## 2 Carrier Aggregation as a Diversity Technique
On the other hand, the idea of CA as a diversity technique considered in this work does not require the addition of new spectrum; instead, it simply divides an existing block of contiguous spectrum into sub-blocks, treating each sub-block as a CC with a separate data sub-stream, and recombines the data sub-streams transmitted through each CC via CA. Note that in the proposed approach the use of CA would in principle not be needed; the operator could choose to transmit using the available block of spectrum as a single channel but, instead, artificially divides the available spectrum into a number of CCs that are recombined via CA in order to benefit from the frequency diversity that would normally be expected in channels with a sufficiently large bandwidth. Such frequency diversity can be exploited via CA because, as stated above, the data transmitted through each CC undergoes an individual instance of the MAC and PHY layers and their associated processes. Consequently, the parameters of the packet scheduler, HARQ retransmission process, transmission power control, dynamic adaptation of the modulation and coding scheme, etc., can be adapted and optimised individually for the instantaneous channel quality conditions of each CC. This individual adaptation and optimisation results in an improved capacity. Conversely, if the available spectrum is exploited as a single channel, then a single instance of the MAC/PHY layers and their processes is run and the selected operation parameters are unlikely to be optimum for the various instantaneous conditions experienced through the whole range of frequencies within the bandwidth of a frequency-selective channel, and as a result a lower capacity is obtained in this case. Thus, by artificially forcing separate data streams via different CCs in the available spectrum, the aim is to exploit the diversity in each frequency interval. As a result, in this context, CA can be effectively employed as a diversity technique to improve the overall performance without requiring additional spectrum. This performance improvement can be obtained regardless of any other diversity techniques that may be implemented in a mobile communication system, for example, at the physical layer. Note that CA was originally proposed by 3 GPP as a technique to increase data rates by incorporating additional bandwidth [8, 9, 10]. On the other hand, the framework proposed in this work as discussed above does not require additional bandwidth since it simply splits the existing spectrum into smaller blocks in order to benefit from frequency diversity, thus effectively exploiting CA as a diversity technique. Therefore, while traditional CA aims to increase the capacity by increasing the available bandwidth (i.e., Hz), the proposed approach aims to increase the capacity by increasing the spectrum efficiency within the available spectrum (i.e., bit/s/Hz). This is a novel point of view for CA that, to the best of the authors' knowledge, has not been considered by other authors before. It is worth noting that the CA-based transmission scheme considered in this work does not involve a higher level of complexity compared to the traditional use of CA, both from the hardware and software points of view. The hardware complexity of CA is determined by the type of frequency deployment of CA, namely (in increasing order of complexity), intra-band contiguous CA, intra-band non-contiguous CA, or inter-band CA. The proposed CA-based scheme falls within the category of intra-band contiguous CA, which is the simplest form of CA with the lowest level of hardware complexity in the radio frequency front-end since all CCs are contiguous and belong to the same band, so they use the same numerology and transmission configuration. From the software point of view, the method considered in this work does not involve additional complexity compared to the traditional use of CA.