RF Connectivity Eases Up on Power Not Security

Sensor networks for consumer and industrial applications can benefit from low-power RF technologies combined with hardware encryption for safeguarding data.

With the spread of wireless connectivity solutions in consumer electronics and the proliferation of Internet of Things (IoT) nodes, bringing similar connectivity to applications in areas such as access control and sensor networks in home and industrial spaces enables more products to become wirelessly connected. It is not too far-fetched to imagine a logistics scenario where a delivery truck pulls up to a warehouse and is granted access through the gates after authentication via wireless communication. Furthermore, as pallets are being offloaded from the truck, each pallet sends its content information, along with its environment conditions such as temperature and humidity, to a central monitoring station where the data is consolidated for inventory management and quality control. All of this is possible today with radio frequency identification (RFID) and radio frequency (RF) communication technology that allows secure wireless communication of data between objects that need to be tracked and a reader station. It requires these objects to be fitted with a tag or transponder that communicates securely with a reader device that is a certain distance away. The communication from the tag can be triggered by a wake-up signal from the reader or can be actively initiated from the transponder side.

By integrating a combination of these technologies in a general-purpose microcontroller (MCU), developers can create solutions for industrial access, home automation, remote monitoring and produce tracking applications, while enjoying the ease of development that a MCU provides. The primary concerns for developers of such applications are power consumption and the cost of bundling multiple features that enable these applications. Other important factors are read range, development effort, noise immunity to environments such as water or metal presence, cost of implementation and also security of data communication.

In that regard, a single package solution that integrates RF technologies, with low power capabilities down to a few µA and advanced encryption standard (AES) hardware would bring immense value to these applications. A low-power solution is important to consider because it can provide extended battery life of up to 6 years with a standard 2032 Lithium coin cell battery and reduce bill of materials cost for the complete system, thereby making it more cost effective to bring RF communication capabilities to many more applications.

Overview of RF technology

Several RF technologies exist and differ from one another in the frequency in which they operate: low frequency (LF) operates at < 135 kHz, Sub-1 GHz operates from 300 MHz to 900 MHz and ultra-high frequency (UHF) operates at up to 5.4 GHz. These communication standards differ in power consumption, battery requirements, read range and immunity to environmental factors. There is also a distinction in whether a transponder can operate passively without a battery, or whether it requires power. In all cases, a reader device initiates the communication and needs to be powered; it can also be used to power a passive transponder using RF energy. Every application is different and requires a careful analysis of system requirements to determine which RF technology does the job best.

This article will focus on a 3-D low-frequency (134.2 kHz) device with wake-up trigger and transponder interface, sub-1 GHz transceiver and integrated AES cryptography co-processor. It highlights what applications can benefit from these RF technologies combined with hardware encryption for data security.

Evaluating the enabling technology

Low frequency (LF) wake-up receiver: An LF wake-up receiver enables the complete system to be shut down and powered-up only when a trigger is received from a reader device. On detecting the RF field from the reader, the LF wake-up interface awakens the rest of the system by generating an interrupt to the core and enables further communication back to the reader. This allows for activation and deactivation of the tag in a dedicated and well-defined read-zone “on-demand” to achieve extended battery life for the whole system. This is in contrast to an active transponder, which transmits continuously and consumes a lot more battery power. It is critical for system design that the wake-up receiver draws very little current when system is in stand-by mode.

3-D antenna: A three-axis antenna on the LF interface can be used to ensure that communication and wake-up is achieved in all orientations of the transponder. Furthermore, it gives the ability to determine the location of the tagged object in the read-zone. The LF wake-up receiver interface uses the return signal strength indicator (RSSI) on each of its three channels to determine the RF energy each antenna receives from the reader and based on that it can determine the position typically with a few centimeters accuracy. It uses the same antenna for the 3-D LF transponder interface.

3-D low frequency HDX transponder interface: The LF communication interface responds by modulating a frequency-shift keying (FSK) signal without an active RF field from the reader and sending data back. It does not require any battery power and operates passively. This interface enables system implementations that require a back-up interface for communication in case the battery dies. It is very common in asset tracking, access control and livestock tracking applications. The read range on this interface is in the order of 10 cm. High sensitivity, excellent blocking performance, low current consumption and compatibility to various ISO standards are the key drivers in component selection here.

