Page 262 - Anatomy of a Robot

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COMMUNICATIONS 247
Code changes
If a communication link begins to deteriorate, another technique
that can be used is a coding change. By prior agreement, the receiver and trans-
mitter can pause and change coding methods. Stronger error correction codes
translate directly to a coding gain that can be added to the Eb/No. As we discussed
before, this generally means that an extra amount of redundant data will be sent
in one form or another. Since extra data will be sent over the channel, and since
the channel’s Eb/No value is already marginal, it makes sense to move to a lower
bandwidth for the data transmission. If less actual data is sent, more redundant
data can be appended, and the channel power per bit remains the same.
A specific example of this can be found in MPEG video transmissions. Most
MPEG transmissions are unidirectional, but some video links do have reverse con-
trol channels of a much lower bandwidth. Although video may be sent over uni-
directional satellite links, the reverse control channel can be established over the
phone.
At the transmitter site, an MPEG compressor takes a video signal and compresses
it using the MPEG algorithms. The compressor has a choice of several compres-
sion algorithms that can squeeze the video picture down to smaller and smaller
amounts of data (at the cost of picture quality). The compressor then encodes the
MPEG data for transmission through the channel using Viterbi and RS codes that
append redundant data. The receiver uses the Viterbi and RS codes to eliminate
errors and then decompresses the video picture.
If the receiver cannot correct all the errors, the picture will begin to break up. The
receiver can use the reverse control link to request a better channel coding method.
The compressor at the transmitter site then uses a stronger compression algorithm
to reduce the amount of data sent and chooses a stronger Viterbi and RS code com-
bination. The channel coding increases the data back to the original amount again.
The receiver will then be able to correct all the errors and present a clean picture.
The video image may not be as good as before (because of the extra compression),
but at least the images are going through.

UNIDIRECTIONAL COMMUNICATION CHANNELS

We’ve already discussed or mentioned many of the methods used to decrease errors in
communication channels. Except for retransmission requests, which are impossible in
a unidirectional communication channel, most of the same techniques can be used.
We’ll discuss a few more of the protocols used, but we won’t go into great depth.
However, to adequately specify a communications link for a robot, we must understand
the options.