DVB-RCT: A STANDARD FOR INTERACTIVE DVB-T

DVB-RCT: A STANDARD FOR INTERACTIVE DVB-T

Gerard FARIAHarris Broadcast Communication, France

Fabio SCALISEST Microelectronics, Italy

ABSTRACTInteractive Digital TV services are considered nowadays as the key element to introducedigital TV services and to encourage customers to invest in new digital TV appliances. Sometime ago, the Digital Video Broadcasting (DVB) forum, which already produced aharmonised family of technical standards [1,2,3], decided to engage the work of definingreturn channels using the same medium than the broadcast “downstream” one. Thus, thecable and the satellite medium have been endowed with their dedicated return channel [4,5].

The last product of this work is called DVB-RCT (Return Channel Terrestrial) [6] and isintended to provide standard specification for interactivity in the UHF/VHF bands. Using thenewly defined DVB-RCT standard as a companion of the world-wide proven, recognised andadopted Terrestrial DVB system [3], DVB-T is no longer limited to unidirectional broadcast, itcan become a true Terrestrial Wireless Interactive system.

The DVB-RCT technical subgroup performed a tremendous work to define the physical layer(RCT-PHY), the Medium Access Control layer (RCT-MAC) and the RF ImplementationGuidelines. As a result, the DVB-RCT standard makes use of a Multiple Access OFDMarrangement, to constitute a high bandwidth Wireless Interactive Terrestrial Digital TVsystem. Officially approved by the DVB forum in April 2001, it will be soon published as anETSI European standard (EN 301 958).

Two European Research projects (WITNESS and IM4DTTV – Integrated Modem for DigitalTerrestrial TV) have been set up to support the roll-out of DVB-RCT through extensive fieldtrials and the design of an integrated VLSI solution for the user terminal. Thus, fast and cost-effective integration of the DVB-RCT standard in the Terrestrial Digital TV set-top boxes areexpected.

Following a presentation of the DVB-RCT technical specifications, the two projects dealingwith the future of the DVB-RCT standard are presented in this paper.

INTRODUCTIONFollowing the selection of the most efficient Digital Terrestrial TV (DTTV) standard, the broadcasters areevaluating several business models able to ensure a successful deployment of the Digital Terrestrial TV system.Surveys have been conducted amongst ‘TV viewers’ to ascertain their vision and expectations of digital TV. Ifthe increase of the available programmes (with a maximum of 30) constitutes a major will, the expectations fornew services and new thematic programmes constitute the main requirements.

Interactivity with new and stimulating content came out top, time and time again. It is now clear that the businessmodel based on ‘Interactive Services’ is the most coherent in regard to the public expectations and therefore, itcan provide additional revenues to the broadcasters. The DVB Consortium, aware of this preference, nowprovides Terrestrial broadcasters additional features in the DVB-T portfolio to implement such interactiveservices; thus opening a new era for the Terrestrial delivery platform.

DVB-RCT constitutes that definitive asset.

WHAT IS DVB-RCT?The transition from Analog to Digital broadcasting is today the main topic of interest in the area of TV industry;new standards have appeared for satellite (DVB-S) [1], for terrestrial (DVB-T) [2] and for cable (DVB-C) [3]broadcasting. Those standards are currently being implemented in various countries, not only in Europe, butalso around the world. In parallel some new services are booming in interest among the users, e.g. e-mail andInternet and some others are seen as going to boom shortly, e.g. e-commerce and Tele-banking.

A general consensus agrees that the main and unique feature that will make Digital TV really appealing to theend users is the provision of interactivity on top of the digital pure broadcasting TV service. This is mostly true forDigital Terrestrial TV, where the user do not want to spend more money for a pure digital TV broadcastingservice, comparing to the existing analogue one. Therefore, the availability of a high bandwidth, highperformance return channel is mandatory to achieve good success for Digital Terrestrial TV deployment.

That is the purpose of DVB-RCT as a companion to the DVB-T broadcasting standard.

ETSI - EN 301 958 - DVB-RCT: A NEW STANDARD IS BORNAfter nearly one and half year of work, the DVB-RCT technical subgroup issued the final DVB-RCTspecifications for Approval in March 2001. The DVB Technical Module and the DVB Steering Board haveapproved them in April 2001; the new specifications are planned to become the EN 301 958 ETSI standardbefore the end of the year.

In the light of the initial DVB commercial requirements for this new standard (especially the ones concerning theportable and even mobile receivers system suitability), the DVB-RCT group decided to offer a flexible system,able to match all the forecasted deployment scenarios of the Terrestrial Digital TV system.

The major characteristics of this new DVB-RCT standard are commented in the following paragraphs.

