1 Introduction

Mobile satellite communications were pre-viously restricted to vehicle-borne and othertypes of mobile stations, due to limited satel-lite capabilities. Handheld terminals—such asthe cell phones used in ground-based wirelesssystems—were thus unavailable. However,with the recent commercialization of the Iridi-um system (using LEO satellites) and the Thu-raya system (using geostationary satellites),the use of such handheld terminals is now areality.

In Japan, the government has led an initia-tive to develop a similar mobile satellite com-munications system using geostationary satel-lites and handheld terminals [1]. At the heartof this system is a novel onboard voice com-munications switch, to be installed aboard theEngineering Test Satellite VIII (ETS-VIII) [2].This switch, featuring a self-controlled switch-ing function and a regenerative transponder,involves the adoption of a circuit primarily

designed to process digital signals, referred toas the “on-board processor,” or “OBP.”

This paper will describe the applicablespecifications, structure, communication con-trol protocol, and performance test results.

2 Outline of the mobile satellitevoice communications system

Because any satellite enabling the use ofhandheld terminals must offer advanced capa-bilities, the ETS-VIII will feature an onboardS-band large deployable reflector 13 meters indiameter. Meanwhile, the OBP will reducethe burden on handheld terminals and groundbase stations by incorporating regenerationand self-controlled switching functions.

Fig.1 is a schematic illustration of themobile voice communications system usingthe ETS-VIII. The 20/30- GHz feeder link isa channel for connection to the ground basestation. The base station connects handheldterminals to the public network and controls

HASHIMOTO Yukio 85

3-7 The On-Board Processor for a VoiceCommunication Switching

HASHIMOTO Yukio

We developed the on-board processor (OBP) used for voice communication switching ofmobile satellite communication systems. It uses multi-carrier time division multiple accessto support high-capacity voice communication systems. Most functions (filtering, switching,carrier composition, and regeneration) are performed by digital signal processing; ASICtechnology is used to reduce device size and power consumption. The switching function iscontrolled autonomously by software (call management, user management, and OBP man-agement, as well as by using statistical and logistical data). We tested the interface func-tions of the OBP after it was installed on engineering test satellite VIII (ETS-VIII) and deter-mined that the OBP proto-flight performance test results met the system requirements.

Keywords Satellite communication, Regenerative transponder, On-board processing, Digital sig-nal processing, Hand-held Terminal

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the channel capacity for each communicationarea, to complement the self-controlledswitching function. The 2.5/2.6- GHz servicelink is the channel for handheld terminals, andthe ETS-VIII features three dedicated beamsfor this purpose.

The channel connecting handheld termi-nals to the public network features a forwardlink from the ground base station to a hand-held terminal and a return link from a hand-held terminal to the ground base station. Across link for regenerative repeating is used incommunication between handheld terminals.

3 Structure and main specifica-tions

The OBP consists of a forward linkprocessor that handles forward link signals, areturn link processor that handles return linksignals, a cross link processor that conductsregenerative cross link repeating and alsomodulates/demodulates communication con-trol signals, and two control processors thatcontrol communication and the OBP itself [3].Fig.2 shows the structure of the OBP.

The feeder link RF line and the servicelink RF line establish connections via a 140-MHz IF. Three channels are used for connec-tion to the service link RF line, correspondingto three beams. Two channels are used forconnection to the feeder link RF line, with acommunication capacity corresponding to thethree beams of the service link. It is possibleto use either of these lines individually.

Table 1 shows the main specifications of

the OBP system. The OBP conducts π/4-shiftQPSK decoding at 70 kbps and error correc-tion using the 1/2-rate convolutionalcoding/Viterbi decoding method. The OBPcan send voice data at 5.6 kbps per carrier(through five channels by MC-TDMA withfrequency intervals of 50 kHz) and can alsosend data at 32 kbps, per carrier.

At the design stage, the bandwidth of eachport was determined to be 5 MHz and thenumber of switchable channels was set to1,000 or more for voice data. Since the Sbandwidth of the ETS-VIII is 2.5 MHz, theservice link transmission capacity was equiva-lent to 720 channels of voice data. In order toreduce spurious S-band radiation towardneighboring satellites, the bandwidth of the

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

Conceptual illustration of the mobilevoice communications system

Fig.1 OBP structureFig.2

OBP main specificationsTable 1

output circuit on the service link side of theforward link processor and return link proces-sor was limited to 4 MHz. Because the samecircuit has been employed on the feeder linkside, 880 channels of voice data are available.The cross link processor that performs regen-erative repeating can modulate/demodulate 32waves, allowing for 160 channels of voicedata. However, since 8 waves are used inexchanging control signals, 120 channels ofvoice data are available for communication.

