Lighting and Full-Duplex Communication using Optical technology
Charles Lim
[1] Department of Electrical Engineering, Ryerson University
Optical Communication – EE 8114
4
Abstract
This paper discusses possible ways of data communication and lighting from the same source, factors affecting it and a very simple project created to demonstrate the true possibility of wireless communications with LEDs. Some of the critical issues in designing and creating an efficient lighting and optical communications system are discussed as well. An array of white and infrared LEDs and photodiodes are used as transmitters and receivers to enable lighting and full-duplex communication as well as communication. Proposals for driving and powering LEDs can be achieved through new power line technologies for existing homes or through installed optical fiber for newer pre-built homes. The lighting and downlink communication are achieved through white LEDs as transmitters and photodetectors as receivers. On the other hand, the uplink communication uses infrared technology to enable operation at totally different wavelengths. As more research occurs, clearly full-duplex communication is becoming more feasible for in-door wireless communication systems.
Figure 1: Plan of the Room
Introduction
White LEDs, a much more power efficient device than that of an incandescent bulb, brings lighting to areas where there is little power source or efficient use of power is required. Under properly built and operated LED systems, with roughly 50,000 or more continuous hours of life, LEDs easily outlast the lighting sources such as fluorescent and incandescent bulbs eliminating material and maintenance cost. With the recent blackout or power crisis that occurred in the USA and Canada due to aging power grids unable to support the increasing power demand, cutting electric consumption through a more efficient lighting source scheme would save both countries millions if not billions of dollars. LEDs are also the main lighting source used in multimode fibers for optical communications meaning this ability of LEDs to transmit data can also be exploited as a means to transfer optical wireless data while still satisfying its main purpose as a lighting system.
LEDs can be used for communications in places such as hospitals and aircrafts where RF electromagnetic interferences are prohibited. Some major advantages of using LEDs for communication are no interference with Radio Frequency (RF) bands, better security capabilities, fast data rates, long life. Communications using white LEDs are very secure compared to RF communication in that it is confined to an area and can be controlled separately. With very little maintenance required, long life as well as dual purpose of lighting and communication, the initial cost of an LED system can be offset quickly. For these reasons, lighting and communication using LEDs as a source (white LED) are discussed in this proposal.
Methods to carry data to the LED system:
PLC and LED System
Power lines can be used to power the LED as well as provide modulating medium. In existing houses, already installed power lines and outlets behave as data networks and ports which can be used to power and modulate LED lighting systems. In PLC (power line communications) existing devices must connect to an outlet to enable communication. With the combination of LED systems and PLC equipment, wireless communications anywhere in the house can be achieved with or without plugging the device onto an outlet.
The data signal is sent along with power through the power line and is received and filtered at the LED circuitry. The filtered data is then used to modulate the power coming in from the power line through a power supply circuit that modulates the DC voltage going to the LEDs. The array of LEDs used to light up the room will all switch at a high bit rates thus, preventing human eyes from detecting any switching. This would allow for easy installation of wireless LED systems into already built and even better for pre-built houses.
The main factor limiting LED communication speed using PLC technology for implementation for lighting and communication are the existing speed of power line modems. Current modem speeds are in the range of a few hundred kbps which aren’t exactly the fastest way to go. The issues limiting the speeds of PLC include noise mostly generated by electric appliances. The 3 main classes of noise are: stationary continuous noise, cyclic stationary continuous noise, and cyclic impulsive noise synchronous to mains. Once the PLC speed increases, higher data rates can be achieved for LED wireless communication as well.
Optical Fiber and LED system
Another way to modulate data to power the LEDs is to route fiber optic cables around the house beside or with the power cables themselves. The fiber carries the data stream to all the power supplies driving the LEDs and this data stream is used to modulate the power supply to turn the LED on and off. This is very advantageous as high data rates can be achieved everywhere in the house with just one fiber optic cable running around. No EMI interference is induced in the cable and since the bandwidth of the fiber is very high, the extra bandwidth can also be used to carry not only data signals but also video, audio, telephone and television signals to the corresponding devices to serve as the main data carrier attaching all the relevant devices in the home.
The disadvantage of routing fiber optic cables is that it is expensive for already existing houses to place the cables and unnecessary. This type of method is more suited for custom houses or designs where in the houses are still in the process of being built and additional installation of fiber optics into the houses will not cause too much trouble.[2]
Factors affecting LED lighting and communication systems:
Thermal temperature
LEDs are generally low power devices and typically consume 1W, thus if the thermal path is poor then the temperature of the LED device and the package will be high. If the junction temperature of the LED goes over 75°C then the long lifetime doesn’t apply. Typically for the LED to achieve 25,000 to 50,000 hours lifetime the junction temperature has to be lower than 75°C. Most LED devices are limited to a die junction temperature of roughly 125°C and anything over that will cause problems to become common.
Packaging material and Integration
Unlike filament based bulbs which radiate a good deal of the heat away from the source, LEDs do not. Thus, this requires conductive cooling as well as proper LED packaging material. Current LED packages have a thermal resistance of around 300°C/W which is terrible for the limitations of 75°C for long lifetime. If we assume room temperature to be 25°C, then the package must be able to support at least 50°C. System integrators must also be careful when designing the system as they have to balance not only light output, fixture design but also thermal path design.
