Clear-day link budgets were calculated assuming a geostationary satellite at 100° West longitude (approximately over the center of the United States) and a ground station in Atlanta, GA. The budgets assumed a worst-case scenario of 86 GHz uplink and 76 GHz downlink (maximizing path loss), and assumed the ground station is at the edge of the satellite HPBW (providing an additional 3 dB of loss) to cover imprecise pointing of the satellite's dish. Clear-day atmospheric attenuation was assumed to be 2 dB, as discussed in the atmospheric attenuation section. Literature sources ([1], [2]) provided estimated antenna noise temperatures of 50 K at the ground station and 280 K at the satellite. The following equation was used to determine received signal power:
where PR is received power, PT is transmitted power, GT and GR are transmitter and receiver gains, d is the path length, f is the frequency, c is the speed of light in a vacuum, and Latm and LHPBW are the losses due to atmosphere and antenna pointing, respectively. All power, gain, and loss values are in dB scale. Table 1 gives the values used for these calculations.
Table 1. Signal Strength Calculation Parameters
Received noise power is calculated as
where PN is received noise power in dBW, k is Boltzmann’s constant (in J/K), B is the channel bandwidth of the modulation in question, and Tant is the antenna noise temperature. Bandwidth values were calculated assuming a 10 Mbps signal with no FEC and 25% roll-off, using an online calculator [3]. Table 2 shows the values used for these equations.
Table 2. Noise Power Calculation Parameters
Finally, the SNR seen at the digital demodulator is given by
where NFsys is the noise figure of the receiver system as described in the equipment section (3.56 dB on the ground, 7.00 dB in the satellite). Table 3 gives the calculated SNR values at the ground station in beacon mode, and at the ground station and satellite in transponder mode.
Table 3. SNR Calculation Results
The design goal in beacon mode is a 36 dB clear day link margin, yielding an SNR of 9.15 dB from our expected 45.15 dB. This corresponds to a BER between 10-5 and 10-6, which is more than sufficient for the small amount of data being transmitted in beacon mode. Thus, the projected link budget provides at least 36 dB of clear day link margin, meeting the stated design goals.
In transponder mode, the design goal is a 10 Mbps data rate in the best case. The modulation bandwidths were selected to accommodate this rate, as described above, and the SNR values calculated above yield BERs of less than 10-6 in all modulations. During testing, the modulation type can be changed and data rates decreased to lower noise power, raise SNR, and maintain a low BER during varying test conditions.
As described above, we expect this system to meet and exceed the stated design goals in all modes. Detailed calculations can be found in the linked Excel file.
References
[1] Fina, S., et al. "Exploitation of the W-band for High Capacity Satellite Communications". IEEE Transactions on Aerospace and Electronic Systems, vol. 39, no. 1, January 2003.
[2]Riera, J., et al. "Low-Cost Millimeter-Wave Beacon Receiver Including Total-Power Radiometer: Design, Implementation and Measurement Calibration". IEEE’s Antennas and Propagation Magazine, vol. 44, no. 1, Feburary 2002.
[3] http://jaunty-electronics.com/blog/2012/05/bpsk-qpsk-8psk-and-qam-calculator/