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WO2003003616A1 - Reduction de differences de frequence - Google Patents

Reduction de differences de frequence Download PDF

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Publication number
WO2003003616A1
WO2003003616A1 PCT/GB2002/002899 GB0202899W WO03003616A1 WO 2003003616 A1 WO2003003616 A1 WO 2003003616A1 GB 0202899 W GB0202899 W GB 0202899W WO 03003616 A1 WO03003616 A1 WO 03003616A1
Authority
WO
WIPO (PCT)
Prior art keywords
local oscillator
mobile communications
predetermined amount
frequency
sign
Prior art date
Application number
PCT/GB2002/002899
Other languages
English (en)
Inventor
Robert Deedman
Original Assignee
Ubinetics Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ubinetics Limited filed Critical Ubinetics Limited
Publication of WO2003003616A1 publication Critical patent/WO2003003616A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W56/00—Synchronisation arrangements
    • H04W56/0035—Synchronisation arrangements detecting errors in frequency or phase
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
    • H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
    • H04B7/2662—Arrangements for Wireless System Synchronisation
    • H04B7/2671—Arrangements for Wireless Time-Division Multiple Access [TDMA] System Synchronisation
    • H04B7/2675—Frequency synchronisation

Definitions

  • the invention relates to a method of reducing the frequency difference between a local oscillator of a mobile communications receiver and a clock of a remote base station.
  • Mobile communications equipment such as mobile telephones
  • UE user equipment
  • a base station it is necessary to provide synchronisation between a local oscillator of the UE and a clock of the base station.
  • part of the .initial synchronisation procedure is to correct for the frequency difference between the local oscillator in the UE and the local oscillator in the base station.
  • the local oscillator in the UE which has to be relatively inexpensive for obvious commercial reasons, will exhibit less accuracy than the more expensive and accurate clock in the base station. Accordingly, there is a frequency difference (sometimes called a "frequency error”) which has to be corrected.
  • UMTS universal mobile telephone system
  • CDMA code division multiple access
  • AFC automatic frequency control
  • the actual frequency error can be as high as +/- 6 kHz.
  • the estimated frequency difference can then be used to correct the local oscillator.
  • the current proposal for AFC is to use the timing information provided in the primary synchronisation channel (PSCH) transmitted by the base station.
  • PSCH primary synchronisation channel
  • the PSCH has to be used since knowledge of a scrambling code is required to demodulate other code-multiplexed channels.
  • the base station transmits a repeating sequence of 256 chips over the PSCH, each sequence occurring for one tenth of the period of a so-called slot (each slot being 2560 chips in length).
  • An estimate of the frequency difference is made by looking at the phase difference between two consecutive correlations of the repeating sequence and the local oscillator signal (a paired phase-estimate).
  • the correlation length used to obtain each phase estimate is 128 chips, and so two correlations of 128 chips are .performed on consecutive halves of the 256 chip PSCH sequence. From the phase difference can be calculated the frequency difference, the frequency of the UE's local oscillator thereafter being modified to account for the calculated frequency difference. This is explained mathematically by:
  • This prolonged period during which there is a frequency difference can cause a timing drift between the UE local oscillator and the base station. If this timing drift becomes large (greater than about half a chip) then the correlation on the PSCH will fail and synchronisation is lost. The position of the PSCH must be re-obtained. The need for re-synchronisation should be avoided.
  • the scrambling code can be more reliably obtained from a so-called Common Pilot Channel (CPICH) without the worry of frequency aliasing.
  • CPICH Common Pilot Channel
  • the CPICH which is transmitted at a higher power that the PSCH can be used to reduce further, with higher accuracy, the frequency difference (reducing the frequency difference to almost zero).
  • the method described herein relates to the more 'coarse' method which enables some initial degree of synchronisation to be achieved.
  • a method of reducing the frequency difference between a local oscillator of a mobile communications receiver and a clock of a remote base station comprising: receiving, at the mobile communications receiver, a synchronisation signal representative of the clock of the base station; performing two consecutive correlation operations on the synchronisation signal to generate an estimate of the sign of the phase difference between the local oscillator and the synchronisation signal; and modifying the local oscillator frequency until the sign of the phase difference changes.
  • the method provides a means by which an initial adjustment of frequency difference is made, without analysing the magnitude of the phase estimate (as relied upon by conventional methods).
  • only an estimate of the sign of the phase difference is used.
  • the local oscillator frequency is modified until the sign of the phase difference changes, which can be, for example, from positive to zero, from positive to negative, from negative to zero, or from negative to positive. Once the sign changes, this provides an indication that the zero error mark has been reached, or crossed, and so a reduced phase difference, and so frequency difference, can be attained.
  • the magnitude of the phase difference which is liable to interference from noise, is not relied upon and no low pass filtering is required.
  • the step of modifying the local oscillator frequency preferably comprises modifying the frequency by one or more steps of a predetermined amount.
  • the method may further comprise: modifying the local oscillator frequency by one or more steps of a further predetermined amount until the estimated sign of the phase difference changes again, the further predetermined amount being less than that of the previous predetermined amount.
  • the local oscillator frequency may be repeatedly modified until the further predetermined amount is reduced to reach a minimum predefined level which is indicative of the phase difference converging around a zero error mark.
  • the method used can be considered a 'successive approximation' method whereby the local oscillator frequency changes in steps of reducing magnitude such that the estimated phase difference converges around the zero error mark.
  • the further predetermined amount may be made equal to approximately one half of the previous predetermined amount.
  • a further two correlation operations are preferably performed to provide an updated estimate of the sign of the phase difference.
  • a further two correlation operations are preferably performed to provide an updated estimate of the sign of the phase difference.
  • the mobile communications receiver may be configured to receive the synchronisation signal over a spread spectrum communications channel.
  • the spread spectrum communications channel may transmit data using CDMA modulation.
  • the mobile communications receiver may be configured for use with UMTS, the synchronisation signal being received by means of the PSCH.
