JOURNAL OF TELECOMMUNICATIONS, VOLUME 29, ISSUE 2, FEBRUARY 2015
1
SFN Monitoring for DVB-T/T2 Networks
Todor Manev
Abstract—This paper is about finding a way to locate any problems in DVB-T/T2 Single Frequency Networks in a fast and
effective manner. The most common problem in SFN networks is lost synchronization which can be caused by defective DVB-T
transmitter, network distribution problems or a faulty GPS receiver at the transmitter site. All these problems lead to an
asynchronous transmission causing reduced quality of reception up to the point of total interference.
Index Terms— DVB-T, DVB-T2, SFN Monitoring.
—————————— u ——————————
1 INTRODUCTION
Nowadays modern DVB-T broadcast networks deliver
a broad spectrum of television programs and have replaced the analog transmission systems. DVB-T often uses
a Single Frequency Network (SFN) in order to have better
coverage and easier mobile TV reception. Such systems
have a lot of benefits. However SFN broadcasts can also
create a variety of problems. Worst case scenario for a
Single Frequency Network is a transmitter that is getting
out of synchronization with the others, thus dramatically
reducing reception quality, up to a point where reception
is not possible anymore. It is not an easy task in a SFN to
pinpoint transmitters that are running out of synchronization. In this article will be presented advanced DVB-T
monitoring receiver technique which effectively assists in
finding problems in SFN broadcasts. It helps broadcasters
to react in time in order to solve the problem before the
viewer is getting aware of it.
2 EXPOSITION
2.1 General schematic of DVB-T/T2 Monitoring
System
In Fig. 1 a typical SFN Monitoring system is displayed. In SFN networks the source of time synchronization is usually done by GPS receiver as a convenient way
to properly synchronize multiple transmitters. At the
transmitter site there is GPS receiver which synchronizes
the broadcast. The synchronization clock is usually 1pps
or 10MHz. This clock is then fed to the transmitter which
synchronizes its broadcast to clock source and inserts MIP
(Mega-frame Initialization Packet) inside the transport
stream to notify receiver for the SFN broadcast and the
GPS time clock.
A monitoring receiver is able to check if the transmitter is in synchronization with GPS clock/time only if it
also has a built-in GPS receiver source as displayed in Fig.
1. The receiver locks to the DVB-T broadcast, receives the
MIP table inside the TS (Transport Stream). It also locks to
the internal GPS 1pps time sync and compares if the MIP
table time corresponds to the GPS time. If the two clocks
are out of synchronization this indicates that the SFN
broadcast is not properly synchronized.
Fig. 1 SFN DVB-T/T2 Broadcast and Monitoring system
There are various scenarios to accomplish the monitoring depending on the placement and connection of the
DVB-T monitoring receiver.
The first and most useful option is to connect RF port
of the monitoring receiver directly to the monitoring output of the transmitter. This approach is very effective as it
is able to diagnose problems both in the GPS receiver and
DVB-T transmitter of the specific transmitter it is connected to.
Second option is to use RF antenna as an input for the
DVB-T tuner. This option may not be very effective as
there could be interference with other SFN transmitters
which could be out of synchronization, thus leading to
loss of TS lock. Failing to lock and receive the transport
stream leads to unavailable MIP making all SFN measurements impossible.
Another scenario is to connect only the 1pps source of
the GPS receiver used for clock synchronization at the
transmitter to the monitoring receiver and compare it
with the internal monitoring GPS receiver. This leads to
identifying any problems with the GPS of the transmitter,
however, problems in the MIP insertion and calculation
cannot be diagnosed.
2.2 DVB-T SFN monitoring receiver
In Fig. 2 internal block diagram of a typical DVB-T
SFN monitoring receiver is displayed. The DVB-T modu-
2
lated signal is fed to the RF input of the DVB-T tuner
which demodulates the COFDM signal and the received
TS is then fed to CPU for processing using transport
stream interface. Then MIP table is extracted and the Synchronization Time Stamp (STS) inside the packet is compared to the GPS pulse received in the GPS receiver connected to the CPU.
Fig. 2 SFN DVB-T/T2 Monitoring Receiver System
Synchronization and transmission information sent by
transmitters are inserted into one TS packet called MIP
packet (or table). DVB normalized its PID to 0x15. The
norm which specified MIP insertion defined a new group
of packet, namely megaframe. The size of the megaframe
depends on the code rate, as well as the constellation
used. The SFN Adapter forms a megaframe (n TSpackets), corresponding to 8 frames (or 2 super-frames) in
8k mode, 16 frames (or 4 super-frames) in 4k mode, and
32 frames (or 8 super-frames) in 2k mode. The MIP inserter will insert exactly one MIP packet per megaframe (with
dedicated 0x15 PID). The position of the MIP packet within the megaframe is signaled by the field 'pointer'. [1]
2.3 Algorithm used to calculate SFN
synchronization timestamp drift
In Fig. 3 a schematic of the relationship between the
timestamp in the MIP table and the GPS 1pps is displayed. This shows how the synchronization timestamp
(STS) is calculated.
Define T=0 as when 1 pps pulse goes high and reference all timestamps relative to this. Use internal CPU
clock timer 1MHz (gives accuracy of +/- 1us) or higher to
accurately timestamp the drift. Receive all MIP packets.
Use the pointer field from the MIP to identify which
packet is the start of the next (M+1) megaframe.
