Repair Mechanisms for Broadcast Transmissions in Hybrid Cellular & DVB-H Systems David G´omez-Barquero1
Aurelian Bria
Mobile Communications Group Polytechnic University of Valencia (UPV), Spain Email:
[email protected]
Radio Communication Systems Lab Royal Institute of Technology (KTH), Sweden Email:
[email protected]
Abstract— This paper proposes a framework for investigating potential infrastructure cost savings in hybrid cellular and DVBH systems by performing efficient error repair of broadcast transmissions. The main idea is to reuse as much as possible the existing network infrastructure to provide mobile TV services. The cellular system is not only employed to collect information, but also to send repair data to the users whose conditions are temporarily affected by noise, interference and fading. To enable an easy and efficient implementation of the repair mechanisms we adopt the use of Application Layer - Forward Error Correction (AL-FEC) with digital fountain coding.
I. I NTRODUCTION One of the challenges that wireless industry faces today is to provide affordable mass multimedia services to mobile phones. Although 3G cellular networks have already started to deliver multimedia services, as video clips from sport events or TV programs, their offer is still limited due to the scarce radio resources and the inefficiency of current point-to-point (p-t-p) cellular architectures to provide multicast and broadcast services. As an alternative to the 3G cellular networks, the technology DVB-H (Digital Video Broadcast - Handheld) is considered as a key element in future wireless networks, due to its capabilities to provide mass mobile multimedia services. DVB-H is an extension of the European terrestrial digital TV standard DVB-T (Digital Video Broadcast - Terrestrial), to reach handheld terminals devices [1]. DVB-H reuses the same physical layer as DVB-T, and adds new features at the link layer, being able to share the same network infrastructure (e.g., transmitters, multiplexes, etc.). Results from commercial DVB-H pilots have showed the willingness of users to consume mobile TV services (both streaming and podcast), and first commercial DVB-H networks are currently under deployment in Italy, Finland and USA. DVB-H enables the convergence of services from broadcast and cellular domains, allowing mobile broadcast interactive services. This is usually referred as IP Datacast (IPDC) over DVB-H. An IPDC system comprises of a unidirectional DVB-H broadcast path combined with a bidirectional cellular interactivity path. The benefits of these hybrid cellular and DVB-H networks are evident, since they can take advantage 1 Supported by a Ph.D. scholarship from the Generalitat de Valencia (Spain). The work presented in this paper was performed while the author was a guest researcher at KTH.
of the intrinsic characteristics of DVB-H to broadcast IP multimedia content to mobile phones at high data rates over large areas, whereas the cellular system can provide bidirectional communication capabilities and a sophisticated billing system that, traditionally, broadcasting networks lack. One of the major concerns about DVB-H is the network deployment cost. As terminals suffer from much more severe propagation conditions than in DVB-T (especially for indoor and vehicular reception), DVB-H networks require considerably more infrastructure investment than DVB-T networks (e.g., more transmission power or number of sites) [2], [3]. This penalty is particulary evident when very high area coverage (e.g., over 90% of locations) is targeted. Most probably, existing broadcasting towers will be used initially to provide the primary coverage, and additional transmitters/repeaters will be deployed in critical areas, where indoor or vehicular reception are required, eventually forming dense Single Frequency Networks (SFN) [4]. Our approach is to look at the cellular system not only as a provider of a feedback/interactive channel, but also as a mean to recover lost information. Typically in a DVB-H session some users will experience significantly worse reception conditions than the majority of users, and could be served more efficiently through the cellular network. The cellular