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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.

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Investigation of the dependence of bit and burst error rate on radio signal level, dispersion and correlation with topography have shown that GSM equalizer specifications rarely are challenged in flat Danish environments. In the Faroe Islands however, strong reflections from cliffs and hill sides are nearly perfectly diffuse and create broadening up to 40 ps of the delay power profiles. These increase bit and burst error rates and require special cautions to maintain satisfactory transmission quality. lntroduct ionTwo separate measurement setups have been operated simultaneously to investigate GSM-link quality dependance on radio dispersion in rural Faroese and Danish environments; A repetitive GSM-data burst sequence system and a correlation sounder. Both operated in 900 MHz band. The measurement campaign in the Faroe Islands were a joint venture between Aalborg University (AUC) and Faroese Telecom (IT). Measurement areasThe Faroe Islands are located in the middle of the North Atlantic. Cliffs up to 700 meters high, steep mountains, valleys and fjords 1 to 3 km wide, characterized the seven different areas specially chosen where long echo delays were expected. Propagation measurements were also made in flat rural terrain with a few gently rolling dwells and hills around Aalborg. Measurement setuDThe GSM system was operated at 910 MHz. A modified GSM super frame was periodically transmitted. Every 8th time slot was a frequency burst for synchronization purposes. The received signal K. Davidsen, M. Danielsen Faroese Telecom Faroe Islands was recorded and off-line decoded by a software Viterbi decoder developed by Benny Madsen, DanCall Radio Ltd. Each measurement consisted of 15 records, one every 1 s, of at least 8 time slots per user. The sensitivity of the receiver was -100 dBm for 10 dB signal to noise ratio (SNR). With 50 dBm transmitted power (including antenna gain) a coverage area of a few km was achieved. A 51 1 bit sequence, 10 Mbit/s correlation sounder developed by RadioTech I/S in cooperation with AUC, was operated at 980 MHz. The receiver provides about 44 dB peak to spurious ratio and a delay window of 51 ps is obtained. In each measurement 30 impulse responses were saved, one every 2.8 m at a vehicle speed of 40 km/h. The coverage area was up to 15 km.The BS antennas had a 60° 3 dB beam width at the GSM transmitter and a 120° at the channel sounder, both with 8 dBi gain. The receiver antennas were 1/4 A monopoles. Radio channel disnersionOnly mean power-delay profiles (PDP) are analyzed because power cross delay-spatial profiles over approx. 2.8 m were measured. The mean profile integrated over the spatial axis is assumed unaffected statistically by the use of cross profiles instead of instantaneous profiles.Statistical parameters are extracted from the mean PDP's obtained by the correlation sounder. The parameters are peak power, total power Pt, delay spread S, the delay window W9 (for 9 dB ratio of in-to outside window power) and interference ratio 0 1 6 (power in-to outside a 16 ps window), follo...

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