video source coder with unequal error protection (UEP) ac- ... work, we concentrate attention on a 2-layer variable bit rate .... sponding residual bit errors.
MULTI-LAYERED VIDEO TRANSMISSION OVER WIRELESS CHANNELS USING AN ADAPTIVE MODULATION AND CODING SCHEME
I: Pei and J. U! Modestino Center for Image Processing Research Electrical, Computer, and Systems Engineering Department Rensselaer Polytechnic Institute, Troy, New York 12180, USA ABSTRACT In this paper we describe a multi-layered video transport scheme for wireless channels capable of adapting to channel conditions in order to maximize end-to-end quality of service (QoS). This scheme combines a scalable H.263+ video source coder with unequal error protection (UEP) accross layers. The UEP is achieved by employing different channel codes together with a multiresolution modulation approach to transport the different priority layers. Adaptivity to channel conditions is provided through use of joint source-channel coding (JSCC) which attempts to jointly optimize the source and channel coding rates together with the modulation parameters to obtain the maximum achievable end-to-end QoS for the prevailing channel conditions. Results indicate that this adaptive JSCC scheme employing scalable video encoding together with a multiresolution modulatiodcoding approach leads to significant improvements in delivered video quality for specified channel conditions. In particular, the approach results in considerably improved graceful degradation properties for decreasing channel S N R . Keywords: Video transport, unequal error protection, scalable video coding, joint source-channel coding, MPSK, adaptive modulation and coding. 1. INTRODUCTION In this paper we describe and investigate an adaptive wireless video coding and delivery system which combines a scalable video codec with UEP across layers achieved through a combination of FEC and use of multiresolution modulation schemes using nonuniform MPSK signal constellations. Figure 1 illustrates the video coding and wireless delivery scheme proposed and investigated in this paper. In our work, we concentrate attention on a 2-layer variable bit rate This work was supported in part by CenSSIS, the Center for Subsurface Sensing and Imaging Systems, under the Engineering Research Centers Program of the National Science Foundation under award num ber EEC-9986821.
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(VBR) H.263+ codec making use of signal-to-noise (SNR) scalability [I]. A scalable H.263+ source coder encodes the input video into two layers, a base layer (Base) carrying the most important information and an enhancement layer (Enh)carrying the less important video information which, in turn, provides two VBR video streams with different priorities. The differential importance of encoder output components from different layers to the reconstructed video quality will be the basis for the proposed prioritized video delivery scheme. The same scalable H.263+ source coder can also be used as a single-layer VBR H.263+ coder together with a single-layer JSCC delivery scheme. This optimized single-layer system will be used as a baseline for comparison purposes. For the %layer system, before the layers are transmitted, they are protected against channel errors according to their relative importance. A set of binary RCPC codes [2] are employed on both layers for forward error correction. The channel coding rates can also be selected adaptively for both the base and enhancement layer based on the channel conditions. Then, the two video streams are modulated by using nonuniform MPSK signal constellations [31 where the data from the base layer are mapped to the coarse resolution
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Fig. 2. Adaptive Nonuniform 8-PSK signaling constellation. layer of the signaling constellation while the data from the enhancement layer are mapped to the finer resolution layer of the signaling constella.tion. Figure 2 demonstrates the adaptive nonuniform 8-PSK signaling constellation'. Finally, the modulated signals are transmitted over a wireless link. During transmission, the modulated bitstreams typically undergo degradation due to AWGN, intra andor interchannel interference and possibly fading, although in this paper we model the channel simply as an AWGN channel. At the receiver side, the received waveforms are demodulated and channel decoded, and then source decoded to form the reconstructed video sequence. The reconstructed sequence may differ from the original sequence due to both source coding errors and possible channel error effects.
