Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Reexamination Certificate
2002-02-25
2004-12-21
Nguyen, Tony T. (Department: 2685)
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
C455S043000, C455S067130
Reexamination Certificate
active
06834184
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method for providing reference data that is to be used in a mobile station of a radio communications network for a frequency update of the frequency used by the mobile station, which reference data is based on bursts received by the mobile station from a base station of said radio communications system. The invention equally relates to such a mobile station, a module for such a mobile station and a radio communications system.
BACKGROUND OF THE INVENTION
In radio communication systems, certain characteristics of the wireless reception environment are unpredictable. For example, when the location of a mobile station is changed from the inside to the outside of a building during a phone call or a packet transmission, the frequency can change on account of a difference in temperature. Resulting errors on the transmission path might lead in the worst case to a disconnection of the phone call or the packet transmission. Therefore, a mobile station has to track certain characteristics of the received signal in order to be able to decode the information from the signal correctly and to be able to transmit an uplink signal that can be decoded correctly by the network.
It is known to provide mobile stations with so called Automatic Frequency Correction AFC capabilities. The purpose of the automatic frequency correction is to track changes in the frequency of the received signal and to cause a corresponding change in the frequency used by the mobile station.
For GSM (global system of mobile communication), the synchronisation of mobile stations with a base station has to be conform with the standardisation by the European Telecommunications Standards Institute described in “Digital cellular telecommunications system (Phase 2+); Radio subsystem synchronisation”, GSM 05.10, Version 5.0.0, May 1996.
According to this standard, the base station sends signals on the broadcast control channel (BCCH) to enable the mobile station to synchronise itself to the base station and if necessary correct its frequency standard to be in line with that of the base station. The timings of timeslots, time division multiple access (TDMA) frames, traffic channel (TCH) frames and control channel frames are all related to a common set of counters which run continuously whether the mobile station and base station are transmitting or not. Thus, once the mobile station has determined the correct setting of these counters, all its processes are synchronised to the current serving base station. The mobile station times its transmissions to the base station in line with those received from the base station. The base station sends to each mobile station a “timing advance” parameter according to the perceived round trip propagation delay base station—mobile station—base station. The mobile station advances its timing by this amount, with the result that signals from different mobile stations arriving at the base station are compensated for propagation delay. This process is called “adaptive frame alignment”.
After synchronisation, the frequency used by the mobile station is continuously updated in order to ensure that it corresponds to the frequency used by the base station. The frequency used by the base station can be determined in the mobile station e.g. by determining the phase difference between two samples of a received burst/block in the digital plane, since there is a direct correspondence between this phase difference and the used frequency. In every used burst/block, an average is done over a certain number of pairs of samples. To get a more accurate estimate, additionally an average over a number of bursts is carried out, while only bursts/blocks that have a good signal-to-noise ratio are used. Depending on the mode, the used bursts/blocks and the average count changes. For example, when receiving system parameters the first time after synchronisation, faster adaptation may work better compared to the operation in traffic channel. An adequate implementation for AFC in TCH could use only slow associated control channel (SACCH) and silent descriptor (SID) frames as basis for reference data for an automatic frequency correction. In SACCH and SID, there is always transmission. The mobile station knows from the frame number when these frames occur, and thus an algorithm using the bursts in those frames can be implemented. As an example, if the averaging count in this case was set to 32 frames, that would result for TCH in an average frequency update time of 1.28 seconds.
In contrast to GSM, in GPRS (General Packet Radio Systems), which is transmitting in packet data traffic channels (PDTCH), there are no SID or SACCH frames ensuring that there is always a downlink transmission. The network is only responsible to provide at least one block containing four bursts every 18 block period for the mobile to be used for automatic frequency correction and other characteristics to be tracked. If these required reference transmission are used for automatic frequency correction, less bursts are available for automatic frequency correction than the minimum number required in GSM. Since the ETSI specification requirements are therefore stricter for the GPRS than for GSM in the sense that in the worst case situation there is less downlink transmission to be used as a reference for automatic frequency correction on GPRS PDTCH, the present possibilities for GPRS automatic frequency correction are not satisfying.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a method, a mobile station, a module for a mobile station and a radio communications system allowing for an improved supply of reference data for a frequency update of the frequency used in a mobile station.
This object is reached on the one hand with a method for providing reference data that is to be used in a mobile station of a radio communications network for a frequency update of the frequency used by the mobile station, which reference data is based on bursts received by the mobile station from a base station of said radio communications system, comprising:
considering all bursts received by the mobile station from the base station as possible basis for the reference data;
determining the signal-to-noise-ratio of at least some of the bursts considered; and
supplying at least some of the bursts for which a signal-to-noise-ratio greater than a predetermined value was determined as basis for the reference data to be used for the frequency update.
On the other hand, the object is reached with a corresponding mobile station mobile station for a radio communications system comprising means for updating the frequency used by the mobile station using reference data based on bursts received by the mobile station from a base station of said radio communications system; and means for considering all bursts received from the base station as possible basis for the reference data, for determining the signal-to-noise-ratio of at least some of the bursts received and for supplying at least some of the bursts for which a signal-to-noise-ratio greater than a predetermined value was determined as basis for the reference data to be used by the means for updating the frequency.
The object is further reached with a module for a mobile station comprising such means for providing reference data to means for updating the frequency used by a mobile station.
Finally, the object is reached with a radio communications system comprising such a mobile station.
The invention proceeds from the idea that all bursts received by a mobile station from a base station can be used as reference data in an accordingly designed automatic frequency correction algorithm used for an update of the frequency presently employed by the mobile station. In order to ensure the quality of the reference data, first the signal-to-noise-ratio is determined in the mobile station for the bursts received from the base station. Each determined signal-to-noise-ratio is compared to a predetermined value. Only the bursts for which a signal-to-noise-ratio
Degn Nils
Harkala Timo
Oksala Jarkko
Ruohonen Jari
Nguyen Tony T.
Nokia Corporation
Perman & Green LLP
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