DTT NETWORK STRUCTURES AND TECHNICAL INNOVATIONS

Transcription

1 DTT NETWORK STRUCTURES AND TECHNICAL INNOVATIONS A. Morello, Co-authors: G. Alberico, P. Forni, V. Mignone, S. Ripamonti, B. Sacco, V. Sardella, M. Visintin RAI Centro Ricerche e Innovazione Tecnologica - Turin - ITALY ABSTRACT The RAI Research Centre (RAI-CRIT), in co-operation with RaiWay, is analysing the architecture of the primary distribution and broadcasting network for DTT (Digital Terrestrial Television) introduction in Italy. The article describes the overall digital platform for encoding, multiplexing and emission of two independent DTT bouquets. The primary distribution network structure is analysed, suitable to differentiate the programme content over the 20 Italian Regions. The network will be capable of evolving progressively from a low-cost solution, based on DTH satellites feeding the terrestrial transmitters, to a final configuration, using optical fibres and radio links as main backbone and satellite links as a backup. A new MUX-adapter concept is introduced, allowing to drop some TV programmes from the DTH satellite bouquet, while significantly reducing re-multiplexing cost. Examples of broadcasting network coverage estimation are presented, according to the studies developed by the Authority on Communications. An enhanced in-band gap-filler scheme is proposed, using adaptive echo cancellation to maintain stability and high performance at increased ERP levels. INTRODUCTION In Italy, debates are ongoing at Government level and within the main broadcasting companies for the definition of the DTT introduction policy. A preliminary outline of the current trends may be summarised as follows: The current TV scenario is dominated by free-to-air broadcasts in the VHF/UHF bands, while CATV has very low penetration and satellite digital TV (with a penetration of the order of 3-4 Million receivers) is mainly driven by Pay-TV services. DTT is considered as the natural evolution of free-to-air TV services by the main public and commercial broadcasters in Italy Law 66/2001 identifies an early date (2006) for analogue TV switch-off, although many actors in the Government and in the broadcasting companies believe that a longer transition period is required. The DTT Regulation, issued by the Authority on Communications (0) defines the main rules for the introduction of DTT, and assigns a full multiplex to RAI for Public Services The Government is aware of the social potential of enhanced DTT receivers, enabling the delivery of interactive T-government services (i.e., access to public administration services using the TV-set), thanks to the universal penetration and to the easy manmachine interface

2 The current Italian market, which amounts to about 3 millions new TV sets per year, enlarged by economic incentives may be the basis for the receiver renewing process towards the digital solution RAI, together with the main commercial broadcasters, is planning to simulcast current programmes, enriched by new thematic TV services, in order to make DTT market more attractive The introduction of Enhanced / Interactive-TV services, and in particular of interactive advertising, is considered an important opportunity to extend business perspectives of free-to-air TV. RAI, the public broadcaster, is directly involved in the activity on digital terrestrial television since 1998, when an experimental plan based on several DTT pilot trials was presented to the Ministry of Communications, M. Cominetti et al (0). The first pilot trials started in Turin in February 1998 by RAI CRIT, in tight co-operation with RaiWay, with the primary goal of assessing the technical performance of DVB-T in the field and identifying the best service configuration to meet a large audience interest, A. Bertella et al (0). The DTT activity of RAI-CRIT in is focused on three main subjects: study of the DTT distribution network infrastructure, at the light of the service requirements study of the broadcasting network coverage evolution in the forthcoming years definition of multimedia services (based on the MHP specification) which will be implemented in the experimental phase, and implementation of the required technological platform for content production study of the receiver MHP technology at the light of the RAI service requirements, towards the specification of the receivers for the Italian market (IDTVs and STBs). This contribution reports the main results achieved on the first two aforementioned subjects. THE CURRENT NETWORK FOR CONTRIBUTION AND PRIMARY DISTRIBUTION Radio Links RAI, through the controlled Company, RaiWay, owns its radio link network to connect the Production and Transmitting/Broadcasting RAI sites spread all over the country (Figure 1). The RaiWay link network consists of about 120 stations along five main trunks, providing distribution and contribution for TV and Radio Programmes, as well as services related to transmission of voice, data and technical management information. For distribution purposes, the network reaches all the main Transmission Sites, while the contribution network connects the four national Production Centres in Rome, Milan, Naples and Turin and the 17 Regional sites; about 60 insertion points are made available to connect Output Broadcasting (OB) units to the contribution network. Figure 1 Typical structure of the Italian radio-link network for TV contribution and distribution

