## 1 Introduction
A range of key issues affecting multiple-CC BS packet scheduler designs have, however, been investigated in various studies published in recent years [15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. This research has addressed the two key aims of 5G systems: spectral efficiency [15; 16; 17; 18; 19; 20] and energy efficiency [21; 22; 23; 24; 25]. The performance of both throughput and delay for elastic traffic in systems utilizing the mechanism of multiple-CC transmission was previously analyzed by [PERSON] and [PERSON] [15]. Various scheduling algorithms for CCs and scheduler structures were proposed in papers by, respectively, [PERSON] et al. [16] and [PERSON] et al. [17], while a user grouping-based algorithm for resource scheduling was proposed in a study by [PERSON] et al. [18]. Additionally, a method of minimizing packet transmission delay was proposed by [PERSON] and [PERSON] [19], who described a quantized water-filling packet scheduling scheme with multiple-CC transmission. A study by [PERSON] et al. [20] presented a number of CC allocation schemes addressing uplink environments. In a 2013 study, the issue of how to configure users on a subset of CCs in order to provide energy-efficient transmissions was investigated by [PERSON] and [PERSON] [21], who specifically addressed the issue by considering both CC selection and packet scheduling. Also in 2013, [PERSON] et al. [22] published a study in which they proposed an energy-efficient coordinated scheduling mechanism that could be adapted for use in multi-cell environments. More recently, another study by [PERSON] [23] addressed the problem of minimizing energy at BS transceivers while maintaining required QoS and fairness for all users, using a rate-and-power control scheme for two-CC transmission. A further study by [PERSON] [24] took a system-level perspective to address energy-efficient two-CC transmission, in an extended framework which was proposed and analyzed. [PERSON] [25] next proposed an efficient multi-CC transmission power-saving scheme with greater flexibility than those considered in his previous studies [23; 24] for application to BSs aggregating more than two CCs. However, while the first eight of these previous investigations (i.e., [15; 16; 17; 18; 19; 20; 21; 22]) of multiple-CC BS packet scheduler designs made significant contributions to the literature, they did not address the issue of how systems deal with simultaneous RT and NRT traffic. Consequently, the previous studies may not meet ideal scheduler requirements, since RT and NRT traffic forms differ considerably from each other with regard to both required QoS levels and also traffic characteristics. In addition, however, none of [PERSON]'s schemes [23; 24; 25] addressed in detail the issue of QoS for different kinds of traffic. Thus far, most of the previous studies have not, to the best of our knowledge, given full consideration to the aforementioned challenges in terms of how they might affect packet scheduler efficiency in 4G/5G systems utilizing multiple-CC transmission. Additionally, given the trend among cellular system operators of co-locating the BSs for mobile networks utilizing various CCs [26], the importance of designing efficient packet schedulers for use in such co-location environments will only increase. With these points in mind, we herein propose a novel packet scheduling algorithm for use in such environments with being focused on downlink transmissions, where the simultaneous transmission of both RT and non-RT traffic is supported; more specifically, this algorithm is based on the classic proportional fair (PF) criterion that has been proposed by [PERSON] et al. [27] and ensures that certain resources are used exclusively for the handling of RT packets. Furthermore, the performance of a system utilizing the proposed algorithm is compared with that of a system utilizing the independent CC mechanism, a mechanism in which the CCs are used independently rather than working in concert. The remainder of this paper is organized as follows.