## 4 Direct Cooperation via In-Band Energy Level Patterns
### Inter-Rat Tdma  
The RTS (Request to Send)/CTS (Clear to Send) method has been introduced in the 802.11 standard as an optional feature to control the access to the medium [33]. If RTS/CTS is enabled, then a Wi-Fi node will not transmit until it completes an RTS/CTS handshake. According to this handshake, before a data transmission, a node first transmits an RTS frame to the destination indicating how long the transmission will last. The receiver should reply with a CTS message after an SIFS period. CTS contains a time value that informs other nodes to postpone their transmission during the length of this value. Using the in-band pattern recognition methodology, an inter-RAT RTS/CTS method can be developed among LTE and Wi-Fi, as is depicted in Figure 11. Using such a method, the co-located wireless technologies can reserve the medium for a maximum time duration. This way, they can operate in an inter-technology Time Division Multiple Access (TDMA) way.  
In this method, we introduce two different energy level patterns that can be used to reserve and release the medium. These patterns are defined as follows:  
* Reservation energy level pattern. This pattern is used by a wireless network to inform other co-located networks that it will reserve the medium for a period of time smaller or equal to a maximum transmission duration.
* Release energy level pattern. This pattern is used by a wireless network to inform other co-located networks that the medium has been released.  
Before an LTE or a Wi-Fi transmission, the eNB or the AP tries to gain access to the medium by broadcasting a reservation energy level pattern. When a network detects and interprets such a signal, it postpones its transmissions. The network that transmitted the reservation pattern can gain access to  
Figure 11: Inter-Radio Access Technology (RAT) TDMA.  
the medium for a period of time smaller or equal to a maximum transmission duration. The choice of the maximum transmission duration that a network can access the medium is very important, as it should guarantee that the medium is not monopolized and every network can access it in a fair way. When the network completes a transmission or when the maximum transmission duration is reached, then it broadcasts a release energy level pattern. At this point, another or the same network can reserve the medium in order to transmit.  
During the transmission duration, an eNB schedules resource blocks the UEs that are attached to it, while in case of Wi-Fi, the nodes compete for the medium using the traditional CSMA/CA technique. Before the start of transmission in the reserved timeslot, the network that requests access to it, must sense the medium for ongoing transmissions by networks that are not part of the cooperation mechanism in order to prevent interference with them.  
From the above description, it is clear that the implementation of an inter-RAT TDMA mechanism requires small modifications for both LTE and Wi-Fi protocols. These modifications will render both technologies capable of transmitting, detecting and interpreting the in-band energy level patterns, so that they can reserve the medium or postpone their transmissions.