From: pbaatech@peg.apc.org
Subject: The Complete DAB, part 6 of 6
Originator: wdp@gagme
Organization: Austraila Public Radio
Date: Thu, 15 Oct 1992 00:18:36 GMT


**************************SECTION 6******************************
*****************EUREKA 147 TECHNICAL BACKGROUND*****************

TYPE OF COVERAGE - NETWORKS REQUIREMENTS
There is a market for a number of services of various sizes:
-super-national
-multi-national
These services may best be served via satellite delivery

-nationa]
-regional
-local
These services may best be served via terrestrial delivery. 

A common feature in most cases would be the requirement for 
several services with the same coverage requirement, which is 
conveniently satisfied by the concept of the DAB system.

For national and regional services, several transmitter sites 
will be required, operating as a Single Frequency Network (SFN). 
For these, a frequency block in a VHF Band would appear 
necessary.

For local services providing coverage to a city or a large town, 
it may be envisaged that coverage can be provided from a single 
main station, possibly with one or more low power relays. 
Although VHF may also be appropriate here, the alternative 
option of UHF may also be possible.

As it is not possible to utilize the same frequency block when 
carrying different programmes in contiguous areas, it follow 
that a minimum of 3 or 4 such frequency blocks is required to 
provide individual network coverages using SFN. The smaller the 
size of the area served by each SFN, the greater will be the 
number of frequency block required, until in the limit, the 
conditions for conventional network planning are approached.

It follows from the above that there are in principle two types 
of terrestrial Digital Audio Broadcasting network  to be 
considered:

NOTE: A more difficult planning requirement is where the 
transmitters of a given service carry a common programme for 
much of the time, but only on certain occasions, e.g. for local 
news items, are required to carry different programmes. Any such 
flexibility would be lost within the SFN concept.


A) Network planned on conventional principles, allowing 
different programmes to be carried by individual transmitters,
with the possible use of gap-fillers or relay, as required.

It is of interest to compare the necessary number of "channels"
required for DAB using conventional planning for extensive area 
coverage to that of FM, recognizing that DAB requires lower 
protection ratio. 

Although the 37 dB co-channel protection ratio used for FM is 
high, the  system is robust in respect of increasing 
interference and intelligibility is still possible, e.g. for 
reception of speech in a car, provided that the protection ratio 
is above 5 dB. 

For digital systems, the protection ratios for high quality 
reception are much closer, with the value for the latter being 
essentially the same as for FM.

Moreover, VHF/FM is based on domestic reception assuming a 
directional antenna at 10 m, whereas any new DAB system should 
guarantee good reception under the more critical conditions of 
mobile reception, using a non-directional antenna at low height.

For this reasons, the reduction in the number of "channels: 
required for DAB relative to that for FM is likely to be less 
than implied by the difference in protection ratios required for 
high quality reception of the two systems. 

Practical experience will be necessary to determine the real 
number of "channels" required for extended DAB network based on 
conventional network planning. It should be recognised that even 
within such a network, spectrum conservation is possible by 
having relays stations using the same channels as their parents 
stations, subject of appropriate programme feed arrangements. 
Provided these relays are of low power, the requirement for the 
extended safe-guard interval necessary for SFNs need not apply.


B) Full area coverage with a Single Frequency Network 
(SFN), where it is essential for all transmitters to 
carry the same programmes.

In this case, all transmitters are operating on exactly the same 
frequency, accommodating exactly the same programmes and datas. 
With regard to its frequency efficiency, an SFN seems highly 
attractive. Therefore, possibilities for its realisation have to 
be considered :

- Guard intervals 
Let us assume a receiver near the fringe of the coverage area of 
one individual transmitter, itself part of an SFN; the receiver 
is fed with the signal of several other transmitters (also part 
of the SFN) emitting the same programme. These signals, although 
showing synchronism at their respective point of emission, will 
reach the receiver with different delays; but they will not be 
distinguished from multipath signals provided that the 
modulation is exactly the same. Since multipath propagation is 
the central problem of a terrestrial DAB system, the problem of 
single frequency capability in principle is solved if the system 
caters for multipath disturbances. 

Signal of more distant transmitters could exceed the maximum 
delay allowed .If an SFN is based on existing transmitter 
separation distances, reflections caused by topographical 
obstacles will not have larger excess delay then the signals 
coming from the various transmitters in the network. 

