Signal Processing for Wireless Transceivers - Signal Processing Systems - page 181

a

Digital
front-end &
RF system

Coding &
modulation

Serial-to-
parallel

-point
IFFT

Add cyclic
prefix

Parallel-to-
serial

Channel

Remove
cyclic
prefix

De-
modulation
& decoding

RF system
& digital
front-end

Serial-to-
parallel

Parallel-
to-serial

Channel
equalizer

-point
FFT

b c

Undi s tor t ed transmitt ed s pec trum

Direct component:

symbol -1 symbol symbol +1

Channel frequency response

1st delayed component:

symbol -1 symbol symbol +1

2nd delayed component:

symbol -1 symbol symbol +1

Distorted receive d s pect ru m

FFT
window

Fig. 5 ( a ) Basic OFDM transmission chain. ( b ) Effect of channel frequency selectivity. ( c ) Effect
of multipath delays not exceeding the channel delay spread in CP-OFDM

As illustrated in Fig. 5 , OFDM solves the problem by splitting the high-rate sym-
bol sequence into a high number ( N ) of lower-rate sequences which are transmitted
in parallel, over a spectrally compact multiplex of orthogonal subchannels. Due to
the increased symbol interval in the subchannels, the effects of channel dispersion
are reduced, and the channel frequency response within each subchannel is, at most,
mildly frequency selective. Furthermore, a cyclic prefix (CP) is commonly inserted
in front of each OFDM symbol. The idea of CP is that it will absorb the variations
in the delays of different multipath components of the channel, preventing ISI if the
length of the CP is at least equal to the maximum delay spread of the channel. In this
case, the effect of the channel can be modeled as a cyclic convolution. Consequently,
the channel effect can be precisely modeled as flat fading at subcarrier level, and can
be compensated by a single complex multiplication for each data symbol modulated
to a subcarrier [ 91 ] .
In existing specifications, the FFT size of OFDM systems ranges from 64 in
IEEE 802.11a/g WLAN to 32k in DVB-T2 [ 128 ] . The subcarrier spacings range,
correspondingly, from 325 kHz to 279 Hz. As an important example, 3GPP-LTE
uses 15 kHz subcarrier spacing and up to 20 MHz bandwidth, the maximum FFT-
size being 2k [ 30 , 117 ] .
The practical implementation of OFDM utilizes inverse fast Fourier transform
(IFFT) for multiplexing each block of parallel data symbols. Correspondingly, FFT
is used for demultiplexing the block of complex sample values corresponding to the