Hybrid Broadcast-Telecom Systems for Spectrum Efficient Mobile Broadband Internet Access Erik Stare, Stefan Lindgren Teracom AB Box 17666, SE-118 92 Stockholm
[email protected],
[email protected]
ABSTRACT
Mobile telecom
The introduction of hybrid systems, including broadcast as well as mobile telecom, for 4G is motivated. A current EU project, MCP, developing and specifying network and receiver architectures for hybrid networks is described. Reasons to use DVB-T for unicast services are explained. A way to increase spectrum efficiency by using broadcast functionality in connection with mobile Internet access is introduced. The idea of using large storage capacity for TV recording is introduced and consequences of time shifted viewing discussed. The possibility of further improving the spectrum efficiency of DVB-T by dynamic capacity allocation between TV and mobile Internet within a DVB-T signal is explained.
With mobile telecom we mean a two-way point-topoint communication without broadcast functionality. In contrast to the broadcast case, where the aim is to get sufficient power (or C/I) everywhere in the coverage area and as little as possible everywhere else, the aim in mobile telecom is to get sufficient power (or C/I) only to one receiver at a time. Just enough power to the mobile and as little as possible everywhere else is the basic principle, which is therefore in direct contradiction with the basic principle for broadcasting. For services, which only involve the two end-points of the bi-directional communication, like a mobile telephony conversation, this is obviously the right way to build a system. However, for services which are of interest for many users, such as e.g. video/audio content (conventional radio/TV, Internet radio/TV) and (a large part of the) www traffic, a pure mobile telecom system is clearly inefficient, since the data has to be repeated a large number of time – one time for each user request. This blocks the spectrum for other data. With broadcast in principle only one transmission is sufficient to reach all receivers.
INTERACTIVE SERVICES USING BROADCAST AND MOBILE TELECOM The ways in which interactive services can be achieved are fundamentally different in the broadcast world and in the mobile telecom world: Broadcast
4G – both broadcast and telecom With broadcast we mean one-way radio based pointto-multipoint transmissions where the information can be received by an unlimited number of terminals simultaneously. For interactive services, using broadcast, data carrousels is the natural way of transmitting data. The cycle time of the data carrousel is e.g. determined by requirements on data access time, amount of available memory in the receiver and the need to correct possible transmission errors. The carrousel principle, although in many ways attractive, is nevertheless a sub-optimum way of using the spectrum, since data is repeated a large number of times. Instead of repetitions the capacity could in principle have been used for other data.
Since a future 4G system has to support the widest possible range of services in the most cost/spectrum efficient way, and since neither broadcast nor mobile telecom are optimised for all types of services, the conclusion must be that 4G has to include both broadcast functionality and traditional point-to-point mobile telecom functionality. In addition, it is difficult to see that a single system could provide all the required services and also be optimum from a cost/spectrum point of view. Instead it is a general trend to look upon future 4G systems as being hybrid systems, based on many existing systems, but also being open for future extensions with new RF systems.
THE MCP PROJ ECT In several on-going EU funded R&D projects under the 5th framework programme hybrid systems are investigated. One of these projects is MCP (Multimedia Car Platform), which includes companies as e.g. NOKIA, Deutsche Telekom, France Telecom, BMW, VW and Teracom. MCP develops and specifies hybrid network and receiver architectures for mobile data services. The starting point is data services to cars, but the architectures are also applicable for general mobile data services. On the network side MCP builds on earlier hybrid system work by the ACTS project MEMO (DAB/GSM) and on the SABINA system (DVBT/GSM) developed by Teracom. In MCP a more generalised approach is taken and a network architecture for a hybrid system which combines any broadcast systems and any mobile telecom systems has been specified [1]. The only requirement on the respective radio access system is that it can tunnel IP packets. The MCP network architecture The MCP network architecture (see fig. 1) fully exploits the enormous spectral and trunking efficiency in the point-to-multipoint link layer of digital broadcast systems like DAB and DVB-T and at the same time maintaining the flexibility of the mobile communication network like GSM/GPRS and UMTS. The MCP architecture supports: •
• • •
IP multicast and unicast over broadcast networks (e.g. DVB-T, DAB) as well as over mobile telecom networks (e.g. GSM/GPRS, UMTS) Dynamic and automated traffic load sharing of multicast and unicast services within and between all attached networks Seamless handover within a radio broadcast network and between any two radio access networks Roaming between different operators
and is a complete platform for broadband mobile Internet access which enables a wide variety of multicast as well as unicast based services. The MCP network resembles MEMO and is an open IP-based architecture based on IPv4 and uses access system like GSM/GPRS and UMTS not only for interactivity but also as an underlying backbone network. The Mobile Node (MN), i.e. the terminal, accesses the services from the Correspondent Node (CN), (e.g.
