Phase detector

Details Phase detector Phase detector

1998-02-02

2001-06-19

Assouad, Patrick (Department: 2857)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Electrical signal parameter measurement system

C324S076770

Reexamination Certificate

active

06249750

ABSTRACT:

The present invention relates to apparatus and a method for measuring the phase of a data sequence.
Sometimes, data is transmitted in a digital format. Usually by modulating a medium with a digital data stream comprising a sequence of (not necessarily binary) symbols.
Increasingly, not only does the data sequence itself contain information but the phase of arrival of the symbols also carries information. For example, in the global positioning system (GPS), measurement of the phase of the spread spectrum symbols (so-called code phase measurements) may be used to perform a position measurement by triangulation. Also, in a mobile telephony system, such as the global system for mobile communications (GSM), a measurement may be made at a base station of the phase of a mobile station's transmissions so that the base station may instruct the mobile station to advance its timing to ensure that its transmissions fit within the allocated time division multiple access (TDMA) slot at the base station.
In order to determine the phase of a digital data sequence it is necessary to take measurements of the incoming signal. Typically such measurements take the form of a periodic amplitude measurement of the incoming signal once it has been demodulated.
Conventional wisdom (as described, for example, in Chapter 6 of “Spread Spectrum Systems—With Commercial Applications” Third edition, Robert C. Dixon, John Wiley & Sons Inc) has it that the measurement (or sampling) of the incoming sequence must occur necessarily at the same frequency as or some integer multiple of the data rate i.e. the frequency of transmissions of each symbol of the sequence. Much work has been performed on finding ways of synchronising the sampling frequency with the inherent clock frequency or data rate of the incoming sequence. However all current methods are based on a complex method and the corresponding apparatus is costly to produce.
It is an object of the present invention to provide improved apparatus and method aspects with reduced complexity.
Therefore, according to the present invention there is provided apparatus for measuring the phase of a predetermined incoming data sequence comprising, sampling means operable under control of a sampling clock to sample the incoming sequence at a frequency not equal to an integer multiple or divisor of the data rate of the sequence, test means operable to record a plurality of the samples in a time-ordered sequence, reference means operable to store a copy of at least a portion of the predetermined digital data sequence, comparison means operable to compare the recorded and stored sequences over a predetermined sequence length and to locate the position of an additional or omitted sample in the recorded sequence, calculation means operable to calculate the time of sampling of the additional sample or the time of sampling of the omitted sample relative to the sampling clock, and output means operable in response to the calculated time of sampling, to output a phase value indicative of the phase relative to the sampling clock, of the incoming sequence.
Generally, the frequency of the sampling clock F
s
may be given by:
F
s
=
X
·
F
c
±
Y
·
F
c
(
N
-
1
)
where Fc is the data rate, N is the predetermined length of comparison, X is the number of samples taken of each incoming symbol, and Y is the number of additional or omitted samples expected to be found in the sequence of N samples (when the fraction is added or subtracted respectively).
X.F
c
is referred to below as the “integer frequency” and X is either an integer or the reciprocal of an integer i.e. 1, 2, 3, 4 etc. or ½, ⅓, ¼ etc. Thus when X=1, one sample per symbol will be taken for most symbols, but because F
5
does not exactly equal F
c
since it is different by
±
Y
·
F
c
(
N
-
1
)
not all symbols will be sampled once. Similarly, when X=2 not all symbols will be sampled twice and when X=½ mostly alternate symbols will be sampled but occasionally consecutive symbols or every third symbol will be sampled when F
5
is greater or less than the integer frequency respectively.
Similarly
Y
·
F
c
(
N
-
1
)
is referred to below as the integer fraction, with Y taking values of 1, 2, 3 and ½, ⅓, ¼ etc. (referred to below as an integer or an integer ratio) so that the accuracy of the system can be improved by offsetting the phase of the received signal to ensure that the extra bit occurs within the length of the correlation register using a phase control on the sampling clock. In this way, the extra bit/s may be forced to occur within N samples from any given instant. This is described in more detail below.
By choosing to sample the incoming digital data sequence at a frequency not equal to the integer frequency of the data rate of the sequence, over time, if the sampling frequency is greater than the integer frequency, eventually one of the symbols will be sampled once more than the adjacent symbols. Similarly, where the sampling frequency is less than the integer frequency, one of the symbols will be sampled less frequently than the adjacent symbols. It will be seen from the description below that the time of sampling of the additional sample or the omitted sample occurs on an edge of the incoming sequence. Thus, by calculating the time of sampling of the additional sample or the time of sampling of the omitted sample, the time of arrival of an edge of the duplicated or omitted sample may be calculated and thus the phase of the complete sequence found in relation to the phase of the sampling clock.
Preferably the test means includes a test shift register operable to shift each successive sample received from the sampling means into the register, wherein the reference means include a register containing the said predetermined digital data sequence, and wherein the comparison means include correlating means operable to perform a multiplication operation on pairs of symbols stored in corresponding positions in the two registers and to sum the result over the length of the test register. In this case, the calculation means may be operable to calculate the product of the sampling period and a value representative of the said summed result to determine the calculated time of sampling in relation to the time of sampling of the first and/or last sample in the test register.
By arranging for the reference means to include a reference shift register operable to shift in the same and in a direction opposite to the test register, additional tests may be made during each sampling period. Preferably the reference shift register is operable to shift at a frequency the same as or greater than the sampling frequency. This is advantageous since the greater the shift frequency, the more comparisons may be made within a sampling period. The results may then be averaged.
Preferably the calculation means is arranged to calculate the time of sampling relative to an external timing reference such as a GPS 1PPS signal. Thus, using a common time reference between a transmitter and a receiver, a distance measurement may be made and triangulation may be used (using several such measurements from different receivers) to perform a position determination.
According to a method aspect of the invention, a method of measuring the phase of an incoming predetermined sequence comprises
sampling the incoming sequence under control of a sampling clock at a frequency not equal to an integer multiple or divisor of the data rate of the sequence,
recording a plurality of the samples in a time-ordered sequence,
comparing the recorded sequence with a local copy of at least a portion of the predetermined data sequence and locating the position in the time-ordered sequence of an additional or omitted sample, and
calculating the time of sampling of the additional sample or the time of sampling of the omitted sample and outputting in response to the calculated time of sampling a phase value indicative of the phase relative to the sampling c

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