those of the code words. Recall that we defined the distance between classic PHY. However, the use of these symbols is sig- two QPSK symbols by reference to the phase-amplitude nificantly different. plane; for a normalized amplitude of 1, two nearest To maintain compatibility with classic systems, neighbors are 2/2 apart; if we square the distance we packets with preambles transmitted at the lowest rate of find nearest neighbors differ by 2 in units of the symbol 1 Mbps, using DBPSK modulation, must be supported. energy, Es. We can similarly define the difference be- However, the new PHY adds the option to use short tween code words by adding up the differences between preambles with 2 Mbps DQPSK modulation to reduce the individual QPSK symbols, after squaring to make the the overhead imposed by the very slow long preamble on terms positive definite (> 0). Having done so, we find short high-rate packets. More importantly, the 802.11b that CCK-11 code words have their nearest neighbors at PHY introduces two completely new approaches to a squared distance of 8. This means that no possible encoding the incoming data onto the QPSK symbols: single-chip QPSK error could turn one allowed code complementary code keying (CCK) and packet binary word into another, SO single-chip errors can always be convolutional coding (PBCC). Each method may be used detected and corrected. In RF terms, the bit error rate is to support two new data rates, 5.5 and 11 Mbps. Both reduced for a fixed (S/N), or equivalently the (S/N) can methods completely abandon the Barker sequence and be increased for the same error rate. The change in (S/N) conventional direct-sequence spreading. is known as coding gain. The coding gain of CCK is about Wireless local area networks CHAPTER 5. 1 even-numbered symbol input bits intermediate phases transmitted codeword Figure 5.1-12 CCK-11 code word encoding scheme (even-numbered symbols). 2 dB. A slight variant of the above scheme is used to The output bits are mapped onto the next QPSK deliver 5.5 Mbps. symbo