TDM Framing for Gap Filler Operation in Satellite Digital Multimedia Broadcasting System A

Chaehag (Steve) Yi Solid Technologies, Inc

Seoul, Korea steveyi@st.co.kr

Abstracl- A new satellite digital multimedia broadcasting system is proposed which deploys additional time division multiplexing (TDM) signal path for gap filler operation over System A. All orthogonal frequency division multiplexing (OFDM) ensembles of System A are multiplexed into one TDM frame with some header information. OFDM frame boundary is used for the reference of TDM framing. This TDM frame is encoded, interleaved, and modulated once more. Both multiple OFDM signals and one TDM signal are transmitted through separate frequency bands. The proposed system can configure efficient and flexible complementary terrestrial gap filler system by introducing mode conversion gap filler which receives TDM signal through Ku-band and transmits multiple OFDM signals through S-band. This proposed gap filler does not have oscillation problem inherently and does not degrade the signal quality. It is shown that it is recommended to deploy large number of gap fillers with low gain power amplifier rather than small number of gap fillers with high gain power amplifier.

Index Terms - Digital multimedia broadcasting, orthogonal frequency division multiplexing, time division multiplexing.

1. INTRODUCTION ITU-R recommends five systems for digital satellite

broadcasting to vehicular, portable and fixed receivers in the broadcasting satellite service (sound) bands in the frequency range 1.4 - 2.7 GHz [I]. Digital System A, also known as the Eureka 147 DAB (digital audio broadcasting) system, has been developed for both satellite and terrestrial broadcasting applications [2]. Tests of satellite DAB in the L-band from Solidaridad 2 geostationary satellite, covering Mexico, confmed that Eureka 147 DAB system is suitable for use in satellite broadcasting [3]. In Korea, by including additional standards to support the video service, Eureka 147 is adopted as national standard for terrestrial digital multimedia broadcasting (DMB) system [4]. This is achieved by Reed- Solomon encoding and convolutional interleaving before convolutional encoding. The service name is changed from DAB to DMB in order to emphasize the video service.

Satellite DMB provides multimedia contents to vehicular and portable users by using satellite such as high quality audio, video, traffic information, car navigation, weather, stock, etc. In order to provide full mobility for users, satellite DMB system should provide general coverage and overcome multi- path fading due to user’s movement [SI. In satellite DMB System A, orthogonal frequency division multiplexing (OFDM) is used to overcome multi-path fading effect.

Complemenary terrestrial gap tiller system is required to provide general coverage. Gap filler transmits DMB signals to the areas such as blocking areas, in-buildings, and subway stations. With current System A specification, gap tiller system may be configured by RF repeater type or by additional wireline network. The latter is an expensive solution since it requires cable laying and its maintenance. RF repeater type gap tiller degrades signal quality and has inherent oscillation problem. Oscillation is usually avoided by spatial separation between receiving antenna and transmitting antenna. The gain of RF repeater is limited by the amount of isolation. This means that RF repeater type gap filler may have difficulty in its deployment.

In this paper, a satellite DMB ‘System A Plus’ is proposed which deploys additional time division multiplexing (TDM) signal path for gap filler operation over System A. In section 11, System A Plus is described and the mode conversion gap filler is proposed. In section 111, the symbol error probability in gap area is derived in order to show the validity on the advantage of the proposed gap filler. In section IV, some numerical results are given.

11. SYSTEM A PLUS DESCRIPTION

Consider one geostationary satellite, 2.6 GHz S-band and 25 MHz bandwidth. According to the outcome of WRC2003 (World Radiocommunication Conference 2003), satellite DMB service can also be provided on 2.605 - 2.630 GHz in Korea. For 25 MHr bandwidth, we can provide up to 12 ensembles if 2 MHz per ensemble is used to avoid inter-ensemble interferences. At the terrestrial station, OFDM transmitter is required for each ensemble. For gap filler operation, all OFDM ensembles are multiplexed into one TDM frame with some header information. Header is composed of synchronization word and ensemble identifier. Dummy bytes are inserted for rate-matching of TDM kame. OFDM frame boundary is used for the reference of TDM frame. This TDM frame is encoded, interleaved, and modulated according to ETS 300 421 [6] which is a commercialized digital video broadcasting satellite (DVB-S) system. TDM signal and OFDM signals are transmitted through different frequency bands. In Satellite, the frequency of TDM signal is converted into Ku-band and the frequency of OFDM signals is converted into S-band. Gap filler receives TDM signal, converts it into OFDM signals, and transmits them through 2.6 GHz. The transmission timing of gap filler’s OFDM signal is aligned to that of Satellite’s OFDM signal. This is accomplished by delaying the OFDM signal

timing at the terrestrial station by the amount of gap filler processing time.

