Broadband Wireless Local Area Networks
and Remote Sensing
With the increasing popularity of multimedia services supplied over the fixed network, services such as: web browsing, video conferencing and video on demand, it is for sure only a matter of time before users will demand higher bandwidth mobile access. Advances in displays, battery technology and processing power have also made it possible for users to afford and carry around laptops and palmtops. The prospects for the delivery of multimedia services to these users is, however, crucially dependent on the development of low cost physical layer delivery mechanisms. Market needs to consolidate the data into one single handset.
To address the bit rate limitation problems of current wireless systems, Dr. Kavehrad, W. L. Weiss Professor of Electrical Engineering, and his research team are examining the concept of adaptive rate delivery of future mobile / portable multimedia services with high bit rates (>100 Mbits/s) for localized areas. Examples of physical layer technologies are GSM, UWB, and Optical Wireless (OW) for the high-bandwidth islands, e.g., classrooms, hotels, future homes, shopping malls, airports, train stations, planes, spacecrafts, etc. Consider the area of home networking, when every home will be illuminated with bright white LED light which can also be a broadband communications carrier. We are entering a new era of always on connectivity. The expectation from consumers for not only ubiquitous but also seamless data, voice and video services presents a significant challenge for today’s telecommunications systems.
[Image Courtesy of : Giga-IR SIG]
Sensor networks are also increasingly being used for monitoring and controlling vital operations in industries, hospitals, and military installations and vehicles. Current research trends concentrate on Radio Frequency (RF) technology for sensor communications. However, OW communications or Visible Light Communications (VLC) can offer a much higher data rate or higher reuse factor.
As we step further into the 21st century, the demand for sustainable energy-efficient technology grows higher. The important area of electric lighting, currently dominated by decades-old incandescent and fluorescent sources, is being taken over by white light emitting diodes, which are solid state devices with much greater energy savings. Replacement of current inefficient lighting by these LEDs will result in reduction of global carbon dioxide emissions, a major cause of global warming, among other things. White LEDs hold the potential, in the field of photonics, to be as transformational as the transistor was in electronics. This core device has the potential to revolutionize how we use light, including not only for illumination, but also for communications, sensing, navigation, imaging, and many more applications. In the tutorials listed below, we highlight some of the potentials.
By: Mohsen Kavehrad
IEEE Communications Magazine, December 2010.
By: Mohsen Kavehrad
Scientific American Journal, July 2007
Wireless communications by infrared (IR) or visible light or ultraviolet light is inherently secure, since it is usually confined within an enclosure. It offers no interference to existing RF sensing or communication infrastructure. It is unregulated worldwide unlike RF spectrum, and small devices can be manufactured that are suitable for miniaturized sensors.
Broadband Optical Wireless Communications for Sensor Networks
Several previous works have investigated the potential of OWC for indoor local area networking. With advancement in solid state optical device manufacturing and increase in availability of off-the-shelf components, OWC needs to be reviewed. We have built a test-bed comprising of Laser transmitter and Avalanche Photodiode (APD) receiver, and measured the optical wireless channel characteristics.
Fig. 1 shows a photograph of the experiment setup. The laser transmitter has a collimating lens attached to it, which allows us to focus a spot on the ceiling, where the infrared light gets diffused. The reflected light is captured with a focusing lens at the receiver. This setup is termed non-directed line-of-sight, as opposed to line-of-sight and diffuse configurations. To obtain frequency responses, a network analyzer is used to modulate the constant laser drive current with RF frequencies swept in the range from 100kHz to 1GHz. Fig. 2 shows the calibrated frequency responses, at 3 different locations in a standard laboratory room, using collecting lenses of 3 different diameters for light collection at the receiver.
Fig. 1 Experimental setup for Indoor Optical Wireless Channel characterization
Fig. 2. Magnitude and phase responses
It is evident from the frequency response plots that the OW channel has an approximately flat frequency response up to 1GHz. The implication of this finding is that the channel contains very little multipath distortion, and would allow transmission of data with little Inter-Symbol Interference (ISI), if the signaling rate is 2 Giga-symbols per second.
