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M. El Ghzaoui and S. Das
of the signal is lesser than the coherence band of the channel which mean that the
channel is practically independent of the frequency, otherwise, the channel will be
frequency selective, and inter-symbol interference will occur in the time domain. In
order to Overcome the problem of frequency selectivity of the transmission channels,
multi-carrier modulation is used.
The multi-carrier modulation technique [15] which has many advantages
compared to the single-carrier which uses equalizers governed by filters of low order
and limited complexity. This limits the performance of this type of system especially
for channels whose characteristics undergo significant variations such as wireless
channel for example. On the other hand, multi-carrier systems operate efficiently on
this type of channel, since multi-carrier modulation (MCM) systems subdivide the
bandwidth of the transmission channel into several subchannels with narrow bandwidths. Its characteristics remain unchanged in terms of SNR, which makes the use
of equalizers simple, sufficient, and at the same time efficient.
Indeed, when a single-carrier signal occupying a frequency band B is sent over
a frequency-selective channel like the THZ channel, this signal is considered to
be expected to be particularly exaggerated by strong frequency fading due to the
propagation channel (frequency selectivity), which generates an increase in BER,
this being even more true if B is large. However, if we use a signal with N carriers,
that mean we will be transmitting data on narrower frequency bands, equal to Bu/N
which is less than the coherence band, so as if we transmit information on flats
channels. Thus, the larger N, the transmission is carried out on frequency bands
where the channel is constant which restrict the result of strong fading of the channel,
and therefore the interference between symbols. System equalization then amounts
to performing a single coefficient equalization per carrier.
4 Baseband OFDM Signal
Multi-carrier transmission systems known as OFDM are based on the principle of
orthogonality of modulation filters. The modulation of a frame realized by a inverse
discrete Fourier transformation (IDFT) [16]. Let I k be the sequence of symbols to
be sent at the rate Ts. These symbols generally belong to a finite alphabet resulting
from a given modulation constellation. The initial data symbol is distributed over N
parallel symbols, each symbol being transmitted on one of the N orthogonal carrier
to one another and modulated at low speed. This modulation class is called classic
OFDM (OFDM based on Weyl–Heisenberg families of density ρ = 1).
Given its easy implementation and its robustness with respect to narrowband
disturbances and frequency selectivity of the channel, multi-carrier modulation has
been very successful. The elementary idea behind OFDM is to distribute the information to be transmitted over different adjoining but disjoint frequency bands. To
understand the concept of orthogonality between subcarriers, it is easier to reason in
the frequency domain. Suppose that two symbols of duration T B are transmitted on
M. El Ghzaoui and S. Das
of the signal is lesser than the coherence band of the channel which mean that the
channel is practically independent of the frequency, otherwise, the channel will be
frequency selective, and inter-symbol interference will occur in the time domain. In
order to Overcome the problem of frequency selectivity of the transmission channels,
multi-carrier modulation is used.
The multi-carrier modulation technique [15] which has many advantages
compared to the single-carrier which uses equalizers governed by filters of low order
and limited complexity. This limits the performance of this type of system especially
for channels whose characteristics undergo significant variations such as wireless
channel for example. On the other hand, multi-carrier systems operate efficiently on
this type of channel, since multi-carrier modulation (MCM) systems subdivide the
bandwidth of the transmission channel into several subchannels with narrow bandwidths. Its characteristics remain unchanged in terms of SNR, which makes the use
of equalizers simple, sufficient, and at the same time efficient.
Indeed, when a single-carrier signal occupying a frequency band B is sent over
a frequency-selective channel like the THZ channel, this signal is considered to
be expected to be particularly exaggerated by strong frequency fading due to the
propagation channel (frequency selectivity), which generates an increase in BER,
this being even more true if B is large. However, if we use a signal with N carriers,
that mean we will be transmitting data on narrower frequency bands, equal to Bu/N
which is less than the coherence band, so as if we transmit information on flats
channels. Thus, the larger N, the transmission is carried out on frequency bands
where the channel is constant which restrict the result of strong fading of the channel,
and therefore the interference between symbols. System equalization then amounts
to performing a single coefficient equalization per carrier.
4 Baseband OFDM Signal
Multi-carrier transmission systems known as OFDM are based on the principle of
orthogonality of modulation filters. The modulation of a frame realized by a inverse
discrete Fourier transformation (IDFT) [16]. Let I k be the sequence of symbols to
be sent at the rate Ts. These symbols generally belong to a finite alphabet resulting
from a given modulation constellation. The initial data symbol is distributed over N
parallel symbols, each symbol being transmitted on one of the N orthogonal carrier
to one another and modulated at low speed. This modulation class is called classic
OFDM (OFDM based on Weyl–Heisenberg families of density ρ = 1).
Given its easy implementation and its robustness with respect to narrowband
disturbances and frequency selectivity of the channel, multi-carrier modulation has
been very successful. The elementary idea behind OFDM is to distribute the information to be transmitted over different adjoining but disjoint frequency bands. To
understand the concept of orthogonality between subcarriers, it is easier to reason in
the frequency domain. Suppose that two symbols of duration T B are transmitted on