24 Link Budget and Coverage Planning

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24.1 Start With the Margin You Need to Trust

A link budget is the story of a signal from transmitter to receiver with every gain and loss written down. It prevents a design from depending on hope, a lucky bench test, or a single reading taken on a good day.

For IoT, the budget becomes a release gate. If the margin is thin before rain, foliage, battery voltage, installation error, or antenna orientation are considered, the network is not ready for the field.

Phoebe’s Field Notes: Where The Other End Of The Budget Comes From

Phoebe the physics guide

Phoebe’s Why

This chapter’s ledger has two ends: transmit power spreading out and losing strength over distance, and receiver sensitivity waiting at the other end to catch what is left. The distance end is spherical spreading – power dilutes over a growing sphere, the same physics behind an inverse-square law. The sensitivity end is not a marketing spec; it is a noise floor set by thermodynamics. Every receiver input resistor, at any temperature above absolute zero, has electrons jittering randomly and generating a tiny, unavoidable voltage – Johnson-Nyquist noise. Sensitivity is simply how far above that thermal noise floor a receiver needs the signal to sit before it can decode it reliably. Wider bandwidth lets in more of that noise, which is why a wideband receiver needs a stronger signal than a narrowband one for the same job – a fact this chapter’s ledger records as a receiver-mode input rather than a derived number.

The Derivation

Spherical spreading turns transmit power \(P_t\) into a power density at distance \(d\), and an aperture \(\lambda^2/4\pi\) collects it back into received power, giving free-space path loss:

\[\mathrm{FSPL} = \left(\frac{4\pi d}{\lambda}\right)^{2}\]

Thermal noise power available from a resistor at temperature \(T\) in bandwidth \(B\) (Johnson-Nyquist):

\[N = kTB\]

At the reference temperature \(T=290\) K this reduces to a constant per hertz, and receiver sensitivity is that floor plus the receiver’s own noise figure \(NF\) plus the minimum signal-to-noise ratio the demodulator needs:

\[10\log_{10}(kT) = -174\ \mathrm{dBm/Hz}\]

\[\mathrm{Sensitivity}_{dBm} = -174 + 10\log_{10}B + NF + \mathrm{SNR}_{min}\]

Worked Numbers: This Chapter’s Own -95 dBm And -118 dBm

\(kT\) at \(T=290\) K: \(1.381\times10^{-23}\times290 = 4.00\times10^{-21}\) W/Hz, which is \(10\log_{10}(4.00\times10^{-21}/10^{-3}) = -174\) dBm/Hz – the constant behind the sensitivity formula.
This chapter’s own overview example – 14 dBm transmit, 2 dB antenna gain, 96 dB path loss, 15 dB site loss – gives received power \(14+2-96-15 = -95\) dBm, matching the chapter’s own figure exactly.
Recovering that 96 dB path-loss line item: at an 868 MHz LPWAN band (consistent with this chapter’s own LoRaWAN-labeled figures), \(\lambda = 0.346\) m, and solving \(\mathrm{FSPL}=96\) dB for distance gives \(d = 10^{96/20}\times\lambda/4\pi \approx 1{,}740\) m – a free-space distance in the range this chapter’s LPWAN coverage figures already illustrate.
This chapter’s own -118 dBm sensitivity example, checked against a catalog-typical 125 kHz LoRa-class channel at NF \(=6\) dB: \(-174+10\log_{10}(125{,}000)+6 = -117.0\) dBm before the SNR term, so -118 dBm implies a required \(\mathrm{SNR}_{min}\approx-1.0\) dB – physically reasonable for a spread-spectrum receiver, and the reason this chapter is right to treat sensitivity as a receiver-mode input: it depends on the bandwidth and coding choice, not just on distance.
With that same received power and threshold, margin \(= -95-(-118)-10\text{ dB reserve} = 13\) dB, matching this chapter’s own overview number.

Overview: A Link Budget Turns Range Into A Checkable Record

A link budget is the radio range ledger for an IoT deployment. It starts with transmit power, adds antenna gains, subtracts cable, path, obstacle, and reserve losses, then compares the predicted received power with receiver sensitivity. The useful output is not a single optimistic range number. It is the remaining margin and the assumptions that created it.

Use the record before choosing gateway spacing, antenna placement, enclosure location, or battery targets. If the ledger has no named environment, no reserve margin, or no field measurement plan, the design is still a guess.

The ledger matters because every term answers a different review question. Transmit power and antenna gain describe how much useful signal leaves the device. Cable loss, connector loss, and antenna orientation describe how much of that launch power actually reaches the path. Path loss, material attenuation, Fresnel clearance, foliage, and body blocking describe what the site does to the signal. Receiver sensitivity and the required packet success target describe how weak the signal can be before the application stops meeting its requirement.

A simple budget also makes trade-offs visible. Suppose a device launches 14 dBm, gains 2 dB from its antenna path, loses 96 dB to distance, and loses another 15 dB to the site. The predicted received power is -95 dBm. If the deployed receiver mode is sensitive to -118 dBm and the design requires 10 dB of reserve, the remaining release margin is 13 dB. That is a reviewable decision. If any input changes, the team can recompute the same ledger instead of debating a vague range claim.

The decision is a chain: radio settings create launched power, deployment losses reduce it, and the remaining margin decides whether the link is ready for field validation.

Power And Gain

Record conducted transmit power, antenna gain in the intended direction, and any cable or connector loss before the signal leaves the device.

Path And Site Loss

Start from path loss, then add realistic losses for walls, foliage, terrain, body blocking, antenna height, weather, or installation quality.

