spectrum, an important requirement is that they do not result in harmful interference with other RF transmitted services, whether current or planned. To ensure this coexistence, the FCC has defined strict EIRP limits on UWB transmission, as shown in Figure 4-34. The maximum permitted power density (EIRP) of -41.3 dBm/MHz is below the FCC Part 15 noise power limit for unintentional emitters such as computers and other electronic devices. As a result of this very low EIRP specification, UWB wireless is suited for applications where very long battery life is required. FCC Part 15 Limit UWB passband 3.1 to 10.6 GHz Indoors Outdoors Frequency (GHz) Figure 4-34: FCC UWB Passband Specification Radio Communication Basics A second characteristic of some UWB implementations is spectral shaping - the capability to control the radiated power spectrum in order to avoid transmission at particular narrow band frequencies. UWB comes in three varieties for data communication applications: time-hopping pulse position modulation (or Impulse radio) direct sequence spread spectrum-UWB (DS-UWB) multiband-UWB (such as multiband OFDM). Of these, MB-OFDM offers the greatest flexibility in spectral shaping, with a wide range of course and fine control options easily implemented in software. Time Hopping PPM UWB (Impulse Radio) Impulse radio (IR) is the name given to UWB radio based on time- hopping, pulse position modulation. Data is transmitted as a discontinuous series of very short pulses, with one pulse per user in each time hopping frame of length Tf. The nominal transmission time of a pulse in a given frame is determined by a pseudo-noise (PN) code that is specific for each user of the communication channel. Finally, whether a pulse represents a 1-bit or a 0-bit depends on the actual transmission time relative to the nominal transmission time (the pulse position modulation). For example, in an early/late PPM system, if the pulse is transmitted a time offset S ahead of the nominal time it represents a 1-bit, or if an offset 8 after t