ON THE WAY TO INTERACTIVE TVSome new services and features have been recently offered to TV viewers, with no need of return channel backfrom the user to the Service Provider: Data carrousel or Electronic Programmes Guides (EPG) are examples ofsuch Enhanced TV services which implement a rough type of “Local Interactivity”.

However, real T-Commerce services require full interactivity, implying data and commands exchange betweenthe users & the Service Providers. The long connection time needed by existing solutions for return channelbased on telecom networks (PSTN, ISDN or GSM) and the very limited bandwidth, limits considerably thecategory of possible interactive services and the interest from users. Pay-per-view is an example of services,which can cope with such long latency connection time. However, to implement real bandwidth demandinginteractive service (e.g. internet) or new services having a strong & real-time relationship with the TVprogrammes (e.g. interactive advertising, Tele-voting, Tele-quiz, etc.) a low latency, high-bandwidth returnchannel technology is mandatory.

DVB-RCT is THE response that offers a wireless interaction channel for these real-time Interactive DigitalTerrestrial Television Services:

! DVB-RCT is spectrum efficient, low cost, and powerful and provides a flexible Wireless Multiple Accessbased on well-known OFDM technique that is well suited for transmission in the terrestrial channel.

! DVB-RCT can serve large cells, up to 65kms radius, providing a typical bit-rate capacity of several kilobitsper second, for each TV viewer, even at the edge of the coverage area. Typically, these large cells closelymatch the downstream coverage area of the Digital Television broadcast signal.

! DVB-RCT can handle very large peaks in traffic, as it has been specifically designed to process up to20,000 short interactions per second in Tele-polling Mode, this in each sector of each cell.

! DVB-RCT can be employed with smaller cells, to constitute denser networks of up to 3.5km radius cells,providing to the user a bit-rate capacity of up to several Megabits per second per user.

! DVB-RCT does not require access to spectrum on a primary basis; the system has been designed to useany gaps or under-utilised spectrum anywhere in Bands III, IV and V without interfering with the primaryanalogue and digital broadcasting services.

! DVB-RCT is able to serve portable devices; bringing interactivity everywhere the Terrestrial Digitalbroadcast signal is receivable,

! DVB-RCT can be used around the world, which uses the different DVB-T system: 6, 7 or 8 MHz channels,

! DVB-RCT does not require more than 0,5W rms transmission power from the User Terminal or Set TopBox to the base station.

DVB-RCT: THE SYSTEM PRINCIPLESThe interactive system consists of a forward interaction channel (downstream) conveyed to the user via a DVB-Tcompliant terrestrial broadcast network, and a return interaction channel based on a Wireless VHF/UHFtransmission (upstream) of the same type. A typical DVB-RCT system is illustrated in the Figure 1.

The downstream transmission from the Base Station to the Terminals (RCTT) provides also synchronisationand information to all RCTTs. That allows RCTTs to synchronously access the network and then to transmitupstream-synchronised information to the Base Station.

T

"

"

"

"

Tf

DTdfupoas

BT

"

BroadcasterCore Network

InteractiveService Provider

InteractiveNetwork Adapter

HomeUser

BroadcastInterfaceModule

InteractiveInterfaceModule

Set TopUnit

Terrestrial Return Channelfrom User to Broadcaster f

Programs & Data toUser

Set Top Box

Figure 1 – DVB-RCT system architecture

he DVB-RCT system is governed by the following rules:

Modulation scheme is OFDM for both downstream (fully DVB-T compliant) and upstream; therefore, severalparallel carriers are available in the upstream to be allocated to different users in different time slots fortransmitting their data and commands back to the Base Station.

Each authorised RCTT transmits one or several low bit rate modulated carriers towards the Base Station, The carriers are frequency-locked and power ranged and the timing of the modulation is synchronised by

the Base Station. On the Base Station side, the Upstream signal is demodulated, using a FFT process, just like the one

performed in a DVB-T receiver.he DVB-RCT standard specification [6] defines and covers the two basic layers (“Physical” & “MAC”) needed

or an interactive session; both layers are presented briefly hereafter.

VB-RCT: THE PHYSICAL LAYERo allow access by multiple users, the VHF/UHF return channel is partitioned both in the frequency and timeomains, using Frequency Division Multiplex (FDM) and Time Division Multiplex (TDM). This grid of time-requency slots can be equipped with carriers, from any RCTT. Each available carrier is allocated to a certainser for a defined time slot (burst); multiple slot allocation is possible (both on the same carrier and onto differentarallel carriers) to cope with bandwidth peak demands. The bandwidth requests are typically bursty in such typef applications; this implies that a permanent allocation of a certain carrier to a certain user would be not efficientt all, thus leading to waste of bandwidth. Therefore, the Base Station will control the allocation policy in order toerve a large number of users in parallel and to make an efficient usage of the available spectrum.

