Digital Signal Processing Reference
In-Depth Information
magnitude response of channel [dBr]
5 km/h
slow-fading channel
coherence time 802.11a = 16 ms
time
50 km/h
fast-fading channel
coherence time 802.11a = 1.3 ms
time
Figure 2.7.
The short coherence time of a fast-varying channel prevents
the use of adaptive bit-loading in ofdm-based systems.
This problem becomes increasingly serious in time-varying (fast fading)
channels with a short
coherence time
as it will degrade the performance
of the adaptive loading technique. As shown in Figure 2.7, the coherence
time for a moving 802
.
11a device is between 16 ms (5 km/h) and 1
.
3ms
(50 km/h) [Sib02]. In the assumption that packets are transmitted without
collisions between co-channel users, the minimum time delay between two
consecutive transmitted packets from an 802
.
11a access point to a mobile
client is 570
μ
s [Gas05]. The rate at which the transmitter gets updated on
changing channel characteristics is thus sufficient for a slowly moving mobile
station, but it is obvious that adaptive loading methods are predetermined to
fail in fast-changing environments.
Remark that the current 802
.
11a/g standard implementations do not employ
adaptive loading at all. The transmitter is not informed on the channel condi-
tions experienced by the receiver and a single modulation depth is used over
all ofdm subbands. In an attempt to gloss over the excessive bit error rates in
some of the subbands, a channel equalization filter partially based on training
data symbols is commonly used in practical ofdm receiver implementations.
12
12
In contrast to the transmitter, the receiver side is not standardized in the 802
.
11a/g system.
