Wireless Communication Lecture 2 – Microwave Communication Fundamentals

A comprehensive set of Q&A flashcards reviewing fundamental principles, formulas, advantages, losses, diversity techniques, and design criteria for terrestrial microwave communication, as covered in Lecture 2.

What frequency range is typically considered "microwave communication" in terrestrial systems?

About 2 GHz to 60 GHz.

According to IEEE, what name is given to electromagnetic waves between 30 GHz and 300 GHz?

Millimeter waves (MMW).

What capacity and frequency range are usually used for short-haul microwave links?

Capacities of 64 kbps to 2 Mbps on frequencies below 3 GHz.

What capacities and frequency range characterize medium- and large-capacity microwave systems?

3 GHz to 15 GHz with capacities from 34 Mbps up to 620 Mbps.

Above what frequency are microwave systems mainly limited to short-haul applications?

15 GHz.

Name three key advantages of microwave systems related to infrastructure needs.

No right-of-way required, fewer repeaters, and minimized underground facilities.

Why is background noise lower for microwave frequencies than for HF frequencies?

Because natural and man-made noise sources are much weaker at microwave frequencies.

Give two reasons conventional lumped components (R, L, C) are unsuitable at microwave frequencies.

Physical size becomes comparable to wavelength and parasitic inductances/capacitances dominate.

Which vacuum-tube devices are commonly used for microwave amplification?

Klystrons, magnetrons, and traveling-wave tubes (TWTs).

List two atmospheric conditions that strongly attenuate microwave signals, especially above 20 GHz.

Rain and fog.

What are the two main categories of terrestrial microwave stations?

Terminals and repeaters.

How does a passive microwave repeater work?

It re-radiates received energy without additional electronic gain (e.g., billboard reflector).

What is the role of an active microwave repeater?

Receives, amplifies/reshapes, then retransmits the signal toward the next station.

Match the common microwave bands with their approximate center frequencies: L, S, C, X.

L ≈ 2 GHz, S ≈ 4 GHz, C ≈ 8 GHz, X ≈ 12 GHz.

What three basic path classifications exist for microwave links?

Line-of-sight (LOS), grazing, and obstructed paths.

Define the K-factor (K-curve) in microwave propagation.

A numerical factor that models atmospheric refraction by defining an ‘effective Earth radius’.

In which K-condition does the effective Earth appear flatter than normal, improving LOS reach?

Super-standard condition (K > 4⁄3).

State the formula for effective Earth radius (re) using true radius (ro) and surface refractivity (NS).

re = ro / (1 – NS × 10⁻⁶).

What is "earth bulge" in path calculations?

The additional height an obstacle appears to have due to Earth’s curvature along the path.

In frequency planning, which band can typically span the longest hop: 8 GHz or 23 GHz?

8 GHz (≈ 30 mi vs 10 mi for 23 GHz).

What are Fresnel zones?

Elliptical regions around the direct path where constructive/destructive interference occurs.

What clearance criterion is normally required for the first Fresnel zone?

At least 60 % of the first-zone radius must be free of obstructions.

Give the formula for the radius of the first Fresnel zone (F₁) in meters.

F₁ = 17.3 × √(d₁ × d₂ / D × f), where distances are in km and f in GHz.

State the general design steps of a microwave link.

Compute losses, fading/fade margins, plan frequencies & interference, assess quality/availability.

What is Effective Isotropically Radiated Power (EIRP)?

Pt + Gant – TLL (transmitter power + antenna gain – line loss).

Write the aperture-based antenna gain formula.

Gant = η(πd/λ)².

Give the simplified antenna gain formula in dB using frequency in GHz and diameter in meters.

Gant (dB) = 20 log f + 20 log d + 17.8.

How is isotropic receive level (IRL) calculated?

IRL = EIRP – FSL.

Provide the free-space loss (FSL) formula for f in GHz and D in km.

FSL (dB) = 92.4 + 20 log f + 20 log D.

What equation yields the unfaded Received Signal Level (RSL)?

RSL = Pt + Gtx – TLLtx – FSL + Grx – TLLrx.

How is carrier-to-noise ratio (C/N) related to RSL and thermal noise power?

C/N (dB) = RSL (dBm) – Pn (dBm).

State the thermal noise threshold formula.

Pn = 174 + 10 log B + NF (dBm, B in Hz).

Define fade margin (FM).

FM = RSL – Receiver Threshold; extra power margin to survive fading.

Write the Barnett-Vigant fade margin equation core term.

FM = 30 log D + 10 log(6ABf) – 10 log(1 – R) – 70 (D km, f GHz).

What terrain-roughness factor (A) is used for smooth terrain or over-water paths?

A = 4.

What B-value applies to hot, humid areas in Barnett-Vigant?

B = 0.5.

Explain "system gain" in a microwave link.

Difference between transmitter nominal output and minimum receiver input (Pt – Cmin); must exceed total path loss minus gains.

What is a link budget?

A tabulation of all gains and losses from transmitter to receiver to verify adequate RSL and SNR.

Name two primary categories of transmission line losses between radio and antenna.

Waveguide loss and connector loss.

What two molecular constituents cause atmospheric absorption loss?

Oxygen and water vapor.

Define diffraction loss.

Loss incurred when a wavefront bends around an obstruction, redistributing its energy.

How much loss is usually allocated for antenna misalignment on a microwave link?

Up to 0.25 dB per antenna (≈0.5 dB per link).

What is flat fading?

Non-frequency-dependent fading caused by rain, ducting, or equipment changes.

What causes frequency-selective fading?

Multipath propagation via reflection and diffraction over terrain or atmospheric layers.

List two countermeasures against flat fading.

Increase antenna size/power (link overbuild) or create shorter multi-hop routes.

What is diversity in microwave links?

Using redundant paths, frequencies, or antennas so at least one signal remains adequate during fading.

Describe frequency diversity.

Same information is transmitted simultaneously on two carrier frequencies separated by 2-3 %.

Describe space diversity.

Two (or more) receive antennas vertically separated receive the same frequency over independent spatial paths.

Define path diversity.

Routing the same traffic over geographically separate links at least about 10 km apart.

State the basic antenna-separation guideline for space diversity.

Vertical spacing ≈ 200 wavelengths or more.

What is time availability in link design?

The percentage of total time that a link meets its performance objectives (e.g., ≥ 99.99 %).

How much fade margin is typically required for 99.99 % Rayleigh availability?

About 38 dB.

How are net path loss and transmitter output related?

Net Path Loss = Pt – RSL; it reflects total attenuation along the path.

Which two main calculations open a microwave link budget worksheet?

Transmitter output power and transmit antenna gain.

What clearance criterion is recommended when k = 4⁄3?

100 % of the first Fresnel zone radius.