The OBP converts the communicationband directly to the base band by quasi-quad-rature detection, digitizes the signals, and thensamples, switches, modulates/demodulates,and combines the communication signalsusing a digital signal circuit employingnumerous large-scale gate arrays or field pro-grammable gate arrays (FPGAs). Table 2shows the number of application-specific inte-grated circuits (ASICs) manufactured fromgate arrays.

The control processor software provides

communication using channel pre-assignment.The normal operating software, featuring self-controlled switching, is downloaded via net-work after the OBP has been activated, so thesoftware can be modified as necessary.

The OBP weighs about 90 kg, and its max-imum power consumption is approximately400 W, depending on the operation mode andthe number of beams employed.

5 Forward and return link proces-sors

Fig.3 shows a block diagram of the for-ward link processor. Both the forward linkprocessor and the return link processor havedemultiplexers (for quasi-quadrature detectionand for splitting waves), a switch, multiplex-ers (for wave combination and quadraturemodulation), an interface, and a power supply.The number of demultiplexers and multipex-ers vary with the number of input/output ports.

The quasi-quadrature detector circuit con-verts the 140- MHz IF signals into two quad-rature base-band signals using 140- MHz localsignals. The 2- MHz-band quadrature signalsare digitized with an analog-digital converter(ADC). This ADC is an 8-bit processor with a48 dB dynamic range.

The demultiplexer circuit adopts a poly-phase FFT circuit in order to divide the 4-MHz MC-TDMA signals at intervals of 50kHz of the carrier wave. The switching circuit

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Number of ASICs usedTable 2

Block diagram of the forward link processorFig.3

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receives signals from the demultiplexer andoutputs them to the multiplexer or cross linkprocessor, subject to control by the controlprocessor. The signals from the cross linkprocessor are handled by the switch in thesame manner. Upon activation, the switch isset to the channel pre-assignment state. Themultiplexer converts the received signals into2- MHz base-band quadrature signals usingthe IFFT and poly-phase filter to conductreverse calculation for the demultiplexer. Thequadrature signals pass the base-band filtersuppressing unnecessary waves (i.e., thosehigher than 2 MHz). The quadrature modula-tion circuit converts the 2-MHz quadraturesignals to analog signals with a digital-analogconverter (DAC) and then conducts quadra-ture modulation using 140- MHz local signals.

The interface sends and receives the con-trol signal and regenerative repeating signal toand from the cross link processor and receivesthe control signal from the control processor.The power supply conducts primary powerconversion from DC 100-V bus power to DC24 V. The voltage required for each circuit isgenerated by a secondary power supplyinstalled in each substrate.

Because there are two or three demulti-plexers or multiplexers, no redundancy is nec-

essary in the circuit. The interface circuit hastwo lines; the line connected to the controlprocessor is the operative one. The 140-MHzlocal signal is used in the quasi-quadraturedetection circuit and the quadrature modula-tion circuit, and each of these circuits featuresa main unit and a redundant unit; the signalcan be switched between these units by theappropriate command. The power supply alsofeatures a main unit and a redundant unit;selection is performed via a command arisingupon OBP activation.

6 Cross link processor

Through the interface, the cross linkprocessor communicates and exchanges con-trol signals with the forward link processorand return link processor. Fig.4 shows a blockdiagram of the cross link processor, consistingof demodulators, modulators, an interface, anda power supply. The demodulators and modu-lators handle 32 waves, 8 of which are used toexchange control signals.

There are four demodulation and modula-tion units. The circuit has been reduced insize by time-division multiplexing of the 8-wave signals in each unit. The demodulationunit consists of a π/4-shift QPSK demodula-

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

Block diagram of the cross link processorFig.4

tor and Viterbi decoder circuits. Fig.5 shows afunction block diagram of the demodulator.

