Link Budget in a Nutshell – Engineer Edition

SECTION 01

Overview — What is a Link Budget?

A power balance equation that defines the viability of a radio link. The output — MAPL — directly drives cell radius calculations in LTE, NR, and 5G planning.

DEFINITION

A link budget tabulates all gains (Tx power, antenna gain, diversity) and losses (path loss, cable loss, body loss, building penetration) between transmitter and receiver to determine whether a link closes at a given distance.

PURPOSE IN NETWORK PLANNING

Determines site density (ISD), coverage radius, and whether indoor/outdoor coverage targets are achievable before deployment. Used in both UL and DL directions independently.

KEY OUTPUT

MAPL (dB)

Max Allowable Path Loss = EIRP − Rx Sensitivity − all margins

LINK TYPES

DL Downlink | UL Uplink
UL often limits coverage (UE Tx power ≪ eNB/gNB)

PROPAGATION MODEL

Okumura-Hata, COST-231, 3GPP TR 38.901, or ITU-R models map MAPL → Cell Radius

SECTION 02

Link Budget Architecture

The complete signal chain from eNB/gNB PA output to UE demodulator, and the reverse UL path.

DOWNLINK PATH — eNB/gNB → UE

Tx Power

eNB Tx Power (e.g. +46 dBm for 20W per carrier) → Cable/Filter Loss → EIRP

Gains

Tx Antenna Gain +18 dBi | Beamforming Gain (NR) | MIMO spatial gain

Losses

Free-Space PL | Shadow Fading Margin | Building Penetration Loss | Body Loss | Feeder/Jumper Loss

Rx Side

UE Rx Antenna Gain | Rx Sensitivity | Interference Margin | SINR Threshold

UPLINK PATH — UE → eNB/gNB (often coverage-limiting)

Tx Power

UE Max Tx Power +23 dBm (Power Class 3) | Body Loss −3 dB

Gains

UE Antenna Gain 0 dBi (isotropic) | eNB Rx Diversity Gain | IRC / MMSE combining gain

Losses

Path Loss | Shadow Margin | Penetration Loss | Feeder Loss (eNB side) | Interference Margin

Rx Side

eNB Noise Figure | Thermal Noise Floor | Required SINR | MAPL_UL

SECTION 03

Core Mechanics — The MAPL Equation

The fundamental balance equation that every link budget reduces to.

MASTER LINK BUDGET EQUATION

MAPL = EIRP + G_rx − Rx_Sensitivity − M_shadow − M_interference − L_penetration − L_body

          EIRP = P_tx + G_tx − L_feeder

          Rx_Sensitivity = Thermal_Noise + NF + Required_SINR

          Thermal_Noise = −174 dBm/Hz + 10·log10(BW_Hz)

GAIN COMPONENTS (+)

P_tx Transmitter output power — eNB: +43–46 dBm, UE: +23 dBm (PC3)

G_tx Tx antenna gain — macro: 15–18 dBi, small cell: 5–8 dBi

G_rx Rx antenna gain — eNB diversity: 2–3 dB, UE ≈ 0 dBi

G_bf Beamforming/MIMO gain — NR mMIMO: up to +10 dB spatial

LOSS & MARGIN COMPONENTS (−)

M_sf Shadow fading margin — 6–10 dB @ 95% coverage probability

L_bpl Building penetration — 10–25 dB (light) to 25–35 dB (deep indoor)

L_body Body loss — 3 dB typical for voice/data calls

M_im Interference margin — 1–3 dB UL, 0 dB DL (SINR absorbs this)

SECTION 04

Propagation Loss Models

Converting MAPL to cell radius requires a path loss model. Each model trades accuracy for applicability range.