3 The radio link network has a strategic importance for RAI. Consequently, a project called Waynet for the digitisation of the microwave radio links was started in 1999, with the target to convert the analogue/fm network into a digital SDH (Synchronous Digital Hierarchy) network within 2002, D Onofrio et al (0), Stroppiana et al (0). Waynet provides a capacity of n*155 Mbit/s (n*stm-1 with n varying from 1 to 3) on the main bi-directional trunks. Each STM-1 stream can include up to three VC-3 containers accessible via PDH 45 Mbit/s interfaces. Network Adaptation from the Transport Stream format (video encoder output) into (unframed) PDH format includes error protection by Reed- Solomon RS(188,204) and interleaving with depth 12, following a proprietary solution directly derived from the DVB approach for 34 Bit-rate Time Satellite MUX 24 National VBR Block DTT MUX Satellite Block to be eliminated Regional VBR Block Time Figure 2 MUX configuration of satellite DTH and DTT broadcasting systems (ATM adaptation as specified by ETSI (0) is not implemented). The 45 Mbit/s network can transport 1, 2 or 3 television channels at 38, 19 or 12 Mbit/s, respectively, multiplexed in a single MPEG Transport Stream. For contribution purposes, typically 2 television channels using the MPEG 422 profile at 19 Mbit/s each are included in a 45 Mbit/s TS, while, for distribution purposes, 3 television channels coded in MPEG MP@ML at 12 Mbit/s each are arranged. Satellite Direct-to-Home (DTH) Network RAI broadcasts two DVB bouquets over the Hot-Bird2 Eutelsat Satellite, including the simulcast copy of the analogue terrestrial RAI1, RAI2 and RAI3 programmes and some freeto-air thematic channels. These broadcasts are intended for Direct-to-Home reception over Europe, and for back-up of the terrestrial radio-link primary distribution network. EVOLUTION OF THE DTT DISTRIBUTION NETWORK The DTT network implementation will be progressive, to dilute the financial effort over the introductory period, while guaranteeing the performance optimisation of the final configuration. Main service requirements are: Two DTT MUXes, deployed over overlapping time scales National coverage, but content differentiation at Regional level (for 20 Regions) Five TV channels allocated per MUX Centralised generation of the SI tables and EPG A typical DTT Multiplex configuration can be as follows (average bit-rates): 23 Mbit/s per 5 TV programmes (Statistical MUX of VBR, Variable Bit Rate, programmes) Mbit/s for SI Mbit/s for conventional teletext, to be released in the long terms via MHP services Mbps for MHP applications

4 In case or Regional SFN broadcasting network configuration the bit-rate should be reduced to about 20 Mbit/s and the number of programmes to 4, in order to allocate the suitable OFDM guard interval. Production Centre Production Centre Contribution Network SATELLITE UP-LINK Rome National MUX GENERATION National Headend DTH REGIONAL MUX GENERATION (statistical remux) NATIONAL DISTRIBUTION NETWORK Satellite DTT TRANSMITTER BROADCASTING NETWORK REGIONAL DISTRIBUTION NETWORK - Radio-links Figure 3 Introductory phase: distribution network configuration Repeaters Gap-fillers Each DTT MUX will be composed of two constant bit-rate blocks, the national and the regional ones. Each block includes a number of statistically multiplexed programmes (VBR), according to future editorial needs. In case of DTT MUX distribution via DTH satellite (offering a bit-rate capacity of 34 Mbit/s, which is larger than the DTT bouquet), Figure 2 shows an example of bit-rate configuration along the distribution path: at the Regional Headend premises, the excess programmes by satellite are dropped, while the regional block is added. The use of constant bit-rate blocks (i.e., groups of VBR programmes) simplifies multiplex adaptations in the Regional Headends, since it avoids changes of bit-rate on the individual VBR programmes (statistical re-multiplexing process). This approach has been selected as the optimum long-term solution (operational phase), while in the short-term the current satellite bouquet will not be modified. Introductory-phase configuration For the introductory phase (from 2002 till the beginning of the DTT operational service), the following service objectives have been identified for the definition of the network architecture: "Quasi"-Regional content differentiation (i.e. coverage areas may include neighbouring Regions) Minimum cost network: - Relaxed reliability and service continuity targets (no redundancy for equipment nor network backup) - For distribution of the National block of each MUX, re-use of the already available satellite DTH signals Maximum population coverage (target 70-80%) and maximum MUX capacity: - by covering the most populated areas - by implementing an MFN broadcasting network configuration (24 Mbit/s per MUX) on the most populated areas ("leopard skin spot" coverage). Easy extension towards the operational configuration. Figure 3 shows the distribution network configuration: the National part of each MUX is generated in Rome, and is distributed via the current DTH satellite service. The Regional Headends re-multiplexes the signals for insertion of regional programming. Currently, the satellite bouquet statistically multiplexes DTT and non-dtt programmes (the scheme of Figure 2 is not followed), and the average programme bit-rates are too high for DTT. Therefore, in a first phase, complex statistical re-multiplexers will be adopted in the Regional

