Prioritisation of Data Partitioned MPEG-4 Video over Mobile Networks Λ

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1 Communication Networks Prioritisation of Data Partitioned MPEG-4 Video over Mobile Networks Λ STEWART T. WORRALL, SIMON N. FABRI, ABDUL H. SADKA, AHMET M. KONDOZ Centre for Communication Systems Research (CCSR), University of Surrey, Guildford, UK GU2 7XH Abstract. Despite much research in the field of mobile multimedia, delivery of real-time interactive video over noisy wireless channels remains a challenging problem. Two of the major issues in providing true end-to-end mobile multimedia capability are interoperability between platforms and networks and the poor performance of video compression algorithms in error-prone environments. This paper presents a method for prioritising data partitioned MPEG-4 video in a way suitable for transmission over a mobile network. The effectiveness of the technique is demonstrated by examining the performance when transmitted using the Real-Time Transport protocol over GPRS (General Packet Radio Service) data channels under varying channel conditions. 1 INTRODUCTION The imminent arrival of third generation wireless networks will dramatically increase the viability of mobile multimedia communications. In particular, it will make it possible to hold video conferencing sessions using portable devices. Unfortunately, the hostile and highly variable channel conditions common to mobile communications makes the realisation of this goal difficult [1]. A large variety of techniques have been proposed to attempt to reduce the sensitivity of compressed video data to corruption [2]. Most of these techniques have focussed on MPEG-2 for high bandwidth applications and H.3 for video telephony. In particular, H.3+ now has a wide variety of error resilience options [3]. Relatively recently, the standardisation of MPEG-4 has introduced a wide range of new features and possibilities [4], [5]. MPEG-4 s error resilience options [6] provide reasonably effective improvements in robustness [7], with little increase in overhead. However, on their own they are still insufficiently powerful for a mobile channel. One technique capable of delivering good results is Unequal Error Protection (UEP). As data partitioning places critical data at the beginning of each packet MPEG-4 can benefit significantly from such an approach. The scheme in [8] protects fixed lengths of data with different strength convolutional codes, with data at the start of the packet receiving the greatest protection. However, as more motion Λ An earlier version of this paper has been presented at the Packet Video 00 Workshop. occurs in the scene, the amount of important motion data at the beginning of the packet grows in size. This results in some of the motion information receiving less protection than required. A similar method for improving error resilience is the prioritisation of different parts of the video bitstream by sending the data as two separate streams. This enables the encoder to demand that the network send the data using channels with different priorities, allocating more important data to more reliable channels. In [9], MPEG-2 was prioritised in a number of different ways for an ATM network. Methods of prioritisation included data partitioning, spatial scalability, and SNR scalability. The work was carried out for high bandwidths, assuming no losses on the highly prioritised base layer. Over a mobile channel it may not be possible to guarantee such a situation, both in terms of bandwidth and base layer loss ratio. The method of data partitioning is different from that used in MPEG-4, while the tests involved cell loss, without corruption of the data inside each cell. The introduction of shape coding in MPEG-4 means that, theoretically, prioritisation of objects can be used [10]. However, it is necessary to solve problems with the syntax of the shape information if this possibility is to be realised [11]. Although the approach to implementing Unequal Error Protection in MPEG-4 described in [8] gives a marked improvement in system performance, it suffers from three main drawbacks. As the channel protection mechanisms are closely integrated with the video compression format- Submission 1