FSPL

Free-Space Path Loss 20log(d) + 20log(f) + 92.4 LOS only, vacuum — theoretical baseline

Okumura-Hata

Empirical macro model 150–1500 MHz Urban/suburban/rural categories — LTE 700/800/900 MHz

COST-231

Extended Hata for 2 GHz 1.5–2 GHz LTE 1800/2100 MHz macro planning — most widely used in 4G

3GPP TR 38.901

5G NR stochastic channel model 0.5–100 GHz UMa / UMi / RMa / InH scenarios — NR planning standard

ITU-R P.1546

Point-to-area prediction 30 MHz–3 GHz Regulatory coverage prediction, terrain-sensitive

3GPP UMa NR

NR Urban Macro formula NLOS PL = 13.54 + 39.08·log10(d) + 20·log10(f) − 0.6·(hUT−1.5)

💡 MAPL → Cell Radius: Solve for d in the chosen propagation model where PL(d) = MAPL. For COST-231 Urban at 1800 MHz with MAPL = 135 dB → typical macro radius ≈ 0.8–1.2 km.

SECTION 05

Key Metrics & Target Values

Typical LTE/NR planning targets — values vary by operator, band, and deployment scenario.

PARAMETER	TYPICAL VALUE	WORST-CASE / LIMIT	FORMULA / NOTE
eNB Tx Power (4G)	+46 dBm	+43 dBm	20W carrier → 43 dBm; per-carrier if split
UE Tx Power (PC3)	+23 dBm	+20 dBm	3GPP PC3 = 200 mW; PC2 = 26 dBm (special)
eNB Rx Sensitivity (LTE)	−121 dBm	−118 dBm	10 MHz BW, QPSK 1/3, NF=3 dB
Shadow Fading Margin	8 dB	10–12 dB	σ × Q⁻¹(p); σ≈8 dB urban, p=95%
Building Penetration Loss	15–20 dB	25–35 dB	Low-e glass, concrete walls; higher at mmWave
MAPL (Outdoor LTE 1800)	135 dB	128 dB	EIRP − Rx_Sens − margins; UL often sets this
Antenna Gain (Macro)	+18 dBi	+15 dBi	65° H-plane 3-sector; mMIMO panel: 24 dBi+
Interference Margin (UL)	2 dB	3–5 dB	Load factor (η): M_im = −10·log10(1−η)
Thermal Noise Density	−174 dBm/Hz	at 290 K	kT = 1.38×10⁻²³ × 290; varies with temp
Required SINR (QPSK 1/3)	−3 dB	−6 dB (Turbo)	MCS-dependent; 64QAM 2/3 needs +22 dB

SECTION 06

Planning Workflows

Standard link budget workflows for both greenfield macro planning and indoor coverage design.

OUTDOOR MACRO CELL PLANNING

1

Define Service Requirements — Set target throughput/service (e.g. VoLTE, 2 Mbps DL), select MCS, derive required SINR threshold.

2

Compute EIRP — Sum Tx power + antenna gain − feeder/cable loss. Confirm regulatory EIRP limit compliance.

3

Calculate Rx Sensitivity — Thermal noise + noise figure + required SINR. Compute for both DL (UE) and UL (eNB) paths.

4

Apply Margins & Derive MAPL — Subtract shadow margin, interference margin, body loss. Take the lower of DL-MAPL vs UL-MAPL. Feed into propagation model → ISD.

INDOOR COVERAGE (BPL SCENARIO)

1

Classify Target Environment — Categorise as outdoor-to-indoor, deep indoor, car-interior, or tunnel. Assign BPL: 10 / 20 / 25 / 30 dB accordingly.

2

Compute Indoor MAPL — Apply outdoor MAPL then subtract BPL. Check if indoor coverage closes with existing macro. If not: IBS/DAS required.

3

Size IBS/pRRU — Run separate indoor link budget for distributed antennas. Account for cable run loss, splitter loss, and DAS antenna gain (~5 dBi).

4

Validate with Drive/Walk Test — Compare predicted RSRP from model against measured CW/pilot signal. Tune BPL/shadow std-dev parameters.

