The convenience of wireless communications and the ever-increasing demand for higher data rates has motivated the proposed study of more efficient methods of transmission. One highly spectrally efficient link architecture is the multiple-input multiple-output (MIMO) architecture, which uses multiple transmit antennas and multiple receive antennas. Under ideal conditions, the theoretical capacity of a MIMO channel increases linearly in the number of transmit/receive antennas. In other words, the theory promises that more bits can be conveyed through the same bandwidth by simply adding more antennas at each end of the link. Orthogonal frequency division multiplexing (OFDM) is a high-data rate modulation technique that is already part of some single-transmitter communications standards and is known for its scalability and its convenient and cost-effective implementation using standard digital signal processing architectures. This study seeks to determine how to combine OFDM with MIMO architectures over real channels. A challenging goal of this research is to arrive at a flexible, scaleable, wireless modem with bandwidth efficiency on the order of 4 to 10 bits per second per Hertz.
The study will include analysis of the major functions of the physical (PHY) and medium access control (MAC) layers. Hardware experimentation will include MIMO channel measurements, including characterization of MIMO interference, and verification of the MIMO OFDM link in real-time on a software radio test-bed. Methods for performing MIMO OFDM channel estimation and synchronization jointly over the spatial channels are being investigated, taking into account noise and channel estimation and synchronization errors. An adaptive transmitter is also being studied that combines space-time processing based on singular-value decomposition (SVD) with adaptive modulation. Solutions to the major transmission impairments in OFDM systems, which include intersymbol interference (ISI) due to insufficient guard interval, interference from co-channel systems, and the effects of amplifier non-linearities on OFDM will be sought for MIMO OFDM systems. To provide data reliability, the combination of space-time coding, Turbo coding with iterative receivers, and hybrid ARQ strategies for error control across the PHY and MAC layers are being considered. Finally, media access protocols will be optimized to work with the parameters of the OFDM air interface, such as the OFDM symbol period.