2 S.T. Worrall, S. Fabri, A.H. Sadka, A.M. Kondoz ting, this approach is suitable mainly for circuit-switched applications as it does not provide for decoupling between the network layers and the higher transport and application layers. In fact implementing such a scheme at the application layer would leave all network and transport layer headers unprotected. A second drawback is that it makes no provision for the occurrence of packet loss. Although noisy channels generally result in high bit error rates, these same channel errors also cause frame and packet erasures, mainly dictated by the operation of error check mechanisms and the loss of payload information caused by the corruption of packet information. The UEP mechanism does not provide any protection against such packet loss. However, the most severe drawback of this approach is the limitations it places on application interoperability. In order to be effective, channel correction mechanisms must be carried out at the physical link layer, where soft-decision information can be used to obtain around 3dB advantage over hard-bit decoding. In order to implement UEP as described in [8], each underlying network must be tailored for the properties of MPEG-4. Not only does this approach greatly hinder transmission across diverse heterogeneous networks, but it also reduces the scope for developing enhanced services, as improving the application capability also requires upgrading all the underlying networks. The approach to providing Unequal Error Protection being presented in this paper addresses these issues. In this scheme, error resilience is improved by the prioritisation of different parts of the video bitstream by sending the data as two separate streams. This allows for the encoder application to allocate a higher priority to the more error-sensitive data and transmit it over higher-quality more reliable channels. This paper presents a data partitioning approach to prioritisation using MPEG-4 over a GPRS channel model. Section 2 delineates the proposed system, section 3 discusses how it can be employed in 3G networks, section 4 describes the method of testing, and section 5 examines the results. 2 PRIORITISATION TECHNIQUE Data partitioning in MPEG-4 divides the data into two sections. Header, motion, and shape data is coded in the first partition, while the less important texture information is placed in the second partition. Figure 1 shows this arrangement, although it should be noted that the code separating the two partitions is different from the code at the beginning of the packet. It is a simple task to produce two streams from this arrangement, using the first partition as the high priority stream and the second partition as a lower priority stream. A major problem with transmitting a single video sequence using a number of streams, is that of synchronisation loss. It is important to ensure that there is some Resync Motion & shape Texture Figure 1: Data partitioning in MPEG-4. way that the decoder can determine whether it is decoding data corresponding to the same frame and packet in both streams. To this end, the proposed scheme exploits the time stamping feature of MPEG-4. Time stamps are present in the VOP header. Therefore, it is unnecessary to place any extra time stamps on the first partition stream. For the second partition stream, Header Extension information is added at the start of every frame. This includes time stamps, and the VOP type. VOP start codes are also placed at the beginning of every frame (see figure 2(a)). VOP start code 1bit H E C Header Extension Information bits 16- bits (a) Second partition stream frame header. Resync marker MB num 17 bits 1-14 bits (b) Second partition stream packet header. Figure 2: Extra data added to second partition stream to ensure synchronisation between the two streams. To ensure synchronisation within a frame, the number of the first macroblock in the packet is added to each packet header in the second partition stream (it is already present in the first partition). This arrangement is shown in figure 2(b). The extra overhead incurred by this technique depends upon the frame rate, packet size (700 bits in the experiments described here), and bit-rate. For the bitstreams tested in this paper the extra overhead generated amounted to approximately 10-13% of the overall rate. This overhead was calculated using the following information: ffl 0 bits for the RTP/UDP/IP header ffl 47 bits for the second partition VOP header (this is an average) ffl QCIF video requires 7 bits for MB num in the second partition packet header (implies header bits per packet) 2 ETT

