Performance Analysis of Optimum Receivers for Differentially Encoded \u3cem\u3eM\u3c/em\u3e-PSK in Low SNR

A recent paper has proved that the classical receiver for coherent detection of differentially encoded M-PSK in AWGN is optimum for the MAP sequence detection criterion. In this letter, we show that asymptotically, as SNR tends to zero, the MAP symbol detection criterion receiver is equivalent to the classical differentially coherent receiver, for M greater than two. An asymptotic relative efficiency figure of merit is defined in order to compare the performances of the classical coherent receiver and the classical differentially coherent receiver

Linear predictive receivers for phase-uncertain channels

In this paper, we propose linear predictive receivers for phaseuncertain channels. These receivers are attractive from a conceptual viewpoint because they generalize previous solutions based on noncoherent sequence detection. On the practical side, the proposed algorithms lend themselves to the implementation of adaptive receivers capable of copying with possible time variations of the statistics of the underlying phase model. 1

Robust multilevel coherent optical systems with linear processing at the receiver

This paper investigates optical coherent systems based on polarization multiplexing and high-order modulations such as phase-shift keying (PSK) signals and quadrature amplitude modulations (QAM). It is shown that a simple linear receiver processing is sufficient to perfectly demultiplex the two transmitted streams and to perfectly compensate for group velocity dispersion (GVD) and polarization mode dispersion (PMD). In addition, in the presence of a strong phase noise of the lasers at the transmitter and receiver, a symbol-by-symbol detector with decision feedback is able to considerably improve the receiver robustness with a limited complexity increase. We will also discuss the channel estimation and the receiver adaptivity to time-varying channel conditions as well as the problem of the frequency acquisition and tracking. Finally, a new two-dimensional (polarization/time) differential encoding rule is proposed to overcome a polarization-ambiguity problem. In the numerical results, the receiver performance will be assessed versus the receiver complexity

A-posteriori symbol probabilities and log-likelihood ratios for coherently detected π/4-DE-QPSK

In this letter, coherent detection of p/4-DE-QPSK is considered, but our analysis also holds for common DE-QPSK. It is shown that maximum a-posteriori (MAP) sequence detection can be regarded as an approximation, based on selecting dominant exponentials, of MAP symbol detection. A better approximation, relying on piecewise-linear fitting of the logarithm of the hyperbolic cosine, is proposed. This approximation results in a performance very close to optimal symbol detection. For the case where the symbols are produced by convolutional encoding and Gray mapping, the log-likelihood ratios are investigated. Again a simple approximation based on selecting dominant exponentials and an improved approximation relying on piecewise-linear fits, are discussed. As in the uncoded case the improved approximation gives a performance quite close to ideal. While the particular examples considered show modest gains in performance, this letter provides a way of improving performance when needed

New Receivers for Differentially Encoded Offset-QPSK

Before a new mobile radio system will be standardised, an intense process of decision making is required concerning the different aspects of the system. One aspect is the definition of the air interface and in particular the employed modulation method. During the standardisation of the TETRA (Terrestrial Trunked Radio) system, the expert group decided to choose the linear modulation format π/4-QPSK. One important point for decision making was the requirement to support power efficient, nonlinear amplifiers. Although the second candidate, Offset-QPSK (OQPSK) shows further reduced envelope fluctuations, the nonavailibilty of a conventional differential demodulator for OQPSK led to a clear preference for π/4-QPSK. Differential demodulation is an at-tractive combined carrier phase estimation and demodulation technique on fast time selective fading channels. The work presented in the thesis is concerned with the synthesis and analysis of receiver algorithms for differentially encoded OQPSK signals, which fulfil the requirements of modern mobile radio systems. The thesis begins with an overview of the theoretical principles of digital modulation and optimum receivers. A description of mobile radio channels and the theoretical performance of PSK signals complete the basic chapter. A new receiver structure for DOQPSK using differential demodulation followed by a Viterbi decoder is then proposed. On this basis, an advanced algorithm suitable for general roll-off factors and with reduced complexity is derived and investigated. Following the differential demodulation, original algorithms for joint coherent demodulation and equalisation, which are based on the principle of per-survivor processing (PSP) are synthesised and discussed. These receiver structures are extended for the use in time and frequency selective mobile radio channels. With the aid of a computer simulation system the derived receiver structures are investigated. Simulation results on different linear and nonlinear channels are presented and compared with results obtained with standard π/4-DQPSK receivers and the theoretical bounds. The thesis concludes with a description of the simulation tool COSSAP and gives examples of created and implemented simulation models. The results of a fixed-point analysis of one model is presented and the possibilities to synthesise a hardware implementation is discussed

Efficient Maximum-Likelihood Based Clock and Phase Estimators for OQPSK Signals

In this paper we propose an algorithm for joint carrier phase and timing estimation with OQPSK modulations. The derivation is based on the maximum-likelihood criterion, and exploits a very efficient algorithm for the detection of differentially encoded $M$-PSK symbols already described in literature. Though we are mainly interested in measuring the phase and clock parameters, estimates of the transmitted symbols are also obtained as by-products. The resulting scheme has a feedforward structure and provides phase and timing information in a fixed time, differently from closed-loop architectures. It can be implemented in digital form and is particularly suitable for burst mode transmissions. Its performance is investigated by simulation and the results are compared with Cramér-Rao bounds. It turns out that the estimation accuracy is very close to the theoretical limits, even with short observation intervals and small values of the excess bandwidth. In such conditions, the proposed estimators largely outperform other schemes already known in literature. Their superiority becomes less significant as the signal bandwidth increases

A space communications study Final report, 15 Sep. 1966 - 15 Sep. 1967

Investigation of signal to noise ratios and signal transmission efficiency for space communication system