalizers with and without time-reversal processing at the transmitter

Time reversal (TR) is a transmission technique that relies on pre-processing the signal at the transmitter (TX) by filtering it through a filter, the impulse response of which is a time reversed and phase conjugated version of the channel impulse response (CIR) to the target receiver (RX). TR is known to have remarkable temporal and spatial focusing properties. Specifically temporal focusing indicates that the perceived channel delay spread is reduced, and therefore we expect a less complicated equalizer to be required at the RX side to guarantee successful signal detection. In this paper we investigate how TR preprocessing affects the performance of finite minimum mean square error (MMSE) equalizers. The results are based on wideband measurements taken in an urban environment. We show that the application of TR is beneficial in the case of low signal to noise ratio (SNR) and low equalizer complexity.