Digital Signal Processing Reference
In-Depth Information
Figure 1.5.
A multipath channel causes constructive and destructive interference.
When consecutive symbols start to interfere with each other, the effect
is called intersymbol interference (isi).
spread results in a flat fading channel for most narrowband communication
systems [Tse05], wideband communications suffer from frequency-selective
fading in their frequency band. This means that some frequencies in the trans-
mission band will be attenuated due to self-interference and other frequencies
experience exactly the opposite effect due to the same mechanism.
From a time-domain point of view, fading is caused by multiple delayed copies
of the transmitted symbol stream which arrive at the receive antenna. If the
symbol period of the transmitted stream is smaller than the delay spread of
the channel, this results in intersymbol interference (Figure 1.5). The problem
becomes increasingly severe as the symbol rate goes up. Some of the more
recent wireless lan systems have abandoned the concept of a single carrier,
exactly because of the intersymbol interference caused by a high symbol rate.
For example, the 802
.
11a standard uses a 52-carrier frequency division mul-
tiplexing system (ofdm [Eng02]), where each of the subcarriers is loaded
with only a fraction of the total symbol rate. Thanks to the increased symbol
duration, the susceptibility of a single subchannel to isi is strongly reduced.
Information is thus spread over multiple parallel data streams, which are trans-
mitted in parallel over the frequency channels. If it happens that some of the
subchannels become unavailable due to fading or narrowband interference, in-
formation can be redistributed over the remaining subchannels.
9
Of course, the
data throughput of the transmission is still affected, but the bit error rate (ber)
9
Adaptive loading of subchannels is not included in the current version of the IEEE 802
.
11a-1999 standard.
All 52 subcarriers use the same modulation depth.