OFDM transmission frame is composed of synchronization channel, fast information channel (FIC), add main service channel (MSC) [2]. TDM frame is generate by the following three rules:

( I ) Ride 1: The Reed-Solomoo (RS) codeword boundary of TDM frame is aligned to the DMB frame. The remaining part of last codeword of every TDM frame is filled by dummy bytes (all I bits: FF in HEX).

(2) Rule 2: At the TDM frame starting point, there are 12 byte synchronization word, SYNC-WORD, and 1 byte ensemble identifier, ENS(C,N), which follow inverted sync byte (or sync byte). SYNC-WORD is defined by lF90CAE06F35073AB6F8C549 (in HEX) and the bit representation ofENS(C.N) is given in Table 1.

( 3 ) Ride 3: Multiple OFDM ensembles are combined into one TDM frame. There are two combination methods:

Method 1 - Data of each ensemble are framed into TDM frame independently by the Rule 1 and Rule 2. Each TDM frame is concatenated into one TDM frame.

Method 2 - Data of each ensemble are concatenated into one DMB frame before TDM framing. TDM frame is generated by Rule I and Rule 2.

Let N-BYTE be the total number of OFDM symbols in one transmission frame. The number of words in a TDM frame is given by

N-WORD-FRAME = INT(((N-BYTE+13)-1)/187)+1 ( I )

where INT(a) is the maximum number which is not greater than a. Let N-DUMMY-BYTE bc the number of dummy bytes inserted for rate-matching in a frame. It is given by

N-DUMMY-BYTE = N-WORI-FRAME * 187

-N-BYTE- 13 (2) TDM framing example is given in Fig. 1 for one ensemble

mode 111 in wbicb there are 7344 byte including phase reference symbol. From the result of ( I ) , TDM frame includes 40 words, in other words, 7520 bytes which is composed of 7344 DMB bytes, 40 inverted sync or sync bytes, 13 header bytes, and 123 dummy bytes. This TDM frame is encoded, interleaved, and modulated according to ETS 300 421 [6]. One word corresponds to one RS codeword. In this case, there is always inverted sync byte at the starting point of transmission frame. This is not guaranteed in general casc since the number of RS codewords is not always multiple number of eight.

TABLE 1. ENS(C,N) BIT REPRESENTATION

ENS( I . I ) ENS( I ,2)

... 0000 0001 00000010

I 'I ensemble a1 method I 2"d ensemble at method I

... ...

ENS( I ,I 5 ) ENS(2,')

TF = 24 ms

0000 I I I I 1 1 1 1 I l l 1 Methad 2

15" ensemblc at method 1

1 - - 1

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Fast I n f m a t w Main Service Channel Channel

DMB Frame

Dummy Byte 123 Byte

Sync Byte

J DMB Frame

I I , 1 I '

ENS(C.N) 1 Byte Inverted Sync Byte

1

Dummy Byte 123 Byte

SYNCWORD(l2Byte) 1 FWCAE06F35073AB6F8C549 (HEX)

N.WORD_FReME i 79 W S K W Rate: E4xUxP1IUm-SV2Mrpa

(b) Method 2

Figure 2. Ensemble combination ~ Two ensembles of mode 111

Two ensemble combination methods are shown in Fig. 2 for two ensembles of mode 111 case. In Method I , TDM framing is done at each ensemble and then TDM frames are concatenated. There are 80 RS codewords in transmission frame so QPSK symbol rate becomes 5.44 Msps if rate 112 convolutional code is used for TDM frame. There is always inverted sync byte at the frame start. In Method 2, DMB ensembles are concatenated into one frame which consists of 14,688 byte. This is converted into TDM frame by (I). There are 79 RS codewords in transmission frame so QPSK symbol rate becomes 5.372 Msps with rate 112 convolutional code. Method 2 gives lower QPSK symbol rate. The same scheme is applied to other modes and to any number of ensembles.

Both multiple OFDM signals and one TDM signal are transmitted through separate 14 GHz Ku-band. The terrestrial station transmitter block diagram is given in Fig. 3. The TDM frame boundary signal is generated from OFDM frame boundary signal. TDM signal suffers processing delay both at TDM transmitter and at gap filler TDM receiver. This processing delay is compensated by delaying OFDM signal intentionally at each OFDM transmitter.

