Abstract: Defective repeaters in a multihop, digital microwave transmission system are located by the use of a special test pattern which is generated at the transmitting terminal. At each repeater station, the two outputs of the phase demodulator are compared to determine if the test pattern has been properly received from the upstream repeaters. The special test pattern, S, has the characteristic that S = S(2N); that is to say, the test pattern equals the complement of itself delayed by 2N symbols. This special characteristic makes it unnecessary to employ a differential decoder in the test apparatus at each repeater station.

Abstract: The downconverter in a digital radio repeater is normally connected to a bias which causes the converter to perform at optimum efficiency in developing an intermediate frequency (IF) signal. This IF signal is then processed in an IF section which is gain controlled by an automatic gain control (AGC) signal. The AGC signal is compared with a reference voltage to develop an error signal which is in turn amplified to produce a stressing bias voltage. During periods of repeater testing a remotely originated command signal causes a switching circuit to be activated resulting in the application of the stressing voltage to the downconverter in place of the normal bias. As a result a predetermined signal-to-noise ratio is established in the digital repeater regardless of variations in the input signal level due to differences in repeater spacing or atmospheric conditions.

Abstract: In a digital transmission system of the type which employs cross-polarized radio channels, the failure of a transmitter associated with a particular channel can cause an undesired signal cross-over into the receiver of the failed channel. According to the instant invention, each channel in the system is uniquely encoded by a combination of bitstream inversion and bitstream delay at the head end of the system. Thus, if a signal crossover occurs an excessive number of parity violations are detected in the failed channel and a switch to a protection channel is requested.