Wireless Network Design: Optimization Models and Solution ...

2 Introduction to Wireless Communications 37
cient system to transmit information over parallel channels. Consider for instance a
simple frequency division multiplexing (FDM) system in which the available bandwidth
B Hz is divided into m = B/b non-overlapping segments of width b Hz. A
traditional modulation pulse and carrier can be used in each of these segments.
However, in an OFDM system, the carrier frequency on each of these smaller segments
(typically referred to as subcarriers) are selected in such a manner that they
are orthogonal with each other as shown in Figure 2.17. As can be seen from the
figure, at the center frequency of each subcarrier, the contributions from the other
subcarriers is 0. This orthogonality is the key to the superior spectral efficiency of
OFDM since it allows numerous subcarriers to be used within the given band without
causing any interference between subcarriers.
There are several variations of OFDM [24] and it has become widely used in
next generation wireless standards such as WiMAX. OFDM based signaling is also
used in wire-line systems where it is frequently referred to as discrete multitone
modulation (DMT). In this chapter, we only focus on a very basic OFDM system
and provide the key insights into its performance.
Consider the basic blocks in an OFDM system as shown in Figure 2.18. The input
data stream is first parsed into groups of N bits. Without loss in generality consider
the transmission of one input vector X of length N. This input is first passed through
Fig. 2.16 Simple illustration of FDMA, TDMA and frequency hopping CDMA