has expcricnced great progress in recent years. Standards for broadcasting digital content to the end user are available and have been proven in large scale commercial deployments or, at least, extensive trial networks. This development can be observed recently also with regard to the standard for digital terrestrial television, DVB-T (Digital Video Broadcasting Tcrrestrial), which is already in operation in inany countries throughout the world. Currently, the system is rolled out in Germany, the Netherlands, France, and Italy; further countries have announced to start services in the near future. The technology decision on the digital terrestrial television system was in many countries based on very peculiar features of the DVB-T standard, among them the possibility to receive broadcast services also with portable and even mobile devices. Meanwhile the benefits of a powerful terrestrial broadcast system like DVB-T have attracted the intercst of the mobile communication industry.
In particular. the ability to reach mobile terminals via a wirclcss link, in conncction with wide geographical coverage and high transmission capacity, are features which make this technology attractive for a completely new environment: transmitting broadband services to mobile radio terminals and comparable pocket-sized portable devices. The international DVB (Digital Vidco Broadcasting) Project has responded to the industry demands by spccifying a new transmission standard: DVB-II (Digital Video Broadcasting – Transmission Sjstern ,/hr llandheld Trrminols) [I]. DVB-H is the latest development within the This work was supported by Panasonic liurupean Laboratories GmbH. Langcn. Germany. M. Komkld is B researcher GI lnstitut her Nachrichtentechnik (Instilute for Communicnlions Tcchnulogy) at Bmunschwcig .Technical University. Gemany (e-mail: [email protected]).
THE process of digitizing the traditional broadcast systems set of DVB transmission standards. Commercial requirements of this system were determined in the first half of 2002 ; technology selection and the specification of the technical elements of DVB-H started in autumn 2002 and wcrc finalizcd in Februaly 2004. It is expected that the DVB-H standard will be published by ETSl (European Tclecommunications Standards Institute) as a European Norm before the end of 2004. The DVB-H technology is a spin-off of the DVB-T standard. It is to a large cxtcnt compatible to DVB-T but takes into account the specific properties of the addressed terminals – small, lightweight, portable, battery-powered devices. The terminal equipment is offered a powerful downstrcam channel in addition to the access to a mobile telecommunications network which will be included in most of the terminals anyway.
Thcrcfore, DVB-H leads to a link between the classical broadcast systems and the world of cellular radio networks. The broadband, high capacity downstream channel provided by DVB-H will feature a total data rate of several Mbitis and may be used for audio and video streaming applications and inany other kinds of services. The system thereby introduces new ways of distributing services to handheld terminals, offering greatly extended possibilities to contcnt providers and network opcrators. The paper provides an extensive introductory survey of the DVB-H technology from the perspective of a contributor directly involved in the development work of the DVB Project. Based on the commercial requirements which arc introduced in section II.the essential technical elcmcnts of the standard are described and explained in detail in section 111.
The main focus is on the power saving technique ensuring reasonable opcrating times for battery powered handheld terminals and on the features of the enhanced error protection scheme of DVB-H. An outline of the system standardization is given in section IV. The paper also includes an analysis of performance characteristics. In the framework of system validation the DVB-H standard was implemented in software for simulations. Detailed investigations are described in section V in connection with a new DVB-H network mode. 11. S Y S I E & l REQUIREMENTS
The commercial rcquirements of the system wcrc determined by the DVB Project in 2002 . DVB-H shall offer broadcast services for powable and mobile usage, including audio and video streaming in acceptable quality. The data rates feasible in practice have to be sufficient for this purpose. For the DVB-H system a useful data rate of up to I O Mbitk per channel is envisaged. Transmission channels are planned in the regular UHF broadcasting band (or alternatively VHF Band 111). The typical user environment of a DVB-H terminal is very much comparable to the mobile radio environment. Thercforc the possibility of a similar geographic coveragc is targeted. The term handheld terminal includes multimedia mobile phones with color displays as well as personal digital assistant (PDA) and pocket PC types. All these kinds of deviccs have in common small dimensions, light weight, and ballcry operation. These properties are a precondition for mobile usage but also implicate several severe restrictions.
