Contents I. Introduction
With a successful launch of 5G services around the world, the academia and industry have commenced with the visioning and planning of 6G networks [1], [2]. Aiming to provide ultra-reliable and high-rate links with extremely low end-to-end latency, a great many researches have been carried out to improve the spectral efficiency. Among them, the permutation-based transmission is a competitive concept, which utilises the indices of activated resource units, including transmit antennas, spectrum bands, and time slots, to convey extra information bits [3], [4]. In this way, the delivery of those extra information bits does not consume any communication resource, which greatly increase the spectral efficiency at low cost of hardware complexity. The idea of permutation-based transmission was initiated at the physical layer [5]–[7], which has been recently developed at the transport layer. In [8], a portion of application-layer data, referred to as opportunistic bits, are not encapsulated into the packet but mapped onto the index of the time slot when the packet is transmitted. The performance of this scheme adopted within short-packet communications has been analysed in [9]. Moreover, a portion of application-layer data is conveyed by the permutation of various packet lengths in a group of packets, rather than encapsulated into the packets to improve the goodput and reduce the latency [10]–[12]. Further, in [13], two packets are grouped if they have a common portion of application-layer data, which allows this portion to be removed from one of the packets.
