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Design Considerations For Microwave RF Repeaters

June 29, 2004
Microwave RF repeaters are designed to transfer signals from one radio terminal to another without loss of quality, data, or traffic, while compensating for multipath and fading loss.
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Microwave on-frequency RF repeaters are commonly used by telecommunications system operators to reliably and cost-effectively relay radio signals at remote locations, typically mountaintops and when bypassing obstructed paths. Understanding the use of microwave on-frequency repeaters requires an understanding of some basic operating concepts and how to apply the latest techniques.

The recently updated RF Repeater Applications Design Tool from Peninsula Engineering Solutions (San Ramon, CA) is a useful program for understanding the operation and application of microwave repeaters. In their simplest form, microwave RF repeaters are fairly simple, linear, on-frequency gain blocks. They can support a wide range of modulation formats and traffic capacity, and the use of channel filters can set the required bandwidth while supporting standard frequency plans. The repeaters, which are often powered by solar- or wind-based energy sources, receive and retransmit signals without loss in quality or capacity.

Organizations that use microwave repeaters include telephone companies, wireless operators, energy companies (water, gas, electric), government agencies (including national, state, county, and local agencies), military, aviation, and national security organizations. Such users expect reliable operation; areas of prime concern include path reliability, repeater-equipment reliability, and power-equipment reliability.

Path reliability normally has to meet the same standards as the rest of the microwave radio relay system. Reliability objectives are often stated on a per hop basis or end-to-end. The most often-used reference objective is the AT&T Short Haul standard, which is defined as 99.98 percent or 6400 s per 250-mile section, end-to end, two-way, with path fading and equipment annual outage combined. Path fading is normally allocated one-half the annual outage budget, 99.99 percent or 3200 s per 250-mile section. The objective applied to each hop is apportioned on a distance ratio basis: d/250 mi. For example, a 30-mile path would have a two-way outage objective of 384 s or less. Some organizations may require more stringent path reliability objectives, such as 99.9999 percent per hop in heavy route applications.

Fading mechanisms considered include fading due to multipath phenomena, obstructions, and rain attenuation. Equipment and power-source reliability demands are dealt with through a combination of highly reliable components and modules plus designs that incorporate redundancy and protection. For example, Peninsula Engineering Solutions addresses these considerations with protected, soft-fail amplifiers and dual, redundant electric power systems as a minimum approach. Supervisory alarm equipment provides reporting of failures or degraded conditions often with enough early warning time for corrective actions to be taken.

The path-transmission-reliability models used for RF repeaters are the same as for most terrestrial, line-of-sight microwave paths. The classic model is the Vigants-Barnett model, with improvements by W. Rummler and others. The International Telecommunications Union (ITU) ITU-R models are frequently used outside of North America. Rain attenuation is normally considered above 9 GHz. Both the Crane and ITU-R rain models can be applied for estimating the path loss due to rain attenuation.

Some of the assumptions that can be applied to microwave RF repeater models include the idea that hops fade independently, so each hop can be calculated separately. Also, rain outages affect two-way communications, and multipath outages do not occur during rainfall. In addition, space- and frequency-diversity techniques can be applied for improved performance in one or two directions.

Determining the equivalent receiver threshold value for a microwave RF repeater is one of the more demanding differences compared to standard transmission engineering. Since microwave RF repeaters do not demodulate traffic, only amplify it, they do not have a designated threshold value even for a specific modulation and traffic capacity. Rather, the equivalent receiver threshold is relative to the terminal radio's threshold and associated noise figure: the net path loss plus the repeater's noise figure and maximum gain. The approach has been to use the cascaded noise figure equation as the basis for determining the equivalent repeater threshold or "minimum receive power":


GainR = the RF repeater maximum gain (in dB);
NFR = the RF repeater noise figure (in dB);
NFT = the terminal radio noise figure (in dB);
NPL = the unfaded net path loss between the RF repeater transmitter and terminal radio receiver (in dB);
Min_Rx_PwrT = the terminal radio threshold (in dBm)
PADIn = the RF repeater input attenuator pad attenuation (in dB); and
PADOut = the RF repeater output attenuator pad attenuation (in dB).

