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What is a Satellite Link Budget and What Factors Influences It?
What is Link Budget Design?
A wireless link budget is drafted prior to data transmission to anticipate the possible losses that could occur during transmission of data through the channel. A link budget takes into account all forms of possible losses including path loss, atmospheric absorption loss and channel noise. Link budget is very commonly used in satellite communications.
E/S – Earth Station; U/L – Uplink; D/L – Downlink; BER – Bit Error Rate; C/N – Carrier to Noise Ratio; G/T- Ratio of receiver antenna gain and receiver system noise temperature
What is a Satellite Link Budget?
Satellites deliver vital information global weather, global news and so on. Satellite communications fall under the category of long distance communications. Since the distance between the communicating entities is very large, proper resource allocation to establish and maintain the link throughout the communication must be ensured.
In satellite communications, the investments are massive and hence it is necessary to meet all the design criteria to serve the purpose to the fullest for which the communication has been set-up.
What Factors Affect Satellite Link Budget?
A satellite link budget will take into consideration several factors such as −
Transmission power (PT)
Antenna Gain (G)
Frequency of operation (f)
Feeder loss (or connector loss)
Atmospheric absorption loss (including rain attenuation if it demands)
C/N ratio (Carrier to Noise ratio)
Gr/Ts ratio
The above mentioned factors or parameters are of prime importance. Along with these parameters, depending on the requirements of the applications, miscellaneous factors are also included in the link budget.
Link Budget Example
A satellite communication operates on different bands for the uplink and downlink.
Link budgets are separately drafted for uplink communication and downlink communication. The uplink budget is discussed here.
Link Budget Analysis for Uplink Communication
Source - The Satellite Communication Applications Handbook, 2nd Edition, 2004
Author- Bruce R Elbert
Some of the important parameters are discussed here relating to link budget.
Transmission Power
The transmission power is a vital parameter in the link budget design. The signal transmitted from the earth station (ES) travels hundreds to thousands of kilometers to reach the receiving satellite. To ensure that the SNR of the signal at the receiver is above the threshold SNR, proper signal power must be allocated.
Therefore, all possible forms of noise and interference are taken into account and most optimum power is selected prior to the transmission. Even if satellite repeaters are used, the received power level at the destination satellite is less than the transmitted power level from that intermediate repeater point.
In the link budget analysis for the uplink taken from a source, the transmitted power is 850 W. This is represented in dBW. The reference power level is 1 W. The ratio of the assigned signal transmission power and the reference power level is mapped on the decibel scale.
$$10log_{10}(\frac{850W}{1W})\sim\:29.3dBW$$
Since the reference power level is 1W, we use dBW for the representation.
Antenna Gain (Gt)
High gain antennas are used in long distance communications. Using high gain antennas reduces the path loss. For long distance communications, parabolic reflector antennas are preferred. This is because, antennas with high diameter offer high gains. From the analysis table, the gain is specified as 50.6 dBi. So, this is the antenna gain with respect to an isotropic radiator. The diameter of the reflector antenna is specified as 7m in the table.
Effective Isotropic Radiated Power (EIRP)
The effective isotropic radiated power (EIRP) is the product of the transmission power and the antenna gain. This product is compared to the product of the transmission power and gain of an isotropic radiator.
$$EIRP=P_{t}G_{t}$$
$$EIRP(dBW)=10log_{10}(P_{t}G_{t})=10log_{10}(P_{t})+10log_{10}(G_{t})=P_{t}(dBW)+G_{t}(dBi)$$
$$EIRP(dBW)=29.3dBW+50.6dBi=79.9dBW$$
Considering the transmitter waveguide loss, the final EIRP is
$$EIRP(dBW)=79.9dBW-2dB=77.9dBW$$
Similarly, we have the link budget analysis for downlink communication.
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