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ABSTRACT
OFDM (orthogonal frequency-division multiplexing) is one of the key digital communication technologies of the current decade. The first part of this paper presents the fundamentals of OFDM and its benefits in the presence of multipath propagation in a tutorial-like fashion. The second part details on some of the most important aspects of OFDM transceiver implementation: concept of receiver channel filtering and A/D conversion, radio impairment compensation (I/Q mismatch), and OFDM demodulator (FFT) design.
OFDM Basics
1. Introduction
The basic principle of OFDM is to split a high-rate DataStream into a number of
lower rate streams that are transmitted simultaneously over a number of
sub carriers. Because the symbol duration increases for lower rate parallel
sub carriers, the relative amount of dispersion in time caused by multipath delay
spread is decreased. Intersymbol interference is eliminated almost completely
by introducing a guard time in every OFDM symbol. In the guard time, the
symbol is cyclically extended to avoid inter carrier interference.
In OFDM design, a number of parameters are up for consideration, such
as the number of sub carriers, guard time, symbol duration, subcarrier spacing,
modulation type per sub carrier. The choice of parameters is influenced by
system requirements such as available bandwidth, required bit rate, tolerable
delay spread, and Doppler values. Some requirement are conflicting. For
instance, to get a good delay spread tolerance, a large number of sub carriers
with small sub carrier spacing is desirable, but the opposite is true for a good
tolerance against Doppler spread and phase noise.
2. Data transmission using multiple carriers
An OFDM signal consists of a sum of sub carriers that are modulated by using
Phase shift keying (PSK) or curvature amplitude modulation (QAM). If i d are
The complex QAM symbol, s N is the number of sub carriers, T the symbol
duration, and
T
f f i i = + 0 the carrier frequency, then one OFDM symbol
starting at s t = t can be written as:
In the literature, often the equivalent complex notation is used, which is
given by (2). In this representation, the real and imaginary parts correspond to
the in-phase and quadrature parts of the OFDM signal, which have to be
multiplied by a cosine and sine of the desired carrier frequency to produce the
final OFDM signal. Figure (1) shows the operation of the OFDM modular in
block diagram.
As an example, figure (2) shows four sub carriers from one OFDM
signal. In this example, all submariners have the phase and amplitude, but in
practice the amplitudes and phases may be modulated differently for each
sub carrier. Note that each sub carrier has exactly an integer number of cycles in
the interval T , and the number of cycles between adjacent sub carries differs by
exactly one. This properly accounts for the orthogoality between sub carriers.