WO2006084361A1

WO2006084361A1 - System and method for implementing a transmit diversity single frequency network without gps receivers

Info

Publication number: WO2006084361A1
Application number: PCT/CA2006/000178
Authority: WO
Inventors: Alex Dolgonos
Original assignee: Alex Dolgonos
Priority date: 2005-02-09
Filing date: 2006-02-09
Publication date: 2006-08-17

Abstract

A transmit diversity single frequency network (SFN) is provided having a distribution network, a head end system, and two or more transmitter circuits. The head end system is configured to provide a signal containing timing information to the distribution network. The two or more transmitter circuits are coupled to the distribution network. Each of the transmitter circuits receives the signal and has a timing component that is synchronized using the timing information contained in the signal.

Description

SYSTEM AND METHOD FOR IMPLEMENTING A TRANSMIT DIVERSITY SINGLE FREQUENCY NETWORK WITHOUT GPS RECEIVERS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims domestic priority from prior U.S. provisional application Serial No. 60/651,075 filed February 9, 2005, the entire disclosure of which is hereby incorporated by reference.

FIELD OF THE INVENTION

[0001] The present invention relates to broadcasting and, more particularly, to a transmit diversity Single Frequency Network (SFN) implemented without the use of Global Positioning System (GPS) receivers.

BACKGROUND OF THE INVENTION

[0002] Some broadcast systems employ a single omnidirectional antenna to floodlight an entire region. A single omnidirectional broadcast antenna generally requires a tall transmission tower so that the signal will not be obstructed by other tall structures in the area. In urban deployments, tall transmit tower structures may not be available or may not be cost effective, thus forcing the system designer to use shorter towers or to install the transmit tower on a roof top. To enhance broadcast coverage, the system designer may opt to employ transit diversity using a Single Frequency Network (SFN). With a transmit diversity implementation, the same signal is broadcast from two or more antennas or transmitters that are coupled to the same signal source.

[0003] A SFN system typically comprises multiple transmitters that use the same frequency to broadcast the same information signal. This technique conserves the electromagnetic spectrum by expanding geographic coverage on a single frequency band and allows broadcasting networks to fill spatial "gaps" in the broadcast, thereby improving the level of spatial coverage in a given area to nearly one hundred percent. The probability that a reflected signal will reach behind a signal obstruction is enhanced when transmit diversity is properly employed. This SFN technique is often applied in Digital Terrestrial Television Broadcast (DTTB) systems.

[0004] In order for the SFN to operate, all signals broadcasted by all transmitters must be synchronized in time, frequency and content. This limitation imposes the following constraints on equipment design: (a) Frequency Synchronization - All local oscillators within a transmitter chain (i.e., modulators, up-converters, etc.) must be frequency and phase locked to a common reference with high stability. In conventional approaches, this common reference is realized using a Global Positioning System (GPS) receiver at each transmit site; (b) Time Synchronization - Time synchronization generally requires that each transmitter broadcast the n^th symbol of information at a specific time T_n, using a very high time resolution to carefully and accurately define the time T_n. Typically, the time resolution employed is about plus or minus one microsecond. The time resolution of +/- lμs is difficult to achieve given the distances that the transmitters are from the head-end of the SFN system; and (c) Content Synchronization - Content synchronization mandates that the same symbol be transmitted at the same time from all transmission sites. Thus, all carrier signals in the modulators of the transmit sites have to be identically modulated, which means that the same bits should modulate the same k^th carrier in OFDM multi- carrier based systems, assuming OFDM is the modulation technique employed.

[0005] In order to fulfill the time and content synchronization constraints, conventional solutions employ a SFN adapter located at the head-end of the system. This SFN adapter periodically inserts specific information into the transport stream provided by the head-end. This specific information is referred to as a Megaframe Identification Packet (MIP), which is used to synchronize the various transmitters. The SFN adapter at the head-end derives the time information from a GPS receiver and inserts this time into each transmitted MIP to convey to each transmitter the correct starting time for each Megaframe, as well as data timing. Each transmitter recovers this information and uses it to perform time and bit synchronization for the SFN transmitters.

