Car Access System
Car Access System is designed to permit or deny access. Advances in microelectronics and RF technologies have improved security functions and featured new capabilities such as automatic vehicle tracking and window roll-up/down. Currently, the technologies most widely and successfully applied in these systems are RKE and PKE. RKE (Remote Keyless Entry) controls car doors and trunk lids by sending wireless signal when users press the buttons on keyfobs. PKE (Passive Keyless Entry), which can be regarded as advanced RKE, allows users to gain entry into the car without the need to press any button.
A typical RKE system has a wireless transmitter (operating at 315 MHz or 433.92 MHz), a MCU embedded in the car keys to realize control over cars. The MCU mostly stays in sleep mode in order to save power. When a button on car keys is pressed, MCU wakes up and controls transmitter to send out a 64-bit or 128-bit data stream after carrier modulation. The data is received by a RF receiver installed in cars and passed on to another MCU which verifies sender’s identity and gives instructions to control mechanism to open doors. The keyfobs with multiple buttons can provide several other functions such as opening trunks and starting engine.
Comparing with RKE systems, PKE has an additional pair of low-frequency (125 KHz) transmitter and receiver which are installed respectively in cars and keyfobs. When users carrying a PKE key pull a door handle of a car, the low-frequency transmitter in car is activated instantly and sends out a signal. The receiver embedded in keyfob receives that signal with a 3-dimentional antenna and makes a comparison between the signal and its stored data. If they match, the key is awaked and sends a high-frequency (315 MHz or 433.92 MHz) signal back to the car. And if this signal is found valid, car doors will be unlocked immediately. The whole procedure only takes several milliseconds so that users will not perceive even a slight delay. Additionally, PKE can provide a good anti-theft performance by, for example, automatic locking doors, closing windows, cutting off fuel supply and power source when users carrying the key are more than 3-5 meters away from cars.