Joint source-channel decoding and unequal error protection aided video transmission

Abstract

Shannon’s source and channel-coding separation theorem has been the basic philosophy followed by most state-of-the-art wireless transceivers, where all source signals are assumed to have the same importance and are protected equally by the channel encoder. However, due to its assumption of using idealized Gaussian channels, potentially infinite encoding/decoding delay as well as complexity, Shannon’s source and channel-coding theorem is not strictly applicable in practical wireless scenarios. This is because it is almost impossible to remove all the redundancy of the video source, therefore there exits a certain amount of residual correlation. Moreover, reallife applications are often required to transmit source signals associated with unequal importance, such as the base-layer and enhancement-layer of layered video codecs. Hence joint source-channel coding (JSCC) was proposed for wireless scenarios by jointly exploiting the residual redundancy of the video source and the intentionally imposed redundancy of unequal error protection (UEP) techniques.

A video codec suitable for lossless video compression and iterative source-channel decoding (ISCD) is proposed by further developing the Markov random field (MRF) soft source decoder for the transmission of video sequences, rather than still video pictures. More explicitly, we used low complexity frame-differencing (FD) for removing the temporal-domain inter-frame redundancy. The low-complexity FD technique does not rely on power-thirsty motion-vector-based motion-compensation and as its other benefit, it does not require the transmission of motion vectors. However, this results in more residual redundancy than the more complex motion-vector based motion-compensation. Furthermore, variable-length code (VrLC) is used for removing the residual spatial redundancy of the FD signal, while exploiting the correlations amongst the FD pixels within the current frame with the aid of our MRF model based soft-in-soft-out (SISO) module. By adopting the MRF for modelling of the video pixels, we can infer maximum a posteriori (MAP) image estimates from the 2-D spatial redundancy between the video pixels, while simpler models like the Markov chain can only explore the 1-D spatial redundancy. Although the estimation of the MRF parameters is a challenge compared to the 1-D Markov models, we conceived novel extrinsic information transfer (EXIT)-chart-based estimation methods, which are shown to be effective. Moreover, a novel three-stage ISCD structure is proposed, which outperforms the two-stage architecture. Furthermore, we examined the convergence of the three-stage iterative decoding process using 3D EXIT charts. The performance of our system operating both with and without FD is compared to our benchmarker schemes.

In support of inter-layer forward error correction (IL-FEC) coded layered video transmission, we conceived an adaptive truncated HARQ (ATHARQ) scheme for minimizing the video distortion under the constraint of a given total number of transmission time slots. More specifically, we investigated the merits of IL-FEC schemes in the context of truncated HARQ (THARQ) transmission schemes, where the packet scheduling arrangements were carefully designed for exploiting the specific characteristics of each IL-FEC coded packet. Furthermore, we developed a method of on-line optimization for our IL-ATHARQ transmission scheme, in order to find the most appropriate FEC code rate distribution among the video layers that reduced the video distortion. Type-I HARQ relying on Convolutional Codes (CC) was used for simplicity, because our focus was on the design of the scheduling schemes. The performance of our IL-ATHARQ scheme as well as of the rate-optimized IL-ATHARQ scheme using a RSC codec were compared to the benchmarkers using different video sequences, followed by characterizing both the effects of the delay as well as of the channel quality prediction errors on the attainable system performance.

Finally, we conceived an UEP scheme for layered video transmission in the downlink of a visible light communication system, explicitly, we proposed a hierarchical colour-shift keying (HCSK) modulation scheme based on the standardized colourshift keying (CSK), which is capable of conveying inter-dependent layers of video signals. More specifically, we proposed the Type I and Type II HCSK arrangements, which can be flexibly configured by according to the channel quality, video quality, etc., where the Type II constellation allows us to use a wider range of FERs for the higher layers upon varying the constellation-shaping parameter dl. Our simulation results show that Type II HCSK provides a high grade of flexibility in terms of both its system configuration and optimization. Furthermore, we provided a detailed design example for the employment of HCSK transmitting scalable video sources with the aid of a RSC code. An optimisation method was conceived in order to enhance the UEP and to improve the quality of the received video. The performance of our optimised M-HCSK-RSC video transmission system using different HCSK constellation sizes was compared to the relevant benchmarker schemes using different video sequences.

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Identifiers

ORCID for Lajos Hanzo: orcid.org/0000-0002-2636-5214

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