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Thursday, 21 September 2017
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OVA Bargellini S.p.a.

ELSA - Emergency Light System Adressable

The object of the research was the realizzation of a wireless infrastructure able to monitor and control complex emergency lighting systems.

Concerning emergency lighting systems, national law requires periodic checks in order to guarantee a near to zero probability to have faulty devices. Those requirements need periodic services that can be really long and expensive, especially if the lighting system is particularly extended. During the past years, in order to overcome this problems, some solutions have been proposed. The simpler solution was the usage of a 2-wire data bus that connects clusters of lamps to a central control unit. On alternative implementations the wiring is optimized and as data bus the lamp use the same mains supply lines.
The control units are so connected to a central server that monitor on real time the status of the network. The server can also control actively the lamps, or on more interesting cases, suggest a lamp check before that a fault happens.
On the sequel we analyze the logical evolution of this network structures realized that moves the information exchange to a wireless connection.
The system is based on ZigBee modules fully integrated on the lamp package. On the network three groups of devices can be identified:
1) Bridge
2) Repeater
3) Lamp
The Bridge is the main device and have the functionality to connect the wireless network to the wired data bus and so to a control unit. All the lamps are connected using multi-hop paths to the reference Bridge and communicate only with its Bridge. The network creation is initiated by the bridge and stopped manually when all the lamps are added to the network. Leveraging on ZigBee standard implementation the radio channel is automatically selected minimizing possible interference from nearest 2.4GHz devices. A network identifier is also present an coded on wireless hardware in order to permit coexistence of different networks on the same area.
Instead the Repeater is a device necessary to enlarge radio coverage on radio signal shaddow zone where group of lamps can't be reachable.

One of the goal of this project was the realization of a sufficiently precise prevision software able to check radio coverage and suggest or automatically estimate the best positions of Bridge and eventually Repeater devices.
The problem is really complex because we have not only to consider propagation issues but also intrinsic limitations on ZigBee stack and control unit limits.
Only as example ZigBee have a limit on the maximum number of hops and the number of children that a single device can connect due to a particular internal memory organization. This limitations are importants because also if radio coverage is guaranteed by neighboring devices there is the possibility that doesn't exists data path to the Bridge due to protocol constraints.
The prediction software uses an ACAD compatible draw as input map.
Some different layers are used to store and load topology information on the considered environment.
A single draw layer is used to record walls positions with different color line where wall type changes.
The lamps are included on different layers of homogeneous type.
The software uses a multi-wall propagation model as a good compromise between precision and computation complexity.
This model consider only the main propagation ray between two devices and add to the path loss an additional attenuation estimated using the number and the type of walls or obstacles crossed.
Initially the software require to positions one or more Bridge devices, so a triangular matrix are filled with the propagation attenuation between all the network devices.
A routing tree are so designed using protocol constraints on maximum number of children and hops, in correspondence to the worst case propagation conditions.
As final phase if some lamps are not covered, the software is able to highlight the affected devices and identify on detail the problem.
The network structure can be modified adding or changing position to Repeaters or Bridges until all the devices are covered by the radio signal.
The prevision software match with a good precision the real network behavior, howewer a more detailed path loss characterization is essentially to guarantee good prevision on wide networks.
In order to tune the propagations model the LQI (Link Quality Index) parameter has considered.
LQI is an index related to the packet error probability and estimated using preamble field of 802.15 PHY frame.
Analyzing the measures realized on different conditions is evident the high dependences from surrounding environment and LQI changes.

Lamp Radio Coverage

On Figure an example of a radio coverage prevision result is reported.
The blue dot is the Bridge device and the orange dots are 3 Repeater used on this case only with test pourpose, all the other devices are lamps.
On the high, left corner is also possible to note the multi-wall  model applications, in fact the link with attenuation of 60,7dB crossing 2 walls have an higher attenuation of the near link at 57,7dB also if distance are lower.
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