The devices lack an in and have to be external power with a limited power budget, L~~ power consumption is necessary to obtain reasonable usage and standby to Mobility is an expected requirement, meaning that services shall be possible not only at almost all indoor and outdoor locations but also while moving in a vehicle at high speed. the handover bemeen adjacent DVB.H radio shall happen imperceptibly when moving along larger distances. However, fast varying channels are very error. prone, ~h~ situation is by thc fact that built-in handheld have limited dimensions and Cannot be pointed at the transmitter if the terminal is in motion. A multiantcnna diversity approach is mostly impossible because of space limitations.
Moreover, interference from adjacent TV channels and GSM mobile radio signals or from electrical devices near by can be very disturbing. As a result, accessing a downstream of several Mbit/s with this kind of dcvices is a vcry demanding task. Finally, the new system is demanded to be similar to thc existing DVB-T systcm for digital terrestrial television. Compatibility with the DVB-T network structure shall exist to a large extent in order to be ablc to re-use the same transmitter cquipmcnt. The given requircment is maintaining compatibility at the physical layer interface which is called the DVB transport steam interface. 111. SYSTEMOVERVIEW network mode, the ‘4Kmode’, assuring more flexibility in designing single frequency networks for mobile reception, and also an cnhanced signaling channel for improving access to the services.
The Physical radio transmission is performed by means of DVB-T standard OFDM modulation [31. There is only one obligatory new feature on the physical layer which makes the DVB-H distinguishable from a DVB-T signal; namely an cxtended parameter signaling for thc DVB-H elementary Streams ill the multiplex. There are few furthcr optional new elements which are described beneath in paragraph D: Physicul /ayer e.rtensions. The signaling is rcalized downwards compatible to the DVB-T system. the DVB-H data is fully compatible with other DVB transport streams. Thcsc properties guarantee that the DVB-H data Stream call be broadcast via dedicated DVB-H transmitter networks as well as via DVB-T networks. For this reason essential new elements like time slicing and the cnhanced forward error correction are deliberatcly put onto the protocol layer above.
This section describes the elements which have been integrated into thc system in order to fulfill the requirements from the previous section. DVB-H, as a transmission standard, specifies physical data transmission as well as mandatoly elements of the lowest protocol layers. DVB-H comprises a bundle of technology elements. The standard uses a power saving algorithm based on timemultiplexed transmission of different services. The technique, called time slicing, allows for selective access to the dcsircd data and results in a large battery power saving effect. Additionally, timc slicing allows soft handovcr with only one receiver unit. The poor signal reception conditions are thwarted with an enhanced error protcction scheme on the link layer. This scheme is called MPE-FEC (Mdri-Protocol Encapsulation Forward Error Correction). MPE-FEC employs powerful channcl coding and time interleaving. Furthermore, the DVB-H standard introduces an additional
B. j-jnle .F[;c;np A particular difficulty of the terminals is the limited battery capacity and the neccssity to handle this capacity cconomically. In a way, bcing compatible with DVB-T is opposcd to this requirement because demodulating and decoding a broadband, high data rate stream like the DVB-T stream involves inevitably a higher power dissipation in the tuner and thc demodulator part. An investigation at thc beginning of the devclopmcnt of DVB-H showed that the total power consumption o f a DVB-T front cnd was quite above I Watt at the time of the examination and was expected not to decrease below 600 mW until 2006; meanwhile a somewhat more optimistic value seems realistic but the envisaged target of 100 mW as a maximum threshold for the cntirc front cnd is still inaccessible for a DVB-T receiver . A considerable drawback for the battery-operated terminals is the fact that with DVB-T the whole data stream has to be decoded before one of the services of thc multiplex can be accessed.
The power saving potential used in DVB-H is derived from the fact that essentially only those parts of the stream have to be received which carry data of the service currently selected. However, the data stream needs to be reorganized in a suitablc way for that purpose. With DVB-H, service multiplexing is performed in a pure time division multiplex. The data of one particular service arc therefore not transmitted continuously but in compact periodical bursts with interruptions in between. Multiplexing of scveral services leads again to a continuous, uninterrupted transmitted stream nfconstant data rate (Fig. I). This kind of signal can be received time-selectively by the terminals synchronizing to the bursts of the wanted service and switching off during the intermediate timc when othcr serviccs arc transmitted. The off-time between bursts givcs the power saving. This technique is called time dicfng.
