RF Design Considerations for Passive Access Control Systems

Passive Entry (PE) systems are leading a new trend in automotive comfort and safety. In terms of completeness, Remote Keyless Entry (RKE) systems are interactive, meaning that the user must press the key to open the door; passive entry systems are passive, meaning that they open the door without any interactive action by the user. When the user is ready to enter the vehicle, the PE system is triggered by the action of pulling the door handle to transmit a low frequency (LF) signal. Within a few milliseconds, the key fob receives the LF signal, encrypts the received data packet, and then sends the encrypted signal to the vehicle via the radio frequency (RF) channel for confirmation. Passive

entry systems can also include a passive engine start function, Passive Entry Go (PEG). As long as the system confirms that the key fob is in the vehicle, the LF circuit is triggered as soon as the driver sits in the driver's seat. After verification and confirmation and completion of the position measurement, the engine can be started by simply pressing the start button.

In both cases, the key fob receives plain text data, encrypts it using a powerful hardware encryption module (such as an AES-28 module), and then returns the encrypted data to the vehicle for verification.

The PE key fob uses a small lithium battery to provide power for data reception, encryption, and transmission. The key fob in the passive access control system is specially designed to ensure the longest possible battery life. If the battery is running low, the key fob will enter an emergency mode to obtain enough magnetic field energy through the LF coil to operate without a battery. This requires the key fob to be placed in a position close to the door coil. In this case, the system only communicates through the LF channel.

Typical PE system

A typical PE system consists of an in-vehicle part and a key fob subsystem, which act as communication peers and establish two communication links: (1) LF uplink: vehicle to key fob, and (2) ultra-high frequency (UHF) downlink: key fob to vehicle (see Figure 1).


Figure 1 Block diagram of passive access control system

In-vehicle part

When the user pulls the car door handle, the antenna driver in the vehicle generates an LF field. This change activates the controller in the center instrument panel, which requests the key fob to initiate LF communication. Typically, an LF antenna coil is installed in each door, driven by an antenna driver unit (one antenna driver unit can drive multiple antenna coils, for example, the ATA5279 can drive up to 6 different antenna coils). The system uses a UHF receiver module to receive the RF data sent from the key fob to support the RF link. The received data is encrypted and sent back to the instrument panel controller, where it is decrypted by software (AES-128).

Key fob

In any PE system, the key fob must be able to measure the strength of the LF signal in three orthogonal axes (X, Y and Z directions) and send this information back to the vehicle via an RF channel using a UHF transmitter to determine the location of the key fob. This signal strength information (also known as the remote signal strength indicator, or RSSI) is collected by three orthogonal antenna coils connected to the 3D LF receiver. Any digital data, such as a wake-up data pattern (preamble, ID), system commands or plain text data passwords as protocol payload, will be received and transmitted to the microcontroller (MCU) in the key fob for processing (return packet, encryption). To save energy, the LF receiver has a dedicated control logic that can analyze and detect the wake-up signal with very low power consumption, so there is no need to fully wake up the entire system, which can greatly extend the battery life of the key fob. The data traffic in and out of the key fob can be controlled by a small 8-bit ultra-low power MCU (such as ATtiny44). The received data can be encrypted by software or by a hardware encryption module with powerful encryption functions (such as AES-128). For increased security, an encryption mechanism is used both internally in hardware and embedded in the MCU. The encrypted data is transmitted to the UHF transmitter and transmitted to the vehicle at a high baud rate.

In the case of a completely exhausted battery, the transponder can operate as a battery-free passive device, which is called emergency mode operation. In this mode, only one of the orthogonal coils couples to the LF magnetic field, obtains sufficient energy from it, and stores it in the form of charge in an external capacitor. The transponder communicates with the base station via the LF link to open the door and is used as an immobilizer to prevent the engine from starting (see Figure 1, where the X-axis coil is equivalent to a 3D LF receiver coil and an emergency/immobilizer transceiver antenna). The analog front end (AFE) block is used for LF communication, while the power management (PM) block manages the field power, i.e., the charge stored on the external capacitor Cbuf. In emergency mode, RSSI measurement, 3D LF data reception, and RF transmission are disabled.

Table 1: Basic parameters of the PE system key fob



Figure 2 3D LF receiver wake-up

Figure 3 RSSI location measurement

Receiving LF signal

The low-frequency field with a carrier frequency of about 125kHz can be used as: (1) a data communication link for transmitting data at a low baud rate; (2) a medium for location information by calculating RSSI values ​​in the three axes; and (3) a contactless electromagnetic medium for transmitting power over short distances. However, each type of application and its transmission quality are closely related to the degree of coupling between the transmitter and receiver antennas. The degree of this coupling depends on many physical and electrical parameters, such as antenna inductance, resistance, distance between coils, degree of resonance tuning, etc. The larger the coupling factor, the stronger the communication link (i.e., the more energy is transferred from coil to coil).

The LF electromagnetic wave signal emitted by the transmitter coil antenna propagates along the direction angle where the magnetic field strength is the largest and gradually attenuates as it moves away from the center. For best antenna coupling performance, the transmitter must be directly facing the receiver antenna. By using three receiver antennas placed orthogonally in the X, Y and Z axes, the directivity problem of a single transmitter antenna is solved. Conversely, multiple orthogonally placed receiver antennas can receive signals from different antenna coils in any direction.