ASIC PARTITIONINGo avoid inter-carrier and inter-symbols interference, two types of sub-carrier shaping are provided:

Nyquist shaping: uses in-time Nyquist filtering on each carrier, to provide immunity against both inter-carrier and inter-symbol interference, as well as immunity against jammers. This configuration is particularlysuited for large cells, where users can be located well far away one from the others and from the BaseStation; misalignments in frequency could lead a user terminal transmitting on one carrier to pollute otherusers transmitting onto neighbouring carriers. When using Nyquist shaping, carrier spectra ar disjointed andmisalignments have limited effects onto other users [8].

RectangularRectangularshapingshaping time

frequency

NyquistNyquistshapingshaping time

frequency

Figure 2 – DVB-RCT RF channel organisation

" Rectangular shaping: makes use of an orthogonal arrangement of the carriers and of a Guard Interval,inserted between modulated symbols, to provide immunity against inter-carrier and inter-symbolinterference, as well as combating multipath propagation effects. This is the typical shaping used in DVB-Tstandard as well and makes use of orthogonal carriers, whose specra are allowed to overlap; however, theorthogonality conditions among carriers allows for correct demodulation and separation of each transmitteddata. Such scheme is more sensitive to misalignments and is well suited for small cells in dense networks.

This organisation of the different carriers is depicted in Figure 2.

TRANSMISSION MODESDVB-RCT standard provides six transmissions modes [6] characterised by a dedicated combination of themaximum number of carriers offered and their inter-carrier distance. The three DVB-RCT inter-carrier spacingvalues are defined in Table 1.

Carrier Spacing (CS) and symbol duration

CS1 ~ 1 KHz symbol duration of ~ 1 000 µs

CS2 ~ 2 KHz symbol duration of ~ 500 µs

CS3 ~ 4 KHz symbol duration of ~ 250 µsTable 1: DVB-RCT approximate targeted inter-carrier spacing for 8 MHz channel

The inter-carrier distance governs the robustness of the system in regard to the possible synchronisationmisalignment of any RCTT. Each value implies a given maximum transmission cell size, and a given resistanceto the Doppler shift experienced when the RCTT is in motion.

The DVB-RCT final bandwidth is a function of the number of carriers used (1K or 2K) and of the Carrier Spacingdefined; it can be roughly 1 MHz, 2MHz or 4 MHz. Each combination has a specific trade-off between frequencydiversity and time diversity, and then between coverage range and portability capability. In addition to that, BandSegmentation can be used to determine the final bandwidth of a specific return channel segment.

The elementary segment for return channel is 1 MHz; this greatly eases the problem of access to the spectrum(this is the main problem when trying to re-use UHF/VHF bands, that are highly congested in some countriesdue to analog services, taboo channels and other interference).

Time

Freq

uenc

y

User SymbolsRanging Carrier

Transmission Frame Type 1

Null Symbol

RangingSymbols

User Symbols carrying or BS1 or BS2 (not simultaneously)

Figure 3 – TF1 – Time domain organisation

Basically, the concept is that any 1MHz segment of spectrum can be used; it is not necessary to allocate anentire 8 MHz channel (or 7/6 MHz outside Europe) to establish a DVB-RCT service. This is particularly importantas long as analogue terrestrial transmissions will continue.

A key commercial advantage of this Band Segmentation is that different Interactive TV Service Providers can beassigned their own 1MHz of spectrum and can thereby remain independent of one another.

TRANSMISSION FRAMESThe collection of physical slots (time partition of each available carrier), which constitutes the basic bandwidthpartitioning, is organised to carry suitable data and commands to implement special protocol functions related tothe various activities of the RCTTs in the system (i.e.: synchronisation, ranging, data transmission). This can beaccomplished through two alternative types of transmission frames TF1 & TF2 [6]:

" Using TF1, a range of complete OFDM symbols is used for each dedicated activity,

" Using TF2, no specialisation of OFDM symbol is used, instead, every OFDM symbols is divided in sub-channels (i.e.: a set of sub-carriers), then each sub-channel is granted for a dedicated activity,

In short, Transmission Frame 1 organises the channel in the Time domain whilst Transmission Frame 2organises it in the Frequency domain.

According to these definitions, the transmission frames provide a repetitive structure, made up of a set of time-frequency slots, in which Null Symbol, Ranging Symbols, Data Symbols and Pilot Symbols (for ChannelEstimation purposes at base Station) are embedded to provide resources for synchronisation, ranging and datatransmission.

