21 - application of mpeg-2 systems to terrestrial isdb-t

Application of MPEG-2 Systems to TerrestrialISDB (ISDB-T)

MICHIHIRO UEHARA

Invited Paper

NHK has developed the band segmented transmission orthog-onal frequency division multiplexing (BST-OFDM) scheme for thetransmission system of Integrated Services Digital Broadcasting-Terrestrial (ISDB-T). This scheme provides the great advantages ofhierarchical transmission and partial reception. To provide com-monality with other systems, the transport signal of ISDB-T adoptsthe MPEG-2 transport stream (TS). However, TS has been designedfor neither hierarchical transmission nor partial reception. Thus,to fulfill the requirements of ISDB-T, the TS has been adapted toprovide effective hierarchical transmissions and partial reception.This paper describes the TS generation methods used by the remul-tiplexer for minimizing receiver processing load. Briefly, they are:1) a method enabling hierarchical transmission and partial recep-tion of a single TS; 2) a method relating a TS packet to a segmentof the OFDM signal; 3) a method for interfacing the remultiplexerwith a modulator at a single constant clock; 4) a method for re-constructing a serial TS at receivers from hierarchical transmissionsignals allotted to layers in parallel by an OFDM multicarrier; and5) a method for correctly recovering the program clock reference(PCR) at a “partial reception” receiver, even if the TS rate of thereceiver is different from that of the transmission side.

Keywords—band segmented transmission OFDM (BST-OFDM),digital terrestrial broadcasting, hierarchical transmission, ISDB-T,MPEG-2 Transport Stream, OFDM, partial reception.

I. INTRODUCTION

NHK has been researching band segmented transmissionorthogonal frequency division multiplexing (BST-OFDM)as a transmission system for both television and audiobroadcasts in digital terrestrial broadcasting [1]. Utilizingthe multicarrier modulation characteristics of OFDM, thesystem enables hierarchical transmission, which combinesfixed-reception and mobile-reception modes in one group ofsegments, and partial reception whereby a receiver picks out

Manuscript received March 3, 2005; revised August 24, 2005.The author is with the Planning Division, Engineering Administration De-

partment, NHK (Japan Broadcasting Corporation), Tokyo 150-8001, Japan(e-mail: [email protected]).

Digital Object Identifier 10.1109/JPROC.2006.859695

only a part of the segments. Here, a segment is the basic unitof BST-OFDM transmission in the frequency domain.

MPEG-2 Systems [2] should be used as the multiplexingscheme for the transport layer to maintain interoperabilitywith other digital broadcasting systems, such as ISDB-S,ISDB-C, and ISDB-T . However, the MPEG-2 transportstream (TS) takes neither hierarchical transmission nor par-tial reception into account.

In the following, we examine of the use of a single TSfor hierarchical transmission and partial reception in BST-OFDM for minimizing receiver processing load, instead ofusing multiple TSs for the corresponding hierarchical layers.First, we describe the frame structure of the OFDM signaland TS in ISDB-T. Next, we describe a method to interfacethe remultiplexer and a modulator by using a single constantclock, and a method to regenerate the same TS sent from thetransmitting side at the receiver by using the frame structure.Finally, we describe a method for setting TS time stamps toaccomplish partial reception.

II. APPLICATION OF SINGLE TRANSPORT STREAM OF

MPEG-2 SYSTEMS TO HIERARCHICAL TRANSMISSION

The transport stream provided by MPEG-2 Systems(ISO/IEC 13818-1) is used as a multiplexing technique inmany digital broadcasting systems around the world. Tomake up a program by synchronizing video, audio, andother monomedia components, MPEG-2 Systems uses timestamps that indicate the time for decoding and presentingeach component. These time stamps are based on a programclock reference (PCR) that is transmitted in the header ofa TS packet (TSP). MPEG-2 Systems does not, however,provide a way for synchronizing multiple TSs, which meansthat all program components need to be multiplexed in asingle TS. In other words, the MPEG-2 TS as it is cannot beapplied to hierarchical transmission. It is therefore desirablethat a single TS be able to accommodate hierarchical trans-mission instead of using multiple TSs for correspondinghierarchical layers with BST-OFDM.

