Alessandra Babuscia

CDMA communications systems with constant envelope modulation for CubeSats

In this paper a communication system for CubeSats in formation to operate in the vicinity of the Lunar Lagrangian L1 is proposed. CubeSats will collect lunar scientific data and will perform surface observations. An improved low complexity CDMA system for CubeSats for communications between the Lunar L1 and Earth station is considered. The complexity of a coded CDMA transmitter is lower than the complexity of the CDMA receiver with decoder therefore for downlink communications it makes sense to use encoders such as space standard LDPC code followed by a spread spectrum transmitter for CDMA systems for CubeSats. For the uplink an uncoded CDMA system is chosen since the uplink transmit power is expected to be high enough to support the use of uncoded CDMA system. The uncoded CDMA yields receivers for CubeSats that have low complexity implementation. For uplink since there would be no multipath the use of orthogonal spreading codes is more appropriate. The choice of orthogonal codes would reduce the multiuser interference at CubeSats. For the downlink, based on the available bandwidth, and the data rates, a reasonable processing gain could be obtained. Thus the multiuser interference degradation due to the other CubeSats could be made small at the Earth station. In this paper we analyzed and simulated the proposed improved CDMA system for a concept Constellation of CubeSats. All system simulations are done using Simulink Matlab platform. For highly efficient nonlinear power amplifiers we use a filtered offset QPSK with phase modulation which is a CCSDS standard for constant envelope signaling. This allows a nonlinear amplifier at CubeSat to operate at saturation point for the highest efficiency. We compare the difference in performance with our current pulse shaped BPSK (using half-sine wave). Certainly if there is no bandwidth limitation (due to the spectral standard masking) we could as well use unfiltered rectangular pulses which would produce constant envelope signaling. But it seems that rectangular pulses will not satisfy the bandwidth limitation imposed by the spectral standard. Filtered offset QPSK with phase modulation is much more bandwidth efficient scheme.

Inflatable Antenna for CubeSat: Extension of the Previously Developed S-Band Design to the X-Band

The inflatable antenna for CubeSat is a 1 meter antenna reflector designed with one side reflective Mylar, another side clear Mylar with a patch antenna at the focus. The development of this technology responds to the increasing need for more capable communication systems to allow CubeSats to operate autonomously in interplanetary missions. An initial version of the antenna for the S-Band was developed and tested in both anechoic chamber and vacuum chamber. Recent developments in transceivers and amplifiers for CubeSat at X-band motivated the extension from the S-Band to the X-Band. This paper describes the process of extending the design of the antenna to the X-Band focusing on patch antenna redesign, new manufacturing challenges and initial results of experimental tests.

Code division multiple access communications systems for CubeSats at Lunar Lagrangian L1

Interplanetary Cubesats would enable low-cost missions for high-quality scientific and exploration programs. In particular cubeSats in formation have been proposed to operate in the vicinity of the Lunar Lagrangian L1 to collect lunar scientific data and to perform surface observation. In this paper we present a low complexity CDMA system for CubeSats (M small spacecraft) for communications between the Lunar L1 and Earth station. It is well known that the complexity of a CDMA transmitter is much lower than the complexity of the CDMA receiver. Moreover, the complexity of a channel encoder is always much lower than the complexity of the channel decoder. So for downlink communications it makes sense to use encoders for modern codes such as Turbo and LDPC followed by a spread spectrum transmitter for CDMA systems for CubeSats. Here we used an LDPC coded CDMA with BPSK modulation with rectangular and half-sine pulse shaping. Except for the PN generator seed numbers, the communication structure of all CubeSats would be identical and operating at one single RF frequency. For the uplink we may choose an uncoded CDMA system since the uplink transmit power is expected to be high enough to support the use of uncoded CDMA system. In addition since there would be no multipath for the uplink (broadcast channel) the use of orthogonal spreading codes such as Walsh codes is appropriate. The choice of orthogonal codes would reduce the multiuser interference. However due to some limitation (bandwidth, data rates, and M) we may be forced to use nonorthogonal PN codes. In addition, one of the spreading codes will not carry any data, which acts as an unmodulated pilot to reduce the complexity of synchronization. The proposed uncoded CDMA yields receivers for CubeSats that have low complexity implementation. Each component of CubeSats could easily extract its own received data with almost no interference from other users in case of orthogonal spreading codes. For the downlink, depending on the available bandwidth, and the data rates, a large processing gain could be obtained if the N is not large. Thus the multiuser interference degradation due to the other CubeSats could be made small at the Earth station. If N is large, and the bandwidth and data rates do not allow large processing gains then the multiuser interference could be high. In such cases we could use a simple parallel interference cancellation method with two stages that dramatically improves the system performance for the downlink. In this paper we accurately analyzed and simulated the proposed CDMA system for a concept Constellation of 20 CubeSats (M=20). All system simulations are done using Simulink platform.

