0 to 1 million times in military systems. Several different methods of spread spectrum transmission have been developed, which differ in the way the spreading function is applied to the information signal. Two methods, direct sequence spread spectrum and frequency hopping spread spectrum, are most widely applied in wireless networking. Chapter Four In Direct Sequence Spread Spectrum (DSSS) (Figure 4-4), the spreading function is a code word, called a chipping code, that is XOR'd with the input bit stream to generate a higher rate "chip stream" that is then used to modulate the RF carrier. Baseband Spread De-spread Baseband signal Spreading function signal Correlation function signal Frequency Frequency Frequency Figure 4-4: A Simple Explanation of DSSS In Frequency Hopping Spread Spectrum (FHSS) (Figure 4-5), the input data stream is used directly to modulate the RF carrier while the spreading function controls the specific frequency slot of the carrier within a range of available slots spread across the width of the transmission band. Time Hopping Spread Spectrum (THSS) (Figure 4-6), is a third technique in which the input data stream is used directly to modulate the RF carrier which is transmitted in pulses with the spreading function controlling the Frequency Frequency Baseband Frequency signal Baseband signal Transmitter frequency Receiver frequency hopping pattern; hopping pattern; 12 > 16 > 5 > 10 > 12 > 16 > 5 > 10 > Figure 4-5: A Simple Explanation of FHSS Radio Communication Basics Data burst transmitted at a position within the time slot determined by the spreading function Time slot; Ts Figure 4-6: A Simple Explanation of THSS timing of each data pulse. For example, impulse radio uses pulses that are SO short, typically in the region of 1 nanosecond (nS), that the spectrum of the signal is very wide and meets the definition of an ultra wideband (UWB) system. The spectrum is effectively spread as a result of the narrowness of transmitted pulses, but time-hopping, with each user or node being assigned a 