Challenges with the V2X Spectrum - Part 1

In a much debated and controversial ruling [1], the United States FCC repurposed the 5850-5895 MHz of the 5.9GHz band for unlicensed use while preserving the upper 30MHz of the band (5895-5925 MHz) for the Intelligent Transportation Systems (ITS) [2]. A developing application for this band is the V2X technology that encompasses numerous communications platforms for connected vehicles, infrastructure, and cellular networks. In contrast to the DSRC technology [3] [4] that has been available and deployed for over 20 years, the automotive and wireless communications industries have been dedicating a substantial investment and eagerness to mature and deploy the V2X technology [5]. This is mainly due to the advanced and ambitious capabilities that V2X is aiming to offer that are considered essential in realizing a reliable and versatile infrastructure for autonomous vehicles. With V2X technology, vehicles, cyclists, pedestrians, traffic control equipment can all be interconnected thorough cellular and/or dedicated short-range networks. As one might expect, this is a massive and dynamic mesh of interconnected devices much like the cellular network but with some unique and challenging differences.

In a series of blog posts, we will be exploring some of these challenges from the spectrum management and RF front-end design prospective…

Let’s return to the frequency band assigned to V2X. As previously mentioned, the lower spectrum of the V2X band has been assigned for unlicensed use such as WiFi, commercial drones, wireless video streaming, etc. This close proximity of the unlicensed band to a band intended for transportation and pedestrian safety poses a major interference risk which may be caused by spurious products generated from devices operating in the adjacent band. While even unlicensed commercial devices are required to comply with certain emission limits, it should be noted that the V2X wireless environment is relatively much more dynamic and more susceptible to noise and interference. Let’s consider a simple example of a vehicle communicating to an intersection traffic light. Excluding the characteristics of the roadside unit (RSU), the signal strength and quality received by the traffic light system from the vehicle depends on a number of factors such as distance, speed, and travel direction of the vehicle as well as the nearby traffic and building and terrain. In more technical terms, the transmitted signal from the vehicle, undergoes power loss caused by multitudes of mechanisms due to the propagation distance, multi-path fading, Fresnel zone blockage, polarization change, and Doppler frequency shift. Subsequently, the signal strength may dynamically vary from an extremely small level to a higher level throughout the communication period. To maintain a reliable link as the signal level fluctuates, the transceivers on both ends must be designed with features that are more resilient to interference such as implementation of modulation and signal processing techniques to enhance the dynamic range of the signal as well as front-ends architectures utilizing MIMO and RF signal conditioning blocks.

It should be noted that similar challenges are also present in the cellular phone networks and have been studied and dealt with for many generations of cellular technology; however, to reiterate, the one exception that makes the V2X spectrum more challenging is the risk of adjacent channel emission due to its extreme proximity to a widely used unlicensed band. In our traffic light system example, if the RSU is located in a dense urban environment with nearby buildings and businesses all utilizing the 5GHz spectrum for WiFi connectivity, the collective out-of-band spurious emissions from their modems and user devices could amount to the rise of unwanted interference that is significantly higher than the levels received by the RSU from the vehicle.