Generalized tamed frequency modulation

Continued investigation of Tamed Frequency Modulation (TFM) has led to an extension, called Generalized Tamed Frequency Modulation (GTFM). The features of GTFM are based on the possibility to select different combinations of parameters which characterize the process of waveform generation. This still allows for the generation of a constant envelope modulated signal with a spectrum similar to TFM. However, a joint optimization of coherent and noncoherent detection with regard to error rate performance can be achieved. Attention is given to a noncoherent receiver based on the discriminator detection of a GTFM signal. Decision on the received binary data is determined from the demodulated signal at the output of the frequency discriminator by a decoder, which takes the form of either a simple bit-by-bit detector or a Maximum Likelihood Sequence Estimator (MLSE) of low complexity. The error rate performance of the proposed discriminator detection scheme can be optimized by an appropriate choice of parameters in GTFM signal generation at the transmitter. The generation of GTFM signals can largely be based on a programmable LSI circuit. 1. Introduetion Tamed Frequency Modulation (TFM) 1) belongs to a class of constant envelope modulation methods 2,3,4) aiming at bandwidth economy with practical equipment in a radio system environment. The superior spectrum of TFM, in comparison with that of the better known Minimum Shift Keyed signal"), is achieved by careful control of the phase argument of the modulated carrier with a particular premodulation filter (a sp.ecific3-tap transversal filter and a Nyquist-3 lowpass filter in cascade). The principles and early implementation of TFM have been described in ref. 1. Generalized Tamed FM (GTFM) is an extension of TFM. It is based on the possibility to select different parameter combinations for the premodulation filter. Within à particular constraint, the tap coefficients of the transversal filter and the roll-off factor of the Nyquist-3 lowpass filter, can appropriately be chosen to achieve a specified radiated spectrum with the desired performance in coherent and noncoherent receivers. Considerations have been given to the realizability of equipment of great flexibility. The type of coherent receiver considered has amply been described *) This paper is based on a paper of the same title presented at the IEEE International Conference on Acoustics, Speech and Signal Processing. (3-5) May 1982, Paris, France. ' Phllips Journalof Research Vol. 37 No.4 1982 165 K. S. Chung and L. E. Zegers in ref. 1. The noncoherent detection scheme discussed in this paper will typically consist of a frequency discriminator which is followed by either a bit-bybit detector or a Maximum Likelihood Sequence Èstimator (MLSE) of low . . complexity. It is noted that the optimization of error rate performance of the noncoherent receiver is based on the parameter adjustment of GTFM signal generation at the transmitter side. This transmitter will largely be based on table look-up processing of Read-Only-Memory (ROM) which forms part of an envisaged programmable transmitter LSI-circuit. 2. Generalized TFM A TFM signal1) is represented by s(t) = sin (2nfct + CP(t», (1) where.fc is the carrier frequency, and Cp(t) is the continuous phase function bearing the transmitted information. With TFM, the allowable change in cp(t) over one bit period is restricted to either 0, ±n/4 or ±n/2, determined by three consecutive input binary data bits. premodulation filter input binary _of---,--j data a