0018-9219/$20.00 © 2006 IEEE

PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006 261

Table 1OFDM Segment Parameters

Table 2Provisional Specifications of Digital Terrestrial Broadcasting

Tables 1 and 2 list the specifications of the BST-OFDMsystem used in this development, and Fig. 1 shows theassignment of OFDM segments in the frequency domain.ISDB-T television broadcasting consists of 13 OFDM seg-ments having a total bandwidth of 5.6 MHz. Although this13-segment format enables a maximum of 13 hierarchicallayers in theory, the number of layers is limited to three atmost in practical applications.

In addition to the television transmissions with the 13-seg-ment format, ISDB-T can provide digital audio broadcastingin either of two formats; a 430-kHz bandwidth one-segmentformat consisting of a single OFDM segment; and a 1.3-MHzbandwidth three-segment format consisting of three OFDMsegments. The one-segment format corresponds to segmentno. 0 in Fig. 1, and the three-segment format corresponds tosegment nos. 0, 1, and 2. In the three-segment format, seg-ment no. 0 is always allocated for partial reception and thenumber of hierarchical layers is always two.

A. Frame Structure of OFDM Signal and TS

Fig. 2 shows the system diagram on the transmit side forhierarchical transmission of a single TS. In the transmissionprocess, TSs inputted to the remultiplexer are combined intoa single TS. Then, the single TS is separated into specifichierarchical layers TSP by TSP, and the TSPs are modulatedthrough a channel coding process on each hierarchical layer.

262 PROCEEDINGS OF THE IEEE, VOL. 94, NO. 1, JANUARY 2006

Fig. 1. Arrangement of OFDM segments.

Fig. 2. Transmit-side system diagram.

Finally, the TSPs of each hierarchical layer are transmittedin parallel by using a multicarrier OFDM signal.

In order to decode the signal with a very short time delay,the system has to periodically transmit synchronizing pointsindicating where the tops of the TSP data align in all hier-archical layers. Since the modulation scheme of each layercan be arbitrarily selected, the transmission capacity of eachlayer will vary depending on the transmission parameters.To cope with this issue, stuffing dummy bits have been intro-duced at the end of the aligned packet data array in all layers.

However, the use of the dummy bits decreases transmis-sion capacity; hence it would be desirable to make the trans-mission capacity compatible with original TS itself. For thispurpose, the amount of data transmitted by using one OFDMsegment in a certain period needs to correspond to an integralmultiple of the number of TSP data in each segment.

Letting denote the number of symbols, the total bits ofone OFDM segment data can be written as follows:

(1)

where

the number of effective carriers in one OFDMsegment,

the number of bits per carrier-modulated symbol and

error-correcting code rate.

When the value of for a certain number of symbolscan be made times TSP ( b, where is a posi-tive integer) for all OFDM-segment transmission parameters,sufficient compatibility between the OFDM segment and thenumber of TSPs can be achieved.

Table 3Number of Symbols in OFDM Segment

Considering this requirement, the following equation isderived from (1):

(2)

Table 3 lists for all combinations of transmission pa-rameters for OFDM-segment Mode 1 in Table 1. This result

UEHARA: APPLICATION OF MPEG-2 SYSTEMS TO TERRESTRIAL ISDB (ISDB-T) 263

Table 4Number of TSPs in 1-OFDM-Segment/1-OFDM-Frame

reveals that sufficient compatibility can be achieved with re-spect to the number of TSPs by giving the OFDM signal aframe structure where one frame consists of an integral mul-tiple of 204 symbols.

On the receiver side, a shorter frame length is desirableconsidering the time required for establishing synchroniza-tion. For this reason, the number of symbols composing aframe is chosen to be the minimum multiple of 204. ThisOFDM-signal frame is called an OFDM frame.

