ELEKTRO INDONESIA                  Edisi ke Lima, Desember 1996
TELEKOMUNIKASI

ASIA Sky-Link : A Wideband Satellite Network for Multimedia information Service amang Asia Pacific Region

By Arifin Nugroho and Tonda Priyanto

Abstract :
The diversity in telecommunications state amongst Asia Pacific countries represent one stumbling block for making the ensemble of the region pacing the same and harmonious economic growth. The less developed nations, represent a vast majority in the region, lamented by its backwardness in catching up with its neighbours in achieving the better infrastructure. Yet, pondering its own national telecommunications, despite huge efforts involved, is not enough in view of global economy to be in reality. It is of regional responsibility for making seamless communications for each citizen therein across any network service irrespective where he or she might be located in the region (or later in globe)

This paper will outline a preliminary design concept of wideband satellite network, called ASIA Sky-Link, designated for delivering both wideband access and gigabit digital pipes applicable for the Asia-Pacific region. The concept is not envisioned to substitute for terrestrial based wideband network within a NII/GII context, rather it will take account for urgent needs in the developing country for wideband access which can hardly be fulfilled by terrestrial system in view of such a large and sometime harsh areas to be covered. Gigabit networking among the major gateway nodes, on the other hand , shall call for major investments in the terrestrial broadband networking. It is of authors conjecture that the situation can in significant part be remedied by the appropriate broadband satellite technology during the first decade of the next century. It is however inevitable to make use of Ka-band or higher frequency band, in view of wider spectrum required.

Market drivers for satellite based broadband networks

In the midst of global telecommunication infrastructure development, countries, especially those in the Pacific Rim, are to catch its proper National Information Infrastructure (NII) . NII is envisioned as a seamless maze of networks composed of different transmission media such as satellites, fiber optic cable, copper wire, coaxial cable and radio that will carry a broad range of telecommunications and information services and information technology applications into homes, business, schools, and hospital [Zuzek]. In general, however, developing countries have difficulties in providing telecommunications infrastructure suitable for NII at the same pace with the more developed countries, due to lack of funding resources. Their existing facilities are somewhat antiquated and incompatible with new standards. On the other hand, the present satellite technology could help moderately in providing partially infrastructures to such countries[Nugroho]. In fact it has inherent capability in reaching the whole national boundary quite easily and fastly within its coverage. But it has also a transparent transponder to accommodate any kind of protocol standards directly from the end user. This falls to the access capability of satellite communications to do networking, interconnecting dispersed nodes. Not necessarily that these nodes are to be at the remote or rural region, it can also those residing in the business area, and therefore require VSAT as terminal for services other than merely telephony : ISDN, multi-media, internet etc. In fact, VSAT or USAT can represent as a massive access system in the developing countries. Unfortunately that the past satellite technology can only bring relatively high cost access facilities, especially broadband terminal (VSAT), at least for citizen in the developing region. On the other hand, different broadband satellite communications are being planned and implemented [[Losquadro], [Elizondo], [Fitzpatrick], [Kadowaki] ….], to respond to various needs of access networks, at affordable costs, spanning from lower to medium and high bit rates. Also being developed gigabit satellite systems in view of gigabit rate requirement in the course of implementing the GII.
This paper will outline a wideband satellite system applicable in the Asia Pacific Region, operating t the Ka band. The objectives of such an undertaking are : It is hoped that this initiative can be of value in guiding further collaborative and concerted works in the region.

System architecture

The most challenging task in designing any satellite communications how efficient would be the use of spectrum. It is of our purpose in this exercise to behold a frequency reuse factor high enough so as to increase the satellite capacity to the point whereby satellite access can be at an affordable price for the user. The approach being exercised is as follows:
  1. multiple-spot beam
  2. repeated frequency reuse for uplink and downlink
  3. orthogonal linear polarisation
  4. for areas with less traffic density, beam hoping will be applied, otherwise fixed beam will be in use
  5. major trunking channel having enough beam separation will use the same frequency spectrum, at smaller size beams.
The potential user of the service will be dispersed and distributed in accordance with certain density distribution. Knowing this function, we could the appropriately design the satellite multi spot antenna beam and its associated link and traffic parameter. For areas with the less number in traffic density, same antenna beam size will be applied, but in view of improving the spectrum efficiency, transponder hoping will be applied to these clusters of beams. There will be three possibilites : one active transponder, will be switched periodically between one, two or four beams. The time dwelling allocation will be dictated by diurnal traffic variation, such that any beam can given 25, 50 or 100% of transponder capacity. This will allow, in average less bandwidth, and therefore transponders, to be shared among those areas. Special Narrow Beams will be designed for (smaller size) areas with higher economic activities, thereby higher traffic requirement, such as metropolitans areas, to account for its higher data rate needs. Here, while transponder hopping scheme will apply, each of HDR beam will represent 2xSTM-1, STM-4 or STM-8 rate link. Antenna coverage along with frequency reuse will be depicted in figure 1.

