A satellite is visible and UE knows when it can transmit. Is the received signal strong enough to decode? This is the question a link budget answers. We start with transmitted power, account for antenna gains and losses, and compare the received signal with noise and the receiver requirement.

For NTN, the distance changes during a satellite pass. UE antenna gain also depends on direction and orientation. So a budget that works directly below the satellite may fail near the edge of coverage. Both the uplink and downlink need to be checked.

This page builds a service-link budget from slant range, EIRP and G/T. The examples use a 600 km orbit and deliberately chosen engineering assumptions. They are not 3GPP reference configurations, product specifications or a claim about a particular satellite network.

• Which Link Are We Calculating ?
• Slant Range, Elevation and Free-Space Loss
  • From Elevation to Slant Range
  • From Slant Range to Free-Space Path Loss
• EIRP, G/T and Noise
  • EIRP: Power Radiated Toward the Receiver
  • G/T: Receive Gain Compared with System Noise
  • C/N0, C/N and Eb/N0
• Which Additional Losses Should Be Included ?
• Worked Uplink and Downlink Examples
  • Downlink: Satellite to UE
  • Uplink: UE to Satellite
• What Changes During the Satellite Pass ?
• From Service Link to End-to-End Margin
• What to Record with a Link Budget
• References

Which Link Are We Calculating ?

First identify the transmitter and receiver. On the service link, downlink means satellite to UE and uplink means UE to satellite. The gateway-to-satellite feeder link has a separate distance, frequency, antenna arrangement and noise contribution.

Figure 1 shows the quantities needed for either direction. EIRP belongs to the transmitter. G/T belongs to the receiver. When we reverse the direction, those values must change even if the propagation distance stays the same.

Figure 1. Use the transmitter's EIRP and the receiver's G/T for the direction being calculated.

A transparent satellite relays the waveform between service and feeder links. Noise from both hops can affect the final receiver. A regenerative payload can decode the signal onboard, so each decoded hop has its own reception requirement. See NTN Architecture for the payload arrangement.

Before calculating, also define what the signal power represents. Total satellite power, total beam power, carrier power and power assigned to one UE are different quantities. The examples below use power allocated to the signal being evaluated.

Slant Range, Elevation and Free-Space Loss

The radio signal travels along the slant range between UE and satellite. Satellite altitude equals this distance only when the satellite is directly overhead. At lower elevation, the path becomes longer and free-space loss increases.

From Elevation to Slant Range

Assume a spherical Earth with radius R, satellite altitude h and elevation angle e measured from UE's local horizontal. The following equation gives the straight-line distance d. It neglects terrain and atmospheric refraction.

d = sqrt((R + h)² - R² cos²(e)) - R sin(e)

Use one length unit throughout the equation. For R = 6371 km and h = 600 km, elevation of 10 degrees gives d = 1931.6 km. That is more than three times the altitude. The coverage geometry diagram shows this relationship for several elevations.

From Slant Range to Free-Space Path Loss

Free-space path loss describes propagation between isotropic antennas. Antenna gains are accounted for separately. ITU-R P.525 gives the free-space relationship; the form below uses kilometres and GHz, with the constant rounded to 92.45.

L_FS = 20 log₁₀(4 pi d / lambda)

L_FS [dB] = 92.45 + 20 log₁₀(d [km]) + 20 log₁₀(f [GHz])

In the first expression, d and wavelength lambda must have the same unit. In the second, the unit conversion is already included. The following values use 2.0 GHz and the unrounded slant ranges before rounding the displayed results.

At 10 degrees, free-space loss is 10.16 dB higher than overhead. Doubling distance adds about 6.02 dB. Doubling frequency also adds about 6.02 dB when antenna gains are held constant. A comparison using fixed physical antenna apertures must also account for their frequency-dependent gains.

The zero-degree row is an ideal geometric horizon limit. It is not an operating recommendation. Local obstructions and additional propagation effects become important before many practical links reach that limit.

EIRP, G/T and Noise

Received power alone does not tell us whether a signal can be decoded. The receiver also collects noise. EIRP summarizes the transmit side, while G/T combines receive antenna gain and system noise temperature.

EIRP: Power Radiated Toward the Receiver

EIRP is the equivalent isotropic radiated power in the direction of interest. It combines transmitter output power, transmit-side losses and antenna gain. Use the gain toward the actual receiver, not automatically the antenna's peak gain.

