Digital Signal Processing Reference
In-Depth Information
Case 1: multipath components arrive within the duration of one symbol period T
s
receiver
transmitter
channel
local scattering causes
intra-symbol interference
T
s
T
s
time
time
frequency-flat
channel
Rayleigh or
Rician fading
|H(f,t
i
)|
2
|H(f,t)|
2
df
∫
t
1
+
t
3
t
2
t
1
t
2
t
3
frequency
time
Figure 4.2.
A transceiver system experiences frequency-flat fading if the duration of
the transmitted symbols is longer than the delay spread of the channel.
The loss of signal that is caused by frequency flat fading is a problem which
is very difficult to avoid. In a digital transmission system, it may be possible
to survive a temporary loss of the signal by employing an error coding mech-
anism which offers a sufficient level of time-diversity. However, this approach
is known to not perform very well in a quasi-static environment. If both the
transmitter and the receiver are stationary or moving very slowly, the duration
of a deep fade interval can become longer than the maximum time frame over
which the error coder is able to spread its data needed to reconstruct the orig-
inal information. Moreover, in a real-time communication system, the use of
coding schemes with a better performing time-diversity is not always allowed.
This is, for example, the case in low-latency communication systems such as
gsm. To avoid long-duration fades, the gsm system employs a slow frequency
hopping mechanism. During the transmission period of a single transmission
slot (0
.
577 ms), the frequency remains unchanged. Different packets however,
are transmitted at different frequencies at a rate of 217 hops/s.
4
Even when
operated under static channel conditions, it seems that the addition of the ar-
tificial extra layer of frequency diversity can preserve the communication link
from unacceptable long outages. By now, it should become clear that a flat
fading channel offers less opportunities to counter the effects of interference
than a frequency-selective fading channel does. In the latter case, of course,
4
The hopping algorithm is distributed by the base station over the broadcast control channel (bcc).
