GB2382478A

GB2382478A - Determining the approximate frequency error of an oscillator

Info

Publication number: GB2382478A
Application number: GB0128303A
Authority: GB
Inventors: Robert Deedman
Original assignee: Ubinetics Ltd
Current assignee: Aeroflex Cambridge Ltd
Priority date: 2001-11-26
Filing date: 2001-11-26
Publication date: 2003-05-28
Also published as: AU2002339154A1; GB0128303D0; WO2003047121A1

Abstract

A method for determining the approximate frequency error of an oscillator of a code division multiple access receiver is described. The method comprises controlling a correlator (10) to correlate a predetermined sequence with a sequence received from an oscillator (23), an operating frequency of the correlator relative to the received signal being dependent on the frequency error of the oscillator. The oscillator (23) is then controlled to adopt plural different frequencies. The frequency error is then estimated by examining outputs of the correlator (10) at the different oscillator frequencies.

Description

Determining the Approximate Frequency Error of an Oscillator This invention relates to a method of and apparatus for determining the approximate frequency error of an oscillator of a code division multiple access receiver.

Before data can be demodulated from signals received at a receiver operating in UMTS (the proposed universal mobile telephone system), the receiver must firstly isolate one or more rays and determine certain characteristics therefrom. The way in which rays are isolated will now be described with reference to Figure 1, which is a schematic diagram of a matched filter. Referring to Figure 1, the matched filter 10 comprises generally an input 11, to which a received signal is applied in use, an output 12 and a clock input 13. The matched filter 10 serves as a correlator. A series of 256 taps ao to a256 are interposed with delay elements 19. The delay elements 19 are commonly connected to the clock input 13, so that they change state substantially simultaneously. The effect is for signals received at the input to be clocked down through the taps, and for it to take 256 clock cycles for a signal to travel from the first tap to the 256th tap.

The taps are programmed with coefficients which correspond to the sequence that is being searched for. The taps each multiply their pre-programmed coefficient (typically either 1 or-1) with the part of the received signal (typically having a resolution of a preceding analogue-to-digital converter) which is at that tap at that time. The results from the taps are added by respective summers 20, to provide an output signal at the output 12. The clock signal provided at the clock input 13 is derived from a local oscillator (shown in Figure 2). Assuming zero frequency error, a peak in the output signal results when the input signal is best matched to the sequence given by the coefficients of the taps. The size of the peak is dependent on, among other things, the signal-to-noise ratio (SNR) of the received signal. The timing of the peak gives the relative timing of the sequence stored in the filter 10 to the received signal.

The local oscillator is initially controlled to operate at a frequency which is expected to be a desired frequency, although there is, at this stage, no external reference which can be used to quantify any error in the frequency of the oscillator. An estimate of the

frequency error can be obtained by evaluating signals transmitted by a UMTS transmitter station (which signals are generated with reference to a reference frequency), but this can only be achieved after synchronisation has been obtained. It will be appreciated that any error in the frequency of the clock signal degrades the size of peaks provided at the output 12 in two ways. Firstly, the phase of the input signal rotates during the length of the correlation, the rate of rotation being proportional to the frequency error. Secondly, the correlation between the sequency worsens as the relative timing drifts. A result of this is that, in the presence of significant frequency errors, it can be difficult or impossible to isolate reliably rays received at the receiver. This is especially so in UMTS, where the primary synchronisation channel (PSCH) is transmitted with a low SNR.

According to a first aspect of the invention, there is provided a method of determining the approximate frequency error of an oscillator of a code division multiple access receiver, the method comprising controlling a correlator to correlate a predetermined sequence with a received sequence, an operating frequency of the correlator relative to the received signal being dependent on the frequency error of the oscillator; controlling the oscillator to adopt plural different frequencies, and estimating the frequency error by examining outputs of the correlator at the different oscillator frequencies.

According to a second aspect of the invention, there is provided apparatus for estimating the frequency error of an oscillator of a code division multiple access receiver, the apparatus comprising a correlator, arranged to correlate a predetermined sequence with a received sequence, an operating frequency of the correlator relative to the received signal being dependent on the frequency error of the oscillator; a controller arranged to control the oscillator to adopt plural different frequencies; and an estimator arranged to estimate the frequency error by examining outputs of the correlator at the different oscillator frequencies.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying Figures (in which reference numerals are re-used for like elements), of which :-

Figure 1 is a schematic diagram of a conventional matched filter used with the invention; and Figure 2 is a schematic diagram of a receiver implementing and according to the various aspects of the invention.

Referring to Figure 2, a CDMA receiver 21 of a UMTS system (not shown) includes a matched filter correlator 10 of Figure 1, a controller and estimator 22, a local oscillator 23 and a clock generator 24. The frequency of the local oscillator 23 is controllable by the controller 22. The clock signal supplied to the matched filter 10 by the clock generator 24 is dependent on the frequency of the local oscillator 23. On power-up of the receiver 21, any error in the (so far uncorrected) frequency of the local oscillator 23 is present also in the frequency of the clock signal provided to the matched filter 10, which could, in normal circumstances, result in rays not being isolated.

