In a cross polarization interference canceller for use in digital radio communications, a signal processing circuit (20) monitors whether numbers of word sync signals and error pulses obtained in error correction decoding exceed specified values, and outputs reset signal RS1 when an abnormality is detected. A signal level detection circuit (21) outputs reset signal RS2 when it is detected that a level of a different polarization side baseband signal from a demodulator is lower than specified. A logical sum RS3 of reset signals RS1 and RS2 is supplied to a cross polarization interference canceller (18). When the word is not synchronized or the number of error pulses generated in error correcting decoding exceeds the specified value, the self-polarization side can be protected from unnecessary interference and disturbance and the self-polarization side data can be protected from disturbance even though an abrupt abnormality occurs in different polarization signals.

In a cross-polarization interference canceller employing a transversal filter, when a tap coefficient of the transversal filter is generated by averaging resultant values of correlation between a residual error component and a data polarity signal of a different polarized wave, a moving average derived by averaging with a sum of most recent given number of bits of correlation results, is employed. Even when offset is caused on the correlation results due to random characteristics of the received signal, the offset cannot be accumulated to allow derivation of the true tap coefficient since the sum of most recent given number of bits of correlation results is used.

In a cross polarization interference canceller for use in digital radio communications, a signal processing circuit (20) monitors whether numbers of word sync signals and error pulses obtained in error correction decoding exceed specified values, and outputs reset signal RS1 when an abnormality is detected. A signal level detection circuit (21) outputs reset signal RS2 when it is detected that a level of a different polarization side baseband signal from a demodulator is lower than specified. A logical sum RS3 of reset signals RS1 and RS2 is supplied to a cross polarization interference canceller (18). When the word is not synchronized or the number of error pulses generated in error correcting decoding exceeds the specified value, the self-polarization side can be protected from unnecessary interference and disturbance and the self-polarization side data can be protected from disturbance even though an abrupt abnormality occurs in different polarization signals.

In a cross-polarization interference cancellation system operable in response to desired and undesired or interference source signals which are obtained from polarized waves having planes of polarization orthogonal to each other and which are produced in accordance with clock signals having a predetermined clock frequency, an interference signal is included in the desired signal and is derived from the interference source signal to be removed from the desired signal by the use of an interference canceller and to be produced as an interference free signal. The interference free signal is demodulated into demodulated signals, error signals, and quadrant detection signals by a demodulator unit. In this event, the quadrant detection signals alone are stably demodulated by the use of specific clock signals which are sent through a frequency doubler operable to double the predetermined clock frequency of the clock signals. The interference canceller comprises a transversal filter and a control signal generator for supplying the transversal filter with control signals stably produced by the control signal generator. As a result, the interference canceller is stably operated in response to the control signals.

In a satellite communications system, a rotatable 90^o phase shifter (4) and a rotatable 180^o phase shifter (3) introduce variable phase shifts to a single pilot signal, which is transmitted to the satellite. Downlink cross-polarization interference cancelling control signals are derived from a beacon signal received from the satellite. A pulse sequence is generated to modulate the amplitude of a carrier to produce an ASK signal as the pilot signal. A correlation calculator (19) provides coarse mode interference cancellation by estimating uplink cross-polarization interference components from the downlink cross-polarization cancelling control signals and adjusting the angle settings of the 90^o and 180^o phase shifters with the estimated components. The pilot signal is transmitted back to the earth and synchronously detected with the pulse sequence to derive a DC signal which represents an uplink cross-polarization interference. A stepwise tracking controller (16) provides fine mode interference cancellation by stepwisely varying the 90^o and 180^o phase shifters by predetermined amounts an detecting previous and recent values of the synchronously detected DC signal resulting from the stepwise variations, and repeating the stepwise variations until a difference between the previous and recent values is substantialy reduced to a minimum.

An apparatus is described for two-way communication between a controller and a communicator. The controller comprises a series of means adapted to generating a carrier signal and to deal with said signal to produce an informational signal. The communicator likewise comprises a series of means directed at producing a electromagnetic wave signal. In accordance with the invention the first means of the controller comprise means to vary the frequency or phase of said carrier signal, and the means for demodulating a second modular carrier signal include means for mixing said carrier signal with said modulated carrier signal.

Data-aided equalization and cancellation in digital data transmission over dually polarized fading radio channels is disclosed. The present invention uses decision feedback structures (150) with finite-tap transversal filters (100). Subject to the assumption that some past andior future data symbols are correctly detected, formulas and algorithms for evaluating the least mean-square error for different structures are disclosed. In a sequence of curves, the performance of various structures for a particular propagation model and several fading events are evaluated and compared.

An improved decision-directed control circuit is disclosed which supplies adjustments to adaptive circuitry in a communications system. The improvement is achieved by only supplying adjustments when there is a high probability that the adjustment is correct. In the disclosed embodiments, this high probability equates, at a minimum, to times when the adaptive circuit output has an amplitude which lies outside of a range extending from the largest to the smallest value of the digital signal. This adjustment approach can be used to adjust a variety of adaptive circuits including transversal equalizers, timing recovery circuits, cross-polarization cancellers and carrier recovery circuits.

In order to reset a cross-polarization interference canceller for use in a digital radio communications receiver, two polarized-signal discriminators are provided in a manner to be coupled respectively to receive reproduced digital data from the corresponding demodulators. In the event that the demodulator produces an output not characteristic thereof, the corresponding polarized-signal discriminator produces a reset signal to reset the canceller.

Data-aided equalization and cancellation in digital data transmission over dually polarized fading radio channels is disclosed. The present invention uses decision feedback structures (150) with finite-tap transversal filters (100). Subject to the assumption that some past andior future data symbols are correctly detected, formulas and algorithms for evaluating the least mean-square error for different structures are disclosed. In a sequence of curves, the performance of various structures for a particular propagation model and several fading events are evaluated and compared.

In order to reset a cross-polarization interference canceller for use in a digital radio communications receiver, two polarized-signal discriminators are provided in a manner to be coupled respectively to receive reproduced digital data from the corresponding demodulators. In the event that the demodulator produces an output not characteristic thereof, the corresponding polarized-signal discriminator produces a reset signal to reset the canceller.

In a satellite communication system a ground station transmits an information carrying plane polarised signal to the satellite where it is broadcast to a number of individual receivers on the earth. The effect of the earth's atmosphere partially de-polarises the plane polarised signal and to provide compensation for this effect, the earth station transmits simultaneously with the information carrying signal an additional plane polarised signal, which is polarised at right angles to the first signal so as to provide forward correction for the de-polarising effect. The additional signal is arranged to cancel the cross-polar components of the de-polarised signal produced within the atmosphere. A number of methods are described for determining the properties of the additional signal which are necessary to achieve compensation.

The present invention concerns a method for determining a power of at least one crosspolar interferer in a frame received by a receiver comprising at least a first and a second receive antennas detecting a first and a second polarization, the frame being transmitted from a satellite transmitter comprising at least a first and a second transmit antennas, the first transmit antenna being used for transferring signal representative of the frames to the receiver on the first polarization, the second transmit antenna being used for transferring signal to another receiver using the same frequency and the second polarization. The method comprises the steps of:

- obtaining the antenna gains between the satellite transmitter and the receiver,

- estimating a value of the atmospheric attenuation between the satellite transmitter and the receiver, a crosspolar attenuation, the channels between the first and second transmit antennas and the receiver,

- estimating the power of the at least one interferer from the obtained antennas gains and estimations.