Direct Sequence Spread Spectrum

A modulation scheme that is related to 8-PSK is jr/4-dlfferentialQPSK(7r/4-DQPSK) (Peterson, Ziemer). It is essentially an 8-PSK format with differential encoding where, from a given phase state, only phase shifts of 7t/4 or 37t/4 are allowed. It has been adopted as the modulation format for one North American standard for second-generation cellular-mobile radio. The other adopted standard uses direct-sequence spread-spectrum modulation. 

Direct-sequence spread-spectrum (DSSS) A modulation scheme employing a transmission bandwidth much wider than that required for the message signal, where the additional spectral spreading is... 

FIGURE 12.75 Direct sequence spread spectrum system. 

For systems such as wireless LANs to operate without a FCC site license, the industrial, scientific, and medical (ISM) frequency bands were created. There are two such bands, 902-928 MHz and 2.400-2.484 GHz. The FCC requires that transmission on these bands must use either frequency hopping or direct sequence spread spectrum transmission. The more restrictive IEEE 802.11 standard, however, limits FHSS to the 2.400-2.484 GHz band, so that the lower frequency band is reserved for DSSS systems. FCC requirements also dictate certain parameters that influence the hopping pattern for FHSS systems. For example, the FCC dictates that in the 2.4-GHz frequency band, a FHSS transmitter must not spend more than 0.4 s in any one frequency slot every 30 s and that the transmitter must hop through at least 75 frequency slots in the frequency hopping pattern (Rigney, 1995). 

Direct sequence A spread spectrum format where a narrow-band signal is multiplied by a high rate pseudonoise code prior to transmission. 

Spread spectrum modidation A modulation format where the bandwidth of the transmitted signal is typically orders of magnitude wider than the bandwidth (or data rate) of the message. Common examples include frequency hopping (FHSS) and direct sequence (DSSS). 

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