Digital Signal Processing Reference
In-Depth Information
1
0.8
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-1
0
5
10
15
20
25
30
35
Time domain training index
Figure 13.41
Time-domain training sequence
13.3.6.2 OFDM-based Digital Transmitter and Receiver
Owing to its robustness for multiple path fading, OFDM (orthogonal frequency-division multi-
plexing) is very effective for high-rate mobile wireless terminals. The scheme is used in many
modern communication system standards for wireless networks, such as IEEE 802.11(a) and
802.16(a), and digital broadcasting such as DAB, DVB-T and DRAM. OFDM uses multiple
orthogonal carriers for multi-carrier digital modulation. The technique uses complex exponentials.
This helps in formulating the modulation as computation of an inverse Fast Fourier Transform
(IFFT) of a set of parallel symbols, whereas FFT is performed at the receiver to extract the OFDM-
modulated symbols:
xðÞ¼
N
1
m¼
0
X
m
e
j
2
pm
T
t
with 0
t T
where
X
m
is the symbol to be transmitted on the subcarrier with carrier frequency f
m
¼
m/T. There are
N multi-carriers in an OFDM system, and the subcarriers
2
pm
T
t
e
j
are orthogonal to each other over interval [0, T]as
T
1
T
2
pl
2
pn
T
t
e
j
T
t
dt ¼ d
ln
e
j
0
At the transmitter, the bits are first mapped to symbol X
m
, and N of
these symbols (for m
¼
0,
...
, N
1) are placed in parallel for IFFT computation. The output of IFFT
is then serially modulated by a carrier frequency. To avoid intersymbol interference, and keeping the
periodicity of the IFFT block intact, a cyclic prefix from the end of the IFFT block is copied to the
start. The addition of a cyclic prefix is shown in Figure 13.42.
where
l; n ¼
01
;
2
; :::; N
1
:
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