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To provide high data rates, the Federal Communications Commission (FCC) in 2002 approved the unlicensed usage of UWB devices in the spectrum of 3.1 GHz to 10.6 GHz, provided that the bandwidth (BW) of the signal is greater than 25% of the carrier frequency (i.e., fractional bandwidth = (FH - FL)/FC > 25%, or the total BW > 1.5 GHz. One approach to this is the UWB-WiMedia protocol, which uses a multiband-OFDM technique. The WiMedia specification divides the UWB frequency spectrum into six band groups, with five different band groups (band groups 1 to 4 and band group 6) consisting of three frequency bands, and the sixth band group (band group 5) consisting of two frequency bands. Each of the frequency bands has a bandwidth of 528 MHz.

The physical layer uses OFDM technology with 122 tones in each of the 528 MHz bands. The OFDM packets are then spread using a time-frequency code (TFC). Two types of spreading are defined: one uses frequency hopping over the three bands and is referred to as time-frequency interleaving (TFI). The other is transmitted in a single band and is referred to as fixed-frequency interleaving (FFI). For band groups 1, 2, 3, 4 and 6, 10 different TFCs are defined, with seven TFIs and three FFIs. For band group 5, three FFIs are defined, thus bringing the total number of channels to 53.

The transmitter WiMedia RF signal is defined in eq. 1:

Where Re{…} represents the real part of the signal. TSYM is the symbol length, Npacket is the number of symbols in the packet, fc(m) is the center frequency for the mth frequency band, q(n) is a function that maps the nth symbol to the appropriate frequency band, and sn(t) is the complex baseband signal representation for the nth symbol, which must satisfy the following property: sn(t) = 0 for t < 0, and t TSYM. The exact structure of the nth symbol depends on its location within the packet.

Unique logical channels are defined by using up to 10 different TFC codes for each band group. TFCs and the associated base sequences for band group 1 are shown in Table 1. A symbolic representation of band hopping among three bands, as defined by TFC 1, is shown in Figure 1.