  • the method may further comprise: after synchronisation is achieved using the PSCH channel, demodulating a further communications channel to provide a further source of synchronisation. As mentioned above, this may be a channel having greater power than the PSCH channel making it possible to achieve a more accurate estimate of phase difference. In effect, the initial method (using the PSCH channel) becomes a coarse AFC method, with the new channel providing a 'finer' AFC.
  • a mobile communications receiver for receiving data from a remote base station, the communications ⁇ receiver comprising: a local oscillator; and a local oscillator controller, the local oscillator controller being arranged to receive a synchronisation signal representative of the clock of the remote base station, to perform two consecutive correlation operations on the synchronisation signal to generate an estimate of the sign of the phase difference between the local oscillator and the synchronisation signal, and to modify the local oscillator frequency until the sign of the phase difference changes.
  • Figure 1 is a representation of a UMTS cellular telephone system
  • FIG. 2 is a block diagram showing the elements of a mobile telephone for use in the UMTS system of Figure 1;
  • Figure 3 illustrates a PSCH channel used as part of the UMTS system of Figure 1;
  • Figure 4 is a phasor diagram showing the phase difference between two correlation operations made using the PSCH channel shown in Figure 3;
  • Figure 5 is a flow diagram representing an algorithm for performing frequency adjustment of a local oscillator forming part of the mobile telephone shown in Figure 2;
  • a UMTS cellular telephone system 1 comprises a base station 3 which is able to communicate with one or more mobile telephones within a cell, represented here by reference numeral 5.
  • a single telephone 7 is shown within the cell 5.
  • FIG. 2 shows the basic elements within the telephone 7.
  • a receiver/transmitter module 9 is connected to a radio frequency (r.f.) antenna 11.
  • the receiver/transmitter module 9 is modulated by a local oscillator 13 which is a crystal having an error specification of 3 parts-per-million.
  • the local oscillator 13 is controlled by a microprocessor 15.
  • the microprocessor 15 itself receives signals sent from the base station 3 by means of the receiver/transmitter module 9.
  • the base station 3 includes a very accurate clock (not shown).
  • a very accurate clock In order to establish a data link between the base station 3 and the telephone 1, it is necessary to establish synchronisation between the clock in the base station and the local oscillator 13 of the telephone.
  • the local oscillator 13 will be of a lower accuracy than the clock in the base station 3.
  • a frequency difference of +/- 6 kHz can occur.
  • the so-called PSCH channel is used to assist in reducing the frequency difference to such an extent that synchronisation can be achieved.
  • the existence of the PSCH channel in UMTS will be well known to those skilled in the art.
  • FIG 3 shows a representation of a PSCH channel 17 established between the base station 3 and the telephone 7. Unlike other UMTS channels, the PSCH channel 17 is located as part of the synchronisation process. This initial stage is often referred to as the "acquisition" stage. As Figure 3 shows, the PSCH channel includes a repeating burst of data (shown in black and represented by reference numeral 19). Each burst last 256 chips and forms one-tenth of each PSCH frame (of length 2560 chips).
  • the microprocessor 9 After powering up the telephone 7, the microprocessor 9 receives data transmitted over the PSCH channel 17, by means of the receiver/transmitter module 9, in its acquisition stage. The microprocessor 9 then obtains an estimate of the sign of the phase difference between the base station clock (represented in the burst 19) and the signal currently being generated by the local oscillator 13. It will be noted that 'phase difference' is referred to above. It will be appreciated that it is straightforward to convert from the phase difference to the frequency difference (a complete explanation of the mathematics involved being given above). The term phase difference is used here for ease of explanation, particularly with regard to the phasor diagram of Figure 4, but it should be remembered that the terms "phase difference” and "frequency difference” may be interchanged.
  • the microprocessor 9 obtains an estimate of the sign of the phase difference by means of performing two consecutive correlations.
  • the first correlation uses the first 128 chips 21a of the burst 19
  • the second correlation uses the second 128 chips 21b of the burst 19.
  • the two correlations are performed using these sets of chips 21a, 21b and the signal generated from the local oscillator 13.
  • the microprocessor 9 stores a so-called frequency modifier (Fmod) variable which is the predetermined amount by which the local oscillator frequency will be initially altered in an attempt to correct the phase/frequency difference.
  • Fmod frequency modifier
  • the initial value of Fmod is set to 6 kHz.
  • SignOld An index known as SignOld is also set. This index indicates the previous sign of the phase/frequency difference, although in the initial set-up it is "+”.
  • the alteration of the local oscillator 13 using Fmod will be in the negative direction (i.e. its frequency will be reduced) if Fmod is a positive value, and will be in the positive direction (i.e. its frequency will be increased) if Fmod is a negative value.
  • Step 27 is the correlation step described previously in which the estimated sign of the phase/frequency difference is obtained. This sign is known as SignNew.
  • step 35 In the case where SignNew is not the same as SignOld, the sign of Fmod is changed in step 35. Thus, if Fmod was previously 6 kHz, it will become - 6 kHz in step 35.
  • step 37 it is determined whether the value of Fmod is less than a value Fmin which is a lower limit set for frequency modification. If Fmod is above this level, then in step 39, the value of Fmod is halved. Accordingly, the -6 kHz will become -3 kHz.
  • the algorithm then returns to step 31 whereby the local oscillator 13 is modified by increasing its frequency by 3 kHz. The process then repeats. In the case where Fmod is less than F in, the value of Fmod will not be halved (since the lower limit is reached) but will remain the same.
  • step 29 the comparison will determine that SignNew equals SignOld and so the local oscillator 13 will be modified by reducing the frequency by 6 kHz, taking the actual frequency difference to -2 kHz (this being unknown to the microprocessor 13).
  • the algorithm is used to provide an initial (coarse) synchronisation in the acquisition stage. Subsequent to this, it becomes possible to access the CPICH on which a known sequence is transmitted. As will be understood by those skilled in the art, the CPICH enables a more fine-tuned synchronisation to be achieved, which is desirable for such a UMTS application. Nevertheless, without the initial, coarse, synchronisation, this would not be possible.
  • reference to a sign change means the sign changes from positive to negative, or vice-versa. However, in the rare event that the sign changes from positive to zero, or negative to zero (so that zero difference is attained) this will constitute a detectable sign change.
  • the above method and apparatus provides a robust way of improving the frequency difference between a clock signal from a base station and a receiver apparatus having a local oscillator.
  • the convergence to an acceptably small frequency difference is provided quickly, and the algorithm intrinsically compensates for timing drift.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