values of the source and channel coding rates, R, and R,, respectively, through JSCC subject to the overall transmission rate R,+c, according to the channel state information (CSI)*.As the channel conditions change, these parameters are adapted to provide the best end-to-end quality of the delivered video measured as the reconstructed P S N R , subject to the overall bit budget. The constrained maximization over 6' determines the optimum choice of 6' as a function of Es l h r I . 3. RESULTS AND DISCUSSION
A block diagram of the p~oposedadaptive modulatiodcoding system is illustrated in Fig. 3. The source encoder encodes the input video into either a single or dual streams. In either case, channel coding is provided by an RCPC channel encoder(s). The encoded messages are then mapped to the nonuniform 8-PSK signaling constellation as described in Section 1. We model the sum of interference and noise as stationary AWGN with one-sided spectral density N I . If E s is the energy per symbol, then 13s/Arr determines the error probability for both layers. Mere specifically, for a fixed value of EsJNl,the probability cf error for the base layer increases as the offset angle 8 is incaeased, while the probability of error for the enhancement layer decreases as the offset angle 8 is increased. As illustrated in Fig. 3, adaptation is accomplished by adaptively adjusting the offset angle 8, switching the encoder on or off for the enhancement layer, and choosing the
We present some selected results for a representative QCIF video-conferencing sequence, Susie at 30 fps. The symbol transmission rate in Fig. 3 is set to be rs = 128 K s p s . For a single-layer system, if uniform QPSK modulation is used, the message bitrate (after channel coding) is r,+, = 256 Kbps; if uniform 8-PSK modulation is used, r,+c = 384 K b p s . For a 2-layer system employing nonuniform 8PSK modulation, the message bitrate (after channel coding) for the base layer is rp2c = 256 Kbp.9, while for the enhancement layer = 128 Kbps. We first evaluate the performance of a single-layer system without channel coding and using uniformMPSK modulation. The results are demonstrated in Fig. 4 for hl = 4 (QPSK) and M = 8 (8-PSK). As expected, QPSK shows better performance in the range of lower EsJN1;however, as channel conditions improve (i.e., E s / N l increases) the P S N R will saturate quickly for QPSK which makes the . the other hand, system very inefficient for large E s / A r ~On 8-PSK will provide better efficiency for large Es/AT1by allowing larger r,+,, but at the expense of poorer performance as Es/hrl decreases compared to QPSK. Intuitively, a simple adaptive scheme could be devised to switch between the QPSK and 8-PSK based on the different values of
'In what follows we restrict attention to M = 8 although the approach is applicable to arbitrary M = T , rn > 1.
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Fig. 4. PShrR as a function of Es/hrl in d B for singlelayer schemes employing uniform MPSK: QPSK and 8PSK, without channel coding. Fixed symbol transmission raters = 128 Ksps.
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Fig. 5. P S N R as a function of ES/hr1 in d B for 2-layer system with adaptive modulation scheme without channel coding. Fixed symbol transmission rate rs = 128 Ksps.
I ' Es/hTI.This scheme will provide performance which follows the upper envelope of the two curves shown in Fig. 4. Instead, if adaptive nonuniform 8-PSK modulation is employed combined with a 2-layer source coding scheme for the uncoded system, we expect to get improved performance in the transition area between QPSK and 8-PSK for an uncoded system. The results are demonstrated in Fig. 5. As can be seen, the adaptive 2-layer nonuniform 8-PSK modulation scheme demonstrates an advantage in keeping the performance at acceptable levels for the lower E s / A rby ~ reverting to a QPSK (0 = 0) modulation scheme, then as Es/hrI increases to approximately 18.5 d B , the enhancement-layerdata can be used to improve the performance. Further increase in Es/hrI causes 0 to increase resulting in a decrease in the bit error rate for the enhancement layer. As Es/hrI becomes large enough, the performance saturates at a level slightly below that of the single-layer system using uniform 8-PSK (0 = x/8) at large Es/hrI. This gap, due to increased overhead, is the penalty to be paid for 2-layer scalable source coding compared to singlelayer source coding. In particular, this performance gap is why we provide a switch in the adaptive modulatiodcoding scheme to revert to a single-layer source coding scheme for large Es/hrI. As Es/hrl becomes large enough, the adaptive nonuniform 8-PSK modulation scheme reverts to a conventional uniform 8-PSK (0 = x/8) modulation scheme supporting a single-layer encoder. So we see that by adjusting 0 adaptively, it provides a more graceful degradation pattern compared to the single-layer system with uniform modulation schemes. This indicates that if CSI is available to the transmitter, the 2-layer encoding scheme employing adaptive nonuniform modulation can be used to obtain a considerable performanceimprovement in the quality of the delivered video in the transition region. In [4], [5J,we demonstrated the advantagesof FEC schemes employing JSCC to improve the overall performance
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Fig. 6. P S N R as a function of Es/hrl in d B for singlelayer JSCC scheme with uniform QPSK. Fixed symbol transmission rate r s = 128 K s p s .
