Pulse or digital communications – Repeaters – Testing
Patent
1986-07-21
1990-10-02
Safourek, Benedict V.
Pulse or digital communications
Repeaters
Testing
375 82, 375 94, 375 97, 375119, H04B 130
Patent
active
049612069
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to data modems, and primarily to modems containing digital filters.
The transmission of digital (typically binary) signals over a communications channel, such as a satellite link, generally involves modulating the digital signal onto a carrier and subsequently effecting a demodulation process to recover the original signal FIG. 1 of the accompanying drawings illustrates this process in block schematic form with the modulator block being referenced 1, the communications channel 2, and the demodulator block 3. Typical time-domain and frequency-domain signal representations for this process are shown in FIGS. 2 and 3 respectively.
Considering first the form of binary signal to be transmitted over the communications channel 2, FIG. 2a shows the time-domain representation of a random binary stream of period T; FIG. 3a shows the frequency spectrum of the binary stream. For a simple frequency-shifting modulation process such as amplitude modulation, the binary stream B is multiplied with a carrier .waveform of frequency .omega..sub.c (represented in FIG. 3b), the logic "1" value of the binary signal producing a modulator output of +A cos .omega..sub.ct (where A is a constant) and the logic "0" value of the binary stream producing a modulation output of -A cos .omega..sub.ct. This form of binary amplitude modulation can also be viewed as binary phase shift keying of the carrier as the latter is keyed between phase states of O and II radians. FIGS. 2c and 3c show the time and frequency domain representations of the modulator output. Suitably implemented, the demodulator serves to recover the original signal (FIGS. 2a, 3a)free of distortion. A major drawback with the simple data modem described above is the excessive bandwidth occupied by the transmitted signal (see FIG. 3c). In order to reduce the system bandwidth and thus allow several channels, with different center frequencies, to occupy the space previously taken by one, it is known to shape the spectrum of the transmitted signal by means of filters. This shaping may be effected either on the unmodulated or modulated signal. Typically, these filters have a frequency response of the form illustrated in FIG. 4 whereby, for a signal of the form of FIG. 2a input into the modem of FIG. 1, the signal at the demodulator output will have the time-domain form illustrated in FIG. 2d and a spectral shape as shown in FIG. 3d. It will be seen from these latter Figures that the occupied signal bandwidth has been substantially limited at the cost of some distortion of the received signal; however, by evaluating the received signal at its period midpoints, it is normally possible to recover the transmitted data virtually error-free.
It can be shown that for optimum system performance, the desired spectral shaping introduced by the modem filters should be equally split between the modulator and the demodulator.
One way of providing filters with the desired spectral shaping characteristics is to calculate the corresponding required filter impulse response and then to construct a filter, known as a transversal filter, which generates the filter output signal as an approximate convolution integral of the input signal with the calculated impulse response. More particularly, for an input signal f(t) and a filter impulse response of h(t), it is known that the filter output g(t) is given by: ##EQU1## where .tau. is the delay variable of the convolution integral. The foregoing can be approximated by ##EQU2## where k is an integer.
This approximation can be seen to be taking samples of the input signal f(t) at delays k.DELTA..tau., multiplying them by the area under the corresponding portion of the impulse response and summating the resultant values (h(.tau.) being, of course zero for negative values of .tau.). The integral on the right-hand side of Equation (3) is frequently approximated to h(k.DELTA..tau.)d.tau..
A physical realization of this convolution approximation is illustrated in diagrammatic form in FIG. 5 where the input signal f(t) is fed to a d
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Bramwell Jonathan R.
Tomlinson Martin
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