GB2391751A

GB2391751A - Clock frequency correction in a mobile communications system

Info

Publication number: GB2391751A
Application number: GB0218287A
Authority: GB
Inventors: Darioush Downer
Original assignee: NEC Technologies UK Ltd
Current assignee: NEC Technologies UK Ltd
Priority date: 2002-08-06
Filing date: 2002-08-06
Publication date: 2004-02-11
Anticipated expiration: 2022-08-06
Also published as: GB2391751B; GB0218287D0

Abstract

Automatic frequency correction (AFC) applies a correction signal to a local clock, and the differences in departure and arrival intervals of data at a transmitter/receiver unit is monitored to derive an additional error correction. The additional frequency adjustment may be carried out before the output is fed to the AFC means. The correction signal may be summed before it is applied to the local oscillator clock.

Description

- 1 - Improvement to Clock Freouenev Correction in Mobile Co unication

Svatoms This invention relates to the correction of clock frequency in a mobile communication system.

s In mobile communication systems, there are typically a plurality of base stations each serving a predetermined area and a plurality of mobile communication devices, eg. mobile telephone handsets.

The base stations are typically synchronized with a lo common clock source. Each mobile device has its own clock source which has to be kept reasonably well synchronized with the clock source serving the base stations when the mobile device is in communication with one of the base stations. This is particularly important when data services Is are being transmitted either from a handset to a base station or vice versa as is the case in 3G mobile telephone systems. The problem arises because a lack of synchronism will lead to corruption of the data. In most handsets, the clock so source is usually compensated with an automatic frequency correction (AFC) scheme as part of a closely controlled system. These type of correction systems can typically achieve accuracy of the order of O.lppm. Although this is a high degree of accuracy, it is inadequate for long term 2s communication using end-to-end synchronous services, such as data services, where differences can accumulate to the point where bit synchronization is lost.

Preferred embodiments of the present invention seek to achieve improved correction of clock frequency when a mobile so device is in synchronous communication with a base station.

This is achieved by making very small short term adjustments in the ARC to avoid the accumulation of clock timing differences which might result in long term loss of synchronization. 5 An example of the clock frequencies and drift that may occur at a mobile communication device is given below: a local clock operating at 19.2MHz (e.g. a voltage controlled oscillator preferably with temperature compensation) has an accuracy of O.lppm, i.e. 1.92Hz.

lo The synchronous clock derived from the local clock source has a frequency of 64kHz which is equivalent to a bit period of 15.625 us.

At an error of O.lppm a cycle slip could occur every 10,000,000 cycles or every 156.25 seconds (2.6 minutes). At 15 lppm cycle slip could occur every 1,000,000 cycles or every 15.625 seconds.

It will therefore be appreciated that at O.lppm maximum error the interval over which errors integrate for the system show that a relatively slow control of the drift 20 needs to be achieved to avoid loss of synchronization.

Thus, the additional compensation process embodying the present invention may be relatively slow.

The present invention is defined with more precision than the appended claims to which reference should now be 25 made. A preferred embodiment of the invention will now be described in detail by way of example with reference to the single figure which shows a block diagram of apparatus in a

- 3 - handset required for implementation of an embodiment of the invention. The figure shows a reference clock source (2) which provides a clock signal to a unit for synthesis of clock 5 signals for data processing functions (4). This down converts the clock frequency to the frequency required for synchronous communication. Thus, the reference clock source may produce eg. 19.2MHz and after synthesis, it is down converted to 64.OkHz.

JO A data processing function unit (6) performs various data processing functions using the synthesized clock frequency to control the speed of these and these are then supplied to buffers (8) from where they may be transmitted by transmitter/receiver (10). When data is received by a 15 communication device it also is first received by transmitter receiver (10) before being supplied to the buffers (8) and passed to the data processing function unit (6). Data arrives and departs at the desired bitrate or JO some lower rate derived from an integer-division of the bit rate. The buffers (8) are used to introduce some delay over which the statistical variation in the bit-rate may be measured for arrival and departure of data. The use of the buffers (8) for upstream and downstream data paths enables 25 the bit slips which occur to be accommodated and therefore enable the measurement of errors to be made. For example, bits can be conveyed in byte or word formats, these in turn can be delivered in packets of a number of such units.

Measurements are preferably applied to some arbitrary but JO specific fixed size of packet containing a fixed number of bits. Therefore the buffer (8) must contain multiple packets for upstream and downstream paths.

( For example, if a frequency requires correction of O.lppm, a buffer of 16 bits could accommodate a slip of up to 16 bits or 16 x 2.6 minutes. Therefore a modest size of buffer can provide a reasonable degree of accommodation 5 without excess resources or without introducing an unacceptable delay.

In order to evaluate the rate of arrival and departure, the synchronous arrival intervals and departure intervals of packets need to be measured. This is done by a data flow lo monitor (12) which receives the arrival and departure interval times (metric) from the data processing function unit (6). It also receives details of the size of packets from the buffers (3).

