Demodulators – Phase shift keying or quadrature amplitude demodulator
Patent
1995-04-03
1997-02-11
Mis, David
Demodulators
Phase shift keying or quadrature amplitude demodulator
375324, 375329, H03D 300, H04L 2722
Patent
active
056025076
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a demodulating method and a demodulator for demodulating an input signal using a replica that is adaptively generated for the transmitted signal from a transmission line which has varying transmission characteristics.
RELATED ART
Transmission characteristics of a communication transmission line, such as impulse response from a transmitter to a receiver, sometimes continuously vary largely. This frequently occurs in micro-wave radio transmission and mobile communications. In such a transmission line, when a signal is received in a relatively high noise level, a desired signal may contain a waveform distortion which varies with time. In addition, when varying interferences of the same channel and an adjacent channel are superimposed, the transmission performance will be remarkably impaired. Thus, the impairment due to such causes should be suppressed so as to realize a receiving system with high reliability.
To receive a transmission signal from a transmission line in which transmission characteristics vary, an adaptive receiver using an adaptive algorithm has been used.
In a transmission line which causes a varying distortion, adaptive equalizers have been used as the adaptive receivers. From the view point of the arrangements of the adaptive equalizers, the adaptive equalizers can be classified into linear equalizers and non-linear equalizers.
First, a linear equalizer will be described. FIG. 1 is a block diagram showing the configuration of a conventional linear equalizer. In the following description, a modulated received signal is represented in a complex notation. In the complex notation, a real part of an input signal x(i) to the equalizer at a time point i in discrete time at intervals of one symbol represents the amplitude of the in-phase component of the received signal. On the other hand, the imaginary part represents the amplitude of the quadrature component of the received signal. The input signal x(i) is supplied from an input terminal 11 to a transversal filter 12 with M taps. By controlling tap coefficients w.sub.1 (i), . . . , w.sub.M (i), the distortion of the input signal x(i) is removed and the resultant signal is sent to a decision device 13. The decision device 13 outputs a decided signal d(i) from an output terminal 12. The input signal and the output signal of the decision device 13 are supplied to an error calculating portion 15. The error calculating portion 15 calculates an error signal e(i). The error signal is sent to a control portion 16. The control portion 16 updates the tap coefficients of the transversal filter 12 based on the error signal e(i) and the input signal x(i). The operation of the linear equalizer is described in, for example, J. G. Proakis, "Digital Communications," 2nd edition, McGraw-Hill, 1989.
A column vector of M tap coefficients w.sub.1 (i), . . . , w.sub.M (i) which are supplied to the transversal filter 12 with M taps is denoted by a coefficient vector W(i). A column vector of M input signals x(i), . . . , x(i-M+1) from a time point i to a past time point (i-M+1) corresponding to the respective tap positions is denoted by an input vector Z(i). The input signal x(i), which is an element of the vector Z(i), is a superimposed signal of a directly received wave signal, a delayed received wave signal, an interfering received wave signal, and noise. Over the radio transmission line, the input signal x(i) continuously varies. A coefficient vector W(i-1) is successively updated to W(i) based on the input vector Z(i) and the error signal e(i).
For example, when the input vector Z(i), which is the M input signals from the time point i to the time point (i-M+1), is applied to the transversal filter 12 at the time point i, the output s(i) of the transversal filter 12 can be given by the following linear expression: *(i).times.(i-M+1) =s(i) (01) of measured values x(i), . . . , x(i-M+1) each set is substituted into Expression (01). An error e(i) between each of the resultant s(i) of the plurality of expressions and
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Mis David
NTT Mobile Communications Network Inc.
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