Margin

Received power minus receiver sensitivity is the headroom. A design with no reserved margin may pass one desk calculation and fail in the first bad location.

Core check: received power = transmit power + gains - losses. The link is plausible only when received power remains above receiver sensitivity after the required reserve margin has been included.

Practitioner: Build The Budget As A Deployment Ledger

A useful link budget is traceable. Each number should have a source, a location, and an owner: radio configuration, antenna datasheet, cable length, path model, measured RSSI, or field survey. Keep the calculation simple enough that a field engineer can update it when gateway height, antenna alignment, enclosure material, or sensor location changes.

Build the record in the same order the signal experiences the path. Start with the transmitter and antenna system, then the open-air or log-distance path estimate, then installation losses, then receiver sensitivity, then reserve. Add a short evidence note beside each uncertain term. For example, wall loss may come from a site survey, a conservative design allowance, or an early RSSI walk test. The review should be able to tell which terms are measured, which are data-sheet values, and which are deliberate safety allowances.

After installation, keep the budget alive. Field measurements should not replace the record; they should calibrate it. If measured RSSI is weaker than predicted, update the loss assumption and check whether similar paths are also at risk. If measured RSSI is stronger than predicted, keep the conservative reserve unless the team has enough representative samples to reduce it. That habit turns the link budget into an operating document for gateway moves, enclosure changes, seasonal foliage, and radio-mode updates.

LPWAN coverage planning depends on a ledger of radio terms, path assumptions, installation losses, and the margin required for reliable reporting.

Field

Example Entry

Why It Matters

Recheck Trigger

Transmit side

Power, antenna gain, cable loss

Defines how much useful power is launched toward the path.

Firmware power change, antenna swap, enclosure redesign, or cable reroute.

Path model

Distance, frequency, line-of-sight condition, obstacle allowance

Separates a clean free-space estimate from real building, terrain, foliage, and body-blocking losses.

Gateway moves, vegetation changes, new walls, new shelves, or different mounting height.

Receive side

Receiver sensitivity, antenna gain, cable loss, required packet rate

Turns predicted power into a pass/fail margin against the actual receiver configuration.

Radio mode, spreading factor, data rate, gateway hardware, or antenna system changes.

Validation

Measured RSSI or SNR at sample locations

Checks whether the model is close enough to release and where extra gateway density is needed.

Any field result that differs materially from the predicted margin.

A waterfall view helps review every gain and loss in order, especially when a design looks good on path loss alone but weak after installation losses are included.

Common failure pattern: the desktop range estimate used free-space loss, but the installed link also had cable loss, antenna mismatch, foliage, wall loss, gateway shadowing, and no reserve margin. Treat those as budget lines, not troubleshooting surprises.

Under The Hood: Margin Is Where Assumptions Become Risk

The budget is arithmetic, but the risk is in the assumptions. Free-space path loss is a baseline, not a site guarantee. Receiver sensitivity depends on data rate, bandwidth, coding, and required error behavior. Antenna gain depends on orientation and mounting. A clean design keeps those assumptions visible instead of hiding them inside one range claim.

dB arithmetic is useful because gains and losses can be added in one chain, but it can also hide weak evidence. A 6 dB antenna gain only helps when the antenna pattern points toward the receiver and is mounted away from detuning surfaces. A receiver sensitivity number only applies to the bandwidth, coding, spreading, data rate, packet error target, and interference assumptions used to measure it. A path-loss number only applies to the environment model and distance convention recorded with it.

Margin is therefore not spare decoration at the end of the spreadsheet. It is the buffer between an assumed world and the deployed world. Fade, multipath, human movement, wet foliage, antenna tilt, component tolerance, enclosure changes, and measurement error all consume that buffer. When the reserve is too small, the system may work during commissioning and fail later under normal site variation. When the reserve is visible, engineers can decide whether the right fix is more gateway density, a different mounting point, a slower radio mode, or a field survey.

For review, separate nominal margin from release margin. Nominal margin is predicted received power minus receiver sensitivity. Release margin subtracts the reserves the team promised to protect: fade allowance, installation uncertainty, site-change allowance, and measurement uncertainty. That second number is the one that should drive approval because it says what headroom remains after known risk has been paid for.

The calculation is additive in dB units: add gains, subtract losses, then compare the result with receiver sensitivity.

Distance And Frequency

Path loss grows with distance and frequency. The free-space formula is useful for an open-air baseline, but real deployments add environment-specific losses before making a coverage decision.

Receiver Mode

Changing data rate, bandwidth, spreading, or coding can change sensitivity. A budget copied from another mode may claim margin that the deployed receiver does not actually have.

Reserve

Reserve margin absorbs fade, installation variation, aging, seasonal foliage, and measurement uncertainty. Low reserve is an operational risk even when the nominal calculation passes.

Margin is the gap between the predicted received power and the receiver threshold after deployment losses and reserve have been accounted for.

The same radio can behave very differently in dense urban, indoor, and open rural paths because the environment changes the loss terms.

Release rule: do not approve coverage from a formula alone. Approve it from a recorded budget, a named margin target, representative field measurements, and a clear trigger for when the budget must be reopened.

24.2 Summary

Link-budget planning turns wireless range from a hope into a record. The record names transmit power, antenna gains, cable and connector losses, path loss, site losses, receiver sensitivity, required reserve margin, and the field measurements used to check the model. Free-space path loss is a useful baseline, but indoor walls, terrain, foliage, antenna mounting, receiver mode, and gateway placement decide whether the deployed link has enough headroom.

24.3 Key Takeaway

A link budget is useful only when the assumptions are visible. Record the gains, losses, receiver threshold, required reserve, and validation points so coverage decisions can be checked when the site changes.