TF1 is, in general, more suited for large cells, where TF2 is optimised for small cells with higher bandwidth.

Transmission frame 1 (TF1)The first type of transmission frame (TF1) carries three categories of symbols:

" One Null Symbol: during which no transmission occur to allow the Base Station to detect jammers,

" Several Ranging symbols to allow the RCTT to request access to the DVB-RCT channel,

" Several User Symbols during which several RCTTs transmit their data bursts using the Bursts Structures.

Figure 3 depicts the organisation of TF1 frame in the time domain.

In the User part of TF1, either Rectangular shaping or Nyquist shaping is used, whilst in the ranging part only theRectangular shaping is used.

Transmission frame 2 (TF2)The second type of transmission frame (TF2) carries only general-purpose symbols. Nevertheless, in the time-domain, groups of six symbols are organised to make sub-channels synchronised (i.e.: they “start” at the same

instant). Then, eight consecutive groups implement a field for Ranging. Accordingly, the sub-channels occurringduring that period can be dedicated either for Ranging functions or to carry the RCTT’s data bursts; this isaccomplished through a dedicated data structure for the elementary burst, i.e. the Bursts Structure. Three BurstStructures are defined in DVB-RCT, as described later in this paper.

Figure 4 depicts the organisation of TF2 frame in the time domain. Note that in Figure 4, the Burst Structures aresymbolised regarding their duration and not regarding their occupancy in the frequency domain.

T

TPo1

TB

Time

Freq

uenc

y

User SymbolsRanging SymbolNull Symbol

Transmission Frame Type 2

User Symbols carrying eight Burst Structure 3

NullSymbols

User Symbols carrying one Burst Structure 2

User Sub-ChannelRanging Sub-Channel

Figure 4 – TF2 - Time Domain organisation

DVB-RCT channel bandwidth

Guard Band DC carrier(not used) Guard Band

1K mode

2K mode

000

001

002

003

004

005

416

417

418

419

420

415

422

423

424

425

426

421

836

837

838

839

840

841

91Unused sub-carriers

91Unused sub-carriers

0000

0001

0002

0003

0004

0005

0851

0852

0853

0854

0855

0850

0858

0859

0860

0861

0862

0857

1706

1707

1708

1709

1710

1711

168Unused sub-carriers

168Unused sub-carriers

427

0856

Figure 5 – TF’s organisation in the frequency domain

F2 frame makes only use of the Rectangular shaping.

F’s organisation in the frequency domainrevious figures depict the organisation of TFs in the time domain; however, as any OFDM system, a dedicatedrganisation is needed in the frequemcy domain as well. Among the available carriers, the 2K mode uses only712 carriers and the 1K mode offers 842 carriers, for carrying information. This is depicted in Figure 5:

he unused carriers, located on each edge of the channel, provide a guard band to protect adjacent channels.oth Transmission frames provide these guard bands and this carrier organisation.

Time

Freq

uenc

y

BS1: 1carrier

BS2: 4 carriers

BS3: 29 carriersIdentical payload :- 144 data modulated symbols- 36 pilot modulated symbols

Figure 6 – Illustration of the Burst Structures

0,1km

1 km

10 km

100 km

0 4 8 12 16 20 24OTP power dBm

BS3 BS2 BS1

Figure 7 – Burst Structure Coverage

BURST STRUCTURESEach RCTT transmits bursts of data based on an integer number of ATM cells. Whatever the protection codingrate and the physical modulation, the data bursts are made up of 144 modulated symbols, which constitute thedata payload. Among this data payload, a collection of Pilot Data are spread to allow coherent demodulation inthe Base Station. DVB-RCT defines three Burst Structures BS1, BS2 and BS3, having their own characteristics,in regard to the partitioning, among the time-frequency axes, of the data bursts and the pilots.

The three Bursts Structures [6], asillustrated in Figure 6, constitute varioustrade-off between burst duration andfrequency diversity they provide. Shorterburst duration is more robust againstinterference but requires more than onecarrier to be allocated in parallel to asingle user for a single bandwidthrequest.

" Burst Structure 1 carries its payloadon consecutive symbols, using onlyone carrier,

" Burst Structure 2 spread its payloadover 4 carriers,

" Burst Structure 3 spread its payloadover 29 carriers,

Accordingly, each Burst Structure provides dedicated characteristic in regard to its robustness, then to the radiusof the cell it cover. Due to the limitation of the transmission power, each Burst Structure has its own maximumcoverage distance, as represented on the Figure 7.

