ADAPTIVE JOINT H.263-CHANNEL CODING FOR MEMORYLESS BINARY CHANNELS

ADAPTIVE JOINT H.263-CHANNEL ING FOR MEMORYLESS BINARY CHANNELS A. Navarro, J. Tavares Aveiro University - Telecommunications Institute, 38 Aveiro, Portugal, navarro@av.it.pt Abstract - The main purpose of this paper is to show that once the video source coding is dependent on the channel characteristics, better performance can be achieved in terms of reconstructed video quality. Our study considers a fixed overall bit rate transmission of 64 kbit/s over error prone memoryless binary channels and the source coder is the ITU-T H.263 video standard. The variable H.263 source coding rate adapts to a particular channel Bit Error Rate () at any instant of time. Firstly, we investigated the error sensitivities of H.263 syntactical elements and grouped them into several classes of different significance. By identifying the most sensitive elements, we developed a data grouping (DG) technique which by itself, exhibits improved error resilience. Secondly, we design an Unequal Error Protection (UEP) scheme and compare its performance with an equivalent (same source coding bit rate) Equal Error Protection (EEP) scheme, using Rate Compatible Punctured Convolutional (RCPC) codes of different rates for forward error protection. Both forward error correction schemes adopted have resulted in PSNR improvements over db for s higher than of 4x 3. Finally, if during the real time encoding process, the encoder is aware of the channel and there is an unlimited number of code rates, this paper shows that UEP and EEP strategies provide quite similar performances. Keywords - adaptive wireless video communications, forward error protection, joint source-channel coding. I. INTRODUCTION Nowadays, commercially available multimedia terminals are not able to provide video based multimedia applications with accepted image resolution and quality. In future mobile multimedia terminals, motion video is the most demanding in terms of bit rate. Therefore, the wideband 3 rd generation mobile networks (UMTS - Universal Mobile Telecommunication Services) as well as the future broadband 4 th generation networks may be able to transmit motion digital video to the subscriber terminals. Although, the 3 rd and 4 th generations overcome the bit rate limitation, wireless video transmission offer other important negative aspects namely caused by the deep fast fading effects. Compressed video is extremely vulnerable to transmission errors since a single erroneous bit may lead to a considerable number of frames to be incorrectly decoded. Video coding standards were developed for error free transmission since prediction and statistical coding techniques have been adopted. Some papers published in the recent years have investigated both compatible standard and non-standard schemes to improve robustness of video coding. Several techniques have been investigated to increase the error resilience of video coding schemes and some examples are, Forward Error Correction (FEC), Error Concealment (EC), Automatic Request (ARQ), Synchronization Markers (SM) or a combination of them. Support for the last three mentioned techniques have recently been included in the H.263 standard as optional annexes [1,2]. FEC schemes are part of the multiplexing standards, H.223 [3] and H.323 [4]. Unlike these standards recommend, this paper results show that FEC codes should be applied dynamically according to instantaneous channel characteristics and different codes should be used to protect bit stream classes according to their significance. In the context of forward error protection of classified bit stream, Yap et al [] have presented an H.263 channel coding scheme based on RCPC codes and on source bit significance which shows the advantage of UEP over EEP. Bystrom and Modestino [6] formulated the problem of optimally allocating bit rate between source and channel coding for a particular value of overall bit budget. However, a mathematical solution for that optimization problem has not been proposed. Therefore, the solution was graphical. This paper obtained a different syntactical element significance order as reported by Yap et al, mainly due to the use of different binary channel modeling. More recently, Pei and Modestino [7] have shown the advantages of combining adaptive coding and modulation techniques. This paper is organized as follows: In Section 2, we investigate the H.263 syntactical element sensitivities subjected to memoryless binary channel errors. Section 3 classifies the H.263 syntactical elements into classes of significance and groups them. In Section 4, we present some forward error protection schemes and discuss the advantages and disadvantages of them in real time video coding applications, mainly in terms of source adaptability to the instantaneous wireless channel characteristics. Finally, Section concludes this paper. II. H.263 SYNTACTICAL ELEMENT SENSITIVITIES The H.263 baseline bit stream is divided into several layers, Picture Layer, Group of Blocks (GOB) Layer, Macroblock (MB) Layer and Block Layer, forming 21 syntactical elements supposing no optional modes. As subjecting to errors, each element has a different negative contribution to 0-7803-789-0/02/$17.00 02 IEEE PIMRC 02

