Optimal beamwidth of periodic reference signals for integrated sensing and communications in cellular-V2X networks

Recently, the technique of Integrated Sensing and Communication (ISAC) has gained great attentions and is expected to enable more advanced applications in cellular Vehicle-to-Everything (V2X). We intend to use the periodical reference signals, such as the synchronization signals in 5G, to actively detect targets at different azimuths by the way of beam scanning. This paper mainly aims to optimize the beamwidth of these reference signals in cellular-V2X to facilitate both sensing and communication functions. Firstly, to address the issue of insufficient beam duration for accurate estimation of Doppler and also velocity in consequent, we combine multiple beam scanning cycles to support better sensing. In this context, an innovative beamwidth optimization algorithm is proposed. Specifically, we deduce the Fisher Information Matrix (FIM) of the sufficient statistic associated with target azimuths. Considering the randomness of target position, we build the objective function based on the expected trace of the derived FIM. Additionally, we formulate the constraint of channel coherent time for efficient communications. It is worth noting that this is the first time to optimize the beamwidth of periodic reference signals for joint communication and sensing in cellular V2X. Experiment results show that the optimal beamwidth varies with the steering azimuth of antennas and is largest when the beam points at the direction perpendicular to the roadside.