Application Layer FEC for Improved Mobile Reception of DVB-H Streaming Services 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 evaluates the capability of Application Layer - Forward Error Correction (AL-FEC) with Raptor coding for improving the reception of DVB-H streaming services for mobile terminals. In particular, we exploit the benefits coming from the spatial diversity introduced by terminals’ mobility. We consider the introduction of an additional burst with parity information transmitted several seconds after the original burst. In this way, mobile terminals which are not able to successfully decode the original burst will have an extra chance to recover the missing information by synchronizing to the additional burst. Simulation results show how it is possible to trade system capacity and streaming delay for increased user satisfaction. A comparison with alternative techniques as using MPE-FEC (Multi Protocol Encapsulation - FEC), reducing the Modulation and Coding Rate (MCR) order at the physical layer, and performing burst retransmissions is also included.
I. I NTRODUCTION One of the challenges that wireless industry faces today is to provide affordable multimedia services to the mobile users. As an alternative to evolved 3G cellular networks, the technology DVB-H (Digital Video Broadcast - Handhelds) is seen as a key element in beyond 3G networks, due to its capabilities to provide mass market multimedia services with a low degree of user interaction. DVB-H is an extension of the European standard for terrestrial digital TV, DVBT (Digital Video Broadcast - Terrestrial), to reach handheld terminals devices, and it allows broadcasting IP multimedia content to mobile phones at high data rates (in the order of several Mb/s) [1]. Results from commercial DVB-H pilots have showed the willingness of users to consume mobile TV (streaming) services, and first DVB-H networks are currently under deployment in Italy, Finland and USA. Although DVB-H systems are, to a large extent, similar to DVB-T, the more severe propagation conditions (especially for indoor and vehicular reception) determine a considerably larger infrastructure requirement in terms of number of sites and transmission power [2]. This penalty is particulary evident when very high area coverage levels (over 90% of locations) are targeted [3]. Results from the first commercial DVB-H pilots in Helsinki (Finland) revealed that one of the most popular times for 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.
mobile TV is while traveling by public transportation [4]. Vehicular users are one of the most critical cases, since they experience Doppler and fast (Rayleigh) fading impairments and additional vehicle penetration loss. Moreover, vehicular users cannot manually optimize the position of their receivers [5]. However, the bursty transmission pattern of DVB-H together with terminal mobility and delay tolerance of the streaming service may offer the opportunity to hide the coverage discontinuities in the service area. How to exploit this advantage is the main topic of this paper. As DVB-H only provides a unidirectional downlink channel, possible solutions to provide higher coverage, as perceived by the mobile users, at the expense of decreased service capacity are: (1) to use a lower Modulation and Coding Rate (MCR) order of the underlying DVB-T standard (i.e., modulation and Forward Error Correction (FEC) at the physical layer), and (2) use additional FEC at the link or application layers. Dynamically changing the MCR is not included in the features of current system implementations, and the MCR is a fixed system parameter. The DVB-H standard specifies an optional coding scheme at the link layer called MPE-FEC (Multi Protocol Encapsulation - FEC). 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 receiving speed [6]. On the other hand, Application Layer - FEC (AL-FEC), using Raptor coding, has been recently standardized for file download services in DVB-H [7]. The advantages of AL-FEC for file delivery in wireless broadcast systems are evident, as they outperform other FEC solutions in terms of reliability, spectrum efficiency and flexibility [8]. In the case of large file downloads that span over several bursts, significative gains were reported when compared to MPE-FEC [9]. However, no differences were found in the case of streaming services. As a result, AL-FEC with Raptor coding was only standardized for file download services. This paper considers streaming services, but proposes the use of AL-FEC together with a slight modification in the transmission pattern: the introduction of an additional burst with parity information transmitted several seconds after the original burst. If an user does not receive an original burst correctly, he will have an extra chance to recover the missing information by synchronizing to the additional burst.
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Application Data Table IP Data 191 columns I
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padding
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Fig. 1.
MPE-FEC sections
MPE-FEC frame.