the reconstructed luminance PSNR. A binary error generator with a constant Bit Error Rate () is used to corrupt the entire syntactical element under evaluation. All other elements are left error free. The mean luminance (Y) PSNR of the reconstructed sequence is used as a measure of sensitivity. By varying the from -1 to - we obtain a plot for each element. The simulation parameters are the following: test sequences- Miss America and Carphone, at Hz; image resolution- QCIF (176x144); target picture frame rate- Hz; target bit rates- 64 kbit/s and 32 kbit/s; GB distance - 1 slice (11 MB); INTRA picture rate - 0. frame/s (every 2 seconds); quantization parameter for INTRA pictures ; rate control used- TMN-8; annexes- all off; elements TR, E and EOS are left error free. As in H.263 bit stream there exist several syntactical elements and to reduce complexity and granularity, different elements with similar functions were grouped together. Those groups are presented in Table 1. Deep explanation of these elements and their meanings can be found in the H.263 video standard [1]. Table 1 Element grouping. Group name Elements P(first 17 bits) and GB P(last bits) and P, G and D P and G Two simulations were accomplished where the first one evaluated the H.263 syntactical element sensitivities of Miss America video sequence at 64 kbit/s and 32 kbit/s, as shown in Fig. 1. For the second one, the video sequence was Carphone and the results are presented in Fig. 2. From the simulation results, we conclude that the transform coefficients of INTRA frames,, are the most sensitive elements, whereas other types like or are nearly insensitive. Nevertheless, the results show element sensitivity dependence on its relative bit rate, whereby the sensitivity depends on the number of erroneous bits and therefore in the number of bits used to encode each element. For example, decreasing the bit rate obviously leads to less bits representing the transform coefficients of INTER frames () and therefore its significance decreases. Since different simulation parameters (concealment, INTRA refreshment, frame rate, bit rate, quantizer, etc.) lead to different element bit rate distribution, it is straightforward to conclude that the sensitivity also depends on those parameters. Any accurate attempt to classify the syntactical elements in a finite number of classes should depend on the channel as well as the source bit rate as mentioned above. Looking at higher s, the following section orders the elements according to their error sensitivities. - -4-3 -2-1 - -4-3 -2-1 Fig. 1.Miss America sensitivities-a) 64 kbit/s, b)32 kbit/s. - -4-3 -2-1 - -4-3 -2-1 Fig. 2.Carphone sensitivities-a) 64 kbit/s, b)32 kbit/s.

III. DATA GROUPING TECHNIQUE In order to improve H.263 error resilience we propose a data grouping (DG) technique which places together the same syntactical elements of each MB in the GOB. However, this technique does not use partitioning markers to separate each partition as in MPEG-4 [8,9]. Thus, unlike MPEG-4, DG does not introduces any additional bit rate overhead due to partitioning markers but nevertheless, it cannot take advantage of Reversible VLC codes (RVLC). As expected in the conventional H.263 decoding method, if errors are detected within a GOB, the decoder starts searching the synchronization marker of the next GOB. However, due to the DG, more valuable data can be recovered because errors in elements occurring close to the end of the GOB cannot corrupt the elements syntactically placed before. Therefore, placing the most important syntactical elements at the beginning of the GOB can increase error resilience. For example, in INTRA pictures placing all the elements at the beginning of GOB prevent any error occurring in, and TCOEF- I to corrupt. Thus, it is possible to reconstruct a low quality picture even if all other elements in the GOB were corrupted. For INTER pictures, placing and elements at the end of GOB allow minimizing crucial data errors for motion compensation such as elements. Our element sensitivity study of the H.263 syntax also led us to place the most sensitive ones at the end of the GOB. The adopted DG