Page 255 - Anatomy of a Robot
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09_200256_CH09/Bergren   4/17/03  11:24 AM  Page 240
                             240 CHAPTER NINE
                               This happens in satellite communications all the time. In fact, most satellite com-
                             munication links are designed and specified with the coding gain built right into the
                             communication protocol. Since many of the codes have parametric options, it is possi-
                             ble for the operator of a satcom link to pick a code on the fly that matches the quality
                             of the channel. If the satcom link has a low No, then little coding gain may be needed
                             and the data rate can go up. If the satellite link has a high No, then a stronger coding
                             gain may be needed to maintain the quality of the data at the expense of a lower
                             data rate.


                             ERROR DISTRIBUTION

                             Robot designers must also take a very careful look at the channel. It’s one thing to pre-
                             dict the BER from the modulation method and coding, but it does no good at all if
                             sunspots wreck the transmission for minutes or seconds at a time. Error rates are con-
                             catenated; all the links in the communication chain must be functioning at the same
                             time. An error in any one link may, or may not, be corrected in another link down the
                             chain.
                               In addition, noise is unpredictable. That’s why they call it noise in the first place.
                             Granted, it has certain mathematical properties that are dependable in the average, but
                             random events can lead to a burst of errors that may not be caught by the coding scheme
                             chosen. We must look at the density and distribution of errors in the channel, in addi-
                             tion to the error rate.
                               One thing further must be said about the distribution of errors. Some coding schemes
                             (like Viterbi, which we’ll get to soon) gather up errors all together in a net and correct
                             them all at once. The problem is, if something goes wrong and they cannot all be cor-
                             rected, the net rips and a local flood of errors happens that would not have occurred nat-
                             urally in such a manner. This type of situation is actually caused by the error-correction
                             coding scheme. The system must be prepared to survive such an event. We’ve probably
                             all seen such error bursts in the middle of soccer games from overseas. The game goes
                             along fine until there’s a massive burst of black and green blocks on the screen. We’ll
                             see why this occurs shortly.
                               Let’s take a look at some of the coding methods that send duplicate data. The differ-
                             ent techniques have the same basic purpose: to decrease the error rate by sending some
                             of the data more than once. The techniques are basically divided into two different meth-
                             ods. Some communication channels are bidirectional, and many are not. A bidirectional
                             communication channel enables the retransmission of data by request of the receiver; a
                             unidirectional communication channel does not.
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