To assess the channel more appropriately, impulse responses can be calculated by taking inverse Fourier transforms, after proper application of smoothing and Kaiser windowing. The resulting impulse responses are illustrated in Fig. 3.
Fig.-3. Impulse response obtained from measured frequency characteristics
The impulse responses indicate how much in time the channel spreads out the signal, which leads to ISI and reduction in channel bandwidth. From the figures, we can see that the bases of the impulses are in the proximity of 1 to 2 ns. This is an initial estimate, which we further make accurate by calculating the delay spread, which are tabulated below.
Table-1. Delay spreads for different receiving locations with lenses used
The delay spread in all the cases are below 0.5ns, which is the resolution of the measurement system, as limited by the rise time of the APD, as well as the Kaiser window applied for transformation. As a result, we conclude that the data rate of the system would be limited by the devices rather than the channel itself.
Taking 0.5 ns as the delay spread, the coherence bandwidth, or the bandwidth over which the channel can be considered flat, can be approximated to be 2 GHz. This allows us to send data at and beyond rates of 1 Gigabits per second, depending on the modulation scheme we choose.
The noise in the system also has to be taken into account when we assess data rate performance. The noise is dominated by shot noise component due to ambient background light for indoor sensor communications, and this degrades the SNR at the receiver. The ambient light noise is calculated and used, in conjunction with the impulse responses obtained before, to generate simulated eye diagrams, shown below, for a 1 Gbps Intensity-modulated/direct-detected (IM/DD) link.
Fig.-4 Error-free eye patterns for 800 Mbps in noise over TOTAL path lengths: ~5 meters, ~6 meters and ~7 meters.
Fig.- 5 Error-free eye patterns for 1 Gbps in noise over TOTAL path lengths: ~5 meters, ~6 meters and ~7 meters.
The eye diagrams clearly indicate that the “open eye” condition is fulfilled, and by visual examination, there is very little ISI overlapping at decision point. As a result, complex equalization schemes are not required to recover distorted signals, as are necessary for conventional RF-based communication systems. Reduction in system complexity leads further to reduction in power consumption, and gives us a ‘greener’ solution to indoor wireless communications or for sensor communications.
The challenges regarding attenuation and multipath distortion can be overcome by using multi-spot diffuse configuration and fly-eye reception ( Kavehrad & Yun, 1992 ). In this configuration, the transmitted beam is split in a control manner into several narrower beams by means of holographic beam-splitters. The narrower beams illuminate selected spots on a reflecting surface. Thus, path loss due to diffusion is reduced, and fly-eye receivers can use diversity combining techniques to increase signal-to-noise ratio.
In summary, the need for future higher data rate can be fulfilled satisfactorily only by optical wireless communication. This potential has been recognized by the infra-red data association (IrDA), who have recently announced the GigaIR standard, for short range infrared communication links operating at 1 Gbps. Further research and development on device manufacturing to improve modulation bandwidth and operating frequency can potentially take data rates to even higher quantities.
M. Kavehrad, J. Fadlullah, "Wideband Optical Propagation Measurement System," Proceedings of the SPIE Photonics-West Conference,
, January 2010. San Francisco, California
IN THE NEWS:
Beaming Broadband Across the Room:
Triple Play Using Power Lines and White Light Emitting Diodes for Home Networking
The increasing interest in modern multimedia applications, such as broadband Internet, HDTV, etc, requires new last mile access and wireless techniques for connecting private premises to communications backbone network. A promising technique, broadband over power lines (BPL) (see: http://cictr.ee.psu.edu/research/bans/index.html ) uses electric power-lines as a high-speed digital data channel to connect a group of private users to a very high data rate fiber backbone.
Channel characteristics of medium voltage overhead power-line grids, a common type of grid in the
, were investigated in details by us in: “Transmission Channel Model and Capacity” and “Medium Voltage Overhead Power-Line Broadband Communications”. It is shown that although the overhead power-line grid is a very low-loss medium, it may suffer from very deep fading caused by multipath due to reflections. Mismatch at the branches of the power-line network reflects signals back and creates several signal paths from a transmitter to a receiver. In the same papers, United States Shannoncapacity limits of overhead MV–lines channels have been investigated. It is shown that the overhead power-line grid network has a high-capacity limit compared to other similar wire-line structures such as cable and twisted pairs. See:
§ M. Kavehrad, J. Fadlullah, “Optical Wireless Networked Systems: Applications to Aircrafts,” SPIE Photonics West, San Francisco-CA., January 2011.
