Each guide opens with a short answer, then follows one original teaching record through definitions, mechanism, worked examples, changed-context checks, misconceptions and bounded reporting.
The examples are stipulated for learning. The standards and specifications define the operational methods; the worked cases keep every input, transformation and limit visible so the reasoning can be repeated.
An outdoor-to-indoor link crosses several boundaries. The free-space reference accounts for frequency and distance in ideal propagation; the building-entry model describes a separate statistical loss at the building boundary and inside the model’s declared scope.
The central 3.5 GHz, 100 m case keeps free-space, outdoor clutter, building entry, indoor path, gains and receiver requirement in separate rows so a changed building does not rewrite the distance calculation.
A defensible link budget preserves every boundary as its own row. Frequency and geometric distance feed the free-space reference; building class, elevation and probability feed the entry model; surrounding clutter, indoor distance, antenna patterns and fading margins keep their own evidence. This separation makes a changed-context calculation reusable instead of hiding several mechanisms inside one unexplained residual.
The worked examples use stipulated inputs so the arithmetic can be checked. They do not predict a named building. A real design must declare the in-force recommendation, the model’s validity range, the chosen percentile, terminal locations, antenna gains and measurement uncertainty before comparing a calculated budget with field data.
This specialist guide belongs to the Wireless Communication root. It also connects to The Gold Standard of Education | Learning That Transfers into Life and the How X Works hub. The worked cases are original teaching examples; their numbers are stipulated so that the reasoning can be checked.
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Read the short answer first: calculate free-space attenuation from frequency and distance, then add building entry as a separate modelled or measured term with its building class, percentile and elevation conditions. One is not a renamed version of the other.
ROUTE 1 · CHAPTERS 1–4
Define the quantities
Name the quantities, reference conditions and reporting rules.
ROUTE 3 · CHAPTERS 9–12
Audit the evidence
Check instruments, algorithms, assumptions and rival explanations.
ROUTE 5 · CHAPTERS 17–20
Report and continue
Repair shortcuts, answer questions and follow verified routes.
Full chapter contents · Wireless Communication subject guide
Chapter contents
Define the quantities · Chapters 1–4
1. Name free-space and building-entry loss
2. Attach frequency and distance to free space
Audit the evidence · Chapters 9–12
Transfer the reasoning · Chapters 13–16
Report and continue · Chapters 17–20
17. Repair path loss means wall loss
18. Repair one wall has one penetration loss
Free-space path loss is the reference attenuation associated with propagation through ideal free space for a stated frequency and distance. Building entry loss is the additional loss associated with entry into a building, statistically defined by the applicable ITU model. They describe different parts of a link budget.
Write the evidence in the order a reader can audit: the Wireless Communication quantity, its reference object, the observation or browser rule, the transformation, and the decision being made. Keeping those fields visible prevents a familiar label from hiding a changed basis.
For 3.5 GHz over 100 m, calculate a free-space reference near 83.3 dB. Add a stipulated 18 dB building-entry loss as a separate term, producing 101.3 dB before indoor clutter and system gains.
First-principles test: ask what physical, observational or algorithmic operation produces each value in this chapter. Then ask whether the second value could change while the first stays fixed. If it can, the two labels are not substitutes. For Wireless Communication, this counterfactual is often clearer than a definition learned in isolation.
Keep distance and frequency fixed but change the building class. Free-space loss stays 83.3 dB while the building-entry distribution changes.
Evidence check: another reader should be able to reproduce the narrow result from the stored definitions, inputs and intermediate steps, then identify the nearest stronger claim that the evidence does not support.
Real links also include antennas, clutter, penetration geometry, indoor path and fading.
CHAPTER 2 OF 20 · Define the quantities
2. Attach frequency and distance to free space
Free-space attenuation rises with both distance and frequency. The familiar logarithmic formula is valid only when units and far-field assumptions are declared; changing MHz to GHz without changing the constant creates a 60 dB error.
Trace the mechanism before interpreting the result. For Wireless Communication, mark which value belongs to the source, which belongs to the measurement or layout algorithm, and which is a reported summary. A calculation is useful only when those roles remain attached.
