Transport Layer Coding for Satellite-Based Audio and Multimedia Services to Vehicular Terminals in Ku-band M. Berioli, H. Ernst, S. Scalise, R. Midthassel and C. Loeillet
Abstract— Satellite radio for vehicles is normally hampered by the problem of the shadowing of the line-of-sight between satellite and receiver. One possible way to overcome this challenge is to provide a new type of radio service, hereafter referred to as personalized satellite radio, in alternative to the streaming approach used for traditional radio: individual audio and multimedia files are broadcasted instead of transmitting a number of distinct continuous streams. The received files are stored in a large cache located in the receiver of each mobile terminal and are sequentially played to generate a service similar to traditional radio/entertainment programmes. The resulting file-based radio approach makes use of higher-layer coding on transport level, to ensure a sufficient file transfer reliability even when some packets are lost due to the shadowing of the line-of-sight path, and of smart techniques to build up an audible programme with no interruptions and full audio quality based upon the available files. The paper presents this novel radio concept and the results of extensive trials which were conducted with a system test-bed developed in the framework of an ESA funded project.
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I. INTRODUCTION
ne of the major strength of satellites is their inherent capability to broadcast data to large amount of users. The most successful example of the usage of communication satellites is broadcasting TV to fixed receivers. An extension to this approach is the broadcast to mobile receivers, especially cars and vehicles. This idea has been realized in the United States by systems like XM-Radio [1] and Sirius [2]. Broadcasting high quality digital radio channels to mobile users by means of dedicated satellites using S-band frequencies (around 2.3 GHz) in the downlink is today an important part of the satellite business. Especially, the great success in terms of subscribers obtained by XM-Radio and Sirius, shows that traditional countermeasures, such as high link margin, time and satellite diversity and usage of terrestrial repeaters within urban areas represent effective though costly solutions to counteract the impairments of the land mobile satellite channel. One possible way to overcome the problem is to provide a new type of radio service, representing an alternative to the streaming approach used for traditional radio: the Personalized Satellite Radio [3]. Instead of transmitting a Manuscript received October 15, 2007. Matteo Berioli (corresponding author) and Sandro Scalise are within the German Aerospace Center (DLR), Institute for Communications and Navigation, PO Box 1116, 82230 Wessling, Germany. Tel. +49 8153 282863, Fax +49 8153 282844, E-mail
[email protected]. Harald Ernst and Rolv Midthassel are within the European Space Agency (ESA). Christophe Loeillet is within SES Global.
978-1-4244-1645-5/08/$25.00 ©2008 IEEE
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number of distinct continuous streams, individual audio and multimedia files are broadcasted. The received files are stored in a large cache located in the receiver of each mobile terminal, and they are sequentially played from the cache to generate a service similar to traditional radio/entertainment programmes. A continuous connection between the satellite and each receiver is therefore no longer required. Instead, it is sufficient to ensure that enough new files to generate an attractive programme are available in the cache. Therefore, the aim is to reach a sufficiently high update rate of the cache content. A failure in the transfer of an individual file has no impact on the audible quality, since the only effect is that this file cannot be used when assembling the programme. Some files like the hourly news should be received with a high degree of reliability by the largest amount of terminals, but for the bulk of data like music, feature stories or even video-clips the overall net throughput is more important than ensuring the reliable delivery of a single file. Besides its robustness, this approach opens new possibilities in terms of enhanced services, like the capability to skip or repeat part of the radio programme independently from any time restriction, and to easily integrate additional data services like e.g. road and weather maps distribution or generic html content. Furthermore, this approach can be combined with the usage of satellite systems with low link margins, which are already available, for instance in the Ku-band. This results in low infrastructure costs for the deployment of such a system, together with the availability of new, attractive personalized features. The paper is divided into four sections. First an overview of the system characteristics is presented focussing on the higher-layer coding aspects and clarifying some functional and architectural aspects. In the third section results of endto-end live trials will be presented. The last section drives the conclusion of the work. II. SYSTEM DESCRIPTION A block diagram of the system architecture is shown in Figure 1. At the provider side, the Content Playout Centre (CPC) collects all the files to be broadcasted. The XML metafile associated to each information file is parsed to extract the relative QoS class. Furthermore, the CPC schedules the transmission of all files over the different transponders. Files to be transmitted over a given transponder are then transferred to the Transport Layer (TL),
where segmentation into IP datagrams takes place.
