A common question often asked of microwave vendors is: “what is the range of your radio?” The only proper answer is: “it depends.” That’s because microwave link distance is only one of several factors that must be optimized simultaneously in a professionally installed microwave radio deployment. And it is rarely the case that one seeks to maximize only range at the expense of all other performance criteria.
The most important questions for any point-to-point microwave deployment are:
- What is the minimum required throughput?
- What is the minimum required availability?
- What is the maximum required latency?
- How far does it need to go?
Only once those requirements are established can alternative radio systems be accurately compared for their suitability to the application.
Range: It Depends
The maximum length, or range, of a point-to-point microwave path is governed by a number of factors, including:
- Radio frequency band (Hz). Signals at lower frequencies travel further and are less susceptible to varying weather conditions.
- Transmit power (dBm). Higher radio transmit power translates into an ability to accommodate more signal degradation and, thus, longer paths.
- Receiver sensitivity or threshold (dBm). The weaker a signal a radio receiver is able to hear, the longer the path can be. Receiver sensitivity is even more important than transmit power in the range equation.
- System gain (dB). The sum of the transmit power and receiver threshold. Typically, longer ranges can be achieved with higher system gain, depending on the amount of fade margin required.
- Antenna gain (dB). Higher gain antennas serve to increase the effective isotropic radiated power (EIRP), or overall link system gain, allowing longer paths and greater fade margins.
- Fade margin (dB): The amount, in dB, that a signal may degrade (fade) before reaching receiver threshold for a given distance. The higher the fade margin, the more resilient the link will be to adverse propagation conditions.
- Required throughput (Mbps). Higher throughput requires higher order signal modulation or greater channel bandwidth. For a given channel size, higher order modulation decreases receiver sensitivity and system gain, thus reducing maximum path length.
- Required path (link) availability. Path length can be extended by lowering the availability requirement, e.g. from 99.999% to 99.99%. Availability is the time in minutes per year that the link will deliver the engineered specifications.
- Required throughput availability. Throughput availability applies a guarantee level to a specified throughput, whereas path availability guarantees only that the link is active. For a given path length, throughput availability is a more stringent performance specification than link availability.
- Climate. As a rule of thumb, the higher the frequency, the shorter the wavelength and the more susceptible the signal is to weather. Transmission at frequencies above 11 GHz is more affected by climate, specifically rain and humidity. Rain region propagation data is required when engineering paths at these frequencies in order to engineer the appropriate fade margin and deliver the required throughput and link availability.
- Terrain. Rugged terrain may partially obstruct signals, lessening maximum path length while smooth terrain (such as water) may cause reflections, also reducing maximum path length.
- Regulatory domain: Parameters such as output power, EIRP levels, channel bandwidth, and modulation are regulated differently depending on the country, the frequency band, and whether they are based on FCC, ITU/ETSI, or other rules. Thus, the regulatory domain also influences the maximum distance of a particular path.
Because path length is dependent upon so many factors, it is impossible to provide a generic answer to the “range question.” Range figures specified without indicating the assumptions applied are not meaningful nor can they be accurately compared with other alternatives. Rather, range must be specified based upon a number of assumptions and calculated using an engineering tool such as Exalt’s link budget calculator.
Your Radio Can Go How Far!?
A common technique that vendors use to claim greater range is to conveniently ignore throughput availability and instead focus only on link availability. This approach is indeed convenient, as it allows these vendors to make just about any range claim they wish, as they guarantee only a connection.
An impressive and popular range claim is 124 miles. The reader will discover that no assumptions are provided to support such claims. Such a range truly would be impressive if it did not require the use of two 2000 foot (600 meter) towers in order to clear the Earth’s curvature. Assuming that small hurdle could be overcome – if the radios were operating in space, for example – then many microwave radios, including Exalt radios, could communicate at this distance, assuming adequately sized antennas and no requirements for throughput or throughput availability.
But such impractical range specifications are of little use to a microwave professional trying to determine whether a microwave system can deliver 100 Mbps of guaranteed user throughput across a link at 99.999% availability. That’s why you won’t see Exalt quoting a 124-mile range and why Exalt range specifications always include realistic underlying assumptions.
Remember: if it sounds too good to be true, it probably is.
Specifying Range
Exalt provides range specifications in a format similar to the table below:
Product | Frequency (GHz) | Aggregate Ethernet throughput | Distance at 99.99% throughput availability1 | Distance at 99.999% throughput availability1 |
EX-2.4i | 2.400 – 2.483 | 100 Mbps | 32.5 mi / 52.3 km | 20.7 mi / 33.3 km |
1 Distance based upon FCC regulations, average climate and terrain, 6’ dish antennas (except for integrated antenna models), 15 dB or greater fade margin and the absence of clearance or multipath issues. Longer or shorter distances will apply for alternative antennas, country regulations, transmission system losses, path topologies and radio configurations. See Exalt’s path planning and path calculator tools to model your scenario.
In this specification, the value of each of the four primary criteria is indicated: throughput, throughput availability, fade margin (in the footnote) and, of course, range. If any one of these factors changes, then at least one other factor must change in response. There are no exceptions to this rule.
Summary
Determining the range of a microwave radio system requires the identification of a number of system requirements, including required throughput, required availability and required fade margin, as well as the appropriate tools.
Thus, any conclusions drawn about the capability of a microwave system or comparisons drawn between two systems will be wrong if such conclusions are made in the absence of information about regulations, application requirements and physical characteristics of the site.
How far will it go? Remember: it depends.
If you would like assistance analyzing your own microwave path, refer to the path profiler tool or contact Exalt customer support at +1 408 871 9890 or support@exaltcom.com.