Use 32.45 + 20log10(3500 MHz) + 20log10(0.1 km) ≈ 83.3 dB. Store units beside every input.
Practical transfer: build two records that share the headline number but differ in the condition discussed here. If the correct interpretation changes, the omitted condition is decision-relevant and belongs in the report. This test turns a vocabulary distinction into a reusable Wireless Communication checking habit.
Double distance to 0.2 km. The free-space term rises by about 6.02 dB.
Practical check: keep one counterexample beside the worked case. If the wording fails when that one condition changes, narrow the sentence until it describes only the evidence actually in hand.
Near-field, obstructed and multipath environments need models beyond the free-space reference.
CHAPTER 3 OF 20 · Define the quantities
3. Attach probability and building class to entry loss
ITU-R P.2109 produces a cumulative distribution of building entry loss using frequency, probability not exceeded, building class and elevation angle. One median value is not a universal wall constant.
Use a compact decision table with columns for condition, input, operation, result and limit. This makes the comparison reproducible and exposes where a missing field would force an assumption instead of an observation.
Record traditional building, 3.5 GHz, 50th percentile and 0° elevation with a stipulated 18 dB output. Preserve the model inputs with the result.
Misconception check: a plausible output does not prove the right reference, denominator, ancestor, spectrum or observation rule was used. Reconstruct one intermediate step and one changed-context prediction. If either cannot be stated, keep the result provisional rather than strengthening its wording.
Move to the 90th percentile while holding the building and frequency fixed. The reported not-exceeded loss changes, not the free-space term.
Transfer check: repeat the reasoning with one consequential condition changed while holding the rest fixed. State what changes, what stays comparable and why the original label alone was insufficient.
Use the current recommendation’s equations or validated software for actual model values.
The P.2109 definition isolates building entry loss from surrounding clutter. Trees, nearby buildings and terrain outside the façade require separate propagation terms or models rather than being hidden inside the entry-loss label.
Preserve the raw inputs beside the derived result. If a later reader changes one condition, the original Wireless Communication reasoning should still be recoverable without reverse-engineering a rounded headline or an unlabeled screenshot.
The teaching link has 83.3 dB free space, 12 dB outdoor clutter and 18 dB building entry. Add them in separate rows before gains.
Evidence design: retain both a positive case and a near-miss. The positive case shows when the rule applies; the near-miss changes one consequential condition and shows when the conclusion fails. Together they teach the boundary of the Wireless Communication mechanism better than another unlabelled example.
Remove the nearby obstruction. Only the clutter row changes.
Audit check: retain the source version, units, settings, exclusions and rounding rule. A future correction should be able to alter one field without rebuilding the entire explanation from memory.
Model terms can be correlated; simple dB addition is a bookkeeping example, not a full stochastic prediction.
A fictional transmitter has 20 dBm power and 3 dBi transmit gain. The receiver antenna is 0 dBi. Subtract 83.3 dB free-space loss and 18 dB building-entry loss to obtain -78.3 dBm before other losses.
Ask what would have to stay fixed for the comparison to be fair. Record those controls explicitly, then name the first uncontrolled condition that could reverse the conclusion. This is the practical transfer step, not an optional footnote.
Calculate 20+3+0-83.3-18=-78.3 dBm. Compare with a stipulated -92 dBm receiver requirement, leaving 13.7 dB before margin and omitted terms.
Communication test: replace any sentence that says two things are “the same” or “different” with a sentence naming the operation, reference condition and supported consequence. The extra words are useful when they let a future reader repeat the decision without access to the original author.
Double distance to 200 m. Free-space loss becomes about 89.3 dB and received level about -84.3 dBm; building entry stays 18 in the stipulated case.
Decision check: end with an action or interpretation that follows from the stated conditions, followed by a boundary sentence explaining when a specialist rule, local procedure or additional measurement is needed.
A practical design needs fading margin, feeder losses, interference, bandwidth and reliability targets.
Free-space loss follows spherical spreading and wavelength relationships in the reference model. It does not imply energy is absorbed by empty space; the receiving aperture captures a smaller fraction as geometry expands.
