Communication System Design
Chosing the Signal and Coding Basics
Binary Phase Shift Keying (BPSK), also known as pass-band Pulse Amplitude
Modulation (PAM), is a standard for deep space communications. It is not very
spectrally efficient, but it requires the lowest normalized power requirement (Eb/N0)
of any signal constellation supported by the Deep Space Network: 10.53dB. [1]
We then chose a Square Root Raised Cosine (SRRC) pulse shape with a roll-off
factor of 0.2 - this is a very spectrally efficient version of the SRRC that is
easily implementable in hardware: 0.89 bits/sec/Hz. [1]
Right Hand Circular Polarization was chosen to mitigate the effects of Faraday Rotation.
Baseband encoding was chosen to be NRZ-L: Non-Return to Zero Level, one of
several encodings supported by the DSN. [1]
Comparisons were made between coding gain and bandwidth requirements for
several Turbo Codes and the rate 1/2, block length 8920 Turbo Code provided the best
impact on the link budget. The (1/2, 8920) Turbo Code has a coding gain of 9.47dB.
Convolutional codes were considered, but all required too much normalized power.
Low Density Parity Check Codes are a viable improvement upon our selected Turbo Code, but
their implementation in the DSN is still years away in an uncertain budgetary environment. Turbo code
performance is shown in the below figure. [1]
Turbo Code Performance [1]
Data Rate Requirements
Based upon existing equipment and projections made in [2], a minimum of 23.5kbps of raw uncoded data is
required to make all the desired seismographic and atmospheric measurements. This translates to a
noise bandwidth of 50kHz and an occupied bandwidth of 60kHz.
Spectrum Management
Through international agreement, a comprehensive band and channel plan has been developped.
Based upon attuation levels determined in [3], S-band 2GHz is the preferred route. All
calculations have been completed for being notionally assigned Ch5 in S-band for uplink and downlink.
Lander Antenna Considerations
Antenna size is determined by a combination of allowable payload volume in the Delta-II rocket,
material strength of a Venus hardened dish, frequency band, and the ability of a Venus hardened
rotator to properly point the dish, especially considering the weakly understood Venusian climate.
A 3m dish was chosen that provides approximately 34dBi of transmit and receive gain. Despite this
very large size, three RTGs are still required to transmit out at 250Watts.
Uplink/Downlink Block Diagram
Downlink Block Diagram
Uplink Block Diagram
Noise
Noise was accounted for at the DSN receiver/transmitter during 90% weather cumulative
distributions and down as low as 5 degrees of elevation angle above the horizon.
The ambient temperature of Venus was assumed to be the noise temperature at the Low
Noise Receiver (LNA) on Venus. Noise temperature due to blackbody radiation from
Earth and Venus were approximated by using noise temperature calculations from [1] for
X-Band. The noise at S-Band will be much less.
Summary
Worst case scenarios were used in each instance. Maximum possible path loss through
the Venus atmosphere as measured in [3]. Greatest incidence angle through the
Venus atmosphere. Greatest possible path loss. This should result in a highly reliable
signal at the desired bit rate and bit error rate.
[1] Jet Propulsion Laboratory, Telecommunications and Mission Operations Directorate, "DSMS Telecommunications Link Design Handbook," Rev. E, 2000.
[2] Jet Propulsion Laboratory, "Mission Concept Study: Planetary Science Decadal Survey:
Mars Geophysical Network," California Institute of Technology, Jet Propulsion Laboratory, 2010, p. 56.
[3] J. M. Jenkins, "Variations in the 13 cm Opacity below the Main Cloud Layer
in the Atmosphere of Venus Inferred from Pioneer-Venus Radio Occultation Studies 1978-1987,"
Doctor of Philosophy in Electrical Engineering, Electrical Engineering, Georgia Institute of Technology, 1992.