unique hopping pattern, is a simple technique for impulse radio to allow multiple user access (see Section "Time Hopping PPM UWB (Impulse Radio), p. 121"). Two other less common techniques are Pulsed FM systems and Hybrid systems (Figure 4-7). In Pulsed FM systems, the input data stream is used directly to modulate the RF carrier, which is transmitted in frequency Linear chirp - frequency sweeping down Linear chirp - frequency sweeping up WWWWW Warble - frequency sweeping up then down WWW.VVA Warble - frequency sweeping down then up Data pulse with FM pattern determined by the spreading function Time slot; Ts Figure 4-7: A Simple Explanation of Pulsed FM Systems Chapter Four modulated pulses. The spreading function controls the pattern of frequency modulation, which could for example be a linear "chirp" with frequency sweeping up or down. Hybrid systems also use combinations of spread spectrum techniques and are designed to take advantage of specific characteristics of the individual systems. For example, FHSS and THSS methods are combined to give the hybrid frequency division - time division multiple access (FDMA/TDMA) technique (see the Section "Wireless Multiplexing and Multiple Access Techniques, p. 87"). Of these alternative spread spectrum techniques, DSSS and FHSS are specified in the IEEE 802.11 wireless LAN standards, although DSSS is most commonly used in commercial 802.11 equipment. FHSS is used by Bluetooth, and FHSS and chirp spread spectrum are optional techniques for the IEEE 802.15.4a (ZigBee) specification. Chipping, Spreading and Correlating The spreading function used in DSSS is a digital code, known as a chipping code or pseudo-noise (PN) code, which is chosen to have specific mathematical properties. One such property is that, to a casual listener on the broadcast band, the signal is similar to random noise, hence the "pseudo-noise" label. Under the IEEE 802.11b standard, the specified PN code for 1 Mbps and 2 Mbps data rates is the 11-bit Barker code. Barker codes are binary sequences that h