Transcript PERFORMANCE OF OFDM IN NAKAGAMI FADING

Digital Video Broadcasting
An Overview
Prof. Dr. Mehmet Şafak
Hacettepe University
Dept. of Electrical and Electronics Engineering
06800 Beytepe, Ankara, Turkey
[email protected]
1
Digital TV
How come a man can be so sensitive as to distinguish
between resolutions 1024 x 678 and 1365 x 768, but can
not see the difference between 15 totally different woman
shoes ?
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DVB - An Overview
I could see the
difference if they
were on the TV
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Outline
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Vision
Data Broadcasting
Integrated Receiver Decoders (IRD)
Transmission on Cable, Satellite and Terrestrially
Interaction Channels
The Multimedia Home Platform (MHP)
DVB-Handheld
Hybrid Networks
Prospects for Future Developments
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Vision
• Initially, DVB concentrated on broadcasting of
audio and video services.
• In later phases, DVB addressed areas which lie
outside of the classical broadcast world.
• Vision defined in 2000:
– DVB’s vision is to build a content environment that
combines the stability and interoperability of the
world of broadcast with the vigor, innovation, and
multiplicity of services of the world of the Internet
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Data Broadcasting
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Data Broadcasting
• A 4:2:2 picture requires a raw transmission rate
of 13.5 Msamples/s x 16 bits/sample =216 Mbps.
• The 216 Mbps just to transmit one digital TV
channel is very high, so the need for
compression is obvious.
• Using compression techniques, the data rates on
the order of 3.2 Mbps per TV channel are
currently used.
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Data Broadcasting
• nn
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Data Broadcasting
• For real-time HDTV encoders, the rate of improvement
in practice has been significantly less than for SDTV:
– smaller number of channels per multiplex means that the
introduction of statistical multiplexing techniques did not yield
as much of a benefit for HDTV as it did for SDTV.
– Today’s real-time HDTV encoders for H.264/AVC or VC-1 do
not yet fully exercise all of the additional tools in the new
algorithms, such as variable block sizes
• Within a year, 8-10 Mbps is expected to be sufficient for
the transmission of HDTV signals.
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Data Broadcasting
Components inside the DVB data container
U. Reimers, DVB-the family of international standards for DVB, Proc. IEEE, vol.94, no.1, pp.173-181, Jan. 2006
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Data Broadcasting
• The output of the MPEG-2 multiplexer (transport
stream) consists of 188-byte packets, where different
video, audio and data channels are multiplexed.
• Depending on the usable data rate of the broadcast
channel, the size of the container varies.
• The program specific information (PSI) provides a list of
packet ID (PID) values of the corresponding program
numbers.
• The service information (SI) contains the modulation
parameters, translates program numbers into service
names and electronic program guide.
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Data Broadcasting
• Data services can be
– program related (e.g., teletext), or
– independent of any other service in the multiplex
(e.g., software download, MHP applications,
information services).
• DVB data broadcasting offers fast Internet
access via satellites.
– Users may be connected to the Internet via standard
modems and in addition install a satellite receiver
card into their PCs for broadband downstream.
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Integrated Receiver Decoders (IRD)
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Integrated Receiver Decoders (IRD)
• SDTV or HDTV
• Baseline IRD or IRD with digital interface
– whether or not they are intended for use with a
digital bitstream storage device such as a digital
VCR
• Video coding formats:
– MPEG-2 video or MPEG-4 AVC (H.264)
• Audio coding formats:
– Dolby AC-3, DTS and MPEG-4 AVC (H.264)
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Integrated Receiver Decoders (IRD)
• Reception of DVB services delivered over IPbased networks
• DVB-TXT replaces Teletext
– which is transported during the period of the vertical
blanking interval (VBI) of analog television.
• DVB developed a generic means for the delivery
of all VBI data, e.g.,
– to enable the control of video recorders,
– the signaling of wide screen programs.
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Integrated Receiver Decoders (IRD)
• As part of DVB signals, it is possible
– to provide a translation of original soundtrack in the
form of subtitles
– add graphic elements to the transmitted images, e.g.,
station logos.
• TV-Anytime information in DVB transport streams
– help personal digital recorders (PDR) to search,
select and acquire the content, the viewer wishes to
record.
