Microwave Communication

Microwave Communication: High-Capacity Point-to-Point Links

Microwave communication uses focused beams of electromagnetic radiation in the 1-300 GHz range to establish high-capacity communication links between fixed points. These directed radio beams carry telephone calls, internet data, and television signals across distances of 30-50 kilometers per hop, requiring no physical cable between endpoints. Microwave links form the backbone of cellular backhaul networks and provide critical communication infrastructure where fiber installation is impractical.

The Concept

Think of it this way: while lower-frequency radio waves spread out in all directions like ripples on a pond, microwaves can be focused into a tight beam like a spotlight. By pointing two dish antennas at each other, you create an invisible bridge of radio energy carrying enormous amounts of data through thin air.

Key characteristics:

• Highly directional (narrow beam using parabolic dish antennas)
• Line-of-sight propagation required (cannot bend around obstacles)
• High bandwidth available (hundreds of MHz per channel)
• Typical capacity: 100 Mbps to 10+ Gbps per link
• No cable needed — rapid deployment possible

Microwave Link Design

System components:

1. Transmitter: Generates microwave signal (modulated with data)
2. Transmission line: Waveguide or coaxial cable connecting to antenna
3. Transmit antenna: Parabolic dish focusing energy into narrow beam
4. Free-space path: 1-50 km through atmosphere
5. Receive antenna: Parabolic dish collecting incoming beam
6. Receiver: Amplifies and demodulates received signal

Frequency bands commonly used:

Fresnel Zone Clearance

For reliable microwave propagation, the path must be clear not only of direct obstruction but also of objects within the Fresnel zone — an ellipsoidal region around the direct path:

First Fresnel zone radius at mid-path:

r₁ = √(λ×d₁×d₂/d)

Where λ = wavelength, d₁ and d₂ = distances from each end to the obstruction point, d = total path length.

Rule of thumb: At least 60% of the first Fresnel zone must be clear of obstacles for full free-space propagation. Obstacles within the Fresnel zone cause diffraction losses even without direct blockage.

Example: 10 GHz link (λ = 3 cm), 30 km path:

• Mid-path Fresnel radius: r₁ = √(0.03 × 15000 × 15000 / 30000) = √225 = 15 meters
• Required clearance above obstacles at mid-path: 0.6 × 15 = 9 meters minimum

Link Budget for Microwave

Received power: Pr = Pt + Gt + Gr - FSL - Lmisc

Free-space loss: FSL(dB) = 92.45 + 20×log₁₀(f_GHz) + 20×log₁₀(d_km)

Example — 18 GHz, 20 km link:

• Transmit power: 20 dBm (100 mW)
• Antenna gain (each): 42 dBi (1.2 m dish)
• FSL = 92.45 + 25.1 + 26.0 = 143.6 dB
• Atmospheric loss: 1 dB
• Received power: 20 + 42 + 42 - 143.6 - 1 = -40.6 dBm
• Receiver threshold (256-QAM): -60 dBm
• Fade margin: 19.4 dB (provides 99.99%+ availability)

Rain Attenuation

Rain significantly attenuates microwaves above 10 GHz:

For high-frequency links in rainy climates, sufficient fade margin must be included or adaptive modulation used to reduce data rate during rain events.

Adaptive Modulation

Modern microwave links dynamically adjust modulation based on atmospheric conditions:

• Clear sky: 1024-QAM or 2048-QAM (maximum throughput)
• Light rain: 256-QAM (reduce rate, increase robustness)
• Heavy rain: QPSK (survive the fade, minimal data rate)

This achieves high average throughput while maintaining availability during adverse weather — called Adaptive Coding and Modulation (ACM).

Applications

1. Cellular backhaul: Connecting cell towers to the core network (70%+ of cell sites use microwave backhaul globally)
2. 5G fronthaul/backhaul: E-band (71-86 GHz) links providing 10+ Gbps for 5G small cells
3. Enterprise WAN: Connecting office buildings across cities
4. Broadcast contribution: Linking remote event venues to broadcast studios
5. Utility networks: Power companies, railways, pipeline monitoring
6. Disaster recovery: Rapid deployment when fiber is cut or unavailable

Advantages Over Fiber

• No trenching, right-of-way, or cable laying required
• Deployment in days (vs. months for fiber)
• Crosses rivers, highways, mountainous terrain easily
• Lower initial cost for moderate-capacity links
• No physical vulnerability to dig-ups or cable cuts

Key Takeaways

1. Microwave links use focused radio beams between directional antennas to create high-capacity point-to-point communication without cables.
2. Line-of-sight propagation with Fresnel zone clearance is required — path surveys and tower height calculations are essential during planning.
3. Rain attenuation above 10 GHz is the primary reliability challenge, requiring adequate fade margin or adaptive modulation for high availability.
4. Modern microwave systems achieve 10+ Gbps using wide channels (2 GHz at E-band) with high-order modulation (up to 4096-QAM in clear conditions).
5. Over 70% of cellular base stations globally use microwave backhaul — it remains essential where fiber deployment is impractical or uneconomical.
6. Adaptive modulation maintains availability during rain fades by trading throughput for robustness dynamically.