An Introduction to Ultra Wideband Communication Systems

6.3. Spectral Encoded UWB Communication System

In this section, we describe a UWB communication system that uses spectral encoding as a multiple access scheme with interference suppression capability [78]. The main advantage of this technique is that the transmitted signal spectrum can be conveniently shaped to suppress narrowband interference. The concept of spectral coding has been proposed both in an optical communication system in [79, 80] and for a wireless communications scenario [81, 82]. 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). See [83] for implementation details. Fourier Transform of the data signal is performed by a well-established implementation that corresponds to the multiplication and convolution of the desired signal by chirp (linear FM) waveforms using surface acoustic wave (SAW) devices. SAW devices have evolved to the GHz range, and currently submicron manufacturing and material techniques are improving greatly to frequencies up to 10 GHz.

A schematic of the transmitter is shown in Figure 6.53a.^[3] The modulated signal f (t) is a real and even function and is only nonzero in . The waveform f₁ (t) is given by

^[3] Figures 6.53a and 6.53b are modified from Figures 6.3, 6.4, and 6.5 of [78].

Equation 6.146

where w_a, b are respectively the center frequency and half slope of the chirp signals, and F_R (t), 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 6.147

where w_c is the carrier frequency. f₁ (t) is valid only in t [T₁ is the duration of the impulse response of the filter labeled (w_at + bt²). The signal f₁ (t) is spread to f₂ (t) by a spreading sequence such that its spectrum is given by

Equation 6.148

where PN (w) is in general a complex waveform bandlimited to . PN_R (w), and PN_I (w) are real waveforms and consist of a sequence of 1s or 1s (and 0s when interference suppression is considered). The function in Figure 6.57a relates to the desired spreading sequence by

Equation 6.149

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

Equation 6.150

where R(w) and I (w) are defined as R(w) = F_R (w)PN_R (w) and I (w) = F_R (w).PN_I (w). W is the bandwidth of the signal f (t). The transmitted signal is valid only in t [T₁ + T]. A receiver for the proposed system is shown in Figure 6.57b. The receiver presented has a RAKE stage due to the multipath channel. In addition to the desired signal, it is assumed that additive thermal noise n_w (t) and interference I (t) are present at the receiver's input. The channel is assumed to have a multipath spread, T_m, and the time window that the Fourier Transform window considers is T + T_m long. The variable t₀ is defined as , where a delay of d was added in the transmitted signal as shown in Figure 6.57b. The RAKE stage has Y_m (w) defined as

Equation 6.151

where

Equation 6.152

for m = 1, 2, . . ., N and a_m is the channel gain.

A rigorous analysis of the receiver with numerical approximations and system performance curves is presented [78]. Figures 6.54 and 6.55 show the performance gains for this novel receiver structure. This method of spectrally encoding the UWB signal can suppress narrowband interference and create spectral nulls where desired, simplifying the requirement to meet the FCC mask.