This paper is concerned with digital modulation for future mobile radio telephone services. First, the specific requirements on the digital modulation for mobile radio use are described. Then, premodulation Gaussian filtered minimum shift keying (GMSK) with coherent detection is proposed as an effective digital modulation for the present purpose, and its fundamental properties are clarified with the aid of machine computation. The constitution of modulator and demodulator is then discussed from the viewpoints of mobile radio applications. The superiority of this modulation is supported by some experimental test results.

GMSK Modulation for Digital Mobile Radio Telephony

Minimum shift keying and offset QPSK are two well-known modulations which can be interpreted as a set of time/phase-shifted AM pulses. We show in this paper that any constant amplitude binary phase modulation can also be expressed as a sum of a finite number of time limited amplitude modulated pulses (AMP decomposition). New methods for computing autocorrelation and power frequency spectrum are derived, which give very simple results for half-integer index modulations. We also show that the signal can be built with good accuracy using only one optimized pulse ("main pulse"). This synthesis is particularly satisfactory for modulations that have good spectral characteristics and/ or low index.

Exact and Approximate Construction of Digital Phase Modulations by Superposition of Amplitude Modulated Pulses (AMP)

The ever increasing demand for digital transmission channels, in the radio frequency (RF) band presents a potentially serious problem of spectral congestion and is likely to cause severe adjacent  and cochannel interference problems. This has, in recent years, led to the investigation of a wide variety of techniques for solving the  problem of spectral  congestion.  Some solutions to this problem include: 1) new allocations at high frequencies; 2) better  management of existing allocations; 3) the use of frequency-reuse techniques such as the use of narrow-beam antennas and dual polarizing systems;  4)  the  use of efficient source encoding techniques; and 5) the use of spectrally efficient modulation techniques [l]. This article will consider the last approach and analyze, in particular, a modulation scheme known as minimum shift keying (MSK). The MSK signal format will be explained and its relation to other schemes such as quadrature phase shift keying (QPSK), offset QPSK (OQPSK), and frequency shift keying (FSK) pointed out. The main attributes of MSK, such as constant envelope, spectral efficiency, error rate performance of binary  PSK, and self-synchronizing capability will all be explained on  the  basis of the modulation format.

Minimum shift keying: A spectrally efficient modulation

An analysis of constant envelope digital partial response continuous Phase modulation (CPM) systems is reported. Coherent detection is assumed and the channel is Gaussian. The receiver observes the received signal over more than one symbol interval to make use of the correlative properties of the transmitted signal. The Systems are M -ary, and baseband pulse shaping over several symbol intervals is considered. An optimum receiver based on the Viterbi algorithm is presented. Constant envelope digital modulation schemes with excellent spectral tail properties are given. The spectra have extremely low sidelobes. It is concluded that partial response CPM systems have spectrum compaction properties. Furthermore, at equal or even smaller bandwidth than minimum shift keying (MSK), a considerable gain in transmitter power can be obtained. This gain increases with M . Receiver and transmitter configurations are presented.

Continuous Phase Modulation--Part II: Partial Response Signaling

M inimum Shift Keying (MSK) [ 11 or Fast Frequency Shift Keying (FFSK) is a digital modulation technique with constant amplitude which has been studied extensively during recent years. The properties of MSK are now understood [2-51. In this article, we report on methods to improve on MSK while maintaining a constant amplitude. By improvement we mean a narrower power spectrum, lower spectral sidelobes, better error probability, or all of the above. The cost of signal processing and the speed and complexity with which it can be performed has steadily been improved over recent years. Further improvements are to be expected. Therefore, it is reasonable to study what can be accomplished with methods that might be too complex for cost effective realization today, but perhaps may be easily achievable tomorrow. In this paper, we consider a number of methods for constructing constant amplitude signals which significantly outperform MSK [5-1031. We also discuss at what level of complexity these improvements are obtained and realized.

Continuous phase modulation

This paper describes a new type of frequency modulation, called Tamed Frequency Modulation (TFM), for digital transmission. The desired constraint of a constant envelope signal is combined with a maximum of spectrum economy which is of great importance, particularly in radio channels. The out-of-band radiation is substantially less as compared with other known constant envelope modulation techniques. With synchronous detection, a penalty of only 1 dB in error performance is encountered as compared with four-phase modulation. The idea behind TFM is the proper control of the frequency of the transmitter oscillator, such that the phase of the modulated signal becomes a smooth function of time with correlative properties. Simple and flexible implementation schemes are described.

C. Dekker

Papers

Tamed Frequency Modulation, A Novel Method to Achieve Spectrum Economy in Digital Transmission