Microwave Backhaul Fresnel Zone Obstruction Case Study

A microwave backhaul link can have visual line of sight and still fail its availability objective. The straight ray between antennas is only the center of the propagation path. If rooftops, trees, cranes, pipe racks or new equipment intrude into the first Fresnel zone, the link can lose several decibels of margin even when a technician can still see the far antenna.

This case study follows a licensed point-to-point microwave link whose clear-weather received signal level becomes weaker after a rooftop mechanical screen is installed near the path. The link does not drop permanently, but it spends more time in lower adaptive-modulation modes and creates service congestion during busy hours.

The engineering problem is not simply “raise the antennas.” The useful task is to prove that the fault is path clearance, estimate the missing margin, separate obstruction from rain fade and interference, calculate the height correction, and define the evidence needed before the link is released.

The calculations are first-pass engineering screens. A real microwave design must use licensed-frequency data, terrain profile, antenna patterns, polarization, earth-curvature and refraction assumptions, regulatory constraints, structural limits, survey uncertainty, vendor receiver thresholds, safety rules and qualified field measurements.

Case Context

Two industrial sites are connected by an 18\ \text{GHz} microwave backhaul path. The link was originally commissioned with high modulation enabled and a clear-sky received signal level close to the design prediction. Six months later, after a rooftop project at an intermediate building, the network operations team observes:

• clear-weather received power about 6\ \text{dB} lower than expected;
• frequent fallback from high modulation to a lower capacity mode;
• queue growth and packet-delay variation during peak traffic;
• no matching increase in co-channel interference;
• degradation during dry weather, not only during rain.

The suspected mechanism is partial first-Fresnel-zone obstruction by a new rooftop mechanical screen.

Field Data

Quantity	Symbol	Value
carrier frequency	f	18\ \text{GHz}
total path length	d	9.6\ \text{km}
distance from site A to obstruction	d_1	3.8\ \text{km}
distance from obstruction to site B	d_2	5.8\ \text{km}
transmitter output power	P_t	22\ \text{dBm}
transmit feeder loss	L_t	1.5\ \text{dB}
transmit antenna gain	G_t	35\ \text{dBi}
receive antenna gain	G_r	35\ \text{dBi}
receive feeder loss	L_r	1.5\ \text{dB}
miscellaneous design loss excluding obstruction	L_{misc}	2.0\ \text{dB}
high-mode receiver threshold	P_{sens,high}	-67\ \text{dBm}
required clear-weather fade reserve above high mode	M_{req}	15\ \text{dB}
measured visual clearance over rooftop screen	c_{vis}	0.8\ \text{m}
clearance survey uncertainty allowance	u_c	0.4\ \text{m}
required first-Fresnel clearance fraction	C_F	0.60

The link is directional and fixed. The problem is therefore dominated by geometry, installation state and propagation margin rather than mobility.

Evidence Review

Evidence	Interpretation
received level is about 6\ \text{dB} below the commissioning prediction	an added path loss is plausible
degradation exists in dry weather	rain fade is not the root cause
spectrum scan does not show a new dominant interferer	receiver desensitization is less likely
adaptive modulation falls back during ordinary short fades	high-mode margin has become too small
the new rooftop screen lies close to the path	partial Fresnel-zone obstruction is credible
antenna alignment peaking improves little	pointing error is not the primary loss

The diagnosis should not be made from a single received-level reading. It should combine path geometry, link budget, modulation logs, spectrum evidence, weather correlation and a survey record of the obstruction.

Step 1: Calculate Wavelength

Use:

With:

and:

the wavelength is:

Engineering Comment

At microwave frequency the wavelength is small, but the Fresnel zone over kilometres is still several metres wide. A path can look optically clear while the radio path is partly blocked.

Step 2: Calculate First-Fresnel-Zone Radius

A convenient first-Fresnel radius approximation is:

where distances are in kilometres, frequency is in gigahertz and r_1 is in metres.

Substitute:

Then:

The 60\% clearance target is:

Engineering Comment

The project does not require the entire first Fresnel zone to be empty for a screening release, but it does require enough clearance to avoid avoidable diffraction loss and seasonal or construction uncertainty. Here the clearance target is far larger than the visible clearance.

Step 3: Apply Survey Uncertainty

The effective clearance is:

The actual clearance fraction is:

The clearance deficit against the 60\% target is:

Engineering Comment

This is not a marginal miss. After uncertainty allowance, the obstruction leaves only about 6.5\% first-Fresnel clearance. Treating this as “line of sight” is an engineering error because the radio path has much larger spatial extent than the visible ray.

Step 4: Estimate Obstruction Loss

For a simple knife-edge screen, the diffraction parameter can be estimated from clearance as:

The negative sign indicates that the obstruction is below the direct ray. With:

and:

the result is:

For v>-0.78, a common screening expression for diffraction loss is:

Substitute:

Engineering Comment

This is a screen, not a substitute for path-profile software. It is still consistent with the field observation: a few metres of missing Fresnel clearance can create roughly the same order of loss as the measured received-level deficit.

Step 5: Calculate Clear-Path Link Budget

Free-space path loss for distance in kilometres and frequency in gigahertz is:

Using:

gives:

The expected clear-path received power is:

Engineering Comment

The boundary is the receiver input after receive feeder loss. Keeping the boundary explicit avoids double-counting feeder losses or comparing antenna-port measurements with radio-reported received level.

