# An Analytical Model for Interference Alignment in Broadcast Assisted VANETs
## 1 Introduction  
The US Federal Communications Commission (FCC) allocated 75 MHz of wireless spectrum (5.850 GHz to 5.925 GHz) bandwidth to dedicated short range communications (DSRC) for intelligent transportation systems (ITS) service. According to FCC regulations, this band is further divided into seven isolated sub-channels, including one control channel and six service channels, where control channel is reserved for transmitting beacons or fundamental safety-related messages. Each vehicle periodically broadcasts traffic state information including its speed, acceleration and GPS on control channel. These types of information help vehicle interacts with surrounding vehicles timely. However, with the number of vehicles increasing, demand for communication between vehicles rises, causing congestion in communication band and decreasing communication efficiency.  
Communication protocols of VANETs are mainly based on IEEE802.11 series protocols, but a large number of performance verification experiments show that competitive protocols cannot meet Quality of communication Service (QoS) requirement of VANETs [1; 2]. Since safety related information should be delivered as quickly as possible, the effective method is that vehicles broadcast their safety-related information directly to surrounding vehicles, which will inevitably lead to information redundancy and flooding [3].  
In the area of wireless communication networks, some research was conducted to improve channel resource use though managing interference between wireless terminals, such as Interference Alignment (IA). Simulation results in [4; 5] show that in interference channel of the K-node communication network equipped with M antennas, if IA is adopted, the total degree of whole network freedom approaches \(KM/2\). However, Channel State Information (CSI) of senders and receivers are required for IA application. The relative positions of vehicles are constantly changing, making it difficult to obtain CSI information.  
Since the location of base station is fixed, in some studies, prediction model of CSI between vehicle with base station are constructed to estimate CSI during a certain period of time [6; 7]. Once CSI information between vehicles and base station can be predicted in advance, IA can be applied to communication of V2I.  
With the advent of Cognitive Radio (CR) and dynamic spectrum access technology, Spectrum Database (SD) including CSI information provided by ITS could be obtained by a user who is about to pass a road segment [8]. Depending on SD and the user's location, CSI between the user and base station could be queried, which could meet time requirement for safety related information in a long enough period [9].  
All above, in this paper, we analyze the communication flow of V2V and V2I in VANETs and construct broadcast model for IA technology. Different communication fashion is adopt for different type senders. Combined with characteristics of safety related information, an assisted transmission model is constructed to further reduce redundancy transmission and improve link use efficiency. Based on these model, we propose an optimization problem of minimizing total number of time slots required for safety information sharing in VANETs. In addition, a clustering algorithm is designed to narrow feasible solution space.