Clear the Way

Designing microwave paths at any frequency is no longer an art; it has become a science. A properly designed path stretches the minimum distance between sites, which means reduced deployment costs for you.

A key question that continues to challenge radio engineers designing microwave radio systems is "How far can you go?" System availability, equipment specifications, local climate and geographic terrain affect the length of a microwave path and its performance. As long as you remember a few basic principles, you can minimize the line-of-sight (LOS) issues that come with designing point-to-point and point-to-multipoint systems.

Clearance

The key to microwave design is having a clear LOS between microwave transmit and receive antennas. However, objects that are spaced specific distances from the path centerline can cause severe system degradations because radio beams spread out from their origin rapidly. Therefore, significant energy can illuminate objects off the main path. If you space these objects from the path improperly, the reflected signal can cause destructive interference.

For the relatively small reflection angles associated with LOS microwave systems, the reflection process results in approximately a 180-degree phase shift. The phase shift differs from atmospheric refraction, which causes no inherent phase shift. Reflection or refraction can cause multiple signals to arrive at the receiver. When the primary and secondary signals are respectively at anti-phase in the passband, the received signal level degrades.

Path clearance is described in terms of Fresnel zones. Fresnel zones are families of ellipsoidal boundaries described by points at which a reflected radio wave would travel an integer multiple of half wavelengths further than by a direct route between the transmitter and receiver. Reflections from odd-numbered Fresnel zones will add in-phase at the receiver. You don't want even-numbered Fresnel zone clearance, because reflections from even-numbered Fresnel zones will be anti-phase at the receiver and will cancel the primary signal.

Obstruction Fading

Obstruction fading, which occurs when large positive radio refractivity gradients develop, occasionally disturbs LOS microwave paths, especially in coastal regions. These subrefractive conditions cause the microwave beam to bend upward, away from the earth's surface and the receive antenna. These conditions can develop when cool, moist air from a sea surface moves across hot, dry land masses. Positive gradients also may develop ahead of cold fronts when moist, temperate air is pushed ahead of the actual front. When obstruction conditions develop, they may persist for several hours and cause fades as deep as 35dB to 40dB.

You can mitigate obstruction fading by choosing antenna heights and path lengths that provide path clearance of grazing at K=1/2 or less. Increased system gain will provide more tolerance to obstruction fading.

Ducting & Interference

Atmospheric refraction differs from surface reflections because there is no inherent phase shift associated with the refraction process. A super-refractive layer above a subrefractive layer forms elevated ducts. Ducts can act like waveguide, trapping and guiding signals.

If both transmit and receive antennas are within the duct, the focusing effect can result in 15dB to 20dB upfades. If only one antenna is within the duct, it may cause 15dB to 20dB downfades. Interference is another impairment for new microwave systems. In the traditional Part 101 bands, interference coordination is required where the system meets interference requirements of TIA Bulletin 10F or other agreed-upon criteria. In auctioned bands such as LMDS, self-induced interference among adjacent hub sites is a bigger problem than external interference from other systems.

However, interference at BTA boundaries can be a challenge. Proper RF planning will ensure sufficient carrier-to-interference ratios (C/I) for the desired operational performance.

Rain attenuation is the single most controlling factor for path availability above 10GHz. This includes the amount of rain, size of the raindrops, density of the rainfall, moisture on tree foliage and sheeting effects off of buildings or antenna radomes. Without judicious engineering and planning, any or all of these factors can degrade the signal's quality and limit path availability significantly.

If you engineer links at millimeter wave frequencies incorrectly, you will see disappointing results. Prudent path engineering that takes all of these factors into account can provide reliable system coverage.

Available Frequency Bands

For point-to-point interconnects, you should first consider 3.7GHz to 4.2GHz, 5.9GHz to 6.4GHz, 10.55GHz to 10.68GHz, 10.7GHz to 11.7GHz, 17.7GHz to 19.7GHz, and 21.2GHz to 23.6GHz. These Part 101 frequency bands require prior coordination and licensing. But other than the minimal costs associated with the license and frequency coordination, these frequencies are essentially free to use.

Broadband and PCS providers may want to use their own frequencies (24GHz, 28GHz, 31GHz, 38GHz and 1.9GHz) for base-station interconnections and system backhaul requirements. However, if these frequencies canbe better used to offer service and generate revenue, you should consider using Part 101 frequencies for interconnect instead.

Although many new wireless applications still are on the horizon, the basic design methodology of a radio path is established. The best plan of action for designing a point-to-point or point-to-multipoint system is to completely understand LOS issues. By planning effectively for those challenges, you can achieve a highly reliable wireless network.

EDX Engineering introduced its Network Design Module, an engineering tool with a section dedicated to designing a wide variety of point-to-multipoint systems. The module is divided into two sections, one for cellular and PCS applications, and one for point-to-multipoint. The point-to-multipoint capabilities offer full-forward and reverse-link signal level, C/I radio, fade margin and percent service availability studies for first-, second- and third-best servers. The link-reliability calculations consider rain outage, terrain and building obstructions, as well as microwave fade. The module includes tools for frequency planning optimization and initial layout of hubs and customer terminals in a user-defined service area.