ew meters, appro- keying (MBOK). In this scheme, the allowed symbols are priate to a PAN application. As the demanded range specific sequences of positive and negative pulses; the increases, even the full UWB band cannot provide enough sequences can be arranged in a hierarchy of mutually coding gain to substitute for transmit power, and the orthogonal symbols, in the sense that if they are multi- performance of the UWB transmitter falls below that of plied together timewise and the sum taken (i.e., if they a conventional 802.1 a system. UWB systems do not re- are correlated), the sum is 0 for distinct sequences. An place conventional WLAN systems but have ideal char- example for M = 4 is shown in Figure 5.1-31. Each pos- acteristics for WPAN applications. sible symbol is composed of four pulses, shown in cartoon At the time of this writing, two distinct proposals are form on the left. In the shorthand summary on the right, under consideration by the 802.15.3a task group as an inverted pulse is denoted "_" and a noninverted pulse Low band High band 5.15 5.825 frequency (GHz) Figure 5.1-29 Pulse UWB band structure. Wireless local area networks CHAPTER 5.1 in frequency in time relative voltage power (dB) frequency (GHz) time (nsec) Figure 5.1-30 High-band modified Hermitian pulse. is "+." Clearly, the first two symbols (00 and 01) are in- and a noise figure of 6.6 dB, the link should still achieve versions of each other, as are the last two (10 and 11). a respectable 75 dBm sensitivity, allowing some margin Either symbol in the first group is orthogonal to either at a few meters range. symbol in the second, for example, 00.10 = -1 +1 -1 The second current proposal is based on multiband +1=0, where we have assumed the product of a positive OFDM, as described in IEEE 802.15-03/267r2, r6 Batra pulse and an inverted pulse to be - -1. et al., with some updates from 802.15-04/0122r4 of The proposed radio architecture uses the MBOK March 2004. In this case, the available UWB bandwidth symbols as chips in a CDMA system. Eac