technique is presented in Fig. 3. As D elements are fixed length coded, any error occurring on them will not affect decoding. This technique is quite different from another proposed by Navarro and Tavares [] which separates the H.263 bit stream into INTRA and INTER GOBs. Although that separation does not lead to any error resilience improvements by itself, it is suitable for designing UEP schemes. Furthermore, a conventional H.263 compatible decoder can be used. INTRA Blocks a) H.263 syntax of an INTRA MB D INTER Blocks b) H.263 syntax of an INTER MB c) Data grouping element organization of an INTRA GOB D d) Data grouping element organization of an INTER GOB Fig. 3. Comparison of bit stream syntax (groups are indicated by bold type) We repeat the sensitivity experiments for this particular DG scheme for Miss America and Carphone sequences. The Miss America results are presented in Fig. 4 for 64 kbit/s and in Fig. for 32 kbit/s. As can be observed, is no longer the most sensitive syntactical element. Due to the DG, the sensitivity of, and are greatly reduced. There is also a considerable reduction in sensitivities of,, and. Elements such as,,,,,, and are nearly unaffected. Similar results were obtained for Carphone. - -4-3 -2-1 Fig. 4. Miss America-H.263+DG syntactical element sensitivities at 64 kbit/s. - -4-3 -2-1 Fig.. Miss America-H.263+DG syntactical element sensitivities at 32 kbit/s. Miss America: H.263 H.263-DG Carphone: H.263 H.263-DG - -4-3 -2-1 Fig. 6. Miss America and Carphone- Error resilience comparison of H.263 and H.263+DG. Due to the sensitivity decrease of most of the elements, the H.263+DG technique is more error resilient than H.263 standard. Figure 6 presents the reconstructed mean luminance PSNR versus for H.263 and H.263+DG bit streams. The performance is almost equal for less than

- and greater than -2. The proposed technique shows its best performance for the middle range. For instance, at of -3, 6.42 db and 4.62 db PSNR gains are achieved for Miss America and Carphone video sequences respectively. The maximum PSNR gain is 7.43 db for Miss America video sequence at 4x -4. Subjective results are presented in Fig. 7. depends on the of the channel. The source coding is modified accordingly to maintain a constant 64 kbit/s transmission bit rate. Table 3 presents several protection schemes evaluated. Table 2 Definition of classes. 1,,,,, 2,, 3, 4,,, Table 3 Schemes used for H.263+DG bit stream protection. Fig. 7. Frame 0, H.263 (left), H.263+DG (right). Miss America, =4x -4. Carphone, =1x -3. IV. FORWARD ERROR PROTECTION In the last section, a video data grouping was devised in order to design UEP schemes. The channel codes used in our simulations are Rate Compatible Punctured Convolutional (RCPC) codes introduced by Hagenauer [11] which are based on a convolutional code of rate 1/4 punctured to produce higher rate codes. Those codes fit our needs in allowing several UEP and EEP schemes. In the simulations performed, it was used a maximum likelihood Viterbi decoder with hard decision. We present two approaches for forward error protection of the H.263+DG bit stream. They are based on Equal Error Protection (EEP) and Unequal Error Protection (UEP) schemes. In UEP, the channel protection is applied judiciously depending on the significance of the source bits. From the results discussed in the previous section, we came up with 4 classes of sensitivity. The classes are presented in Table 2. 1 is the most sensitive and class 4 is the least sensitive. It is assumed that different classes might be transmitted separately and unequal error protection can be applied without any further signaling. Multiplexing and demultiplexing are assumed perfect. For 64 kbit/s overall transmission bit rate, we propose a joint source-channel coding scheme. The level of protection Channel code rate Scheme Source bit rate (kbit/s) 1 2 3 4 NP 64.0 - - - - EEP 1 42.7 8/12 8/12 8/12 8/12 EEP 2 32.0 8/16 8/16 8/16 8/16 EEP 3 21.3 8/24 8/24 8/24 8/24 EEP 4 16.0 8/32 8/32 8/32 8/32 UEP 1 42.7 8/14 8/14 8/11 8/ UEP 2 32.0 8/19 8/16 8/ 8/12 UEP 3 21.3 8/26 8/24 8/23 8/ The results are presented in Fig. 8. As can be observed, the best protection scheme depends on the channel. The unprotected H.263+DG bit stream achieves the best PSNR until 3.x -. For higher s, another protection scheme is more adequate. For channel higher than 4x 3 the PSNR gains achieve more than db over the unprotected scheme (NP). At -2, a PSNR gain of approximately db is obtained by using EEP 2 scheme instead of NP. Figure 9 presents subjective results. By adopting a joint sourcechannel coding scheme, the video quality can be always maintained beyond 31 db over the whole tested range. The PSNR UEP