Use of 2.4 GHz Frequency Band for Communications Based Train Control Data Communications Systems

The increasing deployment of CBTC by public transit agencies that use a data communications systems based in the 2.4 GHz ISM band has raised the concern of RF interference between CBTC equipped trains and the variety of new and existing users of this band. The vast number of RF devices that currently operate in this band (like microwave ovens, cordless telephones, medical devices etc.) have recently been augmented by the proliferation of “Wi-Fi” hotspots and wireless computers permitting untethered internet access by the public and RF identification (RFID) technology. While the popularity of the band is a source of concern, paradoxically, this same popularity yields many advantages and benefits to CBTC systems that use it. Often the fact that a frequency band is “crowded” or heavily used (however that is defined) immediately precludes its use for those systems or services that consider themselves to be mission critical. While avoiding a heavily used frequency band for an important application is a prudent course of action, it might not be possible or, upon closer examination, even necessary: There might not be any other spectrum available or, if there is, the cost and technical risk of obtaining regulatory approval or developing the unique radio equipment could prove prohibitive. Additionally, technical advances in radio equipment and signal processing might completely obviate any perceived interference.