A new, versatile digital microwave radio has been developed for the U.S. Government under the Digital Radio and Multiplexer Acquisition (DRAMA) Program, which will convert transmission facilities of the Defense Communication System from analog to digital signal format. The radio can be configured for either space or frequency diversity in either the 4 or 8 GHz military bands. A choice of quadriphase shift keying or quadrature partial response signaling provides bandwidth efficiencies of 1.0 and 1.9 bits/s/Hz of occupied RF bandwidth. The design of the radio is presented through detailed descriptions of the multiplexer, modem, RF, protection, and timing subsystems. Major tradeoffs leading to the selection of the modulation types and spectral filtering method are discussed.

Pulse-amplitude modulation (PAM) is extended to two and four dimensions by means of a nonlinear, memoryless mapping to obtain pulse-amplitude-and-phase modulation (PAPM). This modulation requires either the same bandwidth as double-sideband PAM or twice as much but yields a signal-to-noise ratio (SNR) improvement factor similar to that of FM with considerably less sacrifice of spectral occupancy. Several signal-space mappings are investigated for Gaussian data. Expressions for output SNR are derived in terms of a linear error above threshold and an anomalous threshold-producing error. PAPM is compared with various modulation techniques in terms of power and bandwidth requirements. The degradations in output SNR caused by carrier phase, carrier amplitude, and sample timing reference errors are evaluated so that requirements for demodulator design can be specified.

Signal sets employing amplitude and phase keying (APK) with large alphabets conserve bandwidth and do not require as high a signal-to-noise ratio (SNR) as phase-shift keying (PSK). Twenty-nine empirically generated APK sets withM-ary alphabet sizes from 4 to 128 are investigated to determine optimum designs. Selected sets are compared on the basis of a symbol-error-probability bound for both average and peak SNR. Results are presented in the form of symbol error curves. The degradation caused by nonlinearities in a typical TWT amplifier is examined. The effects of AM-AM and AM-PM distortion on APK are presented as a function of power backoff for single-carrier operation (when APK is at the greatest disadvantage compared with PSK). Modem implementation methods, including carrier phase reconstruction techniques, are discussed for basic APK designs.