Car access systems permit or deny access to passenger cabins and storage areas. Early car access systems were based on purely mechanical structures designed strictly to prevent theft. Microelectronic and RF technologies have greatly improved security functions and enabled new capabilities such as activating security alarms, setting individualized driver preferences, and even closing windows automatically.
Most cars now include remote keyless entry (RKE), and an increasing number have passive keyless entry (PKE). RKE controls car doors and trunk lids by sending a wireless signal when users press the buttons on key fobs. PKE, which can be considered advanced RKE, allows users who have the key fob to open car doors by pulling the door handles without the need to press any button. Near-field communication (NFC) is rapidly gaining attention from the car industry as a more secure and capable way to deliver PKE functions in the car for a more convenient and a customized user experience.
A typical RKE system has a wireless transmitter operating at 315 or 433.92 MHz and an MCU embedded in the car keys (Fig. 1). The MCU mostly stays in sleep mode to save power. When a car key button is pressed, the MCU wakes up and instructs the transmitter to send out a 64-bit or 128-bit data stream after carrier modulation. An installed RF receiver receives the data and forwards it to another MCU that verifies the sender’s identity and instructs the mechanism to unlock the doors. Key fobs with multiple buttons can perform several other tasks such as opening the trunk, blinking the lights, triggering an alarm, and even starting the engine.
The digital data stream, transmitted between 2.4 and 20 kbits/s, usually consists of a data preamble, a command code, some check bits, and a “rolling code” that ensures vehicle security by altering itself with each use. Without this rolling code, the transmitted signal might accidentally unlock another vehicle or be intercepted by a car thief who could use it to gain entry later.
Several major objectives govern the design of these RKE systems. Like all mass-produced automotive components, they must offer high reliability at low cost. They should minimize power drain in both the transmitter and receiver. In addition, the RKE system designer must juggle receiver sensitivity, carrier tolerance, and other technical parameters to achieve maximum transmission range within the constraints imposed by low cost and minimum supply current.
Design constraints include those defined by local regulations for short-range devices, such as Federal Communications Commission (FCC) mandates in the United States. While the use of short-range devices does not require a license, the products themselves are governed by laws and requirements that vary from country to country.
Passive entry systems enable users to unlock a vehicle door or trunk without pressing any buttons. They are based on a low-frequency/radio-frequency (LF/RF) communication link between the fob and the vehicle (Fig. 2). LF antennas mounted within the outside mirrors or door handles initiate communication and can sense multiple fobs at a range of 1.5 to 2 m.
When the driver pulls a door handle, the passive entry controller sends an LF challenge to authenticate the driver’s fob. The fob then sends an RF response to the controller. If fob recognition is successful, the vehicle automatically opens after a few milliseconds. Vehicles with a PKE system disengage the immobilizer and activate the ignition without the key in the ignition, provided the driver has the key inside the car.
Drivers then can start most vehicles with a PKE system by pressing a starter button or twisting an ignition switch. Also, they can lock these vehicles by pressing a button on one of the door handles, by touching a capacitive area on a door handle, or by simply walking away. The locking method varies between models.
In any PKE system, a key fob must be able to measure the LF signal strength usually on three orthogonal axes (x, y, and z) and transmit this information via an RF channel. This signal strength information, also known as received signal strength indicator (RSSI), is collected using the antenna coils connected to the LF receiver. Any data such as a wakeup data pattern (preamble, ID) used as a payload in the protocol is received and passed to the key fob MCU for processing. The LF receiver includes dedicated control logic that can check wakeup signals with very low power consumption.
A backup mode enables the use of the PKE system even when the key fob battery is low, with power supplied to the device via the LF signal. The device’s response is then transmitted by modulation of the vehicle’s own LF signal. When used in this backup mode, the key fob device must be placed close to the door antenna for entry and exit or in a special area on the dashboard to start the vehicle.
NFC is a short-range wireless technology that facilitates the secure exchange of data. It is being increasingly adopted for secure transactions, including use by mobile phone manufacturers to create an “electronic wallet.” NFC combined with an embedded secure element offers consumers a high level of convenience, interactivity, and security with their mobile devices. It operates at 13.56 MHz at rates ranging from 106 to 424 kbits/s, so NFC-enabled smart phones could be used for RKE functions (Fig. 3).
NFC communication involves an initiator and a target. To transmit data between two NFC interfaces, one NFC interface activates its transmitter and works as an NFC initiator. The high-frequency current that flows in the antenna induces a magnetic field that spreads around the antenna loop and moves through the antenna loop of the other NFC interface located close by. A voltage is then induced in the antenna loop of the other NFC interface, which detects the voltage with its receiver.
If the NFC interface receives signals and the corresponding commands of an NFC initiator, this NFC interface automatically adopts the role of an NFC target. For data transmission between the NFC interfaces, the amplitude of the emitted alternating field is modulated (amplitude shift keying, or ASK, modulation). The transmission direction is reversed to send data from the NFC target to the NFC initiator. If an NFC interface is located close to a compatible RFID reader, the NFC interface adopts the role of an NFC target and can transmit data to the reader using load modulation. This mode is called “card emulation mode.”
An NFC interface also can communicate with compatible passive transponders. The interface can supply these transponders with power, and these transponders can transmit data back to the NFC interface via load modulation. In this case, the NFC interface adopts the role of a radio-frequency identification (RFID) reader.
NFC can be used for car access if a reader is installed in a way that it is accessible from outside the car (e.g., located in the side mirror). This is particularly helpful in communities that share cars or other situations where different people have access to a number of cars (e.g., company car fleets).
ICs are available for RKE, PKE, and NFC applications. For example, the Melexis MLX90132 was designed and qualified for automotive systems, making it robust and durable in retail, consumer, and industrial NFC applications (Fig. 4). This 13.56-MHz, fully integrated, multi-protocol RFID/NFC transceiver IC supports ISO/IEC protocols 18092, 14443A and B, 15693, and 18000-3.
The MLX90132 handles subcarrier frequencies from 106 to 848 kHz and baud rates up to 848 kbits/s. Its digital section handles the low protocol layers from the application programming interface (API) to the physical layer (PHY) using advanced bit and frame decoding functions. The IC embeds tag emulation functionality for NFC support. Enhanced tag and field detection capabilities significantly reduce power consumption in RFID reader configuration and in NFC mode.