Digital Signal Processing Reference
In-Depth Information
in a static environment, where both the transmitter and the receiver are at a
fixed location or are moving only very slowly.
However, in a more or less dynamic multipath environment, the most com-
mon type of slow fading is represented by symbol fading: while the impulse
response of the channel may be quite stable over a period of several tens or
may be hundreds of transmitted symbols, the state of the channel which was
captured during the training burst of the packet will not provide an adequate
accuracy over the entire time frame of a long packet burst. A possible solution
for this would be to increase the refresh rate of channel response estimations
by spreading several training sequences over a single packet. The refresh fre-
quency must be chosen in order to obtain a good balance between the chan-
nel estimation overhead and the accuracy of the estimates. Another possibility
would be to start with a fixed (i.e. known by the receiver) training sequence
and then switch over to a regenerative channel estimation (rce) method. In
a sliding-window rce-based approach, the decoded symbols themselves in-
stead of the training sequence are used to make small but frequent updates to
previous estimates of the channel's transfer function. The response time of the
rce-technique depends both on the frame length of the sliding window and the
cut-off frequency of the noise shaping filter used by the algorithm. In order to
keep track of the changing channel, either the number of symbols in the scope
of the window or the cut-off frequency of the filter should be adjusted in ac-
cordance with the coherence time T
c
. Either way, there is a trade-off between
the speed at which updates become available and the accuracy and frequency
resolution of these channel updates.
If the stability of the channel is further reduced to the duration of only a few
symbols, it will become impossible for the receiver to retrieve useful infor-
mation about the response of the channel: there is no way to discern between
the random changes in the phase rotation of a multipath component and the
modulation of the signal itself. This brings us down again to the case of a fast
fading channel, where it has been already suggested that the failure to track the
channel response leads to a virtual increase of the noise floor of the signal. For
devices working under such conditions, the only recourse is to rely on a form
of diversity that is based on statistics. This is the topic of Section 4.4, in which
the use of statistical multipath diversity techniques is discussed in more detail.
Case study: impact of the coherence time on adaptive bit-loading
The main implementation issue concerning the adaptive bit-loading concept
of ofdm has not been adequately addressed until now. The problem is that
adaptive bit-loading requires an active intervention from transmission side:
the transmitter must adapt the information load of every single subchannel
