# Traffic Offloading in Unlicensed Spectrum for 5G Cellular Network: A Two-Layer Game Approach
## 3 Modified CSAT Mechanism  
Before discussing the CAGT scheme, the basic and modified CSAT will be devised firstly. The primary principle of CSAT is to let LTE in unlicensed bands cycle between state \"on\" and \"off\" within a given single cycle while the LTE in licensed band remains connected all the time. During the activated state \"on\", LTE transmits data on the unlicensed band and blocks the transmission of WiFi systems. However, within the deactivated state \"off\", LAA shuts down its transmission to yield the channel sources to WiFi [11], just as shown in Figure 2. Adjusting \"on\" and \"off\" time dynamically by identifying the number of WiFi terminals can be an affordable way for proper \"on\" and \"off\" time assignment and meet better needs between system aggressiveness and fairness [12]. However, WiFi nodes may not be aware of the LAA activity in some actual scenarios. For example, WiFi causes interference to LTE in the \"on\" state period since WiFi cannot identify the structure of signals from non-IEEE systems. When the Energy Detection for Clear Channel Assessment (CCA-ED) measurement is below the predefined inter Radio Access Technology (inter-RAT) threshold, the ongoing LAA messages cannot be identified and will be regarded as noises. Therefore, a LBTbased CSAT was proposed by Qualcom to deal with the above problem by which LAA should sense the channel (like the LBT) firstly before the cycle period turn \"on\" and ensure that there is no WiFi under transmission [9; 10]. In addition, a Clear To Send to Self (CTS-2-S) message [8; 9] can be inserted to reserve the channel resources for LTE transmission during the next \"on\" state period. Figure 2 illustrates the function of CTS2S message. The duration of upcoming \"on\" state is included in the CTS-2-S message. If WiFi cannot identify the ongoing LAA signals below the inter-RAT threshold, the corresponding WiFi backoff mechanism cannot be triggered in \"on\" period. While after introducing the CTS-2-S signal in LAA, the neighboring WiFi will back off until the LTE completes its transmission during the reserved \"on\" period. Therefore, the possible interference between these two systems is avoided within the time of LAA transmission. In this way, WiFi detect the channel to be busy and collision can be alleviated effectively. However, collision may still occur when the WiFi backoff time is counted down to zero and the LTE is about to send packets simultaneously at the moment of starting \"on\" periods, just as shown in Figure 2.  
In terms of WiFi CCA sensing process, the AP which wants to transmit data will sense the channel for every 9 \(\upmu\)s slot continuously until it counts down to zero and occupies the channel. In addition, a 34 \(\upmu\)s Distributed Inter-Frame Spacing (DIFS) would follow the WiFi data for the general DCF structure. To some extend, the next WiFi message must wait for a 34 \(\upmu\)s DIFS time and then wait until the station finishes the CCA sensing before it enters the channel as shown in Figure 2. To solve the collision problem mentioned above, a modified LBT based CSAT structure is proposed in Figure 3, where a special protection frame is inserted before the coming \"on\" period. ETSI 301 893 [30] specifies that before a transmission, a station must perform a CCA using energy detection for at least 20 \(\upmu\)s. That means CCA sensing period is shorter than the DIFS time. Once the medium is sensed idle before the coming \"on\" period, LAA could occupy the channel before any WiFi station does. In addition, the protection frame we designed is a special almost blank frame with a fixed structure which could be recognized by LTE-LAA. Thus, the channel is reserved for LAA until the following \"on\" stage. By this way, the channel is sensed busy by WiFi before the \"on\" stage and collision would not happen anymore at the starting moment of \"on\" stage.  
Figure 3: Modified LBT-based CSAT frame structure.  
Figure 2: Incumbent LBT-based CSAT frame structure.