IMSA Journal | Jan/Feb 2015
By Robert Kavaler, Ph.D.
Introduction
Wireless communications are playing an increasingly impor- tant role in the deployment of traffic-related infrastructure. Agencies around the world are choosing wireless solutions be- cause they can be easier to deploy, quicker to install and offer a lower total cost of ownership. Wireless systems also provide a wide array of applications such as adaptive control with ad- vance detection, bicycle detection, and travel time that are more difficult or even impossible with other technologies.
But not all wireless systems are designed the same. Let’s take a look at some of the common design trade-offs wireless vendors consider when developing their systems for traffic so- lutions.
Radio Frequency Design Parameters
Some of the more fundamental radio frequency (RF) design parameters include:
- Frequency reuse
- Frequency band
- Antenna design
- Digital radio chipsets
- Implementation cost and efficiency
The common view of RF design is to think of a single trans- mitter sending data to a single receiver. In such a single link model, designers want to transmit at the highest power al- lowed and receive at the best sensitivity allowed for your ap- plication. But this model works only if you are the sole user of the designated RF in your area and you are deploying a single system. Unfortunately this is rarely the case in the real world. Most wireless systems operate in an unlicensed frequency band known as the industrial, scientific and medical (ISM) radio bands. These frequency bands are portions of the radio spectrum reserved internationally for industrial, scientific and medical uses in addition to telecommunications. ISM bands are used in many products, which mean these frequencies are shared by many users, each with a potentially different set of transmitters, receivers, and other equipment located near your equipment. With such real-world designs, it is more important to avoid other interferers in your band than it is to optimize for your own RF link.
Cellular systems have understood this concept from day one and are based heavily on controlling interference via frequency reuse, controlling transmit power, and using sectorized direc- tional antennas. We’ll address these methods individually, but first let’s mention a method that is often discussed but turns out to have limited utility for traffic applications: mesh net- works.
Meshing is a network topology in which each node relays data for the network. All nodes cooperate in the distribution of data in the network.
Meshing has been touted as an automatic way to implement large wireless networks by forwarding packets through inter- mediate nodes. With meshing each packet is received and re- transmitted many times, so the RF performance of each link must be much better than with direct RF transmission. Meshing comes at a cost, in this case power consumption and band- width. In mesh designs each routing/repeating node must be capable of sending both its own data and the data of all other devices on the network. So meshing 10 wireless cameras to- gether requires each camera to support 10 times the bandwidth than a normal camera would require, which is often impossible. Designers typically use two types of devices, routing/repeating devices and end devices. The result requires the design of two networks, one that gives reasonable access to each end device and the other to forward these end device packets to a central hard-wired connection. With good RF design, meshing is not needed, increasing available bandwidth and reducing power re- quirements.
Frequency Reuse
What made cellular telephony different than its predeces- sors was frequency reuse, which is the use of the same fre- quency in adjacent or interleaved geographical areas. For example, the intersection at 1st and Main would use the same frequencies as 3rd and Main, while 2nd and Main would use the same as 4th and Main. Thus Main Street can be covered using two interleaved frequencies.
Frequency reuse requires the power levels and frequency bands to be adjusted so that the edge of the desired coverage is good while the edge of the reused band is greatly attenu- ated. As you move to lower frequency bands this becomes more difficult as RF propagation is increased. For traffic applications, however, it is a good idea to limit RF propagation to less than one block. The use of power amplifiers and low frequency bands actually works against you as they cause overlap into adjacent or alternate areas.
Frequency reuse also requires use of a color code, which is a method of filtering packets from far RF transmissions that may spread over a large distance due to unusual RF reflections or active RF repeating. A typical cell system is designed with a reuse pattern of 7, i.e. 7 separate frequencies are required, and with a minimum of 3 color codes per frequency (21 total). As an example, the Sensys Networks wireless system has 16 frequencies and 254 color codes (4,064 total) which is suffi-cient for operation even in areas where some frequencies are not available due to interference.
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Figure 1. Efficient frequency reuse example across adjacent intersections utilizing varying channels and color codes.
