Adaptive Loading in MIMO/OFDM Systems
Prateek Bansal Andrew Brzezinski
prateek@stanford.edu brzezin@stanford.edu
December 13, 2001
Abstract
Orthogonal Frequency Division Multiplexing (OFDM) is a powerful technique employed in com-
munications systems suffering from frequency selectivity. Combined with multiple antennas at
the transmitter and receiver as well as adaptive modulation, OFDM proves to be robust against
channel delay spread. Furthermore, it leads to significant data rates with improved bit error
performance over links having only a single antenna at both the transmitter and receiver.
This project demonstrates OFDM with adaptive modulation applied to Multiple-Input Multiple-
Output (MIMO) systems. We apply an optimization algorithm to obtain a bit and power al-
location for each subcarrier assuming instantaneous channel knowledge. The analysis and sim-
ulation is considered in two stages. The first stage involves the application of a variable-rate
variable-power MQAM technique for a Single-Input Single-Output (SISO) OFDM system. This
is compared with the performance of fixed OFDM transmission where a constant rate is applied
to each subcarrier. The second stage applies adaptive modulation to a general MIMO system
by making use of the Singular Value Decomposition to separate the MIMO channel into parallel
subchannels. For a two-input antenna, two-output antenna system, the performance is compared
with the performance of a system using selection diversity at the transmitter and maximal ratio
combining at the receiver.
I. Introduction
Evolution of OFDM
Frequency Division Multiplexing (FDM) has been a widely-used technique for signal transmission in fre-
quency selective channels. In essence, FDM divides the channel bandwidth into subchannels and transmits
multiple relatively low rate signals by carrying each signal on a separate carrier frequency. To facilitate
separation of the signals at the receiver, the carrier frequencies are spaced sufficiently far apart so that signal
spectra do not overlap. Further, in order to separate the signals with readily sizeable filters, empty spectral
regions are placed between the signals. As such, the resulting spectral efficiency of the system is quite low.
In order to solve the bandwidth efficiency problem, orthogonal frequency division multiplexing was proposed,
which employs orthogonal tones to modulate the signals [4]. The tones are spaced at frequency intervals
equal to the symbol rate and are capable of separation at the receiver. This carrier spacing provides optimum
spectral efficiency. Although OFDM was proposed in the 1960’s it was not widely employed until the 1990’s,
largely because of significant circuit design issues, such as spurious frequency components and linearity
of amplifiers. Today, OFDM is a major contender for 4G wireless applications with significant potential
performance enhancements over existing wireless technology.
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