# An Analytical Model for Interference Alignment in Broadcast Assisted VANETs
## 2 System Model
### Case Study in VANETs Communication  
CSI is the key data for IA technology application. In VANETs, CSI could be obtained from SD in ITS and should be updated in time. When a vehicle enters a new road, firstly, it registers itself in VANETs. If the vehicle is within communication range of base station, the registration request could be posted to base station directly. Otherwise the vehicle should turn to other vehicles for relaying the request. After registration, base station allocates idle time slot and sends SD containing CSI of this road to the new vehicle.  
The detailed procedure is shown in Figure 3, which contains 4 stages. In the first stage, when vehicle \(A\) enters the range of base station from the boundary _In_ point, it posts an access request to base station in communication 1. During this stage, the base station collects the new access request and allots idle time slot to vehicle \(A\). During the second stage, the base station sends SD and time slot information to vehicle \(A\) in communication 2. During the third stage, vehicle \(A\) runs on this road and collects CSI with base station. At the last stage, when vehicle \(A\) reaches the boundary _Out_ point, the latest CSI information collected by vehicle \(A\) is sent to base station in communication 3 for SD update. When a new vehicle \(B\) enters this road, it will repeat this process as vehicle \(A\). In this way, the base station could keep the latest SD of this road.  
During stage 3, vehicles broadcast their safety related information to surrounding vehicles. These information should be pre-encoded based on CSI before being broadcast if IA is adopt. After the information is encoded with pre-coded vector, the valid signals are mapped to different directions with the interference signals [4]. If the road is reasonably divided, as shown in Figure 4, the CSI in the same segment can be considered as the same value [10].  
Figure 3: Workflow of New Vehicle Registering at Base Station and Uploading CSI to SD.  
In this paper, vehicles adopts half-duplex working mode and safety-related information contains location, speed and acceleration et al., In VANETs, the base station is equipped with multiple antennas for Multiple-Input Multiple-Output (MIMO). A communication task that sending a safety related information from one vehicle to another one, is called a communication session. The fashions of communication sessions sent from different senders to receivers can be categorized as follows:  
\(\boldsymbol{I}\rightarrow\boldsymbol{V}\): For base station, it sends information to multiple vehicles simultaneously by broadcast.  
\(\boldsymbol{V}\rightarrow\boldsymbol{I}\): Since the base station is fixed, vehicles can obtain CSI from SD, thereby making it possible to implement IA MIMO communication with base station.  
\(\boldsymbol{V}\rightleftharpoons\boldsymbol{V}\): Due to vehicle mobility, CSI between vehicles is not stable and difficult to be maintained. For this reason, broadcast is used as communication method for V2V without IA.  
In addition, due to limited coverage of base station, the vehicles among communication range could communicate with base station directly. As shown in Figure 5, coverage of the communication for all vehicles cannot be achieved. Such as the vehicle 2 is out of base station's service. For these vehicles, they should be assisted to upload information to base station.  
Figure 4: CSI Based Road Segments.  
Figure 5: Illustration of Base Station Broadcast Coverage and Assisted Transmission.