W-Band Satellite Transmission in the WAVE Mission A. Jebril, M. Lucente, M. Ruggieri, T. Rossi University of Rome-Tor Vergata, Dept. of Electronic Engineering, Via del Politecnico 1, 00133 Rome - Italy Tel: +39 06 7259 7258; Fax: +39 06 7259 7455, e-mail: ahmed.jebril@uniroma2.it Abstract Satellite communication systems play an important role in the deployment of a global communication infrastructure, and the increasing demand for frequency bands with large bandwidth availability to satisfy satellite communications applications requirements makes the exploration of higher and higher frequency ranges of a vital importance. In this paper, an introduction to the WAVE (W-Band Analysis and VErification) mission is presented, in addition to a general overview of the payload architecture and the related perspective applications and services. WAVE is a new project proposed for the Italian Space Agency (ASI) to design and develop a W-band geostationary payload with the aim of performing the experimental studies of the W-band channel and possible utilization in satellite data communications. Keywords W-band, Satellite, WAVE, Communications, Mission, Data. 1. Introduction Research in Telecommunications systems has been oriented in the last few years towards the development of a global information infrastructure (GII) with the aim of transmitting information with high data-rate and offering broadband access to end users for a wide range of services. Satellite systems, hence, play an important role in the evolving integrated telecommunications architecture due to their geographical coverage and high throughput features. As frequencies around the 60 GHz range cannot be exploited effectively due to atmospheric absorption, the scientific community has expressed great interests towards W-band frequencies (75-110 GHz) in order to develop broadband satellite systems for scientific and commercial services. This was confirmed by the WARC2000 frequency allocations to satellite communications services in W-band. This allocation has suddenly transformed the experimental-only nature of W-band into a range where investments and planning for future commercial operations are strategic for future satellite communications applications. At present, only a few proposals of W-band missions were provided. Among these proposals, we mention the ASI mission named DAVID (DAta and Video Interactive Distribution), that envisages a Data Collection Experiment (DCE) to perform the validation and study of a W- band communication satellite system for data transmission considering a Low Earth Orbit
(LEO) satellite at about 570 km operating at 94.5 GHz in uplink and 83.9 GHz in downlink [1][2]. In addition, a quasi zenithal satellite communication system using W-band was also proposed in Japan [3] to cover Japan and part of Australia for VSAT-VSAT connections via three satellites, adopting frequencies in the 81-85 GHz range for uplink, and in the 71-74 GHz range for downlink. Moreover, the NASA Goddard Space Flight Centre (GSFC) has shown an interest in the exploitation of W-band in future interplanetary missions where an Integrated Interplanetary Network (IIN) is planned [4]. In particular, a telecommunications infrastructure around the Mars planet is foreseen in order to improve data transmission throughput. The ASI mission WAVE is considered as one of the most innovative missions in W-band. Its scope is to design and develop a scientific payload in W-band. During the experimental phase, the payload is expected to be embarked on-board a geostationary satellite platform, and will have the triple aim of: carrying out experimental measures and studies on signal propagation in the atmosphere at W-band frequencies; providing and testing the capacity of data-relay for preliminary commercial purposes; validating W-band technologies (and space-qualification processes) on flight. 2. The WAVE Coverage The large bandwidth availability in W-band will allow the development and distribution of new broadband services on a large scale with high quality of service levels. The W-band payload will provide three coverage beams, as shown in Fig. 1. Figure 1 - WAVE Coverage.
Beam A with a diameter of about 2000 km, is expected to cover a wide area of west and central Europe, in order to perform the propagation experiment measurements. An on board generated beacon signal will be transmitted towards different locations or sites within beam A coverage area to be analyzed at the receiving stations. Beam B is expected to have a diameter of about 700 km covering most of the Italian peninsula (semi-national coverage). The third beam, named C, is expected to have a diameter of about 200 km around the Spino D'Adda fixed station in order to carry out the experimental characterization of the W-band channel and the various experiments for data relay services between beams B and C [5]. Two different types of stations are proposed: a fixed station located at Spino D'Adda site and a transportable station, as shown in Fig. 2. The transportable station will offer the advantage of performing different measurements at different locations or sites. The implementation of the transportable station increases the complexity of the payload (antennas and other technological aspects related to HPA, LNA, etc.), but at the same time permits a particularly large collection of propagation measurements in various places and weather conditions. Furthermore, it could provide the basis for studies concerning future development of services in mobility, the definition of proper transmission strategies for high-rate communications and the design of suitable reconfigurable transceivers for reliable beam switching. The 81-86 GHz and 71-76 GHz will be the frequency ranges for uplink and downlink, respectively. Figure 2 - Frequency range for WAVE mission.