We look at trading system capacity and streaming delay for improved user satisfaction (i.e., perceived coverage while on the move). Power consumption issues are also investigated. Results have been compared with alternative techniques as using MPE-FEC, decreasing the MCR order, and performing burst retransmissions with MPE-FEC. The rest of the paper is structured as follows: Section II briefly describes the main technical features of DVB-H: timeslicing, MPE-FEC and AL-FEC. The proposed transmission technique is described in Section III. The performances measures, system model, and simulation results are described in Section IV. Section V highlights the implications of proposed technique. Finally, Section VI consists on the main conclusions. II. DVB-H T ECHNICAL F EATURES A. Time-Slicing DVB-H employs a discontinuous transmission technique based on time-slicing, where data is periodically sent in bursts at very high bit rates. This allows for a significant reduction in the average power consumption of the terminals, and enables seamless handovers with a single terminal antenna [1]. In DVB-H, terminals synchronize to the bursts of the desired service and switch their receivers (front-end) off when bursts of other services are being transmitted, saving battery. Each burst indicates the time difference to the next burst, off-time. 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. B. MPE-FEC MPE is an adaptation protocol to encapsulate multiple IP streams into the MPEG-2 DVB-T transport stream. MPEFEC was mainly introduced to improve the robustness of the system for mobile users, since as DVB-T does not provide any time-interleaving (it was designed for fixed terminals), its performance is considerably degraded for mobile users. The MPE-FEC scheme consists of a Reed-Solomon (RS) code in conjunction with a virtual block interleaver [1]. Fig. 1 shows the MPE-FEC frame, that represents the information contained in a burst (maximum size is 2 Mb).
The MPE-FEC frame is structured in two parts: the Application Data Table (ADT) with IP data, and the RS Data Table (RSDT) with RS parity data. The RSDT is computed after filling the ADT column-wise, by applying a RS(255,191) code on each row. The MPE-FEC frame is transmitted in the form of sections, containing either one IP packet or a RSDT column. The resulting coding rate depends on the proportion of transmitted columns in each table. In this way, the mother code is 3/4 (when all 191 ADT and 64 RSDT columns are used). To allow different coding rates the DVB-H standard allows padding to make the code stronger (by reducing the number of transmitted columns in the ADT), and puncturing to make the code weaker (by reducing the number of transmitted columns in the RSDT) [1]. The coding rates possible are: 1/2, 2/3, 3/4, 5/6 and 7/8. A time interleaving effect is achieved by the fact that the receiver does not need to receive all sections correctly. For example, assuming exactly one IP packet per column, the receiver only needs to receive correctly a number of sections equal to the number of ADT columns, out of all transmitted (e.g., 191 out of 255 for the mother code 3/4). C. Application Layer - FEC The Raptor code adopted by DVB-H for file download services is a systematic code (i.e., the output data contains the original source IP data and the desired amount of additional parity data) [7]. Raptor codes are a computationally efficient implementation of fountain codes developed by Digital Fountain Inc. that operate at the application layer [10]. As fountain codes, they can generate an unlimited number of encoded data on the fly (i.e., they are rateless), and receivers can recover the file after receiving an amount of encoded data only slightly larger than the original file size. The performance of Raptor codes is very close to an ideal digital fountain code, and the parity overhead required is very small. Moreover, as Raptor algorithms are mathematically efficient, encoders and decoders are typically implemented in software. It should be pointed out that MPE-FEC is disabled when Raptor codes are used. III. P ROPOSED DVB-H T RANSMISSION S TRATEGY This paper proposes to introduce an additional burst containing parity data transmitted several seconds after the original burst, as shown in Fig. 2. A more robust transmission is achieved at the expense of reducing the system capacity and introducing some delay. In this case each original burst indicates not only the starting time of the next original burst with new information, but also when the burst with additional parity data is being transmitted. The effective AL-FEC coding rate over both bursts depends on the size of the additional parity burst. It should be pointed out that original bursts should contain some parity data, otherwise any lost section would determine the loss of the whole burst (similar to DVB-T case). Note also that even more robust transmissions could be achieved by introducing a second additional burst with parity information.
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Fig. 2. Example of the transmission strategy where the additional burst is transmitted 1.5 cycle times (burst duration + off-time) after the original burst.
