Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

122 Hybrid Multiple Access SchemesAmplitudet = 0.5 nsectimeFigure 3-10Gaussian monocycle with duration of 0.5 nsin accordance with a specific PN code, i.e. performing time hopping (see Figure 3-11),this pseudo-random time modulation makes the UWB spectrum a pure white noise in thefrequency domain. In the time domain each user will have a unique PN time-hoppingcode, hence resulting in a time-hopping multiple access.A single data bit is generally spread over multiple monocycles, i.e. pulses. The dutycycle of each transmitted pulse is about 0.5–1 %. Hence, the processing gain obtained bythis technique is the sum of the duty cycle (about 20–23 dB) and the number of pulsesused per data bit. As an example, if we consider a transmission with 10 6 pulses per secondwith a duty cycle of 0.5 % and with a pulse duration of 0.5 ns (B = 2 GHz bandwidth)for 8 kbit/s transmitted data, the resulting processing gain is 54 dB, which is significantlyhigh. The ultra wideband signal generated above can be seen as a combination of spreadspectrum with pulse position modulation.AmplitudeT n T n+1 T n+2 T n+3 T n+4 T n+5TimeFigure 3-11PN time modulation with five pulses