The demodulation and modulation unitfeature no redundancy, although they employtwo different modem patterns—one for con-trol signals and other for communication sig-nals. Switching between these two patterns isperformed via commands. There are twointerface circuits; whichever one is connectedto the activated control processor is used. Thepower supply conducts primary power conver-sion from DC 100-V bus power to DC 24 V.The voltage required for each circuit is gener-ated by a secondary power supply installed ineach substrate. This circuit has a main unitand a redundant unit, and one or the other isselected by a command executed upon OBPactivation.

7 Control processor

The control processor software runs theOBP self-contained switching function; con-trol processors A and B work to monitor andcontrol the OBP as a main one and a redun-dant one performing the same functions.

Fig.6 shows a block diagram of the controlprocessors. This device uses a 20- MHzRAD-6000, equivalent to the IBM PowerCPU. Memory capacity is 128 MB, approxi-mately 1 MB of which is used by the software.The onboard software stored in the ROMchecks the operation of each processor uponactivation and turns on the channel pre-assigncommunications function.

All clock signals for the OBP are suppliedby the control processor, and the interfaces ofthe other processors operate relying on theclock signals supplied by the interface of the

activated control processor. The power supplyconducts primary power conversion from DC100-V bus power to DC 24 V. The voltagerequired for each circuit is generated by a sec-ondary power supply installed in each sub-strate.

Bus commands (other than those to poweron and off the OBP) are received by the con-trol processor and used to control individualprocessors as required.

8 Communications method

This system adopts the MC-TDMAmethod using π/4-shift QPSK modulation.Fig.7 shows the relevant frame structure. Amulti-frame structure is employed in the pres-ent system, and the communication process iscontrolled for each major frame. Only the topof the major frame uses a long preamble invoice communications. The remaining framesuse a short preamble, in order to increase thedata rate. Channel efficiency is increasedthrough appropriate allocation of the referencesignals and the communications control chan-nels. A “super frame” is deemed to exist onthe major frame. This frame functions, forexample, when the channel capacity for eachbeam has changed. This frame is providedwith no other special functions.

Cross link communication involves regen-erative repeating, which results in one-slotdelay. As a result, during the output of regen-erative signals in voice communication, thetop of the major frame features a short pream-ble and the second frame has long preambles.

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Functions of the demodulation circuitFig.5

Block diagram of the control processorFig.6

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Voice data occupies 5 slots. Because of theone-slot delay, the second and latter framesare sent out.

9 Software

The system uses VxWORKS as the Oper-ating System. The size of the software(including this OS) is 261 KB.

Fig.8 shows the OBP mode transfer dia-gram. The software eventually establishestwo modes—wait mode and normal operationmode—as follows. After powering on from

the off mode, the installed software conducts aself check and enters wait mode automaticallyfrom standby mode. After the control signalswitches the OBP from wait mode to down-load mode, software for normal operationmode is downloaded to the OBP through thecommunication link. After downloading, thecontrol signal switches OBP control to thenormal operation software, thus activating thenormal operation mode. Additionally, areconfiguration mode is used to set OBPparameters and to modify status during normaloperation. No communication is possible inthis mode.

The main function of the wait mode is tocheck the status of the OBP and to implementcommunication by channel pre-assignment.In conjunction with timing-control bursts(which are used to measure TDMA timingbetween transmit signal and reference signal),the software activates a function indicating thedelay clock number. The frequency as well asthe transmission timing of handheld terminalsmust be fine-tuned based on the response tothese timing-control bursts. The software

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

Frame structure in MC-TDMAFig.7

OBP operation mode transfer diagramFig.8

indicates a coarse value, but values for eachbeams must be set after the OBP has beenactivated.

The normal operation software runs in thenormal operation mode. Fig.9 shows the func-tions of this software. The call managementfunction carries out call control, mobility man-agement, and radio transmission management.The user management function executes themanagement of phone numbers. The hard-ware management function monitors and con-trols the OBP itself. The statistics and logdata management function records the OBPstatus and communication records and pro-vides this data to ground stations.

The control data is exchanged between theOBP and handheld terminals via two layers—a physical layer corresponding to the actualwireless communication and a logical layercorresponding to the modeled control process.This design enables the system to adapt tochanges in the physical layer. Fig.10 showsan example of a connection sequence. Thehandheld terminal must be registered in theOBP prior to its use. The terminal makes aconnection request for a call, and control(such as paging channel assignment) is carriedout. Burst timing correction is made beforethe connection control sequences, and physi-cal layer control is performed (e.g., a requestfor control channel assignment for communi-cation in the logical layer). Although the

modeling of the control process provides flex-ible control independent of the real channelstructure, traffic is increased because of theextra communication required for control; as aresult the configuration is somewhat redun-dant as applied to satellite communication.

