In recent years, multiple-input multiple-output (MIMO) wireless technologies have captured a
lot of research interest, given the capacity increase achievable with such schemes [1, 2]. On
the downlink, MIMO exploits multiple antennas at both the base station transmitter and the user terminal receiver. In the transmitter, the highspeed data stream intended for the user is encoded in time and space across multiple transmit antennas. In doing so, the same carrier (or spectral resource) is reused at each antenna. Signal processing is then used to decode the composite signals received at the mobile user’s terminal.
The spatial antenna processing at the terminal is able to unravel the effects of complex multipath scattering, and fundamentally provides access to parallel independent propagation paths between the base station and the user. Thus, instead of having access to a single data pipe, as with conventional wireless system design, a wireless system exploiting MIMO technology is able to capitalize on the presence of multiple parallel pipes, improving both the data rate and system capacity. MIMO has now reached a certain maturity, and is being investigated in the Third Generation Partnership Projects (3GPP and 3GPP2) for the evolution of the Universal Mobile Telecommunications System (UMTS) and cdma2000 systems, respectively.
Another technology that has been considered by the industry for 3G systems evolution is
orthogonal frequency-division multiplexing (OFDM). The Wireless World Research Forum
(WWRF) considers OFDM the most important technology for a future public cellular radio access technology [3]. Several wireless networking (e.g., IEEE 802.11 and 802.16) and wireless
broadcasting systems (e.g. DVB-T, DAB) have already been developed using OFDM technology and are now available in mature commercial
products.
Since data is multiplexed on many narrowband subcarriers, OFDM is very robust to typical
multipath fading (i.e., frequency-selective) channels. Furthermore, the subcarriers can easily be generated at the transmitter and recovered at the receiver, using highly efficient digital signal processing based on fast Fourier transform (FFT).