TechWindo

A Link‑Budget in 5G NR is a complete, end‑to‑end accounting of all gains and losses a radio signal experiences as it travels from the transmitter (gNB/UE) to the receiver (UE/gNB).

It answers two core engineering questions:

1. Will the received signal be strong enough to meet the required SNR for decoding?
2. What is the maximum coverage distance for a given transmit power, frequency, bandwidth, and environment?

Link‑budget is essential for:

• Cell planning & coverage prediction
• Network optimization
• gNB/UE sensitivity assessment
• Propagation model comparison
• Determining downlink vs uplink imbalance

Why Link‑Budget Matters in 5G NR

5G uses higher frequencies (FR1 up to 7.125 GHz, FR2 up to 52.6 GHz), massive MIMO antennas, beamforming, and wide bandwidths — all of which impact signal strength.

Higher frequencies → higher free‑space path loss
Wider bandwidth → higher thermal noise
Beamforming → higher antenna gain
NR numerology & MIMO → different SNR and performance targets

This makes link‑budget more important in NR compared to LTE.

Core Components of a 5G NR Link‑Budget

A link‑budget is normally expressed as:

Where:

• Ptx → Transmit power (dBm)
• Gtx / Grx → Antenna gains (dBi)
• Ltx / Lrx → Feeder, cable, and connector losses (dB)
• PL → Path loss (free‑space + environmental losses)

1. Path Loss (PL)

For a first‑order link‑budget, free‑space path loss (FSPL) from ITU‑R P.525 is used.
FSPL formula (GHz, km form):

This is directly from the ITU free-space attenuation model.

Engineers then add more losses:

• Penetration loss (indoor, vehicles)
• Body loss
• Shadowing
• Clutter / foliage loss
• Rain attenuation (especially mmWave)

2. Noise Floor (kTB)

Every receiver has thermal noise based on Boltzmann’s constant (k), temperature (T), and bandwidth (B):

The −174 dBm/Hz value at 290 K is a standard industry reference.

Noise floor increases with bandwidth — which is very important for 5G NR because NR supports up to 400 MHz in FR2.

3. Receiver Noise Figure (NF)

Noise Figure quantifies how much noise the receiver adds on top of thermal noise.

Lower NF → more sensitive receiver.

4. Required SNR (SNR_req)

To decode NR channels (PBCH, PDCCH, PDSCH), the UE or gNB needs a minimum SNR that depends on:

• Modulation order (QPSK/16QAM/64QAM/256QAM)
• Coding rate
• MIMO layers
• Channel conditions

Receiver sensitivity is computed as:

5. Link Margin

Once you compute received power (Prx) and sensitivity (Psens):

• Positive margin → Link is healthy
• Zero margin → Borderline coverage
• Negative margin → Out of coverage

DL vs UL Link‑Budget (Important in 5G)

In 5G NR, uplink is almost always weaker than downlink because:

• UE transmit power is low (23–26 dBm)
• gNB antenna arrays provide high DL gain
• UL beamforming at UE is limited

Therefore UL usually defines:

• Cell edge coverage
• Uplink-limited throughput

This is why UL link‑budget is considered the limiting path in NR planning.

Example (n78, 3.5 GHz)

• gNB EIRP: 61 dBm
• UE gain: 0 dBi
• Distance: 1 km
• BW: 20 MHz
• NF: 7 dB
• Required SNR: 5 dB

FSPL ≈ 103.4 dB (from ITU P.525)

Noise floor (20 MHz) ≈ −101 dBm (from kTB)

Sensitivity = −101 + 7 + 5 = −89 dBm
Prx = 61 − 103.4 = −42.4 dBm
Margin = 46.6 dB (very strong link)

Summary (Easy‑to‑Remember)