Mode 2 and Mode 3, respectively, have twice and fourtimes the number of carriers of Mode 1, which means that thenumber of symbols within the OFDM frame can be made halfand quarter that of Mode 1. However, this number of symbolswithin an OFDM frame is set at 204 regardless of the mode,for the sake of simplifying the receiver’s processing. Table 4lists the number of TSPs in one OFDM segment per OFDMframe.

B. Common Interface Clock for Various TS Bit Rates

The TS can take on a variety of bit rates according tothe transmission parameters of each hierarchical layer. Thismeans that a variety of clock signals would have to be gen-erated at the receiver to accommodate these various bit rates,which places a significant workload on the receiver.

Adding an appropriate number of extra TSPs (null TSPs)to the TS (valid TSPs) so as to interface with a fixed clockis a solution to this problem. Letting denote the OFDMframe length and the interface clock of the TS, the totalnumber of bits to be input from the interface point withinthe duration of one OFDM frame can be written as follows:

(3)

Here, an interface can be achieved with a common clockby selecting to be at least the maximum number of bitsthat can be transmitted by the OFDM signal. The number ofdummy bits to be inserted is equal to the difference between

and the number of bits transmitted in an OFDM frame.

Table 5Results of Calculating Fm/Fs

On the other hand, the process using common 204-B unitsneeds a common interface clock, because RS(204,188) is em-ployed as the outer code and the TSPs of the interfaced TSare arranged in equal intervals for the purpose of synchro-nizing and regenerating TS packets on the receive side.

Considering the 204-B unit process, (3) can be rewrittenin the following conditional form:

(4)

and (5)

(6)

where

the maximum number of TSPs that can betransmitted by the OFDM signal,

an integer equal to or greater than ,

guard-interval ratio and

effective symbol length.

If is related to the fast Fourier transform (FFT) sam-pling clock by a simple integer ratio, only one base signalclock will be needed in the receiver, thereby making theequipment simpler. Given that the number of FFT samplesis , can be written as follows:

(7)

Equations (4), (6), and (7) give the following relationship:

(8)

Substituting (5) and (the number of symbols) in(8), we get

(9)


Table 6Number of TSPs Configuring a Multiplex Frame

Fig. 3. Configuration of receiver model.

To enable (9) to hold for all values of , we set to itsminimum value and get

(10)

Table 5 lists the results of calculating (10) for all threemodes in the 13-segment format, one-segment format, andthree-segment format. These results show that a TS interfacecan be achieved by using a common interface clock that isfour times the FFT sampling clock for the 13-segment andthree-segment formats and two times that for the one-seg-ment format.

The TS here takes on a periodic structure corresponding toan OFDM frame. A TS interfaced by using the clocks givenin the above in one OFDM frame period is called a multiplexframe. Table 6 lists the number of TSPs included in a multi-plex frame.

C. Agreement Between Transmit TS and Receive TS

As shown in Fig. 2, the TS arranged with a multiplex framestructure, which is called transmit TS hereafter, is separatedinto appropriate hierarchical layers TSP by TSP, and eachTSP is modulated with the assigned scheme. The modulatedsymbols are stored in the buffer of each hierarchical layer,mapped to a OFDM frame and transmitted in parallel byusing a multicarrier OFDM signal; therefore, the informa-tion on the sequential order of the transmit TS gets lost in theserial-to-parallel process. At the TSP separator, all null TSPsthat were inserted at remultiplexer so that transmit TS wouldbe kept constant clock are dropped to maintain modulationefficiency in each layer.

To recover the transmit TS at the receiver, the demodulatedTSPs data of each hierarchical layer must be synthesized inthe correct order with null TSPs inserted at the same positionsas in the transmit TS.

