Cryptography Reference
In-Depth Information
in dB, is therefore equal to:
10 log
1
1+Δ
/t
s
(2.190)
The second consequence is that the orthogonality of the carriers must be ensured
so as to be able to separate these carriers at reception, that is, separating them
by a quantity
1
/t
s
. The band of frequencies occupied by the signal OFDM with
a guard interval is therefore:
B
=
N
−
1
(2.191)
t
s
that is, an expansion of the bandwidth compared to a system without a guard
interval of
1+Δ
/t
s
.
In the presence of a guard interval, we should therefore choose
Δ
so as
to minimize the degradations of the signal to noise ratio and of the spectral
eciency, that is, choose the smallest possible
Δ
compared to duration
t
s
.
Transmission with equalization at reception
In the presence of a frequency selective channel, we can use a linear single-
carrier (M-PSK, M-QAM) modulation at emission and correct the ISI created
by the channel by an equalizer. Some equalizer architectures are presented in
Chapter 11.
If we compare the OFDM approach and single-carrier transmission with
equalization, in terms of complexity of implementation, the diculty of real-
izing an OFDM transmission lies in the modulator whereas with equalization,
it lies in the receiver.
