2. Propagation, Link Budgets, and Cellular Infrastructure

The Wireless Link

A basic wireless connection contains a transmitter, a propagation channel, and a receiver. The transmitter converts information into a radio waveform and radiates it through an antenna. The receiver captures only a small part of that radiated energy and must recover the information in the presence of noise and interference.

The channel is not an empty pipe. Buildings, terrain, vehicles, people, weather, and other transmitters alter the signal before it reaches the receiver.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

Power Units: Watt, dB, and dBm

Wireless link calculations are easier in logarithmic units.

• dB expresses a ratio: $10\log_{10}(P_2/P_1)$.
• dBm expresses absolute power relative to 1 mW.
• $0\ \mathrm{dBm}=1\ \mathrm{mW}$, $10\ \mathrm{dBm}=10\ \mathrm{mW}$, $20\ \mathrm{dBm}=100\ \mathrm{mW}$, and $30\ \mathrm{dBm}=1\ \mathrm{W}$.

Gains are added and losses are subtracted in a dB-domain link budget. This avoids repeatedly multiplying very large and small linear values.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

Free-Space Path Loss

In an unobstructed far-field link, received power decreases as the wave spreads. The Friis equation is

$$P_{\mathrm{r}} = P_{\mathrm{t}}G_{\mathrm{t}}G_{\mathrm{r}} \left(\frac{\lambda}{4\pi R}\right)^{2}$$

where $P_{\mathrm{t}}$ and $P_{\mathrm{r}}$ are transmitted and received power, $G_{\mathrm{t}}$ and $G_{\mathrm{r}}$ are antenna gains, $\lambda$ is wavelength, and $R$ is separation distance.

The corresponding free-space path loss is

$$\operatorname{FSPL}_{\mathrm{dB}} = 20\log_{10}\!\left(\frac{4\pi R}{\lambda}\right)$$

Doubling distance adds approximately 6 dB of free-space loss. At the same distance, increasing carrier frequency also increases free-space path loss because wavelength becomes shorter. Directional antenna gain can compensate for part of this loss.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

A Link Budget

A simplified received-power budget is

$$P_{\mathrm{r,dBm}} = P_{\mathrm{t,dBm}}+G_{\mathrm{t,dBi}}+G_{\mathrm{r,dBi}} -L_{\mathrm{path}}-L_{\mathrm{cable}}-L_{\mathrm{other}}$$

Receiver performance depends on whether $P_{\mathrm{r}}$ remains above the receiver sensitivity required for the selected data rate. Engineers normally include a link margin for fading, blockage, hardware variation, and modelling uncertainty.

Example:

Term	Value
Transmit power	+30 dBm
Transmit antenna gain	+15 dBi
Receive antenna gain	+5 dBi
Path loss	−120 dB
Cable and implementation loss	−4 dB
Received power	−74 dBm

If sensitivity is −84 dBm, the nominal margin is 10 dB. A link with no margin may work in a clean calculation and fail whenever the environment changes.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

LOS, NLOS, Reflection, and Diffraction

• Line of sight (LOS): a direct unobstructed path exists.
• Non-line of sight (NLOS): the direct path is blocked, so the receiver relies on reflections, diffraction, scattering, or a controlled path such as RIS.
• Reflection: a wave bounces from a surface.
• Diffraction: energy bends around an edge.
• Scattering: rough or small objects distribute energy in several directions.

At mmWave frequencies, diffraction is weaker and blockage is more severe than at many sub-6 GHz bands. Networks respond with directional beams, denser sites, multi-connectivity, relays, and controllable surfaces.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

Large-Scale and Small-Scale Channel Effects

Large-scale path loss captures average power decay over distance. Shadowing captures slower variations caused by obstacles. Small-scale fading captures rapid constructive and destructive addition of multipath components over distances comparable to a wavelength.

If multiple copies arrive with similar phase, they add constructively. If they arrive out of phase, they can form a deep fade. Mobility changes path lengths, producing time variation and Doppler shift.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

Why Infrastructure Takes Different Forms

Base-station antennas must be placed at useful heights and locations while satisfying cost, land, loading, planning, and coverage constraints.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

Common Site Types

Structure	Strength	Typical constraint	Common use
Monopole	Small footprint and clean appearance	Moderate height and loading	Urban roads, campuses, compact sites
Lattice tower	Strong and able to carry many antennas	Larger footprint and visual impact	High-capacity or long-term macro sites
Guyed tower	Very tall at relatively low structural cost	Needs a large area for guy wires	Rural coverage and broadcasting
Rooftop site	Reuses existing height and land	Building loading and landlord access	Dense urban coverage
Small-cell pole	Fast, compact, close to users	Short range and many required sites	Hotspots, streets, venues, mmWave
Cell on wheels	Rapidly deployable	Temporary capacity and backhaul limits	Events, emergencies, disaster recovery

The tower itself does not create coverage. Coverage depends on antenna height, electrical and mechanical downtilt, carrier frequency, transmit power, terrain, clutter, and the traffic demand around the site.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

Macro Cells and Small Cells

A macro cell covers a comparatively large area and normally uses higher mounting positions and transmit power. Small cells trade coverage radius for spatial reuse and local capacity. A dense network can serve more users because the same spectrum is reused across separated cells, but densification increases interference coordination, backhaul, site acquisition, and handover complexity.

At high frequencies, small cells are especially useful because wide bandwidth is available but propagation range is shorter. A practical network often combines low-band coverage, mid-band capacity, and mmWave hotspots.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

Noise, Interference, and SINR

Thermal noise power over bandwidth $B$ is commonly estimated by

$$N = kTB$$

or in dBm at room temperature,

$$N_{\mathrm{dBm}} \approx -174 + 10\log_{10}(B) + F_{\mathrm{dB}}$$

When other transmitters are present, the useful metric is often signal-to-interference-plus-noise ratio:

$$\operatorname{SINR} = \frac{P_{\mathrm{signal}}}{P_{\mathrm{interference}}+P_{\mathrm{noise}}}$$

Wide bandwidth increases total noise power, so simply widening a channel without maintaining sufficient received signal power can reduce SNR.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.

Takeaway

A reliable link needs more than transmit power. Antenna gain, frequency, distance, obstacles, fading, interference, receiver noise, and deployment geometry all enter the link budget. Cellular infrastructure places antennas where those factors can produce the required coverage and capacity with a practical margin.

References for this section3

• H. T. Friis, "A Note on a Simple Transmission Formula", Proceedings of the IRE, vol. 34, no. 5, pp. 254–256, 1946.
• 3GPP, TR 38.901: Study on channel model for frequencies from 0.5 to 100 GHz.
• ITU-R, Recommendation P.1411: Propagation data and prediction methods for short-range outdoor radiocommunication systems and radio local area networks.