An Introduction to Ultra Wideband Communication Systems

5.5. Spectral Encoded UWB Communication System

In this section, we describe a UWB communications system that uses spectral encoding as a multiple access scheme with interference suppression capability [41]. The main advantage of this technique is that transmitted signal spectrum can be conveniently shaped to suppress narrowband interference. The concept of spectral encoding is not new. It has been proposed both for an optical communication system [42, 43] and for wireless communications scenarios [44, 45]. In this technique, a message signal is multiplied by a spreading sequence in the frequency domain, as opposed to conventional direct sequence code division multiaccess systems in which the encoding is in the time domain. As a result, the signal will be spread in time. Multiaccess capability is achieved by assigning distinct spreading sequences to different users. When narrowband interference suppression is desired, a spreading sequence is used that has a spectral null where the narrowband interference is located. Moreover, the narrowband interference is "notched out" at the receiver when the received signal spectrum is multiplied by the conjugate of the transmitted signal spectrum (correlation receiver). Details can be found in [46]. The Fourier Transform of the data signal is performed via a well-established implementation that corresponds to the multiplication and convolution of the desired signal by chirp (linear FM) waveforms. This method of spectrally encoding the UWB signal can suppress narrowband interference and can create spectral nulls where desired, simplifying the requirement to meet the FCC mask.

A schematic of the transmitter is shown in Figure 5.30. The modulated signal, f (t), is non-zero only in . The waveform f₁ (t) is given by

Equation 5.31

where w_a and b are respectively the center frequency and the half slope of the chirp signals, and F_R (t) and F_I (t) are respectively the real and imaginary components of the Fourier Transform of the modulated signal, f (t), and are given by

Equation 5.32

where w_c is the carrier frequency. f₁ (t) is valid only in t [T₁], where T₁ is the duration of the impulse response of the filter labeled as cos (w_at + bt²). The signal f₁ (t) is spread to f₂ (t) by a spreading sequence such that its spectrum is given by where PN (w) is in general a complex waveform bandlimited to and PN_I (w) are real waveforms that consist of a sequence of 1s or -1s (and 0s when interference suppression is considered). The function in Figure 5.4 relates to the desired spreading sequence by

Equation 5.33

Equation 5.34

The output transmitted signal s (t) is the inverse Fourier Transform of the signal f₂ (t) and is equal to

Equation 5.35

where R (w) and I (w) are defined as R (w) = F (w)PN_R (w) and I (w) = F (w)w. PN_I (w). W is the bandwidth of f (t). The transmitted signal is valid only in t [T₁ + T]. The receiver structure for this mechanism is shown in Chapter 6, Section 6.3.