heir energy in one narrow segment of the one of the contention slots and transmit. band but should spread their radiation over a significant The second backup mechanism for improving the part of the band, presumably a much larger segment than chances of getting a packet through is fragmentation. A actually required by the data bandwidth, that is, users large packet from the wired medium can be split into were required to apply spread spectrum techniques smaller fragments, each of which is more likely to be (a proviso since relaxed). In this fashion, it was hoped transmitted without a problem. If an error is encoun- that interference between collocated systems would be tered in one of the fragments, only that fragment needs minimized. CHAPTER 5.1 Wireless local area networks One approach to spreading the spectrum is to operate on many channels in some pseudo-random sequence; this is known as frequency hopping and is used in the classic FH PHYand the Bluetooth (802.15) PHY. 1 It is obvious that if symbol two neighboring radios are operating on separate channels, interference ought to be minimized except when by chance they happen to select the same frequency. The Figure 5.1-7 Barker sequence. disadvantage of this approach is that the channels must be quite narrow if there are to be a large number of them and multiplied together and the result added (a correlation of therefore little chance of overlap; the 1-MHz maximum the two sequences), the sum is small except when the width specified in the regulations limits the data rate offset is 0. Thus, by trying various offsets and checking that can be transmitted at the modest signal-to-noise the resulting sum, one can locate the beginning of an ratios (S/Ns) expected on low-cost, unlicensed, wireless instance of the sequence readily; that is, synchronization links. to the transmitter is easy. It is also possible to have sev- A quite distinct and rather more subtle approach is eral DSSS signals share the same frequency band with called direct-sequence spread DSSS. Direct-