Pulse or digital communications – Spread spectrum – Direct sequence
Patent
1992-08-14
1994-08-30
Chin, Stephen
Pulse or digital communications
Spread spectrum
Direct sequence
375106, 3701001, H04L 710, H04J 306
Patent
active
053434978
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
This invention refers to the acquisition of synchronization, in the connection phase, between a generic mobile station and a base station, generally called radio base station, in a digital radiomobile system where information is transmitted according to time division multiple access (TMDA) through a phase modulation of a radiofrequency carrier wave, e.g. a MSK, GMSK and similar modulation.
BACKGROUND OF THE INVENTION
While the radio base station is usually equipped with a frequency reference and an highly accurate clock, e.g. in the range of 0.05 ppm, the local oscillator of the mobile station has a lower accuracy (0.1 ppm), mainly for cost reasons. Also, the presence of two oscillators having the same accuracy degree will ending result not practical considering the sum of errors and the error due to the Doppler effect as well.
It is therefore essential that at the start of the connection the mobile station obtains an accurate synchronization from the base station.
In SCPC (Single Carrier Per Channel) systems the problem is solved with the use of a PLL (Phase Locked Loop) or similar system (for instance a so-called Costas Loop); it is sufficient that the mobile system has a frequency not far from the base one more than the link band to be able to link and correctly demodulate the signal received.
In TDMA systems this scheme cannot be proposed since the signal is not a continuous stream, but consists of time slots separated by guard periods during which the transmitter is off and a PLL will loose the link between one slot and the other one.
U.S. Pat. No. 4,653,075 describes a bit synchronizer for a BPSK input signal having a sinusoidal carrier frequency f that is much greater than the modulation signal bit rate. The synchronizer comprises means for repetitively sampling the amplitude of the input signal over a preselected sampling interval portion of each modulating signal bit period.
In a digital radiomobile system this synchronizer cannot be proposed since the signal is very disturbed and the amplitude sampling does not obtain acceptable results.
SUMMARY OF THE INVENTION
The object of this invention is then to propose a device and a method for the synchronization of frequency and internal clock references of the mobile system with those of the base radio station, in a fast and reliable way and at limited costs.
This object is achieved performing a phase sampling instead of an amplitude sampling. The invention comprises a synchronization method between a mobile radio station and a base station in a TDMA digital radiomobile system, in which the base radio station transmits at least one timeslot containing a sinusoid having a frequency equal to 1/p of the bit transmission frequency and a timeslot containing a preamble known by the mobile station, the method comprising: 1/p of the bit frequency and to measuring the phase increases between two subsequent sub-samplings; and 180.degree., assigning a first binary value at each threshold going over and a second binary value in the other cases; subsequent samplings, with n lower than b/p where b is the bit number of actual signal of the sinusoid; V and the sum S at each new sampling; synchronization of the mobile station with the timeslot containing the sinusoid;
VI. calculating the frequency offset between the local oscillator frequency and the one of the base station and correct this offset within .+-.1 kHz; synchronization and message decoding containing the data of the station and of the frame.
The invention comprises furthermore a device for synchronization between a mobile radio station and a base station in a TDMA digital radiomobile system, in which the base radio station transmits at least one timeslot containing a sinusoid having a frequency equal to 1/4 of the bit transmission frequency and one timeslot containing a preamble known by the mobile station, the device comprising: at 4:1 ratio, respectively, a converter circuit equipped with conversion table, whose inputs are connected to the outputs of said decimators; the output of said r
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Canosi Claudio
Rosina Giancarlo
Chin Stephen
Italtel Societa Italiana Telecommunicazioni S.p.A.
Tse Young
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