Literature Review of W-band missions

Prior to the year 2000, the W-band was used primarily for radio astronomy and radar applications. The DAta and Video Interactive Distribution (DAVID) mission was the first to use the W-band for telecommunications purposes.  This was followed by the WAVE (W band analysis and Verification) project. Both of these projects were initiated by the Italian Space Agency (ASI) in collaboration with University of Rome researchers. Experiments performed helped characterize the W band performance to transmit and receive data from a LEO orbit. This section describes the salient features of these projects and their findings as a background for our proposal.

DAVID

Overview of the mission:

The Data Collection Experiment (DCE) based on the DAVID’s LEO satellite involved the collection of data at a high data rate of 100 Mbps from two remote or virtually remote sites using 85.5 GHz in a time window of a few minutes per satellite pass.  This data is then forwarded it through an interorbit link (26GHz) to a GEO satellite ARTEMIS which transmits the data to an Earth station in Italy though a Ka-band link.  The DCE also has the provision for a downlink at 75.7 GHz from DAVID to the ground sites. The two remote sites are located in Spino’Adda in Italy and Antarctica [1].

Hardware implementation:

In terms of the hardware implementation (as shown in Figure 1), one antenna is used for both transmitting and receiving and a diplexer is used to separate the signals. The signal is then passed on to an LNA followed by two stages of down conversion, first to 12.5 GHz and then down to 2 GHz. The 2 GHz IF is chosen because satellite television operates in this frequency, so procuring off the shelf hardware is relatively easy. The down converted signal is demodulated and analyzed in an on board baseband processor which is capable of encoding, decoding, calculating BER and performing ARQ. The transmit procedure follows similar steps in the reverse order with a high power amplifier at the end as can be seen in Figure 1. The Tx and Rx front ends, modulation/demodulation, and baseband processing were identified as critical areas for further exploration in the following phases of the project [1].

Figure 1.  Hardware used in DAVID mission.

Experimental Setup and considerations:

Since DAVID is a LEO satellite, a long measurement time is not allowed as opposed to when a GEO satellite is used. This necessitates the use of lower elevation angles which have a greater attenuation. So, reliable fade mitigation techniques are necessary to use the link efficiently. To this end, adaptive rate convolutional coding with BPSK and Hybrid ARQ were used to allow for the efficient utilization of the W-band spectrum. The link capacity analysis consists of gases, fog, cloud, scintillation and rain and the statistics known for the 50 GHz range are extrapolated to the W band [1].

Beacon reception was used for attenuation analysis and error rate measurements are used to assess the quality of the link. The measurements collected are stored in the on board memory and then are processed by the on board processor. The use of radiometers was strongly recommended by this work to interpolate between successive measurements from the satellite and extending the measurements of the collected data. A total-power radiometer was used along with a double reference temperature to obtain an accurate estimate of the system noise [2].

Data collection and processing:

The Earth station collects the data, applies the convolutional code required to achieve the target BER, and divides the data into blocks. Md5 is then applied to each of these blocks before concatenating them and sending them over the link. The receiver on DAVID has a Mirror provider (MP) software architecture that divides the concatenated data back into blocks and then checksum hashes help in checking if the received data matches the transmitted data and hence one can get an idea of the success of the transmission [3].

Findings from DAVID:

As rain and cloud attenuations are the most significant contributors to the total attenuation, this was the focus of the study using DAVID. The excess attenuation due to rain at 84 GHz was found to be 3.61 dB at zenith and 8.55 dB at a 10-degree elevation.  It was also recommended that it is better to abandon experiments in the scenarios of convective rain (greater than 5mm/h) [2]. Further, total attenuation with varying elevation angle was also calculated using measurements from the earth station in Spino’Adda and are shown in Figure 2 [4].

 Figure 2.  Total Attenuation measurements at the Spino d’ Adda station.

It can be seen from Figure 2 that the attenuation levels are greater than 10 dB over 5% of the year. The key observations made are that obtaining a statistical behavior of the channel in terms of the average fade durations is needed to be able to set the appropriate coding techniques to counteract the attenuation effects. To this respect, this work used a statistical model developed for frequency ranges up to 50 GHz and extrapolated these to the required frequency [4].

WAVE

The WAVE (W band Analysis and VErification) mission was a second project funded by the Italian space agency to characterize the W band after the DAVID mission. Since it uses A GEO satellite, the mission is much closer to the design we are proposing to meet the BAA requirements. 

