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":

where:

Gain_R = the RF repeater maximum gain (in dB);
NF_R = the RF repeater noise figure (in dB);
NF_T = 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_Pwr_T = the terminal radio threshold (in dBm)
PAD_In = the RF repeater input attenuator pad attenuation (in dB); and
PAD_Out = the RF repeater output attenuator pad attenuation (in dB).

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

FFM, dB = [nominal receive signal level (dBm) − minimum receive power (dBm)]

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.