Rateless Joint Source-Channel Coding, and a Blueprint for 6G Semantic Communications System Design

This paper introduces rateless joint source-channel coding (rateless JSCC). The code is rateless in that it is designed and optimized for a continuum of coding rates such that it achieves a desired distortion for any rate in that continuum. We further introduce rate-adaptive and stable communication link operation to accommodate rateless JSCCs. The link operation resembles a "bit pipe" that is identified by its rate in bits per frame, and, by the rate of bits that are flipped in each frame. Thus, the link operation is rate-adaptive such that it punctures the rateless JSCC codeword to adapt its length (and coding rate) to the underlying channel capacity, and is stable in maintaining the bit flipping ratio across time frames. Next, a new family of autoencoder rateless JSCC codes are introduced. The code family is dubbed RLACS code (read as relax code, standing for ratelss and lossy autoencoder channel and source code). The code is tested for reconstruction loss of image signals and demonstrates powerful performance that is resilient to variation of channel quality. RLACS code is readily applicable to the case of semantic distortion suited to variety of semantic and effectiveness communications use cases. In the second part of the paper, we dive into the practical concerns around semantic communication and provide a blueprint for semantic networking system design relying on updating the existing network systems with some essential modifications. We further outline a comprehensive list of open research problems and development challenges towards a practical 6G communications system design that enables semantic networking. The concepts of semantic and effectiveness communication were raised by W. Weaver in a preface to Shannon's mathematical theory of communication--while referring to Shannon's work as a solution to technical communication problem--as what should come next beyond the technical communication [1]. Specifically, a formal definition of the semantic problem that differentiates it against the technical problem towards a meaningfully different communication networking solution, is not available. The notion of "conveying the desired meaning", as opposed to "accurate reconstruction of bits/symbols", was alluded to by Weaver to differentiate semantic against technical problems. The former is thus seen by the literature mostly as a source coding problem with majority effort focused on lossy joint source-channel coding (JSCC), but the impact on what we call communication network is yet unclear. In source coding, the differences are evident and semantic compression has already provided meaningful engineering solutions: for instance, the hierarchical codecs used for image [7]-[10] and video [11], [12] signals can distinguish between semantic vectors and perceptual elements in the signal and compress them at unequal rates according to their importance in reconstruction loss.