## 1 Introduction  
The rapid growth in the demand for high data rate services in wireless networks over the last decades has boosted scientific research towards the implementation and evaluation of efficient transmission strategies, as well as the reduction of overall transceiver complexity. To this end, in 1998 the Wideband Code Division Multiple Access (WCDMA) physical layer protocol was adopted for the third generation (3G) wireless networks [1]. The key idea was to allow mobile users to transmit simultaneously, occupying the whole transmission bandwidth, by assigning different spreading codes per user during transmission phase. Proper code design should ideally result in minimum auto-correlation and cross-correlation properties for any pair of the considered sequences. Therefore, both Multiple Access Interference (MAI) as well as Intersymbol Interference (ISI) can be significantly mitigated in multiuser scenarios [2]. In addition, Bandwidth on Demand (BoD) can also be supported, either by assigning multiple codes per user or by adjusting the corresponding spreading factor. Although 3G networks can provide bit rates per user that reach up to 2 Mbps, it soon became apparent that even higher transmission rates should be supported, as in the era of broadband wireless networks various bandwidth demanding services (e.g., video streaming) are quite popular among mobile subscribers. In this context, a practical solution with no additional spectrum requirements is the deployment of multiple antennas at both ends of the wireless transceiver, also known as Multiple Input Multiple Output (MIMO) [3]. The increased degrees of freedom offered by such topologies, can be used either to improve mean Bit Error Rate (BER) or overall network throughput [4]. In the first case, also known as diversity combining (DC) [4], link outage is significantly minimized, since there are multiple independent replicas of the transmitted signal, especially in rich scattering environments. In the second case, also known as spatial multiplexing (SM), independent data streams are transmitted from different antennas. Therefore, bit rate per user can be increased, at the cost however of increased transceiver complexity, since the diversity order of the system is reduced by a factor equal to the number of transmit antennas used for independent stream transmission. In realistic wireless networks however, the existence of both MAI and ISI can significantly degrade the performance of MIMO transceivers [5]. Therefore, advanced signal processing techniques should be employed at the receiver, in order to maximize DC or SM gain. To this end, various studies in literature examine appropriate transceiver techniques for MIMO orientations, as well as practical limitations from the deployment of MIMO architecture in next generation broadband wireless networks. In this context, in [5] the outage probability and capacity of MIMO-CDMA cellular systems is derived. According to the presented results, intercell interference can significantly degrade the performance of the MIMO orientation in SM scenarios. In [6], the performance of various transmission techniques in MIMO-WCDMA networks is investigated, with and without channel knowledge at the transmitter. As results indicate, adjacent sector cooperation can significantly improve the performance of the cellular orientation, at the cost however of increased signal burden for channel estimation and feedback to the transmitter. In [7], the performance of various precoding matrices is analyzed, when receiver operations are based on Minimum Mean Square Error (MMSE) detection. As results indicate, maximum likelihood detector can improve overall performance when the spatial correlation among the two input streams is taken into account. In [8], the performance of various beamforming algorithms for CDMA systems was evaluated, in terms of algorithmic complexity. In [9], the performance of MIMO-WCDMA downlink systems is investigated with antenna selection at the transmitter in correlated Nakagami fading channels.