## 3 State of the Art
### Proposed Coexistence and Coordination Techniques
#### Other Related Work  
Coexistence between LTE and Wi-Fi in the unlicensed spectrum has attracted the attention of the mobile and the research community. There are several proposed mechanisms, trying to achieve  
Figure 5: LTE Licensed-Assisted Access (LAA) and Wi-Fi coexistence. Figure 6: Carrier Sensing Adaptive Transmission (CSAT) duty cycle periods. fair coexistence between the two technologies. These mechanisms are evaluated on the success of providing the desired fairness. In [18], the authors discuss a preliminary design of a semi-distributed LTE in the unlicensed spectrum scheme, where the eNB senses the carrier before a transmission in a similar way to Wi-Fi. They use a method that exploits the LTE Almost Blank Subframes (ABS). The ABS were initially designed to enhance Inter-Cell Interference Coordination (eICIC) as part of 3 GPP LTE Release 10 [19]. The proposed method evaluates different duty cycles, different distances between an eNB and an AP and different numbers of cells. Different ABS patterns are also studied. In [20], the authors propose an LTE subframe design consisting of three phases named the data transmission phase, the mute phase and the sensing and reservation phase. Based on this subframe design, they proposed three schemes to enable coexistence among LTE and Wi-Fi. The first scheme assumes a fixed mute duration. The second scheme uses a randomized mute duration. The third scheme introduces a random backoff counter during the mute period. The three proposed schemes are evaluated through simulations. The results show that the scheme that uses a random mute duration offers better overall throughput performance, while the scheme that uses a random backoff counter results in smaller throughput difference between LTE and Wi-Fi. The authors in [21] describe an analytical framework for interference characterization of Wi-Fi and LTE. Initially, a first model is described for single LTE and single Wi-Fi AP separated by a specific distance. The results show that Wi-Fi performance is significantly decreased compared to LTE for which the degradation is minimal. They observe that the conventional perception of the inverse proportion of throughput to inter-AP distance is not valid for LTE-Wi-Fi co-channel deployment. A second model with many LTE and Wi-Fi systems is also described. The results show that the overall system throughput first increases and then decreases with growing density. Finally, in order to increase the individual Radio Access Technology (RAT) and system throughput, random channel assignment, intra- and inter-RAT channel coordination are considered. In the intra- and inter-RAT channel coordination schemes, the channel is allocated at an AP as a graph multi-coloring problem. The results show 3.5-5\(\times\) gains in system capacity. In this technique, the networks do not exchange specific information in order to optimize the offered QoS, but different frequencies are assigned to them in order to avoid overlapping frequencies between co-located networks. In [22; 23], the authors use Q-Learning techniques to achieve the desired coexistence. In [22], they propose a Q-Learning-based dynamic duty cycle selection mechanism for configuring LTE transmission gaps. LTE LAA and Wi-Fi performance using a fixed transmission gap is evaluated and then compared with the proposed Q-Learning mechanism. Simulation results show that the proposed scheme enhances the overall capacity performance. The authors in [23] propose a Q-Learning mechanism for advanced learning of the activity within the unlicensed band resulting in efficient coexistence between LTE LAA and Wi-Fi. As a next step, the coexistence is further enhanced through a double Q-Learning method that takes into account both discontinuous transmission and transmit power control of LTE to improve both LTE and Wi-Fi performance. Coexistence of LTE and Wi-Fi when LTE uses an LBT procedure is studied in [24; 25; 26; 27]. The authors in [24] propose an adaptive LBT protocol for LTE LAA.