An experimental investigation is reported of the crosscorrelation of 900 MHz signals received by two spatially separated antennas at a base station. The investigation embraced vertical, horizontal and combined horizontal and vertical separation of the antennas, for transmission from test routes 1.3 km from the base station. It was found that a crosscorrelation <0.7 (i.e. when diversity improvement becomes significant) can best be achieved using vertical separation of the antennas of between 11 X and 13 X, for the 1.3 km cell radius. At 900 MHz such an antenna separation is easily obtained and, in addition, the roof space required is small. Moreover, the crosscorrelation using vertically spaced antennas is independent of the incoming arrival angle (unlike horizontally spaced antennas), and hence low correlation can be achieved while maintaining omnidirectional coverage. List of principal symbolsd' d H dy p ID ft 7s L m 2 N p(z) r v = R(t) = r(t) = T = {z}* = <2> = a = oc H = a,-= oc v = A = 4>i = Penv =

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A description of the physical mechanism causing multipath propagation in built-up areas is followed by a discussion of the various ways in which fading radio communiction channels can be described. It is shown that characterisation in the time-delay/Doppler-shift domain explicitly illustrates the multipath nature of the channel and provides parameters relevant to system design. A brief discussion of channelsounding techniques is followed by a description of a wideband channel sounder using matched-filter signal processing in the receiver.

General expressions for the level crossing rate (LCR) and average fade duration (AFD) are obtained for several diversity combining schemes employing two-branch predetection reception of correlated Rayleigh fading signals. These expressions are obtained from joint and conditional probability density functions (PDFs) of the received signals, and lead to a unified treatment. This simplified method contrasts with the characteristic function approach used in previous investigations. Numerical results are presented for a space-diversity system using horizontally spaced antennas at a mobile station. It is shown that while the angle between the antenna axis and the direction of vehicle motion does not appear in the cumulative distribution function (CDF) of the combined output signal envelopes, it affects the LCR and AFD when the two fading signals are correlated. When the two antennas parallel with the direction of vehicle motion are used, the LCR can be reduced below the value obtainable from signals which fade independently. When the two antennas are perpendicular to the direction of vehicle motion, the AFD is loosely dependent on the antenna spacing and, provided the antenna spacing is not too small, is approximately half that for the no-diversity case. p(a | b) = conditional PDF of a with b given Re (.) = real part of complex value Rj^t) = time-varying signal strength envelope for Nth branch R(t) = time derivative of time-varying signal strength envelope z, = complex amplitude of ith multipath wave zf = complex conjugate of complex ith multipath wave | Zj | = modulus of complex ith multipath wave = ensemble average of complex ith multipath wave = mean matrix = inverse of matrix M -transpose of matrix M = angle between antenna axis and direction of vehicle motion = phase of ith multipath wave = phase difference between ith and ;th multipath waves = wavelength = complex crosscorrelation function between ith andjth branches = average received signal power = variance of R = time delay = average fade duration (AFD) at envelope level R M A/" 1 a Ot X Pill) List of principal symbols d = distance between space-diversity antennas e, = received amplitude of ith multipath wave erfc (.) = complementary error function f D = maximum Doppler frequency J o (.) = zero-order Bessel function nib = mean value of R N/{ = level crossing rate (LCR) at envelope level R n = conditional level crossing rate p(a, b) = joint probability density function (PDF) of a and bPaper 5770F (E8), first received 4th September 1986 and in final revised form 26th August 1987 Dr. Adachi is with the