Wi-Fi & 802.11 · Study deck
Propagation and Link Budgets
A sound site review starts on paper.
Radio Remi is your guide for this deck.
After studying this chapter
Learning objectives
You will be able to:
- distinguish free-space estimates from deployment-ready propagation evidence
- explain why antenna placement, orientation, enclosure, and mounting affect link performance
- identify how obstacles, Fresnel clearance, multipath, and interference change the link record
- review a link budget as a set of evidence assumptions rather than a single computed result
Major section
Start With the Wireless Story
Under the Hood explains path calculations, fading, interference, and why a clean early estimate must still be checked in the field.
- A sound site review starts on paper.
- Leave room for change.
- Recheck after layout, season, or equipment changes.
- It fails after workers mount it behind a metal beam.
Major section
Start With the Wireless Story (continued)
The propagation story follows distance, walls, antenna placement, fading, interference, margin, and retest triggers until the team can explain why packets should still arrive.
- The device did not change.
- The path around it did.
- Wireless design must account for the real place.
- A signal loses strength as it travels.
Major section
In 60 Seconds
Wireless propagation design is the discipline of proving that a radio link will work after the product leaves the lab.
- Free-space calculations are useful early checks, but they are not enough for IoT deployments with walls, people, cabinets, poles, foliage, vehicles, machinery, antenna detuning, interference, and changing gateway placement.
- The goal is not to memorize a path-loss formula.
- The goal is to build a review record that explains why the installed link has enough margin for the actual site.
Major section
Phoebe's Field Notes: How Far Before the Budget Runs Out
The mathematical gist.: This link can tolerate 107 dB total path loss.
- Reserving 8 dB leaves 99 dB for propagation and 92.6 m range: 0.541 times the radius and 0.293 times the idealised coverage area.
Major section
Propagation Evidence Map
A design can pass the formula step and still fail because the antenna is detuned, the gateway is too low, a cabinet blocks the path, or a nearby system occupies the channel.
- The map connects the link budget to field observations and exposes which assumption needs retesting.
Major section
The Link Budget Decides Whether a Link Closes
Propagation is the study of how a signal weakens between transmitter and receiver.
- If received power sits comfortably above sensitivity, the link closes; if not, it drops.
- Path loss is dominated by distance and obstacles.
- The remaining vertical separation is the margin available for fading and installation changes.
Major section
The Link Budget Decides Whether a Link Closes (continued)
This waterfall connects the arithmetic to field validation: every term needs a source, and the final enclosure and location must prove that enough margin remains.
- Free-space loss is about 20 log10(0.1 km) + 20 log10(2400 MHz) + 32.44 ≈ 80 dB.
- Every wireless link is an inequality: received power must exceed sensitivity, with margin to spare.
- RSRP tells a cellular device how that inequality is going.
Major section
Read RSRP, RSRQ, and SINR
RSRP is about strength; RSRQ and SINR are about cleanliness.
- A device can show strong RSRP but poor RSRQ when the cell is congested or interfered — the signal is loud but the channel is noisy.
- Worked example.: A meter reports RSRP = −115 dBm and RSRQ = −18 dB.
- Strength is near the cell edge and quality is poor — connection may work only with heavy repetition and low throughput.
Major section
Read RSRP, RSRQ, and SINR (continued)
Options that raise RSRP (a better antenna, a lower-band cell, relocating off a metal riser) will help more than anything on the network side.
- A device with RSRP around -85 dBm but RSRQ near -17 dB probably has enough reference-signal strength but poor quality from load, interference, or neighboring-cell energy.
- Moving the antenna may not solve that if the local cell is busy.
- A different device with RSRP around -110 dBm and RSRQ near -9 dB has a cleaner channel but little strength margin, so placement, antenna efficiency, band choice, or gateway/base-station geometry matter more.
Major section
Maximum Coupling Loss and NB-IoT's +20 dB
The price is time and energy — deep-coverage messages are slow and costly, which is why NB-IoT targets tiny, infrequent payloads.
- Engineers summarise a technology's reach with maximum coupling loss (MCL) — the largest total path loss a link can survive while still delivering data.
- That extra 20 dB is bought two ways.
- Worked example.: A sensor at the cell edge fails to decode at MCL 150 dB with a single transmission.
Major section
Maximum Coupling Loss and NB-IoT's +20 dB (continued)
NB-IoT is designed for 164 dB — a 20 dB improvement, which is 100× in power terms and roughly the difference between street-level coverage and a signal that reaches a basement water meter.
- Coverage, throughput, and power are the three-way trade every deep-coverage design balances.
- The repeated-copy gain is logarithmic, so it has diminishing returns.
- Moving from 1 to 4 repetitions gives about 6 dB, 4 to 16 gives another 6 dB, and 16 to 128 gives about 9 dB more.
Deck summary
Key takeaways
Under the Hood explains path calculations, fading, interference, and why a clean early estimate must still be checked in the field.
- The propagation story follows distance, walls, antenna placement, fading, interference, margin, and retest triggers until the team can explain why packets should still arrive.
- Wireless propagation design is the discipline of proving that a radio link will work after the product leaves the lab.
- The mathematical gist.: This link can tolerate 107 dB total path loss.
- Propagation is the study of how a signal weakens between transmitter and receiver.
Retrieval practice
Recall check 1 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q1Why are free-space path-loss calculations not enough to approve an IoT radio link?
Show answer
Answer: A Free-space math is an early check; real sites add obstacles and interference, so field evidence is required.
Retrieval practice
Recall check 2 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q2A design review includes a free-space estimate with positive margin, but no final-enclosure antenna test or site measurement. What is the best review decision?
Show answer
Answer: C A free-space estimate is a useful first filter, but approval still needs final-enclosure antenna checks plus representative site, coexistence, and measurement evidence.
Retrieval practice
Recall check 3 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q3What does a link budget determine?
Show answer
Answer: A The accounting combines transmit power, antenna gains, cable/enclosure losses, path loss, and receiver sensitivity.
Retrieval practice
Recall check 4 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q4A cellular IoT device shows strong RSRP but poor RSRQ. What does that indicate?
Show answer
Answer: A RSRP is raw reference-signal strength; RSRQ includes total received power, so congestion, interference, or load can pull it down.
Retrieval practice
Recall check 5 of 5
Radio Remi says: answer from memory, then check your reasoning.
Q5NB-IoT targets a maximum coupling loss of 164 dB versus ~144 dB for legacy networks. How is that ~20 dB mainly achieved, and at what cost?
Show answer
Answer: A Repetition gives roughly 10 log10(N) dB and the narrow carrier raises power spectral density; the cost is airtime, latency, and battery energy.
Print reference
Answers 1 of 2
Answer key.
- A · Free-space math is an early check; real sites add obstacles and interference, so field evidence is required.
- C · A free-space estimate is a useful first filter, but approval still needs final-enclosure antenna checks plus representative site, coexistence, and measurement evidence.
- A · The accounting combines transmit power, antenna gains, cable/enclosure losses, path loss, and receiver sensitivity.
- A · RSRP is raw reference-signal strength; RSRQ includes total received power, so congestion, interference, or load can pull it down.
Print reference
Answers 2 of 2
Answer key.
- A · Repetition gives roughly 10 log10(N) dB and the narrow carrier raises power spectral density; the cost is airtime, latency, and battery energy.