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248 CHAPTER NINE
We need to realize that a unidirectional communications link can only be used suc-
cessfully if the following two conditions are met:
The receiver’s target error rate must be set so it is acceptable given the specifica-
tions for operation. We can pretty well determine ahead of time what error rate
will be acceptable for operation of the robot.
The data received at the receiver must be of sufficient quantity and quality to keep
the data rate high enough and the receiver’s error rate below the acceptable target
value.
To accomplish the second goal, we should review the tools available. In the case of
bidirectional communications, we already talked about block encoding, channel tuning,
and retransmission. Since both channel tuning and retransmission are impossible with-
out a reverse communications channel, we should examine encoding further.
We’ve already discussed block encoding and checksums at some length. Parity bits
and RS encoding are tools that can be used in a unidirectional communications link.
Often, the name given to unidirectional error correction methods is forward error cor-
rection (FEC). It has this name because all error correction information moves forward;
no reverse communication link exists. Here are a few sites about FEC:
www.its.bldrdoc.gov/fs-1037/dir-016/_2298.htm
http://research.compaq.com/SRC/articles/199711/error_correction.html
www.eccpage.com
Two other tools have proven valuable, namely convolution codes and concatenated
codes.

CONCATENATED CODES

The general idea behind concatenated codes is to herd randomly spaced errors into one
spot where we can dispatch them efficiently and reliably. That may be a gross oversim-
plification, but it is the way I view the technique (see Figure 9-12).
Figure 9-12 shows the typical arrangement for a communications system using con-
catenated codes. MPEG video signal data is broadcast in DVB format over satellites
using this type of concatenated coding. We’ll discuss MPEG compression and the DVB
format later. The description of each block within the figure is as follows:
MPEG compressor Broadcast video signals, generated by a video camera, are
accepted by the input to the MPEG compressor. The compressor has several dig-
ital signal processing (DSP) computation engines that compress the signal. We
will discuss data compression later.