[0006] The conventional approach to synchronizing the SFN uses two global external reference signals: a 10 MHz frequency reference and a time reference of one Pulse Per Second (PPS). Conventional solutions use the GPS system as the source of these two references.

[0007] The conventional solution for synchronizing Single Frequency Networks assumes that any Voltage Controlled Oscillators (VCOs) in the modulators and transmitters of the individual transmission stations are locked to the external 10MHz GPS signal. The one PPS from the GPS is used as a starting point to calculate time delay extracted from the MIP to properly time the transmission of each Megaframe.

[0008] It would be desirable to have a SFN broadcast system that does not suffer from the requirement of including a GPS receiver at each SFN transmission station. The improvement of not having a GPS receiver at each SFN transmission station would result in a major network design simplification and reduce the costs of building transmit diversity Single Frequency Networks.

SUMMARY OF THE INVENTION

[0009] Example embodiments of the present invention mitigate the expense and complexity problems of conventional transmit diversity Single Frequency Network (SFN) systems. A system and method are provided for implementing a transmit diversity SFN without the use of GPS receivers. The transmit diversity SFN comprises a distribution network, a head end system, and two or more transmitters. The head-end system is configured to provide a signal to the distribution network. The two or more transmitters each have a local oscillator and are each configured to receive the signal from the distribution network. The two or more transmitters are synchronized by the oscillators using external data contained only in the signal.

[00010] According to one aspect of the invention is a method of synchronizing a plurality of remote transmitters in a single frequency network (SFN) without using GPS receivers at the remote transmitters, including: inserting timing information into a broadcast signal and transmitting the broadcast signal with the inserted timing information over a distribution network to the remote transmitters, and at each of the remote transmitters, receiving the broadcast signal with the inserted timing information and generating, without reference to a GPS receiver at the remote transmitter and in dependence on the inserted timing information, a reference signal used to synchronize the operation of the remote transmitter with the. other remote transmitters enabling the remote transmitters to function as a single frequency network.

[00011] According to one aspect of the invention is a transmit diversity single frequency network (SFN) that includes a distribution network, a head end system providing a broadcast signal containing timing information to the distribution network, and a plurality of transmitter circuits coupled to the distribution network. Each transmitter circuit receives the broadcast signal and has a timing component that is synchronized with timing components in the other transmitter circuits using the timing information contained in the broadcast signal without reference to any further external reference signals. BRIEF DESCRIPTION OF THE DRAWINGS

[00012] Various features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which:

[00013] FIG. 1 is a block diagram illustrating a circuit topology of a conventional solution to transmit diversity SFN systems; and

[00014] FIG. 2 is a block diagram illustrating a circuit topology of an example embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[00015] Referring to FIG. 1, a block diagram is shown illustrating a circuit topology or system 10 of a conventional solution to transmit diversity Single Frequency Network (SFN) systems. The circuit topology 10 generally comprises a head-end system 12, a distribution network 14, and two or more transmitters 16. The head-end system 12 has a number of inputs 20, indicated individually as 2Oa₇ 20b ... 2On, an input 22, and an output 24. The inputs 20 receive source signals to be broadcast such as a video signal at 20a, an audio signal at 20b, and a data signal at 2On. The input 22 receives a Global Positioning System (GPS) signal. In one example, the output 24 may provide an MPEG-2 data stream signal. An MPEG-2 data stream signal is also referred to as a transport stream. The head-end system 12 generally comprises a number of encoders 26, individually indicated as 26a ... 26n, a multiplexer 28, a transport formatter 30, a GPS receiver 32, a SFN adapter 34, and an FEC Encoder 36. In one example, the encoder 26a is a video encoder configured to receive the video signal provided at the input 20a and the encoder 26n is an audio encoder receiving the audio signal provided at the input 20b. The multiplexer 28 receives the encoded video signal from the encoder 26a, the encoded audio signal from the encoder 26n, and the data signal provided at the input 2On. The multiplexer 28 provides a multiplexed content signal to the transport formatter 30. The GPS receiver 32 receives the GPS signal from the input 22 and provides timing information to the SFN adapter 34. The GPS receiver 32 provides the SFN adapter 34 with a 1OM hz reference signal and a one Pulse Per Second (PPS) timing signal. The SFN adapter 34 periodically inserts specific information into the transport stream provided by the transport formatter 30. This specific information is referred to as a Megaframe Identification Packet (MIP), which is used to synchronize the various transmitters 16. The SFN adapter 34 derives the needed timing information from the GPS receiver 32 and inserts this timing information into each transmitted MIP to convey to each transmitter 16 the correct starting time for each Megaframe, as well as data timing. Each transmitter 16 recovers this information and uses it in conjunction with a GPS signal to perform time and bit synchronization for the SFN network 10. The transport stream from the transport formatter 30 is provided to the SFN adapter 34 and then to the FEC encoder 36 that supplies the MPEG-2 data stream signal at the output 24.

