Digital Signal Processing Reference
In-Depth Information
Case 1: coherence time T
c
longer than packet length: block fading
T
s
Solution:
use training symbols
packet length
T
c
to find channel response.
Case 2: coherence time shorter than packet length: symbol fading
T
s
Solution:
- increase channel update rate.
- regenerative channel estimation.
T
c
packet length
Case 3: coherence time shorter than symbol duration T
s
: fast fading
T
c
Solution:
- treat as noise, use time diversity.
- use statistical-based diversity.
T
s
packet length
Figure 4.6.
The ratio between the coherence time (T
c
) of the channel and the
symbol period of a transmission (T
s
) determines whether the re-
ceiver operates under fast or slow fading conditions.
diversity scheme on which multi-antenna receivers are based assumes that an
uncorrelated multipath scattering process takes place at each of the antennas.
As such, the way in which a multi-antenna receiver deals with fast fading is
rather based on statistical probabilities than on some actively controlled chan-
nel tracking mechanism. A more detailed discussion of this topic is delayed
until the next section.
In practice, fast fading only occurs when the symbol rate is very low. Most
data transmission applications use a symbol duration which is below the chan-
nel coherence time. They are thus operated under slow fading channel condi-
tions, which does not necessarily imply that time varying channel parameters
do not form a problem for them. The reason for this is that the estimation of
the channel response is updated only once in a while, for example during the
synchronization preamble at the start of each transmission burst.
7
Basedonthis
consideration, the slow fading subdivision should be split further into symbol
fading and block fading channels [McE84]. In the latter case, the coherence
time is sufficiently long so that the channel response can be considered invari-
ant during the entire period of at least one transmission burst. This is the case
7
In the gsm system, a fixed training sequence is used. This 26-bit training sequence is located in the middle
of each transmission slot, to provide the best channel estimation for both the first and the second part of the
received packet.