an Internet host) via the MCP network. The MCP network is divided into two main sub networks: Unicast network The unicast network supports routing of individually addressed traffic giving the user the possibility to receive personal requested information over both the mobile communication network and over the broadcast network. In the latter case the broadcast network is used as the unicast downlink in an asymmetric telecom system. The Home Unicast Transport Server (HUTS) includes functionality for e.g. optimised TCP performance, and the Visited Unicast Transport Server (VUTS) includes functionality for e.g. Mobility Management. The Media Gateway (MGW) encapsulates the IP stream into the respective broadcast system. Multicast network The multicast network supports routing of multicast addressed traffic giving the user the possibility to receive multi-user information over the broadcast network. The MCP system supports CN initiated multicast (e.g. services providers with fixed bandwidth services) as well as MN initiated services (such as access to an Internet multicast session). In the former case the Multicast Access Server (MAS) routes the traffic to one or more MGWs. In the latter case the multicast traffic is actually tunnelled in the unicast network with the tunnel ending in the MGW. Note that in contrast to multicast over a telecom system, where each user occupies the spectrum, multicast via a broadcast system only occupies the spectrum once, irrespective of number of users. The MCP network supports • • • • • •
IP multicast and unicast over any broadcast networks (e.g. DVB-T, DAB) IP multicast and unicast over any mobile telecom networks (e.g. GSM/GPRS, UMTS) dynamic and automated traffic load sharing of multicast and unicast services within and between all attached networks seamless handover within a radio broadcast network and between any two radio access networks roaming between different operators a future migration from IPv4 to IPv6
and is based on •
an open IP-based architecture - any combination of broadcast and/or mo bile telecom systems can be used
• • • •
scalability - the network has a distributed architecture that allows for easy expansion transparency - the user will not notice any difference from the services that exist on the fixed network mobility - the network support mobility for all types of services unicast as well as multicast security - the unicast traffic is encrypted using IPsec to prevent eavesdropping
WHY UNICAST OVER DVB -T? The broadcast systems obviously have fundamental advantages when it comes to distribution of the same content to many receivers, but what about services where either the content is purely individual and/or the broadcast functionality cannot be exploited? Although DAB and DVB-T were developed for broadcast applications of course they could be used as a high bandwidth downlink in an asymmetric mobile telecom system. Especially DVB-T has in this case some very interesting features which makes it an obvious candidate. •
• • • • • •
• •
DVB-T provides (by far) the largest bandwidth to individual mobile receivers at high speeds than any other radio interface Measured results: Bit rate > 10 Mbit/s at speeds in the range 520-940 km/h (UHF band IV/V – 470-862 MHz, 2K mode) Implemented and tested. Works extremely well with TCP*/IP [1] for web surf or file downloads High TCP throughput (with MCP system) - 10 Mbit/s with 9.6 kbit/s GSM as return channel, measured with real HW Extremely robust - measured TCP throughput > 2 Mbit/s despite a Packet Loss Ratio of 15% on the underlying IP layer. Very low delay – fundamentally a few milliseconds Trunking gain: more users can share a cell when they share one large 10 Mbit/s resource than many small resources Easy and low cost wide area coverage with DVB-T, due to asymmetric capacity and existing broadcast network infrastructure – reduces investment costs and risks in the early phase when the receiver penetration is low and capacity requirement is limited Some rural areas with low population density may have very large cells even with large receiver penetration Capacity can be gradually increased with smaller cells when number of subscribers (and consequent revenue!) increase
•
The end result of this process could be a small cell network with DVB-T transmitters co-sited with UMTS è same cell size!
Finally it should be clearly stated that unicast over DVB-T should be seen as a useful complement to UMTS, but certainly not a replacement. On the contrary, a ‘unicast over DVB-T’ scenario would benefit from the exis tence of wide area coverage UMTS networks, since this would provide a high bandwidth return channel as well as being a fall-back for the downlink traffic. Also UMTS could benefit from the higher bandwidth and wider service availability possible with DVB-T, since it could make the whole “mobile Internet package” more attractive for the consumers. SPECTRUM EFFICIENCY Broadcast push to receiver local storage In combination with large local storage devices in the terminals (e.g. a hard disk) extremely spectrum efficient Internet access can be achieved by an MCP system. Popular web content/files can be pushed to all receivers simultaneously via the broadcast system. The receivers store the received files on the local memory, e.g. a hard disk, and the broadcast system updates the info when new file versions are available or requested. Already today the http protocol on our PCs only fetches the web files that are not already in the local cache memory. To download more individual information and information that somehow has been missed by the receiver, the mobile telecom system, or alternatively unicast over the broadcast system, is used. Optimum spectrum efficiency requires that a file be sent no more than once in a cell. With the above described approach it would theoretically actually be sufficient to transmit a specific file only once in the cell, assuming receivers are always listening and have sufficient local storage capacity, but any degree of listening and storage capacity increases the efficiency. Much more users can therefore exist within a given mobile cell plan. Users will be unaware of when the data is fetched locally and when it is downloaded and will therefore experience a very high “virtual bandwidth” when they use the system. Mobile receivers It should be remembered that e.g. 1 Gbyte of local storage can be filled in 15 minutes with 10 Mbit/s continuous traffic in the air. This means that even if the user for some reason starts with a completely empty memory the top 10 000 www pages could be accessible within 15 minutes.