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Basically, users can decode the content in three different ways. Some users will receive the original burst correctly, and they will neglect the additional one. Some other users will receive only part of the original burst, and will need a part or all of the additional burst. Finally, the users that completely miss the original burst, can be in a better location when the additional burst is transmitted, and receive it completely correct. This last case implies that the additional burst contains enough data to recover the information transmitted in a burst. The proposed technique trades both system capacity and delay by improved mobile user satisfaction, taking advantage of the spatial diversity introduced by users’ mobility, and hiding the coverage discontinuities from them. Obviously, this is not applicable to stationary users, but on the other hand they can optimize their position and do not suffer from Doppler impairments. 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. As an example, Fig. 3 shows the percentage of correctly received bursts, as a function of the average Carrier-to-Noise Ratio (CNR). Shadow fading is modeled with a standard deviation of 5.5 dB and a correlation distance of 70 m. Users move at 18 km/h, and the frequency is 700 MHz, resulting in 10 Hz Doppler frequency. We compare different cases using AL-FEC (i.e., different sizes of the additional burst and times between the original and additional bursts ∆t), with the cases using MPE-FEC with and without burst retransmission. The proposed technique does not imply any modification of the present implementation, since Raptor codes are already standardized for file download services, and all terminals should support them. The only drawback is the introduction of a delay, equal to the time between the original and the additional bursts, the first time a terminal synchronizes to the additional burst. This is motivated by the fact that terminals will paly out the content after the additional burst is received. However, users in good coverage locations able to decode all original bursts will not experience this delay.
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Fig. 3.
Percentage of correctly received bursts vs. average CNR.
IV. P ERFORMANCE E VALUATION The different techniques are compared in terms of: • Effective burst data rate (Mb/s). Defined as the information payload data rate perceived by the user terminal without considering FEC overhead. • Percentage of Satisfied Users (%). The criteria for defining a satisfied user is that the percentage of lost bursts should not exceed 5%. According to [6], this criteria corresponds to a “good/fair” recovery of DVB-H streaming services. • Average time user terminals’ receivers are ON (%). This measure is directly related to the power consumption of the terminals. The percentage of time the terminal’s receiver is ON can be calculated as: TON (%) =
(Bd + St + Dj ) + α · (Bd0 + St + Dj ) ·100 (1) B d + Ot
where Bd and Bd0 are the burst durations of the original and the additional burst, St is the synchronization time, Dj is the allowance for jitter, α is the percentage of synchronized additional parity bursts, and Ot is the off-time between original bursts. If a user does not synchronize to any additional burst, the second term disappears (α = 0). Note that users in poor reception areas will have to synchronize to most additional parity bursts, and thus will have a higher power consumption. A. System Model The settings used for evaluations is a hexagonal service area of 25 km radius with a broadcasting TV tower 250 m height situated in the middle of it. A 10 minutes streaming service at 200 kb/s has been assumed. The DVB-H system parameters used are FFT size 4K and GI 1/4. In the simulations, the number of correctly received sections in each burst is computed (sections size is 1 kbyte). The CNR performance model given by [11] has been used.
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16QAM 1/2 MPE−FEC 3/4 16QAM 1/2 AL−FEC 3/4 16QAM 1/2 AL−FEC 1/2 ∆t 3.8 s 16QAM 1/2 AL−FEC 3/8 ∆t 3.8 s 16QAM 1/2 AL−FEC 3/8 ∆t 26.3 s 16QAM 1/2 MPE−FEC 3/4 retx. ∆t 26.3 s QPSK 1/2 MPE−FEC 3/4
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Fig. 4. Percentage of satisfied users vs. transmission power from TV tower.