[00016] The output 24 of the head-end 12 is coupled to an input 38 of the distribution network 14. The distribution network 14 has a number of outputs 40, individually indicated as 40a, 40b ... 4On for providing the MPEG data stream to the transmitters 16. While one transmitter is shown by way of example, it will be understood by those skilled in the art that any number of transmitters 16 may be coupled to the distribution network 14.

[00017] The transmitter 16 has an input 42, an input 44, and an output 46. The input 42 receives the transport stream provided by the output 4On of the distribution network 14. The input 44 receives a GPS signal. The output 46 presents the final SFN signal for broadcast by an antenna. The transmitter 16 generally comprises a modulator 48, a GPS receiver 50, an up converter 52, a power amplifier 54, and a filter 56. The modulator 48 receives the transport stream provided to the input 42. The GPS receiver 50 receives the GPS signal from the input 44 and provides the lOMhz reference signal and/or the one PPS timing signal to the modulator 48 and/or the up converter 52. The modulator 48 modulates the transport stream signal using OFDM, QAM, or any other modulation technique known in the art. The modulator 48 provides a modulated signal to the up converter 52 that converts the modulated signal to the desired carrier frequency. The up converter 52 supplies the modulated signal at the desired carrier frequency to the power amplifier 54 for amplification to achieve the desired broadcast power. The power amplifier 54 provides an amplified signal to the filter 56. In one example, the filter 56 is a cavity filter. The cavity filter supplies a final signal suitable for broadcasting to the broadcasting antenna.

[00018] Referring to FIG. 2, a block diagram is shown illustrating a circuit topology 100 in accordance with one embodiment of the present invention. The circuit topology 100 generally comprises a head-end system 112, a distribution network 114, and two or more transmitters 116. The head-end system 112 has a number of inputs 120, individually indicated as 120a, 120b ... 12On, an input 122, and an output 124. The inputs 120 receive source signals such as a video signal at 120a, an audio signal at 120b, and a data signal at 12On. The input 122 receives a Global Positioning System (GPS) signal. In one example, the output 124 provides an MPEG-2 data stream (e.g., transport stream) signal. The head-end system 112 generally comprises a number of encoders 126, individually indicated as 126a ... 126n, a multiplexer 128, a transport formatter 130, a GPS receiver 132, a SFN adapter 134, and an FEC Encoder 136. In ope example, the encoder 126a is a video encoder receiving the video signal provided at the input 120a and the encoder 126n is an audio encoder receiving the audio signal provided at the input 120b. The multiplexer 128 receives the encoded video signal from the encoder 126a, the encoded audio signal from the encoder 126n, and the data signal provided at the input 12On. The multiplexer 128 provides a multiplexed content signal to the transport formatter 130. The GPS receiver 132 receives the GPS signal from the input 122 and provides timing information to the SFN adapter 134. In one example, the GPS receiver 132 provides the SFN adapter 134 with a lOMhz reference signal and a one Pulse Per Second (PPS) timing signal. The SFIM adapter 134 periodically inserts specific information into the transport stream provided by the transport formatter 130. This specific information may be referred to as a Megaframe Identification Packet (MIP), which is used to synchronize the various transmitters 116. The SFN adapter 134 derives the needed timing information from the GPS receiver 132 and inserts this timing information into each transmitted MIP to convey to each transmitter 116 the correct starting time for each Megaframe, as well as data timing. Each transmitter 116 recovers this information and uses it to perform time and bit synchronization for the SFN network 100. The transport stream from the transport formatter 130 is provided by the SFN adapter 134 to the FEC encoder 136 that supplies the MPEG-2 data stream signal at the output 124.

