## 1 Introduction  
### Background  
Envisioned and ever-increasing demand for high user data rate and network capacity impose many challenges on existing mobile networks due to the cost of licensing and the scarcity of available radio spectrum. Mobile network operators (MNOs) are facing both technical and business issues, particularly to provide users with high quality-of-service (QoS) at a low cost per bit transmission. Several technologies and techniques have already been proposed in the literature to address both the QoS and the reduction of cost of the use of mobile services. In line with this, instead of dedicating spectrum exclusively to an MNO, the same spectrum is proposed to be shared with multiple players of the same or different systems to improve the spectrum utilization as well to distribute the spectrum licensing cost among the players.  
An important feature of spectrum sharing is spectrum trading involving considerably the commercial aspects in addition to the technical ones [1; 2]. It helps more efficient use of spectrum economically by allowing primary users assigned with licensed spectrum initially to trade all or part of the spectrum with other secondary users. Unlike spectrum sharing where shared users are allowed to get access in a temporary basis to the spectrum of primary users retaining the spectrum license,in spectrum trading, spectrum usage rights are transferred completely to secondary users for a certain period of time [3]. Hence, spectrum trading as a secondary mechanism for assigning the spectrum of primary users has a crucial impact on the overall spectrum utilization efficiency. Spectrum trading may occur due to both macro factors involving external condition changes such as demand, technology and policy, and micro factors such as the economically inefficient assignment of the primary spectrum by company managers, and changes in strategy, service, and other aspects of a company [4]. Irrespective of mechanisms used for the initial allocation of spectrum to primary users, e.g., auction, beauty contests, first-come-first-serve, the rationale to employ secondary spectrum trading to enhance the spectrum efficiency holds.  
However, because of asymmetric and enormous network capacity demand of the future fifth-generation (5G) system, addressing only the spectrum sharing and trading would seem insufficient, which necessitates exploiting other domains of the capacity improvement such as space and spectral efficiency improvement techniques. For the spatial-domain, network densification with small cells, particularly in indoors and hotspots in urban environments within the coverage of a large macrocell, is considered as one of the most effective techniques to serve users within the short distances and low transmit power. Small cells share typically the same spectrum as that of the large macrocell to avoid spending additional spectrum licensing fee. However, one of the major drawbacks of the spectrum sharing is its inherent co-channel interference generated from sharing the common spectrum. Though several techniques have been proposed to address co-channel interference between cells in the literature, the time-domain enhanced inter-cell interference coordination (elCIC) technique employing almost blank subframes (ABSs), primarily proposed for the long-term evolution (LTE) systems, has been considered as one of the most effective ones to address such co-channel interference.