n. tation angle of the whole code relative to the previous The development of the symbol code word or chip symbol (one of four values). There are a total of 256 sequence may be clarified by an example worked out per (64 X 4) possibilities and the chosen one is output as Figure 5.2-13. Let's say the 8-bit data sequence for serial data. At the 5.5 Mbps rate there are four code a symbol is d = do... d7 =10101101. From the phase words to choose from and after code rotation a total of table of Figure 5.2-13 we find the angles Q: Q1 = 180°, Q2 16 choices from which to decide on the output data. = 180°, Q3 = -90°, Q4 = 90°. Now summing up these To maintain compatibility with earlier non high-rate values to get the angle ai of each complex chip, then systems, the DSSS frame format shown in Figure 5.2-11 is taking the cosine and sine to get I and Qi, we summarize retained in 802. 11b. The 128-bit preamble and the header the result in the following table: are transmitted at 1 Mbps while the payload MPDU can be sent at a high rate of 5.5 or 11 Mbps. The long and slow preamble reduces the throughput and cancels some of the advantage of the high data rates. 802.11b defines an op- tional short preamble and header which differ from the standard frame by sending a preamble with only 72 bits and transmitting the header at 2 Mbps, for a total over- head of 96 usec instead of 192 usec for the long preamble and header. Devices using this option can only commu- The code words for 5.5 Mbps rate CCK modulation nicate with other stations having the same capability. are a subset of those for 11 Mbps CCK. In this case, there Use of higher data rates entails some loss of sensitivity are four data bits per symbol which determine a total of and hence range. The minimum specified sensitivity at 6 BIT CODE IDENTIFIER RF IN SERIAL DATA DOWNCONVERTER CORRELATION BANK DATA DEMULTIPLEXER 2 BIT DIFFERENTIAL PHASE Figure 5.2-14 CCK decoding. Short-range wireless applications and technologies CHAPTER 5.2 the 11 Mbps rate is -76 dBm for a frame-error 