PTP and PTMP Wireless Link Planning: Fresnel Zone and LOS

For a reliable PTP or PTMP wireless link, keep at least 60% of the first Fresnel zone clear of obstacles and plan a link budget that leaves a fade margin of typically 10–15 dB for the target modulation. Visual line of sight between the antennas is not enough by itself.

Point-to-point (PTP) links dedicate all capacity to one connection. Point-to-multipoint (PTMP) systems share a base station's capacity among many subscribers. Band selection, antenna type and site survey priorities differ between the two.

PTP vs PTMP

Most ISP networks use both. A PTP link carries capacity from the end of the fiber to a tall tower, and PTMP distributes it to subscribers from there.

Choosing a frequency band

In Türkiye, the conditions for licence-exempt radio equipment are set by BTK's technical criteria for radio equipment exempt from frequency assignment. For example, 5470–5725 MHz allows up to 1 W mean EIRP (50 mW/MHz) indoors or outdoors, and devices must support DFS (radar detection) and TPC (transmit power control). Check the current limits for your band before deployment.

EIRP combines transmit power, antenna gain and cable loss, so transmit power has to come down when antenna gain goes up. A narrow-beam antenna still pays off. It raises the received signal level and rejects interference from off-axis directions.

LOS, nLOS and the Fresnel zone

• LOS (line of sight): the antennas have optical visibility and a sufficiently clear Fresnel zone.
• nLOS (near or non-line of sight): the path is partly or fully blocked, and the signal arrives through diffraction, reflection and scattering. OFDM radios tolerate multipath, but capacity and stability drop. At 5 GHz and above, foliage adds heavy loss. nLOS is usually an acceptable compromise only for short PTMP subscriber links.

What is the Fresnel zone?

The Fresnel zone is an ellipsoid-shaped volume around the straight line between transmitter and receiver. The first Fresnel zone contains all paths whose length differs from the direct ray by less than half a wavelength, and it carries most of the energy that reaches the receiver. Hills, buildings, trees or Earth curvature inside it cause diffraction loss. The practical rule is to keep at least 60% of the first Fresnel zone radius clear.

First Fresnel zone radius:
r1 (m) = 17.32 × √( d1 × d2 / (f × D) )
         d1, d2: distance from each end to the obstacle (km)   D = d1 + d2 (km)   f: frequency (GHz)

At mid-path (d1 = d2 = D/2):
r1 (m) = 8.66 × √( D / f )

Earth bulge (k = 4/3 standard atmosphere):
h (m) = d1 × d2 / (12.74 × k)  ≈  d1 × d2 / 17

Required clearance at mid-path ≈ 0.6 × r1 + h + vegetation/growth allowance

In practice, a 10 km 5 GHz link needs about 6.8 m of Fresnel clearance plus 1.5 m for Earth bulge above the highest obstacle at mid-path. That is roughly 8.3 m, before any allowance for tree growth.

When the tip of an obstacle just touches the direct ray (0% clearance), knife-edge diffraction adds roughly 6 dB. Once it rises above the ray, loss can climb to tens of dB.

Calculating the link budget

FSPL (dB)        = 20·log10(d_km) + 20·log10(f_MHz) + 32.44
EIRP (dBm)       = Ptx + Gtx − Ltx
RSSI (dBm)       = Ptx + Gtx − Ltx + Grx − Lrx − FSPL − Lother
Fade margin (dB) = RSSI − receiver sensitivity (for the chosen MCS and channel width)

Example: a 5 km PTP link at 5600 MHz with 27 dBi integrated antennas at both ends and no cable loss.

FSPL             = 20·log10(5) + 20·log10(5600) + 32.44 = 13.98 + 74.96 + 32.44 ≈ 121.4 dB
Transmit power   = 3 dBm  → EIRP = 3 + 27 = 30 dBm (at the 1 W limit)
RSSI             = 3 + 27 + 27 − 121.4 ≈ −64.4 dBm
Sensitivity      = −75 dBm (example: datasheet value for the target MCS)
Fade margin      ≈ 10.6 dB

If the margin is too thin, use a narrower channel, higher-gain antennas or a lower target MCS. Doubling the channel width increases capacity but raises the noise floor by about 3 dB. Moving from 20 MHz to 80 MHz therefore costs roughly 6 dB of sensitivity.

RSSI, SNR and fade margin targets

RSSI alone does not describe link quality. If interference lifts the noise floor from −90 dBm to −75 dBm, an RSSI of −60 dBm leaves only 15 dB of SNR, and high-order modulation is no longer possible.

Choosing antennas

When many sectors share a tall tower, side lobes become the deciding factor. RF elements Symmetrical Horn antennas have beams from 30° to 90°, with identical horizontal and vertical angles and very low side lobes. Closely spaced sectors on the same mast are therefore well isolated from each other, and channels can be reused more aggressively. TwistPort models attach directly to compatible radios without coaxial cables. On the radio side, MikroTik and Ubiquiti product families cover most PTP and PTMP scenarios.

Include polarisation in the plan too. With dual-polarised antennas (H/V or ±45°), balanced chains are essential for MIMO capacity. Neighbouring systems on the same mast should use separate channels and, ideally, time synchronisation (GPS-synchronised TDD).

Site survey steps

1. Desk study: set coordinates and antenna heights, generate the terrain profile, and check Fresnel clearance and Earth bulge. Terrain models such as SRTM have roughly 30 m resolution and do not include buildings or trees, so results must be confirmed on site.
2. Visual LOS check: confirm both ends with binoculars, a drone, or a mirror by day and a light by night. Note tree growth and planned construction.
3. Spectrum scan: measure channel occupancy and interference at both ends, at antenna height.
4. Mechanical checks: verify wind loading, mounting height and maintenance access for the mast or tower.
5. Power and grounding: plan the PoE budget, UPS needs, and lightning and surge protection.
6. Permits and regulation: confirm landlord permissions and EIRP limits.
7. Acceptance test: compare designed and measured RSSI, SNR, chain balance and throughput, and document the results.

WiMap has the SRTM terrain profile, Fresnel and LOS checks and an RSSI heat map in the same workspace for PTP and PTMP design. Designs can be exported as a PDF engineering report and KMZ. Radios, antennas and mounting hardware are available through wi.com.tr.

Frequently asked questions

Why is the rule 60% clearance and not 100%?

With about 60% of the first Fresnel zone clear, path loss is already very close to free-space loss, and more clearance adds little. Clearance can shrink, though, when atmospheric refraction (the k-factor) changes or trees grow. Critical backbone links are therefore checked against more conservative criteria, such as an extra check at a lower k-factor.

How does the Fresnel zone change with frequency?

The radius shrinks with the square root of frequency. At 60 GHz it is roughly one third of the radius at 5.8 GHz. Free-space loss rises, however, and oxygen and rain losses come into play at higher bands.

Why is 60 GHz used only for short links?

Oxygen absorption at 60 GHz is about 15 dB/km, and rain adds further loss. In exchange, very narrow beams and low interference allow wide channels. That makes the band a good fit for short, high-capacity links in dense deployments.

RSSI looks good, so why is throughput low?

The most common cause is poor SNR, where interference raises the noise floor. Other suspects are chain imbalance, multipath from partial Fresnel obstruction, the wrong channel width and a signal so strong that it saturates the receiver.

Is SRTM terrain data good enough for planning?

It is fine for initial screening and terrain profiles, but not for a final decision. SRTM has roughly 30 m resolution and contains no buildings, trees or masts. Urban and forested routes always need a site visit.