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Transmission on
Cable, Satellite and Terrestrially
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Transmission
Block diagram of the DVB-T encoder
(Blue blocks are used in DVB-C and DVB-S as well)
U. Reimers, Digital Video Broadcasting, IEEE Comm. Mag., pp.104-110, June 1998
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Transmission
• DVB-S
– Published in 1993
– Modulation: QPSK and BPSK
– Convolutional codes concatenated with RS codes
• DVB-S2
– Published in 2003
– Modulation: QPSK, 8-PSK (broadcast applications),
16-APSK and 32-APSK (professional applications)
– Backward-compatibity with existing DVB-S receivers
– Reasonable receiver complexity
– Interactivity (i.e., Internet access)
– Professional services, such as digital satellite news gathering
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Transmission
• DVB-S2
– Best transmission performance
• LDPC codes concatenated with BCH codes
• Variable and adaptive coding and modulation (recovers rain margin)
• Approximately 30 % capacity increase compared to DVB-S
– Maximum flexibility
•
•
•
•
framing structure
variable and adaptive coding and modulation
can operate in any existing satellite transponder
accommodates any input stream format (188-byte MPEG-2 transport
streams (packets), continuous bit streams, IP, ATM)
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Transmission
Performance of LDPC codes over AWGN channel (N=64800)
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Transmission
M. Eröz et al.,
An innovative
LDPC code design
with nearShannon-limit
performance and
simple
implementation,
IEEE Trans.
Communications,
vol.54, no.1,
pp.13-17, January
2006.
Comparison of DVB-S2 (LDPC+ BCH) codes to DVB-S
(convolutional+ RS) and channel capacity
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Transmission
• For each code rate, a parity-check matrix is specified by
listing adjacent check nodes for the first bit node in a
group of M=360.
• Irregular LDPC codes are used, where degrees of bit
nodes are varying.
• DVB-S2 offers more than 30% capacity improvement
• DVB-S2 is, on average, about only 0.7-0.8 dB away
from Shannon limits.
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Transmission
• DVB-C
– Published in 1994
– Modulation: M-QAM with M=16, 32, 64,128 or 256.
– Only RS coding is used (no convolutional coding).
• DVB-H
– Published in November 2004
– Enables the reception of digital TV signals by
handheld devices
– Additional FEC, in-depth interleaving and time slicing
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Transmission
• DVB-T, published in 1997, uses OFDM transmissions
–
–
–
–
OFDM has 2K (subcarriers), 4K and 8K versions
OFDM allows single-frequency network (SFN) operation
Modulation: QPSK, 16-QAM or 64-QAM.
Hierarchical modulation: High and low priority streams are modulated onto
a single DVB-T stream for SDTV and HDTV
• Reception by roof-top antenna, portable and mobile reception
• DVB-T is adopted in large parts of the world with
–
–
–
–
built-in front-ends set-top-boxes
PCI cards and USB boxes for desk-top PCs
PCMCIA modules for lap-top PCs
DVB-T reception in cars in driving speeds
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SFN Range vs Mobility Trade-off
• OFDM with 2K:
– widest subcarrier spacing, hence least susceptible against
Doppler shifts (suitable for high-mobility applications)
– shortest symbol duration, hence provides the minimum range
for SFN.
• OFDM with 8K:
– narrowest subcarrier spacing, hence most susceptible
against Doppler shifts (suitable for low-mobility applications)
– longest symbol duration, hence provides the maximum range
for SFN.
• OFDM with 4K provides a trade-off between 2K and 8K
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Hierarchical Modulation
• Two separate data streams modulated onto a
single DVB-T stream,
– high-priority (HP) (low data rate) stream is
embedded within a low-priority (LP) (high data rate)
stream
• Receivers with good reception conditions can
receive both streams
• Only HP streams are received in bad channel
conditions, e.g., mobile and portable reception
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Hierarchical Modulation
An example
of a
constellation
diagram for
hierarchical
modulation
HP bit
stream
(QPSK)
LP bit
stream
(64-QAM)
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Hierarchical Modulation
• Broadcasters can target two different types of
DVB-T receiver with two completely different (LP
or HP) services
– LP stream is of higher bit rate, but lower robustness
than the HP one;
• hence, a trade-off between service bit-rate versus signal
robustness
• A broadcast could choose to deliver HDTV in the
LP stream.
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Transmission
Choice of parameters for non-hierarchical DVB-T transmission
U. Ladebusch and C.A. Liss, Terrestrial DVB, Proc. IEEE, vol.94, no.1, pp. 183-193, Jan 2006
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Transmission
• Useful bit rate (Mbit/s) for all combinations of guard interval,
constellation and code rate for non-hierarchical systems for 8
MHz channels (irrespective of the transmission modes)
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Transmission
• For the hierarchical schemes the useful bit rates
can be obtained from the table as follows:
– HP stream: figures from QPSK columns;
– LP stream, 16-QAM: figures from QPSK columns;
– LP stream, 64-QAM: figures from 16-QAM columns.
