 the transmitted code word probably was. wideband transmission. UWB radios are allowed to Coding is a separate and distinct operation from modu- transmit right on top of spectrum licensed for other uses lation. TCM mixes coding and modulation operations by dividing the constellation into subsets, within each of which the distance between signal points is large. The choice of subset is then made using a convolutional code 802.11b on some of the input bits, whereas the choice of points 802.15.3 within the subset is made using the remainder of the input bits uncoded. The subset points are widely sepa- rated and noise resistant, SO uncoded selection works -30 dB fine. The subsets are more difficult to distinguish from one another, but this choice is protected by the code, -40 dB 7.5 MHz making it more robust to errors. The net result is typi- -50 dB cally around 3 dB of improved noise margin in the same 11 MHz bandwidth versus a conventional modulation. The benefits of this approach are revealed in the 15 MHz spectral emission mask (Figure 5.1-26). The 802.15.3 22 MHz signal is able to deliver the same data rate as 802.11g but fits completely within a 15-MHz window. The practical Figure 5.1-26 Comparison of 802. 11b and 802.15.3 emission consequence is that one can fit five nonoverlapping spectral mask. CHAPTER 5.1 Wireless local area networks US (FCC) UWB full band (dBm) ETSI (draft) UWB sub-band data rate (Mbps) frequency (GHz) 802.11a space Figure 5.1-27 U.S. FCC and proposed ETSI limits for WLAN space ultrawideband emitters (EIRP is equivalent isotropic radiated power). range (m) (Figure 5.1-27). However, UWB radios operate under several restrictions designed to minimize interference Figure 5.1-28 UWB range VS. data rate, shown for both full band with legacy users of the spectrum. First, the absolute (7 GHz) and subband (500 MHz) approaches (after Gandolfo, power emitted at any band is restricted to less than -40 Wireless Systems Design Conference, 2003). dBm, which is comparable with the emissions allowed by non