Figure 1: System Block Diagram The TL plays an important role in the system. In particular a first stage of packet-based Forward Error Correction (FEC) is applied at TL in addition to traditional FEC and time interleaving at physical layer. This is mainly based on the methods developed by the IETF Reliable Multicast Transport working group [4]: redundancy datagrams are generated for each file by means of coding algorithms with variable code rates. The adopted protocols are specified in RFC 3450 [5] (Asynchronous Layered Coding Protocol, ALC) and in RFC 3926 [6] (File Delivery over Unidirectional Transport, FLUTE). Since the file size can vary, the file will be packetized over a varying number of datagrams. If the TL-FEC code rate is constant, a longer file will have a larger absolute number of redundancy packets, and a shorter one will have fewer redundancy packets. To allow higher flexibility, the code rate can however also change from file to file as shown in Figure 2 depending on the required level of protection. The TL in the receiver is then capable of successfully decoding the received file if a sufficient number of packets is received, regardless of which packets (information or redundancy) have been exactly received. Redundancy packets File 1
Codeword C n = k + h packets n, k, h variable
File 2
additional problems, especially concerning synchronization and memory requirements, bearing in mind that a physical layer interleaver needs to store soft values, with a quantization typically in the order of 4 to 8 bits. The introduction of coding at higher layer is an alternative and appealing solution for counteracting very long and deep fades. Higher-layer FEC is a very powerful technique, as it can be implemented on top of every standardized airinterface, without any modifications to the lower layers, while at the same time it extends the capability of the system to unforeseen situations and environments, especially for mobile applications. Concerning the technical solution adopted in this work, the system makes use of two coding techniques for the TLFEC. Either Reed Solomon or LDPC (Low Density Parity Check) codes can be used, in order to achieve a good tradeoff between computational complexity and coding efficiency. The Reed Solomon, which is performing ideally over a binary erasure channel, is used to encode small files, whereas irregular LDPC codes (based on DVB-S2 [8]) are used for files bigger than 600 kB to reduce the decoding complexity. This threshold has been selected in order to allow a quasi real-time decoding of the received files. Application Layer
FLUTE File Content
FDT-Instance
Building Blocks FEC LCT
ALC individual units
Transport Layer
CC
UDP Packets
Network Layer
IP
Figure 3: FLUTE/ALC protocol overview
Codeword D
Figure 2: File oriented Transport Layer Coding This is one of the most interesting features of this system, permitting to enhance the robustness of the file transfer mechanism and guarantying at the same time a high degree of flexibility. In fact, long time interleaving can be achieved by multiplexing together IP datagrams belonging to different files, thus overcoming the typical memory limitations characterizing interleavers at physical layer. Moreover, in mobile scenarios, the mean duration of fades is typically in the range of some seconds (which already results in the need for relative large interleaver sizes) with a non-negligible probability that fades can even last one to two minutes [7]. Traditional channel coding approaches with the necessary long interleavers require a high complexity and pose 2908
This kind of encoding needs information on the file itself (in particular the file length), therefore file-oriented coding cannot be done at network or data link layer, but needs instead to be implemented at the transport layer. Referring to Figure 3, the FLUTE protocol, allowing file transfers without the need for a return link, discerns between the file content and the meta-information of the file included in the FDT-tables, which contain some additional parameters useful for the transfer (filename, file length, etc…). Both units are delivered as independent objects to the ALC transport layer protocol. The ALC combines different building blocks for the implementation of TL FEC: the signalling of auxiliary information (the so-called Layered Coding Transport, LCT, building block), the FEC building block, and, optionally, a multiple rate congestion control (CC) building block. Lastly, multiplex between packets from different files is also possible. This technique increases the spreading of the file in time and it can be used to achieve virtual long interleavers at the TL, in the order of several
PHY
minutes. For what concerns the Physical Layer (PHY), a dedicated waveform very close to DVB-SH TDM mode [9] has been designed in the context of the ESA Ku-Mobile project [10], capable of coping with Carrier to Noise plus Interference Ratio operating point as low as -10 dB. Direct sequence spreading has been additionally introduced to mitigate adjacent satellite interference in presence of small vehicular antennas. In order to provide different degrees of protection for different types of files, it is possible to provide different combinations of PHY and TL interleaver length and code rate. This might be particularly useful in case the files to be transmitted have different requirements in terms of reliability and/or latency. For this reason the system was designed to provide up to three of these combinations in parallel, so that files with different requirements can be transmitted at the same time with different combinations of settings. We define these combinations “pipes”. Each pipe represents a particular setting of PHY and TL parameters. Table 1 shows pipes’ main characteristics, which are relevant for the rest of the analysis. Pipes are logical entities, since the PHY will multiplex all data in a TDM (time division multiplex) fashion, after applying different coding and interleaving to the data-flow of each pipe. The following comments are in order: • the selection of the code rates was carried out in order to achieve a reasonable trade-off between performance in very different environments such as urban and rural. Altohugh some fine optimisation can be performed, the results in section III confirm the validity of the adopted methodology • pipe 1 is especially optimised to take advantage of very short Line-of-Sight (LOS) intervals typical of the urban environment when transmitting relatively short files. Under the assumption that each burst is either totally correct or totally wrong, no significant penalty is caused by the usage of a repetition code. Property Data Rate (output PHY) (including coding and overhead from TL) Packets per second PHY Code Rate PHY Interleaver length [s]
TL
TL Code Rate TL Interleaver
Pipe 1
Pipe 2
Pipe 3
124 kbps
374 kbps
500 kbps
10 1/12 123 s 10% redund. + file repetition (10/22) File length + repetition
30 1/8 28 s
48 1/5 No interl.
