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When we think of wireless networks, we often picture end-user devices connecting to access points or cell towers. But behind every cellular base station, behind corporate campus networks, and spanning distances where fiber is impractical or too expensive, there exists an invisible infrastructure of microwave links—high-frequency radio connections that form the backbone of modern telecommunications.
Microwave transmission occupies the frequency range from approximately 1 GHz to 300 GHz, encompassing both the traditional point-to-point links that have connected cities for decades and the emerging millimeter-wave (mmWave) technologies that promise multi-gigabit speeds for 5G and beyond. Understanding microwaves is essential for any network professional dealing with wide-area networks, cellular backhaul, or high-capacity wireless infrastructure.
By the end of this page, you will understand microwave frequency characteristics, terrestrial link engineering including path analysis and link budgets, atmospheric effects on microwave propagation, common equipment and deployment configurations, and the emerging role of millimeter-wave technology in 5G and fixed wireless access.
Microwaves are electromagnetic waves with frequencies ranging from about 1 GHz to 300 GHz, corresponding to wavelengths from 30 centimeters down to 1 millimeter. Within this broad range, different frequency bands exhibit distinct propagation characteristics, leading to specialized applications for each segment.
| Band Designation | Frequency Range | Wavelength | Characteristics | Primary Uses |
|---|---|---|---|---|
| L-band | 1–2 GHz | 30–15 cm | Good penetration, moderate bandwidth | GPS, satellite mobile, telemetry |
| S-band | 2–4 GHz | 15–7.5 cm | Balance of penetration and bandwidth | Weather radar, Wi-Fi, satellite |
| C-band | 4–8 GHz | 7.5–3.75 cm | Reliable in rain, moderate capacity | Satellite TV, long-haul backhaul |
| X-band | 8–12 GHz | 3.75–2.5 cm | High resolution, rain-sensitive | Radar, military satellite |
| Ku-band | 12–18 GHz | 2.5–1.67 cm | Higher capacity, rain fade issues | Satellite TV, maritime VSAT |
| K-band | 18–27 GHz | 1.67–1.11 cm | Very high capacity, severe rain fade | Satellite, point-to-point (limited) |
| Ka-band | 27–40 GHz | 1.11–0.75 cm | Very high capacity, rain vulnerable | High-throughput satellites, 5G backhaul |
| V-band | 40–75 GHz | 7.5–4 mm | Ultra-high capacity, atmospheric absorption | 60 GHz: WiGig, short P2P links |
| E-band | 60–90 GHz | 5–3.3 mm | Multi-gigabit capacity, line-of-sight | 5G backhaul, high-density P2P |
| W-band | 75–110 GHz | 4–2.7 mm | Emerging applications | Advanced radar, experimental |
The Microwave Advantage:
Microwaves offer several compelling advantages for network infrastructure:
1. High Bandwidth Availability: The microwave spectrum contains far more bandwidth than lower RF frequencies. A single 80 MHz channel at 38 GHz can carry 3+ Gbps—more than dozens of VHF channels combined. This bandwidth abundance enables high-capacity point-to-point links that rival fiber optic connections.
2. Directional Propagation: Microwave antennas can be highly directional (beamwidths of a few degrees), concentrating power into narrow beams. This directivity minimizes interference, allows frequency reuse, and extends range. A parabolic dish at 18 GHz might have a beamwidth of just 1 degree.
3. Rapid Deployment: Unlike fiber, which requires trenching and physical cable installation, microwave links can be deployed in hours or days. This makes microwave ideal for quick network extensions, temporary events, disaster recovery, and reaching locations where digging is prohibitive.
4. Cost-Effective for Medium Distances: For distances of 1–50 km, microwave is often more economical than fiber, especially when crossing difficult terrain, water bodies, or densely developed urban areas where right-of-way costs are high.
The choice between microwave and fiber often comes down to distance and capacity needs. For distances under ~30 km with moderate capacity needs, microwave is typically more cost-effective. For longer distances or very high capacities (10+ Gbps), fiber usually wins. Many networks use both: fiber for core connections and microwave for access aggregation and remote site connectivity.