BITRATE CAPACITYThe bit-rate capacity of the DVB-RCT channel depends on the transmission mode (i.e.: carrier spacing impliesthe duration of the modulated symbol) and on the robustness of the chosen coded modulation (i.e.: modulationconstellation & coding rate).

Then, in relation with the modulation/coding rate, the RCTT will be able to exchange with the Base Station in a

0 K b p s

2 K b p s

4 K b p s

6 K b p s

8 K b p s

1 0 K b p s

1 2 K b p s

1 4 K b p s

1 6 K b p s

1/2 3/4 1/2 3/4 1/2 3/4 1/2 3/4 1/2 3/4 1/2 3/4 1/2 3/4 1/2 3/4 1/2 3/4

4 Q A M 1 6 Q A M 6 4 Q A M 4 Q A M 1 6 Q A M 6 4 Q A M4 Q A M 1 6 Q A M 6 4 Q A M

Figure 8 – Bitrate per Carrier

range of distance depending on the power it uses. For instance, using 64QAM, 3/4 modulation (highest bitrate,lowest curve), BS3 could be used from 0,6 km to 1.5 km from the Base Station. Conversely, using BS1, thestrongest protection 4QAM, 1/2 (lowest bitrate, highest curve) and the range of power, the RCTT could performexchanges with the Base Station in a range of 30 km to 60 km.

Figure 8 shows the range of available bit-rate when using BS1 or BS2 and the Rectangular shaping [6]. Valuesare given for the three modes (i.e.: carrier spacing) as a function of the coded modulation. Small variationsrepresented in each case come from the four possible durations of the Guard Interval (1/4, 1/8, 1/16, 1/32 of theuseful symbol duration) in the case of Rectangular Shaping.

Globally, in typical applications, the DVB-RCT system offers a net bit-rate per carrier ranging from 0,6 Kbps to15 Kbps. When all carriers are used, the Base Station is able to collect up to 1 Mbbs to 26 Mbps of user data inthe DVB-RCT channel. Obviously the most robust modes offer the lowest bit-rate, over a large radius cell whilstthe weakest modes offer the largest bit-rate over a small radius cell. In addition to these structural definitions, theDVB-RCT standard provides several innovative features, as described hereafter.

DYNAMICALLY-ASSIGNABLE ADAPTIVE MODULATIONDVB-RCT supports within the same cell the simultaneous use of different types of modulation from 4QAM (1/2rate) to 64 QAM (3/4 rate). This feature called “Dynamically-assignable Adaptive Modulation” enables theService Provider to control the level of interference from a given cell into neighbouring co-channel cells while, atthe same time making maximum use of the allocated spectrum.

For instance, the most robust form of modulation (e.g. 4QAM 1/2 rate) can be assigned to users near the outerboundary of the cell allowing them to use the minimum possible amount of power to transmit back to the BaseStation. Conversely, users nearer to the center of the cell can be assigned less robust modulation (e.g.:64QAM, 3/4), at the expense of more power, to transmit back to the Base Station. Even the users, close to thecenter of a cell, will use more power to enjoy higher data throughput, as they are further away from other cells,they will cause less interference into the other co-channel cells.

N=k/2couples of data

Permutation(k/2)

Π

codeword

puncturing

Y1 or 2

B

2

A

1S1A

B

Y

Systematic part

Redundancy part

S2 S3

Figure 9 – Constituent Code for Turbo Coding in DVB-RCT

TURBO OR CONCATENATED CODINGChannel Coding is a key element to define the performances of a transmission system; in particular,transmission in the wireless environment suffers from impairments due to several issues (e.g. multipath,frequency selective fading, etc.). All these elements contribute to produce a high error rate (BER) that has to bedealt with to provide a reliable system.

For instance, DVB-T standard adopted a concatenated channel-coding scheme based on Convolutional Codesand Reed-Solomon Codes separated by an interleaver. However, alternative schemes have been proposed inthe last years, based on iterative decoding algorithms (such as Turbo Codes) that allow, in some cases, for animprovement of BER for a given C/N level. DVB-RCT authorises the usage of both Concatenated Coding andTurbo Coding (alternatively); this choice can give a further reduction in the required C/N ratio of 1.5dB or more.As a result, some Modulation Modes of DVB-RCT require a C/N of only 4dB. Such feature is very important fornetwork planning, because, for a given transmitting power level, a larger cell radius can be achieved. Vice versa,for a given network topology (i.e. cell radius and transmitter placement), the transmitting power can bedecreased; that means, less interference and less pollution.

The constituent codes for the Turbo Encoder block are depicted in Figure 9. It uses a Double Binary CircularRecursive Systematic Convolutional (DB-CRSC) code. A dedicated puncturing law is used to generate rate ½and ¾ codes.

