# A Survey of Resource Allocation Techniques for Cellular Network's Operation in the Unlicensed Band
## 5 Fairness  
For fair co-existence with Wi-Fi devices, 3 GPP mandates that LTE's operation in the unlicensed band must not impact Wi-Fi services any more than an additional Wi-Fi network in the same band [18]. In this regard, fair coexistence of CSAT based LTE-U with Wi-Fi has been studied in [45; 53; 54]. A detailed field trial of CSAT's fair coexistence with Wi-Fi from a normalized throughput perspective for a varying number of users and SC's has been considered in [53]. Both intra-operator and inter-operator scenarios of LTE's operation in unlicensed band with sparse and dense multi-cell deployments show that LTE's operation in unlicensed band outperforms Wi-Fi for all measurement distances considered in [53]. From a delay based analysis, it has been shown in [45], that more work needs to be done to achieve fair coexistence between Wi-Fi and CSAT based LTE-U. In [54], a novel optimal traffic balancing scheme has been proposed for multiple unlicensed bands to achieve fairness between LTE-U and Wi-Fi through optimum power and time allocation in LTE-U. For a large number of devices, the centralized optimization framework proposed in [39] results in fair sharing between Wi-Fi and LTE's operation in unlicensed band by exploiting power control and time division channel access diversity. Using a max-min fairness based approach, the results in [39] indicate that uncoordinated time based operation of Wi-Fi and LTE's operation in unlicensed band results in severe performance degradation. A central coordinator based approach for fair coexistence between LTE-U and Wi-Fi has been analyzed in [35] for the three scenarios where the central coordinator has no information about Wi-Fi users, has information about the distance between Wi-Fi and LTE-U SC, and has the channel state information. A Nash bargaining solution is used for a fair and joint user association plus resource allocation in [35]. To achieve the various possible performance trade-offs between LTE-U and Wi-Fi, a joint licensed and unlicensed spectrum allocation is considered in [55]. In [31], a joint proportional fair scheduling scheme for coexistence of Wi-Fi and LTE's operation in the unlicensed band has been proposed. Several works like [49; 56; 57; 58] analyze CSAT-based channel access. However, they do not consider the impact of a varying number of users, SCs, or multi-operator scenarios on fairness of the system. In [48], the dynamic channel selection, carrier aggregation, and fractional spectrum access for LTE-U networks has been considered, while guaranteeing fair spectrum access for Wi-Fi based on a cross-technology fairness criterion. The per-user average throughput for the LTE users has been maximized in [59], while bounding the minimum average throughput and maximum delay in each Wi-Fi network. For LAA-LBT, the impact of LTE-U on Wi-Fi for multiple cells and multi-operator scenario has been studied in [60]. Given that Wi-Fi's AIFS time (34 us) is longer than LAA-LBT based LTE-U's proposed CCA time (20 us), there exists scenarios in which LTE-U will sense the channel idle and transmit before any Wi-Fi device. To mitigate these scenarios, a fair LAA-LBT mechanism has been proposed in [6] that maximizes the LTE-U's airtime while not degrading the Wi-Fi's throughput. The gain for LTE's operation in an unlicensed band in [6] increases as the number of competing Wi-Fi stations grow. A novel and fair LAA-LBT algorithm to ensure high throughput of both LTE-U and Wi-Fi has been proposed in [61]. Similarly, an adaptive LAA-LBT has been proposed in [62] that scales fairly with an increasing number of users. In [63], the basic LBT mechanism is used to guarantee fairness with Wi-Fi along with a coalition strategy between LTE-U SCs to decrease the high collision probability at a heavy traffic load. A cooperative game theoretic solution is proposed in which SCs in a coalition contend for accessing the channel as single entity. This effectively reduces the number of stations contending for accessing the channel and the collision probability.