§ P. Amirshahi and M. Kavehrad, “Medium Voltage Overhead Power-line Broadband Communications; Transmission Capacity and Electromagnetic Interference,” Proceedings of ISPLC 2005, Vancouver, Canada, April 2005.
§ P. Amirshahi and M. Kavehrad, “Transmission Channel Model and Capacity of Overhead Multi-conductor Medium-Voltage Power-lines for Broadband Communications,” IEEE Consumer Communications & Networking Conference, Las Vegas, Nevada, January 2005. (.PDF)
Homes are connected to electric grid by low-voltage lines (
). Low Voltage lines are distributed to each power plug in every room in a house. More than 99 percent of homes in the LV have access to electricity, whereas connectivity level is far less for cable and phone lines. Thus, a combination of MV and United States power lines can be an appropriate candidate for providing broadband access to every home in the country. The characteristics of LV power lines are very well known, and there are a variety of research activities in this area to exploit different features of LV grids. LV
Indoor wireless connectivity is always appealing to consumers because of its ease of use. One of the conventional wireless access systems is Wi-Fi. But these systems and similar other wireless schemes suffer from many shortages, including interference, not being able to provide quality of service (QoS), adequate coverage and most importantly, security.
A better alternative for high-speed wireless home networking, delivering voice/video/data (Triple Play) is to use optical wireless, indoors. Use of conventional lasers for optical indoor communications has not been feasible as yet because of the high cost of laser sources. Instead of lasers, LEDs can be used as communications transmitters connected to electric grid, receiving high-bit-rate signals via BPL.
Recently, WHITE LEDs emerged in the market and are considered as future “lamps.” Apparently, in the near future, the low cost, efficient and miniature WHITE LEDs will replace the incandescent and fluorescent lamps. Researchers pledge that by 2012, these devices will reach seven watts and 1000 luminescence. This is brighter than a 60-watt bulb, yet draws a current provided by four D-size batteries. A Japanese research team suggested using the same WHITE LEDs not only for lighting the homes but also as light sources for wireless in-house communications. Using this new and developing technology along with MV-LV–power-lines communications can create a revolution in the area of consumer networking because of its efficiency and affordability. Therefore, in future, you turn on the lights for indoor low-cost lighting and you receive broadband via the same through modulated WHITE LED light.
Figure 1: (a) Frequency; (b) Impulse Response (c) capacity of an
MV overhead Power-Line Network
Figure 2: (a) Frequency; (b) Impulse Response (c) Capacity of an LV–Power-Line Network
Figure 3: Visible Light Communications Using Visible Light LEDs
Figure 4: Illustration of Signal Propagation.
The Room Surface is Composed of Three Elements: a, b, and c.
Penn-State Research News: Optical Wireless And Broadband Over Power Lines: High Speed, Secure Wi-Fi
Tuesday, Jan. 10, 2006, at
University Park, Pa. --- Penn State engineers have shown that a white-LED system for lighting and high data-rate indoor wireless communications, coupled with broadband over either medium- or low-voltage power line grids (BPL), can offer transmission capacities that exceed DSL or cable and are more secure than RF.
Eurek-Alert: Optical Wireless……..
Technology News Daily: Optical Wireless………..
Science Daily: Optical Wireless And Broadband Over Power………
CNN_Magazine _Com: Wi-Fi alternative
Innovations-report: Optical wireless………….
Physic-Org: Optical wireless and broadband over power lines..