Bursts entering the receiver have to be buffered and read out with a constant data rate, the service data rate, in case of a streaming service. The amount of data containcd in one burst is sufficient for bridging the front end off-time. For reasons of simplicity and flexibility the position of the bursts is only signaled in terms of the relative time difference between two consecutive bursts of thc same service. Practically, a burst duration is in the range of scveral hundred milliseconds whereas the off-time may amount to scveral seconds. A lead time for power-on. resynchronization etc. has to be taken into account before each received burst; this time is assumed to be less than 250 ms. Depending on the dutyitum-off ratio the resulting power saving may be more than 90 %.
Time slicing requires a sufficiently high number of inultiplcxed services and a certain minimum burst data rate to guarantee cffectivc power saving. Basically, the power consumption of the front end correlates with the servicc data ratc of the service currcntly selected. It is even possible to merge the time sliced DVB-H multiplex with timc-continuous DVB-T services in one single transport stream (Fig. 2). This way both DVB-T and DVB-H can be transmitted within the same nctwork with the capacity of one channel flexibly divided between both. Time slicing offers anothcr benefit for the terminal architccture. The long turn-off times may be used to search for channels with the same service in neighboring radio cells. This way a timcly channel handover can be performed imperceptible for the user at the bordcr between two cells. Both monitoring of adjacent cells and receiving service data can be realized with thc Same frontend [SI.
Fig. 2. DVB-H rodec and transmitter block diagram.
IP data are embedded into the transport stream by means of the Midti P w r c o l Encups~ilatioii (MPE), adaptation protocol an dcfined in the DVB Data Broadcast Spccification [X].. On the level of the MPE an additional stage of forward error correction (FEC) is added. This technique, called MPE-FEC, is the second main innovation o f DVB-H besides the timc slicing. MPE-FEC complements the physical layer FEC of the underlying DVB-T standard. It is intended tn enhance the reliable reception especially with handheld devices which make the reception of high data rate streams in a mobile environment difficult. The MPE-FEC scheme is located on the link layer at the level of the 1P input streams before they arc cncapsulated by means of the MPE. The MPE-FEC, MPE, and time slicing techniques are neighboring in the transmitter block diagram and are directly aligned to each other.
All three elements together form thc DVB-N codec which contains the essential DVB-H functionality (Fig. 2). The IP input streams coming from different sources as single elementary streams arc multiplexed according to the timc slicing mcthod. The MPEFEC error protection is calculated and added separately for each elementary stream. Afterwards encapsulation of IP packets and embedding into the transport stream follow. All relevant data processing is carried out before the transport stream interface in order to guarantee compatibility to a DVB-T transmission nctwork. In more detail, the new MPE-FEC scheme consists of a Reed-Solomon-(RS-)Code in conjunction with an extensive Application data table
C. All’ditiuntrl,/~mcm-d error correction on MPE l e l d In contrast to othcr DVB transmissioii systems which are based on the DVB transport stream  adoptcd from the MPEG2 standard. the DVB-H system is IP (Internet fi-otuco/)-based. thercforc the outer DVB-H interface is the IP interface. This determination allows simple combination with othcr networks as currcntly in the process of being defined in the IP Datucu.~tsystem ,. Nevertheless, the MPEG2 transport stream is still used as a physicat carrier method. The block interleaving. The MPE-FEC coder provides a specific frame structure, the FECframe, for the incoming data of the DVB-H codec (Fig. 3). The FEC frame has a maximum number of 1024 rows and a constant number of 255 columns; cvcry frame cell eorrcsponds to one byte. the whole frame size is approx. 2 Mbits maximum. The frame is separated into two parts, the application data table on the left (191 columns) and the RS data table on thc right (64 columns).
The application data table is fillcd with incoming IP packets of the service to be protected. Aftcr applying the RS(255,191) codc row-byrow to the application data, the RS data table contains the parity bytcs of the RS code. After the coding the 1P packets arc read out of thc application data table and transmitted encapsulated in IP sections, according to the MPE method. Thcse application data are followed by the parity data which are read out of the RS data table column-by-column and encapsulated in separate FEC sections. Thc FEC frame structure also contains a ‘virtual’ block interleaving effect in addition to the coding. Writing to and reading from thc FEC frame is performed in column direction whereas coding is applied in row direction.
