Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2001-08-15
2002-05-28
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S363000, C375S364000, C370S526000
Reexamination Certificate
active
06396883
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for detection of pilot tones. Pilot tones are sinusoidal oscillations at a known frequency, which are used, for example, in communications systems, in particular in mobile radio systems. A frequent task that occurs in such mobile radio systems is to search for pilot tones.
For example, in digital mobile radio systems that operate in accordance with the GSM/DCS1800/PCS1900 Standard, the radio traffic is organized into organization channels. For a mobile station to set up a connection to the network via a fixed station, it first needs to detect and search for this organization channel. The organization channel is detected by searching for specific pulse sequences, which identify this organization channel.
In the system cited above, pulse sequences are referred to as frequency correction bursts (FCB) and have a sequence of
148
zeros.
In the system under consideration here, the GMSK modulation method (Gaussian Minimum Shift Keying) 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, in particular, also with the FCB signal which is of specific interest. The resultant frequency is FT+67.7 kHz, that is to say 67.7 kHz above the carrier frequency. The FCB pulse sequence of 148 zeros is thus converted to a pure sinusoidal signal. In the baseband, this means that the phase difference between adjacent samples is ideally (without channel distortion or noise) ninety degrees (90°), if it is assumed that sampling takes place at the bit clock rate (4*67.7=270.8 kHz).
Various methods for FCB searching 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 G. Frank and W. Koch, PKI Tech. Report 1 (1990), pages 43-49 describes one method for FCB searching. In this method, the FCB search starts with a frequency shift by multiplying all the (I,Q) samples of the received signal by exp(−jkΠ/2). Each sample Z at the time k can be represented, as a complex number, in the form Z(k)=I(k)+jQ(k). This means that the received signal is shifted downward by 67.7 kHz, so that its mid-frequency after frequency shifting is 0 Hz. The signal is then low-pass filtered. If this is the FCB signal, then it passes through the filter; other signals are largely suppressed due to their wide bandwidth. The magnitude of the filtered signal is then formed, ideally resulting in an approximately rectangular pulse of the same duration as an FCB signal. In contrast to this, the modulation with random data bits in the rest of the time results in a signal similar to noise. An optimum search filter can be specified for the approximately rectangular pulse. This corresponds to sliding averaging over the time period of an FCB. An FCB is regarded as having been found when 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 signal.
The method described in this article 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. Therefore, adaptive amplitude control is required for a reliable FCB search. The low-pass filter also must have a high Q factor; therefore, its construction is complex. Furthermore, this method is highly sensitive to frequency mistuning between the mobile station and base station. Thus, in practice, the maximum value has to be averaged over a number of 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 GSM DMCS 900 system (D network)] by H. Neuner, H. Bilitza, S. Gärtner in Frequenz [Frequency] 47 (1993) 3-4, pages 66-72. In this method, the phase difference between every fourth sample of the received signal is evaluated. The method is based on the observation that, ideally, such phase differences are zero for the duration of an FCB signal. Since, as already stated above, the phase difference between two adjacent samples is 90°, the phase difference between four samples is 4×90=360°, or 0°. Interference (fading) is taken into account with a validity range, which is recalculated for each phase difference. An FCB signal is regarded as having been found when a sufficiently large number of negligibly small phase differences occur.
One problem with this method is determining the position of the FCB signal because only every fourth sample is evaluated. Because 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 any hardware support for this, so that the calculation is approximated by a complex series development, which requires a considerable amount of computation time.
A third method from the prior art is a method that was developed by Dr. Ralf Hartmann at Siemens AG, which is similar to the Frank and Koch method. This method uses two frequency-selective filters, one of which filters passes FCB signals at the frequency 67.7 kHz without any attenuation, 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 is 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 quotient minimum marks the end of the FCB signal.
This method already has been used successfully in chip sets for GSM mobile telephones. Because the quotient formation process results in insensitivity to amplitude fluctuations, the amplitude control required in the Frank and Koch method is not necessary. However, the division process required for quotient formation likewise still requires a relatively large amount of computation time. Furthermore, the method is sensitive to frequency mistuning. In the event of frequency mistuning, one filter can no longer pass the signal through completely, while the other filter no longer completely blocks the signal. This means that the quotient minimum value rises considerably and the threshold value, which is configured for the best case of minimum frequency mistuning, is no longer suitable, so that the entire FCB search becomes uncertain.
A further method for searching for such pilot tones is known from German Patent Application DE 197 43 191, corresponding to U.S. patent application Ser. No. 09/539,239 filed on Mar. 30, 2000. The inventors are named R. Hartmann and B. Yang and the invention is entitled, “Verfahren zur Suche nach Pilottönen,” [Method for searching for pilot tones] (date of application Sep. 30, 1997). This method uses what is referred to as differential symbol estimation. In this case, the exact phase differences between successive (I,Q) samples of the received signal are not determined, as in the method by Neuner, Bilitza, and Gartner. Instead of this, all that is investigated is to determine whether the phase differences between successive samples are in the interval (0, Π) or (−Π, 0). Both cases correspond to a transmitted symbol of 1 (“+1”) or 0 (“−1”) from the GMSK modulator. Because a FCB signal has 148 zeros is changed to 147 ones after differential coding at the transmitter end, and a virtually equal number of ones and zeros occur outside the FCB signal, then it is possible to search for an FCB signal by searching for a long, cohesive block of ones.
The advantage of the differential symbol estimation is its simple implementation. If I(k) represents the in-phase component and Q(k) repres
Hartmann Ralf
Yang Bin
Bocure Tesfaldet
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Stemer Werner H.
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