3. Link Budget After defining the preliminary outlines of the study mission, the following link budget of the GEO-Earth link is proposed according to Table 1. Table 1. Summary of the Earth-GEO link data rate Link Data Bit-Rate (Mbps) Permanent-Transportable station 22.9 Transportable-Permanent station 3.3 Propagation Experiment 7.2 TX section RF power 30W Output losses 2dB 0.4m (typical value) Antenna Diameter 0.9m (GEO Feeder Link spot areas) 0.25m (GEO Feeder Link Wide areas) Pointing error 0.05 Antenna beam width 1.04 Antenna efficiency 45% RX section Input losses 2dB Receiver NF 3dB Antenna temperature 270K 0.4m (typical value) Antenna Diameter 0.9m (GEO Feeder Link spot areas) 0.2m (GEO Feeder Link Wide areas) Pointing error 0.05 Antenna beamwidth 1.18 Antenna efficiency 45% 4. Acquisition of Measurements The receiving stations must be capable of measuring both the power of the sinusoidal signals and the bit error rate (BER) in order to evaluate the quality of the transmission. To measure the power of the sinusoidal signals, the satellite must send a stable enough beacon signal. With the proposed configuration, it is appropriate to use also radiometric equipment, operating at lower frequencies - typically in Ka band - in order to perform the so-called "bias removal", specifically to estimate the "zero db" attenuation level [6]. This measurement technique, derived and validated in previous propagation experiments (Olympus, Italsat) [7] allows the calibration of the channel power, considering the clear-air attenuation due to presence of gases in atmosphere, which also affects the signal transmitted by beacon.
The BER measurement must take place, in presence of a modulated signal that will allow evaluating the effect of the tropospheric path length on the transmitted signal if carried out in different locations at different elevation angles. Also in this case evaluations carried out during phases A and B of DAVID project are taken into account [5]. The combined measurements of the power level and BER allows separation of two effects that arise together, namely the additional attenuation (due to rain, clouds, etc...) and the noise (caused by the electronic components at both ends) [6]. 5. The Payload The WAVE payload is composed of two types of transponders, according to the mission definition and requirements: a transparent and a regenerative one. The first is designed to perform the telecommunication experiment and the second is used to carry out the channel characterization. The transparent transponder will work on a variety of selectable channel bands used for transmission on beams B and C to carry out the channel experiments for variable-capacity data relay services and different signal patterns. The propagation experiment will be carried out through beam A, where a W-band beacon will be used for downlink channel characterization. The regenerative transponder will provide the propagation experiments for uplink channel through the beams B and C; moreover, it will carry out a characterization of the channel Bit Error Rate (BER). This is done through an on board processing of the uplink-transmitted signals; the measurements are then sent back to Earth using the beacon. A simple scheme of the WAVE payload, that includes redundancies, is shown in Fig. 3. At the reception section, the front-end should be implemented in an integrated antenna system, to achieve compliance with high requirements of G/T and to reduce the high wave-guide losses in W-band. The transmission front-end is based on HPA technology to accomplish the high power requirements that could not be achieved using solid-state devices. Figure 3 - Basic Architecture of the WAVE Payload.
