ITU - Telecommunications Standardization Sector
STUDY GROUP 16
Video Coding Experts Group (Question 15)
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Sixth Meeting: Seoul, Korea, 3-6 November, 1998	Document  Q15-F-38
Filename: q15f38.doc
Generated: 31 Oct ’98	

Question:	Q.15/SG16	
Source:	Stephan Wenger
TU Berlin, TELES AG, and UBC SPMG
Franklinstr. 28-29
D-10587 Berlin
Germany	
Tel:
Fax:
Email:	
+1-415-297-2063
+1-604-736-7853
stewe@cs.tu-berlin.de	
Title:	Proposed changes for mobile test model	
Purpose:	Proposal	
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Summary
Video coding for error prone mobile channels, using H.223/M as a transport, needs a high amount of error resilience.  While there might be even better mechanisms within the syntax constraints of the current H.263 recommendation, a sufficiently high amount of intra macroblocks in the coded bitstream already yields acceptable quality for many of the error patterns we used n our experiments.  The distribution of those intra macroblocks can be randomly chosen, but can also be determined in a more sophisticated way.  Attached to this contribution is a draft of an academic publication, which outlines a RD-based algorithm for that purpose.  We propose to use either that RD-based mode decision mechanism, or to leave out RD optimization out all.
In addition to this substantial change we propose a few minor changes, mostly to reduce the number of permutations of to-be computed bitstreams and to fix problems with some of the error patterns.
RD optimized error resilient coding
Overview over the algorithm
The proposed RD optimization coder algorithm expects knowledge of the error concealment of the decoder (in our case TCON), and the probability of GOB-losses due to channel errors.  When using this algorithm in conjunction with a TCON-capable decoder and the knowledge of GOB losses, we achieve up to 3 dB higher performance than when using a reasonably chosen, fixed intra update pattern, at the same bit-rate.  But even if the GOB-loss probability is not exactly known, results are significantly better compared to the fixed update pattern defined in Q15-E-51.
For a detailed discussion of the algorithm itself, its complexity constraints and its performance gains, please refer to the attached paper.  The algorithm can be outlined as follows: When a GOB is lost (either because of a packet loss of H.223, or because the CRC check of that packet shows negative results and the content of the packet (at most one GOB) is therefore not used, 
Interference of RD with rate control
This algorithm is subject to the same interference of RD and rate control as reported in Q15-F-35.  Even considering this problem, the algorithm yields, unlike ‘usual’ RD optimization, still substantial additional performance.  Hopefully a solution for the general RD/RC interference problem will be reached, from which this algorithm would benefit, too. 
Alternative Proposal 1
With respect to the test model, we propose to:
•	Include the described algorithm into the mobile test model,
•	Generate anchors based on this mechanism 
In addition we propose to:
•	Add a codepoint into H.263++ annex L to signal the use of this algorithm.
Furthermore we will, within Q.11, argue for a similar codepoint in the H.245 once H.263++ is on its way.
Arguments pro: best possible performance based on current syntax.  Contra: high complexity of (any) RD algorithm does not reflect the power consumption constraints of a mobile terminal.  
Alternative Proposal 2
We propose to
Use anchors as generated by UCLA, which means D, F, I, J, T, but no RD optimization.
Arguments pro: anchors available, low complexity matches mobile demands.  Contra: not best possible performance within the syntax constraints of H.263V2. 
Minor issues
Error patterns
Currently 6 error patterns are mandated for core experiments for both 32 kbit/s (Raleigh, DECT) and 64 kbit/s (WCDMA).  Discussions of representatives of some interested parties showed that out of those 12 error patterns 8 seem to be redundant.  In particular:
•	The 4 dB Raleigh patterns do not allow for a reasonable picture quality based on all currently evaluated mechanisms.
•	The 10 dB and 20 dB Raleigh patterns are available at two different Doppler speeds.  For both Doppler speeds, comparable results are achieved.
•	The same is the case for the three different Doppler speeds of the WCDMA patterns,
We therefore propose to delete the 14 km/h Raleigh patterns, as well as the 3 km/h and 40 km/h WCDMA patterns and mandate only the following patterns:
1.	dect/1_4/ray0_10.seq
2.	dect/1_4/ray0_20.seq
3.	wcdma/wcdma-64kb-211hz-3
4.	wcdma/wcdma-64kb-211hz-4
Raleigh pattern initial burst
It was found that there is an initial burst of 80 bytes of erroneous data.  Per reflector discussions all involved groups moved this data to the end of the error pattern file.  We do not propose to skip the full 80 burst, because that would have significant impact to the BER of the whole error pattern, once we use a larger number of pictures (as proposed in the next point).  
Length of sequences
In Whistler we decided for a length of 900 coded frames.  Discussions between UBC/TELES and UCLA/Samsung personnel lead to consensus between the two groups that we need longer sequences for eliminating statistical errors.  It was found that, although the algorithm descriptions are very similar, UCLA got for some sequences about 2.5 dB worse results than UBC.  While there is always the possibility of a bug in one of the various pieces of software, we currently think that this is the result of the negative impact of several dropped frames due to errors in their picture header.  This might have happened because of some form of alignment of the error bursts to the picture header for the data from UCLA, which was not observable in the data of UBC.  If the observed behavior is the result of a statistical error then this discrepancies could be minimized in the future by using longer sequences.
We therefore propose to use at least 4000 instead of 900 coded frames.