KOSS-18 Universal method for determining digital amplitude-phase modulation type under conditions of a priori uncertainty

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KOSS-18 Universal method for determining digital amplitude-phase modulation type under conditions of a priori uncertainty

At present, a need in radio channel resources increases sharply. This has provoked a rapid progress in methods for transmitting information with spectral-effective signals. Signals using digital multi-positional amplitude-phase modulation relate to such a class. In the last five years, the number of various types of signals having modulation of such class increases several dozen times, and now reaches several thousand according to very modest estimations. A characteristic feature of modern systems is a capability to change, during operation, form one type modulation to another one depending on varying external conditions. In this case, the channel capacity is employed as much as technical possibilities allow.

Since the digital signal processing technology permits to realize rather complicated algorithms, many processing methods appeared to be available, which are capable to perform effective exchange at maximum rates. Thus, equalizer circuits capable to eliminate substantive intersymbol interference, adaptive apparatus for testing channel, fine precision synchronization units have been widely applied. Hundreds of amplitude-phase signal structures (V. 90 Recommendation) have been established as standards. However, all issued apparatus of communication systems, prior to reach the optimal multi-positional modulation structure, perform procedures of adjustments, mutual synchronization and matching of characteristics of transceiver units with channel characteristics. Monitoring systems substantially lack such possibility and could not generally orient themselves to a possibility of matching with both channel and monitored sources. On the other hand, air monitoring apparatus could not get access to the a priori information on employed irradiating sequences and precise synchronization characteristics generated by remote monitored objects during mutual operation in adjustment mode. Examples of such systems can be V.34, V.90, Link-11, Link-16, etc. systems.

Conventional monitoring methods, which assume operation with rather simple modulation laws, e.g., PM-n signals, are absolutely useless for aforementioned modern communication systems. Channel band operation ratio common for up-to-date communication systems (90-100% of Nyquist limit) leads to intersymbol interference of such a level that probability of symbol distortion without use of equalizers becomes near 1/30-1/20, even in the absence of noise. Extraneous modulation due to intersymbol interference does not substantially permit to distinguish signal having phase and amplitude-phase modulation using known methods. And the worst thing is that due to intersymbol interference having determined nature, they could not be suppressed using conventional methods of statistical processing. At the same time, parameter characteristics of real channels are so complicated that it is impossible to create an intersymbol interference model. All this leads to the urgent necessity of searching and creating essentially new, universal processing methods which, under conditions of a priori uncertainty relative to parameters of carrier frequency, amplitude, phase, modulating digital sequence and communication channel characteristics, could to:
  • form estimates of unknown synchronization parameters of signals being received (carrier and clock frequencies);
  • determine a type of amplitude-phase modulation (to form a pattern of employed amplitude-phase constellation);
  • form automatically a trustworthiness degree of obtained solutions.

The proposed algorithm for automatic recognizing type of amplitude-phase signal modulation meets above operation conditions. The problem solution of principle has been succeeded to obtain on the base of introduced method for preprocessing observations and choosing character sections of signal trajectory. In fact, "experiment planning" with preliminary sifting data is proposed. After such "planning", "hampering data" are excluded from consideration, and stochastic methods are quite applicable for processing remained data. Universality of obtained solutions was found out to be so large that it become possible to identify various modulation types in the presence of arbitrary (analog) transformations of coordinates and position number in original signal amplitude-phase constellations. The method has enveloped all signals having conceivable and inconceivable structures of amplitude-phase constellations with position number up to 128. Only restriction of modern computing means on memory reserve prevents a further raising of combination power. Synchronization parameter recovery was found out to be possible even without pre-adjustment and without elimination of intersymbol interference. Such results are unknown in literature even for issued operation mode of communication systems, although they suppose full a priori information on a type of complex amplitude-phase modulation being employed. The accuracy of synchronization parameters estimates being obtained by the new method was found out to be commensurable with potentially achievable border which could be reached in monitoring schemes operated in conditions of full a priori certainty and at small deviations of synchronization parameters themselves. Automatic mode for forming trustworthiness measure for solution being made has opened a possibility to organize an operatorless scanning of monitored ranges. Essentially new methods for organizing operation of equalizers which are "blindly" adjusting only in accordance with solution certainty at an output. It was confirmed experimentally that their use leads to reducing probability of intersymbol interference from 1/20 to 10-4 (having in mind the cases with sufficient energy reserve).

 
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