3 Prioritisation of Data Partitioned MPEG-4 Video over Mobile Networks 3 USE IN 3G MOBILE NETWORKS This arrangement of video information into different streams will allow for true Unequal Protection mechanisms to be employed for MPEG-4 in a UMTS (Universal Mobile Telecommunications System) scenario. In order to ensure the long-term viability of any of the IMT-00 candidates, a decoupling between higher-level functions such as source encoding mechanisms and the lower-layer networkdependent functionality is necessary [12]. This means that 3rd Generation mobile access technologies, be they based on TDMA or W-CDMA schemes will offer a number of bearer services to the user. These will be defined in terms of the bearer characteristics and bearer quality [13]. The latter includes parameters such as end-to-end latency, delay variation, throughput and bit and frame error rates. Bearer channels do not offer any particular type of service, such as speech, data transfer protocols or video services, but instead offer links conforming to a given set of Quality of Service parameters. It is required that a single terminal should have access to more than one bearer channel, with each separate channel being able to offer different levels of Quality of Service. In the scheme being proposed, each stream will be transported over a different mobile bearer channel as offered by the underlying network. While this scheme allows for the error-protection mechanisms to be tailored to the specific formatting of the MPEG-4 video bitstream syntax, it also offers independence from the type of access technology that is used. As long as the bearer channels can meet the QoS parameters set by the application, then the prioritisation scheme is completely independent from the underlying network. This facilitates interoperability between similarly configured terminals communicating via different networks. QoS management will be carried out by integrating the end-to-end requirements set by the end applications with the mechanisms employed by the underlying networks [14]. In this scenario, UEP is implemented by transmitting the two separate streams over different bearer channels meeting different QoS levels, thereby optimising performance over the access networks. This is achieved without integrating the higher-layers with network-layer mechanisms, thereby guaranteeing interworking functionality. 4 SIMULATIONS The stream prioritisation scheme described above was tested on simulated GPRS mobile access channels. GPRS (General Packet Radio Service) is an end-to-end mobile packet data service standardised within the framework of ETSI s SMG (Special Mobile Group) activities and currently being deployed in several countries worldwide. Unlike existing mobile networks, GPRS does not transmit any of the user information via circuit-switched channels, re- High-priority segment Video Frame Low-priority segment GPRS/UMTS radio interface protocl protocol stack MPEG-4 Encoder Separate into two segments Transport & IP layers Encapsulate segments into separate IP packets Figure 3: Prioritised streams operation for MPEG-4. Table 1: GPRS Channel Coding Schemes. Transmit over two different radio bearer channels Coding Code Radio Block Data Rate/Slot Scheme Rate Payload (bits) (kbit/s) CS-1 1/ CS-2 2/ CS-3 3/ CS lying instead on IP-based switching networks for the operation of its core network. The radio access network on the other hand, is shared with the current GSM circuitswitched voice and data services. This not only brings about a considerable saving in the cost of the required radio infrastructure, but also allows for a dynamic sharing between voice and data services. Although GPRS was not initially designed for real-time multimedia services, its multislotting capabilities allow for data rates of up to around 60 kbit/s where traffic conditions allow. In addition, its end-to-end packet transfer capability and the migration path being designed by ETSI to lead to 3G candidates for IMT-00, mean that the GPRS access channel is a suitable environment for testing novel video transmission mechanisms. GPRS currently supports four channel protection mechanisms [15], each offering different levels of protection. CS-1 employs a 1/2 rate convolutional code and a 40-bit FIRE redundancy check sequence. CS-2 and CS-3 both use punctured versions of the CS-1 code, thereby allowing for a greater user payload at the expense of reduced performance in error-prone environments. CS-4 however only uses a cyclic redundancy check for error detection and does not offer any correction capabilities. A summary of these schemes is shown in Table 1. The video packets were encapsulated within RTP packets [16] and transmitted using the UDP transport protocol. Submission 3