M. Kavehrad, P. Amirshahi, “Hybrid MV-LV Power Lines and White Light Emitting Diodes for Triple-Play Broadband Access Communications,” IEC comprehensive report on; Achieving the Triple Play: Technologies and Business Models for Success, ISBN: 978-1-931695-37-4, January 2006. (.PDF)
and M. Kavehrad,
“Broadband Access over Medium and Low Voltage Powerlines and use of White Light Emitting Diodes
for Indoor Communications,” IEEE Consumer Communications & Networking Conference,
Y. Alqudah and M. Kavehrad, “MIMO characterization of indoor wireless optical link using a diffuse-transmission configuration,” IEEE Transactions on Communications, Vol. 59, No. 9, September 2003, 1554–60.
Dominic C O’Brien and Marcos Katz, “Short-Range Optical Wireless Communications,” Wireless World Research Forum. (.PDF)
In this research, we investigate the capabilities of each of these techniques for providing broadband communications.
Indoors Broadband Wireless Optical Local Area Networks
“Huge Bandwidth & Huge Bandwidth Reuse”
It is commonly agreed that the next generation of wireless communication systems, will not be based on a single access technique but it will encompass a number of different complementary access technologies. The ultimate goal is to provide ubiquitous connectivity, integrating seamlessly operations in most common scenarios, ranging from fixed and low-mobility indoor environments in one extreme to high-mobility cellular systems in the other extreme. Surprisingly, perhaps the largest installed base of short-range wireless communications links are optical, rather than Radio Frequency (RF), however. Indeed, ‘point and shoot’ links corresponding to the Infra-Red Data Association (IRDA) standard are installed in 100 million devices a year, mainly digital cameras and telephones. It is argued that Optical Wireless (OW) has a part to play in the wider 4G vision.
In large open environments where individual users require 100 Mbps or more, OW is a more sensible solution because of its limited cell size. Today's Radio Frequency (RF) LANs realistically cannot support more than one or perhaps two high capacity users per cell. With cell sizes of ~100 meters which could accommodate ten's of users, this is highly wasteful. Multiple high capacity users could only be serviced by deploying a similar number of systems, all within the same locale. This would create a situation where the multiple cells almost completely overlap, which then raises concerns with regards to interference, carrier re-use, etc. In contrast, OW could deliver the necessary capacity to each user through multiple user-sized cells, and because of the intrinsically abrupt boundary of these cells, interference would be negligible and carrier re-use would not be an issue. Indeed, OW is a future proofed solution since additional capacity far beyond the capabilities of radio could be delivered to users as their needs increase with time.
We have been examining the potential of infrared (IR) for transmission of information packets in broadband indoor multimedia wireless communications. The current focus is on the feasibility study of broadband indoor infrared wireless systems for very high-speed transmissions, as in broadband wireless local multimedia access.
We originated and designed concepts for a Multi-Input-Multi-Output (MIMO) wireless optical architecture referred to as Multi-Spot Diffuse (MSD) configuration with Multi-element optical transmitters and multi-branch optical receivers, proposed in:
M. Kavehrad, “Spot Diffusing and
Fly-Eye Receivers for Indoor Infrared Wireless Communications," Proceedings of IEEE Wireless Communications Conference,
G. Yun, M. Kavehrad, " Indoor Infrared Wireless Communications Using Spot Diffusing and Fly-Eye Receivers," The Canadian Jour. on Elect & Comp. Eng., Vol. 18, No. 4, October 1993.
Professor Joseph Kahn of Stanford University and Mr. P. Djahani, in an overview paper in December 1998 IEEE Communications Magazine entitled; “Imaging Diversity Receivers for High-Speed Infrared Wireless Communication,” describes these contributions as:
- Implementation of multi-branch angle diversity using non-imaging elements requires a separate optical concentrator for each receiving element, which may be excessively bulk and costly. Yun and Kavehrad proposed the fly-eye receiver, which consists of a single imaging optical concentrator (e.g., a lens) that forms an image of the received light on a collection of photo-detectors, thereby separating signals that arrive from different directions. Implementation of an angle-diversity receiver using imaging optics offers two advantages over a non-imaging implementation. First, all photo-detectors share a common concentrator, reducing size and cost. Second, all the photo-detectors can be laid out in a single planar array, facilitating the use of a large number of receiving elements or pixels.
