Signal Processing for Wireless Transceivers - Signal Processing Systems - page 180

De-
inter-
leaver

LS
ch.est

FFT FFT

Equalizer Decoder

y y

SAGE
ch.est.

Soft
decisions

Inter-
leaver

IFFT

Fig. 4 Decision-directed channel estimation in MIMO receiver

3.1 Waveform Processing in OFDM Systems

The coding and modulation block produces a sequence of typically QAM modulated
symbols, and the purpose of the waveform generation block is to produce a digital
sample sequence which corresponds to the discrete-time baseband version of the
final RF signal to be transmitted. Likewise, on the receiver side the waveform
processing block receives the corresponding digital sample sequence, but effected
by additive noise and interferences as well as various distortion effects, and produces
a sample sequence corresponding to the QAM modulated symbol sequence at the
coding & modulation block output.
In today's wireless communication system, various waveforms are utilized
including linear single carrier modulation, i.e., QAM-type symbol sequence with
Nyquist pulse shaping, Gaussian minimum shift keying (GMSK), and various types
of spread-spectrum techniques, including direct sequence (DS) spread-spectrum
with code-division multiple access (CDMA) [ 13 , 118 ] . However, we focus here
on the celebrated multicarrier transmission technique called orthogonal frequency-
division multiplexing (OFDM) [ 17 , 76 , 91 , 96 , 132 ] , which is the basis for most of the
recent and emerging high-data-rate wireless systems, including 802.11a/g/n WLAN,
DVB-T/H, WiMAX and 3GPP-LTE.

3.1.1 OFDM Principle

A fundamental issue in wireless communications with increasing data rates is
the complexity of the channel equalization. Channel equalization is needed in
practically all wireless communication systems for compensating the effects of the
multipath propagation channel, which appears as frequency dependency (frequency-
selectivity) of the channel response experienced by the transmitted waveform. More
importantly, this effect introduces dispersion to the symbol pulses which appears as
inter-symbol interference (ISI), and eventually as errors in detecting the transmitted
symbol values [ 13 ] . Traditional time-domain techniques for channel equalization,
based on adaptive filtering or maximum likelihood sequence detection, would have
prohibitive complexity at the signal bandwidths adopted in many of the recent
communication standards.