system could also be used for sending repair information to the mobile terminals whose DVB-H reception is temporarily affected by noise, interference, slow fading (shadowing) or fast (Rayleigh) fading. Especially evolved 3G cellular networks present a very good potential to provide efficient error repair for broadcast transmissions due to the recent enhancements of the cellular 3G standard, that allow high speed p-t-p connections with HSDPA (High-Speed Downlink Packet Access), and pointto-multipoint (p-t-m) transmissions with MBMS (Multimedia Broadcast Multicast Services) [5]. This paper proposes a framework for investigating potential infrastructure cost savings in hybrid cellular and DVB-H networks, based on a more general approach towards affordable wireless systems which involves the usage of smart multimode terminals and reuse of the existing infrastructure for broadcasting and cellular systems. We propose to take advantage of the users’ mobility and the bursty character of DVBH transmissions in order to hide the coverage discontinuities from the user perception, either by sending more parity infor-
mation in DVB-H, or by serving the problematic users through the cellular network. We describe how efficient error repair mechanisms for broadcasting transmissions can be provided in hybrid cellular and DVB-H networks. To enable an easy and efficient implementation of the repair mechanisms we adopt the use of Application Layer - Forward Error Correction (ALFEC) with digital fountain coding. Both streaming and file download services are considered. We also provide examples of the potential DVB-H infrastructure savings, in terms of transmission power and number of sites. The rest of the paper is structured as follows: Section II highlights the differences between streaming and file download DVB-H services. Section III briefly describes the FEC schemes in DVB-H at the link and application layer. Section IV describes the repair mechanisms alternatives. Several numerical evaluations are provided in Section V. Conclusions are summarized in Section VI. II. DVB-H S ERVICES DVB-H employs a discontinuous transmission technique based on time-slicing, where data is periodically sent in bursts at very high bit rates. Terminals experience a constant service data rate equal to the burst size divided by time between bursts. Terminals synchronize to the bursts of the desired service and switch their receivers (front-end) off when bursts of other services are being transmitted. This allows for a significant reduction in the average power consumption of the terminals and enables seamless handovers. Two basic types of DVB-H services have been identified: streaming and file download. Additionally, they can be classified according to their tolerance to errors and delay constraints. For streaming services terminals play the information received in the last burst in such a way that users do not notice a discontinuous transmission. If one burst is lost, the service is interrupted until the next burst is received. The time difference between bursts, also known as off-time, depends on the amount of source data transmitted in a burst and the streaming service data rate. Streaming services allow some error tolerance (for example, according to [6], a percentage of lost bursts of 5% corresponds to a “good/fair” recovery of DVB-H streaming services). We can distinguish between real-time services (e.g., live transmissions) and non-real time, where some delay is accepted (e.g., telenovela episode). File download services require an error-free transmission of the file (i.e., even a single bit error corrupts the whole file and makes it useless for the receiver). A file download service simply consists of the successful reception of a number bursts. Requirements on time delay are rather relaxed, and the offtime can be chosen arbitrarily. III. L INK AND A PPLICATION L AYER FEC IN DVB-H The physical layer of the underlying DVB-T standard does not provide any time interleaving, as it was designed for fixed terminals. Therefore, in DVB-H, due to the very challenging reception conditions, it is very likely that most of the terminals will temporarily experience packet losses at the physical layer.
