Your Line-of-Sight Into Backhaul Planning
Almost daily, there are new software applications, department processes, and newly approved policies that demand bandwidth intensive voice, data, and video to computers, hand held devices, and even connected televisions.
The good news is that many viable communication options are available to use for wireless backhaul solutions. Providers today can choose from copper, fiber, or microwave systems. That said, the choice regarding which method to use for each specific application is one that must be evaluated on different levels.
“It is not unreasonable to expect a network reliability of 99.995% while using a microwave backhaul solution digital communication network.”
New technologies requiring higher data rate circuits include Voice over Internet Protocol (VoIP), Internet Protocol Television (IPTV), Radio over Internet Protocol (RoIP), IP gaming, and multimedia. Today, Global Positioning Systems (GPS) and Location Based Services (LBS) utilizing Ethernet networks are commonplace and are utilized for services such as locating and tracking fleet, personnel, and equipment. These Internet Protocol communication suites utilize Ethernet and Transmission Control Protocol/Internet Protocol (TCP/IP) network topologies. TCP/IP, of course, provides a very good Quality of Service (QoS) or reliability built in to the network.
Clearly, these data intense applications are placing a greater demand for increased capacity in existing backhaul systems. As a result, Ethernet networks are becoming mandatory at many existing locations and in many new service areas.
When analyzing the options, it’s a given that copper and fiber network solutions can be very expensive due to construction costs. Additional expenditures can increase even further when traversing rivers, mountains, or other geographical and topographical challenges. Therefore, a microwave backhaul is an economical method of providing high-speed data service while maintaining 99.995% availability for Ethernet applications. The microwave backhaul solution is becoming more prevalent in communication services at reduced costs.
Licensed or Unlicensed Frequencies
Microwave signals are synonymous to light rays and must have a clear path, or line of sight, between microwave antennas for optimum performance. There can’t be any obstructions (buildings, trees, hill tops) to limit the radio wave in between the 2 antennas. Therefore, having a true line of sight path, near line of site path, or non line of sight path are a few of the planning considerations to determine if a microwave backhaul system will utilize licensed or unlicensed radio frequency bands.
To understand this further, let’s look at the difference between licensed and unlicensed frequencies.
Licensed Bands: These bands will require an FCC license before transmit operation can begin. Licensing requires more planning and coordinating along with additional upfront costs for frequency coordination. It is critical that licensed band microwave systems strictly adhere to the FCC parameters for trouble free operation and to prevent costly fines for violation of FCC assignments.
The FCC application may be a joint effort with a licensing agent to select the correct frequencies, tower heights, power levels, antennas sizes and polarization of antennas. This process is not much different than licensing radio systems encountered in commercial or two-way radio networks.
The cost factor for licensed radio links is commonly higher than that of unlicensed radio links. The parabolic dish antenna, required tower hardware, transmission line and high data rate modulation radios utilizing Quadrature Amplitude Modulation (QAM) will drive up the initial cost of capital expenditure.
Several common licensed frequencies used for point-to-point radio systems are in the bands of 6 GHz, 11 GHz, 18 GHz, 23 GHz, 28 GHz, and can go up to 80 GHz. The amount of spectrum available for microwave link communication will vary by location dependent on the amount of existing licensed links in operation. Frequency coordination for microwave links is critical, and good planning will result in an error-free, highly reliable link with minimal interference.
Keep in mind that when higher frequency microwave bands are in use it will place limits on the distance design factor. For example, a 60 GHz or 80 GHz system may have an effective link distance from 1 to 2 miles. An 11 GHz link could be utilized for a 6 to 8 mile link. A 6 GHz link design could span 15 miles or longer dependent on the radio modulation scheme, antenna height, link topography, and other engineering contributing factors to be considered in the planning stages.
Unlicensed Bands: These are commonly used in topographical areas of limited LOS, often called near Line of Sight (nLOS) and even Non Line of Sight (NLOS) links.
The unlicensed bands have several frequencies which do not require an FCC operating license. However, the microwave system power levels and antenna size must be within the FCC specifications for legal compliance. These bands are known as the Unlicensed National Information Infrastructure (U-NII) and the Industrial Scientific and Medical (ISM) bands. The most common unlicensed frequencies used for point-to-point radio systems are the ISM bands in the 900 MHz, 2.4 GHz and 5.8 GHz frequency range.
The cost factor for unlicensed radio links is commonly much less than that of licensed radio links. This factor alone makes the unlicensed point-to-point microwave backhaul very affordable and popular in many applications. Proper planning and engineering practices will provide the unlicensed microwave backhaul link with high data rates, reliability and long term operation. An unlicensed, 5.8 GHz system, is often the best performing microwave systems for an nLOS or NLOS link, due to many uncontrollable factors.
Network Architecture Design, Topology, Infrastructure, and Reliability
The network architecture selected by the user will determine the design criteria of a microwave link. Ethernet, or Electrical and Electronics Engineers (IEEE) standards organization IEEE 802.3, has evolved to reliable and dependable network architecture and is currently the most popular physical layer (OSI Model Layer 1) network topology used today. Additional network architectures include Token Ring, Fast Ethernet, Fiber Distributed Data Interface (FDDI), and Asynchronous Transfer Mode (ATM).
