# A Survey of Resource Allocation Techniques for Cellular Network's Operation in the Unlicensed Band
## 3 Channel Access Mechanisms
### License Assisted Access with Listen before Talk (LAA-LBT)  
The commercial availability of LAA was first discussed in [5]. The LAA requires discontinuous transmission on a carrier with limited maximum transmission duration. The channel selection for LAA devices should select carriers with low interference and is required for radar avoidance in certain regions/bands (discussed in detail in next section). Transmit power control is another regulatory requirement in many regions that requires the transmitting LAA device to reduce the transmit power compared to the maximum nominal transmit power [17]. The LAA utilizes carrier aggregation such that the secondary carrier comprises of the unlicensed spectrum in the 5 GHz U-NII bands and the primary carrier is the licensed spectrum [7]. The LAA based systems are required to perform LBT that includes a clear channel assessment (CCA) before accessing the channel using energy detection for a pre-determined time (20 \(\upmu\)s). Based on the energy detectors output over CCA time, the channel is considered as idle or busy. For an idle channel, the LTE-U device starts transmitting immediately, whereas, for a busy channel, more CCAs are performed based on the type of LBT category as follows [18]. * In _Category 1_, no LBT is performed (similar to CSAT discussed in the next subsection). * The _Category 2_ consists of LBT without random back-off. * An LBT with a random back-off with fixed size of contention window is performed in _Category 3_. * The _Category 4_ consists of LBT with random back-off with variable size of contention window. The basic LAA-LBT based channel access mechanism is depicted in Figure 2 for the last three categories. The defer period in Figure 2 is the minimum time a device must wait after the channel becomes idle before its transmission [3]. In Figure 2, the four key resource allocation parameters that govern the performance of the LAA-LBT based channel access mechanisms are:  
1. the channel access time,
2. the transmit power,
3. the backoff window,
4. the defer period and extended clear channel assessment (ECCA). A MAC protocol design that to select LAA-LBT's channel access time (also termed as transmission time) such that the LAA-LBT's throughput is maximized while maintaining fairness to existing Wi-Fi devices has been proposed in [19]. Given existing Wi-Fi networks, the feasible region of LAA-LBT's transmission time has also been determined. The impact of ECCA has been neglected in the computation of optimal channel access time in [19]. Joint optimization of the LAA-LBT's channel access time and its transmit power has been considered in [20; 21; 22]. The optimal traffic balancing between licensed and unlicensed band for LAA-LBT based SCs has been considered in [20]. The traffic balancing for LAA-LBT and Wi-Fi has been modeled as a coalition game in [23]. Min-max and Shapley fairness has been used as objectives to improve resource utilization with almost blank sub-frames and user offloading for LAA-LBT. A joint subcarrier level power allocation along with channel access time allocation has been formulated as an optimization problem in [21]. Both [20; 21] consider a waterfilling algorithm based power allocation. The tuning of the minimum contention window can have a significant impact on the performance of the overall system. The resource allocation problem for selection of minimum contention window without affecting the aggregate throughput of the WiFi users has been framed in [24]. A Genetic Algorithm based solution has been proposed in [24] to search for the optimal contention window size size of every LAA-LBT SC. Furthermore, a multi-agent reinforcement learning (Q-learning) based contention window adjustment for LAA-LBT has also been proposed in [24] to achieve the dual goals of maximizing the LAA-LBT utility and ensuring the Wi-Fi network's throughput. In [25], a Markov chain-based approach has been used to obtain the optimal contention window size of LTE-U SCs, which maximizes the total throughput of Wi-Fi and LTE-U networks.