Is Coded OFDM really useful in short range applications?
Coded OFDM has been promoted by a number of vendors as the
"only" solution for delivering high performance wireless
communications for video, but is COFDM the only solution?
In
OFDM
data is transmitted over several subcarriers. These subcarriers
are evenly spaced and are read by a receiver during an "active
symbol period" where the signal is effectively demodulated.
Since the subcarriers are evenly spaced, each subcarrier can be
read independently.
The
receiver reads the signal to noise ratio of each sub carrier and
calculates a
bit error ratio. All of the bit error ratios for all
of the subcarriers are averaged to create the bit error ratio for
the data stream.
Soft
decision error correction can be applied to account for noise.
For instance, if 1V is expected to be a "1" but only .5V is
received, this would be interpreted to be a "1" rather than a
-1V which is equal to "0". Additionally,
convolutional coding is used for forward error correction.
COFDM
goes further and looks at the signal to noise ratio of each
subcarrier and sets a "priori" known as channel state
information or "CSI". The CSI can then be used to correct
for interference.
COFDM
usually corrects for single carrier interference such as CW or
analog TV interference, very well. Over long distances or in the
presence of a persistent RF interference source, there is no
question that COFDM is helpful. But in the majority of
applications this is simply not the environment that you operate
in.
Most
high bit rate video applications are short range (less than 2km)
with rapidly changing sources of obstruction, reflection, and
interference. In these environments are are no persistent
interference sources and the sampling time available to create a
CSI is just too short to be meaningful. In these types of
environments Coding, or the C in COFDM really is not that
helpful.
What
does work is antenna
diversity! With a receiver that is able to
simultaneously sample from two, space diverse antennas,
determine best
SNR from each antenna and then use the antenna
with the best SNR for that frame, or until the SNR shifts, the
system can immediately respond to changes in reflection,
refraction, interference and obstruction, without waiting for
enough sampling to create a CSI. Multipath can be compensated
for effectively through diversity.
In
short range, rapidly changing environments diversity is more
effective than coding.
Obviously if cost were no issue, then using both coding and
diversity is preferred, but generally COFDM solutions are
expensive as they do not use off the shelf components. Diversity
can be achieved with off the shelf components at a much lower
cost.
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