Pulse or digital communications – Synchronizers – Phase displacement – slip or jitter correction
Reexamination Certificate
2001-08-08
2004-06-22
Chin, Stephen (Department: 2634)
Pulse or digital communications
Synchronizers
Phase displacement, slip or jitter correction
Reexamination Certificate
active
06754296
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus and a method for generating a reference frequency.
BACKGROUND OF THE INVENTION
In a satellite TDMA network the various user stations need a common time and frequency reference for deriving their first transmission instance and their carrier frequency. Therefore, time and frequency synchronization of the user stations is an important feature which must respect severe constraints in order to lead to an efficient system with minimum interference between users and maximum throughput.
From WO 98/13969 time stamping clocks are synchronized in an ATM-network for compensating or measuring delays in the network. In an ATM-network the time value is sent in a field designated TSTP. The TSTP field consists in 32 bits and the time value is stored as the 32 least significant bits of the number of micro seconds that have passed since Jan. 1, 1972. By simply calculating the time difference of these values time stamping clocks can be run in each node which are phase-locked to the network synchronization clock. However, timing of the time stamping clocks can vary due to delays within the ATM-network. Delays in packet switched networks occur e.g. when switching a packet from one node in the network to another node. These delays can vary significantly and are due to the degree of utilization in the network and in individual nodes. Therefore, it is both necessary to measure and compensate the delays so as to enable a network to be trimmed or to ascertain which parts of the network are subjected to greater or smaller delays.
Firstly, in order to synchronize the time stamping clocks in different nodes these clocks are phase-locked to network-synchronization clocks and additionally, an absolute time is obtained from a GPS receiver. The GPS receiver supplies a TOD (Time Of Day) and PPS (Pulse Per Second) to a synchronization function. The synchronization function uses TOD to provide an absolute time for the time stamping clock in a node. There is thus obtained a synchronized absolute time between the time stamping clocks in the different nodes with a resolution of one second. The PPS pulse is used to obtain a degree of accuracy of one micro second between the clocks. Because the time stamping clocks are phase-locked to the network synchronization clocks and not to the GPS, the time stamping clocks will continue to have a high degree of accuracy even if the GPS equipment should malfunction or if the signal from the GPS satellite should be disturbed.
Secondly, delays are measured by time stamping a packet, i.e. by storing in the packet a value which represents the time at which the packet leaves the node. The packet is then sent to another node in the network and this node reads the stored value and compares the set value with the value on the time stamping clock in the own node. This provides a value of the delay.
From EP 0 564 220 A2 a clock synchronization system for synchronizing the performance of a number of clocks with a reference clock is disclosed. Each clock of this synchronization system includes a counter that indicates the current time and that is sequentially incremented by clock signal. A time, counter controller both initializes the counter and generates the clocking signal that controls the advancement of the counter. The time counter controller further monitors the time indicated by the counter and compares it to a reference-time signal received from a reference clock. Based on the comparison, the time counter controller selectively reinitializes the counter and adjusts a rate at which a clocking signal supplied to the counter so as to ensure that the counter advances at a rate equal to the rate at which the reference clock advances. Preferably, a reference-time signal is received from a global positioning system (GPS).
From EP 0 671 828 A1 a clock circuit is known to recover timing and transmitted information signals in a data receiver. Thereby, the accuracy of a low cost local master clock can be increased. The timing relationship can be derived from horizontal synch or colour burst pulses provided in a broadcast television signal. Another possibility of receiving a timing relationship is using a data packet communication scheme as digital HDTV data packet format or MPEG-2 format.
From EP 0 836 282 A1 an adapted phase-lock loop circuit is known for extracting time stamps from a MPEG-2 transport stream for obtaining a synchronized reference time. In a MPEG-2 transport stream a PCR (Program Clock Reference) time stamp is periodically transmitted. The PCR is detected from the transport stream by a PCR extracting circuit. When the time reference value is detected a counter counts a clock oscillated by a VCO (Voltage Controlled Oscillator) and a comparator compares the value of the counter and the value of the PCR. The phase difference between both values is fed back to the VCO through a digital filter. In the control start stage, the gain of the digital filter is designated to a large value. Thus, the phase difference is quickly converged to the allowable difference range. In the lock stage, the gain is designated to a small value. Thus, the control operation is stably performed.
For generating a time reference in a user station of a satellite TDMA network it has to be observed that the user station generally consists of an indoor unit (IDU) and an outdoor unit (ODU). The outdoor unit comprises a satellite antenna with an upconverter and/or a downconverter wherein the indoor unit comprises an encoder/decoder and a QPSK modulator. Having this separation-between an indoor unit and an outdoor unit in the past different mechanisms have been employed to achieve synchronization especially for satellite systems:
The IDU uses an “expensive” Oven Controlled Oscillator (OCXO) as an extremely stable reference frequency. The IDU sends the signal at a fixed IF carrier frequency to the Outdoor Unit (ODU). The IDU also sends a reference frequency to the ODU, which is used by an “expensive” microwave oscillator in the ODU in order to generate the upconverter frequency. The Indoor Unit (IDU) exchanges messages with the Hub in order to correct its IF frequency.
Same as in above, but the Outdoor Unit (ODU) uses another independent “expensive” OCXO to have an extremely stable upconverter frequency. This avoids the use of a reference frequency sent from the IDU to the ODU.
The IDU locks its local oscillator to the down link signal, and exchanges messages with the Hub in order to generate the correct IF frequency. The IDU sends a reference frequency to the ODU, which is used by an “expensive”, microwave PLL in the ODU in order to generate the upconverter frequency. This avoids the use of an expensive OCXO in the IDU.
In frequency agile systems, the IDU may also generate the IF signal directly in L-Band (959-2150 MHz), thus avoiding ODU upconverter frequency agility. The ODU upconverts the signal by a fixed oscillator frequency. This provides frequency agility but still requires an “expensive” microwave PLL in the ODU in order to control the upconverter frequency.
From EP-A-0836282 an adapted phase lock loop circuit is known for extracting timestamps from a MPEG-2 transport stream for obtaining a synchronised reference time. The adapted phase lock loop comprises a voltage controlled oscillator (VCO) for generating an internal reference frequency.
From U.S. Pat. No. 5,841,987 a system for generating a signal for coupling digital audio, video and data signals in compressed form via a bus is known. A processing means formats the digital audio, video and data signals into super packets for transmission via the bus. Each super packet comprises a timestamp, wherein receiving devices utilise the timestamp for clock synchronisation. The realisation of the clock synchronisation is not explained in more detail.
From P. A. SARGINSON: “MPEG-2: A Tutorial Introduction to the Systems Layer” IEE COLLEQUIUM ON MPEG-2, 1995, pages 4/1-4/13 and 500 IN LEE ET AL.: “Implementation of MPEG-2 TS Remultiplexer and Data Transport Unit for HDTV Satellite Broadcasting” IEEE
Bethscheider Gerhard
Schulz Detlef
Siebert Peter
Chin Stephen
Kim Kevin
Societe Europeenne des Satellites S.A.
Wolf Greenfield & Sacks P.C.
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