10/11
2/3
File length
180 s
Table 1: Exemplary configuration of the three pipes III. TRIALS RESULTS Using this approach, extensive trials were performed to assess the effects of transport layer coding under realistic conditions. These will be now discussed for two typically 2909
scenarios corresponding to trials itineraries in a rural environment and inside Munich city. In order to understand the results of this section it is important to introduce a parameter which will be extensively used in the following, the relative received file size. It represents the portion of encoded file which was correctly received. In order to do that we first need to define the relative encoded file size, i.e. the size of the encoded file considering the TL redundancy with respect to the original file size (without redundancy). So the relative encoded file size for pipe 1 will be 220%, as the file is encoded with code rate 10/11 and transmitted twice. Analogously, the relative encoded file size for pipe 2 is 110% and 150% for pipe 3. In the same way, the relative received file size is the correctly received portion of encoded file size with respect to the original file size. Thus, the relative received file size cannot be bigger than the relative encoded file size for the relative pipe. Figure 4 shows, the amount of successfully received data for the three pipes, averaged over a window of 10 s for the two environments: the drops in the throughput are due to packet losses when the LOS is not available. The plots clearly show the trade-off between robustness and throughput. The pipes with higher protection (e.g. pipe 1) have a lower throughput, but they show fewer failures. In Figure 5, the complementary Cumulative Distribution Function (CDF) of the relative received file sizes is shown. As we already said, the relative received file sizes must be in the range between 0 and the relative encoded file size for each pipe (220%, 110%, and 150%). For a concrete file size x[%], the value of the complementary CDF gives the probability to receive a relative file size equal or greater than x. Ideally, if 100% or more of the relative file size is received the file can be decoded. In reality this decoding threshold is slightly bigger than 100% (up to ca. 106%), due to the slight inefficiency of the employed LDCP codes. Therefore the point corresponding to 100% on the x-axis should represent the overall theoretical probability of successful file decoding for each pipe in case ideal codes are used. The measured file decoding probabilities (marked with dotted lines in the figure) correspond to points slightly above 100% in the figure, due to the mentioned inefficiency. Looking at the performance it can be observed that pipe 1 has the best results (92%), followed by pipe 2, which decoded 79% of the files, and finally pipe 3, which got almost 50% of the files through. This figure also gives important information on the efficiency in the use of the satellite capacity. The complementary CDF should remain as high as possible before the decoding threshold, but it should drop as steeply as possible after that threshold. In this particular measurement, pipe 2 seems quite efficient, even if it does not get the highest probability of successful file decoding. By defining the code rate R as the ratio between the relative received file size and the relative encoded file size of each pipe and operating the random variable substitution, the
cumulative CDFs of Figure 5 can be referred to a generic code rate R. These new set of cumulative CDFs represent the probability to decode a file in case of perfect code efficiency at TL for a certain code rate R. We will call this decoding probability Pd(R). We can then analyze the performance of the system for different code rates R at TL. In particular, the product Pd(R)·R can be seen as the system net throughput normalized to the LOS conditions (with no packet loss). In case of no packet errors, the system net throughput is limited by the TL coding, so the maximum theoretical limit is the diagonal in this figure. In the range of low code rates the curves of the three pipes are all close to the theoretical maximum throughput (the diagonal), whereas in the higher range they remain below the maximum, to different extents according to the TL correction capabilities of each pipe. This picture shows a trade-off between protection and efficiency. The behaviour in the urban environment (see Figure 6) is relatively similar to the profiles shown in the other environment, though in the urban one, the probabilities of successful file decoding are 83%, 68% and 29%, for pipe 1, 2 and 3, respectively. For pipe 1 and 2 these results are slightly worse than the rural environment, so actually it seems that the system is well dimensioned to overcome severe channel conditions also in presence of frequent outages. Pipe 3 pays the trade-off of a higher throughput, and therefore less protection, and it provides slightly worst results. In an optimized configuration, attention should be paid to efficiently exploit the satellite resources and, maybe, to try increasing the throughput without reducing the probability of successful decoding. For the urban environment, there is