In each simulation, 1000 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 compute the number of satisfied users as a function of the EIRP (Effective Isotropic Radiated Power) from the TV tower. Presented results are an average of 100 independent simulations. Link budget values used are the ones proposed in [5] for DVB-H coverage planning. DVB-H terminals are characterized by an omni-directional antenna with -7 dBd gain and a noise figure of 6 dB. The operating frequency is 700 MHz. The shadowing has been implemented by means of a lognormal distribution with a standard deviation value of 5.5 dB, and a correlation distance of 70 m. Fast (Rayleigh) fading has also been considered. A vehicle entry loss of 7 dB has been assumed. The ITU-R P.1546 path loss model has been used with a height loss correction factor of 18 dB, to account for 1.5 m receiver antenna height in a sub-urban environment. No external interferences have been considered. The mobility model accounts for arbitrary urban street patterns and realistic movements by a limited number of parameters that can be easily derived for a particular city. The mobility model parameters are the following: direction change probabilities p0o , p90o , p−90o and p180o , standard deviation of direction distributions (equal for all four distributions) σϕ , average length of major roads d, variance of the length of minor city roads σd2 , mean velocities of city and major roads, v and vmr , velocity deviation σv , and percentage of cars on major roads pmr . The values used in our simulations are: p0o = 0.695, p90o = 0.2, p√−90o = 0.1, p180o = 0.005, σϕ = π/32, d = 250 m, σd2 = d · (2/π), v = 15 km/h, vmr = 40 km/h, σv = 15 km/h, and pmr = 0.7. A coding rate 3/4 has been considered in the original bursts (i.e., the coding rate 3/4 means that no additional parity burst is used). The AL-FEC coding rates 1/2 and 3/8 are achieved with additional burst sizes of 1 Mb (half burst) and 2 Mb (full burst), respectively. The additional bursts are transmitted ∆t
16QAM 2/3 MPE−FEC 3/4
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16QAM 1/2 AL−FEC ∆t 3.8 s 16QAM 1/2 AL−FEC ∆t 11.3 s 16QAM 1/2 AL−FEC ∆t 18.8 s 16QAM 1/2 AL−FEC ∆t 26.3 s 16QAM 1/2 MPE−FEC
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Fig. 5. Transmission power from TV tower vs. effective burst data rate for 95% satisfied users.
seconds after the original bursts. A 1% reception overhead has been assumed, as in the standardization work of AL-FEC in DVB-H [9]. B. Numerical Results Fig. 4 shows the percentage of satisfied users as a function of the transmitted power from the TV tower for different cases using AL-FEC and MPE-FEC. We can see the improvement in user satisfaction due to the additional burst, and how it depends on ∆t. Note that AL-FEC does not perform worse than MPEFEC when no additional burst is used, in agreement with [9]. We can also see that the difference between transmitting a complete additional burst with AL-FEC and with MPE-FEC is negligible. However, MPE-FEC do not provide the same flexibility offered by AL-FEC, since there is no other option than retransmitting the whole burst. Fig. 5 shows the transmission power from the TV tower needed to achieve 95% satisfied users as a function of the effective burst data rate. The different capacities shown with AL-FEC are achieved for coding rates 3/4, 2/3, 3/5, 1/2, 3/7 and 3/8 (corresponding to additional burst sizes of 0, 0.25, 0.5, 1, 1.5 and 2 Mbits respectively). The capacities shown with MPE-FEC are achieved for coding rates 3/4, 2/3 and 1/2. We can see how the gain obtained by delaying the additional burst is negligible once the correlation between reception conditions of the original and additional burst is very low. Note that the coverage improvement (reduction in the EIRP requirement) obtained by trading capacity is higher with AL-FEC than with MPE-FEC (without burst retransmissions). Moreover, it provides higher flexibility (i.e., it can trade more capacity). Also note that the performance obtained with AL-FEC and 16QAM 1/2 is better than QPSK 2/3 MPE-FEC 3/4, and almost the same than QPSK 1/2 MPE-FEC 3/4. It should be pointed out that the coverage improvement increases for more demanding user satisfaction criteria. In our investigation, we additionally considered the case when
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Terminal Average ON Time (%)
probably continue being dominant, while real-time streaming services will be offered only during specific time of the day [13]. On the other hand, zapping time is currently seen as a crucial parameter for DVB-H usability [5]. However, our technique does not affect it as long as original bursts are not lost while zapping. Compared to the case when no additional bursts are transmitted, this technique increases the chances of recovering all information. If an original burst is lost, but the information is recovered after receiving the additional burst, the stream will start from the point where was interrupted.
16QAM 1/2 AL−FEC 16QAM 1/2 MPE−FEC MPE−FEC 3/4 MPE−FEC 2/3 MPE−FEC 1/2 16QAM 2/3 QPSK 2/3 QPSK 1/2
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VI. C ONCLUSIONS
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Fig. 6. Power consumption measure vs. effective burst data rate for 95% satisfied users.