4 S.T. Worrall, S. Fabri, A.H. Sadka, A.M. Kondoz Each video frame was placed into a single RTP/UDP/IP packet. This means that a combined transport-network header overhead of 0 bits was required for every transmitted video frame. It was assumed that an error occurring anywhere within the combined RTP/UDP/IP header would be uncorrectable and would result in the contents of the entire packet being discarded. Errors were assumed to occur only within the radio access part of the network, with no packets being lost due to congestion or other mechanisms. The radio conditions simulated were for an interferencelimited scenario, which is the most common operating scenario for mobile terminals. The noise at the receiver was set at db, and the propagation conditions were those specified in GSM as TU50 Ideal Frequency Hopping at 900MHz. The TU channel model represents the multipath propagation conditions found in typical urban conditions, and in the particular conditions used in these experiments, a mobile terminal velocity of 50 km/hr was assumed. CS2singlestream,noIP CS1 1st stream, CS3 2nd stream, no IP Figure 5: PSNR of Suzie for single stream and prioritised stream transmission without IP header corruption. CS2 single stream, with IP Table 2: Effectiveness of error concealment for Suzie, with transmission over a 15 db C/I channel, with coding scheme CS 2. Error PSNR Concealment (db) Yes 29. No CS1 1st stream, CS3 2nd stream, with IP Figure 4: PSNR of Suzie for single stream and prioritised stream transmission with IP header corruption. Each of the points on the graphs shown in figures 4, 5, 6, and 7 correspond to the average PSNR taken from decoding the same sequence times. The video sequences were coded so that the combined source-channel coded bit-rate was approximately 64 kbit/sec. Suzie and Foreman were the standard sequences selected for testing. Both are in QCIF format. Suzie was coded at 10 frames/sec, while Foreman was coded at 7.5 frames/sec. The coded sequences had INTRA frames inserted every two seconds for Foreman, and every three seconds for Suzie. The decoder employs simple error concealment upon encountering errors, which involves replacing incorrect or missing macroblocks with their corresponding motion compensated macroblocks. This is accomplished in the decoder implementation by setting the prediction errors for the motion vectors and pixel values to zero for each affected macroblock. This is a standard technique employed in many decoder implementations. The effectiveness of the error concealment scheme can be seen in the results in Table 2. Simulations were run as described above for the non-prioritised case, but with the error concealment functionality turned off at the decoder. Without error concealment, picture quality is completely unacceptable. However, turning error concealment on gives results that are reasonably acceptable. 5 RESULTS This section examines the performance of the prioritisation technique under the previously described test conditions. Results are compared with those for normal, single stream encoded video data. The comparison is made between two different schemes. One using CS-2 to protect a single stream and another using CS-1 and CS-3 on the first and second partition streams, respectively. These 4 ETT

Prioritisation of Data Partitioned MPEG-4 Video over Mobile Networks CS2 single stream, with IP CS2singlestream, no IP 18 CS1 1st stream, CS3 2nd stream, with IP 18 CS1 1st stream, CS3 2nd stream, no

16 Figure 7: PSNR of Foreman for single stream and prioritised stream transmission with IP header corruption. two schemes use approximately the same source-channelcoding ratio.

5 Prioritisation of Data Partitioned MPEG-4 Video over Mobile Networks CS2 single stream, with IP CS2singlestream, no IP 18 CS1 1st stream, CS3 2nd stream, with IP 18 CS1 1st stream, CS3 2nd stream, no IP 16 Figure 6: PSNR of Foreman for single stream and prioritised stream transmission with IP header corruption. 16 Figure 7: PSNR of Foreman for single stream and prioritised stream transmission with IP header corruption. two schemes use approximately the same source-channelcoding ratio. The objective results in figures 4 to 7 indicate clear benefits from using the technique. Of course, the additional overhead generated from using prioritised streams leads to a small reduction in error-free quality. In relatively clean channel conditions, single stream encoding will often give superior performance. This is demonstrated by the results for 18 db C/I for Suzie in figure 6. However, such good channel conditions are unlikely in real life. It should be noted that the crossover in the results shown for Foreman in figures 6 and 7, also occurs in the results for Suzie. The Suzie sequence s results cross between 18 and 21 db, which are, as previously stated, conditions that are unlikely to be encountered when using a real network. Differences in crossover position are due to the amount of motion in each sequence. The high degree of motion present in the Foreman sequence means that, on average, the first partition of each foreman packet takes up a greater proportion of the packet than it does for Suzie. Therefore, texture data must occupy less space in order to achieve the same bitrate as Suzie. This leads to the use of coarser quantisation, resulting in lower quality video. The effect is exacerbated using prioritised streams, because the size of the first partition with respect to the second is increased by the difference in convolutional codes applied to each stream. Thus, the reduction in quality from using the proposed scheme is much greater for Foreman than it is for Suzie. Subjective assessments also tend to reveal performance improvements from using prioritised streams. Figure 8 shows a frame from the middle of the Suzie sequence. It is (a) Single stream transmission. (b) Prioritised stream transmission. Figure 8: Frame 75 of Suzie with C/I=12 db. corrupted with a C/I of 12 db. This frame is shown because the high degree of movement at this point makes this part of the sequence highly sensitive to error. Although degradation is still visible when using prioritised streams, the quality is much more acceptable than when a single stream is used. 6 CONCLUSION The work presented here has demonstrated a method of increasing the error resilience of MPEG-4 while retaining interworking capability by using prioritised streams. The scheme proposed allows for unequal error protection to be implemented in a way that is transparent to the underlying networks and consequently can be used across diverse networks. Tests were carried out using on simulated GPRS Packet Data Traffic Channel. Subjective and objective results have Submission 5