Selon l'invention, un système téléphonique cellulaire UMTS (1) comprend une station de base (3) qui peut communiquer avec un ou plusieurs téléphones mobiles dans une cellule (5), laquelle comprend un seul téléphone (7). Dans ce téléphone (7), un module récepteur-émetteur (9) est relié à une antenne HF (11). Le module récepteur-émetteur (9) est modulé par un oscillateur local (13) qui est un cristal ayant une spécification d'erreur de (3) parties par million. L'oscillateur local (13) est commandé par un microprocesseur (15) qui règle la fréquence de l'oscillateur local. Ce microprocesseur (15) reçoit lui-même des signaux envoyés par la station de base (3) au moyen du module récepteur-émetteur (9). La station de base (3) comporte une horloge à haute précision (non illustrée). Afin d'établir une liaison de données entre la station de base (3) et le téléphone (7), il est nécessaire d'établir une synchronisation entre l'horloge de la station de base et l'oscillateur local (13) du téléphone. A cet effet, on utilise un algorithme qui obtient des estimations du signe de l'erreur de phase déterminée par la corrélation d'une partie d'une rafale de données (19) sur le canal de synchronisation primaire (PSCH) (17). En fonction du signe de l'erreur de phase, la fréquence de l'oscillateur local (13) est modifiée de façon itérative afin de diminuer la fréquence d'erreurs.
PCT/GB2002/002899 2001-06-27 2002-06-25 Reduction de differences de frequence WO2003003616A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB0115771.8 2001-06-27
GB0115771A GB2377126B (en) 2001-06-27 2001-06-27 Frequency difference reduction