[00019] One aspect of the present invention synchronizes Voltage Controlled Oscillator (VCO) reference generators 149 in the transmitters 116 using a 40-bit time stamp pattern. In one example embodiment, the 40-bit time stamp pattern is inserted into the transport stream that is generated at the head end 112. The time-stamp may, for example, be inserted by the SFN Adapter 134. The 40-bit time stamp pattern presents the time to each SFN transmitter 116. In one example, the 40-bit time stamp pattern presents the time on a 24 hour clock system and provides an accuracy of 100 nanoseconds, which corresponds to the commonly used lOMhz reference frequency. The number of bits used for the time stamp may vary depending on the details of the particular SFN network being designed. A time stamp pattern of any bit length may be used.

[00020] The output 124 of the head-end 112 is coupled to an input 138 of the distribution network 114. The distribution network 114 has a number of outputs 140, indicated individually as 140a, 140b ... 14On, for providing the transport stream to the transmitters 116. One transmitter is shown in greater detail in Figure 2, and it will be understood by those skilled in the art that any number of transmitters 116 may be coupled to the distribution network 114. [00021] The transmitter 116 has an input 142 and an output 146. The input 142 receives the transport stream provided by the output 14On of the distribution network 114. The output 146 presents the final SFN signal for broadcast by an antenna. The transmitter 116 generally comprises a modulator 148, a VCO reference generator 149 (which includes a VCO 150), an up converter 152, a power amplifier 154, and a filter 156. The modulator 148 receives the transport stream provided to the input 142. In one example, all or parts of the VCO reference generator 149 may be incorporated into the modulator 148. The VCO reference generator 149 of modulator 148 also has a VCO 150, counter 158, an extractor 160, and a comparator 162. The extractor 160 receives the transport stream provided at the input 142, extracts the 40-bit time stamp pattern from the transport stream, and provides the timing signal to the comparator 162. The counter 158 receives the lOMhz reference signal from the VCO 150 and generates the 1 PPS timing signal. The comparator 162 compares the timing information extracted from the transport stream with the counter 158 and provides a synchronization information signal back to the VCO 150. In one example, the VCO 150 may generate an output frequency of lOMhz similar to the signal generated by a GPS receiver. However, the VCO can be configured to generate any reference signal based on the design criteria of a particular application. The modulator 148 includes a modulator circuit 151 for modulating the transport stream signal using Orthogonal Frequency Division Multiplexing (OFDM), Quadrature Amplitude Modulation (QAM), or any other suitable modulation technique known in the art. In one example, the counter 158 may be a 40 bit counter. The modulator 148 provides a modulated signal to the up converter 152 that converts the modulated waveform to the desired carrier frequency. The up converter 152 supplies the modulated waveform at the desired carrier frequency to the power amplifier 154 for amplification to achieve the desired broadcast power. The power amplifier 154 provides an amplified signal to the filter 156. In one example, the filter 156 may be a cavity filter. The cavity filter may provide a final signal suitable for broadcasting to a broadcasting antenna. [00022] In one aspect of the present invention, the VCO reference generator 149 including internal reference VCO 150 may be implemented in each Single Frequency Network (SFN) transmitter 116. In one example, a lOMhz reference VCO may be employed. To synchronize the internal reference VCOs .150, the 40- bit time stamp pattern is inserted into the transport stream by the head end 112 and presents the time to each SFN transmitter 116. In one example, the 40-bit time stamp pattern presents the time on a 24 hour clock system. The number of bits used for the time stamp may vary depending on the details of the particular SFN network 100 being designed. A time stamp pattern of any bit length may be used, depending on the level of accuracy (e.g., the time resolution) desired. The reference VCO 150 in each transmitter 116 is used for synchronizing all local oscillators in the transmitters 116 of the transmitter chain. The 10MHz reference clock generated by the VCO 150 runs the 40-bit counter 158. The 40-bit time stamp pattern is extracted from the incoming transport stream at the SFN transmitter 116 and is compared with the local 40-bit counter 158. The difference between the local 40-bit counter 158 and the 40-bit time stamp is used to lock the internal reference VCO 150 to the time stamp. In one example, the time delay from the head-end 112 of the SFN system 100 to the transmitter 116 is constant for each transmitter 116. More typically, however, the time delays through distribution network 114 to each of the transmitters 116 will vary from transmitter to transmitter and regardless, any differential delay between the transmitters 116 in the SFIM network 100 may be compensated in the modulators 148 using an adjustable delay to ensure that the effective delays between all transmitters 116 and the head-end 112 are equal. Therefore, all modulators 148 used in the SFN network 100 are locked to the time stamp pattern inserted into MPEG-2 stream by the SFN adapter 134 and all 40-bit counters 158 are synchronized. In this case, the one Pulse Per Second (PPS) signal is generated by the counter 158 that is the timing reference for the Megaframe.