Alternatively, a Concatenated Coding scheme is possible; the data shall first enter a RS(63,55,t=4) encoder andare then passed to a tail biting Convolutional Encoder. The coding produced in the first stage, although basedupon the systematic RS(63,55,t=4), can be varied by the number of parity symbols sent. Less parity symbols canbe used, by deleting some of the parity symbols from the end of the block at the transmission side (in the RCTT)and then using erasures at the receiver side (in the Base Station). The Convolutional Encoder has constraintlength 9 and uses mother codes: G1=561oct for X and G1=753oct for Y. Both constituent codes in theconcatenation can be punctured to generate a total puncturing rate for the concatenation of, again, ½ and ¾.

TIME INTERLEAVINGTime Interleaving, performed through adequate data mapping onto the Bursts Structures, can give at least anadditional 5dB improvement against the Impulsive Interference (the actual figure depends on the repetition rateof the interference).

This ensures that the coverage area of the Multiple Access OFDM signals can be designed to closely match theservice area of Digital Terrestrial Television Broadcasting, thus dramatically reducing the need for newinstallations. A Pseudo Random Binary Sequence (PRBS) is used to build a Random Interleaver.

POWER RANGINGDVB-RCT uses a Power Ranging system, similar to the one used in Cellular phones, to ensure that the lowestpower is used by the RCT Terminals at all times. This provides a powerful feature to reduce interference and toincrease spectrum efficiency of the system.

As a conclusion, from the technical point of view, DVB-RCT is built around the most recent developments inalgorithms and techniques for digital transmission and it constitutes the first system in the world using MultipleAccess OFDM technique to provide a Wireless Interactive Terrestrial channel.

Cell 1using RF 1

Figure 10 – One RCT upstream in one DVB-T cell

Cell 1using RF 1

Cell 2using RF 2

Cell 3using RF 3

Figure 11 – Several RCT upstream in one DVB-T cell

DVB-RCT: THE MAC LAYERThe DVB-RCT MAC layer [6] is built over the physical layer to provide a reliable and shared transmission linkbetween the RCTTs and the Base Station. The MAC layer implements all the functions needed to allow theBase Station to optimise the repartition, among RCTTs, of the bi-dimensional data bursts (i.e. in time &frequency) onto the physical layer. The main issue here is to control efficiently the contention access to thereturn channel in order to allocate the available bandwidth (i.e. the carriers) to the largest number of contendingusers and to handle their various traffic types (depending on the supported application). DVB-RCT has defined aMAC layer with the objective to re-use as much as possible of the MAC specification designed for the cablereturn channel [4]. At a certain extent, DVB-RCT MAC layer can be considered as a superset of the Cable MAClayer. Accordingly, this could provide an easy way to use the Cable MAC to control the RCT Physical layer (RCT-PHY) or, better, a subset of it.

In this specific scenario (that is meant as an extension of the DVB-RCT specification), it would be possible tointroduce DVB-RCT boxes using the cable infrastructure, then to insure a smooth transition from cable toterrestrial. It will be also possible to enlarge the cable system capabilities, giving to it a wireless system to coverthe last mile to the users.

Such scenario is seen as very attractive to speed-up the rollout of DVB-RCT system and a dedicated annex tothe DVB-RCT specification is currently under finalisation by the DVB-RCT group.

DVB-RCT: DEPLOYMENT SCENARIOSThe DVB-RCT group performed a deep analysis of the possible deployment scenarios to cope with constraintsderived from the existing broadcasting allocation and frequency planning in various countries. Real life scenarioscan be derived from these basic examples reported hereafter.

" One omni directional DVB-RCT channel:The basic configuration is to use one single DVB-RCT upstream channel to cover the whole DVB-T downstreamcell, as illustrated in Figure 10.

Such scenario is suited for a limited number of users per cell, because all traffic has to be managed by onesingle upstream/downstream channel. This can be applied to the service introductory phase, a small urban cellor a large rural cell.

" Several omnidirectional DVB-RCT channels:To increase the collection capacity inside a single DVB-T downstream cell (more bit-rate to the users), severalDVB-RCT upstream channels can be implemented on the same area, using different DVB-RCT frequencies.This is illustrated in the Figure 11.

" SectorisationTo further increase the upstream capacity, it is possible to sectorise the upstream cell. It can be also consideredto protect against ‘jammers’ and to prevent interference in the coverage area.

C2

C2

C3

C1.1C1.2

C1.3

C1 jammer

Figure 12 – Sectoring DVB-RCT cells in one DVB-T

For instance, if an upstream channel is not available for use within the complete DVB-T cell coverage, it ispossible to configure the cell with several upstream channels each covering a sector of the cell. In the Figure 12,the frequency C1 is not available on the whole covering area, as polluted by jammers. In that sector, a C3frequency is used instead. Other sector could be created using a C2 frequency.

