5. MOBILE SENSORS AND COMMUNICATIONS
5.3. Mobile Sensing in City.Risks
5.3.5. Theft detection sensor technical consideration
From the previous description it is clear that BLE consumes much less energy than its predecessors. Not only does it significantly extend the battery life of traditional Bluetooth devices, but it also enables wireless communication for a class of low- power devices that run on as little as coin-cell batteries. Besides, lower energy consumption leads to a more robust, efficient product.
Because BLE is super energy-efficient, manufacturers have been aggressively building BLE capability into modern devices such as phones, and tablets. On the supply chain side, major manufacturers make BLE modules and chips widely available and in large quantities. A wide and growing range of applications that communicate with BLE- supported devices are now available in both business and consumer markets.
However the most challenging issue as emerged from the project development process is the Radio-Link mechanism (RLM) to be used for triggering a remote interrupt at a ‘sleeping’ beacon device. The basic thing here is to combine BLE module with an RF chipset operating in 2.4GHz or other frequency band.
Normally the radio network should be used to convey the Wake up signal from the Web platform to theft detection sensor so as to wake up the BLE and then it should broadcast the alarm signal. Of course radio network shall provide a local coverage within a reasonable range of signal reception.
5.3.5.1. Sensor use case
The sensor shall be used in the following use case as described below:
A citizen attaches a theft detection sensor, a small and discrete sensor coupling Bluetooth and radio-based technologies, to a personal item, e.g. mobile phone or bicycle.
He / She registers the sensor with the authorities.
The responsible authority remotely activates the sensor from its hibernation mode by multicasting a short-range signal that triggers the sensor to periodically broadcast signals to mobile devices in proximity.
The signal broadcasted by the sensor is picked up by a mobile device with the City.Risks mobile application installed and the authorities are notified by the application that the stolen item has been located.
5.3.5.2. Operational scenario and working modes
BLE module should work in peripheral mode and in beacon mode. It shall simply transmit information that is stored internally. Because the beacon device does not activate any receiving capabilities, it achieves the lowest possible power consumption by simply waking up, transmit data and going back to sleep.
Once an item (bicycle, motorbike, mobile asset) with such a module installed is stolen, the owner shall inform the authorities which in turn shall send through an application a broadcasting signal via Internet to certain local RF base station infrastructure.
RF chipset –co-existing with BLE module on the same board-should be able to receive a notification message from a nearby base station antenna within a certain range of coverage.
Then through the interface between RF chipset and BLE the BLE module shall turn from sleep mode to wake up mode and it will start transmitting the alerting signal to the nearby mobile applications allowing for Detection of a stolen item in the area. The amount of information that is going to be forwarded from RLM to BLE is not that big, i.e. a wake up signal, therefore the interface should be quite simple and mostly reliable. Radio chipset should normally work in receiver-only mode.
BLE should normally be in sleep/wake up mode as peripheral and should turn to beacon mode only upon receiving alert message from the RF chipset. Again RF chipset can also be put to sleep mode and then periodically wake up to be on "listening-for-alert" mode. The last two working conditions shall be followed to preserve module's battery.
5.3.5.3. Radio link mechanism approach
City.Risks shall define the required functions that the Radio-Link mechanism (RLM) should encompass to trigger the ‘sleeping’ beacon device.
The radio link mechanism is an add-on that is included to the sensor in order to formulate a compact, discrete battery-powered application.
Under this context, the development of such a prototype combining BLE module and RF chipset must take into deep consideration that the integrated board should be small-enough so as to be easily installed as a theft detection sensor.
RLM must be integrated in a chipset consuming as less energy as possible. Since the power consumption is a key factor in sensor design, global RF networks such as GSM, TETRA, UHF etc. although they might be suitable for functional prototyping they are still too large as a deployment platform for our application, and are considered as high energy -consuming devices.
A key point is the coverage distance that the RF network can cover. This certainly means that if the user perceives the theft of his/her item by the time it has left the coverage area then the broadcast signal originated by the authorities shall never reach the module RF chipset as it will be out of coverage zone.