5 Headends, capable of changing the MUX configurations and the programme bit-rates. The transportation of the digital signals from the Regional Headend to the terrestrial transmitters is performed via SDH radio-links (the alternative of DVB-T modulation at the Headend premises, and OFDM transportation via analogue FM radio-links, may be considered in some cases, for economical reasons). The configuration shown in Figure 3 offers some key advantages with respect to a solution characterised by DTT re-multiplexing at each transmitter site: possibility to fulfil the requirements of the DTT Regulations, Authority on Communications (0), requesting a separation between the three subjects: Content Provider, Service (MUX) Operator, Network Operator reduction of the high complexity nodes (re-muxing functions) in the network. This offers significant cost savings in view of the operational phase, when the number of transmitters will significantly exceed the number of regions possibility to evolve towards a regional SFN configuration for the operational phase (if independent re-multiplexing is performed at each transmitters, the DVB-T requirements for SFN feeding may not be fulfilled). Operational-phase configuration For this phase, the following objectives have been identified: Achievement of a Regional coverage of the broadcast network (Regional SFN configuration, if required by the national digital plan) Maximum security, reliability and service continuity through: - use of a main terrestrial distribution digital network based on hybrid technologies (optical fibres and radio links); - use of a satellite network as (National-only) backup network; - equipment redundancy. Coverage extension to reach more than 90% of population (if required by the RAI / State Service Contract, and upon definition of suitable public financing mechanisms). Figure 4 shows the operational network structure, resulting as an evolution of the introductory phase network. The terrestrial FO / PR network offers high service availability. The network segmentation reduces the impact of hypothetical terrorist attacks, and the satellite alternative path further reduces these risks. The satellite-based back-up network for the national bouquet offers a good trade-off between cost and benefits, using the already operational DTH satellite service (as an alternative, a specific satellite link may be leased for DTT distribution). Instead, the implementation of Regional satellite back-up networks has been discarded for economical reasons. For the main DTT transmitters, the backup network should be terrestrial, in order to make available the Regional programmes also during the main network interruptions. This also preserves the possibility to achieve full serviceavailability in Regional SFNs. Conversely, in the cases in which the satellite national backup signals will be used to reach the final DTT transmitters, only the national programmes will be available during interruptions of the main network (a further negative drawback is the service coverage reduction in Regional SFN configurations, because of the network self-interference during interruption of the main distribution network).

6 To reduce the equipment cost, a simplified MUX adapter (as described in Section: New Developments) has been developed by RAI-CRIT. This avoids the installation of complex remultiplexers at each transmitter site. Production Centre Production Centre Contribution Network SATELLITE UP-LINK Rome National MUX GENERATION NATIONAL DISTRIBUTION (SDH) NETWORK FO/ Radio-links DTH REGIONAL MUX GENERATION Regional Headend Optional ad-hoc Transponder National Back-up Network DTT TRANSMITTER Repeaters Gap-fillers BROADCASTING NETWORK REGIONAL DISTRIBUTION NETWORK - Radio-links Figure 4 Operational phase: distribution network configuration THE BROADCASTING NETWORK The evaluation of the potential coverage of a DTT broadcasting network in Italy started within the DTT National Committee of the Authority on Telecommunications, and the main results of the study are reported in the White Book (0). Several scenarios have been considered for the delivery of each multiplex (64 QAM rate 2/3; Tg=1/4; 8K; 4-5 TV programmes and additional data & multimedia applications): 1-SFN network (1 frequency, nation-wide SFN; constant content at National level), 3-SFN networks (3 frequencies, Regional SFN coverage; content differentiation at Regional level); 4-MFN networks (4 frequencies, MFN; content differentiation at local level). The following results have been obtained: 1-SFN: About 87% population coverage, by using 306 (VHF) 391 (UHF) transmitters; 3-SFN: About 97% population coverage, by using 430 (VHF) 460 (UHF) transmitters; 4-MFN: About 93% population coverage, by using 360 (VHF) 400 (UHF) transmitters. The White Book indicates a potential capacity for a Fully-Digital Plan (55 channels in VHF and UFH bands) of multiplexes per service area (4-MFN or 3-SFN, respectively), with content differentiation at Regional/Local level. This work has set the basis for the development of the Italian Digital Plan, which will be developed by the Authority starting from Considering the reduction of the available spectrum imposed by the International Regulations to avoid interference to neighbouring Countries (the revision of the ST 61 Plan is planned for ), and the needs for additional frequency resources to solve local coverage problems, it is likely that the real Digital Plan will make available a reduced number of multiplexes compared to the theoretical figures of the White Book. To optimise investments in the DTT broadcasting network (acquisition of channels, infrastructures and transmitter hardware), the most attractive areas in terms of audience and economical potential have been identified by RAI. The RAI plan for the introduction of DTT aims at a target of about 60%-80% of population coverage within the next four to six years, requiring the installation of about 40 to 60 DTT transmitters per MUX. Figure 5 shows the

