Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
2000-03-30
2002-05-14
Vo, Don N. (Department: 2631)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
Reexamination Certificate
active
06389089
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the signal processing field. More specifically, the invention relates in general to a method for searching for pilot tones. These are sinusoidal oscillations at a known frequency in any given signal.
In communications systems, in particular in mobile radio systems, it is frequently necessary to search for pilot tones. For example, in digital mobile radio systems which operate in accordance with the GSM/DCS1800/PCS1900 Standard, the radio traffic is organized using organization channels. A mobile station which wishes to set up a connection to the network via a fixed station must firstly detect and search for the organization channel. This is done by searching for defined pulse trains. In the system mentioned above, so-called frequency correction bursts (FCB) are used for this purpose. Frequency correction bursts are a pulse train of 148 zeros.
In the system under consideration here, the so-called GMSK (Gaussian Minimum Shift Keying) method is used for transmission. In this case, a carrier frequency FT (for example 900 MHz) is modulated with the signal to be transmitted, that is to say in this case with the FCB signal of specific interest as well. The resultant frequency is FT+67.71 kHz, that is to say 67.71 kHz above the carrier frequency. The FCB pulse train of 148 zeros is in consequence changed into a pure sinusoidal signal. In baseband, this means that the phase difference between adjacent samples is ideally (without channel distortion and noise) 90°, if it is assumed that sampling takes place in time with the bits (270.83 kHz).
Various FCB search methods are known from the prior art. For example, the article “Anfangssynchronisation der Mobilstation im D-Netz” [Initial synchronization of mobile stations in the D network] by Frank and Koch, PKI Tech Report 1 (1990), pages 43-49 describes an FCB search method. There, the FCB search starts with a frequency shift by multiplying all the (I,Q) samples of the received signal by exp(−jk&pgr;/2). Each sample Z at the time k can be represented, in the complex plane as Z(k)=I(k)+jQ(k). This means that the received signal is shifted downward by 67.71 kHz, so that its mid-frequency after frequency shifting is 0 Hz. After this, the signal is low-pass filtered. If this is the FCB signal, this signal passes through the filter; other signals, in contrast, are largely supressed owing to their wide bandwidth. The magnitude of the filtered signal is then formed which, ideally, results in a pulse similar to a rectangle, with the duration of an FCB. On the other hand, the signal for the rest of the time is like noise, owing to the modulation with random data bits. An optimum search filter can be specified for a pulse similar to a rectangle. This corresponds to sliding averaging for the duration of an FCB. An FCB is regarded as having been found if the maximum value of the filtered signal exceeds a previously defined threshold. The position of the maximum value marks the end of the detected FCB.
The method described in that prior art approach has the disadvantage that the maximum value of the filtered signal depends on the instantaneous signal amplitudes, and is therefore subject to severe fading fluctuations. Adaptive amplitude control is thus necessary for a reliable FCB search. Furthermore, the low-pass filter must have a high Q-factor, and its implementation is therefore complex. In addition, the method is very sensitive to frequency mistuning between the mobile station and base station. Thus, in practice, the maximum value must be averaged over several observation intervals.
A further method is described in the article “Synchronisation einer Mobilstation im GSM-System DMCS 900 (D-Netz)” [Synchronization of a mobile station in the DMCS 900 GSM system (D network)] by Neuner, Bilitza, and Gärtner in Frequenz [Frequency] 47 (1993)3-4, pages 66-72. In that prior art method, the phase difference between every fourth sample of the received signal is evaluated. The method is based on the observation that such phase differences are ideally zero for the duration of an FCB. Since the phase difference between two adjacent samples is 90 degrees, the phase difference between four samples is 4×90=360 degrees, or zero degrees. Disturbances (fading) are taken into account by means of a validity area which is recalculated for each phase difference. An FCB is regarded as having been found when a sufficiently large number of negligibly small phase differences are present.
With that method, there are problems in determining the position of the FCB, since only every fourth sample is evaluated. Since the method described here makes it necessary to determine the phase difference between samples the arctan function must be used in order to calculate the phase of the sample from the quadrature components of the sampled received signal. However virtually no signal processors provide hardware support for this, so that the calculation is approximated by means of a complex series, which correspondingly requires computation time.
A third prior art method, which is described in European published patent specification EP 0 387 720, is similar to the above-mentioned method described by Frank and Koch. Two frequency-selective comb filters are used, one of which filter passes FCB signals at a frequency of 67.71 kHz without any impediment, while the other filter completely blocks FCB signals. Magnitudes and then sliding averages are formed from both filtered signals. The quotient of the two averages is then formed, and compared with a previously defined threshold value. If the quotient is below the threshold value, then an FCB is regarded as having been found. The position of the minimum of the quotient marks the end of the FCB.
The latter method has already been successfully used in chip sets for GSM mobile telephones. Since the quotient formation results in insensitivity to amplitude fluctuations, there is no need for the amplitude control that is required in the Frank and Koch method. However, the division required for quotient formation for this purpose likewise still involves a relatively large amount of computation time. Furthermore, the method is sensitive to frequency mistuning. If the frequency is mistuned, the one filter no longer passes the signal through completely, and the other filter no longer blocks the signal completely. This means that the minimum value of the quotient rises considerably and the threshold value, which is designed for the ideal case of minimal frequency mistuning, is no longer appropriate. The entire FCB search thus becomes uncertain.
German published patent disclosure DE 43 28 584 describes a circuit for synchronization of an electronic angle position transmitter. The angle position transmitter is continuously synchronized to the change in the angular position of a rotor. German published patent disclosure DE 38 06 428 describes a method for determining a bit combination contained in a serial bit stream. A search cycle is used to compare the bit combination successively with bits from comparison bit combinations. The comparison is carried out at the possible phase angles. European published patent disclosure EP 0 228 771 describes a full-duplex data transmission system for subscriber loops. The receiver in this case has a pilot phase detector which determines the phase of the incoming pilot tone. The phase detector does this with respect to a local pilot phase reference. Further, European published patent disclosure EP 0 489 880 discloses a diversity combination method, which relates to RF radio information connection systems. There, pilot symbol samples are separated from information symbol samples in the received, sampled and demodulated signal. The pilot symbol samples are then processed at predetermined pilot sampling times, in order to determine estimated values of a channel gain.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a search method for pilot sequences in a
Hartmann Ralf
Yang Bin
Siemens Aktiengesellschaft
Vo Don N.
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