OFDM Signal N

OFDM Signal 1

I /

Figure 3 . Terreslnal sfation transmitter block diagram.

Figure 4. Gap filler block diagram.

Satellite translates the frequency of TDM signal to 1 I GHz and OFDM signals to 2.6 FHz. Gap tiller block diagram is given in Fig. 4. Gap tiller receives TDM signal only through 1 I GHz. Gap filler restores TDM frame first. This TDM frame contains header information, OFDM ensembles data and dummy data. By using the header information, OFDM frame boundary is detected. Gap tiller re-generates each OFDM ensemble after removing header information and dummy data at TDM to OFDM converter. So this type gap filler is called 'mode conversion gap filler'. And transmitter identification information (TII) signal can he inserted at each gap-tiller. TI1 signal will be used for gap filler network management system. The timing of OFDM transmitter is obtained from the TDM receiver. The OFDM signal timing of gap filler is synchronized to the OFDM signal timing o f the satellite.

111. PERFORMANCE ANALYSIS In satellite signal reception, there are open area and gap

area. Open area corresponds to area with unobstructed view of satellite, unsbadowed areas, and gap area corresponds to area where the direct satellite signal is shadowed by obstacles. In both cases, satellite signal is reflected from a large number of objects in the surroundings of a mobile receiver. These signal components are received with independently time varying amplitude and phase. These components add up to a complex

Rayleigh process. In open areas, this multipath signal is superimposed on the direct satellite signal. This forms a Rician process. In gap areas, there is no direct signal path and the multipath fading has a Rayleigh distribution with short-term average signal to noise ratio (SNR). The shadowing process results in a time varying short-term average SNR which has a lognormal distribution. The received signal is described by a Rayleigh-lognormal distribution which is often used for the terrestrial land mobile channel.

There are several parameters which are different values according to the environment such as the time portion A that a mobile receiver is in gap areas, the direct to multipath signal power ratio c (Rician factor), and the variance a2 of the signal level due to shadowing. The parameters A , c, and o2 are varied according to satellite elevations, types of environment, antennas. Estimated values are given in [7] which have been determined from the statistics of the recordings. In this paper, we consider only Rayleigh fading. And DPSK (differential phase shift keying) is assumed for modulation for simplicity though 4DQPSK is used in System A. The goal of this work is to show the advantage of gap filler. This assumption may not affect the validity on the advantage of gap filler. The symbol error probability is derived as a performance measure.

Simultaneous reception of the same information from multiple transmitters, one satellite and multiple gap tillers, is possible. This can be used to provide more reliable coverage as it introduces macro spatial diversity. In OFDM demodulator, the strongest multipath component is selected for demodulation. This kind of diveristy reception can he modeled as a selection combining. Average SNR from gap filler can be controlled independently compared with that of satellite. In gap areas with mode conversion gap tiller, a mobile receiver has additional signal source from gap tiller whose SNR has a Rayleigh distribution. Signal from gap filler can have different average SNR from that of satellite.

Consider the case that there is one gap tiller with high gain amplifier for gap area. A mobile receiver in gap area bas two Rayleigh faded signals: one is ys from satellite with average SNR rS and the other is E from a gap filler with average SNR rc. The probability densitiy function (PDF) of 7s is given by

The PDF of y. is given by the same equation with different average SNR llc. Let y he the selection combined signal of ys and yc in gap area defined by y = max(y, , ). By using (6.55) of [SI, the PDF of selection combined signal yis derived by

For an instantaneous SNR x the symbol error probability of DPSK is given in [9] by

0-7803-8255-2/04/$20.00 02004 IEEE. 2784

P e ( Y ) = l e - Y 2 (5)

The average symbol error probability in gap area for this case is given by

Let K be the gap filler relative gain factor defined by K = rG/rs. Then (6) becomes

Consider the case that there are multiple gap fillers with equal gain power amplifier for gap area. A mobile receiver in gap area has multiple Rayleigh faded signals: one is from satellite with average SNR rs and the others are from gap fillers with average SNR rc. Assume that rc = r,. As the number of gap filler increases, the diversity order of selection combining is increased. With selection combining with equal average SNR r,, the PDF of combined signal is given in [IO] by

The average symbol error probability in gap area for this case is given by

By using the binomial formula,

(9) becomes

Let Pb, be the resulting bit error probability of TDM link after RS decoding. Then, the average symbol error probability in gap area is given by

The value of Pb. TDh, is known as 10.'" - 10." in [6] so the effect of TDM link error can he neglected.