SECTION 07

Reference — Scenarios & Loss Values

Standard deployment scenarios with indicative loss and margin values used in real operator planning.

PENETRATION LOSS BY ENVIRONMENT

Car 3–8 dB — standard automotive glass penetration

Indoor Shallow 10–15 dB — timber/brick, near window, ground floor

Indoor Mid 15–22 dB — concrete/steel structure, mid-floor

Deep Indoor 25–35 dB — basement, low-e glass, reinforced concrete

Tunnel 30–50 dB — road/rail tunnel; leaky feeder or RRU required

SHADOW FADING MARGIN BY SCENARIO

Rural σ = 6 dB → 7–8 dB margin at 90–95% coverage

Suburban σ = 8 dB → 9–10 dB margin at 95% coverage

Urban σ = 8–10 dB → 10–12 dB margin at 95–99% coverage

Dense Urban σ = 10 dB → 12–14 dB margin at 99% coverage

Formula M_sf = σ × Q⁻¹(p); Q⁻¹(0.95) ≈ 1.645, Q⁻¹(0.99) ≈ 2.326

FREQUENCY IMPACT ON LINK BUDGET

Sub-1 GHz Best MAPL — macro radius 2–5 km, superior indoor coverage

1–3 GHz Capacity layer — radius 0.5–1.5 km, moderate penetration

C-Band +20–25 dB FSPL vs 700 MHz — dense macro, BPL >25 dB

mmWave Extreme path loss; BPL >40 dB; range <200 m, LOS only

COMMON PLANNING MISTAKES

UL/DL Mismatch Using DL-MAPL only — UL almost always more restrictive in 4G/5G macro

Wrong BPL Using generic 20 dB for high-spec office glass (actual: 30–35 dB)

No Fading Margin Omitting shadow margin results in actual 50% coverage at cell edge

ISD Overestimate Hex grid oversimplification ignores terrain — validate with ray-trace

SECTION 08

Values Cheatsheet

Quick-reference parameter cards for LTE macro, NR C-Band, IBS, and noise calculations.

LTE 1800 MHz MACRO (OUTDOOR UL)

UE Tx Power+23 dBm
UE Antenna Gain0 dBi
Body Loss−3 dB
EIRP_UL+20 dBm
eNB Rx Sensitivity−121 dBm
Shadow Margin−9 dB
MAPL_UL132 dB

LTE 1800 MHz MACRO (OUTDOOR DL)

eNB Tx Power+46 dBm
Feeder Loss−1.5 dB
Tx Antenna Gain+18 dBi
EIRP_DL+62.5 dBm
UE Rx Sensitivity−100 dBm
Shadow + BPL−9 dB
MAPL_DL153.5 dB

NR 3.5 GHz C-BAND (OUTDOOR UL)

UE Tx Power (PC3)+23 dBm
Body Loss−3 dB
gNB NF3 dB
BW (100 MHz NR)−164.8 dBm
Required SINR−3 dB (QPSK)
Shadow Margin−10 dB
MAPL_UL (NR)~124 dB

NOISE FLOOR CALCULATION

Thermal Noise Density−174 dBm/Hz
BW 10 MHz (LTE)+70 dB·Hz
Noise Floor @10 MHz−104 dBm
eNB Noise Figure+3 dB
Receiver Noise Floor−101 dBm
Required SINR (QPSK)−3 dB
Rx Sensitivity−104 dBm

INDOOR IBS / DAS LINK BUDGET

pRRU Tx Power+24 dBm
Cable + Splitter Loss−6 dB
DAS Antenna Gain+5 dBi
EIRP Indoor+23 dBm
Floor Attenuation−15 dB/floor
Target RSRP> −95 dBm
Coverage Radius~25–40 m

RSRP TARGET vs COVERAGE CLASS

Excellent> −80 dBm

Good−80 to −90 dBm

Fair−90 to −100 dBm

Poor−100 to −110 dBm

No Service< −120 dBm

Link Budget in a Nutshell Radio Network Planning