A generalized technique for generating quadrature amplitude modulated (QAM) signals that permits nonlinear amplification is presented. With this technique a high-power2^{2n}-state QAM signal is generated by combiningnunfiltered, nonlinear amplified, QPSK signals,nbeing a positive integer. The specific methods of generating 16- and 64-state signals using this technique are presented. An attractive feature of the technique is that despite significant difference with the conventional method of generating QAM signals, the same straightforward demodulation techniques apply to both, and the PeversusS/Nperformances are essentially identical. In the 16-state version employing traveling wave tubes (TWT's) or GaAsFET amplifiers, the technique is shown to result in a transmitter output power advantage that is on the order of 5 dB compared to the conventional method. This advantage is achieved, however, at the expense of an additional output power amplifier.

The effects on spectral densities, symbol waveshapes and PeversusS/Nperformances, resulting from the addition of filtering followed by hard limiting on raised cosine filtered QPSK, offset QPSK, and MSK point-to-point radio systems are studied. A mathematical model and physical insight are presented into the crosstalk phenomenon between quadrature channels, created in the systems by the effect of limiting on filtered signals. This crosstalk is shown to result whether the filtering is ideal or otherwise. Computergenerated and measured eye diagrams showing crosstalk as predicted on a filtered, then limited, offset QPSK signal are given. Measured and computed spectral density results are given which are in close agreement with each other, indicating that the computer model provides a good representation of the real system. In addition, an explanation of the shape of the power spectra associated with filtered, then limited, modulated signals is provided by studying the symbol waveshapes of these signals. Using the spectral density results and theP_{e} (S/N)performance findings, it is shown that for a microwave system which: 1) incorporates an amplitude limiting amplifying device in the transmitter, 2) must operate within the FCC limits for radiated spectrum, and 3) must operate at a spectral efficiency greater than 1 bit/s/Hz, offset QPSK modulation is the best choice of the three modulation methods studied.

In this paper a model of Minimum Shift Keying (MSK) is introduced, an analysis of which shows that the theoretical maximum bandwidth efficiency of MSK is 2 bits/s/Hz, the same as for Quadrature Phase Shift Keying (QPSK) and Offset Quadrature Phase Shift Keying (Offset QPSK). The filtering necessary to achieve minimum spectrum width as well as a raised cosine spectrum while maintaining the condition of zero intersymbol interference for Offset QPSK is reviewed, and the equivalent filtering for MSK derived. In addition, partioning of this filtering between transmitter and receiver for optimum probability of error (Pe) versus receiver input signal to noise (S/N) ratio is given. Computer simulation results are presented for a MSK system and an Offset QPSK system, both filtered for 0%, 50% and 100% excess bandwidth raised cosine spectrum. The results show fully open eyes at the decision threshold input for all cases. They also show that when filtered to give the same spectrum the different system have identical PeversusS/Nperformance,S/Nbeing defined at the input to the receiver decision threshold, i.e., after all the filtering for the desired raised cosine spectrum. This performance is best for 100% excess bandwidth filtering and worst for 0% excess bandwidth filtering.

Amplitude and delay slope are significant components of inband distortion resulting from multipath fading. In this paper, we present and compare computer simulated results of signal to noise (S/N) degradation for probability of error (Pe) of 10-4, due to these distortion components on Quadrature Phase Shift Keying (QPSK), Offset Quadrature Phase Shift Keying (Offset QPSK) and 8 Phase Shift Keying(8 PSK)systems. For all systems the transmitter and receiver filtering is modeled so as to provide a 50% excess bandwidth raised cosine spectrum at the receiver detector input. The QPSK and Offset QPSK systems are studied with bit rates of 44.7 Mbits/s, the so-called T3 rate, and the 8 PSK system is studied with bit rates of 44.7 Mbits/s and 67 Mbits/s, a 67 Mbit/s 8 PSK system having the same modulated frequency spectrum as 44.7 Mbit/s QPSK and Offset QPSK systems. When the frequency spectrum is the same for all systems, then for a given amplitude slope, results show that QPSK is the least degraded, followed by Offset QPSK, then 8 PSK, and for a given delay slope, Offset QPSK is the least degraded, followed by QPSK, than 8 PSK. When the bit rate is the same for all systems, then for a given amplitude slope, QPSK is the least degraded. 8 PSK is less degraded than Offset QPSK for values of amplitude slope less than 0.47 dB/MHz, but more degraded for higher values. For equal bit rates and a given delay slope, Offset QPSK is the least degraded, followed by 8 PSK, then QPSK. By relating degradation results to published information on the occurrence of amplitude and delay slope distortion components during multipath fading, estimates ofS/Ndegradation for a Peof 10-4due to these distortion components as a function of frequency selective fade depth are made. Finally, measuredS/Ndegradations for a Peof 10-4due to amplitude and delay slope on a 44.7 Mbit/s Offset QPSK system with filtering similar to the computer model are presented and compared to the computer simulation results.