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Transmission
Minimum C/N ratio in the
transmission channel
required for quasi-errorfree (QEF) reception for
DVB-T
• QEF reception:
BER <10-11 at the output
of the RS decoder
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Interaction Channels
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Interaction Channels
• The data belonging to a certain interactive
service is transmitted in the broadcast channel
• The interaction channel enables the user to
respond in some way (for instance via the
standard remote control ) to the interactive
service.
• The service provider or network operator listens
and reacts to that response.
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Interaction Channels
Generic system reference model used by DVB for interactive services
U. Reimers, DVB-the family of international standards for DVB, Proc. IEEE, vol.94, no.1, pp.173-181, Jan. 2006
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Interaction Channels
• The user’s response may take the form of some
simple commands, like
– voting in a game show,
– purchasing goods advertised in a shopping program.
• Interactive services may take the form of full
Internet access at the receiver.
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Interaction Channels
• DVB broadcast channels can deliver information
at typical rates of
– 20 Mbps per channel for terrestrial broadcast
networks,
– 38 Mbps per channel for broadcast networks via
satellite and cable.
• Capacity of interaction channel may range from
a few kbps to up to 10 Mbps in cable networks.
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Interaction Channels
• Return Channel Terrestrial (RCT):
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Multiple access: OFDMA
Coding: Turbo or RS+ convolutional
Several kbps per TV viewer in cells with 65 km radius
Can handle large peaks in traffic
Use any gaps or under-utilised spectrum
Serve portable and mobile devices
Can operate in 6, 7 and 8 MHz channels
Transmit power < 0.5 W rms
Time interleaving against impulsive interference
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Interaction Channels
V. Paxal,
DVB with
return
channel via
satellite,
DVBRCS200,
www.dvb.org
Simplified diagram of a network architecture for DVB return
channel satellite systems (RCS)
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The Multimedia Home Platform (MHP)
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The Multimedia Home Platform
• The MHP specification defines an interface between a
digital TV and the network to be connected to in order to
support interactive services.
• It provides features and functions required for the
– Enhanced Broadcast,
– Interactive Broadcast,
– Internet Access.
• The right to use the MHP logo is only granted to those
MHP implementations that pass some 10000 tests,
defined by ETSI.
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The Multimedia Home Platform
• MHP offers true multimedia services to TV users.
• MHP provides a technical solution for the user
terminal enabling the reception and presentation
of applications in an environment that is
– independent of specific equipment vendors,
– application authors,
– broadcast service providers.
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The Multimedia Home Platform
• Some examples:
– Electronic program guides for the channels/services
provided by a broadcaster
– Information services (superteletex, news tickers,
stock tickers)
– Enhancements to TV content (sporting and voting
applications and local play-along games)
– E-commerce, e-government and other applications
relying upon secure transactions
– Educational applications
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DVB over IP-based Networks
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DVB over IP-based Networks
• A typical IPTV service involves the delivery of broadcast
television, radio and similar on-demand services over IP
networks,
– hence, a bi-directional IP communication.
• An open IP infrastructure is used to reach the
customers that can not be reached via classical
broadcast networks.
– Thus, the geographical reach of DVB services can be
extended using broad-band IP networks.
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DVB over IP-based Networks
Basic IPTV architecture
www.dvb.org
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DVB over IP-based Networks
• The work on IPTV can be divided into three areas:
– Set-top boxes and personal video recorders (PVR),
– Home networking,
– Additions to the Multimedia Home Platforms (MHP).
• A specification has been developed that describes the
transport of MPEG-2 based DVB services over IPbased networks.
• Creation of a wireless home network segment is soon
expected.