A typical method for doing the above is to add informationthat indicates the packet order so that the packets can be cor-rectly regenerated at the receiver. The following describesone possible method of attaching a sequence number to all


Fig. 4. Configuration of input signal to the hierarchical divider.

TSPs in every multiplex frame. From Table 6, one sees thatthe total number of TSPs included in one multiplex frame isat most 5120, which means that this method would require aminimum of 13 bits for a sequence number to each TSP.

To avoid adding extra data such as the sequence number,an algorithm to prescribe the TSP order within a multiplexframe so that the transmit TS can be recovered at the receiveris introduced.

Since an actual receiver treats the data in serial order afterthe FFT process, the two relations in the following enable theorder of TSPs in a multiplex frame, i.e., the multiplex framepattern, to be uniquely determined from the OFDM signalconfiguration.

1) the relation between carrier symbols of the OFDMsignal and the serial signal output from the FFT;

2) the relation between the serial signal and the TSP orderin a multiplex frame.

These relations are specified by the simple operation ofthe receiver model shown in Fig. 3. If the transmit TS is con-structed according to this specification at the transmitter, thereceiver can recover it correctly. Moreover, an actual receiverdesigned to operate according to this receiver model wouldhave the advantage that it does not require any computationto know the multiplex frame pattern.

1) Prescribing the Relation Between OFDM Carrier andFFT-Output Serial Signal: After carrier demodulation anddeinterleaving, the serial signal, i.e., the signal input to the hi-erarchical divider, is made in ascending order of the segmentnumber, and also in ascending order of the carrier frequencyof the information symbol within a segment (obtained byexcluding the carriers of the control symbol). Fig. 4 showsthe configuration of this serial signal. Here, dummy data in-cluded in one OFDM symbol correspond to the sum of sam-pling (equivalent to pilot signals), FFT sampling (sampling inexcess of the net signal band), and guard-interval sampling.

The serial signal for digital audio broadcasting is made ina similar manner; here, the segment data included in eachOFDM symbol consist of segment 0 in the one-segmentformat and segments 0-2 in the three-segment format.

2) Prescribing the Relation Between the Serial Signal andTSP Order in a Multiplex Frame: The serial signal, dividedinto multiple hierarchical layers, is then subjected to depunc-turing before being stored in the hierarchical buffer. At thistime, the number of bits that are depunctured and stored in thehierarchical buffer is determined by the convolutional-code

rate and the modulation scheme of each hierarchical layer.Here, the processing delay time is assumed to be the samein each hierarchical layer and can be treated as zero. SwitchS1 changes the hierarchical buffer when the stored data reachthe amount of one TS packet, and the data are instantaneouslytransferred to the TS buffer.

The TS reproduction part checks the existence ofTS-buffer data or not. If data exist, it switches S2 overto the TS-buffer position and reads out one TS packet data.If it does not, the TS reproduction part switches S2 over tothe null TSP position and inserts a null TSP. This processforms a signal having consecutive TSPs. Readout of the TSis carried out at the TS bit rate, that is, at four times fordigital television broadcasting and the three-segment digitalaudio broadcasting and at two times for one-segmentdigital audio broadcasting.

Switch S3 is used to alternately move between twoTS reproduction units for inputting a hierarchical-com-biner-output-signal at the beginning of an OFDM frame.Switch S4 is used to move between TS reproduction-unitsignal outputs. Depending on the pattern of the input serialsignal, i.e., the hierarchical configuration of the OFDMsignal, TSP data might be left in the TS buffer at the endof an OFDM frame. All data should be output within theOFDM frame duration, which is the same length as amultiplex frame. Accordingly, in order to delay outputtingTS from the TS reproduction unit, the movement of S4 isdelayed relative to S3.

To examine the delay time, an algorithm was run to searchfor the number of residual TSP data in the TS buffers at theend of an OFDM frame for all configurations of the OFDMsignal. Table 7 shows the results of this examination. Themaximum number of residual TSPs is two, which means thatS4 has to be delayed by at least two TSPs.

