IF signal combiner for minimizing the BER in a space or angle diversity digital radio receiver

In a digital radio signal receiver operating in digital radio links and protected with space or angle diversity, the digital radio signal receiver having two radio receivers and a demodulator providing a demodulated output signal, a signal combiner connected between the two radio receivers and the demodulator, the signal combiner comprising:

• an adder for summing intermediate frequency signals issuing from the radio receivers and forming a combined signal having a residual power attenuation and a residual amplitude dispersion due to propagation conditions;
• a network connected to said adder for measuring the residual amplitude dispersion and a relative position of a notch frequency, whereby the notch frequency is defined such that a residual amplitude attenuation in a spectrum of the combined signal is a maximum;
• a measuring device connected to said adder for measuring the residual power attenuation;
• a digital signal processor connected to said measuring device and receiving measurement results therefrom, said digital signal processor generating control signals setting an amplitude and a mutual phase shift of the intermediate frequency signals, and said digital signal processor adaptively perturbing the control signals for minimizing a bit error rate BER in the output signal of the demodulator, by minimizing a polynomial function of a normalized residual amplitude dispersion and of the residual power attenuation, the polynomial function approximating a parametric logarithmic representation of the bit error rate BER also being a function of the normalized residual amplitude dispersion and of the residual power attenuation, wherein constant BER values are taken as many values of a parameter of said parametric representation, and said digital signal processor normalizing the residual amplitude dispersion with respect to the relative position of the notch frequency and reducing a further dependency of the representation of the BER on the relative position of the notch frequency.