Typically, this results in an even higher loss rate on the application layer. Error correction in DVB-H can only be achieved by means of Forward Error Correction (FEC) mechanisms. They protect loss events on an underlying level without a need for feedback, and rely on the transmission of additional parity data that allows reconstructing the original data when transmission errors occur. To increase the robustness of the DVB-H transmission while keeping compatibility with DVB-T, two additional FEC mechanisms have been specified on the link and application layers. A. Link Layer - FEC The DVB-H standard specifies and optional FEC scheme at the link layer called MPE-FEC (Multi Protocol Encapsulation FEC). MPE-FEC was mainly introduced to improve the robustness of the system for mobile users. Field measurements have shown that MPE-FEC increases the robustness of reception for mobile terminals, such that the service availability becomes quasi-independent of the speed [6]. The MPE-FEC scheme consists of a Reed-Solomon (RS) code in conjunction with a virtual block interleaver. MPEFEC works at the burst level, and each burst contains source IP data and RS parity data. The maximum burst size is 2 Mb, of which 1.5 Mb are IP data and 0.5 Mb parity data. The resulting coding rate depends on the proportion of parity data transmitted . To allow different coding rates, the DVB-H standard allows to reduce the amount of IP data and parity data transmitted in a burst. The coding rates possible are: 1/2, 2/3, 3/4 (mother code), 5/6 and 7/8. Bursts are transmitted in the form of sections, containing either one IP packet or RS parity data (maximum size 1 kbyte). A time interleaving effect is achieved by the fact that receivers do not need to receive all sections correctly. Basically, MPE-FEC can cope with a section error rate equal to 1 minus the coding rate. B. Application Layer - FEC FEC mechanisms operating on the application layer can recover packet losses of all underlying layers and protocols, providing end-to-end error recovery (e.g., they can even recover IP packets lost in the core network or the Internet). Digital fountain codes are a special class of FEC codes that operate at the application layer. They were originally designed to allow asynchronous download over broadcast channels very efficiently [7]. As fountain codes, they can generate an unlimited number of encoded data on the fly (i.e., they are rateless). An ideal fountain code has the property that the source file can be reliable reconstructed after receiving an amount of encoded data equal to the file size. It makes no difference which specific encoding packets are received, any encoding packet correctly received can be used to recover the source data. The advantages of digital fountain codes for data delivery in wireless broadcast systems are evident, as they outperform other FEC solutions in terms of reliability, spectrum efficiency and flexibility [8]. Raptor codes are a computationally efficient
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Example of file download using MPE-FEC and AL-FEC.
implementation of fountain codes, that can be implemented in software without the need of dedicated hardware. Moreover, Raptor codes achieve close to ideal performance [9]. In the standardization process of AL-FEC for DVB-H, significant gains were found compared to MPE-FEC for large files download [10]. However, no differences were found for small files and for streaming services. As a result, Raptor AL-FEC was only standardized for file download services. It should be pointed out that when AL-FEC is used, MPE-FEC is disabled. To illustrate the difference between MPE-FEC and AL-FEC, the delivery of a file that requires 3 bursts with MPE-FEC is considered, see Fig. 2. Note that with MPE-FEC each burst contains both source and parity data, whereas with AL-FEC the original file is transmitted first, followed by parity data (i.e., the Raptor code adopted in DVB-H is a systematic code). With MPE-FEC files are broadcasted in a carousel (i.e., the file is transmitted repeatedly), whereas with AL-FEC more parity data is transmitted. In the case of using MPE-FEC, each of the unique bursts must be successfully decoded to recover the file. If, for example, a receiver fails to recover one burst, it must wait until that burst is retransmitted. Bursts containing data already received are then discarded. Note also that if one burst is completely received (i.e., all source and parity data), it cannot be used to correct errors in other bursts. The key difference when using AL-FEC is that all data correctly received is useful. IV. R EPAIR M ECHANISM A LTERNATIVES We identify three different error repair alternatives that are possible in an hybrid cellular and DVB-H network. They involve different types of transmissions of the repair data: • p-t-m in the whole service area with DVB-H. • p-t-p with the cellular network (e.g., HSDPA). • p-t-m in a cell with the cellular network (e.g., MBMS). The use of AL-FEC is key, since the repair session consists on transmitting additional parity data that can be used by all users. As a consequence, the amount of repair data needed in a p-t-m repair session is minimized. In addition, the repair data transmitted through cellular could be generated by a different server. Note also that terminals only need to specify
