4-7-3 an ofdm terminal for experiment of a satel- lite

1 Introduction

For Engineering Test Satellite VIII (ETS-VIII), the Communications Research Labora-tory is planning high-quality digital audiobroadcasting experiments for mobiles byOFDM (Orthogonal Frequency Division Mul-tiplexing). Since OFDM provides excellenttransmission characteristic even in the statewhere there is delay wave, it is used for high-quality digital audio broadcasting (DAB: Digi-tal Audio Broadcasting) for mobiles inEurope. Moreover, OFDM has been adoptedas a transmission system for digital TV broad-casting in Japan. One significant reason forthis adoption is that the transmission system iseffective against interference by delayedwaves caused by multipath in environmentssuch as urban areas, where high-rise buildings

stand close together.On the other hand, for satellite communi-

cation, since the equipment on board a satel-lite requires high electric power efficiency,amplifiers are often operated in their satura-tion region. Under these conditions, deteriora-tion of transmission characteristics resultingfrom intermodulation, a drawback of theOFDM transmission system, may presentproblems.

This paper reports on the OFDM terminalequipment used for the high-quality satelliteaudio broadcasting experiments by an OFDMtransmission system that uses ETS-VIII, andreports the performance results of a ground-test using an SEM (System EngineeringModel) of the ETS-VIII.

YAMAMOTO Shin-ichi 211

4-7-3 An OFDM Terminal for Experiment of a Satel-lite Audio Broadcasting

YAMAMOTO Shin-ichi

An OFDM terminal, with sound quality equivalent to that of a compact disk, is used forexperiments on satellite audio broadcasting using Engineering Test Satellite VIII (ETS-VIII).

The terminal consists of an OFDM signal generator, and a test receiver for estimatingOFDM signals. The modulation method used is a multi-carrier OFDM system. A feature ofthis OFDM method is that good transmission characteristics are acquired even if a delaywave exists in a transmission path. It is suitable for reception of signals in urban areaswhere delay waves often exist as a result of reflections from buildings, etc. However, sinceit is a multi-carrier, when there are nonlinear characteristics in the transmission path, thetransmission characteristic deteriorates due to the influence of inter-modulation. For thisreason, when a satellite transponder designed to achieve electric-power efficiency is used,it is thought that a bit error rate may deteriorate under this influence.

This paper describes an outline of an OFDM terminal for satellite audio broadcastingexperiments. The results of a ground test using an electric model of ETS-VIII (SEM) and anOFDM terminal are also reported.

Keywords ETS-VIII, Satellite audio broadcasting, OFDM

212

2 Overview of satell ite audiobroadcasting experiments

Overview of the satellite audio broadcast-ing experiments is shown in Fig.1.

The OFDM signal transmitted from a Ka-band feeder-link earth station [1] is frequencyconverted to an S-band signal by the satelliteand re-transmitted to the ground. On theground, by receiving signals via the mobilestation [2] or S-band earth station [3], we willobtain several types of data on the effects ofmultipath caused by structures, such as build-ings, and the effects of nonlinear transmissionpaths.

3 Experimental system

The OFDM terminal for the audio broad-casting experiments consists of an OFDM sig-nal generator in the transmission system and atest receiver for estimating an OFDM signal inthe reception system.

The OFDM signal is transmitted from theKa-band feeder-link earth station, convertedinto an S-band signal by the satellite, re-trans-mitted to the ground, and received by themobile station and the S-band earth station.Therefore, the OFDM signal generator isinstalled in the Ka-band feeder-link earth sta-tion, and the test receiver for estimating anOFDM signal is installed in the mobile stationand the S-band earth station.

Fig.2 shows the appearance of the OFDMsignal generator.

The upper side is the OFDM signal gener-ator and the lower side is the vector signalgenerator (vector SG). OFDM modulated Iand Q signals which are output from theOFDM signal generator are quadrature modu-lated by the vector SG, and obtain the 140-MHz band IF signal.

Fig.3 is a block diagram of the OFDM sig-nal generator.

The OFDM signal generator consists of anFIC (First Information Channel) data genera-tor, an MSC (Main Service Channel) data gen-erator, and an OFDM modulator.

The FIC transmits additional informationabout the service. Its data generator can selectand output fixed data prepared in advance,user-created data, and a pseudo noise code

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

Overview of satellite audio broadcast-ing experiments

Fig.1

Appearance of OFDM signal genera-tor

Fig.2

Block diagram of OFDM signal gener-ator

Fig.3

(PN code). Moreover, it has rewritable mem-ory for saving user-created data. For the PNcode, the FIC data generation section has gen-erators for three types of CCITT-compliantcodes (PN9, PN15, and PN23).

The MSC is a channel through which actu-al service information is transmitted. Its datagenerator converts input digital audio interfacedata into DAB audio frames, and carries outpredetermined scrambling, convolution codingand time interleaving together with externalserial data and the PN code in an energyspread scrambler, a convolutional encoder,and a time interleaver. The data is then multi-plexed with a data from the padding generatorand a data in static memory B, by multiplexerA according to the contents of FIC, and outputas MSC data.

