Digital Signal Processing Reference
In-Depth Information
according to the signal quality of that particular subchannel as seen by the re-
ceiver. Unfortunately, telepathy is one of the things the transmitter does not
have at his disposal, so in a certain sense the receiver is obliged to keep the
transmitter informed over the current status of the channel. The most conve-
nient way for the receiver to do this is to release regular updates on the status
of the channel. The updates can be send in the first available transmission slot
or are piggybacked onto the next outgoing data packet. For a static channel of
which the frequency response varies slowly over time, the feedback system is
able to keep track of changing channel conditions and can avoid transmission
over subchannels with an insufficient snr.
The real problem emerges when the transmitter or the receiver are moved or
are being operated in a non-stationary environment. If one or more of these
conditions are met, it follows that the coherence time will shorten. As a con-
sequence, by the time the transmitter is ready to send the next ofdm packet to
the receiver, the adaptive bit-loading mechanism is already acting on outdated
information.
For example, in a fairly static wlan environment, with a receiver moving
slower than 5 km/h (3 mph), measurements have shown [Sib02] that the co-
herence time is better than 10 ms. Suppose that the 802
.
11a/g system transmits
10 ofdm symbols in a single transmission burst. Taking into account the mini-
mum interframe spacing (sifs/difs) [Gas05] and the length of a single ofdm
symbol including the cyclic prefix, it follows that the minimum time frame be-
tween two consecutive packets from the transmitter is 570
s. An 802
.
11a/g
system using the adaptive bit-loading technique should thus be able to cope
with a coherence time of 10 ms.
However, these calculations were performed under the assumption that no
packet collisions have been occurred and in the mean time, and no other com-
peting systems were able to claim the next transmission slot. Also, for this
example, a very low number of 10 ofdm symbols was chosen as the payload
of one packet burst, which generates a lot of overhead data (e.g. the synchro-
nization preamble). The situation becomes even worse when the receiver is
moving at speeds above 35 km/h (22 mph), which reduces the coherence time
even further to less than 1
.
37 ms [Sib02]. It should be clear that under such cir-
cumstances, the slow feedback channel between receiver and transmitter won't
be able to keep up with the rapidly changing environment. The only way left
for an ofdm-based system to cope with a block fading channel is to avoid
the slow feedback channel in the first place and to use a combination of fec,
receiver-side channel equalization and bicm to prevent that high concentra-
tions of errors emerge at the input of the decoder.
μ