1V. NUMERICAL RESULTS The average symbol error probabilities in gap areas with

proposed gap filler are evaluated for two cases. In the case of one gap filler with high gain amplifier, numerical results shown in Fig. 5 are obtained by using (7). In the case of multiple gap fillers with low gain amplifier, numerical results shown in Fig. 6 are obtained by using (1 1 j.

In Fig. 5, the average symbol error probability in gap area is shown for the case of one gap filler with high gain amplifier according to the gap filler relative gain factor K. Since the mode conversion gap filler does not have oscillation problem inherently, it can increase the gain of power amplifier up to what is wanted. Without gap filler, the symbol error prohabildy remains IO.' order even with high average SNR. Even with gap filler with low gain, K = I , the error performance is moderately improved. As K increases, the error performance is gradually improved.

In Fig. 6, the average symbol error probability in gap area is shown for case of multiple gap fillers with low gain amplifier according to the number of gap fillers, in other words, the diversity order L. The number of gap fillers is L - 1 and all of

&7803-8255-2/04/$20.00 02004 IEE.

Figure 6. Symbol error probability according to the diversity order L.

them have equal gain amplifier. The results ofL = 1 correspond to those of without gap filler. As L increases, the error performance is improved remarkably.

Comparing the results of Fig. 5 with those of Fig. 6, we can estimate that the case of multiple gap fillers with low gain amplifier improves the error performance much better than that of one gap filler with high gain amplifier. This implies that it is recommended to consider large number of gap fillers with low gain power amplifier rather than small number of gap fillers with high gain power amplifier at the gap filler deployment.

V. CONCLUSIONS

In order to penetrate satellite DMB serivce like cellular mobile communication service, satellite DMB System should provide ‘general coverage’ comparable to that of cellular mobile communication. In System A Plus, this is accomplished by the proposed mode conversion gap filler which deploys TDM path of the commercialized digital video broadcasting satellite (DVB-S) system over System A. This gap filler does not have oscillation problem inherently since the frequency of

receiving TDM signal is different from that of transmitting OFDM signal. And this gap filler does not degrade the signal quality since OFDM signals are re-generated at each gap filler. Re-generation can control the coverage with large degrees of freedom. Therefore, the efficient and flexible complementary terrestrial system can be achieved by this mode conversion gap filler in System A Plus.

By comparing one gap filler with high gain amplifier with multiple gap fillers with low gain amplifier, it is shown that the latter improves the error performance much better than the former. This implies that it is recommended, at the gap filler deployment, to consider large number of gap fillers with low gain power amplifier rather than small number of gap fillers with high gain power amplifier.

REFERENCES

Rec. ITU-R B 0 . I 1304, Systems for digital satellite broadcasting to vehicular, portable and fixed receivers in the bands allocated to BSS (sound) in the frequency nnge 1400 - 2700 MHz. ETS 300 401, Radio broadcasting systems: Digital Audio Broadcasting (DAB) to mobile, portable and fined receivem. I. T. Zubrzycki, R. H. Evans, P. Shelswell, M. A. Gama, and M. Gutierrez, “Expefimental Satellite Broadcast of Eureka 147 DAB from SOLIDARIDAD 2,” BBC Rearch and Developemnt Repon 199615. T l A 2003SG05.02-046, VHF Digital Radio Broadcasting Video TransmissioniReception Specification. (Korean Edifion) R. De Gaudenzi and F. Giannetti. “Analysis of an advanced satellite digital audio broadcasting system and complementary gap filler single frequency network.” IEEE Trans. Vehic. Techno/.. Vol. 43. no. 2, pp. 194-2lO.May 1994. ETS 300 421, Digital brondcsting systems for television. sound and data services: Frame ~tmcture, channel coding and modulation for I 1/12 GHz satellite services. E. Lutz, D. Cygan, M. Dipplod, F. Dolainsky, and W. Papke, “The land mobile satellite communication channel ~ recording. statistics and channel model,” IEEE Trms. Vehic. Techno/,, vol. 40, no. 2, pp. 375- 386,May 1991. A. Papaulis, Pwb&/i,y, Rondom Yoriablcs. and Stochastic Pmcesses, McGraw-Hill, 1984. J. G. Proakis. Diairal Communicarions. 2‘“ Edition. McGnw-Hill, 1989. -

[ I O ] G. T. Chyi, J. G. Proakis, and C. M. Kelller, “On the symbol m m probability of maximum selection diversity reception schemes over a Rayleigh fading chnnnel;’lEEE Tmnons. Con~mun., vol. 21, no. I , pp. 79- 83, 1989.

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