A fine-grained analysis is required so as to examine what could be the most suitable radio link mechanism that can be coupled together with the BLE kit thus turning a complete end-to-end powerful kit to powerful sophisticated theft detection sensor. Under development process the most important aspects needed to be taken into account are the following:
Integration of RLM with the BLE board
Definition of RLM and BLE inter-operational modes Protocol handling.
The above aspects involve technologically-based subtasks. In particular the development of the board is based on:
Schematics - How to wire things together Layout - How to organize parts on a board
Manufacturing - How to get boards assembled in bulk
Code - How to establish communication from the BLE chip to RF chipset and so forth.
The average power consumption of the RF chipset should also be kept very low enabling quite a long time range operation. The power consumption is strongly related to the operational logic and to the time periods where the radio mechanism is “listening” for alarm broadcasting message and the BLE module is awake for transmitting the alarm message to smartphones in proximity.
The use of Wi-Fi™ as an RLM is considered as a first choice option for our application extending wireless connectivity to enable sensor’s communication with cloud services and backend server, thus improving usability and reducing maintenance needs.
Besides,2.4GHz unlicensed band and Wi-Fi™ are wireless standards with either a specific focus on–or recent additions addressing–simpler but low power or ultra low power wireless technologies. These are the main technologies that have been examined as RF candidate mechanisms for the theft detection sensor implementation.
The different technologies can roughly be split into the following categories: Low power (average current consumption in a node 5-50+ mA):
• Bluetooth® versions prior to v4.0.
Ultra low power (average current consumption <1 mA) • ANT+™
• Bluetooth® v4.0 (which includes Bluetooth® low energy as a hallmark feature). This segmentation also roughly corresponds to the applications that can use the different standards. Due to the limited data transfer capacity, ultra low power standards are mainly used in applications where the data throughput demand is low (< 100 kbps) such as in sensor & actuator networks, user control input and for limited size file transfers. This also applies to our specific application.
Lithium coin cell batteries, traditionally used for low power applications, are the battery technology of choice for most of ultra low power wireless applications today. These batteries are simple to fit in small enclosures and replacements are easily accessible for the end user.
To this respect, as our application involves ultra low power wireless solution it is absolutely imperative that the average current consumption shall be as low as possible. But, focusing only on the average current assumes that the battery capacity found in a battery data sheet is fixed for all conditions.
Even in sleep mode WiFi based RLM consumes much more power than BLE technology resulting in sooner battery drainage. Besides once wake up, WiFi needs to associate Access Point - this procedure might can take several seconds even to minutes.
By examining all possible wireless candidate solutions that are available, it was clear that critical point of sensor design involved the state when the radio circuitry is activated. By that time it can draw large amount of current depending on technology, vendor and implementation. This drain could far exceed the rated drain current condition (~200 uA for a CR2032) for which the battery capacity is foreseen to provide, according to the battery specifications.
5.3.5.4. Assessment of key parameters
There are specific key parameters which should carefully be taken into account: Overall Size
The total size of the sensor must be kept very small since it is supposed to be placed and attached to bikes, bicycles, hand-bags etc.
Total Power Consumption
Perhaps the most challenging issue here, since we must accommodate RLM kit along with BLE kit, therefore a fine-grained analysis of power consumption is required.BLE module should work as a peripheral device which is assumed to be a low power device that exposes short piece of information. Despite of the peak current during transmission can reach relatively high values the user should take into account that
In most cases a peripheral device spends the majority of its life asleep only waking up when it needs to send data. This is a good perspective though since the operation of the BLE device is limited in only short time intervals.
Modular Scalable and easily customizable open platform
Application Logic structure should be scalable, expandable and allow for easy configuration and over the air firmware download.
5.3.6. Research directions
City.Risks theft detection sensor technical examination is currently in progress and key findings have been already presented in previous paragraphs. In its current phase, the on-going analysis of the state of the art includes the identification of commercially available BLE hardware platforms that should be suitable for our application development. The evaluation of characteristics, the consideration of coupling mode between BLE and wake up radio mechanism, the design of functionality and operational mode as well as the definition of application structural logic have also been examined and addressed.
Planned future work include further hardware platforms performance characterization, in-depth exploration of candidate wake up radio integration and interoperability with BLE stack, and optimization of the implementation bottlenecks.