The COFDM system can be adapted to SFN operation by adjusting 
its guard interval and symbol time, Assuming an average 
transmitter distance of 60 km, the duration of the guard 
interval should be in the order of 250 microseconds, Thus the 
system will operate reliably under all multipath propagation 
conditions. This guard interval corresponds to a useful symbol 
period of 1 ms, that means a total symbol time in the order of 
1.25 ms.  A preliminary estimate of tolerance for synchronous 
condition is 5% of the guard interval, i.e. about 12.5 
microseconds, this being practically realisable with fixed 
programme lines.


- Gap-fillers
Gaps in coverage of an SFN can be done by low power relays using 
the same frequency block; these need not be fully synchronous 
with the main transmitters provided that delays are within the 
guard interval capability in terms of multipath tolerance. 
If a certain isolation between receiving and transmitting 
antennas is possible, the fill-in transmitters (gap-fillers) can 
work as normal rebroadcast transmitters, using the same 
input/output frequency. An alternative to gap-filling would be to 
increase the powers of certain transmitters in the network, 
typically by 10 dB. Apart from cost implications, this would also 
increase interference to co-channels services, precluding 
implementation of such high power DAB network.


- Distribution of programmes
All transmitters require identical and synchronous modulation 
with the COFDM.

a) One possibility is individual feeds of all programmes to each 
transmitter site.in digital, preferably  reduced bit-rate 
format.

- A  first problem is the need to maintain correct time-
coherence of the feed, bearing in mind that the feed circuits 
may change, this would require exclusive lines even for gap-
fillers. 

- Secondly, from these sound-signals, a synchronising signal 
must be derived in order to multiplex signal and data and to 
produce the COFDM signal. All this would be required to be done 
in exactly the same way at all transmitters sites. 

Although  possible in principle, some form of automatic 
monitoring would be required in order to make corrections when 
even  slight differences in modulation to those of adjacent 
transmitters occurs. This monitoring would not seem possible 
because of the co-channel operation of the SFN.

The sole reasonable method seems to have a central production of 
the COFDM signal, being distributed to each individual 
transmitter in the network in digital form, requiring about 12 
times the data rate of a corresponding number of base-band codec 
sound signals. Thus the cost of programme feed for DAB/SFN would 
be greater than for VHF/FM, particularly since the simple re-
broadcast feed of relays is not possible. 

b) a more cost effective solution consists of satellite 
distribution for programme distribution. A COFDM signal can be 
converted down to within a normal video-frequency range, and 
contained within one FM-TV transponder channel using frequency 
modulation. The appropriate technology is already available for 
11 GHz satellite TV.

Thus the requirement at individual transmitter sites when fed by 
satellite with the complete COFDM signal will be confined to FM 
demodulation, filtering, frequency conversion and power 
amplification. This will also apply to relays.

Some disadvantages of this method are:
- additional delay of about 240 ms in the transmission, adding 
  to coding delay (estimated min 400 ms).
- satellite failure means total collapse of network, thus 
  standby requirement at some additional costs.
- cost of providing the feed to satellite earth station, 
  possibly reduced by siting earth station close to DAB combining 
  centre.


SERVICE MULTIPLEX ,CODING AND MODULATION TECHNIQUES

The conceptual diagram of a system A digital transmitter 
generally revolves around the main service multiplex block.

- Each audio sound sources are firstly applied to the source 
coding in order to reduce the data-stream according to ISO/IEC 
MPEG layer II system, formally called Musicam. The bit rate 
available for a monophonic sound signal are 64, 96, 128 or 192 
Kbit/s with 2 Kbit/s allocated to programme associated data. 
Stereophonic signal may be conveyed as two co-phased mono 
signals or be jointly encoded within one of the available bit 
rates either in the form of L and R or M and S signals for 
better error protection. 

This signal is than applied to  an error protection coding block 
(energy dispersal and convolutional coding) before being fed to 
the main service multiplex block. 

- Data sources, in stream or packet mode are also possible, 
the packet mode follows the lines of the packet multiplex system 
used in the full-channel mode of the Mac/packet family systems 
outlined in CCIR Rec. 712. A data service channel has a source 
bit rate of n x 16kbit/s. 

Each data source signal is also applied to  an error protection 
coding block (energy dispersal and convolutional coding) before 
being fed to the main service multiplex block. 