Publications (1)

Publication Number Publication Date
WO2003003616A1 true WO2003003616A1 (fr) 2003-01-09

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PCT/GB2002/002899 WO2003003616A1 (fr) 2001-06-27 2002-06-25 Reduction de differences de frequence

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WO (1) WO2003003616A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE202009003176U1 (de) 2009-03-10 2010-07-22 Glabete Ag Befestigungsmittel

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
TWI324023B (en) 2002-07-31 2010-04-21 Interdigital Tech Corp Start-up automatic frequency control (afc) method and apparatus

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5384550A (en) * 1992-09-18 1995-01-24 Rockwell International Corporation Loop transient response estimator for improved acquisition performance
US5450447A (en) * 1992-09-30 1995-09-12 Rockwell International Corporation Adaptive variable-gain phase and frequency locked loop for rapid carrier acquisition
EP1035665A1 (fr) * 1999-03-10 2000-09-13 Robert Bosch Gmbh Méthode pour la synchronisation initiale d'un système radio mobile numérique
DE19918373A1 (de) * 1999-04-22 2000-11-02 Siemens Ag Korrektur von Frequenzfehlern in Teilnehmerstationen

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH07312549A (ja) * 1994-05-19 1995-11-28 Toshiba Corp クロック補正方法及びその装置
JP2000332678A (ja) * 1999-05-21 2000-11-30 Kokusai Electric Co Ltd 同期維持方法及び無線装置及び無線通信システム

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5384550A (en) * 1992-09-18 1995-01-24 Rockwell International Corporation Loop transient response estimator for improved acquisition performance
US5450447A (en) * 1992-09-30 1995-09-12 Rockwell International Corporation Adaptive variable-gain phase and frequency locked loop for rapid carrier acquisition
EP1035665A1 (fr) * 1999-03-10 2000-09-13 Robert Bosch Gmbh Méthode pour la synchronisation initiale d'un système radio mobile numérique
DE19918373A1 (de) * 1999-04-22 2000-11-02 Siemens Ag Korrektur von Frequenzfehlern in Teilnehmerstationen

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE202009003176U1 (de) 2009-03-10 2010-07-22 Glabete Ag Befestigungsmittel
US8764932B2 (en) 2009-03-10 2014-07-01 Glabete Ag Mounting means

Also Published As

Publication number Publication date
GB2377126A (en) 2002-12-31
GB0115771D0 (en) 2001-08-22
GB2377126B (en) 2004-08-25

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