Nippon Telegraph and Telephone Corporation

Takehiro Murase

Hideaki Matsue

Tadashi Shirato

Hiroyuki Ohtsuka

In a digital demodulation system (FIGS. 2, 3, 4, 7, 8) in a dual polarization radio system, having a first demodulator (4) and a first A/D converter (14) for a main signal, a second demodulator (5) and a second A/D converter (15c) for an interference signal (cross polarization signal), a transversal filter (18) for providing a compensation signal for cancelling an interference component in the main signal depending upon the demodulated interference component; the second A/D converter (15c) for providing the interference component in digital form is operated with a clock signal (102) regenerated in the main signal branch (4, 14) so that system operates correctly even when the main signal and cross polarization signal are in an asynchronous condition. The A/D converter (14, 15c, 105) is provided with an attenuator (104) for compressing amplitude of an input signal to avoid saturation of the circuit (105) in case of fading and/or distortion, and a ROM table (107) for correcting digitalized output after equalization and/or cancellation. Tap coefficients of the transversal filter (18) are obtained by a control (19) through a ZF method when inter-symbol interference is small, and a MLE method when inter-symbol interference is large. Further, an integration constant for the coefficients is adaptive depending upon inter-symbol interference.

Dual polarization demodulation with cross polarization cancellation

An interference cancellation system at a receiver side for digital radio system has a first quadrature phase detector for demodulating the main signal. A second quadrature phase detector (or a phase detector) is supplied with the same reference carrier and clock signal as that of the first quadrature phase detector, for phase detecting an interference signal. A decision circuit is coupled with the first quadrature phase detector to provide a baseband signal. An error signal detector provides a difference between a decision level of a digital signal and an input of the decision circuit due to the interference signal. A correlation device provides a correlation signal between an error signal and a phase detected interference signal. A control device provides a compensation signal which has the same amplitude and anti-phase as those of the interference component included in the main signal according to the correlation signal. A combiner combines the main signal with the compensation signal to provide a compensated main signal which is free from interference. A combiner and a control means may be inserted either in an RF stage or an IF stage, or in a baseband stage. An error signal detector and decision circuit are implemented by an A/D converter. A correlation means is implemented by a digital or analog multiplier and integrators.

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Kanagawa, Japan

Takehiro Murase