Connecting the dots

After the user-needs assessment has been completed, the radio sites have been located and a radio-frequency plan has been approved, the next step in the design of a private land-mobile radio system is to connect the sites to each other, as well as to dispatch and command-and-control.

There are many different methods and tools to transfer the information from the radio sites to the dispatch and command-and-control points: landline, microwave radio, free space optics, fiber optics and RF control links. Let’s examine each of them.

Landline. Traditional communications systems are linked by landlines, i.e., copper wires. These copper lines are in pairs or in sets of four wires, and are found throughout the world as a primary means of transferring communications-based information.

Copper wires as a medium have advantages and disadvantages. One advantage is that they are available virtually everywhere; another is that their price is low compared with other media. But the recurring costs of transmitting over landlines can be significant over a long period of time. Also, if a problem occurs, a wireline link can be out of service for days before the service is restored.

Microwave. Microwave radio links are the next most common means of connecting radio sites, and of connecting to dispatch and command-and-control. These links are either licensed or unlicensed by the FCC and typically are in the range of 2 GHz to 23 GHz.

Link designs, bands, ranges and bandwidth vary depending on whether the system is licensed or unlicensed and on the frequency band. But there are some basic guidelines for all instances. Let’s start with the link design and the radio link budget. The link design and radio link budget are similar to those in LMR systems with the exception that both ends of the link are fixed in a microwave path, while one end typically is mobile for LMR systems.

The path fade margins that are required are much higher in microwave systems to accommodate for the deep fades that may occur and the inability to move out of the fades. Physical diversity of antennas at either end help, but fade margins of 30 dB are typical. Another design difference concerns the antenna gains. The antennas normally are parabolic dishes and have gains in the 20 to 30 dB range at both ends. This allows path lengths from 10 miles with line of sight for the higher frequencies, to 50 miles or more for the lower frequencies.

In the microwave region of the spectrum, the following bands are used:

Today’s typical microwave systems often consist of an indoor unit (IDU) located inside of a building or cabinet, and a corresponding outdoor unit (ODU) located on the building rooftop, on a tower or on a water tank. The IDU and ODU will be interconnected via an Ethernet LAN cable or a small coaxial cable that carries the information between the IDU and the ODU.

You can find unlicensed channels in the 2.4 GHz and 5.8 GHz bands, but in most instances you will want to use licensed radios in one or more of the other bands.

When designing a microwave system, the transmitter power, transmission line loss (if Ethernet is not used), antenna gain, path loss, receive antenna gain, transmission line loss (if Ethernet is not used) and receiver minimum sensitivity are considered. The signal level must be at least 30 dB above the minimum receiver sensitivity for a path to be viable. One thing to watch out for is a fade margin that is in the order of 60 dB or higher, which can destroy the intermediate frequency amplifiers in the microwave radio receiver. It is imperative to know the maximum and minimum signal levels for a microwave receiver. Sometimes more signal is not better.

The path of a microwave signal should be plotted on a map and also should be visually observed to ensure that there always is a line-of-sight path between the two nodes of the system. Most modern mapping programs will allow you to perform a path analysis and profile of the ground elevation. The 4/3 earth contour must be taken into consideration when plotting the path.

An interference reflection zone exists — called a Fresnel Zone — that can have no obstruction or any metallic objects within it. This will force you to have a larger clearance path than just the line-of-sight tunnel between the two sites.

When an antenna is placed on a structure, it is very important that there is absolutely no movement in any direction of the antenna once it is aimed and optimized for a given path. This includes the movement of the tower itself. Many towers must be guyed for sway once a microwave dish or antenna is placed on that tower. There are special outrigger guy brackets that are available for this purpose.

The bandwidth of a microwave system is directly proportional to the capacity of that system. As you increase the capacity of a given system, the bandwidth will increase. For licensed systems, you are required to stay within your authorized bandwidth.

Many of the newer microwave radio systems use the Internet Protocol (IP) as a means for communications; consequently, all nodes and connections must be part of the same network addressing protocol. Some of the older microwave systems use analog modulation schemes, but all of the new systems are using a digital modulation scheme of various types.

Free space optics. A technology exists where a pair of lasers is used to provide a transmission path across a span of up to one mile. Focused lenses and fiber-optic cables are utilized to provide a high-speed Ethernet LAN connection between two sites.

The main advantage of using free space optics (FSO) is that you can create a path across town without having to bury cable, apply to the FCC for a microwave license or use telephone lines that have recurring charges. The bandwidth varies from 100 Mbps to 1,000 Mbps between the two nodes.

An FSO system consists of a laser encoder, laser receiver, fiber-optic cables, two or four antenna lenses, and a secure mounting system. The installation is simple and the units on the market today can be self-aligning.