The local measurement of time is restricted to the 5 accuracy of the local reference clock system (by definition), the limitation of whose accuracy creates the original requirement for further correction. However, the data flow monitor detects the relative changes between the arrival and departure intervals for upstream and downstream so data paths. From this, it is possible to detect any variation and therefore determine whether or not correction is required. If correction is required then a bias adjustment signal is fed into the AFC loop.

The AFC loop is linked to the reference clock source 25 and commences with a frequency measurement unit (14). The measured frequency then has the clock bias adjustment produced by the data flow monitor and clock biassing unit (12) subtracted from it in a subtraction unit (16). The reason for this is that otherwise, the addition of the small I so bias might cross a detection threshold which would result in a counter- correction by the existing AFC process. Thus, its

( - 5 effect must be subtracted from the AFC loop before the frequency correction is generated.

After the subtraction unit (16), the frequency is supplied to the AFC unit (18) which generates an AFC control s signal. This is then summed with the bias adjustment generated by the data flow monitor and clock biassing unit (12) in a summation unit (20). This sum signal is then used as a frequency control input to the reference clock source (2). The bias adjustment must be sealed to an appropriate lo value both for subtraction from the frequency measurement at 16 and for the summation with the AFC signal at 20.

The weighting of the correction generated by the data flow monitor and clock biassing unit (12) could be a three level correction (positive, zero, or negative) or a multi IS level correction depending upon the method by which the different forms of AFC are combined. The maximum magnitude of the correction should be limited such that it does not invalidate the accuracy achieved by the existing AFC unit (18). so A short algorithm written in pseudo-code which applies a three level correction to the AFC loop is now described.

Key! Tai - arrival interval Tdi - departure interval as Dab - arrival buffer depth Ddb - departure buffer depth Thrsh - a hysteresis threshold for testing The rate of applying this algorithm is at the highest packet rate, or a suitably higher frequency

- 6 - If (Dab>Ddb) Then apply positive bias to frequency correction Accumulated slip with respect to far end must be recovered} 5 Else If (Ddb>Dab) Then apply positive bias to frequency correction {accumulated slip with respect to far end must be recovered} Else If (Dab=Ddb) 0 Then If Magnitude (Tai-Tdi)<Thrsh) Then apply zero bias {near and far end clocks are reasonably matched} Else If (Tai-Tdi≥Thrsh) {far end is faster than near end] Is Then apply positive bias to frequency correction (speed up our clock by a very small amount} Else If (Tdi-Tai≥Thrsh) {far end is slower than near end} Then apply negative bias to frequency correction Slow down our clock by a very small fraction} 20 An alternative algorithm could be produced to apply a varying, multi-level control over the bias adjustments.

Such an algorithm would also have to avoid producing unstable dynamics. This would have to be considered on a case by case basis.

25 The units for subtracting and summing the bias adjustment, (16 and 20) could be digital or analog summation/subtraction unit. Providing the clock accuracy at each end is commensurate, or is better at the base station

- 7 - end, the resulting AFC can be steered towards an improved estimate of centre clock frequency by this method, based upon a statistical average of end-to-end network performance. 5 Thus it can be seen that the embodiment of the present invention described provides an additional feedback input into the AFC feedback loop for the reference clock source. i This additional input comprises a small bias adjustment generated from the rate of arrival and/or departure of data lo during synchronous fixed rate data communication. The direction of correction applied by the AFC unit is advanced or retarded in dependence on the reserved rates to ensure that data rates are closely matched end-to-end.

Claims

( - 8 - CLAIMS

1. Apparatus for correcting local clock frequency at a mobile communication device for transmitting and receiving data to and from a network comprising: 5 a local clock source; means for measuring the frequency of the local clock source; automatic frequency correction (AFC) means for applying a frequency correction signal to the local clock source in lo dependence on the output of the frequency measuring means; means for monitoring differences in departure and arrival intervals of data at a transmitter/receiver units means for deriving an additional local clock frequency correction signal in dependence on the monitored 15 differences; and means for applying the additional local clock frequency correction signal to the local clock.

2. Apparatus for correcting local clock frequency according to claim 1 including means to subtract the 20 additional local clock frequency correction signal from the output of the frequency measuring means prior to application of that output to the AFC means.

3. Apparatus for correcting local clock frequency according to claim 1 or 2 in which the means for applying 25 the additional local clock frequency correction signal to the local clock applies a summation of the output of the AFC means and the additional local clock frequency correction signal.

4. Apparatus for correcting local clock frequency at a 30 mobile communication device for transmitting and receiving

( - 9 - data to and from a network substantially as herein described with reference to Figure 1 of the drawings.

5. A method for correcting local clock frequency at a mobile communication device for transmitting and receiving s data to and from a network comprising the steps of: measuring the frequency of a local clock source;: applying an automatic frequency correction (AFC) signal to the local clock source in dependence on the measured frequency; lo monitoring differences in departure and arrival intervals data at a transmitter/receiver unit; deriving an additional local clock frequency correction signal from the monitored differences; and applying the additional local clock frequency 15 correction signal to the local clock.

6. A method for correcting local clock frequency at a mobile communication device substantially as herein described.