- In non-Line-of-Sight (LoS) wireless optical links, Yun and Kavehrad also proposed the spot-diffusing transmitter, which utilizes multiple narrow beams pointed in different directions, as a replacement for the conventional diffuse transmitter, which utilizes a single broad beam aimed at an extended reflecting surface. While the diffuse transmitter provides considerable immunity against beam blockage near the receiver, it yields a high path loss. The spot-diffusing transmitter is expected to reduce path loss compared to the diffuse transmitter, because the narrow beams experience little path loss traveling from the transmitter to the illuminated reflective surfaces.
Effectively, this is equivalent to using Multi-Element Antennas at both transmit and receive ends (MIMO).
Today, MSD-MIMO utilizing multi-beam transmitter and multi-branch angle diversity detection is one of the most promising ways of achieving very high digital transmission capacities in places as classrooms, hotel lobbies, shopping malls, train stations, etc., where the roaming flexibility for the users is imperative. The multi-beam transmitter while improving the power efficiency significantly, it maintains its robustness to transmitted beam blockage possibility. The multi-branch angle diversity detection further reduces the power requirements due to reduced ambient light reception and multipath-induced distortions. Typically, the receiver optical front-end consists of a concentrator to increase the received optical signal power, and an optical band-pass filter to reject the ambient light. Several types of optical concentrators for the multi-branch angle diversity receivers have been suggested, i.e., ball lens, compound parabolic concentrator and imaging lens. Interference filters have been used to reduce the ambient light reception.
We have designed a novel optical transceiver design in which we exploit unique advantages of holographic optical elements. Eye-safety limits on the transmit power and the limits imposed by the background noise, e.g., sunlight or in-building lights on the receiver field-of-view (FOV) are the constraints we are considering for the implementation of a practical MSD-MIMO wireless local access IR architecture.
Articles below quote this project:
- Penn-State Research News: Low Power, Highly Reliable, Wireless, Infrared LAN
- New York Times Article: Beaming Data Holds Promise, With Limits, for Networking
- Center Daily Times Article: Professor sees the future in infrared
- The Digital Collegian: Researchers look into ways to network computers through infrared light
- Electronics Times: Inside job for network system
- IEEE Communications Society Industry NewsCache (INC): Scientists are working on high-speed
- Science Daily Magazine: Low Power, Highly Reliable, Wireless, Infrared
- iG - NY TIMES: As ondas de rádio não conseguirão competir com a luz," afirmou Mohsen Kavehrad
- ICON News: Beaming data could be networking's saviour
- Wireless News Factor: Does Infrared Have a Chance?
- Uni-Science: Indoor Wireless LANs Minus Line-of-Sight IR can work
- Penn-State Intercom: Wireless IR LAN
- Photonics Spectra: IR Wireless signals High Bandwidth at Low Power
- TRN News: Holograms control data beams
- INFORMATICA 2.0: Redes conectadas con rayos infrarrojos
- EE TIMES,
: Inside job for low power network system with reduced error rate UK
- Laser Focus World: Free-space optical communications
- Burrus Research Associates Inc: Reliable, Broadband, Infrared Communication System
- EurekAlert: Low power, highly reliable, wireless, infrared local area networks demonstrated
- December 1998 IEEE Communications Magazine: Imaging Diversity Receivers for High-Speed Infrared Wireless Communication
- Book: Wireless Optical Communication Systems
Channel modeling by computer simulations as well as experimental measurements and optical transceiver design for actual communications are other aspects that have a major influence on the system architecture design.
M. Kavehrad, J. Fadlullah, “Optical Wireless Networked Systems: Applications to Aircrafts,” SPIE Photonics West, San Francisco-CA., January 2011.
M. Kavehrad, "Sustainable Energy-Efficient Wireless Applications Using Light," IEEE Communications Magazine, pp. 66-73, December 2010.
J. Fadlullah, M. Kavehrad, “Indoor High-Bandwidth Optical Wireless Links for Sensor Networks,” IEEE Journal of Lightwave Technology, Vol. 28, No. 21, pp. 3086-3094, November 1, 2010.