The Microwave Challenge: Line of Sight:
The fundamental constraint of microwave communication is the requirement for line-of-sight (LOS) between transmitter and receiver. Unlike lower-frequency radio waves that can diffract around obstacles or reflect from the ionosphere, microwaves travel in nearly straight lines and require a clear path.
Factors affecting LOS requirement:
Earth Curvature: Over longer distances, the Earth's surface curves away from the straight-line path between antennas. The bulge height increases with the square of distance.
Fresnel Zone Clearance: Electromagnetic waves spread as they travel. The Fresnel zone is an elliptical region around the direct path where obstructions significantly affect signal quality. At minimum, 60% of the first Fresnel zone must be clear of obstructions for reliable communication.
Atmospheric Refraction: The atmosphere bends microwave paths slightly, typically extending the effective horizon beyond the geometric line of sight. This is accounted for using an 'effective Earth radius' (typically 4/3 of actual radius).
Terrain and Obstacles: Hills, buildings, and even tall vegetation can block microwave paths. Path surveys identify obstacles and determine required antenna heights.
Designing a reliable terrestrial microwave link requires careful engineering analysis covering path geometry, link budget, equipment selection, and environmental factors. This section introduces the fundamental concepts every network professional should understand.
Path Analysis and Fresnel Zone:
The Fresnel zone is an elliptical region of space between transmitting and receiving antennas where electromagnetic wave diffraction effects are significant. Obstructions within the first Fresnel zone cause signal degradation even if they don't physically block the direct line between antennas.
The radius of the first Fresnel zone at its midpoint is:
$$r = 17.3 \times \sqrt{\frac{d}{4f}}$$
Where:
For a 10 km link at 18 GHz: r = 17.3 × √(10/(4×18)) = 17.3 × √0.139 = 6.45 meters
At minimum, 60% of the first Fresnel zone should be free from obstructions; ideally, 100% clearance provides the best performance. This requirement directly influences tower heights and antenna mounting positions.
The Fresnel zone radius is inversely proportional to the square root of frequency. A 38 GHz link needs only 70% of the clearance required at 18 GHz for the same distance. This is one reason why higher frequencies are preferred for urban environments where achieving clearance is challenging.
Link Budget Analysis:
A link budget is an accounting of all gains and losses in a communication system, from transmitter to receiver. It determines whether a link will achieve the required reliability. The fundamental equation:
$$P_{received} = P_{transmitted} + G_{tx} - L_{path} - L_{misc} + G_{rx}$$
Where all values are in decibels (dB or dBm).
Key Components:
| Component | Symbol | Description | Typical Values |
|---|---|---|---|
| Transmit Power | Ptx | Power output from radio | +20 to +30 dBm |
| Transmit Antenna Gain | Gtx | Focusing capability of transmit antenna | +25 to +45 dBi |
| Free Space Path Loss | FSL | Loss due to spreading over distance | 120–150 dB at microwave frequencies |
| Atmospheric Absorption | Latm | Loss from oxygen and water vapor | 0.01–15 dB/km (frequency dependent) |
| Rain Fade Margin | Lrain | Additional attenuation during rain | 5–30 dB (link availability dependent) |
| Miscellaneous Losses | Lmisc | Cable, connector, radome losses | 1–5 dB total |
| Receive Antenna Gain | Grx | Focusing capability of receive antenna | +25 to +45 dBi |
| Received Power | Prx | Signal power at receiver input | Must exceed receiver threshold |
Free Space Path Loss (FSPL):
The dominant loss component, FSPL describes how signal power spreads over an ever-increasing area as it propagates:
$$FSPL_{dB} = 20 \log_{10}(d) + 20 \log_{10}(f) + 92.45$$
Where:
For a 20 km path at 18 GHz: FSPL = 20×log₁₀(20) + 20×log₁₀(18) + 92.45 = 26 + 25.1 + 92.45 = 143.55 dB
This enormous loss explains why microwave links require high-gain antennas and relatively powerful transmitters, even for moderate distances.