6 S.T. Worrall, S. Fabri, A.H. Sadka, A.M. Kondoz been analysed. These test results have shown that the proposed technique provides considerable improvements in quality when transmitting over a mobile channel. Manuscript received on... REFERENCES [1] S. Fabri, A. Cellatoglu, and A. Kondoz. Transmission of Multimedia Services over GPRS using MPEG-4 Coded Video. In IEEE VTC 1999-Fall, Amsterdam, pages [2] Yao Wang, Qin-Fan Zhu. Error Control and Concealment for Video Communication: A Review. Proceedings of the IEEE, Vol. 86, No. 5, May 1998, pp [3] S. Wenger, G. Knorr, J. Ott, and F. Kossentini. Error Resilience Support in H.3+. IEEE Transactions on Circuits and Systems for Video Technology, Vol. 8, No. 7, November 1998, pp [4] A. Puri and A. Eleftheriadis. MPEG-4: An object-based multimedia coding standard supporting mobile applications. ACM Mobile Networks and Applications, Vol. 3, June 1998, pp. 5. [5] ISO/IEC JTC 1/SC 29/WG 11. Information technology - Generic coding of audio-visual objects - Part 2: Visual, ISO/IEC MPEG Vancouver meeting, Doc. N02, July [6] R. Talluri. Error-Resilient Video Coding in the ISO MPEG- 4 Standard. IEEE Communications Magazine, June 1998, pp [7] ISO/IEC JTC 1/SC 29/WG 11. Report Of The Formal Verification Tests On MPEG-4 Video Error Resilience. Doc. N04, December [8] W. Rabiner, M. Budagavi, and R. Talluri. Proposed extensions to DMIF for supporting unequal error protection of MPEG-4 video over H.4 mobile networks. Doc. M4135, MPEG Atlantic City meeting, October [9] R. Aravind, M. R. Civanlar, and A. R. Reibman. Packet Loss Resilience of MPEG-2 Scalable Video Coding Algorithm. IEEE Transactions on Systems for Video Technology, Vol.6, No. 5, October [10] A. Cellatoglu, S. Fabri, S. Worrall, and A. Kondoz. Use of Prioritised Object-Oriented Video Coding for the Provision of Multiparty Video Communications in Error-Prone Environments. In IEEE VTC 1999-Fall, Amsterdam, pp [11] L. Soares and F. Pereira. An Alternative to the MPEG-4 Object-Based Error Resilient Video Syntax. In IEEE Int. Conf. Image Processing Conference (ICIP), 1998, pp [12] 3GPP. 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects, Service Aspects; Services and Service Capabilities. 3G TS.105, Version 3.6.0, October [13] 3GPP. 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects, Service Aspects; Mobile multimedia services including mobile Intranet and Internet services. 3G TR.960, Version 3.0.1, April [14] 3GPP. 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects, Service Aspects; QoS Concept and Architecture. 3G TR , Version 3.0.0, October [15] SMG. GSM v6.1: Digital cellular telecommunications system (GSM Radio Access Phase 3); Channel Coding. January [16] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson. RTP: A Transport Protocol for Real-Time Applications. RFC 1889, Audio-Video Transport Working Group, January ETT

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