7 current coverage of the Turin test-bed, managed by the RAI Research Centre. The investments to achieve 60%-80% population coverage may be financed directly by RAI, while universal coverage of the Italian territory/population comes out to be economically non attractive, since the number of required transmitters becomes huge (currently about 1500 transmitters are in operation for each RAI analogue TV network). Therefore suitable financing mechanisms need to be defined in the regulatory framework to reach universal coverage for public services NEW DEVELOPMENTS The MUX-adapter: a new low-cost remultiplexing concept As described above, distribution of national DTT signals by DTH satellites is a cost effective solution A/V Data Mux SI(DTT) Additional Satellite services DTT Bouquet MUX processor A/V Data 24Mb/s SI(SAT) DTT end-users Mux Figure 5: Coverage of the Turin DTT test bed (e.g. backup system for the operational phase), although the use of re-multiplexers may increase costs and network complexity. To avoid the needs for statistical re-multiplexers, the satellite multiplex may be re-arranged according to Figure 2, to include fixed bit-rate blocks. A conventional technological platform at each DVB-T transmitting site includes: DVB-S satellite receiver re-multiplexer at 24 Mbps dropping the unused satellite programmes; COFDM modulator. An innovative RAI-CRIT solution allows a further chain simplification, replacing the remultiplexers with a simple and cheap equipment (MUX-adapter) that is able to extract a virtual DTT bouquet embedded in the complete DTH bouquet. This approach also allows centralised SI generation for the terrestrial services, to simplify network management. The overall chain architecture including the MUX-adapter is shown in Figure 6: the DTT constant bit-rate block, including DTT Service Information (SI) tables, is generated by a conventional statistical multiplexer, at the target DTT bit-rate (e.g. 24Mb/s); a novel MUX processor hides the original DTT PSI/SI tables in ghost PIDs (reserved for private data) and tags all the DTT packets (for example by setting the transport priority in the TS headers). The satellite conventional remultiplexer adds the extra services (and new PSI/SI tables) to generate the complete satellite bouquet, which is fully DVB-S compliant and therefore available for DTH users. In real-world implementations, this scheme can be further 34Mb/s DVB-S Mod DTH satellite 24Mb/s (useful) DVB-T Mod DTH end-users 34Mb/s DVB-S RX 34Mb/s MUX Adapter DTT Transmission Site Figure 6 Overall network architecture using the RAI lowcost multiplex adapter.