EIRP [dBW] = P_TX [dBW] - L_TX [dB] + G_TX [dBi]

For example, 12 dBW at the transmitter, 2 dB feed loss and 20 dBi antenna gain give 30 dBW EIRP. This does not mean the amplifier produces 1000 W. Antenna directivity contributes to the equivalent radiated power.

Remember the reference unit: dBm = dBW + 30. A 23 dBm transmitter is -7 dBW, approximately 0.2 W. Mixing dBm and dBW creates a 30 dB error. If EIRP is already supplied, do not add transmit antenna gain a second time.

G/T: Receive Gain Compared with System Noise

G/T measures the receive system's ability to collect a signal relative to its noise. A larger value improves the link. The temperature must be the equivalent system noise temperature at the same reference plane used for receive gain.

G/T [dB/K] = G_RX [dBi] - 10 log₁₀(T_sys [K])

With 0 dBi receive gain and T_sys = 316.2 K, G/T is approximately -25 dB/K. With 30 dBi gain and the same temperature, it becomes +5 dB/K. These are the deliberately selected UE and satellite values used later.

T_sys includes antenna noise and receiver contributions referred to the selected plane. It is not simply the air temperature or receiver noise figure. For a receiver alone, equivalent noise temperature is T_e = 290(F - 1) K, where F = 10^NF/10. Antenna noise and input losses still need to be included.

Receive feed losses can reduce signal power and change equivalent noise temperature. Account for them consistently in the effective gain and T_sys, or in an explicitly defined receive-chain model. Do not subtract the same loss again after using a G/T value that already includes it.

C/N0, C/N and Eb/N0

The receiver sees noise across its measurement bandwidth. C/N0 removes that bandwidth dependence and expresses carrier power relative to noise power density. We can then convert it to C/N or Eb/N0 for the chosen bandwidth or bit rate.

C/N0 [dB-Hz] = EIRP - L_FS - L_other + G/T + 228.60

C/N [dB] = C/N0 [dB-Hz] - 10 log₁₀(B [Hz])

Eb/N0 [dB] = C/N0 [dB-Hz] - 10 log₁₀(R_b [bit/s])

The 228.60 term comes from Boltzmann's constant expressed in logarithmic units. EIRP must be in dBW for this form. B is the noise-equivalent bandwidth used for the evaluated signal. R_b must match the bit-rate definition used by the required Eb/N0, such as information bits rather than coded bits.

C/N0 is not a power and is not measured in dBm. C/N also does not include interference. When interference is relevant, compare the applicable SINR with a receiver requirement based on the same signal and bandwidth definition.

Which Additional Losses Should Be Included ?

Free-space loss is only one term. A practical link also depends on antenna pointing, polarization, weather and the UE environment. Keep these contributions visible so the same impairment is not counted twice.

The following list identifies typical entries. Their numerical values depend on deployment, frequency, equipment and the required availability. A generic allowance cannot establish a coverage guarantee.

A fade allowance should correspond to the target availability and environment. A positive clear-sky margin alone does not establish 99.9% availability. Atmospheric attenuation can also increase received noise, so a detailed calculation may need a weather-dependent G/T.

Worked Uplink and Downlink Examples

Use the 600 km satellite at 10 degrees elevation: slant range 1931.6 km and free-space loss 164.19 dB at 2.0 GHz. Both examples use this frequency only to simplify comparison. This is not a proposed uplink/downlink frequency allocation.

Assume 3 dB of additional path loss, split into 1 dB polarization loss, 1 dB pointing loss and 1 dB other propagation loss. These are illustrative allowances, not environmental predictions. The stated antenna figures exclude these three losses, and no interference or feeder-link noise is included yet.

Figure 2 follows the arithmetic for the two directions. The uplink has less C/N0, but its smaller assumed bandwidth collects less noise. Consequently, the two C/N results are similar in this particular example.

Figure 2. Uplink and downlink use different EIRP, G/T and bandwidth assumptions; compare the final margin under each assumption.

Downlink: Satellite to UE

Assume satellite EIRP of 30 dBW allocated to the evaluated signal and UE G/T of -25 dB/K. Use a 1 MHz noise bandwidth. The following calculation shows every contribution to C/N0 and the remaining margin.

Cross-check with receive power. Using 0 dBi UE gain and 316.2 K system noise temperature, C = 30 - 164.19 - 3 + 0 = -137.19 dBW, or -107.19 dBm. Noise is -228.60 + 25 + 60 = -143.60 dBW, or -113.60 dBm. Their difference is again 6.41 dB.

