# Joint Cell Association and User Scheduling in Carrier Aggregated Heterogeneous Networks
## 1 Introduction
### Related Work and Motivation  
Both industry and academia are convinced that the HetNets would play an important role in improving the cellular network capacity by extremely dense small cell BS deployments. However, such ultra density has made the network design more complicated in several ways.  
Intercell interference has always been a major concern for modern cellular networks. The LTE-A specifies several strategies for enhanced intercell interference coordination (eICIC), one of which is to explore cooperative [6] and coordinated transmission approaches involved with multiple antenna techniques [7; 8; 9]. Along with the ultra dense small cells deployment, massive antenna regime are considered the most crucial technologies in the 5G mobile systems [10; 11].  
In the HetNets, challenges on interferences both intra-layer and inter-layer should be handled in a more spectrally efficient way that a successful cell deployment must offer a perfect balance between preferred network utility and the scarcity of radio resources. In the literature, a bunch of work has been done for spectral efficiency improvement in the HetNets by interference mitigation approaches [12; 13]. Joint solutions of network MIMO are proved to have great potentials in interference management [14] for higher spectrum utilization.  
To deal with the severe co-channel interference in the HetNets, another straightforward approach for the network design is the joint cell association and user scheduling problems aiming to maximize the overall network utility [15; 16; 17]. Traditional cellular scheduler associated the users with maximal signal to interference plus noise ratio (SINR) provider which may not be optimal, especially when multiple layer of small cells were engaged in the communication network [18; 19; 20]. One direct mend on this dilemma is to take the unbalanced traffic loads between the high transmit power layers and low power layers' BSs into account [21; 22; 23] when formulate the cell association problem based on max-SINR criterion. The small cell BS would benefit from such joint association and scheduling strategies from global optimization to locate the optimal cell load [24]. Moreover, game theory approaches [25; 26] were also proved to offer the best negotiation on the cell range expansion balance among BSs with different transmit power level.  
Most works in the literature are based on the assumption that all users associated on the same BS would share the power and other radio resources equally [27]. Full frequency reuse policy suffered heavily due to the dense deployment of small cells and the complicated interferences in the HetNets, where the spectral efficiency was already in jeopardy. In order to support high data rate and overcome the challenges on the spectrum scarcity in the same time, carrier aggregation (CA) has been introduced into future networks design [28; 29]. Different frequency bands are involved in the downlink transmission of the HetNet with different coverage abilities and different propagation characteristics [30]. Under such circumstances, the joint problem of cell association and user scheduling in the HetNets would be more flexible if the users could be engaged with more than one carrier band with proper carrier aggregation strategies [31; 32; 33].