According to the invention, the frequency of the local oscillator 23 is controlled to oscillate at a frequency 6kHz less than the expected centre frequency of the received signals. The correlator is then controlled to correlate the received signal with the sequence defined by the taps 14 to 18 of the filter 10 over a period of time corresponding to four frames, which is 40ms in UMTS. The controller and estimator 22 examines the output of the matched filter 10 over that period of time, and estimates or determines the amplitude of the largest peak, and estimates or determines the average signal level over the time period. The controller and estimator 22 then estimates the ratio of the magnitude of the largest peak to the average signal level. This ratio is used by the controller as a measure of the quality of correlation, which is dependent on, among other factors, the frequency error of the local oscillator 23. Next, the controller and estimator 22 controls the local oscillator 23 to increase its frequency by 2kHz, so that it is 4kHz below the expected centre frequency. A correlation is then performed over a period corresponding to two frames of data, and the ratio of the magnitude of the largest peak to the average level of the output of the filter 10 is determined. This is repeated for local oscillator frequencies, compared to the expected centre frequency, of - 2kHz, OkHz, + 2kHz, + 4kHz and lastly + 6kHz. The ratios estimated for each of these local oscillator frequencies are then examined by the controller and estimator 22,

and the largest ratio is identified. The local oscillator frequency giving the largest ratio is treated as being the frequency giving rise to the lowest frequency error, and an appropriate output signal is given for use by other parts of the receiver 21.

In a further embodiment. The frequency of the local oscillator 23 is controlled to oscillate at a frequency 6kHz less than the expected centre frequency of the received signals. The correlator is then controlled to correlate the received signal with the sequence defined by the taps of the filter 10 over a period of time corresponding to four frames. After determining or estimating the amplitude of the largest peak, the average signal level and the ratio of the two, the controller and estimator 22 controls the local oscillator 23 to oscillate at the expected centre frequency. Following the determination or estimation of the same parameters, the oscillator 23 is controlled to oscillate at 6kHz above the expected centre frequency, following which the parameters are again determined or estimated. In this embodiment, only three local oscillator frequencies are used, which allows frequency estimation in a reduced period of time.

It will be appreciated that the period of time over which correlation is performed determines the effectiveness of the procedure. Increasing the time period results in a greater difference between the ratio at low and at high frequency errors, and thus a more reliable and accurate result. However, this is achieved at the expense of an increased time to perform the procedure.

The above described procedures result in coarse frequency error estimation which is accurate and which is obtained in a relatively short period of time and with relatively little dedicated hardware or software. The procedures also do not require any measurements to be made, other than the measurements of signals provided by the correlator 10, which is considered to be a significant advantage.

It will be appreciated that the above described local oscillator step sizes and ranges are purely illustrative, and that any convenient step size and range could be used.

Preferably, the matched filter 10 is a heirarchiral correlator, which type of correlator is known in the art. Using a heirarchiral correlator provides advantages in efficiency.

Typically, a heirarchical correlator will require eight times fewer multiply and accumulate operations to implement than the equivalent matched filter constructed according to Figure 1.

Claims

Claims 1. A method of determining the approximate frequency error of an oscillator of a code division multiple access receiver, the method comprising : controlling a correlator to correlate a predetermined sequence with a received sequence, an operating frequency of the correlator relative to the received signal being dependent on the frequency error of the oscillator ; controlling the oscillator to adopt plural different frequencies, and estimating the frequency error by examining outputs of the correlator at the different oscillator frequencies.
2. A method as claimed in claim 1, in which the estimating step includes determining the magnitude of a peak of the correlator output.
3. A method as claimed in claim 2, in which the estimating step includes estimating the ratio of the peak to the average correlator output signal magnitude.
4. A method as claimed in any preceding claim, in which the controlling step includes controlling the oscillator to step in frequency with approximately equally sized steps.
5. A method of estimating the frequency error of an oscillator of a code division multiple access receiver substantially as hereinbefore described with reference to the accompanying figures.
6. Apparatus for estimating the frequency error of an oscillator of a code division multiple access receiver, the apparatus comprising : a correlator, arranged to correlate a predetermined sequence with a received sequence, an operating frequency of the correlator relative to the received signal being dependent on the frequency error of the oscillator ; a controller arranged to control the oscillator to adopt plural different frequencies ; and

<Desc/Clms Page number 7>

an estimator arranged to estimate the frequency error by examining outputs of the correlator at the different oscillator frequencies.
7. Apparatus as claimed in claim 6, in which the estimator is arranged to determine the magnitude of a peak of the correlator output, and to use the magnitude to estimate the frequency error.
8. Apparatus as claimed in claim 7, in which the estimator is arranged to estimate the ratio of the peak to the average correlator output signal magnitude, and to use the ratio to estimate the frequency error.
9. Apparatus as claimed in any of claims 6 to 8, in which the controller is arranged to step the frequency of the oscillator with approximately equally sized steps.
10. Apparatus for estimating the frequency error of an oscillator of a code division multiple access receiver substantially as hereinbefore described with reference to and/or as shown in Figure 2 of the accompanying drawings.
11. A radiotelephone including apparatus as claimed in any of claims 6 to 10.