Multipath dispersion
Multipath dispersion is mainly due to the large amount of LEDs required to light up the entire room. For low data rates, multipath dispersion may not be an issue as the speed of light and the distance is minimal for lower rate applications. Multipath dispersion can be easily taken care of by adaptive digital signal processing techniques with the complexity of the algorithm depending on the communication system requirements. Also, interference from the white LEDs and IR LED transmitters will be minimized due to spectral separation. The narrow linewidth of the LED for downlink and the IR for uplink allows for good spectral separation between both the downlink and uplink systems. Signals from white LEDs will only mainly cause multipath dispersion with the white light photo receivers as well as the IR signals for the IR photodiode.
Reflectivity of the room
Increasing the amount of reflective materials in the room will increase the power of the received reflected signals increasing BER. Even if we only assume we take into account the first reflection(as the second reflections will be of much lower intensity) this could still be a problem or issue causing dispersion for high speed systems. This problem can also be easily taken care of by the adaptive digital signal processing techniques as well.
Downlink/Uplink bit rate
The maximum downlink bit rate is determined by the LED power and receiver circuitry which should not be an issue. The uplink data rate needs to be minimal due to the fact that the uplink system consisting of a few diffused IR transmitters cannot support high data rates unlike the vast number of LEDs used to light up the room. For most home usage purposes like the internet and television, this should not be a problem as data is usually downloaded to the home user. Uplink data rate can afford to be minimal as possible since for normal home purposes, uplink is not that critical unless it is used for a server.
LED Power
LED power is not only critical in lighting the room but also in ensuring the optical receiver is able to receive enough power to support the current data rate. The optical transfer function is given by
Optical TF = (PR / PE)
where
PE = PR(ph2)/(Acosd) * sin2(q)
d = angle between the radiation axis and the normal to the receiver area.
q = p/2 for a lambertian source
A= area exposed at the rx emitted from the transmitter light.
h[m] = distance of the rx to tx
[3]In the Lambertian emission, radiant intensity Pointer depends on the angle of radiance q.
Ptr(q) = (m+1)Ps [Cosm(q) / 2q]
Ps = total power transmitted
m = directivity of emission pattern.
Also,
PR = wAsin2(FOV)
FOV = receiver field of view
0 o < FOV £ 90o
w = uniform radiant emittance
The received optical power is independent of position and angular orientation of the photodetector with respect to the radiating surface. (Surface should cover the entire FOV).
DOWNLINK USING WHITE LEDs UPLINK USING INFRARED:
The design of the downlink is to use the existing white LEDs used primarily for luminance to modulate the transmitted signal and get received on the receiver of the device. The uplink design is to use diffused IR to allow for spectral separation of the downlink and uplink carriers. The IR transmitter will be on the device while a single IR receiver will be installed along with the array of white LEDs for every ceiling section of the room to ensure full coverage of the area. Depending on how large the room is, it will determine how many IR detectors are needed to be installed.
Infrared and visible light are free of government regulations, immune to radio interference and secure (cannot penetrate walls). The key factor in infrared is its spectral region; it offers a virtually unlimited bandwidth while another key factor for white LEDs is the abundance of optical power throughout the room and the ability to modulate it at high data rates.
With all this in mind, optical signals(IR and visible light) like other light sources also have some interference when transmitting or receiving signals. The major sources of interference affecting infrared are; sunlight, and indoor light. Assuming we are operating an LED system throughout the house, IR interference will be minimal unless a large amount of sunlight is allowed inside the area. As the lighting mechanism is powered by the LED with a narrow linewidth limited to the visible band, IR wavelengths should encounter minimal interference. Same can be said of the white LEDs and as long as the visible light receiver is not placed directly on the sunlight there should be minimal interference with the downlink signal as the optical power generated by the white LEDs are good. To ensure all possible cases of interferences are checked and to increase the reliability of the system an LMS adaptive digital signal processing technique is proposed.
Figure (2): Optical Power Spectral Densities of common light sources.
Simple LED Downlink System using RS232
To prove the theory of LED lighting and communication, a simple circuit was designed utilizing RS232 serial communications, a couple of superbright white LEDs as well as visible light photodiodes. RS232 is a well known serial communications technology and components to interface to this technology are easier to find, as such, this has been chosen to be the interface to the computer.
TX Circuit
The transmitter circuit was realized by driving the white LEDs according to their forward bias current using the correct resistor values all attached to the emitter of the same transistor. The transistor gets turned on and off by the incoming RS232 signal generated by the computer serial port.
RX Circuit
The receiver circuit was realized by using 3 visible light photodiodes and an op-amp used as a comparator circuit. The comparator inverting terminal is referenced by a known voltage level with the non-inverting terminal voltage being controlled by the photodiode circuit. Any changes in the photodiode voltage(i.e. data from light received) causes the comparator to switch high or low.
Glue Logic
To interface both the transmitter circuit to the serial port circuit. A serial RS232 line transceiver was used to convert the input of the RS232 to logic that can drive the LED transistor circuit. On the other side, the receive input of the transceiver was attached to the output of the photo-detector circuit and is used to drive the connecting receiver pin on the RS232 port.