The RF repeater receive flat fade margin (FFM, dB) thus becomes:

FFM, dB =

Linear RF repeaters are compatible with a wide range of modulation formats. The transmit power level for a particular repeater model depends on two parameters: the frequency modulation (FM) or fully rated power level and the backoff amount for the modulation used by the terminal radios. The table offers examples of transmit power setting per modulation type. The appropriate transmit power level is selected by looking up the modulation format. Terminal radio traffic capacity is not a consideration when selecting the RF repeater transmit power level.

Frequently, the RF repeater transmit power rating will be less than the associated terminal radio. This difference is one cause of asymmetrical receive signal levels and fade margins per hop. Since the fade margins are different per direction per hop, it is necessary to calculate the reliability per direction per hop as well.

Normal transmission engineering practice is to assume that each path fades independently. This makes sense when dealing with radio terminals that have high-enough gain and automatic gain control (AGC) to compensate for 60 dB unfaded net path loss plus 40 to 50 dB deep fast fades and maintain full transmit power. Microwave repeaters normally have less gain and AGC as a consequence of on-frequency operation. Typical microwave radio terminals have 100 to 120 dB system gain and 50 dB down-fade AGC range, while microwave RF repeaters have 50 to 70 dB system gain and 20 dB down-fade AGC range.

A classic characteristic of multipath fading is that the deeper the fade, the shorter the fade duration below a given depth. This is also known as "time below level." Fades greater/deeper than 30 dB are very short and hence less likely to occur simultaneously. Shallow fades less than 10 dB occur often with some paths constantly in shallow fade. When considering shallow fades, it is somewhat likely that multiple hops will experience simultaneous fades less than 10 dB.

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A microwave RF repeater with 10 to 20 dB down-fade AGC reserve will fully compensate for this shallow fading. Path fading in excess of the RF repeater's AGC reserve will be passed on to the following terminal radio. When more than one microwave repeater is used in tandem, the cumulative shallow fading may exceed the cumulative AGC reserve. As an example, consider three microwave repeaters in tandem on a four-hop section between terminals. Each repeater has 5 dB of AGC reserve and each hop has 8 dB of shallow fading. The end-to-end link will fade (4 × 8 dB) − (3 × 5 dB) = 32 − 15 = 17 dB. While the terminal radios can easily compensate for 17 dB of fading, the concept does affect designs that use multiple tandem microwave repeaters. A conservative approach is to limit the number of tandem microwave repeaters to three or possibly four when the expected fading conditions are favorable.

Diversity improvement techniques apply to microwave RF repeaters much in the manner in which they are used with standard microwave radios. It is important to consider that most microwave RF repeaters do not directly provide diversity switching or combining. Terminal microwave radios associated with microwave repeaters provide diversity switching or combining for the whole multihop microwave repeater section. Microwave RF repeaters supporting diversity-receive functions must provide two or more orthogonal channels to the terminal radio receivers. Finally, space diversity can be applied in one direction.

Frequency diversity is somewhat simpler to understand than space diversity. In frequency diversity, two channels on different frequencies carry the same traffic information. Two receivers switch between or combine the two channels to provide the best-quality signal to the receiver demodulators. A microwave RF repeater supporting frequency diversity only needs two parallel amplifying channels per direction, one on each frequency. The two frequencies are the two orthogonal channels. Frequency-selective or multipath fading on any hop in the microwave repeater section is passed to the two receivers at the terminal radio where the diversity action takes place. Diversity receivers on microwave repeater sections work normally except they may be "busier" than normal due to dealing with fades from several hops. Microwave repeaters supporting frequency diversity can be used in tandem without undue concern.

