Automotive wireless is more than cell phones and music

"Designers are limited on what can and cannot be used, as regulated by the FCC with regard to bandwidth and frequency," says Klaus Seiberts, director of RFID, automotive for Texas Instruments.For instance, RKE (remote keyless entry) uses the 350-MHz band freed up for short range applications, which he notes is getting more crowded with devices including garage door openers. The eventual scope of the challenge, for example, might encompass a parking lot filled with cars all having tire pressure monitoring where drivers are entering and locking their cars, not to mention some extraneous noise signals.

"Frequency hopping within the band, from 340 to 360 MHz" can ease the situation somewhat, Seiberts feels. "We are also looking at sending out two frequencies (the same content, twice), one at say 1 MHz above or below to ensure a command or information is received. Or you could make communications two-way for certainty, as well as switch frequencies if a link is weak." Modulation schemes could be amplitude shift keying or frequency shift keying (for the high/low bits), analogous to amplitude and frequency modulation in broadcast radio.

Power management
Any wireless communication scheme also has the challenge of minimizing power consumption to conserve battery life, say in a tire for pressure monitoring. The next step in power management could be to make the sensors or the key passive—providing them with power only when needed from a valid electromagnetic field signal sent out by the car, adds Seiberts. A fob, say, would decode the signal and power up for functioning.

Powering a tire pressure monitoring sensor mounted in a wheel would probably be relatively easy since it would always be within 1-2 m of a signal source. The situation for a key in someone's hand or pocket is more problematic with respect to range. Here astute design with regards to antennas and sensitivity is needed. For antennas, Seiberts cites materials and care in assembly methods.

"Directionality can be managed with three orthogonal antennas," adds Youbok Lee, technical staff engineer in the Security, Microcontrollers, and Technology Division of Microchip Technology (see below). Increased circuit sensitivity could increase useful range for keys up to 3 m, notes Lee. But TI's Seiberts says read range would likely be restricted to reduce power consumption, and that smaller silicon devices with sensitivity are prone to higher leakage current.