Link Budget for a Typical VSAT Operation at KuBand

The model VSAT system in question is a two-way operation using Ku-band frequencies. The outbound link is 128 kbps in a TDM format employing QPSK modulation with coherent detection using rate 2 convolutional coding, K = 7 and 3-bit quantization, and Viterbi decoder. The inbound (traffic) link has a transmission rate of 32 kbps using a HDLC*-type frame format, QPSK with similar FEC. The 128-kbps link with its rate -j coding has a coded symbol rate of 256 symbols/s. This link requires 200-kHz RF channel; the 32-kbps information rate and 64-kbps coded symbol rate require a 50-kHz RF channel. The BER, under clear-sky conditions, is 1 X 10 9; for degraded conditions the BER may drop to 1 X 10 6. There is a 2-dB modulation implementation loss so that the Eb/N0 for clear-sky operation is 8.5 dB; for degraded operation, it is 6.7 dB for the 128-kbps outbound channel. The inbound 32-kbps channel also requires 8.5 dB for clear-sky and 6.7 dB for degraded operation. To counter rainfall loss at Ku-band, there is a 4-dB margin or both links. The elevation angle at both the hub and outstation VSAT is 10°. The range (distance) to the satellite (from Figure 6.5) is 25,220 sm.

The outbound uplink frequency is 14,100 MHz; its equivalent downlink operates at 11,800 MHz. The inbound uplink frequency is 14,300 MHz; its equivalent downlink frequency is 12,000 MHz. The satellite transponders in question each have an EIRP of +44 dBW over a 72-MHz bandwidth, assuming full loading. Transponder/satellite G/T in either case is 0.0 dB/K.

The inbound carrier downlink has an EIRP of + 12.4 dBW; for the outbound downlink, the EIRP for the VSAT carrier is + 18.4 dBW. These EIRP values were calculated assuming a uniform power density across the entire transponder bandwidth of 72 MHz. Therefore the EIRP =+ 44 dBW - 10 log(72,000/200) = +18.4 dBW.* The +12.4-dBW value is calculated in a similar fashion or EIRPdBW = +44 dBW - 10 log(72,000/50).

The hub facility has the following terminal parameters: transmitter power output, 500 W or + 27 dBW; line loss, 2 dB; antenna aperture, 5 m or 16.25 ft. Its gain at 14,100 MHz is 53.5 dB and at 11,800 MHz it is 52.0 dB; Tsys = 200 K, so the hub G/T is +29.0 dB/K. The EIRP = +78.5 dBW.

Postulated parameters of the VSAT terminal to operate in this system are as follows:

G/T = ? The antenna aperture is unknown. We will assume its efficiency is 65%.

The Tsys for the receiving system consists of the sum of Tant and Tr. Tr = 100 K and Tant = 120 K. Thus Tsys = 220 K. These are typical values.

The VSAT EIRP is unknown. The transmission line losses are 1 dB; the transmitter power output is unknown (in the range of 0.5-10 watts). The downlink (outbound) link budget will determine the antenna aperture.

The free-space loss values are:

(14,100 MHz) FSLdB = 36.58 + 20log 14,100 + 20log25,220

= 36.58 + 82.98 + 88.03 = 207.59 dB (14,300 MHz) FSLdB = 36.58 + 83.11 + 88.03

= 207.71 dB (12,000 MHz) FSLdB = 36.58 + 81.58 + 88.03

= 206.19 dB (11,800 MHz) FSLdB = 36.58 + 81.44 + 88.03 = 206.05 dB

*This is the same as dividing 25,188 watts by 360 because there are 360 200-kHz segments in 72 MHz.

Outbound Link Budget



Polarization loss Terminal pointing loss Satellite pointing loss Atmospheric loss Isotropic receive level Satellite G/T Sum

Boltzmann's constant C/N0 Downlink EIRP satellite FSL

Polarization loss Satellite pointing loss Atmospheric loss Terminal pointing loss Isotropic receive level VSAT G/T Sum

Boltzmann's constant C/N0

-130.89 dBW 0.0 dB/K -130.89 dBW/K -(-228.6 dBW) 97.71 dB

What net C/N0 is required for an Eb/N0 of 8.5 dB?

N0 = -228.6 dBW + 10 log Tsys Tsys = 220 K (given above)


Eb must have a level 8.5 dB higher than -205.17 dBW or -196.67 dBW. The bit rate on the channel is 128 kbps; thus C = RSL = -196.67 dBW + 10 log(128 X 103) or -196.67 + 51.07 dB = -145.6 dBW. Then the objective

Neglecting satellite generated noise (IM products);

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  • clotilde
    What is typical eirp for vsat terminal?
    3 months ago

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