Digital Phase Modulation Research Articles

A digital phase modulator and demodulator for a biomedical telemetry system.

The paper describes a digital phase modulator for use with a crystal-controlled biomedical telemetry transmitter, and a demodulator which accepts the phase-modulated output, at intermediate frequency, from a commercial receiver. The demodulator incorporates a phaselocked loop to provide a reference carrier for demodulation.

The Effect of a Bandpass Nonlinearity on Signal Detectability

When an angle-modulated signal plus noise constitute the input to a bandpass device exhibiting a nonlinear input-output power characteristic and AM to PM conversion, the noise component of the output has altered first- and second-order statistics. A method of evaluating the two-dimensional first-order statistics of this noise is presented. The effect on signal detectability of a nonlinearity inserted between two channel noise sources is studied; expressions for the mean square received phase error and probability of error (for coherent digital phase modulation) are derived. The hard limiting satellite channel, with Gaussian noise on the up and down links, is examined in detail, and it is demonstrated that the limiter can significantly affect signal detectability.

Properties of a Class of Signaling Waveforms for Digital Phase Modulation

This paper is concerned with the performance of partially coherent digital phase modulation (DPM) systems employing a class of binary signaling waveforms containing a zero crossing within each bit period. The class of waveforms includes the Manchester II bit code and sinusoidal bit code as well as an infinite number of other nonrectangular bit shapes. We wish to study the tradeoff between the RF bandwidth and the probability of bit error <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P(E)</tex> as a function of bit shape. The equation describing the RF power spectrum for these DPM signals is obtained using a two-dimensional Fourier transform technique and is shown to contain both continuous and discrete terms. The spectra are evaluated numerically for a typical bit rate showing the effect of different bit shapes. An original technique is presented for including the effects of modulation interference on the extraction of the carrier-phase reference from these DPM signals using a partially coherent phase-locked-loop receiver. The expression for the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P(E)</tex> is obtained for the class of signals and is a function of the phase deviation angle and the modulation interference. This expression is evaluated numerically to obtain sets of curves that show the optimal phase deviation angles for five members of the class. The tradeoff between the RF bandwidth and the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P(E)</tex> is given by a curve that indicates the increase in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">P(E)</tex> for a reduction in RF bandwidth as a function of the bit shape.

Error-Rate Considerations for Digital Phase-Modulation Systems

The performance of digital phase-modulation systems has been investigated by many authors, and we first present a simple and alternate method of evaluating the character error rates of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</tex> -phase coherent and differentially coherent phase-shift keyed (DCPSK) systems when the receiver is ideal. An ideal receiver is assumed to operate with zero-width decision thresholds, but in any practical receiver, the voltage (or current) in question must differ from the threshold value by a finite amount in order for the device to respond properly and reliably. This finite-width decision threshold for any practical receiver can be expressed in terms of a certain parameter γ, and we give methods of evaluating the character error rates of coherent phase-shift keyed (CPSK) and differentially coherent phase-shift keyed systems when the receiver has a finite nonzero γ. The receiver may also introduce a certain phase perturbation δ in the detected phase of the signal because of imperfections present in various parts of the receiver. The methods of evaluating its effect on both CPSK and DCPSK systems have also been given in this paper.

Reciprocal latching phase modulator for microwave frequencies

This paper describes the design and operation of a new type of latching (or digital) phase modulator in standard rectangular waveguide. This new device uses the square-loop magnetic properties of toroidal ferrite materials to obtain digital increments of reciprocal phase shift. By latching or switching the magnetization of this square-loop ferrite material from a zero-field value to either one of its two remanent states, a corresponding phase shift of the microwave energy is produced. The electrical characteristics of these latching-type phase modulators, typical of those obtained for early test models designed for X band, include reciprocal phase shifts of approximately 180°/in, insertion loss less than 0.8 dB, input VSWR no greater than 1.5, amplitude modulation less than ±;0.2 dB and an operating bandwidth of five percent. They can be switched to either of two stable states in approximately 1 μs with little switching energy. They are also capable of digital operation with noncritical driver current pulses, requiring no holding power to maintain phase in either state.

Combined Digital Phase and Amplitude Modulation Communication Systems

The performance of phase modulation communication systems is investigated for the asymptotic case of a high signal-to-noise ratio. A simple and excellent approximation to the probability of error is derived for both coherent and phase-comparison demodulation. In addition, a simple expression is derived for the asymptotic degradation of phase-comparison demodulation. When the number of bits per sample of the signal is large, a combination of digital amplitude and phase modulation is found to make more effcient use of transmitter power than phase modulation alone. The number of amplitude levels may be optimized for either minimum peak power or minimum average power, using the approximate expressions for probability of error. While an analytical solution can be found for the peak power case, a numerical solution with restriction to powers of two appears preferable, and is necessary for the average power case. Accordingly, tables showing the optimum number of levels with this restriction are presented.

Performance of Digital Phase-Modulation Communication Systems

This paper analyzes the performance of digital phase modulation systems in Gaussian noise and determines required signal-to-noise ratio as a function of the number of discrete phases and the desired error rate, under conditions of no fading. Both coherent detection with a locally-derived reference carrier and phase comparison detection are considered. The calculations show that multiphase modulation provides an efficient trade of bandwidth for signal-to-noise ratio in comparison with multilevel amplitude modulation. It is also found that phase comparison detection introduces about a 3-db degradation over coherent detection except with binary modulation, for which the degradation is less than 1 db for error rates not exceeding about 0.001.