of course a decrease of the maximum normalized throughput in all pipes. As we saw for the rural environment, the net throughput of pipe 2 is better than the one of pipe 1, but the rate of successfully decoded files is still the best one in pipe 1 (83%). One could remark that in such an environment decreasing the code rate increases the reception probability relatively slowly compared to the rural case, so very low code rates would be needed to reach the same decoding probabilities as in the rural case. On the other hand it is especially interesting to notice that the three functions for the three pipes have similar behaviours in the two environments, in particular for pipe 2 and 3, which have maxima in the same regions (around 0.8-0.9 for pipe 2 and 0.5-0.6 for pipe 3). One can conclude that from the throughput analysis, the same code rate setting would result in a quite optimal tradeoff in very different environments such as rural and urban. At system level, this means that if 2 similar programmes are transmitted, the users located in the rural environment can make use of the fine distinctions between each programme, or use the additional material for a better personalization. In turn, the users in the urban environment can combine the contents of these two programmes to achieve a still interesting radio application. Taking into account that 10-20 such programmes per Ku-band transponder are possible, this would on the one side still allow an acceptable service even
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in cities and without any repeaters and on the other side enable a user-friendly highly personalized service in less challenging environments. IV. CONCLUSIONS We presented in this paper a mobile satellite system, called Personalized Satellite Radio, for file-based digital radio broadcasting, and the results of some end-to-end trials conducted with a demonstrator of such a system operating in the Ku-band. The paper focused on the use of higher-layer FEC coding techniques to mitigate the problem of the shadowing of the line-of-sight between satellite and receiver. The results show that, depending on the setting of particular parameters, the system can guarantee the required performance in terms of reliability and a good trade-off between protection and throughput in different operating conditions (i.e. in different environments like urban and rural areas) without resorting to terrestrial repeaters. The system is very flexible since it works with several “pipes” in parallel, where different settings can be configured and over which files with different requirements can be transmitted. The different characteristics of three exemplary pipes were shown in the paper together with some theoretical insights, also highlighting some room for further optimization. ACKNOWLEDGEMENTS This work was conducted in the framework of the ESA (European Space Agency) project “Mobile Ku-Band Receive only Terminal Demonstrator”, ESTEC Contract No. 17769/03/NL/US [10]. The authors acknowledge the work of all the team members of the Ku-Mobile project and the respective companies. REFERENCES [1]
J. Snyder and S. Patsiokas, “XM satellite radio - satellite technology meets a real market,” in Proceedings of the 22nd AIAA International Communications Satellite Systems Conference & Exhibit, May 2004. [2] R.D. Briskman and S. Sharma, “DARS satellite constellation performance,” in Proceedings of the 20th AIAA International Communications Satellite Systems Conference & Exhibit, May 2002. [3] S. Scalise, H. Ernst, T. Hack, C. Loeillet, and R. Midthassel, “Kumobile: satellite multimedia services for cars in the Ku-band”, International Journal of Satellite Communications and Networking, vol. 24, no. 2, pp. 137{151, 2006 [4] IETF Reliable Multicast Transport (rmt) working group (http://rmt.motlabs.com/) [5] M. Luby, J. Gemmell, L. Vicisano, L. Rizzo, J. Crowcroft, “Asynchronous Layered Coding (ALC) Protocol Instantiation,” RFC3450, December 2002. [6] T. Paila, M. Luby, R. Lehtonen, V. Roca, R. Walsh, “FLUTE - File Delivery over Unidirectional Transport,” RFC3926, October 2004. [7] S. Scalise, H. Ernst, and G. Harles, “Measurement and Modelling of the Land Mobile Satellite Channel at Ku-Band”, IEEE Transactions on Vehicular Technologies, to appear in January 2008. [8] ETSI EN 302 307 v1.1.1: Digital Video Broadcasting (DVB): Second generation framing structure, channel coding and modulation system for Broadcasting, Interactive Services, News Gathering and other broadband satellite applications [9] DVB Bluebook A110, “System specifications for satellite services to handheld devices below 3 GHz”, Mar. 2007. [10] Mobile Ku Band Receiver Demonstrator, ESA-Contract No. 17769/01/NL/DS, http://telecom.esa.int/mobilekureceiver
Figure 4: Successful Throughput for the Three Pipes for the Rural (left) and Urban (right) Environments
Figure 5: Cumulative CDF of the Received File Size (left) and Throughput Normalized to LOS as a Function of the Code Rate (right) – Rural Environment
Figure 6: Cumulative CDF of the Received File Size (left) and Throughput Normalized to LOS as a Function of the Code Rate (right) – Urban Environment
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