 for DSSS as being the chip rate divided by the data microseconds, required to transmit the data-carrying bandwidth doesn't apply here. In fact, the processing field labeled MPDU (MAC protocol data unit). An error gain requirement that for years was part of the FCC check field, labeled CRC, protects the integrity of the Rules paragraph 15.247 definition of direct sequence SIGNAL, SERVICE, and LENGTH fields. The last field spread-spectrum was deleted in an update from August MPDU (MAC protocol data unit) is the data passed 2002, and at the same time reference to DSSS was down from the MAC to be sent by the physical layer, or replaced by "digital modulation." to be passed up to the MAC after reception. All bits in The mandatory high-rate modulation method of the transmitted frame are pseudo-randomly scrambled 802.11b is called complementary code keying (CCK). to ensure even power distribution over the spectrum. An optional mode called packet binary convolutional MATCHED RECEIVED FILTER SIGNAL QUADRATURE DIFFERENTIAL DEMODULATOR DECODER MATCHED FILTER LOCAL OSCILLATOR Figure 5.2-10 DSS reception. Short-range wireless applications and technologies CHAPTER 5.2 VARIABLE SIGNAL SERVICE LENGTH MPDU (PAYLOAD) PREAMBLE PLCP HEADER Figure 5.2-11 DSSS frame format. coding (PBCC) is also described in the specification. from the 65536 possibilities. The chosen 256 or 16 Although there are similarities in concept, the two complex words have the very desirable property that modes differ in implementation and performance. First when correlation detectors are used on the I and Q lines of the general principle of high-rate DSSS is presented the received signal, downconverted to baseband, the below, applying to both CCK and PBCC, then the details original 8-bit (11 Mbps rate) or 4-bit (5.5 Mbps rate) of CCK are given. sequence can be decoded correctly with high probability As in the original 802.11, a pseudo-random noise se- even when reception is accompanied by noise and other quence at the rate of 11 Mcps is the basis of high-rate