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Unidirectional distribution systems

Asymmetric communication environments

bandwidth limitations of the transmission medium
depends on applications, type of information
examples

wireless networks with basestation and mobile terminals
client-server environments (diskless terminal)
cable TV with set-top box
information services (pager, SMS)

Special case: unidirectional distribution systems

high bandwidth from server to client (downstream), but no bandwidth viceversa (upstream)
problems of unidirectional broadcast systems

a sender can optimize transmitted information only for one group of users/terminals
functions needed to individualize personal requirements/applications

Mobile Communications: Broadcast Systems

6.1.1

4 of 18

Unidirectional distribution

Mobile Communications: Broadcast Systems

service provider

service user

sender

receiver

unidirectional

distribution

medium

optimized for expected

access pattern

of all users

individual access

pattern of one user

≠

6.2.1

5 of 18

Structuring transmissions - broadcast disks

Sender

cyclic repetition of data blocks
different patterns possible (optimization possible only if the content is known)

Receiver

use of caching

cost-based strategy: what are the costs for a user (waiting time) if a data block has been requested but is currently not cached
application and cache have to know content of data blocks �and access patterns of user to optimize

Mobile Communications: Broadcast Systems

flat disk

skewed disk

multi-disk

6.3.1

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DAB: Digital Audio Broadcasting

Media access

COFDM (Coded Orthogonal Frequency Division Multiplex)
SFN (Single Frequency Network)
192 to 1536 subcarriers within a 1.5 MHz frequency band

Frequencies

first phase: one out of 32 frequency blocks for terrestrial TV channels 5 to 12 (174 - 230 MHz, 5A - 12D)
second phase: one out of 9 frequency blocks in the L-band�(1452- 1467.5 MHz, LA - LI)

Sending power: 6.1 kW (VHF, Ø 120 km) or�4 kW (L-band, Ø 30 km)
Date-rates: 2.304 Mbit/s (net 1.2 to 1.536 Mbit/s)
Modulation: Differential 4-phase modulation (D-QPSK)
Audio channels per frequency block: typ. 6, max. 192 kbit/s
Digital services: 0.6 - 16 kbit/s (PAD), 24 kbit/s (NPAD)

Mobile Communications: Broadcast Systems

6.4.1

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DAB transport mechanisms

MSC (Main Service Channel)

carries all user data (audio, multimedia, ...)
consists of CIF (Common Interleaved Frames)
each CIF 55296 bit, every 24 ms (depends on transmission mode)
CIF contains CU (Capacity Units), 64 bit each

FIC (Fast Information Channel)

carries control information
consists of FIB (Fast Information Block)
each FIB 256 bit (incl. 16 bit checksum)
defines configuration and content of MSC

Stream mode

transparent data transmission with a fixed bit rate

Packet mode

transfer addressable packets

Mobile Communications: Broadcast Systems

6.5.1

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Transmission frame

Mobile Communications: Broadcast Systems

synchronization

channel

main service

channel

FIC

MSC

null

symbol

phase

reference

symbol

data

symbol

data

symbol

data

symbol

. . . . . .

symbol

T_u

frame duration T_F

guard interval T_d

L-1

fast information

channel

FIC

6.6.1

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DAB sender

Mobile Communications: Broadcast Systems

Trans-

mitter

Trans-

mission

Multi-

plexer

MSC

Multi-

plexer

OFDM

Packet

Mux

Channel

Coder

Audio

Encoder

Channel

Coder

DAB Signal

Service

Information

FIC

Multiplex

Information

Data

Services

Audio

Services

Radio Frequency

FIC: Fast Information Channel

MSC: Main Service Channel

OFDM: Orthogonal Frequency Division Multiplexing

6.7.1

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DAB receiver

Mobile Communications: Broadcast Systems

Packet

Demux

Audio

Decoder

Channel

Decoder

Independent

Data

Service

Audio

Service

Controller

Tuner

OFDM

Demodulator

User Interface

FIC

Control Bus

(partial)

MSC

6.8.1

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Audio coding

Goal

audio transmission almost with CD quality
robust against multipath propagation
minimal distortion of audio signals during signal fading

Mechanisms

fully digital audio signals (PCM, 16 Bit, 48 kHz, stereo)
MPEG compression of audio signals, compression ratio 1:10
redundancy bits for error detection and correction
burst errors typical for radio transmissions, therefore signal interleaving - receivers can now correct single bit errors resulting from interference
low symbol-rate, many symbols

transmission of digital data using long symbol sequences, separated by guard spaces
delayed symbols, e.g., reflection, still remain within the guard space