Frequency Band
Frequency bands are defined by each country, in the U.S. by the Federal Communications Commission (FCC) and in Canada by Industry Canada. The frequencies within each band are assigned to allow both commercial and military or govern- ment users to operate successfully. Most frequency bands are licensed which means that nobody can use them except the owner of the license. Cellular bands, for instance, are all li- censed. There are only a handful of bands that are unlicensed, available for everyone to use, but at the cost of following the rules. Rules include maximum transmit power, minimum band- width (i.e. spread spectrum), randomization algorithms, and modulation types. In the U.S. such unlicensed bands include 433 MHz, 900 MHz, 2.4 GHz and 5.8 GHz. Rules for each band are different. The higher the frequency band the more band- width is available. Bandwidth is important because the wider the bandwidth the more immune you are to interference and the more channels available for operation. For example, the 2.4 GHz band has over three times the bandwidth of the 900 MHz band.
The lower the frequency band the more area the RF signal will cover at a given power level. But when you consider all equipment that might be present, the fact that the RF signal can extend further might be more of a problem than a benefit! This is because the transmitting equipment that is far away might overpower weaker local equipment. In fact, the 900 MHz band is notorious for such issues since FCC rules in that band allow narrow band transmitters with high power while such narrow band transmitters are not allowed in the 2.4 GHz and 5.8 GHz bands.
To overcome the problem network designers usually imple- ment frequency hopping, wherein each packet of data is broken into pieces and each piece is sent on a different carrier fre- quency. While this helps to avoid the interfering signals, the duration of the pieces, the frequencies used to transmit these pieces, and the methods used to reconstruct the packet from pieces is not standardized. Additionally, regulations dictate an upper limit for the transmit duration in the 900MHz ISM band. For these reasons, frequency hopping is not a complete solu- tion.
Antenna Design
The most efficient way to reduce interference is to use a di- rectional antenna. While most people view a directional an- tenna as a way to increase the link budget, improving both receive and transmit gain, directional antennas also reduce in- terference by attenuating unwanted RF signals. As long as the interferer is “behind” the antenna (i.e. the antenna is pointing away from the interferer) its signal can be reduced signifi- cantly, by a factor of 100 or 1000 or more. While omni-direc- tional antennas seem easier to install, you have no recourse when an interfering signal is causing system performance to degrade. Thus the use of directional antennas provides a simple method that allows the installer to avoid their biggest site- related problem – interference while improving the second biggest issue-link budget.
Link budget is the RF power loss that can be suffered from a transmitter to a receiver while still transmitting data with reasonable success. This is computed as transmit power divided by receive sensitivity. Loss may be due to range, atmospheric conditions, or other environmental causes such as obstacles or reflections from objects. Directional antennas improve link budget by focusing RF energy, boosting both transmit power and receive sensitivity.
Digital Radio Chipsets
Another considerable implementation issue for RF system designers is the design and cost of integrated circuits cus- tomized to an application. If a designer chooses to use a stan- dard in the 2.4 GHz band such as Wi-Fi or 802.15.4, then there are many off-the-shelf chipsets available on the market and many more being developed every year. Riding this trend low- ers costs and improves features in the long run.
Directional Antenna / Omni Antenna
Figure 2. Directional Antennas are more effective at reducing interference and maintaining link budget than Omni Antennas.
The 900 MHz band, however, is re- plete with proprietary RF solutions using either customized chipsets or do-it- yourself radios. These solutions are more expensive and are usually stuck in time. since any initial investment takes a long time to recover. Good chipsets for the narrow 900 MHz band were abandoned, for many of the reasons addressed here, some 20 years ago when Wi-Fi began to dominate the wider 2.4 GHz band.
Bringing It All Together
RF designers know there is no single optimal design without consideration of the application. This is true for indus- tries like cellular telephony, but equally true for traffic infrastructure applica- tions. Be sure to consider these RF de- sign trade-offs when evaluating wireless systems!
