Multiple Input Multiple Output – Orthogonal Frequency Division Multiplexing
INTRODUCTION
The growing demand of multimedia services and the growth of Internet related contents
lead to increasing interest to high speed communications. The requirement for wide bandwidth
and flexibility imposes the use of efficient transmission methods that would fit to the
characteristics of wideband channels especially in wireless environment where the channel is
very challenging. In wireless environment the signal is propagating from the transmitter to the
receiver along number of different paths, collectively referred as multipath. While propagating
the signal power drops of due to three effects: path loss, macroscopic fading and microscopic
fading. Fading of the signal can be mitigated by different diversity techniques.
To obtain diversity, the signal is transmitted through multiple (ideally) independent
fading paths e.g. in time, frequency or space and combined constructively at the receiver.
Multipleinput- multiple-output (MIMO) exploits spatial diversity by having several transmit and
receive antennas. However the paper “MIMO principles” assumed frequency flat fading MIMO
channels. OFDM is modulation method known for its capability to mitigate multipath. In OFDM
the high speed data stream is divided into Nc narrowband data streams, Nc corresponding to the
subcarriers or subchannels i.e. one OFDM symbol consists of N symbols modulated for example
by QAM or PSK. As a result the symbol duration is N times longer than in a single carrier
system with the same symbol rate.
The symbol duration is made even longer by adding a cyclic prefix to each symbol. As
long as the cyclic prefix is longer than the channel delay spread OFDM offers inter-symbol
interference (ISI) free transmission. Another key advantage of OFDM is that it dramatically
reduces equalization complexity by enabling equalization in the frequency domain. OFDM,
implemented with IFFT at the transmitter and FFT at the receiver, converts the wideband signal,
affected by frequency selective fading, into N narrowband flat fading signals [1] thus the
equalization can be performed in the frequency domain by a scalar division carrier-wise with the
subcarrier related channel coefficients. The channel should be known or learned at the receiver.
The combination MIMO-OFDM is very natural and beneficial since OFDM enables support of
more antennas and larger bandwidths since it simplifies equalization dramatically in MIMO
systems. MIMO-OFDM is under intensive investigation by researchers.
