Front-End Technology Selection and Downlink Budget
The beacon downlink budget analysis is contained in Table 1. All values used in this analysis correspond to a worst-case clear-sky scenario: a maximum downlink frequency of 76 GHz between the satellite and the Earth station with the most path loss, Virginia Tech. It is assumed that the additional losses incurred by moving from 71 to 76 GHz are greater than the increased antenna gain at 76 GHz caused by the decreasing wavelength for a constant aperture size, so 76 GHz is the worst case scenario. The directivities and beamwidths of the antennas are assumed to be fixed in these calculations (see the Antennas & Coverage section for more details).
Our goal is to satisfy the objective downlink SNR of 36 dB in a low-risk, cost-effective manner. Because the antennas are fixed, the main “tuning knobs” in the system design are the beacon saturated output power, the Earth station receiver bandwidth, and the Earth station receiver noise temperature (dominated by the front-end LNA).
Achieving high output powers (> 50 W) at moderate millimeter-wave frequencies (such as V-band and W-band) and above is quite technologically challenging. These frequencies push the limits of most affordable technologies, as shown in Figure 1. Traveling wave tubes can produce output powers of up to around 100 W at V-band, but above 100 W technologies such as klystrons and gyrotrons are required, which are far too large and power-intensive for use on a satellite. It is difficult to dissipate heat from high-power amplifiers in space as well. The most effective heat transfer mechanism, convection, cannot occur in a vacuum. This heat dissipation problem is minimized by using a lower-power amplifier which generates heat that can be manageably dissipated by conduction and radiation. Overcoming these challenges can be quite expensive.
Figure 1. Frequency dependence of output power for various high-frequency amplifying technologies (after [1])
We aim to balance the cost-performance tradeoff by first minimizing the Earth station receiver noise power and then by selecting a PA technology that will satisfy the objective SNR. We first select a narrow receiver noise bandwidth of 1 kHz, as proposed in similar experiments [2]. The most significant challenge is to select an LNA with an extremely low noise temperature at V-band. V-band LNA MMICs in a 35 nm InP HEMT technology with more than 15 dB of gain and a noise temperature of less than 250 K across most of W-band have been recently demonstrated [3]. This noise temperature enables a 6 dB improvement in SNR over typically proposed LNAs which only enable a noise temperature of around 1200 K [4]. Even further reductions in the noise temperature can be achieved by cooling the LNAs, as is typically done in radio astronomy applications. Moderate cooling to improve the amplifier performance may be feasible and will be explored during Phase 1. Procuring ultra-low-noise V-band LNAs in this technology should be readily feasible. For the purposes of this link budget analysis, a conservative system noise temperature of 300 K is assumed.
Only a single tuning knob remains in the link budget – the beacon transmit power. When these InP LNAs are used along with a 1 kHz noise bandwidth, the PA output power required to satisfy the 36 dB SNR requirement is a manageable 22 W. We propose, however, using a recently demonstrated space-qualified V-band traveling wave tube amplifier (TWTA) with a saturated output power of 75 W and an efficiency of around 50% [5]. This pushes the final projected SNR up to over 41 dB. However, to ensure reliable operation, we propose operating this TWTA at a roughly 3 dB backoff from the saturated output power. This will improve the long-term reliability of the TWTA, reduce DC power consumption and heat dissipation, prevent the generation of undesired harmonics, and minimize AM-PM distortion which would introduce uncertainty into phase measurements at the receiver [6]. The additional SNR margin above 36 dB is a safety buffer, as there are surely more SNR degradation mechanisms that are not included here (antenna feed losses, depolarization, etc).
Table 1. V-band GEO Satellite Downlink Budget, Clear Sky Conditions
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] A. Paraboni, A. Vernucci, L. Zuliani, E. Colzi, and A. Martellucci, “A New Satellite Experiment in the Q/V Band for the Verification of Fade Countermeasures based on the Spatial Non-Uniformity of Attenuation,” European Conference on Antennas and Propagation (EuCAP), Nov. 2007.
[3] 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,” in Proc. 22nd International Symposium on Space Terahertz Technology, Tucson, AZ, April 2011.
[4] R. Acosta, J. Nessel, R. Simons, M. Zemba, J. Morse, and J. Budginer, “W/V-Band RF Propagation Experiment Design,” 18th Ka and Broadband Communication Conference, Ottawa, Canada, Sept 2012.
[5] N. R. Robbins, D. R. Dibb, W. L. Menninger, Z. Xiaoling, D. E. Lewis, “Space qualified, 75-Watt V-band helix TWTA,” IEEE Vacuum Electronics Conference (IVEC),pp.349-350, April 2012.
[6] S. C. Cripps, RF Power Amplifiers for Wireless Communications, 2nd ed. Boston: Artech House, 2006.