how many additional encoding packets should be sent in the repair session. If AL-FEC is not used, terminals would have to notify the server the specific packets they need. Therefore for a high number of independent requests, the repair session would consist on the retransmission of the whole file. A. Streaming Services One possibility to perform error repair for streaming services in DVB-H would be to transmit pre-scheduled additional bursts with parity information several seconds after the original bursts. In this way, mobile terminals which are not able to successfully decode an original burst have an extra chance to recover the missing information by synchronizing to the corresponding additional burst. User velocity, fading correlation distance and time between the original and additional bursts determine the statistical correlation between reception conditions of both bursts. The lower the correlation between reception conditions, the higher the probability of successful mitigation of coverage discontinuities. The coverage improvement will thus increase as a function of the users’ velocity and the time between bursts. The performance of such solution is investigated in [11]. The cellular network can be used as well as a repair mechanism of bursts for streaming services. Users not able to decode a burst could receive repair data through the cellular network until the next burst is received. The amount of repair data that can be transmitted, and hence the number of users that can recover the information in the repair session, will depend on the time difference between bursts (off-time). For example, service data rates of 128 kb/s, 256 kb/s and 384 kb/s determine off-time values of approximately 12 s, 6 s and 4 s. One alternative is to let the terminals notify the media server about the amount of repair information needed for each burst. The main problem here is when users miss most of the burst content, and thus could only be served in time if high speed p-t-p connections are possible (e.g., users close to the cellular base station with HSDPA). Another alternative would be to continuously push repair data (e.g., with MBMS) for each DVB-H burst in the already known problematic areas, so that users needing only a small amount of repair data can directly listen to the cellular transmission. Once a burst is lost but recovered during the following offtime, the stream visualization will be shifted from real-time with a time equal to the off-time. However, the stream will start from the point where it was interrupted. This will happen the first time a burst is lost, and it may differ in time from one user to another. If the recovery of a burst cannot be done in the offtime, the burst will be discarded unless a higher visualization time shift is allowed. This approach could be interesting for streaming services which are not real-time constrained (e.g., telenovela episode). B. File Download Services If only the DVB-H transmission is assumed, the media server has to generate a fixed AL-FEC overhead, that is sufficient for nearly all users in the radio environment. How
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much overhead is transmitted (which determines the number of bursts per file) depends on the anticipated network conditions, service characteristics and target users. On the other hand, if MPE-FEC is used, the only parameters to play with are the MPE-FEC coding rate, and the number of transmissions of the file (similar to a carrousel). Note that the Modulation and Coding Rate (MCR) at the physical layer is usually a fixed system parameter. As mentioned before, one advantage of AL-FEC is that users not able to recover the file need only to specify how many encoding packets they have missed. The cellular network can be used to collect this information to decide the optimum repair mechanism: p-t-p or p-t-m through cellular, or more bursts in DVB-H. Intuitively, only sending more repair data through DVB-H will not help users which are stationary in bad coverage areas. The system must decide whether these users should be ignored or served completely through the cellular system.
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V. P ERFORMANCE E VALUATION We consider vehicular (in-car) users in our numerical evaluation. To account for a practical implementation of an ALFEC code, a 1% reception overhead has been assumed as in the standardization work of AL-FEC in DVB-H [10]. A. Streaming Services In our evaluation for streaming services we investigate the reception of a single burst in a cellular cell as a function of the average Carrier-to-Noise Ratio (CNR) of the DVB-H signal (defined for outdoor terminals), see Fig. 2. Shadowing (characterized by a lognormal distribution with a standard deviation value of 5.5 dB and a correlation distance of 70 m) and fast fading are also considered. We look at how many users receive the burst correctly, and the amount of repair data needed for users not able to decode the burst. The coding rate considered in the burst is 3/4. For example, from Fig. 2 we can see that for an average CNR of 25 dB around 73% of the terminals do not need any repair data (i.e., this value corresponds to the DVB-H area coverage for vehicular users in the cell), and 85% of them miss up to 120 kbytes (about 1 Mb). In other words, if the cellular system is capable to send up to 1 Mb of repair data to any terminal, an extra 12% of users would decode the burst, but they will visualize the content with a delay equal to the time needed to receive the repair data. Note that this is equivalent to improving the link budget in 2 dB. In our investigation we observed that higher improvements are achieved for low values of the standard deviation of the shadowing, σ. The reason is that the probability of partially receiving the burst is higher. Larger values of σ lead to a situation where most users either receive the whole burst or nothing at all. Examples of such scenarios are dense SFN or line-of-sight environments.
