Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
1998-09-22
2003-09-30
Ghayour, Mohammad H. (Department: 2631)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
C375S226000, C375S355000
Reexamination Certificate
active
06628738
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method to determine during a tracking mode a clock timing error used for synchronisation purposes in a multi-carrier transmission system an arrangement adapted to perform this method as and synchronisation units to be used in such a multi-carrier system.
BACKGROUND OF THE INVENTION
Such a method and related equipment to perform this method are already known in the art, e.g. from the European Patent Application EP 0 453 203, entitled ‘Method and apparatus for correcting for clock and carrier frequency offset, and phase jitter in multicarrier modems’ from applicant Telebit Corporation. Therein, phases of a few pilot carriers are detected and used to calculate a clock timing error named a phase correcting signal (see page 3, lines 11-35 of the cited European Patent Application). As is indicated on page 6, lines 24-26 of EP 0 453 203, the phase correcting signal is used in a phase-locked loop (PLL) to realise synchronisation between a transmitting multi-carrier modem and a receiving multi-carrier modem.
In case of a norrowband interferer in the vicinity of one of the pilot carriers whose phases are detected to calculate the clock timing error, use of phase information extracted from this pilot carrier renders the so called clock timing error or phase correcting signal less accurate as a measure for the timing difference between transmitting and receiving modem. As a consequence, synchronisation between the transmitting and receiving modem may be lost in the known system when one of the pilot carriers is affected by noise.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method, arrangement and synchronisation units similar to those known, but whose robustness for narrowband noise near the pilot carriers is optimised.
According to the invention, this object is achieved by a method to determine during a tracking mode in a multi-carrier system a clock timing error used for synchronisation purposes, the method comprising the steps of detecting phase errors for a plurality of pilot carriers and calculating the clock timing error from the phase errors, wherein a share of a phase error of the phase errors in the clock timing error depends on a value of a transmission quality parameter measured for a pilot carrier of the pilot carriers for whom the phase error is measured.
It is also achieved by an arrangement to determine during a tracking mode in a multi-carrier system a clock timing error used for synchronisation purposes, the arrangement comprising: phase error detection means whereto a multi-carrier signal is applied, the phase error detection means being adapted to detect phase errors for a plurality of pilot carriers; and calculation means, coupled to the phase error detection means, and adapted to calculate the clock timing error from the phase errors, wherein the arrangement further comprises: share determination means, having an output coupled to the calculation means and being adapted to receive via an input thereof values of a transmission quality parameter related to the plurality of pilot tones and to determine shares of the phase errors in the clock timing error from the values of the transmission quality parameter; and further in that the calculation means is adapted to receive via an input thereof the shares and to calculate the clock timing error from the shares and the phase errors.
It is further achieved by a synchronisation unit to be used in a multi-carrier system, the synchronisation unit comprising: skip and duplicate means, adapted to remove a sample from or to duplicate a sample in a multi-carrier signal when a clock timing error becomes larger than or equal to a sample period; phase rotation means, coupled in series with the skip and duplicate means, and adapted to apply a phase shift to each carrier in the multi-carrier signal proportional to the clock timing error and proportional to a frequency of the carrier; a clock timing error determination arrangement, coupled to the phase rotation means, and adapted to determine during a tracking mode of the multi-carrier system the clock timing error, the clock timing error determination arrangement comprising phase error detection means whereto a multi-carrier signal is applied, the phase error detection means being adapted to detect phase errors for a plurality of pilot carriers; and calculation means, coupled to the phase error detection means, and adapted to calculate the clock timing error from the phase errors; and a feedback loop coupled to the clock timing error determination arrangement and having an output coupled to inputs of both the skip and duplicate means and the phase rotation means, the feedback loop being adapted to feed back the clock timing error to both the skip and duplicate means and the phase rotation means, wherein the clock timing error determination arrangement further comprises share determination means, having an output coupled to the calculation means and being adapted to receive via an input thereof values of a transmission quality parameter related to the plurality of pilot tones and to determine shares of the phase errors, in the clock timing error from the values of the transmission quality parameter; and further in that the calculation means is adapted to. receive via an input thereof the shares and to calculate the clock timing error from the shares and the phase errors.
Indeed, giving the phase error detected for a first pilot carrier which is transferred with a low transmission quality a relatively low share in the clock timing error used for synchronisation, and giving the phase error detected for a second pilot carrier which is transferred with a high transmission quality a relatively high share in the clock timing error used for synchronisation, has a filtering effect on the norrowband noise which affects the transmission quality of the first pilot carrier for this clock timing error. As a consequence, the variance of the clock timing error is reduced according to the present invention resulting in a better tracking of the timing-locked loop whereto the clock timing error is applied as input. This implies that the synchronisation process is made less sensitive for narrowband noise.
It is to be noticed that the term ‘comprising’, used in the claims, should not be interpreted as being limitative to the means listed thereafter. Thus, the scope of the expression ‘a device comprising means A and B’ should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
Similarly, it is to be noticed that the term ‘coupled’, also used in the claims, should not be interpreted as being limitative to direct connections only. Thus, the scope of the expression ‘a device A coupled to a device B’ should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.
An additional feature of the present invention is such a method where the value of the transmission quality parameter is determined during an acquisition mode preceding the tracking mode.
Indeed, it can be expected that the transmission quality of the medium in between two multi-carrier modems at a certain frequency does not change abruptly, unless impulse noise disturbs the medium. If the transmission quality of the medium at that certain frequency is measured once during an acquisition mode or initialisation procedure, the measured quality can be used for a long period. In Discrete Multi Tone (DMT) systems such as an Asynchronous Digital Subscriber Line (ADSL) system, the transmission quality of the medium has to be measured as a function of frequency during initialisation of the system to be able to execute the bit allocation procedure: the process wherein each carrier is assigned a number of bits depending on the transmission quality of this carrier. In such systems
Peeters Miguel
Pollet Thierry
Al-Beshrawi Tony
Ghayour Mohammad H.
Ware Fressola Van Der Sluys & Adolphson
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