## 2 Carrier Aggregation as a Diversity Technique  
When CA is utilised, the data stream from the user application layer is divided into a number of sub-streams at the medium access control (MAC) layer and each sub-stream is transmitted through a different CC at the physical (PHY) layer. In this process, the PHY layer divides and maps higher-layer data onto physical resources while the MAC layer is in charge of the scheduling and resource management processes, with the radio resource control (RRC) layer controlling and configuring the aggregated CCs. At the receiver side, the data of the different sub-streams are recombined (aggregated) at the MAC layer and passed forward to the higher layers, as illustrated in Figure 1. Thus, multiple chunks of (possibly non-contiguous) spectrum can be combined in a transparent manner so that they are effectively perceived as a single (larger) block of spectrum by the higher layers of the protocol stack. This enables the network to simultaneously transmit and receive data across multiple CCs, thereby enhancing data rates, increasing capacity, and optimising network performance. Since the user's main data stream is split into sub-streams at the MAC layer, the sub-streams transmitted through each CC will run a separate instance of the MAC and PHY layers and their associated processes. Consequently, each sub-stream will run an individual, dedicated instance of packet scheduler, HARQ retransmission process, transmission power control, dynamic adaptation of the modulation and coding scheme, and so on. As a result, these processes are dynamically adapted and optimised individually to the instantaneous channel quality conditions experienced in each CC. This observation suggests the possibility of utilising CA as a way to exploit the frequency diversity obtained through the use of different CCs. In general, the transmission through each CC will experience different propagation conditions due to frequency diversity, and each CC can, therefore, be seen as a different diversity path. By transmitting the user data through multiple CCs, frequency diversity can be exploited. In order to employ CA as a diversity technique, a sufficiently large block of spectrum is divided into a number of sub-blocks, each of which is employed as a CC and combined via CA. The user data stream is then divided into the same number of data sub-streams when transmitted through the set of spectrum sub-blocks (CCs) via CA. For instance, a mobile operator with a spectrum block of 20 MHz could exploit the existing block of 20 MHz, not as a single carrier of 20 MHz, but instead as two CCs of 10 MHz each, or four CCs of 5 MHz each, which would then be combined via CA. The number of possibilities is actually much larger, in particular if the new bands introduced for 5G are considered. In Frequency Range 1 (FR1) [23], a block of 100 MHz could be exploited based on the proposed framework as 2 CCs \(\times\) 50 MHz, 4 CCs \(\times\) 25 MHz, 5 CCs \(\times\) 20 MHz, or 10 CCs \(\times\) 10 MHz (the option of 20 CCs \(\times\) 5 MHz is not allowed by the standard given that up to 16 CCs can be aggregated). In Frequency Range 2 (FR2) [24], a block of 400 MHz could be exploited as  
Figure 1: Transmission and reception with carrier aggregation. The data stream from the user application layer is represented by the solid blue line, while the sub-streams into which it is divided are represented by the dashed blue lines. PHY: physical layer; MAC: medium access control; RLC: radio link control; PDCP: Packet Data Convergence Protocol; RRC: radio resource control; HARQ: hybrid automatic repeat request; AMC: adaptive modulation and coding. 2 CCs \(\times\) 200 MHz, 4 CCs \(\times\) 100 MHz, or 8 CCs \(\times\) 50 MHz. Many other combinations are also possible following a similar principle. Note that this is different from the use of CA in classical sense. The traditional definition of the CA concept is to enable the incorporation of new additional frequency bands as a way to increase the data rate.