scheme curves have smoother slope as the most sensitive classes are better protected. The PSNR UEP curve is always better than the corresponding EEP curve. However, the PSNR improvements with UEP are not very promising, especially if source bit rate is allowed adaptively changing in real time according to the channel characteristics (). For instance, at.4x -2 where the intersection between EEP 3 and EEP 4 occurs, the UEP 3 scheme gives only a PSNR improvement of 0.77 db. However, this small PSNR improvement will tend to zero as the number of available code rates increases. From Fig. 8, the optimal bit allocation between source and channel encoders can be obtained for both EEP and UEP schemes. However, the source and channel encoders must know the channel conditions. We point out that smoother PSNR transitions can be obtained if more combinations of source and channel coding rates were used. Then, considering a

curve flying over all curves dispatched in Fig. 8 and touching them in the curvature starting points, x -6,1x -3, 1x -2, 4x -2 and 7x -2, we derive an approximate expression of the channel code rate in function of the, given by, R c = -0,0219 (Log()) 2-0,3184 Log() - 0,0738. (1) From a particular, the selected channel code should correspond to one of the available codes, 8/9;8/;8/12;8/14;8/16;8/18;8/;8/22;8/24;8/26;8/28;8/; 8/32, with the closest rate but greater than R c calculated from (1). For example, for =4x -3, R c =0.328 corresponding to an 8/14 code. The H.263 video source bit rate, R s is given by, R s = 64 R c kbit/s, (2) and for this particular example, R s = 36.7 kbit/s. NP EEP 1 UEP 1 EEP 2 UEP 2 EEP 3 UEP 3 EEP 4 - -4-3 -2-1 Fig. 8. Miss America-Forward error protection of H.263+DG bit stream. Fig. 9. H.263+DG technique. Frame 0, = -2. NP(left), EEP 2 (right). V. CONCLUSIONS The H.263 baseline syntax is very vulnerable to channel errors. We have been able to identify the sensitivities of all H.263 syntactical elements at the transmission rates of 32 and 64 kbit/s. The significance ordering is almost independent of the H.263 source bit rate. The most sensitive elements are the AC coefficients of INTRA blocks (). By using a DG technique we improved error resilience, due especially to decreasing, and sensitivities. The DG technique proposed imposes no overhead. We show that it is very important to adapt the bit rate in an optimal way, i.e. allocating channel coding and source coding rates according to the channel conditions. It was also shown that UEP does not yield any noticeable improvement compared to EEP, especially if we operate on the optimal EEP curve. This paper also presents an expression to calculate the optimal channel and source code rates as a function of the channel for an overall transmission rate of 64 kbit/s. AKNOWLEDGEMENTS J. Tavares is sponsored by the Portuguese Science and Technology Foundation, Portugal. The authors gratefully acknowledge this support. REFERENCES [1] Telecommunication Standardization Sector of ITU, Video coding for low bitrate communication, ITU-T Recommendation H.263, February 1998. [2] S. Wenger, et al, Error Resilience Support in H.263+,IEEE Trans on Circuits and Systems for Video Technology, pp. 867-877, Nov. 1998. [3] Telecommunication Standardization Sector of ITU, Multiplexing protocol for low bit rate multimedia communication, ITU-T Recommendation H.223, 1996. [4] Telecommunication Standardization Sector of ITU, Packet based multimedia communications systems, ITU-T Recommendation H.323, February 1998. [] C.W. Yap, et al, Error Protection Scheme for the Transmission of H.263 Coded Video over Mobile Radio Channels, Proc. SPIE Conf. on Visual Communication and Image Processing, pp. 1241-1249, 1997. [6] M. Bystrom and J. W. Modestino, Combined Source- Channel Coding for Transmission of Video Over a Slow- Fading Rician Channel, Int. Conference on Image Processing, 1998. [7] Y. Pei and J. W. Modestino, Multi-layered Video Transmission over Wiereless Channels Using an Adaptive Modulation and Coding Scheme, Int. Conference on Image Processing, pp. 09-12, 01. [8] Raj Talluri, Error-Resilient Video Coding in the ISO MPEG-4 Standard, IEEE Communications Magazine, pp. 112-119, June 1998. [9] L. Ducla-Soares and F. Pereira, Error Resilience and Concealment Performance for MPEG-4 Frame-based Video Coding, Image Comm., pp. 447-472, 1999. [] A. Navarro and J. Tavares, Statistical Analysis of H.263 Video Coding and Forward Protection in Error Prone Channels, Int. Conference on Signal Processing Applications and Technology, 00. [11] J. Hagenauer, Rate-Compatible Punctured Convolutional Codes (RCPC Codes) and their Applications, IEEE Transactions on Communications, vol. 36, no. 4, pp. 389-0, April 1988.