ans, SO that this drift is small same for two consecutive chips, a 0 has been transmitted. compared with the /2 radian changes we're trying to 2.4835 12 MHz 21.5 MHz channel: Figure 5.1-9 Nonoverlapping 802.11 channels, U.S. ISM band. CHAPTER 5.1 Wireless local area networks amplitude = 1 amplitude = 1 phase= phase= phase of previous PLCP header, basic rate extended rate Figure 5.1-10 802.11 Classic modulations: DBPSK (left) and DQPSK (right). resolve between chips, we need the oscillator to maintain propagation indoors is unlikely to achieve the same per- phase to about 3 parts per billion. This is implausibly formance as unimpeded ideal propagation in free space. difficult and expensive. (Of course, more clever methods We investigate the distinctions in more detail elsewhere; are available to perform such a task at more modest for the present let us simply add 30 dB of path loss for requirements on the hardware; the calculation is done to a long indoor path, leaving us with an allowed range of provide a frame of reference.) On the other hand, to (2000/101.5) = 63 m. The transmit power levels and re- maintain phase lock between successive chips at ceiver sensitivities envisioned in the 802.11 standard 11 Mbps, we only need hold this accuracy over 1/11 usec: make a lot of sense for indoor communications over ranges 220 cycles, or 1400 radians. The required accuracy in this of 10-100 m, just as one might expect for a technology case is 200 parts per million, which is easily achieved designed for the LAN environment (see section 5.1.1). with inexpensive crystal-referenced synthesized sources. The receive sensitivity requirement also tells us some- The price of this alternative is merely that we lose any thing about the radio we need to build. Recall that a QPSK information present in the first symbol of a sequence: signal needs to have an (S/N) of about 12.5 dB for rea- because we can exploit our synchronization sequence sonably error-free reception. At first blush, one would (which doesn't carry any data anyway) f