Earth Station Design
Figure 1 shows the preliminary block diagram of the proposed Earth station transmitter which can transmit signals using 3 frequencies 81 GHz, 84.5 GHz and 86 GHz. A fixed pseudorandom bit sequence is generated by the processor which also applies an appropriate rate Turbo code. A software defined radio then performs the required modulation and the digital to analog conversion. These operations are assumed to happen at 1 GHz in the software defined radio and components will be chosen accordingly in this stage. The modulated analog signal is then up converted to an intermediate frequency in the range 31-36 GHz using a VCO that operates within 30-35 GHz. The second stage of up conversion is performed by the use of a 50 GHz PLDRO to bring the signal up to a frequency range of 81-86 GHz. This is then filtered and passed onto a driver amplifier. A klystron is proposed to be used as the high power amplifier because of its capability to put out high output powers in the range of tens of kW[1]. A preliminary analysis of the clear day link budgets for the W band uplink at 86GHz led us to choose such a high power amplifier despite its high cost (see the W band uplink & downlink budgets section for more details).
Figure 1. Preliminary earth station transmitter block diagram
The receiver on the Earth station (as shown in the block diagram in Figure 2) is designed to receive the 71 GHz downlink signal from the satellite transponder and is proposed to have a cryogenically cooled InP HEMT LNA at its front end to reduce the receiver noise temperature to less than 36 K (at a physical temperature of 20 K) [2]. The reason for using cryogenic cooling for the LNAs is also based on our preliminary clear day link budgets for the V band downlink. The limitations on the transmit output power (75W using TWTAs) from the satellite transponder coupled with the high path and atmospheric losses predicted a high system outage probability (see W band uplink & downlink budgets section for more details). Hence, devices that would give us any SNR improvement (although expensive) are proposed to be used. The amplified signal is filtered and is down converted in two stages to bring it down to 1 GHz baseband. Once again, a software defined radio is proposed to be used to demodulate followed by the BER and SNR calculation in the processor. The data is then stored to be used for statistical characterization of the W-band. Plans to integrate this receiver with that of the beacon for optimization will be further investigated in Phase 1.
Figure 2. Preliminary earth station receiver block diagram
References:
[1] R. J. Trew, “High-Frequency Solid-State Electronic Devices,” IEEE Transactions on Electron Devices, vol. 52, no. 5, pp. 638-649, May 2005.
[2] L. Samoska, S. Church, K. Cleary, A. K. Fung, T. Gaier, P. Kangaslahti, R. Lai, J. Lau, X. B. Mei, M. M. Sieth, R. Reeves, and P. Voll, “Cryogenic MMIC low noise amplifiers for W-band and beyond,” inProc. 22nd International Symposium on Space Terahertz Technology, Tucson, AZ, April 2011.