## 1 Introduction  
In an increasingly connected world, where information flows at unprecedented speeds, the demand for seamless and lightning-fast mobile communications has become paramount. Mobile networks have evolved from being simple voice communication platforms to sophisticated data networks that can be employed to deliver a wide and heterogeneous range of services, from video streaming and online gaming to remote healthcare and autonomous vehicles [1; 2; 3; 4; 5]. Since the introduction of the first data services in mobile networks, the evolution of mobile communication technologies has been mainly characterised by an exponential growth in data-intensive applications and the emergence of transformative technologies for achieving higher data rates in mobile communication systems [6]. The advent of 5G/6G introduced a paradigm shift in the way service requirements are perceived and prioritised, by means of encompassing a broader vision beyond mere data throughput and recognising that different applications have diverse needs that extend beyond raw data speed. Nevertheless, achieving higher data rates in mobile communication systems undoubtedly continues to be crucial for future services [7]. Carrier aggregation (CA), a key technology in modern mobile communication systems, emerged as a response to the ever-increasing demand for higher data rates and improved network capacity. Initially introduced as part of the 3 GPP specifications for 4G LTE-Advanced [8; 9; 10], CA has become an integral part of subsequent mobile communication standards, including 5G/6G and beyond [11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. CA allows mobile devices to simultaneously use multiple frequency bands, or component carriers (CCs) in 3 GPP terminology,effectively combining the available bandwidth and boosting the overall data throughput. By aggregating multiple carriers, mobile operators can efficiently utilise their spectrum resources, enhance the user experience by enabling faster download and upload speeds as well as reduced latency, seamlessly support bandwidth-intensive applications by providing the necessary data rates, and improve the overall network performance. Despite being introduced over a decade ago, CA continues to be a relevant technique in mobile communications and an active area of research. Several aspects of CA have recently been explored such as its delay performance [12], energy efficiency [12; 13; 14; 15], spectrum orchestration [16], integration with O-RAN [17], integrated sensing and communications [18], sidelink communications [19], and THz communications [20]. The classical definition of the CA concept assumes that higher data rates are enabled through the aggregation of additional spectrum. In this work, a different approach is considered, where CA is employed to increase the data rates without requiring additional spectrum (the gain is obtained from a higher spectral efficiency). To this end, the proposed approach is to artificially divide an available block of spectrum (that would otherwise be used as a single carrier) into a number of sub-blocks, each of which is treated as a separate CC. The set of CCs into which the available spectrum is divided is then aggregated via regular CA. The motivation for this approach is to benefit from the frequency diversity available in frequency-selective channels with a sufficiently large bandwidth. Such frequency diversity can be exploited via CA because the data transmitted through each CC undergoes an individual instance of the MAC and PHY layers and their associated processes, which are individually adapted and optimised for each CC. Thus, CA can be effectively utilised as a diversity technique. Frequency diversity has traditionally been exploited by means of PHY-layer techniques, mainly through diversity receivers such as maximum-ratio combining or selection combining [21].