Terminals Technology assessment

ASIA Sky-Link will differentiate the following type of terminals:
  1. High Data Rate Terminal: Which serves as digital pipes bridges amongst major economic centres in a meshed network basis, at 1.244 Gbps rates
  2. Medium Data Rate Terminal: Which will cope for multi media communications; typical DirectPC terminal as well as Direct TV terminal is conceivable at maximum of 155 Mbps rates.
  3. Low Data Rate Terminal: Mainly meant for voice, facsimile and data at 2 to 12 Mbps
  4. Brief Case Terminal; which will be a single channel BRA ISDN portable (mobile) terminal at 64 to 2 Mbps.
Physical network architecture can be seen in figure 2.
Each of the smaller terminal (TIT and LDRT) can be principle, be connected one another by an on-board processing unit, subject to some regulatory aspect of networks, subject to resources management carrier out by Network Operation Centre (NOC). By doing this, double hopping can be avoided. For example, a call originating from a terminal to be forwarded to another terminal at different administrative area, will be legal as long as regulations permits. LDRT, MDRT as well as HDRT will also be capable of gateway function, as an interface to the PSTN, or any other network. Each of these terminals will also be capable of other functions such as Remote Concentrator to a PSTN, Bridge or Router to a LAN/MAN, or ATM/SDH terminal access. The most part of the architecture revealing the highest complexity would be the ground segment architecture. This segment will address among others the following functions: In view of multitude of networks upon which the Asia-Link will be interconnected with, this interface problem will represent the utmost challenge ever. Intensive co-ordination works shall be of prominence for such an undertaking.
The available frequency for Ka band is about 2.4 GHz but this frequency band should be shared with the mobile or fixed communication where the regulation is differ from each country. Therefore, the available band from each country could be different. For example in Japan, the available band for Ka band GSO satellite is only 750 MHz[Kadowaki]. In this paper we assume that 1.5 GHz band is available with the worst case is 750 MHz..
In view of this spectrum limitation, the transmission of 1.244 Gb/s bit rate will require an appropriate modulation type along with the flexible frequency reuse technique to optimise the use of the spectrum. The design for the High Data Rate will based on a 16 QAM with coding rate at 7/8 using pragmatic trellis code and Reed Solomon. The modem of this kind is already in the market with a maximum bit rate of 44.4Mb/s, and we hope within 5 years this type of modem will be available for higher bit rate.
The test results revealed a minimum Eb/No of 10.3 dB is required for BER of less than 10-9[Stewart]. The 1.244 Gb/s will be transmitted by using 16 QAM with 7/8 coding rate which needs about 350 MHz bandwidth. Therefore, the 750 band may be divided into two(2) transponder of 375 MHz bandwidth each polarization, hence there will be a total of 8 transponder in 1.5 GHz bandwidth, for one spot beam only.
Presuming that there will be 10 areas having enough spatial separation to be bridge by 1.244 Gb/s, a total of 80 transponder at 1.244 Gb/s will be available for 1.5 GHz spectrum. This present scheme however will limit the use of the same spectrum by MDR and LDR as well as TIT bigger beam in the vicinity of the HDR spot beams. To alleviate the problem, spectrum sharing will be inevitable between HDR and other rate spot beams, which will reduce the total transponder available dedicated for STM-8 operation to some 75% of the capacity, leading to a capacity 60 transponder available for STM-8. Table 1 is ASIA Sky-Link typical ground station network performance.
MDRT and LDRT will serve for bandwidth on demand terminal, transmitting from 12 Mbps up to 155 Mbps and from 2 to 12 Mbps respectively, whereas the TIT will transmit in bandwidth on demand basis from 64 kbps to 2 Mbps. QPSK will be applied for MDRT, while BPSK modulation will be designed for both LDRT and TIT.
For the wide band satellite link to be an adequate alternative for trunk connections, ATM/SDH protocol shall be incorporated. Adoption of a satellite link for conveying an ATM protocol would call for different criteria to be met. Those which are considered of importance are Network Performance Objectives [Gedney1] which are under discussion within ITU. Two ratios tend to predominate the objectives, first the so called Cell Loss Ratio (CLR) and secondly Cell Error Ratio (CER). The ITU-R is about to approve a CLR of 3x10-6 and CER of 4x10-6as performance objectives. Measurements made by NASA Lewis Research Centre has shown that a satellite link of 45 Mbps ATM with BER of 1.2 x 10-7has shown in agreement with the CLR (burst) criteria but not CER (random). But the report has also suggested that reed Solomon block code can readily achieve a nominal BER of < 10-11, as has been successfully demonstrated by ACT High Data Rate experiments for an ATM OC-3 (155 Mbps), and therefore the solution could palliate the CER problem.