M. Kavehrad, J. Fadlullah, "Wideband Optical Propagation Measurement System," Proceedings of the SPIE Photonics-West Conference, San Francisco, California, January 2010.
M. Kavehrad, " Broadband Room Service by Light," Scientific American Journal, pp. 82-87, July 2007.
M. Kavehrad, "Let there be light and broadband internet - - You will have a happier life I bet," Proceedings of SPIE Optics East, Boston, Mass., September 2007.
M. Kavehrad, P. Amirshahi, “Hybrid MV-LV Power Lines and White Light Emitting Diodes for Triple-Play Broadband Access Communications,” IEC Comprehensive Report on; Achieving the Triple Play: Technologies and Business Models for Success, ISBN: 1-931695-51-2, pp. 167-178, January 2006.
P. Amirshahi and M. Kavehrad, “Broadband Access over Medium and Low Voltage Powerlines and use of White Light Emitting Diodes for Indoor Communications,” IEEE Consumer Communications & Networking Conference, Las Vegas, Nevada, January 2006.
S. Jivkova, M. Kavehrad, “Transceiver Design Concept for Cellular and Multispot Diffusing Regimes of Transmission,” EURASIP Journal on Wireless Communications and Networking, Vol. 2005, No. 1, pp. 30-38, March 2005.
Y. Alqudah, M. Kavehrad, “On Optimum Order of Angle Diversity with Maximal Ratio Combining Receivers for Broadband Indoor Optical Wireless Communications,” Annual Review of Communications, Vol. 57, ISBN: 1-931695-28-8, November 2004.
Y. Alqudah, and M. Kavehrad, S. Jivkova, “Optical Wireless Multi-Spot Diffusing; a MIMO Configuration,” Proceedings of ICC'04, Paris, France, June 2004.
Dr. S. Jivkova, Dr. B. A. Hristov and Dr. M. Kavehrad, “Power-Efficient Multi-Spot-Diffuse Multi-Input-Multi-Output Approach to Broadband Optical Wireless Communications,” IEEE Trans. on Vehicular Tech., Vol.-53, No. 3, pp. 882-889, May 2004.
Y. Alqudah, M. Kavehrad, “Optimum Order of Angle Diversity With Equal-Gain Combining Receivers for Broad-Band Indoor Optical Wireless Communications,” IEEE Trans. on Vehicular Tech., Vol.-53, No. 1, pp. 94-105, January 2004.
Y. Alqudah, M. Kavehrad, “On Merits of Spatial Coding in Multilevel Wireless Infrared Links,” Proceedings of GLOBECOM’03, San Francisco – CA., December 2003.
S. Jivkova and M. Kavehrad, “Shadowing and Blockage in Indoor Optical Wireless communications,” Proceedings of GLOBECOM’03, San Francisco – CA., December 2003.
Y. Alqudah, M. Kavehrad, "Orthogonal Spatial Coding in Indoor Wireless Optical Link," Proceedings of the OptiComm 2003 Conference, Dallas - Texas, October 2003.
S. Jivkova, S. Shurulinkov and M. Kavehrad, “Optical Wireless Multi-Spot Diffusing Configuration: Link Quality Assessment Using Statistical Approach,” Proceedings of 8th Int’l Conf. on Laser and Laser Information Technologies, Plovdiv – Bulgaria, September 2003.
Y. Alqudah, M. Kavehrad, “MIMO Characterization of Indoor Wireless Optical Link Using a Diffuse-Transmission Configuration,” IEEE Trans. on Commun., Vol. 51, No. 9, pp. 1554-1560, September 2003.
Y. Alqudah, M. Kavehrad, “Assessing the Feasibility of New Diffused Configuration for Broadband Wireless infrared Links,” Proceedings of the IEEE WCNC'2003, New Orleans – Louisiana - USA, March 2003.
M. Kavehrad, S. Jivkova, “Indoor Broadband Optical Wireless Communications: Optical Subsystems Designs and Their Impact on the Channel Characteristics,” IEEE Wireless Communications Magazine, Vol. 10, No. 2, pp. 30-35, April 2003.
