, the first eleven Differential quadrature phase shift keying handles channels are used. Figure 5.2-8 shows how channels one, 2 Mbps of data. Each sequence of two input bits creates six and eleven may be used by three adjacent independent four permutations on two outputs. The differential en- networks without co-interference. When there are no coder outputs the differences from symbol to symbol on more than two networks in the same area, they may the lines that go to the inputs of the exclusive OR gates choose their operating channels to avoid a narrow-band shown in Figure 5.2-9. The outputs on the I and Q lines transmission or other interference on the band. are the Barker sequence of 11 Mcps inverted or sent In 802.11 DSSS, a pseudo-random bit sequence phase straight through, at a rate of 1 Msps, according to the modulates the carrier frequency. In this spreading se- differentially encoded data at the exclusive OR gate quence, bits are called chips. The chip rate is 11 mega- inputs. These outputs are spectrum shifted to the RF chips per second (Mcps). Data is applied by phase carrier frequency (or an intermediate frequency for modulating the spread carrier. There are eleven chips per subsequent up-conversion) in the quadrature modulator. data symbol. The chosen pseudo-random sequence is Reception of DSSS signals is represented in a Barker sequence, represented as 1,-1,1,1,-1,1,1,1,-1, Figure 5.2-10. The downconverted I and Q signals are -1,-1. Its redeeming property is that it is optimally applied to matched filters or correlation detectors. These 802.11 DSSS QPSK REFERENCE 0 dBr -30 dBr/11 MHz -50 dBr/22 MHz Frequency (MHz) Figure 5.2-7 802.11 DSSS spectral mask. CHAPTER 5.2 Short-range wireless applications and technologies DIFFERENTIAL QUADRATURE MODULATED ENCODER 1 OR 2 MODULATOR CARRIER LOCAL SYMBOL BARKER OSCILLATOR CLOCK SEQUENCE 1 MBPS 11 MBPS Figure 5.2-9 DSSS modulation. circuits correlate the Barker sequence with the input Data is returned to its original form by descrambling in signal and output an 