From the preliminary analysis of the RF payload performances, it results that the compliance with EIRP requirements can be obtained using a 25 W HPA. The WAVE payload will be placed on board a host platform dedicated to other mission into a GEO orbit so the mass, size, and power requirements are reduced to achieve this goal. From the first analysis, the maximum expected DC power requirements is 300 W and the mass is about 30 kg. The characterization of the uplink channel will be performed sending known sequences through the uplink channel; then the uplink signals is captured through a coupler and demodulated with a variable data rate (2-10 Mbps). The BER is then calculated by comparing the sequences with a standard sequence stored on board and the measurements are thereafter retransmitted to Earth using the beacon (Beam A). The required specifications of the antennas are shown in Table 2. Coverage Beam Center Table 2 - Specifications of the antennas Spot 9.29 E, 45.23 N (Spino d'adda) Wide (Seminational) 10.50 E, 43.46 N European 10.50 E, 48.88 N Beam Width 200 km 700 km 2000 km Beam Width from Satellite 0.3 x 0.3 0.68 x 1.05 1.64 x 3.00 Area 0.07 (0.043 ) 0.56 3.86 Receiving Band 81-86 GHz 81-86 GHz N. A. Transmitting Band 71-76 GHz 71-76 GHz 71-76 GHz Rx Gain > 52.9 dbi > 43.1 dbi N. A. Tx Gain > 51.9 dbi > 42.1 dbi > 33 dbi The on board section allows a large flexibility of configuration; the idea is to identify a hardware platform where it is possible to program various types of processing architectures. It is possible to assume modulation techniques that have great robustness, rather than spectral efficiency, to obtain a good channel characterization. For the propagation experiment, the data should be uncoded to verify channel performances but at the same time permits to add different coding schemes to study the reaction to errors produced by the W-band satellite channel.
6. Future Services As discussed above, a new perspective was born with the recent frequency allocations for satellite communication services issued by the World Administrative Radio Conference 2000 (WARC2000). The experimental nature of W-band turns into a range where investments on key-technologies and knowledge-achievements are strategic for future commercial operations related to communication services. In fact, the increasing demand of large bandwidth availability, that is requested for advanced multimedia applications, data-oriented services, high-volume data transfers and in general broadband services, needs the exploitation of new higher frequencies ranges; as frequencies around the 60 GHz range cannot be used effectively, due to oxygen atmospheric absorption, W-band could be considered as the new frontier of satellite communications in order to support those services. Moreover, W-band telecommunication services could be prospected in one or more of the following classes: Global Direct Broadcast Satellite (DBS) Fixed Satellite Services (FSS) Digital Audio Broadcast Satellite (DABS) Very Small Aperture Terminal (VSAT) networks Broadband Data Services Global Positioning Systems Mobile Satellite Services These services could be directed towards a number of specific applications and end-users such as: Private users Business users Public users Scientific users 7. Conclusion W-band with its broad bandwidth is a promising new frequency range for high data rate transmission via satellite. The advanced technologies in the satellite system design can be considered as the key towards the realization of these systems at elevated frequencies such as the W-band. Therefore, it is important to study the signal behavior and the channel propagation effects at this band in order to evaluate its use for future applications of satellite communications and services. WAVE mission may represent an important step towards the knowledge and the exploitation of the challenging W-band channel. The obtained experiment results will provide a comprehensive view of the different atmospheric effects in W-band on both the uplink and downlink at each terminal of the proposed Earth stations Spino D Adda and Rome.
In this paper, the W-band WAVE project has been introduced and an overview of the proposed payload design has been provided concerning the important aspects towards an optimum performance. Moreover, a new development-line has been outlined for the next forthcoming WAVE A2 phase. 8. Acknowledgement The work was performed in the frame of the ASI contract number (I/032/2003/0), that is gratefully acknowledged. 9. References Bonifazi C., Ruggieri M., Paraboni A., (2002), The DAVID Mission in the Heritage of the SIRIO and ITALSAT Satellites, IEEE Transactions on Aerospace and Electronic Systems, vol. 38, no. 4, pp. 1371-1376. M. Ruggieri, S. De Fina, M. Pratesi, A. Salomè, E. Saggese, C. Bonifazi, (2002), The W- band Data Collection Experiment of the DAVID Mission, IEEE Transactions on Aerospace and Electronic Systems, vol. 38, no. 4, pp. 1377-1387. Takahashi T., Tanaka M., Wakana H., The quasi zenithal satellite communication system using the W-band. White Paper on Integrated Interplanetary Network (IIN) : http://nmsp.gsfc.nasa.gov/news/iin.htm Ruggeri M., De Fina S., Bosisio A. V., (2003), Exploitation of the W-band for High Capacity Satellite Communication, IEEE Transactions on Aerospace and Electronic Systems, Vol. 39, No.1. Paraboni A., Testing of rain attenuation prediction methods against the measured contained in the ITU-R data bank, ITU-R Study Group 3 Document, SR2-95/6, Geneva, Switzerland, 1995. Polonio R., Riva C., (1998) ITALSAT Propagation Experiment at 18.7, 39.6, and 49.5 GHz at Spino D Adda: Three years of CPA Statistics, IEEE Trans. Antennas and Propagation, Vol. 46, No. 5.