he nominal time then it represents a 0-bit. In the example shown in Figure 4-35, a TH code of length 4 is used, SO that four pulses are transmitted for each bit, each pulse in one of the eight code slots (Tc) in each of four successive frames. Pulse amplitude modulation (PAM) or pulse shape modulation (PSM) can be used as alternatives to PPM, with 1-bit and 0-bit then being determined by the amplitude or shape of each individual pulse. Impulse radio is one of two optional physical layer specifications selected in March 2005 by the IEEE 802.15 Task Group 4a as part of the enhancement of the 802.15.4 standard. (The other optional PHY is a chirp spread spectrum operating in the 2.4 GHz ISM band.) Chapter Four User n bit stream = 1101. User n TH code = 2767 Four pulses transmitted per bit Nominal pulse transmission time Early pulse (t = T2/2 - 8) signifies 1-bit Late pulse (t = Tc/2 + 8) signifies 0-bit Figure 4-35: Pulse Train in a TH-PPM Impulse Radio Transmission Direct sequence UWB (DS-UWB) Direct sequence, discussed in the Section "Direct Sequence Spread Spectrum in the 2.4 GHz ISM Band, p. 83" as the spread spectrum technique underlying the IEEE 802.11b and 802.11g physical layer, can also be applied in UWB radios. Instead of the chipping code being used to spread the carrier spectrum by increasing the symbol transmission rate, the spectrum is spread to UWB proportions as a result of the very narrow pulse that is used to transmit each symbol. The chipping code then plays a multiple access role (CDMA) with individual user codes determining the exact times at which individual users of the channel will transmit or receive a pulse. A variety of modulation methods (PAM, PSM) can be used to code the data stream onto the pulse stream. So far, DS-UWB has not been identified as a target technology for any wireless networking applications. Multiband UWB In multiband UWB, ultra-wide bandwidth is achieved by dividing the frequency band of interest into multiple overlapping or adjacent bands, Radio Communication Basics Chan