of video delivery. Here we first applied JSCC to singlelayer uniform modulation schemes. The results for a selected subset of RCPC channel codes used together with QPSK is demonstrated in Fig. 6. It is clear that the lower the channel code rate R, the better the performance for the smaller values of Es/hrl because the more powerful channel codes employed can substantially decrease the corresponding residual bit errors. However, as the channel quality improves (i.e., Es/hrr gets larger), the scheme using the more powerful codes will suffer a quality loss since for a given r s (e.g., rs = 128Ksps in this work) the corresponding source coding rate R, must be reduced to accommodate the channel coding overheads. This results in poorer reconstructed video quality for an error-free situation. Through a JSCC approach which adaptively chooses the channel codes based on the CSI, we can achieve a much more graceful degradation pattern as shown by the solid curve in Fig. 6 representing the convex hull of all operating points. The same property holds for the single-layer 8-PSK schemes. If we put the results obtained so far together, we can compare the performance between a single-layer uncoded system and a single-layer system employing FEC codes in conjunction with JSCC as shown in Fig. 7. The schemes
4. CONCLUSION
Fig. 7. Comparison of :ISCC coding schemes and uncoded schemes for a single-layer system. Fixed symbol transmission rate rs = 128 Ksps.
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We have described and investigated a wireless video coding and delivery system which combines a scalable video codec with unequal error protection (UEP) across layers through a combination of FEC and multiresolution modulation schemes using nonuniform MPSK signal constellations. The results clearly demonstrate that FEE is required to maintain the video quality at an acceptable level for relatively small values of Es/ArI. Furthermore, in order to maintain the video quality at acceptable levels over a relatively wide range of Es/ArI (i.e., the case for typical timevarying wireless links), JSCC is required to adaptively choose source and channel coding rates based on CSI, in order to protect the data against channel errors while operating within a fixed bandwidth allocation. Finally, adaptive modulatiodcoding schemes can be used to obtain improved performance for smaller ES/ArI,while allowing higher throughput for larger ES/hTI. More specifically, 2-layer adaptive modulatiodcoding schemes can provide much more graceful degradation characteristics between these two extreme ranges of ESlIVI. Hence, multilayered video encoding and delivery with adaptive modulatiodcoding approaches, such as described here, should provide a significant system advantage for future wireless multimedia transmission systems.
Fig. 8. P S N R as a function of Es/ATI in dB for 2-layer adaptive modulation and coding scheme. Fixed symbol transmission rate TS = 128 K s p s . employing JSCC show a large gain in energy efficiency compared to the uncoded :system for both uniform QPSK and 8-PSK. Finally, we investigate the performance of our proposed adaptive 2-layer modulatiodcoding scheme employing JSCC compared to those using only single-layer coding and uniform MPSK either with or without JSCC. The results are demonstrated in Fig. 8. We see that for lower values of E,y/ArI (e.g., Es/hTI 5 8 dB), the adaptive scheme performs essentially the same as single-layer coding with JSCC and uniform QPSK. On the other hand, for larger values of ES/ArI (e.g., Es/hTf 2 15 dB), the adaptive scheme performs essentially the same as single-layer coding with JSCC and uniform 8-PSK. However, in the intermediate transition range (e.g., 8 d B < iYs/hrI ‘< 15 d B ) , the 2-layer adaptive scheme demonstrates a significant advantage and provides a much more graceful performance degradation pattern achieved by means of adaptively adjusting the modulation parameter 8 toj;ether with the use of JSCC. Specifically, as shown in the figure there is a gain of approximately 1.8 dB in Es/ArI for a fixed quality level P S h T R= 37 dB . This improvement in energy efficiency can lead to a significant improvement in overall system capacity.
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5. REFERENCES [ l ] ITU-TISG15, “Video Coding for Low Bitrate Communication,’’ Draft RecommendationH.263 version2, Sep.
1997. [2] J. Hagenauer, “Rate-Compatible Punctured Convolutional Codes (RCPC Codes) and their Applications,” IEEE Trans. Commun., vol. COM-36, no. 4, pp. 389400, April 1988. [3] M.B. Pursley and J.M. Shea, “Adaptive signaling for multimedia transmission in CDMA cellular radio systenq,” in Proc. 1998 IEEE Military Commun. Con5 Oct. 1998, vol. ,pp. 113-1 17, Boston, MA.
[4] Y. Pei and J.W. Modestino, “Multilayered H.263+ Video Encoding and Delivery over an AWGN Channel: a Joint Source-Channel Coding Approach,” Submitted to IEEE Trans. on Image Pmc., 2000.
[S] M. Bystrom, J.W. Modestino and Y. Pei, “Combined Source-Channel Coding for Wireless Transmission of H.263 Coded Video,” in UCSD Conference on Wireless Communications.Feb. 1999,pp. 36-49, SanDiego, CA.