Space diversity is used in several ways with a microwave RF repeater. The first is simple receive diversity at the terminal microwave radio. Here, space diversity can be implemented as a "one-way" improvement in the path from the microwave repeater to the terminal. What about the other direction? It should be remembered that a microwave repeater often has less transmit power than the terminal radio. The resulting asymmetrical fade margins can be balanced better by provisioning space diversity receive at the terminal radio sites especially on the longer hops.

The second application of space diversity with RF repeaters is in the case where improvement is needed in both directions. Space diversity receive at the microwave repeater works best under the following conditions: the path configuration includes one RF repeater; the paths are one long hop needing improvement and one short hop having lower fading probability; and normal space-diversity antennas are provisioned on the longer path. RF repeaters configured for space diversity may use either a co frequency, dual-polarized pair of links on the short path or in the case of hybrid space- and frequency-diversity techniques, two frequency channels. Either approach provides the necessary orthogonal channels. There may be additional considerations for the co-frequency, dual-polarized configuration regarding antenna cross polarization discrimination (XPD) and receiver co-channel carrier-to-interference (C/I) or threshold-to-interference (T/I) values. Hybrid space and frequency diversity can be easily applied in frequency-diversity routes that need extra improvement. The resulting hybrid improvement is the product of the two individual improvement factors.

On-frequency microwave repeaters must manage the feedback or echo signals that can occur on site. Management is mainly focused on the antennas selected and their relative mounting locations. Digital radios typically require a co-channel C/I or T/I value of 25 to 40 dB depending on modulation, traffic capacity, forward-error correction (FEC), and receiver equalization. The antenna isolation C/E should be greater than the radio co-channel C/I or T/I specification. Antennas with adequate front-to-back (F/B) ratios and side lobe suppression are selected for each application. The task of the transmission engineer is often to select the most economical antenna configurations for the project. Often one or more antennas may be size constrained as well. The design tool used by Peninsula Engineering Solutions uses a proprietary C/E prediction method that results in recommending antennas that meet the needs of the project with enough margin to work in the real-world field installation. The recommended antennas are not "over engineered."

Antenna spacing is an available technique that doesn't add much or any cost to projects. When the repeater site antennas are separated by greater distances, either horizontally or vertically, the antenna isolation increases. Towers with 8-to-12-ft faces are often provisioned at RF repeater sites to provide greater antenna separation.

Links using microwave RF repeaters typically require antennas one size larger than if the repeater site used back-to-back microwave radios. Considering the economics of the RF repeater site with remote alternative power sources, the increased antenna costs are easily accommodated.

A significant consideration to selecting microwave RF repeaters is their low power consumption and optimization for alternative energy sources. Duplex microwave RF repeaters may only use 25 to 50 W of DC power. Reasonably sized solar arrays and storage batteries can provide reliable site power almost anywhere in the world. Solar electric battery systems are designed for the particular site location and microwave repeater load. Most systems have a battery reserve time of 7 to 10 days without solar charging. Adding a 500-W wind turbine generator can provide that extra confidence that battery-charging power is being generated during prolonged storms. Currently available solar panels have life expectancies of 25 years or more. Solar rated batteries typically last 5 to 8 years due to the daily charge and discharge cycles. Charge controller and wind turbines normally last 20 years or more at remote sites.

Beyond the typical duplex configurations where one frequency is used per direction, RF repeaters may be configured for multiline service such as 2 + 1 to 7 + 1 or in a Y-junction function where one repeater site may connect three end points.

Certain higher capacity radios have spectral bandwidths that fill the available channels. When the radio channel and the repeater filter bandwidth are close, it may be beneficial to provision delay equalizers in the RF repeater. The delay equalizers can compensate for the parabolic group delay shape of the channel filters that otherwise may cause distortion and a low error rate.

Microwave RF repeaters follow the same basic design rules as standard microwave radios with a few specific techniques needed for successful applications. Microwave repeater design tools can supplement commercial programs in automating the transmission engineering work. Microwave RF repeaters can meet or exceed the stringent system reliability demands of the user organizations. For more information on repeaters, or a free copy of the RF repeater applications design tool, contact the author or Peninsula Engineering Solutions.

See associated figures 1 and 2

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