This paper deals with orthogonal Space-Time Block Coded MC-CDMA systems, in outdoor realistic downlink scenarios with up to two transmit and receive antennas. Assuming no channel state information at transmitter, we compare several linear single-user detection and spreading schemes, with or without channel coding, achieving a spectral efficiency from 1 to 2 bits/s/Hz. The different results obtained demonstrate that spatial diversity significantly improves the performance of MC-CDMA systems, and allows different chipmapping without notably decreasing performance. Moreover, the global system exhibits a good trade-off between complexity at mobile stations and performance. Then, Alamouti's STBC MC-CDMA schemes derive full benefit from the frequency and spatial diversities and can be considered as a very realistic and promising candidate for the air interface downlink of the 4 th generation mobile radio systems.I.

The combination of multiple antennas and multi-carrier code division multiple-access (MC-CDMA) is a strong candidate for the downlink of the next generation mobile communications. The study of such systems in scenarios that model real-life transmissions is an additional step towards an optimized achievement. We consider a realistic MIMO channel with two or four transmit antennas and up to two receive antennas, and channel state information (CSI) mismatches. Depending on the mobile terminal (MT) class, its number of antennas or complexity allowed, different data-rates are proposed with turbo-coding and asymptotic spectral efficiencies from 1 to 4.5 bit/s/Hz, using three algorithms developed within the European IST-MATRICE project. These algorithms can be classified according to the degree of CSI at base-station (BS): i ) Transmit space-frequency prefiltering based on constrained zero-forcing algorithm with complete CSI at BS;ii ) transmit beamforming based on spatial correlation matrix estimation from partial CSI at BS; iii ) orthogonal space-time block coding based on Alamouti scheme without CSI at BS. All presented schemes require a reasonable complexity at MT, and are compatible with a single-antenna receiver. A choice between these algorithms is proposed in order to significantly improve the performance of MC-CDMA and to cover the different environments considered for the next generation cellular systems. For beyond-3G, we propose prefiltering for indoor and pedestrian microcell environments, beamforming for suburban macrocells including highspeed train, and space-time coding for urban conditions with moderate to high speeds.

Abstract-This paper investigates a downlink multiple-input multiple-output (MIMO) multi-carrier code division multiple access (MC-CDMA) system with pilot aided channel estimation (PACE) and iterative channel estimation (ICE) in the receiver. Due to the superposition of different users' spread data signals, zero-valued chips can occur after spreading, which can cause noise enhancement when using data estimates as reference signals in ICE. Hence, we propose a MIMO channel estimation method to overcome the above problem and avoid noise enhancement in ICE. Simulations results for PACE and ICE were computed for a realistic outdoor MIMO channel model. The results show that frame error rates of ICE are smaller than the ones of PACE.

In this paper, we compare the performance of Alamouti's space-time block coded MC-CDMA systems for indoor and outdoor realistic scenarios with zero forcing or minimum mean square error detection schemes. Two different configurations of the system are considered for the two scenarios. The different results obtained as well for indoor as for outdoor scenarios demonstrate that spatial diversity improves significantly the performance of MC-CDMA systems. Then, Alamouti's STBC MC-CDMA schemes derive full benefit from the frequency and spatial diversities and can be considered as a very realistic and promising candidate for the air interface downlink of the 4th generation mobile radio systems.

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