Fade Margin:
The difference between the received signal power under normal conditions and the receiver's minimum threshold is the fade margin. This margin must be sufficient to maintain the link during adverse conditions—primarily rain fade at frequencies above 10 GHz.
Target fade margins depend on required link availability:
Higher availability requirements demand either more robust equipment (higher power, larger antennas) or lower-frequency bands less affected by rain.
The atmosphere is far from an ideal transmission medium for microwaves. Various atmospheric constituents absorb and scatter microwave energy, causing additional attenuation beyond free-space spreading. Understanding these effects is essential for reliable link design, particularly at higher frequencies where atmospheric interactions become significant.
Gaseous Absorption:
Oxygen and water vapor molecules absorb microwave energy at specific frequencies, converting electromagnetic energy into molecular motion (heat). This absorption creates 'windows' of relatively low attenuation and 'peaks' of high attenuation across the microwave spectrum.
Key Absorption Features:
22 GHz water vapor peak: A significant absorption line where H₂O molecules resonate. Attenuation can reach 0.2 dB/km at sea level in humid conditions.
60 GHz oxygen peak: A very strong absorption band (~15 dB/km at sea level) caused by O₂ molecular resonance. This makes 60 GHz unsuitable for long-distance links but provides natural frequency reuse isolation.
118 GHz oxygen peak: Another strong O₂ absorption line limiting practical use of this frequency range.
Between these peaks lie 'atmospheric windows' where attenuation is manageable: 35–50 GHz and 70–95 GHz are commonly used for high-capacity links despite some residual absorption.
| Frequency | O₂ Absorption | H₂O Absorption (1% humidity) | Total (dB/km) |
|---|---|---|---|
| 10 GHz | ~0.007 | ~0.004 | ~0.01 |
| 18 GHz | ~0.010 | ~0.030 | ~0.04 |
| 23 GHz | ~0.015 | ~0.180 | ~0.20 |
| 38 GHz | ~0.035 | ~0.060 | ~0.10 |
| 60 GHz | ~15.0 | ~0.100 | ~15.1 |
| 73 GHz | ~0.50 | ~0.40 | ~0.90 |
| 80 GHz | ~0.35 | ~0.60 | ~0.95 |
While 60 GHz's extreme oxygen absorption limits range to a few hundred meters outdoors, this 'flaw' becomes a feature for certain applications. The WiGig (802.11ad/ay) standard uses 60 GHz precisely because signals don't travel far, enabling extreme frequency reuse without interference. Multiple 60 GHz links can operate in adjacent areas without coordination—something impossible at lower frequencies.
Rain Attenuation:
Rain is the most significant atmospheric impairment for microwave links, especially above 10 GHz. Water droplets absorb and scatter microwave energy, causing attenuation that increases dramatically with rain intensity and frequency.
Rain Attenuation Factors:
Rain Rate: Measured in mm/hour. Light rain (~5 mm/hr) causes modest attenuation; heavy tropical downpours (100+ mm/hr) can cause extreme fading.
Frequency: Attenuation increases roughly with the square of frequency. A 38 GHz link experiences about 4× the attenuation of an 18 GHz link for the same rain intensity.
Drop Size Distribution: Larger drops cause more scattering. Tropical rainfall (larger drops) typically causes more attenuation than temperate rainfall of the same rate.
Path Length: Rain cells are typically 1–5 km in extent. Longer paths are more likely to encounter rain cells and experience attenuation.
| Frequency | Light Rain (5 mm/hr) | Moderate Rain (25 mm/hr) | Heavy Rain (50 mm/hr) | Tropical (100 mm/hr) |
|---|---|---|---|---|
| 10 GHz | 0.1 | 0.6 | 1.4 | 3.1 |
| 18 GHz | 0.6 | 2.5 | 5.0 | 10.0 |
| 23 GHz | 1.0 | 4.0 | 7.5 | 15.0 |
| 38 GHz | 2.0 | 7.5 | 14.0 | 26.0 |
| 60 GHz | 4.0 | 12.0 | 20.0 | 35.0 |
| 80 GHz | 5.5 | 15.0 | 25.0 | 42.0 |
Mitigating Rain Fade:
Network designers employ several strategies to maintain link availability during rain events:
1. Adequate Fade Margin: Building sufficient margin into the link budget is the primary defense. For 99.99% availability in temperate climates, 15–25 dB of rain fade margin is typical at Ku-band and above.
