Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1999-11-10
2002-05-14
Tse, Young T. (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S341000
Reexamination Certificate
active
06389079
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of digital modulated radio signals and more particularly to a method for the implementation of a non-coherent sequence estimation receiver for linear digital modulations.
BACKGROUND ART
In the relevant technical field, there are different classes of receivers. A first class of receivers is based on the structure of an optimum coherent receiver, that is, a receiver that minimizes the error probability on decided symbols when the synchronism is perfectly known, and in particular, the phase of the received signal, which will be dealt with hereafter. The implementation of such a receiver does not exhibit particular problems in a laboratory environment, where the modulation carrier is always available, but it cannot be followed up in practice when this receiver is placed in the field and the carrier is not available. In these cases, a preferred solution is to supply the receiver with a synchronization device enabling it to “recover” the information on the phase of the modulated carrier. The devices often used for this purpose are phase locked loops (Phase Locked Loop, or PLL). Such a receiver shall be hereinafter referred to as “pseudocoherent”, since it is implemented according to the configuration of a coherent receiver to which a phase reference is supplied by a synchronization device. In these receivers, the phase is recovered but for multiples of 2&pgr;
, where n depends on the type of adopted modulation. As a consequence of the ambiguity in the phase introduced by the PLL, a differential encoding must be used in transmission, that is a coding where the information is not associated with the absolute phase of the modulation carrier, but to the phase difference between two consecutive symbols. As an alternative to the differential encoding it is possible to use pilot symbols during transmission, as described hereafter.
A second class of receivers consists of non-coherent receivers, that is those not requiring the information on the absolute phase of the transmitted signal. These receivers have different advantages compared to pseudocoherent receivers, namely:
1. they can be employed in situations where the synchronization recovery is difficult, such as for instance in the case of fading channels, or in presence of Doppler shift or frequency jumps due to the instability of oscillators;
2. they are simpler and cost effective since they have no PLL;
3. the synchronization state cannot be lost, contrarily to receivers with PLL where this loss can occur due to phase jumps, false locking or loss of the locking state;
4. after an out-of-duty interval caused by deep fading they are immediately operative, contrarily to receivers with PLL that require a transient period to recover the locking condition;
5. they can be employed in time division multiple access communication systems (Time Division Multiple Access, or TDMA), where coherent detection is not recommended due to the comparatively long acquisition time of the synchronism.
The first non-coherent receivers considered in technical literature were differential receivers, often employed in the detection of modulated phase digital signals, or PSK (Phase Shift Keying), where a differential coding ties the information to the phase difference between two consecutive PSK symbols. The receiver estimates this phase difference, not requiring therefore to be locked in phase with the received signal. A possible interpretation of the operation of these receivers is the following: with the differential coding process, the phase reference necessary for the data estimate is contained in the preceding symbol. Therefore it is not necessary to determine an absolute phase reference, since the preceding symbol can be used for this purpose. However, this involves a degradation of performance compared to a coherent receiver, due to the fact that in differential detection the phase reference is noisy, while in coherent detection this reference is perfectly known and therefore noise free. We could say that in the case of differential detection the signal to noise ratio (Signal-to-Noise Ratio, or SNR) of the reference signal is the same as the SNR of the information signal. In the case of a coherent receiver, on the contrary, the SNR of the reference signal is theoretically infinite. For instance, in the case of PSK modulations with two phase values only, or BPSK, (Binary PSK) the loss is small, that is 0.8 dB approximately at bit error rate, or BER, (Bit Error Rate) of 10−
5
. On the contrary, in the case of PSK modulations with M >2 phase values, or M-PSK, the performance loss can reach 3 dB. Starting from the above considerations, differential receivers have been conceived, drawing the phase reference from a given number of past symbols, in order to “filter” the noise effect. In this way the SNR of the phase reference is of higher quality and the performance approaches that of a coherent receiver. This type of receivers employing a so-called “decision feedback” are described, for instance in the following papers:
“The phase of a vector perturbed by Gaussian noise and differentially coherent receivers”, authors: H. Leib, S. Pasupathy, published in IEEE Trans. Inform. Theory, vol. 34, pp.1491-1501, November 1988.
“Bit error rate of binary and quaternary DPSK signals with multiple differential feedback detection”, author: F. Edbauer, published in IEEE Trans. Commun., vol. 40, pp. 457-460, March 1992.
They can be considered the forerunners of block differential receivers, or N-differential receivers, described below.
Block differential receivers fill the performance gap between coherent performance and simple differential ones, and are well described in the following papers:
“Multi-symbol detection of M-DPSK”, authors: G. Wilson, J. Freebersyser and C. Marshall, published in the Proceedings of IEEE GLOBECOM, pp.1692-1697, November 1989;
“Multiple-symbol differential detection of MPSK”, authors: D. Divsalar and M. K. Simon, published in IEEE Trans. Commun., vol. 38, pp.300-308, March 1990;
“Non-coherent block demodulation of PSK”, authors: H. Leib, S. Pasupathy, published in the Proceedings of IEEE VTC, pp.407-411, May 1990;
and in the volume under the title “Digital communication techniques”, authors: M. K. Simon, S. M. Hinedi and W. C. Lindsey, published by Prentice Hall, Englewood Cliffs, 1995, for the case of M-PSK modulations.
Block differential receivers, as well as those adopting decision feedback, are based on the idea of extending the observation interval on which decisions are based, compared to the observation interval of two symbols only, typical of simple differential receivers. For the latter, there is an additional peculiarity, which is deciding on multiple symbols at the same time, instead of symbol by symbol. N-differential receivers use an observation window of N symbols, and simultaneously make the decision on N-1 information symbols. This decision strategy can be seen as an extension of the decision strategy of differential receivers, which in fact correspond to case N=2. It has been demonstrated that in the case of M-PSK modulations, for N→+∞ the performance of this type of receiver tends to be like that of the coherent receiver. A number of examples of block differential receivers can be found in the literature, suitable to the different modulations; some of which are described in the papers mentioned above. In addition, we point out that:
M-PSK modulations with channel coding are described in the paper under the title “The performance of trellis-coded MDPSK with multiple symbol detection”, authors: D. Divsalar, M. K. Simon and M. Shahshahani, published in IEEE Trans. Commun., vol. 38, pp.1391-1403, September 1990;
M-QAM coded and uncoded modulations (Quadrant Amplitude Modulation) are addressed in the paper “Maximum-likelihood differential detection of uncoded and trellis coded amplitude phase modulation over AWGN and fading channels metrics and performance”, authors: D. Divsalar and M. K. Simon, published in IEEE Trans. Commun., vol. 42,
Colavolpe Giulio
Raheli Riccardo
Siemens Information and Communications
Tse Young T.
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