Finally, to increase bit-rate available in some sectors (i.e.: the ones with more population) several upstreamchannels, using additional frequencies, could coexist as it has been illustrated in Figure 11.

DVB-RCT has been designed with a lot of flexibility, to cover several deployment scenarios to reduce and copewith RF spectrum congestion. Several features, including RF engineering (i.e.: antennas sectorisation), largeand flexible collection of operational modes (i.e. cell radius, adaptive modulation, code robustness, etc.) havebeen incorporated to provide the best system configuration even in the (highly) congested UHF/VHF bands.

COSTS OF DVB-RCTThe overall costs of the DVB-RCT system is made up of the User Terminal Costs and the Base Station Costs.There is also, of course, the cost of the backhaul linking network from the Base Stations to Service Provider’smain hub – although this is normally already in place for any existing broadcasting network. The reason whyDVB-RCT is so cost effective is that a single low-cost receiving system (DVB-RCT Receiver and MAC Machine)at the Base Station can process up to 20,000 short interactions per second.

For instance, in the 2K mode, 59 interactions from different subscribers can be processed at a rate up to 600every second! According to complexity estimation performed by the members of the DVB-RCT group, DVB-RCT can be deployed for a small fraction of the cost of any competing system – for example PSTN, XDSL, GSMor UMTS. Such cost includes user terminal cost (for an integrated VLSI solution in mass production), networkcost per user (Base Station cost and network installation cost).

Thanks to the fact that the same terrestrial network is used for both broadcasting and return channel, the costhas been estimated in a few percents more than conventional pure broadcasting terrestrial Set-top Boxes.Furthermore, the ability of the system to cope with different scenarios allows for tuning the investment to the realramp of number of users (no big initial investment for a new network), thus minimizing investment risks.

DVB-RCT: TWO RESEARCH PROGRAMMESDVB-RCT largely benefits from the experience established by the European-funded research project iTTi(“interactive Terrestrial TV integration” – 1997-1999), where a first solution for terrestrial interactivity wasdeveloped and demonstrated. Both field experimentation and public demonstrations have been performed byiTTi consortium and have been already reported [8]. Two new projects have recently been set up to supportDVB-RCT implementation: WITNESS (“Wireless Interactive Terrestrial Network System and Service”) and IM4DTTV (“Integrated Modem fo(u)r Digital Terrestrial TV) which are briefly presented hereafter.

IST “WITNESS” PROJETWITNESS (2000-2002) [9] aims to upgrade, test, and validate equipment and planning algorithms to aid in thestandardisation of digital terrestrial TV return channel (DVB-RCT). WITNESS main mission is to validate theoperation of a Terrestrial Return Channel service by deploying up-graded interactive wireless terminals in thetwo test sites of Ireland (Dublin) and France (Rennes); then to develop spectrum planning and frequency usagerecommendations for Terrestrial TV services.

MEDEA+ “IM4DTTV” PROJETIM4DTTV project (2001-2003) [10], aims at validating DVB-RCT specification, developing a prototype VLSIsolution for the DVB-RCT terminal, as well as to support all necessary investigations and tests needed to assessthe DVB-RCT specification towards the ETSI standardisation process.

IM4DTTV will provide a 1G-silicon solution and a complete hardware validation platform covering all aspects ofthe DVB-RCT system, including both user terminal side (i.e.: RCTT) and broadcast side (i.e.: Base Station).Main objectives are the following:

" To add seamless interaction capabilities to the DVB Digital Terrestrial TV system (DVB-T), providing fastaccess from the TV set to Internet, Interactive TV applications, E-commerce and, concretely, to implementthe new DVB-RCT standard for the first time ever.

" To design and manufacture low-cost integrated interactive terminals for DVB-RCT technology by makinguse of dedicated silicon solution (both BB and RF parts) designed and developed within the project itself.

" To test and validate in realistic conditions the integrated prototype platform.

" IM4DTTV consortium comprises, among others, some world-wide leading silicon manufacturers (Philips,STMicroelectronics), Set-top Box manufacturers (Philips), head-end equipment manufacturers (Thomcast,ITIS). Two European broadcasters (France Telecom and RTE) will allow the consortium to test the systemin the real environment. Canal+ Technologies will support software development for DVB-RCT set-topboxes.

The DVB-RCT standard has been presented for the first time to the public during NAB2001 Conference &Exhibition in Las Vegas in April 2001. A White Paper [7] has been also distributed to the visitors of DVB boothgathering very high interest.

