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1.4 UWB Signals

As defined by the FCC's First Report and Order, UWB signals must have bandwidths of greater than 500 MHz or a fractional bandwidth larger than 20 percent at all times of transmission [3]. Fractional bandwidth is a factor used to classify signals as narrowband, wideband, or ultra-wideband and is defined by the ratio of bandwidth at –10 dB points [4] to center frequency. Equation 1-4 shows this relationship.

Equation 1-4

01equ04.gif


where fh and f1 are the highest and lowest cutoff frequencies (at the –10 dB point) of a UWB pulse spectrum, respectively. A UWB signal can be any one of a variety of wideband signals, such as Gaussian, chirp, wavelet, or Hermite-based short-duration pulses. Figure 1-5 represents a Gaussian monocycle as an example of a UWB pulse in the time and frequency domains. The Gaussian monocycle is the first derivative of a Gaussian pulse and is given by

Equation 1-5

01equ05.gif


01fig05.gif

Figure 1-5 A 500-picosecond Gaussian monocycle in (a) the time domain and (b) the frequency domain

where t represents time and t is a time decay constant that determines the temporal width of the pulse.

As shown in Figure 1-5, a 500-picosecond pulse generates a large bandwidth in the frequency domain with a center frequency of 2 GHz. In Figure 1–5b, the lowest and highest cutoff frequencies at –10 dB are approximately 1.2 GHz and 2.8 GHz, respectively, which lead to a fractional bandwidth of 80 percent; this is much larger than the minimum Bf required by the FCC:

01equ06.gif

Here is the classification of signals based on their fractional bandwidth:

Narrowband

Bf < 1%

Wideband

1% < Bf < 20%

Ultra-Wideband

Bf > 20%

For example, 802.11 and Bluetooth have fractional bandwidths of 0.8 percent and 0.04 percent, respectively.

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