Minimizing Interference of Microwave and Millimeter Wave
Many companies are looking into easy-to-deploy, point-to-point wireless alternatives typically used for urban private networks and for backhaul-to-trunk lines. Millimeter wave (mmWave) radio is popular for this application because it provides a multiple gigabit transmission rate similar to fiber optic cables but at lower costs.
The Federal Communications Commission (FCC) has standardized mmWave E-band spectrum and rules for the technology where “pencil beam” narrow frequencies are highly re-usable at sites with multiple antennas. However, when collocating mmWave equipment on a tower or building, stakeholders should be concerned about minimizing potential interference with existing radiating equipment. Antenna installations must incorporate safe design controls to ensure compliance with both public and occupational RF exposure limitations.
In evaluating mmWave, your organization will need to carefully examine potential interference for the collocation. This brief explains major causes of interference. It also provides best practices to minimize interference for the successful deployment and use of mmWave.
Transmitters and receivers may experience either near or far interference. Two kinds of near interference may occur at a collocated site. Intermodulation is interference within a single equipment at a site; coupling interference occurs between different gear at a site. Far interference occurs with gear located away from a site; it occurs when other signals of the same channel cross paths. The figure below shows where possible interference may occur between two systems.
- From transmit equipment (Tx) in one housing unit to receiver equipment (Rx) in the same cabinet or other cabinets in a room
- From transmitter’s antenna to receiver’s equipment housing
- From transmitter’s antenna to receiver’s antenna
- From transmitter’s housing to receiver’s antenna
- As spurious signals in power supply
Minimizing the amount of frequency used is important to keep the quality of service protected from interference; here are some of the ways to plan frequency assignment:
- Examine channels used to ensure there is adequate duplex spacing between each band—whether link-by-link or channel block is used
- Consider traffic and capacity before assigning the duplex half for transmitters
- Check azimuths (directions) and elevation to determine possible near interference
- Understand antenna radiation patterns, polarization and front-to-back ratio to know roughly where the antenna transmits
- Start planning with lower output power
- Understand the network configuration and topology specific to the location
- Have microwave systems engineers review your plan for accuracy and potential mitigating issues, such as areas with unusual rainfall or temperature variations or the presence of many radio reflections
3. Check Fresnel Zone Clearance
The Fresnel Zone is an ellipsoid of space between two antennas. This is “free space” that must be clear of obstruction, such as buildings located between a pair of antennas. If a building even slightly grazes the Fresnel Zone boundary, it can cause a reflection that interferes with radio transmission. If the distance between two antennas is more than about five miles, earth curvature is another potential source of interference—particularly if terrain in between grazes the Fresnel Zone. Atmospheric refraction may also cause signal degradation.