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DVB-Handheld
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DVB-Handheld
• The system takes into account the specific properties of
typical DVB-H terminals:
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–
–
–
–
–
–
Battery-powered
User mobility
Handover between cells
Mobile multipath channels (antenna diversity)
High levels of man-made noise
Indoor and outdoor operation
Flexibility to operate in various transmission bands and
channel bandwidths (to operate in various parts of the world)
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DVB-Handheld
• DVB-H requires some additional features in the
link layer of the existing DVB-T standard;
– Existing receivers for DVB-T are not disturbed by
DVB-H signals
• The additional elements in the link layer:
– Time slicing
• to reduce the average power in the receiver front-end
significantly (significant power savings in the receiver)
• to enable smooth and wireless handover when the users
leave one service area as they enter a new cell
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DVB-Handheld
The capacity of one
DVB-T channel is
split between three
TV programs and an
additional eight
DVB-H services
625 ms
Slice duration:
625 ms
Bit rate:
3.2 Mbps
(=2 Mb/625 ms)
Average bir rate:
0.4 Mbps(=3.2/8)
U. Reimers, DVB-the family of international standards for DVB, Proc. IEEE, vol.94, no.1, pp.173-181, Jan. 2006
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DVB-Handheld
– Additional forward error correction (FEC) gives an
improvement in
• carrier-to-noise (C/N) performance
• Doppler performance in mobile channels
• tolerance to impulsive interference
– e.g., ignition noise in cars.
– In view of the restricted data rates and small displays
of handheld terminals, it is suggested to exchange
MPEG-2 video by H.264/AVC.
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DVB-Handheld
• The extensions to the physical layer of DVB-T:
– Bits in transmitter parameter signaling (TPS) are upgraded to
indicate the presence of DVB-H service
– A new 4K OFDM mode adopted for trading off mobility and
single-frequency network (SFN) size
• All modulation formats (QPSK, 16QAM and 64QAM) with
nonhierarchical or hierarchical modes can be used
– A new way of using the symbol interleaver of DVB-T has
been defined (to provide tolerance against impulsive noise)
– The addition to DVB-T physical layer of a 5-MHz channel
bandwidth to be used in non-broadcast bands.
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DVB-Handheld
• DVB-H is intended to use the same broadcasting
spectrum, which DVB-T is currently using.
• DVB-H services can be introduced
– in a dedicated DVB-H network
• now it is possible to select 4K mode or in-depth interleavers
– by sharing an existing DVB-T multiplex between DVB-H and
DVB-T services
– by using the high-priority part of the DVB-T hierarchical
modulation
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DVB-Handheld
• Possible applications for DVB-H:
– IP datacasting service to handheld terminals like mobile
phones
– Broadcast services for the mobile phone users
• frequency allocation for simultaneous operation?
• DVB-H is very spectrum-efficient when compared with
the traditional TV services:
– One 8-MHz channel can deliver 30-50 video streaming
services to the small screen terminals
• 10 times more than SDTV with MPEG-2
• 20 times more than HDTV with AVC
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Hybrid Networks
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Hybrid Networks
• Hybrid networks exploit the benefits of both DVB
and mobile communications to enhance services
provided to the consumer:
– Broadcast networks typically involve wide area and
high throughput at the expense of high Tx powers.
– Mobile communications offer low-power transmitters
covering smaller areas (cells)
• Network cost per user is higher than for a broadcast
network
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Hybrid Networks
Hybrid Networks
www.dvb.org
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Hybrid Networks
• The IP datacast, used by DVB for a system
under development, integrates DVB-H in a
hybrid network structure consisting of
– a mobile communications network such as GPRS or
UMTS, and
– an additional DVB-H downstream.
• In the process of being standardized
• A paid service
• Possibility of handover
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Hybrid Networks
• Architecture of
the IP datacast
system
U. Reimers, DVB-the
family of international
standards for DVB,
Proc. IEEE, vol.94,
no.1, pp.173-181, Jan.
2006
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Prospects for Future Developments
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Future Developments
• Mobile communications, digital broadcasting and
Internet are converging.
• Current achievements of DVB:
–
–
–
–
Broadcast delivery to fixed, portable and mobilr terminals
Interactivity-capability in receivers
Data broadcasting over IP-based networks
Multimedia home platform (MHP) to run software applications
on all sorts of terminal devices.
• To understand a person’s current location, availability,
and preferred method of communication at that moment
e.g., a mobile phone or a DVB-H terminal.
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Future Developments
• The focus is now moving to the content itself:
– The ubiquitous access to media content requires
• content management, and
• copy protection measures
– Portable content formats
• To deliver or update the content over fixed and mobile IP
networks (for portable video players)
– TV anytime/anywhere
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References
•
•
•
•
•
•
Special issue of Proc. IEEE on global digital
television, vol.94, number 1, January 2006.
DVB-T: ETSI EN 300 744 V1.5.1 (2004-11)
DVB-S2: Draft ETSI EN 302 307 V1.1.1 (200406)
DVB-S: EN 300 421 V1.1.2 (1997-08)
http://www.dvb.org
http://pda.etsi.org/pda/queryform.asp
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Thanks
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