Exponential growth of hard disk capacity Hard disk capacity is growing exponentially. For ICs Moore’s law predict a doubling of performance per cost unit every 18 months. For hard disks this pace has actually been significantly exceeded the last 10 years, and there is no sign that this exponential development will not continue for another 5-10 years, or more. We can therefore expect that in course of time there will be room for more and more content to be stored locally, with a corresponding increase in spectrum efficiency. A POSSIBLE SCENARIO FOR THE NEXT 10 YEARS Assuming a doubling every 18 months, a common 20 Gbyte hard disk of today will correspond to a 2.5 Terabyte (Tbyte) hard disk in 10 years! If 1 Tbyte of the 2.5 Tbyte capacity is allocated to store web content – received via broadcast push or individually downloaded, then the 10 million most popular (in my cell) www pages could be stored locally and continuously updated, assuming the size of a web page is about 100 kbyte. The remaining 1.5 Terabytes could be used to store a full day’s (24 hours) broadcast of 6 digital TV multiplexes, each carrying 24 Mbit/s. In this way most TV viewing could be completely time shifted. A user would always have all the content from the last 24 hours of TV (from all multiplexes) immediately accessible with full fast forward/rewind etc functionality and will therefore experience a kind of local video-on-demand (LVOD). This could revolutionise TV broadcasting industry because most of the TV viewing would not have to be real time viewing, although real time viewing will always exist to some extent (sports, live events etc). This development would more or less make all hours of the day “Prime time”, in the sense that the exact hour of transmission is not of that much interest, except for broadcasts of real-time content.
viewing while the programme continues to be recorded at a later point in the programme. Similarly it is possible to record a programme while viewing another previously recorded programme. This hard disk recording functionality will probably be a standard feature of future set-top-boxes and will probably accelerate the move to digital TV for the consumers. With the ever increasing capacity development the broadcast capacity will soon (much sooner than 10 years) be much better utilised than today, since all 24 hours will be of more equal value. In the future, the broadcast channels will therefore probably have a constant load and be filled 24h a day with TV content. The difference in programme choice, as perceived by the user, between terrestrial distribution of digital TV, compared to satellite and cable distribution, will in practice be reduced since the choice will be limited more by storage capacity than by actual number of multiplexes. DYNAMIC CAPACITY ALLOCATION BETWEEN TV AND MOBILE INTERNET Alternatively, the broadcast of TV content could be deliberately tailor-made to fit the variations of DVB-Tbased mobile Internet traffic during the day. The mobile Internet traffic is a function of user behaviour, and can be assumed known statistically. In this way a dynamic capacity allocation TV/mobile Internet could be achieved within DVB-T, so that 100% of the spectrum is utilised 100% of the time (see fig. 2). This approach could be implemented independently of the ‘broadcast push of www pages’ approach described above. Both approaches provide important spectrum savings and together they would mean that digital terrestrial TV and mobile Internet is provided by DVB-T in an extremely spectrum efficient way – we see the true convergence of broadcast and mobile telecom! CONCLUSIONS
The revolution will start now! It is important to note that this trend of time -shifted viewing will not appear only when we have 2.5 Tbyte hard disks, but is a process that is starting now, or within the next year, with emerging set-top-boxes having in-built 20-30 Gbyte hard disks for TV recording. Already this will constitute a minor revolution since hard disk recording provides very significant improvements for the user compared to good old linear VCR recording. With hard disk recording the user can have random access to any stored content via a suitable user interface, in a similar way that we access files on computers with e.g. Microsoft Explorer. The user can also pause viewing of a programme and later resume
Broadcasting and mobile telecom are both optimised for certain kinds of services but are clearly suboptimum for others. Only by combining broadcast and mobile telecom functionality can a future 4G system provide full cost/spectrum efficiency for widest possible range of services. In the EU funded MCP project an open hybrid system based on any combination of broadcast network(s) and mobile telecom network(s) is being developed and specified. With an MCP system an increase in spectrum efficiency can be achieved for content that is relevant for many users, such as a large part of the www traffic, by using the broadcast functionality together with large amounts of local storage. Thanks to the emerging existence of hard disks in set-top-boxes and to the
forecast continuous exponential increase in storage capacity, consumption of TV will be more and more time shifted in the future. This could open the door for dynamic capacity allocation between digital terrestrial TV and mobile Internet within a DVB -T signal. This would further increase the spectrum efficiency of DVB-T.
REFERENCES [1] MCP WP3, Overall Network Architecture, MCP D10, 2000-10-20
Communication network Point to Point
Broadcast network Point to Multipoint MN
CN
INTERNET VUTS
MGW
HUTS
INTRANET MAS
Fig. 1 – The MCP Overall Network Architecture
100%
DVB-T/TV
DVB-T/Mobile Internet
00.00
06.00
12.00
18.00
24.00
Fig. 2 – Possible dynamic capacity allocation during the day between digital terrestrial TV and mobile Internet. (Please note that the shape of the curve in fig.2 and the proportions between TV and mobile Internet is only an example in order to show in principle how TV and mobile Internet could share spectrum)