In the modulator section, FIC and MSCdata are multiplexed by Multiplexer B accord-ing to a transmission mode, and the multi-plexed data is modulated with an OFDM mod-ulator, and I - Q signals are output.

Although four transmission modes are pre-pared for OFDM, the ETS-VIII experimentuses MODE III. Table 1 shows the transmis-sion parameters of the respective transmissionmodes. These modes are chosen according tothe difference in the frequency band and thetransmission method to be used, and MODEIII is used for satellite broadcasting at 3 GHzor below.

Fig.4 shows a composition of a transmis-sion frame.

The NULL symbol with l = 0 is a sectionof signal off, and is used in order to adjust

timing on the reception side.The symbol with l = 1 is called PRS, and

is fixed data. The reception side uses thissymbol to estimate offset frequency and tim-ing, and a reference phase signal in differentialdetection. The symbols of l = 2 to L containtransmitted data.

Fig.5 shows the appearance of the testreceiver for estimating an OFDM signal. Theequipment can be mounted into a 19-inchrack.

Fig.6 is a block diagram of the test receiv-er for estimating an OFDM signal.

The received S-band RF signal is convert-ed into the 190-MHz band IF signal by the S-band down-converter. This IF signal is inputinto the tuner unit through a U-link.

The IF signal input into the tuner unit isfurther converted into the 3.072-MHz IF sig-nal, and converted into a baseband signal by


Parameters of respective transmis-sion modes

Table 1

Composition of transmission frameFig.4

Appearance of the OFDM signal esti-mation receiver

Fig.5

Block diagram of OFDM signal estima-tion receiver

Fig.6

214

quadrature detection in a demodulator. Thissignal is FFT transformed in an effective sym-bol period, and complex data transmitted oncarriers of OFDM is subjected to differentialdetection to obtain demodulated data. Thisdemodulated data undergoes frequency de-interleaving, time de-interleaving, error cor-rection, and energy spread removal in thedecoder to obtain service data.

If a service selected in the front panel isaudio, the data is MPEG-decoded in the audiosection and output as analog voice signals. Inthe case of the data, this is output in the formof data with clock to the outside.

Synchronization is performed so thatcoarse frame synchronization is established bya NULL symbol of a frame, its frequencydeviation is detected based on PRS (PhaseReference Symbol), and the VCXO of thetuner section is controlled by AFC. AGC cal-culates the electric power of the OFDM sym-bol subjected to differential detection and out-puts an AGC voltage to the tuner section sothat its value coincides with the target value.

Table 2 shows the electrical performancefor the test receiver for estimating an OFDMsignal.

Fig.7 shows a BER characteristic (beforeerror correction) of the loop test of this system

in IF. The reception frequency is 176 MHz,and the signal is input into the test receiver atan RF input (1). BER measurement was per-formed in all transmission modes.

Inherent deterioration of system to a theo-retical value of BER up to 1.0 E-4 is 1 dB orless.

Fig.8 shows BER characteristics (beforeerror correction) of a loop test with IF in theS-band. The signal was input into the testreceiver at an RF input (2). In this measure-ment, noise was added to the output signal(190 MHz) from the S-band down-converter.

In the S-band, inherent deterioration ofsystem to the theoretical value of a BER at1.0E-4 is approximately 1.2 dB, indicating aresult slightly worse than the measurement at176 MHz.

4 Ground test using SEM

The ground test was performed to measuredeterioration of the transmission characteris-tics caused by the nonlinear characteristic of


Electrical performanceTable 2

Loop back BER characteristics in IF(before error correction)

Fig.7

Loop back BER characteristics in S-band (before error correction)

Fig.8

the transponder (especially a power amplifier)of the satellite [4].

In addition, the SEM (System EngineeringModel) of ETS-VIII used for the examinationconsists of orbital flight models. Therefore,this experiment can be regarded as a verifica-tion of whether or not the audio broadcastingexperiments can be conducted normally whilein orbit.

Fig.9 shows an example of a system dia-gram of the ground test which used the SEM.Several routes inside the satellite can beselected for the audio broadcasting experi-ments. A typical system diagram is shownhere.

Since the input frequency of the SEM is inthe Ka band, the S-band OFDM signal outputfrom the OFDM signal generator is convertedinto the Ka-band by an up-converter, and isinput into the SEM.

In the SEM, the signal passes through aninput circuit, a low noise amplifier, and a

down-converter, and is converted into an IFfrequency of the 140-MHz band. The signalthen passes through an S-band up-converter, abeam-forming network, and an SSPA (SolidState Power Amplifier), which outputs a signalof the S band. The SEM has 31 SSPA sys-tems, the combined outputs of which serves asa transmission signal in orbit. In this test, theone output of these SSPA systems is taken outto the outside.

Fig.10 is an input-output characteristic ofthe SEM.

