Coding for suppression of noise and interference in airborne PCM telemetering systems

Abstract

For the transmission of data from aircraft and missiles to a ground station, compact, lowpower equipment in the air is mandatory, but considerable complexity can be acceptable for the receiver. To transmit data reliably with low transmitter power under conditions of noise and interference normally encountered in air-to-ground transmissions, one may apply noise suppressing coding techniques such as PCM. In standard PCM a signal consists of a sequence of pulses with positive or negative amplitude, and the signals are detected by sampling the amplitudes and ascribing the value +1 to a positive amplitude and the value −1 to a negative amplitude. An improvement of about 7 db in signal-tonoise power ratio over this system may be obtained by using a detection method based on correlation. The received signal is multiplied by a certain function, and the integral of this product taken over the length of the whole signal assumes the value +1 or −1. For each bit of information in the signal a different multiplying function is used. A signal with eight bits of information will thus be represented by eight integrals of value +1 or −1 each. This “integration system” behaves — as far as noise is concerned-like an “amplitude sampling system” having a bandwidth equal to one half times the pulse rate. Since amplitude sampling systems usually are operated with five times that bandwidth in order to keep the filter distortions sufficiently low, the gain of 7 db mentioned above is obtained.