There are a few caveats to consider when using this technology:

Fiber. Fiber-optic connectivity is one of the most common methods of tying communications systems together today. The largest expense of a fiber optic (FO) system is the installation. Because of this, it is cheaper over the long run to install more fiber cables than you need when putting in the required cables. These unused cables are called dark cables. Almost all FO systems contain such extra cables for future use.

The FO cable diameter determines how long a run can be made of a single strand of the cable. The 62.5-micrometer diameter cable is called multi-mode, and the run can be up to 1,000 feet. A 9.0-micrometer diameter cable is called single mode, and it can run over 30 miles for a single strand.

A multi-mode cable can be used with either 850 nm or 1300 nm systems, which are the two most common bands used for FO systems. Multi-mode cable will be found in a single building, but single-mode cable will be used to connect multiple sites together in a city or wider area. Single-mode cable usually uses the 1300 nm or 1550 nm bands. Most systems offer a 200 Mbps bandwidth, but the newer systems deliver a 1 Gbps bandwidth.

Just as radio systems have a link power budget, so do FO systems. The transmit output power of +10 dBm to +30 dBm will be put into the FO distribution cable, cable attenuation will range from 5 to 50 dB, and the receiver will have a sensitivity of approximately –30 to –50 dBm for its bottom-line receiver noise floor. If a signal needs to be enhanced because of attenuation, amplifiers are available to boost the signal.

Many cities and right of ways have FO cables running throughout them, and they are available as an alternative to other communications paths for data and voice systems. Some cities install such cable when they do any kind of excavation for water or sewer pipes, so that they can utilize these systems for their internal communications without having to pay any recurring charges to the telephone or cable companies. Other agencies have required the local utility and cable companies to put in extra capacity and dark fibers into their networks as a condition of getting a permit to install cables in their jurisdictions.

A newer form of FO systems uses a technology called Wave Division Multiplex. The 1550 nm band is used for such systems; it has capacity that is 10 to 20 times that of a normal single-mode system. This will allow for extremely high-density data systems to be utilized for current and future needs.

Satellite. In some circumstances, there is no connectivity to a site over land or water. In these cases, satellite systems will be required for the system to work. The geo-stationary satellites reside at 22,000 miles above the equator, and there are dedicated channels available for connectivity. In addition, there are low earth orbiting satellites, called LEOs, and these also work, especially in mobile environments.

As with any communications system, there is a link budget that must be determined for the system to work. You will need to know the transmitter power, antenna gain, coaxial loss and receiver sensitivity in order to engineer a reliable path. In addition, cloud cover can interfere with satellite communications, unless you have sufficient antenna gain, transmitter power or receiver sensitivity, or a combination of all three factors to make the path reliable.

Satellite system bandwidth is available, but it always comes with a high recurring price tag.

RF control links. If you only have a few channels to control, conventional radio channels will work for this purpose. In addition, most repeater systems use RF control base stations for the fixed locations in the system. The simple systems and trunking systems will use mobile units with a base station power supply that provides the 12 VDC needed for the mobile unit from the 120 VAC in the building. Some systems will use regular base stations as the control stations for the repeater links. This normally is the case when there are many other radios in close proximity to each other.

In the U.S., most control links use one of the following bands for control stations:

All VHF, UHF, 800, and 900 MHz systems require the control links to be licensed. The FCC does allow you to license control stations under the 6.1 Meter Rule. This rule lets the licensee install a control base anywhere in the state in which it is licensed, as long as the antenna is no more than 20 feet above ground or 20 feet above an existing building.

In every radio system, the control station must use a directional antenna, unless the system itself has multiple repeaters in different directions, or the control station must operate on different channels with multiple sites involved.

Control stations can be licensed to go on buildings, towers, water tanks or any other place. If the antenna is required to be over 20 feet above one of the structures, the station’s location must be specified in the license.

Editor's note: The last entry in the LMR series, "The lynchpin," included some factual errors. See the corrected version online. Urgent Communications regrets the errors.

Part 1: Class is in session: Basic LMR and FCC definitions
Part 2: Start at the beginning: Understanding LMR user needs
Part 3: The devil's in the details: Conducting a user-needs survey
Part 4: Decisions, decisions: Understanding the LRM procurement process
Part 5: Let's get started: System engineering begins with RF planning
Part 6: The lynchpin: Receiver planning and noise interference
Part 8: The next piece of the puzzle: Understanding dispatch communications
Part 9: Now the real work begins: How to select a suitable LMR site
Part 10: The bane of your existence: How to deal with RF interference
Part 11: Winning the battle: What causes radio frequency interference

Ira Wiesenfeld, P.E., is a consulting engineer who has been involved in the radio communications business since 1966. He is a senior member of the IEEE and has been a licensed amateur radio operator since 1963. He can be reached at [email protected].

Robert C. Shapiro, P.E, is the senior manager-systems engineering for PlantCML, an EADS Company. He serves on the TIA TR8 committee as the TSB-88.4-C task-group chair and is a senior member of the IEEE. He can be reached at [email protected].