Step 6: Include Obstruction Loss

With obstruction:

The high-mode clear-weather margin is:

The required margin is:

So the margin shortfall is:

Engineering Comment

The high modulation mode can still work in stable conditions, but it no longer has enough clear-weather reserve. Ordinary fading, wet radomes, alignment drift or interference bursts can push the radio below the high-mode operating margin, forcing fallback before a rain-event threshold is reached.

Step 7: Explain the Service Symptom

The radio log shows high-mode capacity of 420\ \text{Mbit/s} and fallback-mode capacity of 150\ \text{Mbit/s}. During peak intervals the offered traffic is:

At high mode:

At fallback mode:

Engineering Comment

The link degradation first appears as latency and jitter, not as a complete outage. When capacity falls below offered traffic, queues grow. This is why packet performance data belongs in the diagnosis: RF margin loss becomes a service-quality problem through adaptive modulation and queueing.

Step 8: Calculate Antenna-Height Correction

The line height at the obstruction changes as a weighted average of antenna-height changes:

The correction target should include the required clearance and the uncertainty allowance:

Current visible clearance is:

So the line at the obstruction must rise by:

Suppose the feasible site change is:

Then:

The new visible clearance is:

The new effective clearance is:

The new clearance fraction is:

Engineering Comment

The proposed height correction exceeds the 60\% first-Fresnel screen after uncertainty allowance. Structural capacity, wind loading, mast sway, lightning protection, grounding, access safety and regulatory constraints still have to be checked before the change is approved.

Step 9: Predicted Post-Correction Margin

With adequate clearance, the obstruction loss screen returns approximately to zero:

Predicted received power:

High-mode margin:

Margin above the release requirement:

Engineering Comment

The margin is acceptable but not excessive. If the service requires stronger rain availability, the design may still need larger antennas, a lower-frequency path, a protected fiber route, route diversity, lower feeder loss, a shorter hop, or more conservative adaptive-modulation policies.

Engineering Decision

The link should not be treated as healthy in its obstructed condition. It is not failing because the radio cannot close the path; it is failing because the installation lost enough geometric clearance to consume the operating margin needed for high availability and stable high-capacity modulation.

The decision is:

Hold unrestricted acceptance, correct the path clearance by raising antennas or moving the path, verify structure and regulatory limits, re-peak antenna alignment, repeat received-level and spectrum measurements, and release the link only after modulation logs and packet performance confirm stable margin under normal traffic.

Rain fade remains a design consideration, but it is not the root cause of this clear-weather degradation.

Failure Modes and Controls

Failure mode	Evidence	Control
visual line of sight accepted as RF clearance	low Fresnel clearance fraction	use terrain/profile and first-Fresnel screen
new rooftop equipment enters path	received level drops after construction	change-control review for rooftop and tower work
survey uncertainty ignored	clearance looks marginally acceptable	include measurement and seasonal allowances
adaptive modulation hides RF loss	no hard outage but service degrades	review modulation-state logs and packet metrics
rain fade blamed incorrectly	dry-weather degradation exists	correlate RSL with weather and obstruction timing
antenna-height fix creates structural risk	taller mounts increase wind load	structural review, grounding and access controls

Risk Review

Risk item	Severity	Occurrence	Detection	RPN
continuing operation with obstructed Fresnel zone	8	5	5	200
accepting visual line of sight without clearance screen	7	5	6	210
underestimating traffic impact of modulation fallback	7	4	5	140
raising antennas without structural validation	8	2	4	64

The highest risks are not pure RF calculations. They are governance risks: site modifications, weak survey control, missing acceptance criteria and service metrics that hide physical-layer margin loss until traffic increases.

Release Criteria

Release should require both RF and service evidence.

Criterion	Required evidence
path geometry	updated path profile with obstruction heights, antenna heights, distance split and clearance fraction
uncertainty	survey tolerance, mast movement, vegetation or construction allowance included
received power	measured RSL within accepted tolerance of predicted post-correction value
antenna alignment	azimuth and elevation peaked with final hardware state
spectrum condition	no dominant co-channel or adjacent-channel interferer masking the result
modulation stability	high mode remains stable during representative clear-weather operation
packet performance	latency, jitter and packet loss remain inside service limits during peak traffic
weather separation	rain-fade events are distinguished from clear-weather obstruction behavior
structural and safety review	taller mounts approved for wind, access, grounding and lightning protection

Transferable Lessons

Fresnel-zone clearance is not a decorative detail in microwave engineering. It is part of the operating margin.

The practical workflow is:

1. compare measured received level with the commissioned link budget;
2. rule out interference, rain-only effects and simple antenna mispointing;
3. calculate first-Fresnel radius at the suspected obstruction;
4. apply survey uncertainty, not only visible clearance;
5. estimate obstruction loss and margin shortfall;
6. connect RF margin loss to adaptive modulation and packet performance;
7. calculate the height or path correction;
8. release only after RF measurements and service metrics validate the correction.

This case is distinct from a rain-fade availability case. Rain fade asks whether weather attenuation consumes margin during precipitation. Fresnel obstruction asks whether the installed geometry has already removed clear-weather margin before weather, interference and traffic variation are considered.