Mobile Communications: Broadcast Systems

6.9.1

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Bit rate management

a DAB ensemble combines audio programs and data services with different requirements for transmission quality and bit rates
the standard allows dynamic reconfiguration of the DAB multiplexing scheme (i.e., during transmission)
data rates can be variable, DAB can use free capacities for other services
the multiplexer performs this kind of bit rate management, therefore, additional services can come from different providers

Mobile Communications: Broadcast Systems

6.10.1

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Example of a reconfiguration

Mobile Communications: Broadcast Systems

6.11.1

Audio 1

192 kbit/s

PAD

Audio 2

192 kbit/s

PAD

Audio 3

192 kbit/s

PAD

Audio 4

160 kbit/s

PAD

Audio 5

160 kbit/s

PAD

Audio 6

128 kbit/s

PAD

DAB - Multiplex

Audio 1

192 kbit/s

PAD

Audio 2

192 kbit/s

PAD

Audio 3

128 kbit/s

PAD

Audio 4

160 kbit/s

PAD

Audio 5

160 kbit/s

PAD

Audio 7

96 kbit/s

PAD

DAB - Multiplex - reconfigured

Audio 8

96 kbit/s

PAD

D10

D11

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Multimedia Object Transfer Protocol (MOT)

Problem

broad range of receiver capabilities �audio-only devices with single/multiple line text display, additional color graphic display, PC adapters etc.
different types of receivers should at least be able to recognize all kinds of program associated and program independent data and process some of it

Solution

common standard for data transmission: MOT
important for MOT is the support of data formats used in other multimedia systems (e.g., online services, Internet, CD-ROM)
DAB can therefore transmit HTML documents from the WWW with very little additional effort

Mobile Communications: Broadcast Systems

6.12.1

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MOT structure

MOT formats

MHEG, Java, JPEG, ASCII, MPEG, HTML, HTTP, BMP, GIF, ...

Header core

size of header and body, content type

Header extension

handling information, e.g., repetition distance, segmentation, priority
information supports caching mechanisms

Body

arbitrary data

DAB allows for many repetition schemes

objects, segments, headers

Mobile Communications: Broadcast Systems

6.13.1

header

core

header

extension

body

7 byte

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Digital Video Broadcasting

1991 foundation of the ELG (European Launching Group)�goal: development of digital television in Europe
1993 renaming into DVB (Digital Video Broadcasting)�goal: introduction of digital television based on

satellite transmission
cable network technology
later also terrestrial transmission

Mobile Communications: Broadcast Systems

6.14.1

SDTV

EDTV

HDTV

Multimedia PC

B-ISDN, ADSL,etc.

DVD, etc.

DVTR, etc.

Terrestrial

Receiver

Cable

Multipoint

Distribution

System

Satellites

DVB

Digital Video

Broadcasting

Integrated

Receiver-Decoder

17 of 18

DVB Container

DVB transmits MPEG-2 container

high flexibility for the transmission of digital data
no restrictions regarding the type of information
DVB Service Information specifies the content of a container

NIT (Network Information Table): lists the services of a provider, contains additional information for set-top boxes
SDT (Service Description Table): list of names and parameters for each service within a MPEG multiplex channel
EIT (Event Information Table): status information about the current transmission, additional information for set-top boxes
TDT (Time and Date Table): Update information for set-top boxes

Mobile Communications: Broadcast Systems

multimedia

data broadcasting

MPEG-2/DVB

container

single channel

high definition television

MPEG-2/DVB

container

HDTV

multiple channels

standard definition

MPEG-2/DVB

container

SDTV

multiple channels

enhanced definition

MPEG-2/DVB

container

EDTV

6.15.1

18 of 18

Example: high-speed Internet

Asymmetric data exchange

downlink: DVB receiver, data rate per user 6-38 Mbit/s
return channel from user to service provider: e.g., modem with 33 kbit/s, ISDN with 64 kbit/s, ADSL with several 100 kbit/s etc.

Mobile Communications: Broadcast Systems

6.16.1

DVB adapter

Internet

TCP/IP

leased line

service�provider

information�provider

satellite �provider

satellite receiver

DVB/MPEG2 multiplex�simultaneous to digital TV