Write the evidence in the order a reader can audit: the Wireless Communication quantity, its reference object, the observation or browser rule, the transformation, and the decision being made. Keeping those fields visible prevents a familiar label from hiding a changed basis.
Draw spheres at 100 and 200 m. Surface area grows fourfold, matching roughly 6 dB additional spreading loss.
First-principles test: ask what physical, observational or algorithmic operation produces each value in this chapter. Then ask whether the second value could change while the first stays fixed. If it can, the two labels are not substitutes. For Wireless Communication, this counterfactual is often clearer than a definition learned in isolation.
Keep distance fixed and double frequency. The isotropic receiving-aperture relation yields about 6 dB more path loss in the standard link formula.
Evidence check: another reader should be able to reproduce the narrow result from the stored definitions, inputs and intermediate steps, then identify the nearest stronger claim that the evidence does not support.
Antenna physical size and gain conventions affect how frequency comparisons should be interpreted.
Building entry loss is a statistical end-to-end entry term, not necessarily a measured loss of one wall sample. The model does not separate exterior-wall penetration from the path within the building because detailed building information is often unavailable.
Trace the mechanism before interpreting the result. For Wireless Communication, mark which value belongs to the source, which belongs to the measurement or layout algorithm, and which is a reported summary. A calculation is useful only when those roles remain attached.
A receiver 3 m inside and another 20 m inside may both be assessed with a generic building-entry distribution only under the model’s scope. Keep any explicit indoor path term separate.
Practical transfer: build two records that share the headline number but differ in the condition discussed here. If the correct interpretation changes, the omitted condition is decision-relevant and belongs in the report. This test turns a vocabulary distinction into a reusable Wireless Communication checking habit.
Move the receiver deeper while holding the façade point fixed. Do not automatically call the added indoor loss wall penetration.
Practical check: keep one counterexample beside the worked case. If the wording fails when that one condition changes, narrow the sentence until it describes only the evidence actually in hand.
Site-specific ray tracing and measurements can be more appropriate when detailed geometry exists.
CHAPTER 8 OF 20 · Work the mechanism
8. Audit thermally efficient and traditional classes
Modern thermally efficient construction can have different radio-frequency attenuation from traditional buildings because of coated glass and insulation systems. The class is an input to the statistical model, not an aesthetic judgement from a photograph.
Use a compact decision table with columns for condition, input, operation, result and limit. This makes the comparison reproducible and exposes where a missing field would force an assumption instead of an observation.
Run the same 3.5 GHz link for the two model classes at the same percentile and elevation. Store two building-entry outputs.
Misconception check: a plausible output does not prove the right reference, denominator, ancestor, spectrum or observation rule was used. Reconstruct one intermediate step and one changed-context prediction. If either cannot be stated, keep the result provisional rather than strengthening its wording.
Replace glazing while keeping distance fixed and treat the building classification as an evidence question.
Transfer check: repeat the reasoning with one consequential condition changed while holding the rest fixed. State what changes, what stays comparable and why the original label alone was insufficient.
Mixed construction and renovations may not fit one class cleanly.
Building-entry loss predictions depend on the path elevation angle at the façade. A rooftop base station and a street-level transmitter can therefore have different entry distributions even at similar slant distance.
Preserve the raw inputs beside the derived result. If a later reader changes one condition, the original Wireless Communication reasoning should still be recoverable without reverse-engineering a rounded headline or an unlabeled screenshot.
Compare 0° and 30° elevation inputs for the same building, frequency and percentile. Keep free-space slant distance in its own row.
Evidence design: retain both a positive case and a near-miss. The positive case shows when the rule applies; the near-miss changes one consequential condition and shows when the conclusion fails. Together they teach the boundary of the Wireless Communication mechanism better than another unlabelled example.
Raise the transmitter and recompute both slant distance and model elevation rather than changing one label only.
Audit check: retain the source version, units, settings, exclusions and rounding rule. A future correction should be able to alter one field without rebuilding the entire explanation from memory.
Azimuth is not represented explicitly in the generic P.2109 model and remains a limitation.