The OBP is designed to conduct many ofthe traffic controls by itself, so that the overallsystem will continue to work only by allocat-ing a channel capacity for each beam throughthe control station.

10 Performance check test

After each device was subject to electrical,environmental, mechanical testing, OBP com-bined with S-band transponder was checked[4][5]. And an overall communication per-formance check for communication equip-ments installed in the ETS-VIII was per-formed by NASDA (National Space Develop-ment Agency of Japan, now JAXA: JapanAerospace Exploration Agency) [6]. Fig.11shows a picture of the OBP installed in thesatellite.

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Functions of the normal operationsoftware

Fig.9

Call control sequenceFig.10

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Filtering is performed during signalexchange between the ground base station andhandheld terminals for carrier separation,exchange, and combination. Fig.12 shows theOBP filtering performance in the non-regener-ative repeating channel. Because of the priori-ty placed on filtering performance, the filter’sbandwidth is narrower than the modulationspectrum and the roll-off shaping effect isweakened in the modulator/demodulator.

Regenerative repeating is performed incommunication between mobile terminals.The system is equipped with a modu-lator/demodulator to transcribe switching con-trol signals. Fig.13 shows the spectrum of anS-band SSPA output of regenerated signals.

Although the spectrum presents line-likespurious noise due to quantization and calcu-lation errors, this noise level is lower than thenoise level of reception during non-regenera-tive repeating. In addition, some leakage oflocal signals and signal ghosts are producedby mismatching in the quadrature modulator.This problem could probably be solved byadopting a digital circuit in the hardware con-figuration (including the quadrature modula-tor).

Fig.14 shows the bit error rate in the over-all communication performance test. Signalsdo not degrade during non-regenerativerepeating, whereas a signal degradation ofabout 4 dB is seen during regenerative repeat-ing, relative to the non-regenerative period.The OBP digitizes numerous processes. Since

Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

Picture of the OBPFig.11

Filtering performance in non-regener-ative repeating channels

Fig.12

S-band spectrum of regenerated sig-nals

Fig.13

Bit error rateFig.14

it performs conversion of the 4-MHz analogsignals to base-band signals and executesquadrature modulation, significant signaldegradation is seen. Such digital processesmay be optimized, for example in terms ofcalculation bit length.

Overall communications performance test-ing has indicated that handheld terminals cancommunicate with each other in the test mode,that the normal operation software can bedownloaded successfully, and that the self-controlled switching function works normallyin the normal operation mode.

11 Conclusions

We have developed an OBP serving as asatellite-borne, self-controlled switch formobile voice communications. We installedthe OBP in the ETS-VIII and examined theperformance of the entire communicationssystem. With the exception of an increase inthe bit error rate, performance was as expect-ed.

Many ASICs and FPGAs are used in thehardware of the OBP. Because such semicon-

ductor devices designed for use in space mustbe durable enough to resist radiation and otherharsh conditions, it is particularly troublesomethat these devices are two to three generationsbehind those designed for ground use. It isanticipated that continued rapid progress insemiconductor technology will produceFPGAs that can be re-programmed to act asmodulators/demodulators or as demultiplex-er/multiplexer circuits. It will also becomepossible to correct an increased error rate inthe OBP even after launch.

This OBP was developed by the AdvancedSpace Communications Research Laboratory(ASC), and taken over by the CRL after theformer body completed its mission. If theground base stations are well prepared, thisOBP system will even be capable of connec-tion to public telephone networks. For thetime being, three prototype handheld terminalswill be manufactured. One such terminal,with no RF unit, will be used as a control sta-tion for software loading and monitoring ofOBP status. Another will be used as a termi-nal in the ground base station.

HASHIMOTO Yukio 93

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Communication Research No. 85, ISSN 0912-5094, May 2000. (in Japanese)

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munications", Acta Astronautica, pp. 365-373, Sep. 1999.

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Test Satellite VIII", Technical Report of IEICE, SAT2000-78, Dec. 2000. (in Japanese)

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94 Journal of the National Institute of Information and Communications Technology Vol.50 Nos.3/4 2003

HASHIMOTO Yukio

Senior Researcher, Broadband SatelliteNetwork Group, Wireless Communica-tions Division

Satellite Communication