K. Akhavan, M. Kavehrad and S. Jivkova, “High‑Speed Power‑Efficient Indoor Wireless Infrared Communication Using Code Combining, PART-- II,” IEEE Trans. on Communications, Vol. 50, No. 9, pp. 1495-1502, September 2002.
K. Akhavan, M. Kavehrad and S. Jivkova, “High‑Speed Power‑Efficient Indoor Wireless Infrared Communication Using Code Combining, PART-- I,” IEEE Trans. on Communications, Vol. 50, No. 7, pp. 1098-1109, July 2002.
S. Jivkova, M. Kavehrad, "Receiver Designs and Channel Characterization for Multispot High Bit Rate Wireless Infrared Communications," IEEE Trans. on Communications, Vol. 49, No. 12, pp. 2145-2153, December 2001.
M.R. Pakravan, M. Kavehrad, H. Hashemi, "Indoor Wireless Infrared Channel Characterization by Measurements," IEEE Trans. on Vehicular Tech., Vol.-50, No. 4, July 2001.
S. Jivkova, M. Kavehrad, " Multi-spot Diffusing Configuration for Wireless Infrared Access," IEEE Trans. on Communications, Vol. 48, No. 6, pp. 970-978, June 2000.
S. Jivkovaand M. Kavehrad, “Wireless Infrared In-House Communications: How to Combat Multipath Distortion,” Proceedings of Photonics East’2000, , November 2000. Boston
M. Pakravan, M. Kavehrad, H. Hashemi," Effects of Receiver Rotation on the Path Loss and the Delay Spread in Indoor Infrared Channel," Proceedings of ICC'98, Atlanta, GA, June 1998.
M.R. Pakravan, E. Simova, M. Kavehrad, "Holographic Diffusers for Indoor Infrared Communication Systems," Journal of Wireless Information Networks, Vol. 4, No. 4, pp. 259-274, October 1997.
M.R. Pakravan, E. Simova, M. Kavehrad, "Holographic Diffusers for Indoor Infrared Communications Systems," Proceedings of GLOBECOM, London-England, Nov. 1996.
E. Simova, M. Tai, M. Kavehrad, "Indoor Wireless Infrared Link with a Holographic Multiple-Spot Diffuser," Proceedings of ICAPT, , August 1996. Montreal
E. Simova, M. Kavehrad," Light Shaping Diffusers for Indoor Wireless Infrared Communications via a Holographic Approach," Proceedings of SPIE Photonics West'96, , February 1996. San Jose, California
Q. Jiang, M.Kavehrad, M.R. Pakravan, M. Tai, "Wideband Optical Propagation Measurement System for Characterization of Indoor Wireless Infrared Channels," Proceedings of ICC'95,
, June 1995. Seattle, Wa
M.R. Pakravan, M. Kavehrad, "Design Considerations for Broadband Indoor Infrared Wireless Communications Systems," International Journal of Wireless Information Networks, Vol. 2, No. 4, pp. 223-238, October 1995.
H. Hashemi, G. Yun, M. Kavehrad, F. Behbahani, P. Galko, "Indoor Propagation Measurements at Infrared for Wireless Local Area Networks Applications," IEEE Trans. on Veh. Tech., Vol. 43, no. 3, pp. 562-576, Aug. 1994.
G. Yun, M. Kavehrad, " Indoor Infrared Wireless Communications Using Spot Diffusing and Fly-Eye Receivers," The Canadian Jour. on Elect & Comp. Eng., Vol. 18, No. 4, October 1993.
M. Kavehrad and G. Yun, United States Patents: Optical taper for increasing the effective area of a photo diode in atmospheric free space communications applications (U.S. 5,192,863), awarded March 1993.
G. Yun, M. Kavehrad, " Indoor Infrared Wireless Communications Using Spot Diffusing and Fly-Eye Receivers," Proceedings of IEEE Wireless Communications Conference, Vancouver, June 1992.
This research has been supported by the National Science Foundation (NSF) Grant number ECS-9820604, the NSF Grant number ECCS-0824052, the IBM Shared University Research (SUR) Program and the Pittsburgh Digital Greenhouse.