Overview of the mission: 

The WAVE mission involves deploying a satellite to the GEO orbit to perform the necessary studies of the propagation at W-band using an 81-86 GHz uplink and a 71-76 GHz downlink.  Two Earth stations are used for uplink here, one of them is a permanent Earth station located at Spino d’ Adda (previously used for DAVID) and the other is a mobile station that will provide the ability to take measurements at different sites around Rome. The downlink coverage provided is three fold (using three different antennas on board the satellite): first by the use of a beacon to cover a 2000 km area in Europe, second gives coverage of 700 km around Rome and the third is 200 km around the Spino d’Adda Earth station [5].

Hardware Implementation:

The hardware used in this mission involves both transparent and regenerative transponders.. The transparent transponder is used to evaluate the W-band for the data relay services while the regenerative transponder (of greater interest to us as it is used to perform the propagation experiments to characterize the W-band using BER calculations) [5]:

Figure 3. WAVE payload architecture.

The WAVE payload as shown in Figure 3 uses a regenerative transponder in which the received signal is filtered, demodulated and processed after which it is remodulated and sent back to the Earth station. The antennas used are smooth horns (aperture of 3 lambda) with the use of an OMT or a diplexer to separate vertical and horizontal polarizations. Key suggestions in terms of hardware are: (1) to use high power amplifiers like the already available gyrotronic Travelling Wave Tube technologies, (2) to use space qualified ADCs that can operate at high frequencies,  (3) to use phase-locked oscillators to maintain frequency stability, and (4) to use space qualified FPGAs to perform decoding, de-multiplexing and BER measurements [6].

Modulation and coding:

A QPSK modulation scheme is used in WAVE mainly because of its simplicity of analysis to obtain a good channel characterization despite its spectral inefficiency. Further, turbo codes are proposed for WAVE because of their efficiency in reaching the Shannon limit. However, the authors were concerned with the latency that might be introduced with using this kind of coding and hence various other forms of traditional coding like convolutional codes and block codes are also proposed to be investigated [6].

The receiving stations measure the power of the received signal and also the BER to evaluate the quality of the link. The use of radiometric equipment operating at the lower Ka band to calculate the 0 dB attenuation so as to calibrate the clear day channel attenuation and thus to maintain stability of the signal transmitted by the satellite beacon is suggested [5]. The data is collected over an extended period of time and stored. Statistical characterization of the W-Band in terms of link availability, fade margin etc are proposed to be evaluated along with the capacity analysis of the W-band.

Findings from WAVE:

A model for characterizing the behavior of the W-Band uplink and downlink communications link is presented by the WAVE Phase 1. Various aspects of the hardware are discussed in detail. In Phase 2, this mission divided into Aero-WAVE (uplink 94 GHz and downlink 92 GHz) which involves placing the WAVE payload on a Geophysica M55 in LEO and IKNOW (uplink 94 GHZ and downlink 84 GHz) which involves the use of a nano satellite also in LEO [7]. Results in terms of channel characterization are still limited to simulations like the one showed in Figure 2.

References:

[1] The DAVID mission of the Italian Space Agency (ASI- Agenzia Spaziale Italiana), IEEE Transactions on Aerospace and Electronic systems, October, 2002, Vol. 38, No. 4.

[2] A. De Luise, A. Paraboni and M. Ruggieri, “Satellite Communications in W- Band: Experimental setup for Channel Characterization,” IEEE Aerospace conference proceedings, 2004.

[3] C. Bonifazi, M. Ruggieri, M. Pratesi, A. Deluise and P. Crosetti, “Scientific operations of the W-Band Experiment during the DAVID Satellite Mission,” IEEE Aerospace Conference proceedings, 2002.

[4] S. De Fina, M. Ruggieri and A. V. Bosisio, “Exploitation of the W-band for High Capacity Satellite Communications,” IEEE Transactions on Aerospace and Electronic systems, January, 2003, Vol. 39, No. 1.

[5] A. Jebril, M. Lucente, M. Ruggieri and T. Rossi, “WAVE – A new satellite mission in W-Band,” IEEE Aerospace Conference, March 2005.

[6] Jebril et al., “The WAVE mission Payload,” IEEE Aerospace conference, 2005.

[7] A. Jebril, M. Lucente, T. Rossi, M. Ruggieri, V. Dainelli and L. Zuliani, “New Developments in the WAVE W band Mission,” IEEE Aerospace Conference, 2006.