CONCLUSIONSThis paper has highlighted the work performed by DVB to provide terrestrial broadcasters with a flexible wirelessreturn channel for interactive DVB-T Services. More than a technical standard, DVB-RCT will become soon acommercial reality. Regulation policies remain to be addressed in regard to the use of the UHF/VHF bands forInteractive DTV service. The issue is already in the agenda of forthcoming meetings of ITU-T, EBU and othersimilar bodies and will be dealt with in the next months.

Even if works remains, from now, the DVB-RCT constitutes a real asset for a successful deployment of theinteractive terrestrial digital TV worldwide.

ACKNOWLEDGEMENTSAuthors would like to thank their colleagues and partners of the WITNESS and IM4DDTV European Projects, aswell as the members of the DVB-RCT Technical Working Group of the DVB Forum, who contributed to thediscussions, analysis and finalisation of the DVB-RCT standard presented here.

SUMMARY OF DVB-RCT PHYSICAL PARAMETERSDownstream Channel (DS) OFDM, ETS 300 744 (DVB-T) compliantReturn Interaction Channel Multiple Access OFDM (MA-OFDM)Forward Interaction Channel (US) Embedded in DS, compliant with ETS 300 744 (DVB-T)OFDM Carrier set 1024 (1K), 2048 (2K)OFDM Carrier spacing (CS) ~1KHz, ~2KHz, ~4KHzTransmission modes 6 modes (as combination of 3 CS and 2 Carrier set)Carrier shaping Nyquist, RectangularGuard Interval 1/4, 1/8, 1/16, 1/32 (for Rectangular shaping only)Transmission Frames TF1, TF2Data randomization PRBS with polynomial: 1+X14+x15

Modulation QPSK, 16QAM, 64QAMEncoding rates ½ , ¾Useful data payload per burst 18, 27, 36, 54, 81 bytes (1 burst = 144 modulated symbols)Channel codes Turbo or concatenated (Reed-Solomon + Convolutional)Interleaving Random Interleaver – PRBS with polynomial: 1+X3+x10

Burst Structures BS1, BS2, BS3Frequency hopping for BS1 only (optional)Medium Access Schemes MAS1, MAS2, MAS3 (as combinations of BS and TF)Net Bit rate /carrier (range) 0.6 Kbps – 15 Kbps (depending on the mode)Maximum nb carrier per user No limit (in the case of Rectangular Shaping)Service range Up to 65 km (cell radius)Channelisation 6, 7, 8 MHz channels are supported

PROTOCOLS incorporated in DVB-RCTMedium Access Control (MAC) Specifications mostly derived from EN 200 800Access options Fixed rate access, Contention access, Reservation accessSecurity Supported (derived from EN 200 800)

REFERENCES[1] DVB Project, ETSI: “Digital Broadcasting System for Television, Sound and Data Services; Framing Structure,

Channel Coding and Modulation for 11/12 GHz Satellite Services” - EN 300 421 DVB-S standard, 1993[2] DVB Project, ETSI: “Digital Broadcasting System for Television, Sound and Data Services; Framing Structure,

Channel Coding and Modulation for Digital Terrestrial Television” - ETS 300 744 DVB-T standard, 1996[3] DVB Project, ETSI: “Digital Broadcasting System for Television, Sound and Data Services; Framing Structure,

Channel Coding and Modulation for Cable Systems” - EN 300 429 DVB-C standard, 1993[4] DVB Project, “DVB Interaction Channel for Cable Distribution Systems” – EN 200 800 DVB-RCC standard, 2000[5] DVB Project, “DVB Interaction Channel for Satellite Distribution Systems” – ETS xxx xxx DVB-RCS standard, 2000[6] DVB Project, “Digital Video Broadcasting (DVB); Interaction channel for Digital Terrestrial Television (DVB-RCT)

incorporating Multiple Access OFDM”, DVB-TM document TM2361r3, March 2001 and ETSI, EN 301 958 DVB-RCTstandard, 2001

[7] DVB-RCT group, Scalise F., Branagan P, Sofer E, Faria G, MacAvock P, “White paper on DVB-RCT”, NAB 2001, April01, Las Vegas (USA)

[8] F.Scalise et. Alt. “A New Solution for Wireless Interactive TV Based on DVB-T Standard and SFDMA Technique”,Proceedings of SMPTE 1999 Conference, Sydney Australia, July 1999

[9] see at : www.wireless-interactive.org[10] see at: www.medea.org

Gerard FARIA – Harris Broadcast Communications Fabio SCALISE – STMicroelectronics([email protected]) ([email protected])