- Multiplex controller. 
The Multiplex Configuration Information (MCI) contains timing 
information, informations about the ensemble itself, and 
informations about the major service components of the multiplex, 
allowing a receiver rapid access to these service components. 
Moreover, the number and type of service components to be carried 
in the ensemble may change. 

It is the another function of the MCI to ensure that these 
changes can occur without weakening the integrity of those 
service components which continue during such changes. The data 
information is error protected before being applied to the main 
service multiplex block.

- Service information assembler. The system is also capable 
of complying with the ISO open system interconnect model for 
data broadcasting; In order that the receiver and user can have 
adequate information about the service being carried, a service 
information facility is provided to give textual comments and 
future programmes, together with machine-readable data to assist 
programme pre-selection.

This service information data is also applied to an error 
protection coding block (energy dispersal and convolutional 
coding) before being fed to the main service multiplex block. 


MAIN SERVICE MULTIPLEX is where all source components of a 
DAB service,(except for the fast information channel) are 
processed and combined into a common virtual frame.

the transmitted signal of digital system A is build up around a 
frame structure corresponding to the juxtaposition in time of a 
synchronisation channel, the service information channel and a 
data field. The data field comprises a multiplex of sound and 
data channels which is defined  in the service information 
channel.

The multiplexing of the various sources is performed within 
successive time slot of 24 ms, during which the data blocks 
resulting from the encoding process of each sound or data 
channel are sequentially multiplexed.

Depending of the network configuration, 3 different modes are 
defined, each having its particular set of parameters.

MULTIPLEX FRAME STRUCTURE.
The frame is composed of elementary time slots called symbols.

- the 1st symbol of the frame is a null symbol 
- the remaining part of the frame is a juxtaposition of symbols, 
  each of these symbols consists of a set of equally spaced 
  orthogonal carriers.

Based on a 2 MHz bandwidth for a given DAB block, allowing up to 
6 stereo channels plus some data capacity, typical parameters 
for each of the 3 modes defined for system A are:

					Mode I	Mode II	Mode III
max frequency of operation	340 MHz	1.38 GHz	2.76 GHz

Overall frame duration		96 ms		24 ms		24 ms
Null symbol duration		1 ms		250 5s	250 5s
Overhall symbol duration		1.25 ms	312.5 5s	156.5 5s
Guard interval duration		250 5s	62.5 5s	31.25 5s
Useful symbol duration		1 ms		250 5s	125 5s

Symbols per frame *			76		76		152
Number of carriers			2048		512		256

* excluding null symbols.

These parameters for mode II and III can be optimised depending 
of frequency of operation.

SYNCHRONISATION CHANNEL
The first symbol of the frame is the null symbol, it can be used 
to provide approximate time synchronisation of the receiver, and 
also estimate the characteristics for noise and interference 
present in the radio-frequency channel.

the second symbol is the frequency reference symbol, it can be 
used to lock the local oscillator of the receiver on the 
received signal frequency.

the third symbol is the phase reference symbol which provides 
the reference for the differential demodulator applied in the 
receiver.
It can also be be used to estimate the impulse response of the 
radio-frequency channel in order to improve the time sync.of the 
receiver.

TIME AND FREQUENCY INTERLEAVING 
The convolutionally encoded data issue from the main service 
multiplex block are finally applied to the time and frequency 
interleaving block  The frequency interleaving spreads the data 
over all available carriers, while the time interleaving is of 
the 
convolutional type and spreads the data over the appropriate 
range.

MODULATION
The resulting data stream (main service channel) is finally 
applied into a final modulation block The modulated symbols 
belong to the service information channel and to the data field. 
Each symbol constitutes a multiplex of orthogonal carriers (OFDM 
+ Orthogonal frequency division multiplex) with a carrier 
separation of 1/ symbol duration. Differentially encoded 
quadrature phase shift keying of each individual carrier (4-
DPSK) is applied, so that the phase rotation of a given carrier, 
between one symbol and the next, conveys an elementary dibit.


Christian Burnat
Public Broadcasting Association of Australia

REFERENCES:
EBU Compilation for the first International Symposium on DAB
Strategies for introduction of DAB world-wide
Experimental evidence and recent system developments
Concepts for DAB into the 21st Century
Montreux 8 and 9 June 1992





Newsgroups: rec.radio.broadcasting