Sub-1 GHz radio frequency transceiver: In an active transponder application, the communication range can reach up to several hundred meters depending on the environment. It requires battery power to send and receive data with the corresponding sub-1 GHz receiver on the other end. There are multiple regional frequencies that are permitted and a developer must ensure the right frequency is chosen.

Data encryption: Security of data is critical. It ensures any information read or sent is only to and from the intended and authorized reader. Another benefit is that the AES security feature can also provide efficient product authentication for the tagged object. Before the data is transmitted, the hardware encryption engine can encrypt the data using one of the available encryption standards such as AES-128 and decrypt the data using the same scheme on receiving it. Hardware-based encryption saves on valuable CPU bandwidth in performing this task.

Low-power MCU with peripherals: Any MCU integrating these RF functionalities needs to provide low-power capabilities so the system can be put in standby- or off-state until it is triggered for communication. The MCU core is central to the whole system, as it collects data received or sent on LF and sub-1 GHz interfaces. With additional peripherals like an analog-to-digital converter (ADC) or analog sensor interface, the MCU can collect sensor data such as temperature and send it over LF or RF interface.

Enabling a broad range of applications

Figure 1. Low-power RF technology can be applied in a variety of connected applications.

Asset tracking: High-value assets in a hospital or construction site can be tagged with transponders that remain in standby mode most of the time, but as a reader device is activated and moved around the property, each transponder will activate when it comes in the read-zone. When the transponder becomes active, it will send the unique ID of the asset to the reader for inventory management. This eliminates the need for resource-intensive visual or manual inventory tracking. With the location tracking, the reader can log the exact location where each asset was detected.

Container tracking: Similar to the asset tracking application, shipping containers can be fitted with the transponder that remains in standby mode until it passes through a read-zone at a shipping channel or the docks. In the read-zone the transponder is activated by the LF trigger and sends the identification content information and temperature measurements securely over the RF interface.
Access control: Similar to the familiar use case of vehicle access, the combination of LF wake-up and an RF transceiver can be used for home and industrial access control. The key fob is activated by the LF trigger in the access zone and sends the encrypted information over RF interface to authenticate access. Furthermore, in a home setting it can be used to automatically pick up the light settings as the person carrying the key fob enters the house or a specific room.

Wireless sensor networks: Wireless sensor nodes remain in the standby mode and periodically wake up to transmit measurements to the reader. Industrial environments where monitoring and data logging are important can benefit from remote monitoring capabilities.

Health and fitness devices and sports timings: Fitness devices are fitted with sensors that take measurements of vital signs like temperature, hydration, pulse rate, etc. They continue to log data and communicate to the reader via RF interface. In case the battery dies, the LF transponder can be powered passively and transmit the data back to the reader. It is important that the data transmission is done securely.

There are countless other applications such as product authentication through the AES engine, remote monitoring systems and more that can benefit from the cost advantage and easy development of RF technologies.

Figure 2. Diagram showing LF trigger and RF communication.

Conclusion: evaluating an RF system-in-package solution

Developers have many concerns when evaluating an RF system. Key areas to focus are reducing power consumption with the LF wake-up function because it extends battery life and the ability to shut down the radio and start-up on the LF trigger. Developers also need to consider a 3-D LF receiver, since it can show position and distance of objects with the 3-D LF receiver, delivering RSSI on all three channels to determine the coupling and orientation of respective antennas, as well as orientation of objects tagged with this device. Determining the exact location (+/-5 cm) of a device within the LF trigger near field (up to 4m radius) is critical.

Finally, developers should consider security. Protecting sensitive data and preventing signal interference with high-security can be enabled by an AES co-processor. It allows data encryption for product authentication and/or high security access control capabilities. And, selecting a product that accommodates a wide range of regional Sub-1GHz frequencies with wide band (300-950 MHz) frequencies will help developers create solutions that support many countries.

One solution that is available in the market today is the TI RF430F5978. It is a unique system-in-package solution that is based on TI’s ultra-low-power MSP430™ MCU, integrates a 3-D LF wake-up receiver and transponder interface and a sub -1Ghz transceiver with an AES hardware encryption for data security.

Diwakar_BansalDiwakar Bansal works in Product marketing, Safety and Security MCU, at Texas Instruments.

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