[00023] It will be understood by those skilled in the art that the distribution network 114 may be implemented in a number of ways. In one example, the distribution network 114 is a cable system. In another example, the distribution network 114 may be comprised of a geo stationary satellite link and the transmitters 116 may be repeaters that receive the transport stream from the satellite. It will also be understood by those skilled in the art that the internal configuration of the modulator 148 is not illustrated in its entirety. Only components and interconnections that are needed to explain the functioning of the present invention have been illustrated.

[00024] In one aspect of the present invention, a GPS receiver 50 is not needed at each transmitter 116. The elimination of the GPS receiver 50 at each of the transmitters 116 represents a major network design simplification and reduces the costs of implementing SFN networks.

[00025] The head-end SFN adapter 134 may derive the 10 MHz reference signal from the GPS receiver 132. In another aspect of the present invention, the head-end SFN Adapter 134 may derive the 10 MHz reference signal from a 10MHz high stability Temperature Controlled Crystal Oscillator (TCXO), thus completely eliminating the need for any GPS receiver in the system 100.

[00026] In another aspect of the present invention, and in the same manner as discussed above, a 42-bit Program Clock Reference (PCR) that exists within the MPEG-2 stream may be used to synchronize the SFN network 100. In this case, an extra 27MHz VCO may be used inside the modulator 148. This 27 MHz VCO may be locked to 42-bit PCR and may provide the reference signal for 10MHz VCO 150.

[00027] The present invention may be embodied in other specific forms without departing from the spirit or characteristics thereof. Certain adaptations and modifications of the invention will be obvious to those skilled in the art. Therefore, the presently discussed embodiments are considered to be illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

1. A method of synchronizing a plurality of remote transmitters in a single frequency network (SFN) without using GPS receivers at the remote transmitters, including: inserting timing information into a broadcast signal and transmitting the broadcast signal with the inserted timing information over a distribution network to the remote transmitters, at each of the remote transmitters, receiving the broadcast signal with the inserted timing information and generating, without reference to a GPS receiver at the remote transmitter and in dependence on the inserted timing information, a reference signal used to synchronize the operation of the remote transmitter with the other remote transmitters enabling the remote transmitters to function as a single frequency network.