CDMA communication system for mars areostationary relay satellite

Stationary orbits around Mars, which are known as areostationary orbits, have similar characteristics as Earths geostationary orbits. An areostationary Relay Satellite is expected to be used for future communication network for the exploration of Mars. An areostationary Relay Satellite at 17,000 km above Mars surface is always in the same place in the sky on Mars, receiving data from the rovers/landers and low Mars orbiters and transmitting it to an Earth station. The areostationary satellite enables almost all time communication, except for certain durations when Mars is in between the areostationary Relay Satellite and the Earth. In this paper we define a code division multiple access (CDMA) communication system between various assets such as rovers/landers on Mars surface, low Mars orbiters such as MRO, CubeSats in vicinity of Mars, and an areostationary relay satellite at 17,000 km above Mars surface. We assume data rates between 50 Kbps and 1 Mbps operating at 8.40 GHz frequency. We define the transmit power, antenna size, and data rates for the assets communicating with the areostationary relay satellite. Since the areostationary relay satellite simultaneously receives data from many users we have chosen the code division multiple access (CDMA) as a multiple access scheme, which does not require time/frequency coordination among users. Further-more we compute the link budgets for various communication links, we define the modulation format, the PN spreading and chip rates. We provide a new theoretical analysis for multiuser interference, and simulate the performance of coded CDMA communication system. For coded system we use standard CCSDS LDPC codes proposed by NASA for space applications. Using CDMA, the communication structure of all assets on Mars operates at one single RF frequency. However the initial states of spreading PN codes assigned to users are different such that if all users communicate simultaneously with the areostationary relay satellite over a given band will have low cross correlation that reduces the multiuser interference.

Telecommunication system design for interplanetary CubeSat missions: LunaH-Map

The1 Lunar Hydrogen Mapper (LunaH-Map) will be one of 13 CubeSats to launch on the first integrated flight of NASAs Space Launch System and Orion spacecraft in 2018. The goal of the LunaH-Map mission is to map the hydrogen content of the entire South Pole of the moon, including permanently shadowed regions at high resolution. The spacecraft is a 6 U CubeSat and the main instrument is a neutron spectrometer which will be used to perform the measurements required to accomplish the primary science objective. Once LunaH-Map reaches the Moon, the spacecraft will perform a 60-day science mission, consisting of 141 science orbits during which the measurements will be taken. In the design of the LunaH-Map spacecraft, one of the main challenges is the telecommunication system as the spacecraft needs the ability to relay data during the long cruise phase and during the science phase. In addition, the telecommunication system needs to support the main navigation functions required to reach the Moon and to insert in the highly elliptical orbit required for the science phase. This paper covers the main aspects of the telecommunication design for the mission including: link and coverage analysis, waveform selection and spectral constraints, hardware selection and ground station coordination.

An experimental platform for multi-spacecraft phase-array communications

The emergence of small satellites and CubeSats for interplanetary exploration will mean hundreds if not thousands of spacecraft exploring every corner of the solar-system. Current methods for communication and tracking of deep space probes use ground based systems such as the Deep Space Network (DSN). However, the increased communication demand will require radically new methods to ease communication congestion. Networks of communication relay satellites located at strategic locations such as geostationary orbit and Lagrange points are potential solutions. Instead of one large communication relay satellite, we could have scores of small satellites that utilize phase arrays to effectively operate as one large satellite. Excess payload capacity on rockets can be used to warehouse more small satellites in the communication network. The advantage of this network is that even if one or a few of the satellites are damaged or destroyed, the network still operates but with degraded performance. The satellite network would operate in a distributed architecture and some satellites maybe dynamically repurposed to split and communicate with multiple targets at once. The potential for this alternate communication architecture is significant, but this requires development of satellite formation flying and networking technologies. Our research has found neural-network control approaches such as the Artificial Neural Tissue can be effectively used to control multirobot/multi-spacecraft systems and can produce human competitive controllers. We have been developing a laboratory experiment platform called Athena to develop critical spacecraft control algorithms and cognitive communication methods. We briefly report on the development of the platform and our plans to gain insight into communication phase arrays for space.

CDMA communication system performance for a constellation of CubeSats around the Moon

In this paper a communication system for CubeSats in formation that operate in the vicinity of the Moon is proposed. A CDMA system for the fleet of CubeSats in the vicinity of the moon to communicate with the Earth station is considered. This is an extension of our previous proposed CDMA system for a concept Constellation of CubeSats. In this paper the Doppler effects on CDMA communication system performance for a constellation of CubeSats around the Moon will be investigated. As an example we have estimated the maximum Doppler and Doppler rate profile of a SOLARA/SARA CubeSat scenario. We investigate the Effects of Doppler Shift/Rate on the CDMA system performance as a result of the CubeSat constellation orbiting in a halo orbit around Earth-Moon Lagrange Point L1. A detailed analysis and simulation of system in presence of Doppler frequency and unknown carrier phase will be performed. First we define the CDMA system for uplink and downlink between the vicinity of the Moon and Earth station. Link budgets will be provided both for uplink and downlink. Bandwidth limitations imposed by the spectral standards will be investigated for modulation formats. All system simulations are done using Simulink Matlab platform. For highly efficient nonlinear power amplifiers, we prefer to use a filtered offset QPSK with phase modulation, which is a CCSDS standard for constant envelope signaling. This allows us to use a nonlinear amplifier at CubeSat to operate at saturation point for the highest efficiency. Filtered offset QPSK with phase modulation is much more bandwidth efficient scheme. We demonstrate that this modulation format satisfies the two international spectral standards. For estimated and specified Doppler frequencies and Doppler rates we design frequency-tracking loops to track the Doppler frequency and Doppler rate in the presence of data with filtered offset QPSK with phase modulation. For carrier phase offsets a well-designed tracking loop is derived with specified loop bandwidth for the same modulation format. We use the CCSDS standard LDPC codes for space applications to meet the link budget margins.