Information Theory, Inference, and Learning ... - Inference Group

Copyright Cambridge University Press 2003. On-screen viewing permitted. Printing not permitted. http://www.cambridge.org/0521642981You can buy this book for 30 pounds or $50. See http://www.inference.phy.cam.ac.uk/mackay/itila/ for links.226 13 — Binary CodesRelationship between bit error probability and block error probability: The bitguessingand block-guessing decoders can be combined in a single system: wecan draw a sample x n from the marginal distribution P (x n | r) by drawinga sample (x n , x) from the joint distribution P (x n , x | r), then discarding thevalue of x.We can distinguish between two cases: the discarded value of x is thecorrect codeword, or not. The probability of bit error for the bit-guessingdecoder can then be written as a sum of two terms:p G b = P (x correct)P (bit error | x correct)+ P (x incorrect)P (bit error | x incorrect)= 0 + p G BP (bit error | x incorrect).Now, whenever the guessed x is incorrect, the true x must differ from it in atleast d bits, so the probability of bit error in these cases is at least d/N. Sop G b ≥ d N pG B.QED.✷Solution to exercise 13.20 (p.222). The number of ‘typical’ noise vectors nis roughly 2 NH2(f) . The number of distinct syndromes z is 2 M . So reliablecommunication impliesM ≥ NH 2 (f), (13.55)or, in terms of the rate R = 1 − M/N,R ≤ 1 − H 2 (f), (13.56)a bound which agrees precisely with the capacity of the channel.This argument is turned into a proof in the following chapter.Solution to exercise 13.24 (p.222). In the three-player case, it is possible forthe group to win three-quarters of the time.Three-quarters of the time, two of the players will have hats of the samecolour and the third player’s hat will be the opposite colour. The group canwin every time this happens by using the following strategy. Each player looksat the other two players’ hats. If the two hats are different colours, he passes.If they are the same colour, the player guesses his own hat is the oppositecolour.This way, every time the hat colours are distributed two and one, oneplayer will guess correctly and the others will pass, and the group will win thegame. When all the hats are the same colour, however, all three players willguess incorrectly and the group will lose.When any particular player guesses a colour, it is true that there is only a50:50 chance that their guess is right. The reason that the group wins 75% ofthe time is that their strategy ensures that when players are guessing wrong,a great many are guessing wrong.For larger numbers of players, the aim is to ensure that most of the timeno one is wrong and occasionally everyone is wrong at once. In the game with7 players, there is a strategy for which the group wins 7 out of every 8 timesthey play. In the game with 15 players, the group can win 15 out of 16 times.If you have not figured out these winning strategies for teams of 7 and 15,I recommend thinking about the solution to the three-player game in terms