A loss not exceeded at probability p is a distribution statement, not the chance that one fixed wall will change its loss from moment to moment. It represents variability in the modelled population and conditions.
Ask what would have to stay fixed for the comparison to be fair. Record those controls explicitly, then name the first uncontrolled condition that could reverse the conclusion. This is the practical transfer step, not an optional footnote.
At the median, half the modelled cases do not exceed the reported value. At 90%, nine tenths do not exceed the larger threshold under the model.
Communication test: replace any sentence that says two things are “the same” or “different” with a sentence naming the operation, reference condition and supported consequence. The extra words are useful when they let a future reader repeat the decision without access to the original author.
Hold the measured building fixed and change only the reporting percentile. The physical wall does not change.
Decision check: end with an action or interpretation that follows from the stated conditions, followed by a boundary sentence explaining when a specialist rule, local procedure or additional measurement is needed.
Do not convert model percentiles into unsupported outage probabilities without the rest of the link model.
Loss terms are subtracted from a dBm budget; antenna gains are added relative to their stated references. Writing building entry as -18 dB and then subtracting it would add power by mistake.
Write the evidence in the order a reader can audit: the Wireless Communication quantity, its reference object, the observation or browser rule, the transformation, and the decision being made. Keeping those fields visible prevents a familiar label from hiding a changed basis.
Use columns type, magnitude and operation: transmit +20, gains +3 and +0, free-space loss 83.3 subtract, entry loss 18 subtract.
First-principles test: ask what physical, observational or algorithmic operation produces each value in this chapter. Then ask whether the second value could change while the first stays fixed. If it can, the two labels are not substitutes. For Wireless Communication, this counterfactual is often clearer than a definition learned in isolation.
Represent every loss as a positive magnitude and rerun the budget.
Evidence check: another reader should be able to reproduce the narrow result from the stored definitions, inputs and intermediate steps, then identify the nearest stronger claim that the evidence does not support.
Cable, mismatch and polarization terms need their own signs and reference conditions.
A field measurement combines free-space reference, actual outdoor propagation, façade, indoor path, antennas and temporal fading. Subtracting calculated free space from one received level does not isolate universal building-entry loss.
Trace the mechanism before interpreting the result. For Wireless Communication, mark which value belongs to the source, which belongs to the measurement or layout algorithm, and which is a reported summary. A calculation is useful only when those roles remain attached.
Measure -82 dBm with 23 dBm EIRP at 100 m. A residual after free-space subtraction still contains clutter, entry, indoor path and measurement uncertainty.
Practical transfer: build two records that share the headline number but differ in the condition discussed here. If the correct interpretation changes, the omitted condition is decision-relevant and belongs in the report. This test turns a vocabulary distinction into a reusable Wireless Communication checking habit.
Add an outdoor reference receiver at the façade to improve the decomposition.
Practical check: keep one counterexample beside the worked case. If the wording fails when that one condition changes, narrow the sentence until it describes only the evidence actually in hand.
Synchronized calibrated measurements and statistical sampling are needed for defensible component estimates.
Lower frequency changes free-space loss and usually changes the building-entry distribution. Both rows must be recalculated; it is unsafe to scale only the wall term or only free space.
Use a compact decision table with columns for condition, input, operation, result and limit. This makes the comparison reproducible and exposes where a missing field would force an assumption instead of an observation.
At 700 MHz and 100 m, free-space loss is about 69.3 dB, roughly 14 dB below the 3.5 GHz reference. Obtain the entry term from the applicable model.
Misconception check: a plausible output does not prove the right reference, denominator, ancestor, spectrum or observation rule was used. Reconstruct one intermediate step and one changed-context prediction. If either cannot be stated, keep the result provisional rather than strengthening its wording.
Keep the same EIRP and receiver requirement and compare the remaining margin.
Transfer check: repeat the reasoning with one consequential condition changed while holding the rest fixed. State what changes, what stays comparable and why the original label alone was insufficient.
Spectrum rules, antennas and bandwidth differ across bands and can dominate a real comparison.
Millimetre-wave links face a much larger free-space reference term and often severe building-entry loss. A high-gain beam can offset part of the budget but does not rename either loss.