2. Adaptive Coding and Modulation (ACM): Modern microwave radios can dynamically reduce data rate during fade events, transitioning from 256-QAM to QPSK or even BPSK. This 'graceful degradation' maintains connectivity (at reduced speed) during conditions that would otherwise cause outages.
3. Lower Frequency Bands: For maximum reliability, C-band (4–8 GHz) links are preferred in high-rainfall regions. C-band experiences minimal rain fade, though at the cost of reduced bandwidth availability.
4. Space Diversity: Using two receiving antennas separated vertically (by 100+ wavelengths) or horizontally (by hundreds of meters) provides uncorrelated fade paths. When one antenna experiences deep fade, the other often maintains adequate signal.
5. Route Diversity: For critical links, deploying two geographically separate paths ensures that localized rain cells don't simultaneously affect both paths.
Tropical and equatorial regions experience rainfall intensities that are rare or unknown in temperate zones. A 100 mm/hr rainfall rate that might define a 99.99% availability in London could occur daily during monsoon season in Singapore. Always use region-specific ITU rainfall data when designing microwave links.
Other Atmospheric Phenomena:
Ducting and Anomalous Propagation: Temperature inversions can create atmospheric 'ducts' that bend microwave signals beyond the normal horizon, potentially causing unexpected interference or temporary extreme range extension. Ducting is unpredictable and cannot be relied upon for operational links.
Fog and Clouds: Water droplets in fog and clouds cause attenuation similar to rain but typically at lower levels due to smaller droplet size. Fog attenuation becomes significant above 50 GHz.
Snow and Wet Snow: Dry snow causes minimal attenuation, but wet snow (melting layer) can significantly attenuate signals, particularly when accumulated on antenna surfaces.
Sand and Dust: In desert regions, sandstorms can cause modest attenuation (few dB) and, more importantly, can physically abrade antenna surfaces, degrading performance over time.
A microwave link comprises specialized radio equipment, antennas, mounting infrastructure, and network integration components. Understanding this equipment enables informed procurement decisions and effective troubleshooting.
Microwave Radio Configurations:
Microwave radios come in three primary form factors:
1. Split-Mount (Indoor/Outdoor Unit - IDU/ODU): The traditional architecture separates the radio into two units:
2. Full Outdoor (All-Outdoor): The complete radio is integrated into a single weatherproof unit mounted on the tower, directly behind or integrated with the antenna.
3. Integrated (Radio + Antenna): Radio electronics built directly into the antenna housing, creating a single 'radio antenna' unit.
| Antenna Type | Gain Range | Beamwidth | Characteristics |
|---|---|---|---|
| Parabolic Dish (Standard) | 25–45 dBi | 1–4° | Most common; high gain; requires precise alignment |
| Parabolic Dish (High-Performance) | 30–48 dBi | 0.5–2° | Shielded design; superior sidelobe suppression |
| Flat Panel (Planar Array) | 15–30 dBi | 3–15° | Low profile; easier installation; some gain trade-off |
| Cassegrain (Dual-Reflector) | 40–55 dBi | 0.5–1.5° | Very high gain; used for very long paths |
| Horn Antenna | 15–25 dBi | 10–30° | Simple; often integrated with radio; E-band/mmWave |
Capacity and Modulation:
Modern microwave radios support a wide range of capacities through flexible channel widths and modulation schemes:
Channel Widths:
Modulation Schemes: From QPSK (~2 bits/Hz) up to 4096-QAM (~12 bits/Hz), with Adaptive Modulation switching dynamically based on conditions.
Current Capacity Benchmarks:
Cross-Polar Interference Cancellation (XPIC) enables two independent data streams on the same frequency using orthogonal polarizations (horizontal and vertical). Advanced signal processing separates the streams despite cross-polar leakage. This effectively doubles capacity without additional spectrum—a critical technique for bandwidth-constrained links.