no losses of bursts are allowed (i.e., a satisfied user receives all bursts correct), and we found a maximum gain with one complete additional burst delayed of 6.1 dB instead of 4.5 dB. Fig. 6 shows the average time user terminals’ receivers are ON as a function of the effective burst data rate, for 95% satisfied users. These values have been computed using (1). It has been assumed a synchronization time of 120 ms, an allowance for jitter of 10 ms, and a 4% overhead due to section and transport packet overhead [5]. The AL-FEC curve shown corresponds to the case when ∆t is equal to 26.3 s. The advantage of AL-FEC over MPE-FEC is evident. This is due to most users only synchronize to few additional bursts (the average percentage of synchronized additional bursts is always less than 6%). Reducing the MCR order implies a higher terminal power consumption because the burst duration increases, whereas the drawback of MPE-FEC is that the number of bursts increases when more robust coding rates than the mother code 3/4 are used. V. D ISCUSSION The proposed technique allows to flexibly and dynamically trade service capacity and streaming delay for improved mobile reception at the application layer. In this way, multiple services with different Quality of Service (QoS) requirements (area coverage, delay, data rate, burst error rate, etc.) could efficiently share the same infrastructure, eventually in a competitive manner. This technique can be used, for example, to provide streaming services to vehicular users in areas where, otherwise, only outdoor pedestrian reception is possible. In the case of mobile TV, the main drawback of the proposed technique is the introduction of a delay equal to the time between the original and additional bursts. This delay could be translated into a larger service access time and zapping time between channels. For most streaming services, a larger service access time will not be an issue, as non-real time streaming services will
Our investigation shows that transmitting additional bursts, as a complement for the original ones, has the potential to hide coverage discontinuities to the mobile users. Transmitting additional bursts provides increased user satisfaction compared to traditional MPE-FEC usage, by trading both system capacity and streaming delay. Moreover, it provides a considerably lower terminal power consumption. Results show that MPE-FEC with burst retransmissions performs similar to AL-FEC with one additional burst with the same size than original bursts. However, AL-FEC provides a larger flexibility, since with MPE-FEC there is no other option than retransmitting the whole burst. ACKNOWLEDGMENT The authors would like to thank Erik Stare (Teracom AB, Sweden) and Mark Watson (Digital Fountain Inc., USA) for their valuable input and discussions regarding this topic. R EFERENCES [1] G. Faria, J. A. Henriksson, E. Stare, and P. Talmola, “DVB-H: Digital Broadcast Services to Handheld Devices,” Proceedings of the IEEE, vol. 94, no. 1, pp. 194-209, January 2006. [2] D. G´omez-Barquero and A. Bria, “Feasibility of DVB-H Deployment on Existing Wireless Infrastructure,” International Workshop on Convergent Technologies (IWCT), Oulu, Finland, 2005. [3] A. Bria and D. G´omez-Barquero, “Scalability of DVB-H Deployment on Existing Wireless Infrastructure,” IEEE Personal, Indoor and Mobile Communications (PIMRC), Berlin, Germany, 2005. [4] Finnish Mobile TV Project, “Consumers also want to watch TV programs on their mobile,” Press Release 30th August 2005. http://www.finnishmobiletv.com/ [5] ETSI, TR 102 377 v1.2.1, “Digital Video Broadcasting (DVB); DVB-H Implementation Guidelines,” November 2005. [6] ETSI, TR 102 401 v1.1.1, “Digital Video Broadcasting (DVB); Transmission to Handheld Terminals (DVB-H); Validation Task Force Repor,” May 2005. [7] ETSI, Draft Technical Specification, “IP Datacast over DVB-H: Content Delivery Protocols,” December 2005. [8] M. Luby and A. Shokrollahi, “Mobile Data Broadcast Delivery using FEC codes,” Digital Fountain Technical Report, March 2003. [9] M. Watson, “Application Layer Forward Error Correction. Summary of Simulation Results,” DVB TM-CBMS1397, August 2005. [10] A. Shokrollahi, “Raptor Codes,” Digital Fountain Technical Report, June 2003. [11] TeamCast and DiBcom, “DVB-H Calculator - Mobile Performance Evaluator,“ Available online http://www.teamcast.com/ [12] P. I. Bratanov and E. Bonek, “Mobility Model of Vehicle-Borne Terminals in Urban Cellular Systems,” IEEE Trans. on Vehicular Technology, vol. 52, no. 4, pp. 947-952, July 2003. [13] B. Karlson, A. Bria, J. Lind, P. L¨onnqvist, and C. Norlin, “Wireless Foresight: Scenarios of the Mobile World in 2015,” Wiley, 2003.
