# A Survey of Resource Allocation Techniques for Cellular Network's Operation in the Unlicensed Band
## 4 Channel Selection for LTE Operation in the Unlicensed Band  
The objective of channel selection is to choose one or more 20 MHz channels for LTE operation in the unlicensed band that would cause least interference to the existing devices operating in these channels [43]. The channel selection for LTE operation in the unlicensed band is performed periodically over a time scale larger than the channel access mechanisms discussed in the previous section. Varying numbers of neighboring devices and their corresponding traffic loads play an important role in designing appropriate channel selection schemes. A comprehensive set of scenarios for LTE's channel selection in the unlicensed band is as follows [44]:  
* _Fully distributed scenario_ with each LTE based SC independently selecting the channel in the unlicensed band. * Scenario with _intra-operator coordination_, such that all SC's of an operator share information and jointly perform channel selection for the LTE's operation in the unlicensed band. * _Inter-operator coordination_ scenario, where all operators in the vicinity of each other share LTE's channel selection information for the unlicensed band. * _Inter-RAT coordination_ scenario in which multiple LTE operators share channel selection information even with nearby Wi-Fi networks. Given the unlicensed channel selection by the LTE operating in the unlicensed band, the selected channel can be used in one of the following three modes [45]:  
* _Supplemental Downlink_: The unlicensed channel is used to carry only data traffic in downlink. The uplink and control channels remain on the licensed spectrum. * _Carrier Aggregation_: The uplink and downlink data can be transmitted over both unlicensed and licensed channel. However, the control channel remains in the licensed spectrum. * _Standalone_: Data and control information on the downlink and uplink is sent completely on the unlicensed spectrum. The different modes of operation for LTE in the unlicensed band are illustrated in Figure 4. The fully distributed scenario of channel selection for LTE-U with the utilization of the unlicensed band in the supplemental downlink mode has been explored in [44]. A distributed Q-learning based mechanism has been proposed in [44] in which each LTE-U based SC measures and stores an expected reward that can be achieved by using any channel based on its past experience. These reward values are used for efficient channel selection that ensures fair coexistence with Wi-Fi. A Q-learning based channel selection scheme for the fully distributed scenario with supplemental downlink mode has been considered in [46] for non-stationary users. The supplemental downlink mode with the fully distributed scenario has also been modeled as a non-cooperative repeated game in [47]. An iterative trial and error learning based approach for selection of best action has been presented in [47] that drives each SC towards a Nash equilibrium of channel selection. A cognitive spectrum sharing scheme for carrier aggregation mode in the intra-operator coordination scenario has been proposed in [48]. The spectrum sharing for LTE-U in [48] has been framed with utility maximization as the objective to ensure proportional fairness between the SCs. The resulting formulation turns out to be a mixed-integer nonlinear optimization problem (MINLP) because of the binary nature of channel assignment. Traditionally, the MINLPs are difficult to solve optimally in polynomial time. Hence, a solution algorithm based on a combination of a branch and bound and successive convex relaxation techniques for channel selection has been presented in [48]. A spatial adaptive play based channel selection method has been presented in [49] for the distributed scenario with carrier aggregation mode. The distributed algorithm in [49] tries to achieve the optimal action profile in which each LTE-U SC adapts its channel selection based on neighboring LTE-U SC usage, and this channel selection information is shared in the group.