This paper describes and summarizes the characteristics of the modulation techniques most applicable to digital radio. The modulation techniques discussed are on-off-keying (OOK) with coherent and noncoherent detection, quadrature amplitude modulation (QAM), quadrature partial response (QPR), frequency-shift-keying (FSK) with noncoherent detection, continuous phase FSK (CP-FSK) with coherent and noncoherent detection, minimum-shift-keying (MSK), binary and quaternary phase-shift-keying (BPSK, QPSK) with coherent and differentially coherent detection, offset-keyed QPSK (OK-QPSK), M -ary PSK with coherent detection ( M = 8, 16 ), and 16-ary amplitude and phase-shift-keying (APK). Functional descriptions of these schemes are provided and their performance is compared in a series of tables summarizing the results of the literature of the past 20 years. The modulation schemes are compared with respect to ideal (white Gaussian noise) performance, spectral properties, signaling speed, complexity, and the effects on performance of interference, fading and delay distortion.

A Comparison of Modulation Techniques for Digital Radio

This paper deals with the problem of synchronization of bursty offset QPSK signals in the presence of frequency offset and additive white Gaussian noise. The main tasks of the receiver are to detect a burst (a frame of data) with a low acquisition time, to estimate and cancel the frequency and phase offset (carrier recovery), perform timing recovery, matched filtering and finally recover the data such that the bit-error-rate (BER) is close to the theoretical limit. The key contribution of this paper is to develop synchronization techniques in discrete-time that perform effectively even at signal-to-noise ratio (SNR) close to 0 dB in additive white Gaussian noise (AWGN) channels. These techniques are well suited for implementation on a digital signal processing (DSP) platform.

Synchronization of bursty offset QPSK signals in the presence of frequency offset and noise

The key contribution of this work is to develop transmitter and receiver algorithms in discrete-time for turbo- coded offset QPSK signals. The proposed synchronization and detection techniques perform effectively at an SNR per bit close to 1.5 dB, in the presence of a frequency offset as large as 30 % of the symbol-rate and a clock offset of 25 ppm (parts per million). Due to the use of up-sampling and matched filtering and a feedforward approach, the acquisition time for clock recovery is just equal to the length of the preamble. The carrier recovery algorithm does not exhibit any phase ambiguity, alleviating the need for differentially encoding the data at the transmitter. The proposed techniques are well suited for discrete-time implementation.

Design and Development of a Burst Acquisition System for Geosynchronous Satcom Channels

A recovery loop includes an analog circuit that mixes a fixed frequency signal from a temperature controlled oscillator (20) with an IF input to produce product signals corresponding to the quadrature components of data signals with a frequency offset. A digital complex multiplier (32) is responsive to the product signals and to the output of a number controlled oscillator (34) to produce a digital output corresponding to the data signals. The output of the number controlled oscillator (34) is controlled by a digital phase lock loop.

Demodulator carrier recovery loop and method for demodulating a signal

A parallel phase locked loop (PLL) system includes a first chain of a plurality of pre-locking PLLs that operates from a free-run state to a locked state; and a second chain of a plurality of PLLs to work from the locked-state to recover signal output.

Systems and methods for performing parallel digital phase-locked-loop

The Canadian RD-3 digital radio system will utilize duobinary encoded, offset QPSK modulated signals. This complex signal structure is such that a conventional fourth order nonlinearity and phase-locked loop cannot be used for carrier recovery. In this paper, a decision-directed phase-lock loop is proposed for carrier recovery and demodulation of these signals. The equations describing loop operation are obtained, and the effective phase-detector characteristic is examined. Fokker-Planck techniques are then employed to approximately find the steady-state probability density function of the phase error, and this is used to find an upper bound on the average probability of error.

D. Cheung

Papers

A Decision-Directed Carrier Recovery Loop for Duobinary Encoded Offset QPSK Signals