# A Survey of Resource Allocation Techniques for Cellular Network's Operation in the Unlicensed Band
## 6 Coverage, Capacity, and Coordination in LTE-U  
The SINR coverage of an LTE-U operator is defined as the probability that a randomly chosen UE can achieve an SINR through LTE-U SC higher than a pre-determined SINR threshold as given by [65]. The analytical results in [65] are presented for a single carrier, random carrier selection, and adaptive carrier selection schemes. In [76], SINR and rate coverage probabilities have been analytically derived and numerically evaluated for a stochastic geometry based framework. It has been shown in [76] that WLAN and LTE-U can jointly achieve better rate coverage probability than typical WLAN networks through lower channel access priority along with a more sensitive sensing threshold for LTE-U. In [77], network capacity has been optimized for LTE-U and Wi-Fi coexistence with an optimal spectrum splitting ratio. Furthermore, the coverage probabilities and average achievable rates for each radio access technologies have been derived based on the user association in the networks. In [53], coverage and capacity performance has been evaluated through field trials for LTE-U and WLAN for single and multi-operator scenarios. A similar field trial has been considered in [78] for ultra-dense networks. Four different scenarios for the LTE-U SCs and corresponding transmission powers have been considered in [53] as follows:  
* For coverage test: 20 dBm,
* For capacity test: 16 dBm,
* For single-operator test: 8 dBm,
* For multi-operator test: 16 dBm. The authors in [53] show that direct introduction of LTE-U is not recommended and interference avoidance technology is essential to maintain coverage and capacity in LTE-U. Furthermore, it has been shown in [4] that existing water-filling algorithms are not suitable for LTE-U, due to computation load and possible iteration divergence. Hence, to reduce complexity and achieve capacity, a successive cap-limited water-filling algorithm has been proposed in [4] that maximizes the weighted capacity within the power and interference constraints. In [79], a weighted Voronoi tessellation-based approach has been utilized to maximize network capacity over an optimal SC operation scheme in LTE-U. Typically, the WLANs are distributed networks based on the DCF. However, the LTE-U SC based cellular networks can be centralized or distributed. The hardware based results in [39] indicate that both WLAN and LTE networks suffer due to interference from each other. Hence, inter-network coordination between WLAN and LTE-U through a centralized radio resource management entity has been proposed in [39] for improved system performance. A novel approach of applying coordinated multipoint (CoMP) based LTE-U has been considered in [80]. The simulation results in [58] show that interference coordination can significantly improve the ergodic throughput of LTE-U without compromising on WLAN's throughput. Coordinated and distributed channel selection for collocated WLAN and LTE-U cells has been analyzed in [57] for a single operator scenario. However, a similar study for multi-operator scenario is required. A detailed study of intercell interference coordination (ICIC) for LTE-U and WLAN has been performed in [7] with the following four scenarios. * Scenario 1 (Worst Coordination) Non Co-Located SCs with fully time aligned CSAT gating cycle. * Scenario 2 (Worst Coordination) Co-Located SCs with fully time aligned overlapping CSAT gating cycle. * Scenario 3 (Best Coordination) Co-Located SCs using proposed ICIC in [7] to get orthogonal non-overlapping CSAT gating cycle. * Scenario 4 (Random No Coordination) Co-Located SCs using randomly partially aligned overlapping CSAT gating cycle. The scheme proposed in [7] results in improvement of the overall LTE-U system performance. In [35], a central coordinator based approach for fair coexistence between LTE-U and WLAN has been analyzed based on the availability of distance and channel state information of the users at the centralcoordinator. A novel cloud based centralized resource allocation for LAA-LBT has been proposed in [81].