Tower and Mounting Infrastructure:
Microwave antennas require stable, properly aligned mounting structures:
Tower Types:
Mounting Considerations:
Alignment: Microwave antennas must be precisely aligned to within a fraction of a degree. Alignment is typically performed using signal level measurements while slowly adjusting azimuth and elevation. Some modern systems include electronic alignment assistance or auto-tracking.
Millimeter waves (mmWave) occupy the frequency range from 30 GHz to 300 GHz, where wavelengths shrink to between 10 mm and 1 mm. Once considered exotic and impractical, mmWave technology has become mainstream with advancements in semiconductor technology, driving its adoption for 5G mobile networks, high-capacity backhaul, and fixed wireless broadband.
Key mmWave Bands for Networking:
V-band (57–71 GHz): Falls within the 60 GHz oxygen absorption peak. While atmospheric attenuation limits range to a few hundred meters outdoors, it enables extreme spectrum availability (up to 14 GHz in some regions) for short-range, high-capacity links. WiGig (802.11ad/ay) operates here, as do short urban backhaul links.
E-band (71–76 GHz and 81–86 GHz): Two 5 GHz wide bands (10 GHz total) sitting in an atmospheric window with moderate attenuation (~1 dB/km). E-band has become the workhorse for multi-gigabit backhaul, with channels up to 2 GHz wide supporting 10+ Gbps. Light-licensing regimes in many countries have accelerated adoption.
D-band (110–170 GHz): Emerging spectrum for next-generation backhaul. Very wide bandwidth available but significant propagation challenges. Active research is underway, with early commercial products appearing above 130 GHz.
| Band | Frequency Range | Typical Range | Channel Width | Capacity | Primary Use |
|---|---|---|---|---|---|
| V-band | 57–71 GHz | 100–500 m | Up to 2.16 GHz | 1–10 Gbps | WiGig, short P2P, street-level backhaul |
| E-band | 71–76, 81–86 GHz | 1–5 km | 250–2000 MHz | 1–20 Gbps | 5G backhaul, enterprise WAN |
| W-band | 75–110 GHz | 0.3–2 km | Up to 5 GHz | 10–25 Gbps | Emerging high-capacity links |
| D-band | 110–170 GHz | 0.1–1 km | Up to 10 GHz | 25–100 Gbps (future) | Research, future 6G backhaul |
mmWave Propagation Characteristics:
Millimeter waves present unique propagation challenges that distinguish them from traditional microwave bands:
1. Higher Free-Space Path Loss: Path loss increases with the square of frequency. An 80 GHz link has 12 dB more FSPL than an 18 GHz link over the same distance—requiring higher antenna gain or shorter distances to compensate.
2. Atmospheric Absorption: While E-band sits in a relatively clear window, even 0.5–1 dB/km of clear-air attenuation becomes significant over multi-kilometer paths. The 60 GHz oxygen peak makes V-band impractical beyond a few hundred meters.
3. Rain Attenuation: mmWave links are severely affected by rain. At 80 GHz, heavy rain can cause 25+ dB/km attenuation. This is the primary factor limiting E-band link distances in tropical climates.
4. Limited Diffraction: With wavelengths of just a few millimeters, diffraction around obstacles is negligible. Strict line-of-sight is essential—even small obstructions or misalignments can cause outage.
5. Sensitivity to Alignment: High-gain mmWave antennas (35–50 dBi from small apertures) have very narrow beamwidths (<1°). Tight alignment tolerances and structural stability are critical. Tower sway that would be invisible at lower frequencies can cause significant fading at mmWave.
Antenna gain for a given aperture size is proportional to frequency squared. A 30 cm dish at 80 GHz provides the same gain as a 1.2 m dish at 18 GHz. This allows mmWave links to achieve very high gain from compact, lightweight antennas—essential for dense urban deployments and rooftop installations.
mmWave for 5G Access:
Beyond backhaul, mmWave spectrum is fundamental to 5G New Radio (NR) for user access. 5G mmWave deployments differ significantly from sub-6 GHz:
Fixed Wireless Access (FWA): mmWave enables 'fiber without the fiber'—delivering gigabit-class broadband to homes and businesses wirelessly. Carriers deploy mmWave FWA as an alternative to last-mile fiber construction, particularly where trenching is difficult or uneconomical.