B. File Download Services We investigate the delivery of a 30 Mb file in a hexagonal service area of 25 km radius. We consider two DVB-H deployment scenarios. First scenario consists of a DVB-H network with only one transmitter mounted on a 250 m height TV tower situated in the middle of the service area. The second scenario assumes a dense SFN deployed on existing cellular sites. The antenna height is 35 m and the cell radius is 2 km (this yields 157 cellular sites in the service area). In our simulations, users are initially uniformly distributed over the service area and move according to the mobility model proposed in [12] (they can also be interpreted as different trajectories a user might have). We look at the number of users able to decode the transmitted file (acquisition probability), as a function of the EIRP (Effective Isotropic Radiated Power) from the TV tower and the number of sites of the cellular SFN. The operating frequency considered is 700 MHz. DVBH terminals are characterized by an omni-directional antenna with -7 dBi gain and a noise figure of 6 dB. Shadowing and fading are considered as for streaming services. A vehicle entry loss of 7 dB has been assumed. The ITU-R P.1546 path loss model has been used for the TV tower with a height loss correction factor of 18 dB, and the sub-urban Okumura-Hata model for the cellular sites. An EIRP of 30 dBW at the cellular sites has been assumed. The mobility model parameters employed are: direction change probabilities p0o = 0.695, p90o = 0.2, p−90o = 0.1, p180o = 0.005, standard deviation of direction distributions σϕ = π/32, average and √ variance of the length of major roads d = 250 m, σd2 = d · (2/π), mean velocities of city and major roads v = 15 km/h, vmr = 40 km/h, velocity deviation σv = 15 km/h, and percentage of cars on major roads pmr = 0.7. Fig. 3 shows the acquisition probability of the 30 Mb file as a function of the EIRP from the TV tower. We can clearly see
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Fig. 3. Acquisition probability vs. EIRP from TV tower. File size is 30 Mbits. Transmission mode FFT 4K, GI 1/4, 16QAM 1/2.
Fig. 4. Acquisition probability vs. Number of sites of the SFN. File size is 30 Mbits. Transmission mode FFT 4K, GI 1/4, 16QAM 1/2.
the significant improvement obtained with AL-FEC compared to MPE-FEC. Note how the performance of MPE-FEC 3/4 is close to sending no parity at all. This is due to the increase of the number of bursts that the file is divided (recall that with MPE-FEC each burst contains parity data). As all bursts must be correctly received to decode the file, the probability of missing at least one will be quite high if the area coverage is not that high. A significant improvement is already noted when AL-FEC is used and one additional parity burst is transmitted (16 bursts). The difference in EIRP between MPE-FEC 3/4 and AL-FEC 3/4 (20 bursts) to achieve a 95% acquisition probability for 20 bursts transmitted is 6.5 dB. Note also that AL-FEC with 20 bursts performs better than MPE-FEC with one retransmission (40 bursts). Fig. 4 shows the acquisition probability of the 30 Mb file as a function of the number of sites in the SFN. In this case, around 50% of the sites can be saved to achieve a 95% acquisition probability when using AL-FEC 3/4 (20 bursts) compared to MPE-FEC 3/4. In our investigation we also considered smaller file sizes, and the outcome was lower gains of AL-FEC compared to MPE-FEC. Fig. 3 and Fig. 4 give also an idea of the infrastructure savings in a hybrid system compared to the DVB-H only deployment case. For example, the curves for 16 bursts are valid for a hybrid system where the DVB-H network transmits the source file and the cellular network is able to deliver 1 burst (2 Mb) with parity data. This represents only a 6% of the file, and looks feasible with 3G systems). In this case, we can save around 3 dB EIRP and 25% of the sites.
investigations are needed to evaluate the infrastructure savings in a complete deployment scenario. From a file download perspective, savings in transmitted power and number of sites are possible even with low amount of data (e.g., 6%) delivered though the cellular system. The results provided in this paper should encourage DVBH service providers to start their services in cooperation with cellular operators.
VI. C ONCLUSIONS Our initial evaluations show that it is possible to reduce the DVB-H infrastructure investment if a cellular system, as 3G, is employed for error repair. From a streaming service perspective, the benefits are larger if higher visualization time shifts are accepted. However more
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