The selected network design will have a large effect on system reliability and performance. Consider the differences between the following topologies:
• Microwave point-to-point (P2P) network topology designs include the use of star, ring, or mesh network topologies, much like wired network systems.
• Microwave Ethernet networks can be designed similar to fiber optic rings having self-healing capabilities that will reroute data traffic in the event of a microwave link outage. Microwave radio networks are able of achieving Gig-Ethernet speeds for data-intensive applications.
• The star network design has only 1 available path from the central tower site to all of its remote sites.
• The ring network design is a topology in which each tower site connects to 2 other tower sites forming a continuous path, or ring, for signals through each tower with each tower handling every packet of data.
• The mesh network design topology is a connection of tower sites whereby each tower must capture and disseminate its own data and also serve as a relay for other towers.
The economics of site-to-site microwave connections will be determined by microwave radio equipment costs, local zoning codes, new tower construction, utilization of existing tower structures and the possibility of using building rooftops. Overall infrastructure costs will also be dependent on the physical topography of the area to be served. The selection of the antenna support structure will also determine distance between tower sites, radio type, antenna size, and operating frequency.
Though microwave reliability has been questioned in the past, its reliability is really about proper design. Critical communication systems require the utmost care in design and engineering to provide consistent system reliability and performance. The use of a Network Management System (NMS) utilizing Simple Network Management Protocol (SNMP) to manage devices on an IP network will greatly aid in the proactive management of the microwave backhaul network. With proper design, it is not unreasonable to expect a network reliability of 99.995% while using a microwave backhaul solution digital communication network.
This fundamental information is presented as a starting point in the consideration of a microwave backhaul solution for the initial design, expansion or upgrade to your network. There are several backhaul solutions available to communication network planners and designers -- and smart designs come from diligent research.
Basic engineering economics will point out reduced data costs in megabytes of data per month by utilizing microwave networks over leased lines, both copper and fiber. Actual expenses can be factored into the equation including any tower leases, Federal Communications Commission (FCC) licensing costs, and electrical or backup power at the tower site. Even with all of these items factored in, the cost difference is still substantial, and the microwave backhaul link is favorable in many cases.
There is a wide selection of microwave equipment types, name brands, and system integrators available. Today, manufacturers offer systems that can deliver up to 99.999% availability in virtually any environment.
Unlike just a few years ago, today's microwave backhaul solutions can span short or long distances, over water and open terrain, and through extreme weather conditions and climates.
That said, the best strategy is to do your homework. Seek the expertise of Radio Frequency (RF) consultants and engineers who are well-versed in microwave radio backhaul engineering to ensure YOUR network reliability and performance is exactly what your customers and shareholders demand.
Pointing Out 6 Point-to-Point Fundamentals
A complete list of design criteria for a microwave backhaul network is beyond the scope of this article. However, below are 6 important design items to consider while contemplating the move to a microwave backhaul link:
Calculated RSL: The Receive Signal Level (RSL) of the microwave signal anticipated to be received from a transmitted signal source. The calculated received signal strength is comprised of system antenna gain, feed line loss, and the free space loss of the electromagnetic wave as it propagates through the Earth's atmosphere on its way to the receive antenna. The RSL value is required to indicate proper antenna alignment during the installation phase of both antennas to ensure network reliability.
Fade Margin: A design allowance providing sufficient gain to compensate for expected losses to accommodate the receiver's level of sensitivity. An example of Rain Fade Margin may include 15dB-s20dB of fade to accommodate for the signal reliability during a heavy rain shower, fog, or icing of the antenna.
Fresnel Zone: An area between microwave antennas similar to the shape of a foot ball, or Zeppelin, that must have a clear LOS (no trees, hill tops, buildings, or physical obstructions) for optimal performance. The Fresnel Zone is critical for error free delivery of microwave signals for optimal performance and high reliability. The determining factors of a Fresnel Zone are the operating frequency and the distance between the antennas. For example, a 5.8 GHz microwave system with a 3-mile-long span has a Fresnel Zone measuring 16' in diameter at the midpoint of the span.
Link Budget: The power level (measured in dBm) of a signal at the receive site to ensure maximum signal is available to meet the receiver's sensitivity. The Link budget is performed using a software tool utilizing the selected frequency allocation, transmit power, antenna gain, receiver sensitivity, free space loss, transmission line loss, antenna heights, span topology, reflections, refractions, geometric distance to horizon, and line of sight impairments.
LOS: The Line of Sight propagation refers to the path an electromagnetic wave must travel on its way from a transmitting antenna to a receiving antenna. Lower frequency radio waves often follow the curvature of the Earth as a ground wave.
Twist and Sway: The affect of wind on a tower's structure produces a Twist and Sway (T&S) momentum of the tower. A tower's structural T&S is measured in degrees of movement. Microwave antennas are designed to direct the majority of their signals in a very narrow beam, often less than 3 or 4 degrees in width. Using a 3' or 4' dish antenna on a tower with a T&S of greater than 2 - 3 degrees may affect the RSL by 10 dB, or greater, which affects signal levels and network reliability.
Dane Brockmiller is an independent consultant who has been in the wireline and wireless industries for more than 37 years. His work experience includes Clearwire, Sprint, Verizon, T-Mobile, AT&T, SBC, and Southwestern Bell Telephone companies. His services include designing and managing voice, data, and video systems along with engineering copper, microwave, and fiber networks. He can be reached at firstname.lastname@example.org.
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