Figure 4 RSSI resolution

RF communication in passive access control systems

Atmel offers a wide range of UHF ICs designed for one-way or two-way communication in the ISM frequency range for automotive applications such as car access control systems. For example, the T5750/53/54, ATA5756/57 transmitter series and the ATA5723/24/28, ATA5745/46 receiver series are used for one-way communication. For two-way communication, there are the transceiver series ATA5811/12 and ATA5823/24. Table 1 summarizes the RF issues that must be considered when designing a passive access control system.

Table 2: Comparison of the advantages and disadvantages of different types of antennas


The most discussed issue in RF design is the receivable distance and, of course, the system reliability. Generally speaking, an RF system consists of a transmitter module (in this case, the key card) and a receiver module. To design the best solution, two main parameters must be considered: transmission power and sensitivity. For example, Atmel's transceiver IC ATA5824 can provide an excellent frequency shift keying (FSK) sensitivity of typical value of -109 dBm (at a data rate of 2.4kBps) and a typical transmission power of 10dBm, which helps the car access system achieve a very ideal coverage distance.


Figure 5 Example of 2D radiation pattern of vehicle antenna

Antenna performance

To achieve an optimal system link cost budget and the greatest possible coverage distance, in addition to RF parameters such as transmit power and sensitivity, the performance of the antenna is also critical. In most cases, the antenna design must be a trade-off between the available space and the antenna size. For this reason, the optimal antenna shape is often not achieved in the key fob, and a small loop antenna is more often used. Loop antennas are magnetic antennas that are more useful than whip antennas in key fob applications because they are less sensitive to human contact. However, some applications may require high-efficiency antennas due to long transmission distances, in which case (folded) whip antennas may be a suitable key fob solution. Some antenna manufacturers offer chip antennas that have higher quality factors (Q factors) and gain than printed antennas. This is also a good solution if the cost of the system is not a key factor. In vehicles, the size of the antenna is not very important, and some cars place the antenna on the window (such as the rear window), but the most popular solution is a printed antenna placed on the PCB of the receiving module. Table 2 summarizes the advantages and disadvantages of these two types of antennas.


Figure 6 Example of 2D radiation pattern of key fob antenna

Ground noise reflections

In real life, the environment affects the attenuation of the system link budget due to reflection and fading effects. These factors must be taken into account when defining the link budget of the system. The following calculation example illustrates the impact of ground noise (ground bounce) reflections on the receivable distance:

Example:

• Receiver sensitivity: typical value

-109dBm, 433.92MHz

• Transmit power: typical value 10dBm

• Transmitter antenna gain: -18dB (close to the performance of a small size loop antenna)

• Receiver antenna gain is assumed to be -6dB in this example

If ground noise reflections can be ignored, according to the free space equation, the distance can be calculated to be about 3km. However, in this example, ground noise reflections are taken into account and the typical receivable distance is reduced to 300m. Of course, ground noise reflections in reality are much more complicated than this example. Figure 7 shows how the reflection effect affects the received power of the vehicle antenna. The red curve represents the ideal situation under free space conditions, while the blue curve represents the behavior when someone slowly approaches the car.


Figure 7 Effect of reflection on the received signal power of the system at 868MHz

Blocking

performanceRF systems are always subject to environmental interference; and the situation is more obvious in the presence of a lot of noise and interference inside the car. Atmel's car access devices have excellent blocking performance and can provide the best solution for this type of application. Figure 8 shows the 3dB blocking curve of the ATA5824 at 433.92MHz. However, in some cases, this blocking requirement is far beyond what an integrated circuit can meet. In order to meet the requirements of such scalable applications, an external front-end surface acoustic wave (SAW) filter can be used to improve the blocking performance.


Figure 8 Broadband 3dB blocking characteristics of the ATA5824 at 433.92MHz

IF filter bandwidth

Another important indicator in the system definition is the intermediate frequency (IF) filter bandwidth. For this parameter, all system frequency tolerances must be fully considered. The crystal tolerance and crystal of the receiver and transmitter must be specified very carefully so that the transmitter spectrum signal within the IF filter bandwidth can still be received in the worst case. For systems with very narrow IF bandwidths, the data rate and modulation type must also be considered (in addition to tolerance).

Power consumption

Power consumption has always been a major issue in car access systems, especially in key fob modules. The typical battery life requirement is now around 7 years. Even if the vehicle power consumption requirements do not seem to be so stringent, low-power solutions are still necessary because the number of electronic modules inside the car is constantly increasing. Atmel's UHF devices are specifically designed to meet such low-power requirements.

The following are examples of power consumption of Atmel ICs:

• Transparent receiver IC ATA5745: 6.5 mA (typical) in active mode

• Receiver IC ATA5724: 8 mA (typical) in active mode

• Transmitter IC T5754: 9 mA (typical) at 7.5 dBm power

• Transceiver IC ATA5824: 10.5 mA (typical) in both receive and transmit modes (P=5 dBm)

In addition, there are some methods that can be used to further reduce the average power consumption. For example, by increasing the data transmission rate, the transmission time becomes shorter, thereby reducing current consumption. In terms of reducing the average power consumption of the transmitter, On/Off Keying (OOK) modulation is more advantageous than frequency shift keying (FSK). The best way to reduce the power consumption of the receiver is to switch between sleep and working modes so that the entire circuit remains in a working state when a valid signal enters. For this reason, most Atmel receiver and transmitter ICs have polling modes.