Parallel receiver channel sounder for spatial and MIMO characterisation of the mobile radio channel S. Salous, P. Filippidis, R. Lewenz, I. Hawkins, N. Razavi-Ghods and M. Abdallah Abstract: The paper briefly compares the architecture of commonly used channel sounders for single input multiple output (SIMO) and multiple input multiple output (MIMO) applications. The architecture of a novel dual UMTS band parallel receiver sounder developed for the purposes of such studies is outlined. The performance of the sounder from back-to-back measurements and calibration considerations are discussed. Outdoor SIMO measurements and indoor/outdoor MIMO measurements obtained with different antenna arrays demonstrate its capability. 1 Introduction Important considerations in radio system design are the presence of multipath and spectral efficiency. To enhance the capacity of present and future radio communications networks, multipath propagation and co-channel interfer- ence effects can be reduced using adaptive antennas with intelligent base stations. A necessary input to the estimation of performance of smart antenna systems is the angle of arrival, AOA of the multipath components [1] . Recently, Bell Labs have proposed the use of MIMO systems for efficient use of the spectrum [2] . Such systems require the characterisation of the channel with multiple antennas at both the transmitter and at the receiver. Direct evaluation of MIMO systems has been obtained from an indoor prototype at Bell Labs [3] . Subsequently, measure- ments to estimate MIMO channel capacity have been conducted using either narrowband channel measurements with multiple CW tones, or orthogonal sequences [4, 5] or using wideband double directional measurements for the estimation of the multipath parameters including AOA in azimuth and in elevation, angle of departure, AOD in azimuth and in elevation, time delay, amplitude and Doppler shift [6] . The parameters are then used to evaluate MIMO capacity by introducing random phase variations. Apart from the narrowband measurements which used multiple parallel receive channels [4, 5] , all the wideband channel measurements reported for the AOA and for MIMO studies use a sequential single input single output, SISO sounder with an RF switch to multiplex the antennas at either the receiver (SIMO) or at both the transmitter and at the receiver (MIMO) [6, 7]. While this avoids the cost and complexity of the multiple receiver architecture, this technique requires short duration pulses and fast switching or that the measurements are performed in quasi stationary environments such as those reported in [6] where each measurement was taken over a 5-minute period. This limits the observation window of the multipath structure to a microcell or a picocell application. To avoid the limitations of the commonly used sequential technique; a semi-sequential architecture, which uses eight parallel channels at the receiver and switching at the transmitter was developed. The semi-sequential MIMO sounder initially envisaged for 8-channel SIMO measure- ments [8] is upgraded from the dual frequency architecture and still operates at the two 60 MHz UMTS frequency division duplex bands [9] . It transmits simultaneously at the two 60 MHz bands and receives alternately at the two bands on eight parallel channels or continuously on all the 8-channels at either frequency band. For SIMO applica- tions the sounder has the same sampling rate as a SISO system while for MIMO applications the sampling rate is reduced by the number of transmit antennas. By connecting an 8-element antenna array to the eight receive channels, it is possible to deduce the AOA of the multipath components and to estimate the delay spread for different sectors. This requires precise knowledge of the characteristics of the antenna array. Due to the mutual coupling between the antenna array elements, it is necessary to measure the radiation pattern of the entire array simultaneously. Two eight-channel calibration units have been constructed to measure the arrays characteristics. One gives the amplitude function only and the other unit gives the quadrature components for four channels to estimate the amplitude and the phase [10, 11] . The paper presents the architecture of the sounder and results of its performance from back to back tests. SIMO measurements in outdoor environments with eight direc- tional antennas each, with 451 beam width, are processed to study the time variations of rms delay spread for the different sectors for a stationary user. MIMO capacity results, obtained for both indoor and outdoor environments with different antenna arrays, are also presented. 2 Multiple antenna channel measurement techniques The main channel sounding requirements are determined by the time delay resolution, and the maximum expected Doppler shift f d max . The first determines the transmitted S. Salous, R. Lewenz, N. Razavi-Ghods and M. Abdallah are with the School of Engineering, University of Durham, South Road, Durham DH1 3LE, UK P. Filippidis is with the Siemens, Greece I. Hawkins is with the School of Electrical and Electronic Engineering, University of Manchester, UK E-mail: sana.salous@durham.ac.uk r IEE, 2005 IEE Proceedings online no. 20045346 doi:10.1049/ip-com:20045346 Paper first received 22nd December 2004 and in final revised form 22nd April 2005 912 IEE Proc.-Commun., Vol. 152, No. 6, December 2005