Network planners often face the choice between microwave and fiber for point-to-point connectivity. Neither technology is universally superior—the optimal choice depends on capacity requirements, geography, timeline, and budget. Understanding the trade-offs enables informed decision-making.
| Factor | Microwave | Fiber Optic |
|---|---|---|
| Capacity | 100 Mbps – 20 Gbps typical; 40 Gbps+ with aggregation | 10 Gbps – 100+ Gbps per fiber pair; near-unlimited with DWDM |
| Latency | Speed of light in air (~3.3 μs/km) | Speed of light in glass (~5 μs/km); slightly higher than microwave |
| Deployment Speed | Days to weeks (after site preparation) | Months to years (includes civil works, permits, construction) |
| Capital Cost | Moderate: radio equipment, towers, installation | High: fiber cable, trenching, splicing, rights-of-way |
| Operating Cost | Low: power, spectrum fees, occasional maintenance | Very low: minimal power, very low maintenance |
| Reliability | 99.99% – 99.999% typical; affected by weather | 99.999%+ typical; affected by fiber cuts, construction damage |
| Upgrade Path | Replace radios or add channels/bonds | Light additional wavelengths on existing fiber (DWDM) |
| Physical Vulnerability | Towers visible targets; sensitive to severe weather | Buried cables at risk of dig-ups, construction damage, flooding |
| Spectrum/Rights | Licensed spectrum or light-licensed E-band required | Rights-of-way, easements, pole rights required |
When Microwave Wins:
Time-Critical Deployments: When connectivity is needed in days, not months. Disaster recovery, temporary events, and rapid business expansion favor microwave.
Difficult Terrain: Crossing rivers, mountains, highways, or dense urban areas where trenching is impossible or prohibitively expensive.
Moderate Capacity Needs: When 1–10 Gbps is sufficient and unlikely to grow beyond 20 Gbps.
Financial Trading: Where nanoseconds matter, microwave's lower latency provides competitive advantage.
Developing Regions: Where fiber infrastructure is limited and deploying it would take years.
Regulatory Challenges: Where securing fiber right-of-way is difficult or politically impossible.
When Fiber Wins:
High Capacity Requirements: When current or future needs exceed 10 Gbps, fiber's scalability is unmatched.
Long Distances: For paths beyond 30–50 km, fiber avoids multi-hop microwave complexity.
Urban Last-Mile: Where fiber is already prevalent and connection costs are reasonable.
Extreme Reliability Requirements: Where 99.999%+ availability is non-negotiable and weather-independent.
Dense Fiber Aggregation: Connecting dozens of sites to a central hub typically favors fiber's aggregation economics.
Many networks combine both technologies strategically. Fiber provides backbone capacity and connects major hubs. Microwave extends the network to remote sites, provides redundant paths for resilience, and enables rapid deployment for new locations while fiber is being installed. The most resilient networks are those where both technologies can independently serve critical connections.
Microwave transmission is a mature, essential technology for wireless infrastructure, providing high-capacity connectivity where fiber is impractical, too slow to deploy, or simply not cost-effective. From traditional licensed bands to cutting-edge millimeter-wave frequencies, microwave continues to evolve alongside network demands.
What's Next:
In the next page, we'll explore infrared transmission—the portion of the electromagnetic spectrum between microwaves and visible light. We'll examine how infrared enables short-range communication in remote controls and IrDA, as well as cutting-edge free-space optical links that can achieve fiber-like speeds through the atmosphere.
You now have a comprehensive understanding of microwave transmission—from fundamental frequency characteristics through link engineering, atmospheric effects, equipment, and the emerging mmWave landscape. This knowledge is essential for anyone designing or managing wireless infrastructure, cellular backhaul, enterprise WANs, or understanding the physical layer of modern telecommunications.