V.I. Simongauz – Ph.D.(Eng.), Associate Professor, Department 402, Moscow Aviation Institute (National Research University)
The carrier regeneration, the phase and symbol synchronization before the selection of symbols are necessary operations when re-ceiving a radio signal with suppressed carrier. In practice, the problem of carrier regeneration, phase and symbol synchronization solved sequentially. Initially it provided the carrier recovery and the phase synchronization, and then the symbol synchronization and the selection of symbols. Used to practice methods provide an effective solution to these problems only when the signal to noise ratio at the input is large enough (at least about 10 dB). With a decrease in signal/noise ratio increases energy losses. Steady work is disrupted when the signal/noise ratio exceeds the threshold. There is suppression of the weak signal due to noise.
The article deals with the problem of constructing a system of joint phase and symbol synchronization for a radio signal with quadrature phase modulation with time offset (OQPSK). The optimal algorithm is based on the maximum likelihood method. In this case the likelihood function is from the functional of two-dimensional probability density for the components of the complex envelope of the input process. Relations defining the maximum likelihood estimates for phase and delay of the modulating signal used as a basis for the construction of tracking algorithms. In contrast to the methods used in practice, considered algorithms provide reliable tracking options for small values of the energy relationship without suppressing small-signal noise, which is particularly important in radio deep space communications. Energy losses in the selected symbols depend only on the tracking error, the allowable level of which can provide a choice of loop bandwidth of phase tracking and loop bandwidth of delay tracking. Presented relations determining the energy ratio in the symbol band, ratio defining bandwidth of tracking systems and tracking error variance.
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