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 method is multiple-output (MIMO)
orthogonal frequency division multiplexed (OFDM) wireless access systems that
use M transmit/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.
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.
The proposed study addressed the problem of combining OFDM with MIMO
architectures over real channels.
A challenging goal of this research was 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 has included analysis of the major functions of the physical and medium
access control (MAC) layers.
Hardware experimentation has consisted of 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 have been investigated, taking into account
noise and channel estimation and synchronization errors. An adaptive transmitter
was 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 have been developed 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 physical (PHY) and media access control (MAC) layers have been
considered.