Furthermorc, MPE-FEC is closely aiigncd to time slicing. Both techniques are applied on elementary stream Icvel, and the size of one time slicing burst exactly corresponds to the content of one FEC frame, enabling re-use of memoly in the recciver chips. Separating IP data and parity data within one burst involves that the use of MPE-FEC decoding is optional in the receiver since the application data can be utilizcd while ignoring the parity information as well. Some signaling of MPE-FEC parameters is nccessary in the MPE section headers: Thc position of each transmitted datagram (IP packet or RS data table column, respectively) within thc FEC frame is noted in an address field; thc border bctween IP data and parity data and the end of a burst are each signaled by diffcrent flags. Also the timing parameter of the time slicing, i.e. the time diffcrcnce until the ncxt burst of the same servicc, is comprised in the section headers.
D. Pl?y.sical layer e.rten.sions Thc signaling of parameters of the DVB-H elcmentary streams in the multiplcx is an extension of the Transmission Paranjeter Signaling (TPS) channel known from the DVB-T standard. This is a rescrvcd information channel which provides tuning parameters to the recciver. The new contents of the TPS channel provide the information if time sliced DVB-H elementary streams arc availablc in the multiplex and if MPE-FEC protcction is uscd at least in one of the elcmentary streams. Also thc ncw physical transmission modes being described in this paragraph are signaled in the TPS channcl. Finally, broadcasting of thc cell identifier is mandatory, which simplifies discovcring neighbored network cclls where thc same servicc is available. Thc ncxt new clcment is an additional OFDM transmission mode. DVB-T already provides a 2K and an 8 K mode for differcnt network topologies. DVB-H also disposcs of an intermediate 4K mode with a 4096-point Fast Fourier
Transform (FFT) in the OFDM modulation. Table I shows relevant parameters of thc three differcnt OFDM transmission modes. The 4K mode represents a compromise solution betwccn the two othcr modes. It allows for a doubled transmitter distance in single frequency nctworks (SFNs) compared to the 2K modc, and it is less susceptible to Doppler frequencies in case of mobilc rcception compared to the 8K mode. The 4K mode shall offer more network planning flexibility. Since DVB-T does not include this mode, it is an option only in dedicated DVB-H networks. The 4K mode made a new 4K symbol interleavcr necessary which is specified for 4096 OFDM carricrs. Furthermorc, in connection with the three network modes a new symbol interleaving scheme is defined (Fig. 4).
As standard compliant tcrminals include the X modc and the 8K symbol interleaver, K it suggests itself to cxploit the relatively big memory of the 8K symbol interleaver in all thrce network modes. This symbol interlcaver is able to process the amount of one complete XK OFDM symbol or alternatively two 4K OFDM symbols or four 2K OFDM symbols. The new scheme allows this usc of the memory and results in an increased interlcaving depth in the 2K and 4K modes, which can be cxpccted to improve performance. Using the full memory interlcaver solution is denoted as in-depth iriterleuving whereas the symbol interlcavers belonging to the individual modes arc dcnoted as native interleavrrs. A further option is the specification of DVB-H for a channcl bandwidth of 5 MHz. The DVB-T standard provides the thrce diffcrcnt VHFiUHF bandwidths used worldwidc (6 MHz, 7 MHz, 8 MHz) which arc therefore also supported in DVB-H. The 5 MHz bandwidth solution enables using this transmission standard outside of classical broadcast bands as well.
Chain is able to accept arbitraly standard conformant DVB-H transport streams, to simulate the physical Iaycr and the radio channel transmission and to perform the whole terminal-sided decoding up to the IP output interface. In view of the maximum acceptable length of this article only one set of simulation results can be reported. This is the analysis of the performancc o f the 4K network mode and of the ‘in-depth’ symbol interleaving. The simulations werc performed assuming two different characteristic user environments, i.e. signal interference by an impulse noisc source (in practice a radiating electrical device or a car ignition) and mobile rcception in a multipath cnvironment. In the first case impulse noise according to the model defined in [I21 was added to the signal, in the second case a COST207 radio channel model with 6-tap ‘typical urban’ (TU6) profilc and classical Dopplcr spectrum was used. The parameters used for the simulation were: 16-QAM modulation, convolutional +2K