Digital Signal Processing Reference
In-Depth Information
Case study: frequency diversity in the OFDM system
It was shown earlier that ofdm solves the problem of isi by splitting up the
data stream over
n
parallel narrowband transmission channels. If the symbol
duration T
s,n
applied to subchannel
n
is longer than the delay spread of the
channel, then isi is avoided and - in theory - will not cause any additional
signal losses. The multipath resolution capabilities of ofdm lie in the fact that
it is possible - at least in theory - to redistribute the data load from subchan-
nels with an insufficient signal quality to channels with a better signal-to-noise
ratio. No signal energy is lost or created due to isi in a frequency-selective
channel, so the Shannon capacity is not affected under ideal circumstances.
Of course, this is only true when the appropriate subband bit-loading tech-
niques are employed. Failing to do so indeed leads to performance loss,
but this effect should be classified as a problem of implementation loss (il)
rather than a fundamental limitation of ofdm. Since adaptive bit-loading re-
quires a difficult-to-implement support from transmission side, it is currently
not supported by the 802
.
11a/g standards. All subchannels share the same
modulation depth, which has a serious impact on the performance of the
802
.
11a/g system. Where in the ideal case of adaptive bit-loading the loss of
snr due to destructive intersymbol interference in one band is regained in
another subband, failing to support adaptive loading downgrades the perfor-
mance of ofdm: channels with constructive interference are not used at their
peak capabilities, while the modulation depth is too high for subchannels with
destructive fading.
For these reasons, bit-interleaved coded modulation (bicm) was introduced
in the 802
.
11a/g standard [Cai98, Set06]. Interleaving information across the
ofdm subchannels merely spreads the excessive ber experienced in some of
the subchannels uniformly over a larger sequence of data bits. Bit-interleaving
in a non-adaptive ofdm scheme must be seen as a method which transforms
the problem of isi into a problem of white noise, an issue which can be eas-
ily solved by standard (modulation-unaware) error coding schemes. In this re-
spect, the way in which the 802
.
11a/g system employs ofdm has very little
to do with true rake receiver architectures, in which it is attempted to re-
solve and recombine as much of the energy of several multipath components
as possible.
4.3
Coherence time
Apart from a mobile transmitter-receiver setup, the varying characteristics of
a wireless channel are also caused by moving objects in the propagation path
from the transmitter to the receiver. In either of the foregoing cases, the net
result is a changing length of the propagation path over time. From the point of
