Page 254 - Anatomy of a Robot

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Symbol 'X' is under each crossing

X X COMMUNICATIONS 239

FIGURE 9-11 An open-eye diagram showing received signal traces crossing
two symbol space X’s

In the same manner, engineers can plot the recent data points to see how tightly they
cluster around the symbol points. A healthy communication link will have a very tight
clustering around the symbol points, and a sickly system will have them spread out in
a sloppy manner.
These are all ways to try to keep the physical link healthy, but steps can be taken in
the design of the communication link that will make it more robust. Many different
ways are available for looking at what these techniques represent. I prefer to think of
them all the same way: sending the data more than once.
In a situation where noise might ruin data inside the channel, the receiver is more
likely to get the data if it’s sent more than once. If the receiver is smart enough to rec-
ognize when data is corrupt, it can just wait for the second helping of the same data.
This becomes particularly important for robots in remote locations.
Sending duplicate data can be done in many different ways. Clearly, it’s possible to
just send the data twice or three times. But believe it or not, it’s possible to send the data
1.5 times, 1.1 times, or even 1.01 times.
Within certain bounds, robot designers can choose among communication protocol
codes that enable them to pick the amount of redundancy built into the communication
link. Since redundant data consumes bandwidth, this allows the designers to decide how
much of the bandwidth is wasted. Sending extra data effectively lowers the BER, since
errors are corrected at the receiver. Getting a lower BER is almost the equivalent of hav-
ing a better Eb/No. Thus, designers can say they get coding gain out of different com-
munication protocol codes. This coding gain can actually be realized since the coding
gain can be subtracted off the Eb/No in the actual channel to get the same BER in a
given situation. Add coding gain, decrease the Eb/No gain, and come out even. In prac-
tice, however, most engineers take the coding gain on top of the existing Eb/No and
realize their profit as a lower BER.