2. The method of claim 1 wherein at each of the remote transmitters generating the reference signal includes: causing a voltage controlled oscillator to produce an output signal; extracting the timing information from the broadcast signal; comparing the extracted timing information with information derived from the output signal from the voltage controlled oscillator and adjusting the operation of the voltage controlled oscillator until the information derived from the output signal matches the timing signal in a predetermined manner, the reference signal including the output signal or information derived therefrom.

3. The method of claim 2 wherein the timing information inserted into the broadcast signal includes a multi-bit time stamp pattern that is extracted at each remote transmitter, and including at each remote transmitter: applying the output signal from the voltage controlled oscillator to a counter to generate a timing signal, wherein comparing the extracted timing information with information derived from the output signal includes comparing information from the extracted multi-bit time stamp pattern with information from the timing signal generated by the counter.

4. The method of claim 3 wherein the multi-bit time stamp pattern includes N-bits and the counter is an N-bit counter.

5. The method of claim 4 wherein N=40, and the multi-bit time stamp pattern represents a time on a 24-hour clock.

6. The method of any one of claims 3 -5 wherein the counter generates a 1 pulse per second timing signal.

7. The method of any one of claims 2-5 wherein the output signal provided by the oscillator is substantially a 10 MHz signal.

8. The method of any one of claims 2-6 including, at each of the remote transmitters, using the reference signal generated at the remote transmitter to adjust the operation of local oscillators at the remote transmitter.

9. The method of any one of claims 2-8 including, at each of the remote transmitters, using the reference signal generated at the remote transmitter to perform time and bit synchronization on the broadcast signal, with all of remote transmitters broadcasting substantially the same broadcast signal at the same time on the same transmission frequencies.

10. The method of anyone of claims 2-9 wherein the timing information is inserted into a mega frame identification packet of an MPEG transport stream signal.

11. The method of anyone of claims 1-10 wherein inserting timing information into the broadcast signal includes receiving GPS frequency and timing reference signals through a GPS receiver and generating the timing information in dependence on the GPS frequency and timing reference signals.

12. A transmit diversity single frequency network (SFN) comprising: a distribution network; a head end system providing a broadcast signal containing timing information to the distribution network; and a plurality of transmitter circuits coupled to the distribution network, each transmitter circuit receiving the broadcast signal and having a timing component that is synchronized with timing components in the other transmitter circuits using the timing information contained in the broadcast signal without reference to any further external reference signals.

13. The transmit diversity SFN according to claim 12, wherein the timing component of each transmitter circuit comprises: an extractor for receiving the broadcast signal and extracting the timing information; an oscillator for providing a reference signal; a counter for receiving the reference signal and generating a timing signal; and a comparator for comparing the timing signal to the timing information and generating a feedback signal that is provided back to the oscillator for adjusting the operation of the oscillator to cause the timing signal and the timing information to match in a predetermined manner, and wherein the reference signal is for synchronizing the operation of a modulator of the transmitter circuit.

14. The transmit diversity SFN according to claim 13, wherein the head end system further comprises a global positioning satellite (GPS) receiver coupled to a SFN adapter for adding the timing information to the broadcast signal.

15. The transmit diversity SFN according to claim 14, wherein the SFN adaptor is configured for adding the timing information as a time stamp to a mega frame identification packet of the broadcast signal.

16. The transmit diversity SFN according to claim 15, wherein the time stamp is an N-bit time stamp and the counter is an N-bit counter.

17. The transmit diversity SFN of claim 16 wherein N=40, and the time stamp represents a time on a 24-hour clock.

18. The transmit diversity SFN of any one of claims 13-17 wherein the counter generates a 1 pulse per second timing signal.

19. The transmit diversity SFN of any one of claims 13-18 wherein the reference signal provided by the oscillator is substantially a 10 MHz signal.

20. The transmit diversity SFN of any one of claims 12-19 wherein the broadcast signal includes an MPEG stream.