Digital Signal Processing Reference
In-Depth Information
Remote reflections
Time
t
3
t
1
t
2
t
3
t
1
Receiver
t
2
Mobile user
Local scatterers
Noise
Transmitter
Remote reflections
Interferers
FIgure 1.1
Illustration of some of the effects of a radio channel. Local scatterers cause fading;
remote reflectors cause multipath and time dispersion, leading to ISI; mobility of the user or scatter-
ers causes a time-varying channel; reuse of frequencies and adjacent carriers cause interference.
of service. Then the system is adapted to the varying channel and interference condi-
tions so that the BER is maintained below the target value. In order to ensure this for
all types of channel and interference conditions, the system changes power, modula-
tion order, coding rate, spreading factor, etc. Note that this changes the throughput as
the channel quality changes. On the other hand, for the constant throughput case, the
adaptations are done to make sure that the effective throughput is constant, where the
BER might change.
In general, it is possible to classify the adaptation algorithms as link and transmitter
adaptation, adaptation of system resource allocation, and receiver adaptation. In the fol-
lowing sections, brief discussions of these adaptation techniques will be given.
1.2.1 Link and Transmitter Adaptation
A reliable link must ensure that the receiver is able to capture and reproduce the trans-
mitted information bits. Therefore, the target link quality must be maintained all the
time in spite of the changes in the channel and interference conditions. As mentioned
earlier, one way to achieve this is to design the system for the worst-case scenario so that
the target link quality can always be achieved.
If the transmitter sends more power for a specific user, the user benefits from it by
having a better link quality, but the level of interference for the other users increases
accordingly. On the other hand, if the user does not receive enough power, a reliable link
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