At an assumed information rate of 500 kbit/s, Eb/N0 = 66.41 - 56.99 = 9.42 dB. That value is another representation of this example, not a second independent gain. Any required Eb/N0 must use a compatible coding and bit-rate definition.

Uplink: UE to Satellite

Assume UE EIRP of 23 dBm, which is -7 dBW, and satellite G/T of +5 dB/K. Use a 180 kHz noise bandwidth. The UE EIRP assumption already includes its transmit antenna and feed effects.

With 30 dBi satellite receive gain and 316.2 K system temperature, C = -144.19 dBW, or -114.19 dBm. Noise over 180 kHz is -151.05 dBW, or -121.05 dBm. Their difference is 6.86 dB.

At an assumed information rate of 50 kbit/s, Eb/N0 = 59.41 - 46.99 = 12.42 dB. The 180 kHz bandwidth does not specify an entire NB-IoT configuration. Likewise, the 23 dBm value is an example assumption, not a statement that every NTN UE has that EIRP.

The 3 dB ideal C/N thresholds in both examples are chosen for arithmetic. Real thresholds depend on waveform, MCS, coding, channel conditions and the target error rate. For NR, use the appropriate BLER performance data rather than assigning this threshold to an MCS.

What Changes During the Satellite Pass ?

Now keep EIRP, G/T, bandwidth and the 3 dB additional loss fixed. Only change elevation and therefore slant range. This isolates the distance effect; a real pass can also change pointing loss, antenna gain and system noise.

The following table recalculates the two examples with the same 4 dB total C/N requirement. The zero-degree result is included only as the ideal horizon limit.

The downlink example has 12.57 dB margin overhead but only 2.41 dB at 10 degrees. An extra 3 dB of unmodelled loss would make the 10-degree downlink margin -0.59 dB. This shows why a budget should identify the limiting elevation and UE orientation.

If total received signal power stays constant, halving noise bandwidth improves C/N by about 3.01 dB. This does not give free throughput. A narrower allocation changes the supported signal and rate. If power spectral density stays constant instead, narrowing the bandwidth also reduces signal power, so the same improvement does not follow.

Repetition and stronger coding can reduce the quality needed for a target error rate, with resource and latency costs. Timing advance and frequency compensation help reception remain synchronized; they do not cancel free-space attenuation. See HARQ and Scheduling and Timing for the related protocol trade-offs.

From Service Link to End-to-End Margin

The worked examples stop at the service-link receiver. For a transparent payload, the gateway path and forwarded noise still matter. A positive service-link margin is therefore not sufficient to declare that the full connection works.

For a simple linear transparent relay, combine independent noise contributions referred to the same final carrier and noise bandwidth. If gamma denotes a linear C/N ratio, the thermal-noise-only relation is:

1 / gamma_total = 1 / gamma_hop1 + 1 / gamma_hop2

For example, two contributions each equivalent to 10 dB C/N give gamma = 10 each. The total is 1 / (0.1 + 0.1) = 5, or 6.99 dB. They do not add to 20 dB. Interference, intermodulation and other impairments need their own consistent treatment.

For a payload that decodes onboard, check each decoded hop against its own receiver threshold. The transparent-relay noise sum is not the way to combine two independently decoded hops. Overall delivery still depends on both hops, their error performance and scheduling.

Finally, decide whether the budget covers only data or also acquisition and control. Successful PDSCH decoding does not establish that UE can acquire synchronization, decode control or complete RACH at the same location. Each required channel needs suitable power allocation and receiver performance.

What to Record with a Link Budget

A budget is useful only when another engineer can reproduce its result. Record the geometry, reference planes, allocated power and receiver requirement with the final margin. These inputs often explain disagreements between two apparently similar calculations.

Use Coverage and Beam Movement to establish the geometric service window. Then apply the link budget across that window. The usable interval can be shorter than the interval during which the satellite is above the horizon.

References

The references below support the free-space equation, satellite link-budget quantities and Earth-space propagation treatment. The numerical equipment assumptions, thresholds and resulting examples on this page are illustrative calculations.

• ITU-R P.525-5 : Calculation of free-space attenuation; Annex section 2.3.
• NBSIR 86-3058 : Calibration Requirements for EHF Satellite Communication Systems; section 2, link-budget quantities and system performance. Used for the general RF relationships, not modern NTN equipment assumptions.
• ITU-R P.618-14 : Propagation data and prediction methods required for the design of Earth-space telecommunication systems.