Preserve the raw inputs beside the derived result. If a later reader changes one condition, the original Wireless Communication reasoning should still be recoverable without reverse-engineering a rounded headline or an unlabeled screenshot.
At 28 GHz and 100 m, free-space loss is about 101.4 dB. Add a separately modelled building-entry term and declared antenna gains.
Evidence design: retain both a positive case and a near-miss. The positive case shows when the rule applies; the near-miss changes one consequential condition and shows when the conclusion fails. Together they teach the boundary of the Wireless Communication mechanism better than another unlabelled example.
Narrow the antenna beam while keeping path distance fixed and record gain plus alignment sensitivity.
Audit check: retain the source version, units, settings, exclusions and rounding rule. A future correction should be able to alter one field without rebuilding the entire explanation from memory.
Blockage, material variability and directional antennas require models and measurements beyond this simple case.
When the transmitter and receiver are both inside, building entry may not belong in the link budget. Indoor partition, corridor, floor and clutter models answer the new geometry.
Ask what would have to stay fixed for the comparison to be fair. Record those controls explicitly, then name the first uncontrolled condition that could reverse the conclusion. This is the practical transfer step, not an optional footnote.
Move the base station from outdoors to the same room as the receiver. Remove building-entry loss and replace free-space-only reasoning with an indoor model.
Communication test: replace any sentence that says two things are “the same” or “different” with a sentence naming the operation, reference condition and supported consequence. The extra words are useful when they let a future reader repeat the decision without access to the original author.
Place the transmitter in another room and add a partition-loss term.
Decision check: end with an action or interpretation that follows from the stated conditions, followed by a boundary sentence explaining when a specialist rule, local procedure or additional measurement is needed.
Model choice must follow terminal locations and the validated frequency range.
CHAPTER 16 OF 20 · Transfer the reasoning
16. Transfer to a window-mounted receiver
A receiver at the window can experience a different entry path from one deep inside. The generic building-entry model deliberately avoids detailed site geometry, so a site-specific measurement may be more useful.
Write the evidence in the order a reader can audit: the Wireless Communication quantity, its reference object, the observation or browser rule, the transformation, and the decision being made. Keeping those fields visible prevents a familiar label from hiding a changed basis.
Compare a window receiver and an interior receiver while recording façade, depth, orientation and antenna pattern.
First-principles test: ask what physical, observational or algorithmic operation produces each value in this chapter. Then ask whether the second value could change while the first stays fixed. If it can, the two labels are not substitutes. For Wireless Communication, this counterfactual is often clearer than a definition learned in isolation.
Open the window in a controlled teaching setup. The changed path does not alter free-space distance appreciably but can change entry evidence.
Evidence check: another reader should be able to reproduce the narrow result from the stored definitions, inputs and intermediate steps, then identify the nearest stronger claim that the evidence does not support.
Safety, access and radio regulations govern real measurement campaigns.
Path loss is often used loosely for the whole end-to-end attenuation. For a checkable report, separate the free-space reference, outdoor excess, building entry, indoor path and fading margin.
Trace the mechanism before interpreting the result. For Wireless Communication, mark which value belongs to the source, which belongs to the measurement or layout algorithm, and which is a reported summary. A calculation is useful only when those roles remain attached.
Rewrite ‘path loss is 101.3 dB’ as ‘83.3 dB free-space reference plus a stipulated 18 dB building-entry term, before clutter and indoor loss.’
Practical transfer: build two records that share the headline number but differ in the condition discussed here. If the correct interpretation changes, the omitted condition is decision-relevant and belongs in the report. This test turns a vocabulary distinction into a reusable Wireless Communication checking habit.
Add 12 dB outdoor clutter and update only the total and its wording.
Practical check: keep one counterexample beside the worked case. If the wording fails when that one condition changes, narrow the sentence until it describes only the evidence actually in hand.
A total can be useful, but only when its component scope is declared.
CHAPTER 18 OF 20 · Report and continue
18. Repair one wall has one penetration loss
Building-entry performance varies with frequency, material, glazing, angle, location and building class. A single measured wall coupon or one phone reading cannot establish a universal building value.