III. CORRECTLY RECOVERING PCR DURING

PARTIAL RECEPTION

The partial reception mechanism picks out only oneOFDM segment at position no. 0 from the 13-segmentformat signal or the three-segment format signal. Using thesignal structure, low-power handheld receivers that receiveonly one OFDM segment can be implemented.

In terms of the OFDM signal, partial reception means thatthe receiver demodulates only the carriers of OFDM segment


Table 7Maximum Number of Residual TSPs

Fig. 5. (a) PCR packet transmission in partial reception. (b) PCR packet transmission under transmission restrictions (Mode 1).

no. 0 in the multicarrier signal. However, in terms of the TS,it extracts only TSPs assigned for the partial-reception hierar-

chical layer to reconstruct a low-bit-rate TS from the 13-seg-ment or three-segment high-bit-rate TS. In general, when the


TS bit rate changes, the PCRs must be replaced in accordancewith the new TS bit rate.

PCRs that were correctly stamped for a high-bit-rate TSdo not have correct values for a reconstructed low-bit-rateTS. In Fig. 5(a), for example, TSP0 and TSPn, which carryPCRs, are extracted from the high-bit-rate TS and reconstructa low-bit-rate TS. Here, 0 and n denote locations in the multi-plex frame. Given that the beginning of the frame representsa reference time point, the PCRs of TSP0 and TSPn are re-ceived at time points different from their original time points,and therefore, the PCR interval between TSP0 and TSPn dif-fers between the high-bit-rate and low-bit-rate TS.

To avoid this PCR jitter, the location of the TSP that carriesPCR, i.e., the PCR packet, is restricted in the multiplex framewhen it is remultiplexed. In Mode 1, for example, only onePCR packet should be multiplexed per service for the dura-tion of a multiplex frame, and the multiplexing position mustremain constant for all multiplex frames [see Fig. 5(b)].

In this way, although some difference in offset may occurin the low bit rate TS, the PCR interval will always be equiv-alent to the multiplex frame period as shown in Fig. 5(b).Consequently, no PCR jitter occurs and no special processingsuch as PCR correction is necessary at the receiver side.

IV. CONCLUSION

In this paper, we presented the issues associated with hier-archical transmission by BST-OFDM using MPEG-2 TS anddescribed how we deal with them by using methods such asbuilding the OFDM signal and multiplex signal for ISDB-Tand minimizing receiver workload. The following summa-rizes the issues that were dealt with in this paper.

• Compatibility between OFDM segments and transportstream packets can be ensured by introducing a frameconfiguration of the OFDM signal and multiplex signal.

• An interface can be established with a fixed trans-mission clock by inserting null packets in a multiplexframe.

• Packets can be correctly separated and synthesizedwith respect to their appropriate hierarchical layers byprescribing a packet arrangement in a multiplex framebased on the receiver operation.

• In partial reception, setting restrictions in the PCR trans-mission on the transmit side enables simplified recep-tion without special processing such as PCR correctionat the receiver.

REFERENCES

[1] T. Kuroda and M. Sasaki, “Terrestrial ISDB system using bandsegmented transmission scheme,” in Proc. Int. Television Symp.ITVS 20 1997, pp. 641–654.

[2] Generic coding of moving pictures and associated audio: Systems,ISO/IEC 13818-1, Nov. 1994.

Michihiro Uehara received the B.E. degree inphysical electronics and the M.E. degree in elec-tric and electronics engineering from the TokyoInstitute of Technology, Tokyo, Japan, in 1987and 1989, respectively.

In 1989, he joined NHK, Tokyo, and from1994 to 2004 was with NHK Science and Tech-nical Research Laboratories, where he workedon channel coding and signal multiplexingfor satellite digital broadcasting systems andterrestrial digital broadcasting systems. He is

now Senior Engineer of the Planning Division, Engineering AdministrationDepartment, NHK.


21 - application of mpeg-2 systems to terrestrial isdb-t

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