# A Survey of Resource Allocation Techniques for Cellular Network's Operation in the Unlicensed Band
## 3 Channel Access Mechanisms
### Carrier Sensing Adaptive Transmission (Csat)  
In CSAT, the LTE-U based SC senses the channel for a longer duration compared to LBT and CSMA (around 10 to 200 ms) [5]. Then, based on the sensed activity on the channel, the CSAT algorithm selects the proportion of time to be used for LTE as shown in Figure 3. Thus, for a proportion of the gating cycle, the CSAT based LTE-U SC actively utilizes the unlicensed spectrum, while, for the rest of the cycle, the SC switches off the transmission. Note that the CSAT has longer latency then CSMA or LBT, an impact that is mitigated by avoiding channels that Wi-Fi APs use for discovery signals and QoS traffic (i.e., primary channels) [5]. Thus, CSAT can also ensure fair channel sharing between LTE-U and neighboring Wi-Fi devices by suitably selecting the LTE-U ON period as depicted in Figure 3, utilizing optimal transmit power, and balancing the load between the licensed and the unlicensed LTE users. In [31], a proportional fair rate allocation scheme has been considered for a mixed Wi-Fi and LTE-U network. Optimal LTE-U ON time has been derived using the famous Karush-Kuhn-Tucker (KKT) conditions [32] on the rate allocation problem. Similarly, a proportional fair rate allocation for CSAT and LBT has been framed as a convex optimization and solved using the KKT conditions for optimal LTE-U ON time. Joint resource allocation for LTE-U based SC's in the licensed and unlicensed bands has been considered in [33]. A weighted Tchebycheff method based algorithm has also been presented that can find the complete set of Pareto optimal solutions for the energy efficient resource allocation in [33]. In [34], a fair time-sharing model based on the ruin theory has been proposed to perform efficient duty-cycle allocation in LTE-U. Figure 3: CSAT based channel access for LTE operation in the unlicensed band. In [35], a multi-objective joint optimization for load balancing between Wi-Fi and LTE-U has been framed. The optimal solution to the multi-objective optimization includes both the number of users to be offloaded as well as the optimal LTE-U ON time. Even the scenario of multiple Wi-Fi APs has been considered in [35]. Both throughput and fairness for the LTE-U system have been maximized by a multi-objective optimization problem in [36]. In [37], a multi-armed bandit approach for dynamic duty cycle selection has been considered along with downlink power control for higher energy efficiency and interference reduction. In [38], a minority game inspired distributed user association mechanism has been proposed for LTE-U SCs that aims to achieve load balancing under different resource contention schemes between the LTE-U and Wi-Fi networks. Joint optimization of transmit power, available bandwidth, and CSAT channel access time has been considered in [39]. A max-min based objective function has been used to ensure fairness in the joint optimization across both Wi-Fi and LTE networks' throughput. Utilization of the Wi-Fi point coordination function protocol by LTE-U to facilitate fair coexistence with Wi-Fi has been considered in [40]. A network utility maximization problem based on the Nash bargaining solution has been considered in [40], along with joint optimization of user association and resource allocation to improve system throughput and user fairness. A closed-form expression for the optimal contention period of Wi-Fi under a given user association has also been derived in [40]. In [41], a hybrid approach using both CSAT and LAA-LBT has been considered. Joint spectrum and transmission power allocation along with dynamic adaptation of the back-off window size and the duty-cycle time fraction based on the Wi-Fi traffic and available licensed spectrum resource has been considered in [41]. Similarly, analytical derivation of dynamic switching between CSAT and LBT for LTE-U has been considered in [42]. Both CSAT and LAA-LBT as channel access mechanisms can ensure channel sharing between existing devices. However, the 5 GHz band has multiple channels available.