Robert Kavaler, Ph.D., Senior VP, Co- Founder
” Sensys Networks co-founder, Dr. Robert Kavaler is a born inventor. With over 25 years of experience in developing wireless, ultra-low-power sensors, Dr. Kavaler is both the architect of the Sensys Networks product line, and an individual contributor in the soft- ware and hardware implementations. Prior to Sensys ptenen Networks, Dr. Kavaler led the Berkeley Design Center at InnoMedia, where he served as chief architect of the company’s telecom server and VoIP gateway prod- ucts. Before his work at InnoMedia, Dr. Kavaler was CTO and VP of Engineering at Diva Communications, a wireless local loop startup he co-founded with Dr. Amine Haoui in 1993. Dr. Kavaler has also developed new technologies for UPS, Mitsubishi, AT&T, Sharp, and Digital Microsystems, and received his B.Sc., M.Sc., and Ph.D. in Electrical Engineering from the University of California at Berkeley. “
In addition to consultation, FirstNet also worked with the Public Safety Ad- visory Committee (PSAC) on key issues and grew its organization in the past year. The FirstNet staff has increased from a handful of employees at the out- set of 2014 to more than 75 full time employees at the end of the year. First- Net also secured its headquarters and technical facilities to house the staff and developed a Strategic Program Roadmap to drive program priorities. Moving forward, FirstNet has built the foundation necessary to implement the Roadmap and in 2015 will continue to focus on achieving milestones in the areas of consultation and the develop- ment of an acquisition strategy for the network.
Amanda Hilliard
” Amanda joined the First Responder Network Au- thority (FirstNet) in January 2014 as its first Direc- tor of Outreach. In this capacity, Amanda oversees FirstNet’s education and outreach efforts with the public safety community across all levels of govern- ment. Prior to joining FirstNet, Amanda worked at the Department of Homeland Security Office of Emergency Communications (OEC) for 6 years, ini- tially as Senior Consultant with Touchstone Con- sulting Group and then a federal employee, leading public safety stakeholder engagement and outreach and statewide planning efforts. She most recently served as the OEC Partnerships Branch Chief, over- seeing the the Office’s engagement with key stakeholder groups at all levels of government such as SAFECOM, the National Council of Statewide Interoperability Coordinators, the Emergency Communications Pre- paredness Center, and the One DHS Emergency Com- munications Committee. The branch predominantly executed its mission through stakeholder meetings, the delivery of annual statewide planning work- shops, and the development of emergency commu- nications products. While working j for Touchstone, Amanda also provided support to the Metro-Boston Homeland Security Region in implementing their Five-Year Strategic Plan for Communications Inter- operability and to the Office of Management and Budget’s Office for E-Government and Information Technology in implementing its Government to Gov- emment portfolio initiatives. Amanda holds a Bach- elor of Science in Finance from the Pennsylvania State University. She is a certified Project Manage- ment Professional and Six Sigma green belt. “
So why utilize the frequency coordination services offered by IMSA? First, the basic coordination rates are the lowest available. Second, no interservice surcharges apply to PF, PM, PS or PX channels. Third, any excess income comes right back to IMSA in support of its membership programs rather than producing income for a for- profit business. Finally, our processing time is usually less than 20 days.
The next time your agency is making changes to your radio system or planning a new system, look to IMSA to assist in that process and handle all of your frequency coordination needs. You can enter your application electronically by following the frequency coordination tabs on the IMSA web site. IMSA can also keep track of your license renewal and construction notification dates so you do not lose your license or channels for failing to make timely notifications to the FCC.
If you have any questions, call our frequency coordination office at 855-803-1465. Our staff is ready to assist you, whatever your radio communication needs may be. You can also visit FreqEasy, www.freqeasy.com, our on-line application portal, to cre- ate a free account and immediately file applications electronically. If you are modi- fying a license, the system downloads your current license information from the FCC, populates the modification application with that data, and allows you to enter de- sired changes.
If you are not using IMSA for frequency coordination services, you are throwing money away! Give us a try for your next FCC licensing need. We are confident you will be pleasantly surprised.
Ralph A. Haller
” Ralph Haller is a 25-year veteran of the Federal Communications Commission. He held several positions ranging from inspector in the Los Angeles field office to Chief of the Commission’s Private Radio Bureau for nearly nine years. Haller left the FCC in 1996 and formed a telecommunications consulting company, Fox Ridge Communications, Inc. Fox Ridge specialized in assisting public safety organizations in improving their radio systems. For the past three years, he has been general manager of the IMSA frequency coordination op- eration. He can be contacted at (717) 398-0814 or ralph.haller@frequencycoordination.org. “