The figure shows the input-output charac-teristics of the CW signal and the OFDM sig-nal. The input-output characteristic of OFDMsignal tend to exhibit lower gain than the CWsignal at low input level. This is characteristicof cases in which multiple signal waves arecommon amplified. Since the OFDM signal iscomposed of 192 carriers, intermodulationeffects are expected to become prominent.

In this test, the maximum input level is set


An example of an SEM test system diagramFig.9

216

to －79.6 dBm due to limitations of SEM.This input level is more or less identical to theinput level (－78.9 dBm) at the same interfacepoint, estimated from the link budget exempli-fied in Table 3.

Each characteristic was obtained using asa parameter the input-back-off (IBO) from themaximum input level of the SEM into a refer-ence level.

Fig.11 shows the spectrum of an output ofthe Ka-band up-converter input into the SEM.In the transmission path from the OFDM sig-nal generator to an SEM input, the signal istreated in a linear region so that it is notaffected by intermodulation.

Fig.12 is an output spectrum of SEM at a

reference input level (IBO = 0 dB). The spec-trum was observed at an output of the S-banddown-converter built into the test receiver forestimating an OFDM signal (in the 186-MHzband).

As shown in the figure, slopes exist in thenoise level of the both sides of the OFDM sig-nal. This is considered to be caused by inter-modulation.

Figs.13 and 14 are SEM output spectrumwhen IBO was set to －3 dB and －6 dB,respectively.

The larger the IBO, the flatter the slope ofthe noise level becomes, indicating reductionof the influence of intermodulation. Inciden-tally, noise levels decrease on both sides of thespectrum in Fig.14, which is attributed to thecharacteristics of a band pass filter installed on


An example of SEM input-outputcharacteristics

Fig.10

An example of a link budget foraudio broadcasting (good weather)

Table 3

Output spectrum of Ka-band up-converter

Fig.11

SEM output spectrum (IBO = 0 dB)Fig.12

a signal route in the SEM.Fig.15 shows the BER characteristics of

several IBO (before error correction).A tendency is observed in which BER

somewhat improves for higher C/No withincreasing IBO, but for lower C/No, no signif-icant difference can be observed.

It is thought that the communication quali-ty required in the audio broadcasting experi-ments is 1.E－4 or less in BER after error cor-rection. The BER before error correction is1.E－2 or less, and the overall C/No requiredfor the link is 72.6 dBHz or more at IBO = 0dB.

From one example of the link budget ofthe audio broadcasting experiments of Table 3,

although the required C/No is 74.4 dBHz, itincludes a value of inherent deterioration.From the results of the SEM test, the requiredC/No becomes 72.6 dB or more, indicatingthat obtaining a margin of 1.4 dB is possible.Performance of the mobile station assumed inthe link budget is an antenna gain of 6 dBi anda LAN NF (Noise Figure) of about 3 dB. Thislink budget assumes a state free of rain attenu-ation. Moreover, intermodulation noise andinterference noise are assumed to be part ofthe inherent deterioration obtained in the test.

5 Concluding remarks

This report discussed the OFDM terminalused for the audio broadcasting experimentswith the ETS-VIII. We have obtained BERcharacteristics using this terminal and theSEM of ETS-VIII, confirming the feasibilityof experiment in orbit.

Although the electric power efficiency ofthe equipment on a satellite is important, inthe satellite audio broadcasting which uses anOFDM, deterioration of the channel qualitydue to the nonlinear characteristic of on-boardequipment poses a problem. However, it isthought that OFDM is effective in the measureof disturbance by the delay wave in environ-ment with many multipass like ground broad-casting, and experimental results using ETS-VIII are expected.


SEM output spectrum (IBO = －3 dB)Fig.13

SEM output spectrum (IBO = －6 dB)Fig.14

BER characteristics of several IBOsFig.15

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

References1 S. Yamamoto, N. Obara, and H. Ohashi, "4-1 Ka-band feeder-link earth station", This Special Issue of NICT

Journal.

2 A. Miura, S. Yamamoto, and M. Satoh, "4-5 Land Mobile Station for ETS-VIII Mobile Satellite Communica-

tions Experiments", This Special Issue of NICT Journal.

3 S. Yamamoto, N. Obara, and I. Yamazaki, "4-2 S-band earth station for mobile communication", This Spe-

cial Issue of NICT Journal.

4 S. Yamamoto, K. Takano, H. Mitsumoto, S. Sakai, S. Ichihashi, and S. Hama, "Experiments on satellite

mobile broadcasting using OFDM -Results of a ground test using System Engineering Model of ETS-VIII-",

Communications Society Conference IEICE, B-3-20, Sep. 2000.

5 T. Shiomi and M. Hatori, "Wave Summit Course: Digital Broadcasting", Ohmsha, Ltd. 1998.

6 T. Iida, "Wave Summit Course: Satellite Communications", Ohmsha, Ltd. 1997

YAMAMOTO Shin-ichi

Senior Reseacher, Mobile SatelliteCommunications Group, WirelessCommunications Division

Mobile Satellite Communications

4-7-3 an ofdm terminal for experiment of a satel- lite

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