Use a compact decision table with columns for condition, input, operation, result and limit. This makes the comparison reproducible and exposes where a missing field would force an assumption instead of an observation.
Three façade points measure residual losses 9, 18 and 27 dB under controlled geometry. Report a distribution rather than selecting 18 as a law.
Misconception check: a plausible output does not prove the right reference, denominator, ancestor, spectrum or observation rule was used. Reconstruct one intermediate step and one changed-context prediction. If either cannot be stated, keep the result provisional rather than strengthening its wording.
Change polarization and repeat while retaining the original data.
Transfer check: repeat the reasoning with one consequential condition changed while holding the rest fixed. State what changes, what stays comparable and why the original label alone was insufficient.
Generic ITU models and site-specific measurements answer different planning questions.
Store frequency, distance, coordinates, antenna heights and gains, EIRP, free-space formula and units, outdoor clutter model, building-entry recommendation version, class, percentile, elevation, indoor path, receiver bandwidth, measured level and uncertainty.
Preserve the raw inputs beside the derived result. If a later reader changes one condition, the original Wireless Communication reasoning should still be recoverable without reverse-engineering a rounded headline or an unlabeled screenshot.
Package the 3.5 GHz, 100 m central case so another reader can reproduce 83.3 dB and identify the stipulated 18 dB entry term.
Evidence design: retain both a positive case and a near-miss. The positive case shows when the rule applies; the near-miss changes one consequential condition and shows when the conclusion fails. Together they teach the boundary of the Wireless Communication mechanism better than another unlabelled example.
Delete the model percentile and show why the building-entry number loses its statistical meaning.
Audit check: retain the source version, units, settings, exclusions and rounding rule. A future correction should be able to alter one field without rebuilding the entire explanation from memory.
Protect site and network security information when publishing measurement records.
A defensible conclusion names free-space attenuation as a reference calculation and building entry as a separate modelled or measured term with its building and probability conditions. It does not claim the sum predicts every indoor point.
Ask what would have to stay fixed for the comparison to be fair. Record those controls explicitly, then name the first uncontrolled condition that could reverse the conclusion. This is the practical transfer step, not an optional footnote.
Write: ‘At 3.5 GHz and 100 m, the free-space reference is about 83.3 dB. Adding the stipulated 18 dB building-entry term gives 101.3 dB before clutter, indoor loss and margin.’
Communication test: replace any sentence that says two things are “the same” or “different” with a sentence naming the operation, reference condition and supported consequence. The extra words are useful when they let a future reader repeat the decision without access to the original author.
Change distance to 200 m and update only the free-space and total rows.
Decision check: end with an action or interpretation that follows from the stated conditions, followed by a boundary sentence explaining when a specialist rule, local procedure or additional measurement is needed.
Final radio design requires current regulations, validated models, interference analysis and qualified engineering.
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Continue through the verified reading route
Receiver noise figure and sensitivity
Receiver noise figure and sensitivity. Use this for receiver-added noise versus the final detectable level.
Occupied bandwidth and out-of-band emissions
Occupied bandwidth and out-of-band emissions. Use this for in-band signal extent versus unwanted emissions.
Receiver selectivity and sensitivity
Receiver selectivity and sensitivity. Use this when interference rejection and weak-signal response must remain separate.
Frequently asked questions
Is building entry loss the same as one wall’s penetration loss?
No. The generic ITU model is a statistical building-entry term and does not isolate one wall sample.
Does a lower frequency reduce both losses?
Frequency changes both the free-space reference and the building-entry distribution, so recalculate each row.
Can I subtract free-space loss from one field measurement to get building entry loss?
Not by itself; the residual also contains clutter, indoor path, antennas, fading and measurement uncertainty.
Authoritative sources and further reading
ITU-R P.525-5. Provides the in-force calculation methods for free-space attenuation.
ITU-R P.2109-2. Provides the statistical prediction model for building entry loss.
ITU-R P.2040. Defines building-material and building-entry propagation terms used by the model.
Return to the Wireless Communication subject guide, The Gold Standard of Education, or the How X Works hub.