8 simplified, by using a single multiplexer and injecting the DTT PSI/SI tables directly as private data, and setting the priority bit of the relevant DTT packets. At the DTT transmission site, the received 34Mbit/s stream is processed by the MUX Adapter, that retrieves the original PSI/SI DTT tables (changing PIDs) and replaces the additive DTH services (identified by transport priority = 0) with null packets. This operation leaves unchanged the gross bit-rate (34Mb/s) while reducing the useful one, thus reducing the Mux adapter complexity (avoiding time stamp adaptation and clock re-generation). A conventional DTT modulator for MFNs, including an input transport stream rate adapter to decouple network from broadcasting bit-rates, removes the null packets overhead and provides time-stamp adaptation. A laboratory demonstrator of the MUX adapter has been implemented by very simple PLD technology, and successfully operated. A new in-band gap filler with active echo cancellation In DTT network planning, the use of in-band gap fillers may be envisaged to cover shaded areas (e.g. in hilly and mountainous regions). They receive the signal from the main transmitter and amplify it for re-transmission on the same radio-frequency channel. The advantage of this solution is its lower cost and the possibility to re-use the same frequency of the main transmitter. On the other side, disadvantages come from the output power limitations, due to stability problems caused by the non-perfect isolation of the transmit/receive antennas and the reflections from the surrounding environment. To achieve a good input/output isolation (in the range of 90 db), normally the transmit / receive antennas are placed far apart on the antenna mast(s). Simulations and laboratory tests indicate a required protection ratio of the DVB-T signal of 12 db in presence of a feed-back selfinterference, therefore the maximum gap filler amplification should be of about 78 db. Assuming that the received field strength at the margin of the service area of a DVB-T network is of about 57 dbµv/m, the maximum output power of an in-band gap-filler is of only 36 mw (assumptions: channel 66, receive antenna gain 14 db, transmitting antenna gain 10 db). According to ITU-R Rec propagation curves, this allows to extend the service into a spotted area of about 20 km. In order to reduce the stability and power limitations of current in-band gap-fillers, a new scheme has been studied and simulated, allowing to adaptively cancel the echoes produced by feedback interference and multi-path propagation (see annex 1 for a technical description). Simulations indicate that this canceller can maintain the loop stability also with positive loop gains, under the condition that, at the start-up, the amplification begins from 0 and is slowly increased till the maximum level. As an example, for the same configuration described above, the possibility to set the loop gain (protection ratio) to 0 db allows to increase the gap-filler output power to about 600 mw, extending the coverage area to about 30 km. Alternatively, lower antenna decoupling figures can be accepted. In the future, the proposed canceller will be implemented in hardware and tested in real situations. CONCLUSIONS The development of a new DTT network infrastructure represents an extraordinary technological and economic challenge for broadcasters / network operators. The identification of network structures and technological solutions suitable for a progressive deployment of the DTT coverage is a key factor of success. This allows gradual investments, while guaranteeing the network functionality evolution from the introductory to the operational phases.

9 REFERENCES 1. Autorità per le Garanzie nelle Comunicazioni. Libro bianco sulla televisione digitale terrestre, Novembre Regolamento sulla televisione digitale, M. Cominetti, A. Morello, R. Serafini, Current plans for DTT implementation in a densely utilised frequency spectrum. Proceedings of the Montreux TV Symposium A. Bertella, M. Cominetti, S. Ripamonti, M. Visintin, The RAI DTT pilot trials in Turin. Proceedings of IBC 2000 Conference, Amsterdam 4. D Onofrio M., Cianfa M. and De Carolis A., Il progetto RAINET nella rete dei collegamenti televisivi della RAI. Elettronica e Telecomunicazioni, No 1, Aprile M. Stroppiana, M. Cianfa, M. Visca, Assessment of a digital SDH network for digital TV contribution and distribution. Proceedings of IBC 2000 Conference, Amsterdam. 6. ETS : DVB Interfaces to Synchronous Digital Hierarchy (SDH) networks. 7. H. Hamazumi et al., A loop Interference Canceller for the Relay Stations. NHK Laboratories Note, November, 2000, No ANNEX 1 DESCRIPTION OF THE CANCELLER The canceller scheme (Figure 7) is based on a previous work of NHK Laboratories (H. Hamazumi et al (0)). The signal received by the in band gap-filler is corrupted by the interference coming from the transmit antenna, and by the multipath propagation from the main transmitter. The canceller, through conventional channel estimation methods of DVB-T receivers, evaluates the channel impulse response in P (Figure 7) and uses it for adaptation of a FIR filter, which generates the cancellation signal. If F(f) is the channel transfer function evaluated in P, the FIR filter transfer function, which ideally cancels the loop interference is C n (f) = C n-1 (f)+1-1/f n (f). The cancellation algorithm can promptly adapt to the channel variations. In order to avoid lock-in problems in presence of multipath propagation affecting the received signal, the proposed canceller performs a initialisation procedure, with the transmitter off, setting the FIR filter τd x(t) xrit(t) + Noise + - τd+τ FIR Filter Tap Update P Estimation chain βe jθ α y(t) Figure 7 In band DTT gap-filler structure coefficients to equalise the multipath channel distortions. In this case the FIR filter transfer function is set to C n (f) = C n-1 (f)+ F n (f)-1. The difference in the adaptation formula is due to the different scheme of the interference to be modelled by the FIR filter: feedback for the loop interference, feedforward for the multipath one. When this lock-in phase is completed, the output power is gradually increased using the feedback adaptation formula. To be able to cope with long multipath echo delays, the FIR filter should be very long (and thus very complex). In order to limit this complexity, a simplified structure, based on a cascade of short FIR filters separated by variable delays, has been proposed and tested by simulations (e.g. the combination of five 9-taps filters could be used instead of a single 200- taps filter).