Timestamp - using local timer - the reception of the first
(M+1) mega-frame packet. The timestamp is relative to
the T=0 point in time occurring as the 1pps pulse goes
high. Call this value ACTUAL. Extract the corresponding
fields from the MIP packet and call it IDEAL. This is a
calculated value and represents the time from when the
1pps pulse goes high to when the start of the (M+1)
mega-frame should be transmitted out of the antenna of
each transmitter tower. ACTUAL is larger than IDEAL in
a real-life system due to the transmission delay from the
transmitter antenna output to the COFDM demodulator
antenna input. A second delay factor is the delay in the
COFDM demodulator stage of the DVB-T tuner which is
internally calibrated in the device [4].
If the GPS sync of the transmitter is failing then the
transmitter will slowly start to drift out. We will see this
as a gradual change in (ACTUAL-IDEAL). This is causes
an alarm condition User specifies alarm criteria as a number (units: time) that the absolute value of (ACTUAL –
IDEAL) difference should stay within. For example: Generate alarm if ABS (ACTUAL – IDEAL) > 10 us.
The alarms are logged inside the DVB-T Monitoring
receiver and can be forwarded as SNMP Trap or e-mail.
In Fig. 4 configuration table of SFN related alarms is displayed [3]. Minimum and maximum time of impulse response could be set depending on the placement of the
receiver. Also maximum SFN impulse response drift
could be configured this is the absolute maximum deviation of the impulse response value. It is also very useful to
have alarms in the monitoring receiver if there is no MIP
table inside the transport stream or no GPS lock, which
makes SFN measurement of the impulse response impossible.
Fig. 4 SFN alarms configuration table in embedded web site
Fig. 3 Megaframe/GPS pulse timing relationship [2]
In Fig. 5 the final result of the SFN monitoring is displayed. The SFN impulse response is calculated using the
timestamp inside the MIP table and the timestamp from
3
the built in GPS receiver [3]. This also makes it possible to
estimate the proximity of the transmitter to the monitoring device if RF antenna feed is used. If the SFN measurement is done using the monitoring RF output port of
the transmitter (direct feed) this value should be 0uS. The
distance to the transmitter is calculated using the SFN
Impulse Response (uS) * 300 meters due to the speed of
light with which the broadcasted wave travels from the
transmitter to the receiver.
coverage (strong Inter-carriers interferences).
The use of good monitoring equipment is vital in finding and solving such problems. Timely discovery of any
temporary or permanent problems in the SFN synchronization is very important for broadcasters that wish to
supply faultless round-the-clock service to their clients. In
this article we have shown a possible approach for creating and implementing such device.
ACKNOWLEDGMENTS
Fig. 5 SFN Measurements for one carrier view in embedded web site
It is also useful if the monitoring receiver can do
measurements for multiple DVB-T carriers using roundrobin check of each selected frequency. The approximate
time needed for one carrier is 20 seconds as the receiver
needs to tune and lock to the specific frequency monitored. Needs to decode and timestamp the MIP tables for
this period and thus calculate the impulse response for
each MIP. After that it calculates the drift of the impulse
response and checks it with the configured thresholds.
Besides these measurements also some RF measurement
are performed like level, CNR (carrier to noise ratio),
MER (Modulation Error Rate), CBER (Channel Bit Error
Rate), and VBER (Viterbi Error Rate). In Fig. 6 the final
result of the SFN and RF monitoring is displayed. This
HTML table is automatically refreshed to show any
changes or alarms that arise. In yellow the currently monitored carrier is displayed all values that are out of
boundaries are displayed in red, in Fig. 6 all values are
OK [3].
Fig. 6 SFN and RF monitoring for multiple DVB-T carriers
3. CONCLUSION
Optimizing spectrum and bandwidth is made possible
with Single Frequency Network topology: all the transmitters will radiate synchronously based on information
provided by Single Frequency Network (SFN) adapter [5].
The more accurate SFN synchronization provided, the
more precise RF coverage is. It is to be noted an inaccuracy of frequency synchronization will result in very bad RF
The present document has been produced with the financial assistance of the European Social Fund under Operational Programme “Human Resources Development”.
The contents of this document are the sole responsibility
of “Angel Kanchev” University of Ruse and can under no
circumstances be regarded as reflecting the position of the
European Union or the Ministry of Education and Science
of Republic of Bulgaria.
Project No BG051PO001-3.3.06-0008 “Supporting Academic Development of Scientific Personnel in Engineering and Information Science and Technologies”
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
ETSI TR 101 191 V1.4.1, Digital Video Broadcasting (DVB); DVB megaframe for Single Frequency Network (SFN) synchronization, 2004.
ETSI TR 101 290 V1.2.1, Digital Video Broadcasting (DVB), Measurement guidelines for DVB systems, 2001.
http://www.kvarta.net/DVB_MONITOR_A_C_T
http://www.bridgetech.tv/pdf/sfn-drift-a.pdf
http://www.enensys.com/documents/whitePapers/ENENSYS%20T
echnologies%20-%20Single_frequency_network%20Overview.pdf
http://www.2wcom.com/fileadmin/redaktion/dokumente/Produkt
e/DVB-T_DTT_SFN_White_Paper
Todor Manev received his B.S. and M.S. in
Informatics from Sofia University in 2007 and
2010. During 2010, he was Technical Student in CERN, Geneva participating in LHC
Computing Grid project. Since 2013 he started working on his PhD in Technical University, Gabrovo in Telecommunications Systems
Monitoring, working on various projects regarding FM, DVB-T/C/S/S2/T/T2 monitoring.
He is now with Kvarta Soft, Ltd.