The payload

Assuming that the HDR antenna gain can be designed at approximately 55 dBi and the EIRP is 69 dBW, a link budget calculation suggest an output power of 50 Watts per beam per carrier at 375 MHz each. A MDR/LDR antenna is designed for 49 dBi at the same EIRP 69 dBW, the output power would be 200 Watts per transponder of 125 MHz. Having 24 HDRT transponder and 24 MDRT/LDRT transponder on board, last figure translates to a total DC power requirements at the end of life of about 20 kWatts, at efficiency of 40%. Single aperture antenna system (each for transmit and receive ) will be utilised to permit a larger overall aperture than is possible with multiple aperture, in view of such big area to cover and obvious physical constraint that spacecraft can offer to accommodate such a complex payload. This assessment follows [Garland]. There will be a total of 10 (pencil) beam to ~ 10 Asia/Australia covering major economic centres at approximately 0.3 degree width each (pencil) beam and 84, 0.55 degree (spot) beam in operation to cover 5 regions of Australia, India/Thailand/Indochina, Indonesia/ Papua New Guinea / The Philippines, China and Korea/Taiwan/ Japan.
The onboard processing system will consist of two unit. One will be devoted to process the HDR signals, the other will cope for the rest. Either HDR-OBP (On borad Processing) or MDR-OBP system will be of ‘standard’ type, consisting of multicarrier demultiplexer demodulation and decoder. Recombining function and ATM switching function shall also be incorporated in these OBP, at an aggregate of more than 52 Gbps and to be distributed into a) 29.8 Gbps HDR OBP and b) 22.4 Gbps MDR/LBR-OBP .

Rain induced degradation considerations

ASIA Sky Link will utilize Ka band throughout the region spanning from 60 degree S to 60 degree north. Such vast areas will represent extremely severe propagation variation, from rain intense tropical condition, to low slant angle range subtropical range. Whereas rain attenuation has been subject to such extensive measurement campaigns therefore making its prediction quite tractable, scintillation is another major source of error [Gremont] and thus can not be neglected. For rain attenuation[Gedney2] has analysed that ITU-R model has been showing convergence with measurement better than other model (that of Crane and of Dissanayake). These last two models tend to be over estimated. Although [Gedney2] does not recommend any model to work with, it is of importance for designer to refer to certain model. Authors would like to suggest that the ITU-model is an appropriate reference for statistical model of rain prediction.
Following Crane model, however, the rain attenuation considered in the link budget allows the availability of 99.5% for station at 30 latitude and 55 degrees elevation ( attenuation will be lower than 12 dB for that percentage of time). Station at 15 deg. latitude and 72 elevation will have a 99% availability and attenuation will be lower than 11 dB for that percentage of time, whereas a station at the equator and 90 elevation will have an availability of 98.5% with attenuation of less than 10 dB.
The above analysis demonstrates lack of homogeneity in the availability budget of services, at the detriment of those residing in tropical region. Such a difficulty can be overcome by using permanent fading margin associated at the link involving the tropical region’s terminal, but then such an approach will tend to squander satellite power unwisely. Other approaches would be : This represent the a challenge to satellite communications designer for Ka band system to be successful.

Conclusion

The design of a wideband satellite communication system, called ASIA Sky-link applicable for enhancing the wideband communications capability is outlined. The purpose of such a system would be both delivering wideband digital access to Asia Pacific Region countries, - which are predominated by developing nations -, and for bridging the major economic centres in the region with High Data Rate satellite based links.
Technological challenges are still open among others: adaptive controlled VSAT / USAT and satellite payload to combat rain induced attenuation impairments, high speed m-ary-QAM modems, satellite resources traffic optimum on board control.
The satellite platform shall be capable in delivering more than 20 KW DC power. Higher level of heat dissipation resulting from the such a high degree of power involved shall be taken into careful design. Along with this is the problem of beam accuracy resulting from thermal distortion which can affect the HDR performance. Lastly, total network architecture associated with terrestrial network interoperation will always pose an extremely difficult and tedious tasks.
The concept will eventually be extended to other regions as well, which shall demand for the need of Inter Satellite Links.
The concept of ASIA Sky-link is a proprietary of PT Telekomunikasi Indonesia.

References

By Arifin Nugroho and Tonda Priyanto
Satellite Technology & Planning Division
PT Telekomunikasi Indonesia (PT TELKOM)
Jl Japati no 1 Bandung, Indonesia 40133
Phone ++62-22-700051, Fax, ++62-22-7101787,
e-mail : arifin_nugroho@telkom.net.id

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