A study of alternative broadcasting technologies for the extension of DTV coverage in Taiwan

Correspondence

Extension of DTV Coverage in Taiwan

Hsiao-Cheng Yu, Jenn-Hwan Tarng, Hsueh-Ming Hang, and Zon-Yau Lee

Abstract—In Taiwan, there are six major NTSC terrestrial broadcasting

stations and 129 relay stations, which cover about 98% of Taiwan’s popu-lation. This paper addresses the planning of DTV broadcasting in Taiwan. DTV signals will be broadcast from the same six major transmitting sites and are expected to provide similar coverage. To extend DTV coverage be-yond the major transmitting sites, only the existing 9 NTSC relay stations with over 100W transmitting power would relay DTV signals. Residences living beyond the above coverage areas could rely on CATV or DTH to re-ceive terrestrial DTV programs.

Index Terms—CATV, coverage, DTH, DTV, LMDS, relaying, subsidy.

I. INTRODUCTION

An experimental ATSC transmitting station has been on the air since June 2000. Full coverage of DTV terrestrial broadcasting from the six major transmitting sites is scheduled to happen in late 2001.

This paper first summarizes how, in the past, Taiwan’s NTSC cov-erage was extended to hard-to-reach places. Alternative broadcasting technologies were assessed for the extension of DTV coverage in Taiwan. The findings of this research have been recommended to Taiwan’s Di-rectory General of Telecommunications (DGT) to be considered as part of Taiwan DTV implementation plan, including: the UHF/VHF taboo channels assignment, the government subsidy policy to households out-side of terrestrial TV coverage, and the policy regarding the use of the VHF band after the shutdown of NTSC, etc. Detailed information can be found in Taiwan DGT Report SEC88043 [1].

II. NTSC COVERAGEEXTENSION

In Taiwan, NTSC signal is broadcast from six major transmitting sites. Because Taiwan has mountainous terrain, there are close to 4 mil-lion people, out of the 21.7 milmil-lion populations, who cannot receive terrestrial TV signal from the six transmitting stations. The DGT of Taiwan has allocated 12 UHF channels (Chan. 37–48) for the purpose of relaying NTSC signals to areas out of reach from the six major trans-mitting stations. Over the past twenty years, the government of Taiwan has spent 656 million NT dollars (about 21 million US$) building a total of 129 relay stations. These relay stations extended the NTSC coverage to an additional 3.5 million people. Consequently, there still exist about half a million people who have always been deprived of terrestrial TV broadcasts.

Most of the relay stations are located on mountains in remote areas. Due to the high humidity of Taiwan’s semi-tropical climate, and the Manuscript second revision, Nov. 17, 2000. The work was partly sponsored by the Ministry of Education and the National Science Council of ROC, under contract no. 89-E-FA06-2-4; also by the Directorate General of Telecommuni-cations of ROC, under contract no. SEC 88043.

H.-C. Yu and Z.-Y. Lee are with the Institute of Management of Technology, Chiao-Tung University, 1001 Ta-Hsueh Rd. Hsinchu, Taiwan 30010.

J.-H. Tarng is with the Department of Communications Engineering, Chiao-Tung University, 1001 Ta-Hsueh Rd. Hsinchu, Taiwan 30010.

H.-M. Hang is with the Department of Electronics Engineering, Chiao-Tung University, 1001 Ta-Hsueh Rd. Hsinchu, Taiwan 30010.

Publisher Item Identifier S 0018-9316(00)11689-0.

strong typhoon gusts in the summer, maintenance of transmission equip-ment and the antenna towers has been a difficult and costly task. The maintenance and utility expenses of these relay stations totaled approx-imately 15 million NT dollars a year (about 500 000 US$). This amount of money used to be paid by the local township government. Recently, Taiwan legislators have cut government budget for this purpose, and ruled that commercial TV broadcasters should take this responsibility. The TV broadcasters wish that less costly alternative broadcasting tech-nologies could be used to extend DTV coverage in Taiwan.

III. ALTERNATIVEDTV COVERAGEEXTENSIONTECHNOLOGIES The DGT of Taiwan has assigned two extra UHF channels to each of the four commercial TV broadcasters for DTV broadcasting. Be-cause the existing six NTSC transmitting stations are located at ideal sites carefully surveyed, the same sites turned out to be the chosen sites for DTV transmission. Other benefits of co-locating DTV and NTSC transmitting stations include availability of land and efficiency in maintenance.

A 2-D wave propagation computer model was used to predict DTV signal coverage. The 2-D model uses ray optics approach, which de-scribes wave propagation in the vertical Transmitter-Receiver plane, including all relevant ray paths along the plane. In addition to the di-rect ray in the case of a Line-of-Sight link, reflected rays, reflected-diffracted rays, and multiple reflected-reflected-diffracted rays are considered. The method used for the calculation of diffraction effect is based on Uniform Theory of Diffraction. The total DTV field strength at a re-ceiver location is the summing of the field strengths of the line-of-sight ray, and the diffracted-reflected rays. The [2, eq. (1)–(8)] were the equa-tions implemented in the 2-D wave propagation computer model to cal-culate the field strength of multiple diffracted-reflected rays from the DTV transmitter to the receiver. The validation of the 2-D wave prop-agation computer model is shown in Appendix A.

Both the FCC guideline [3]–[5] and the Canadian ATV field test re-sults [6] suggested that if the DTV emission power should be 12 dB below the corresponding NTSC power, then DTV is expected to pro-vide equal or better service than the NTSC Grade B contour with avail-ability F(50,90). Taiwan NTSC broadcast is in the VHF band, while DTV broadcast is in the UHF band. Moreover, UHF signals suffer more severe diffraction losses than VHF signals, especially in the moun-tainous terrain typical of Taiwan. Through computer simulation ex-plained in Appendix B, the DTV emission power is suggested to be set at 7 dB below that of each of the corresponding NTSC transmitters in order to have similar coverage areas. The computer estimated cov-erage map of the six DTV transmitting stations is shown in Fig. 1. The comparison of NTSC and DTV coverage areas is listed in Table I. Nev-ertheless, the challenge of extending the DTV coverage beyond the six major transmitting stations remains to be resolved.

The following technologies have been assessed as candidates to extend DTV coverage in Taiwan, including, terrestrial relaying (UHF/VHF), LMDS (Local Multi-point Distribution System), MMDS (Multi-point Multi-channel Distribution System), DTH (Direct-To-Home), and cable TV system.

A. Terrestrial Relaying

Using UHF channels to relay NTSC signals has been a standard tech-nique for over two decades in Taiwan. Due to the recurrent

Fig. 1. Coverage map of the six DTV transmitting stations in Taiwan. ( = 90, E = 41 dB  V/m).

TABLE I

THENTSCANDDTV COVERAGEAREAS OF THESIXMAJORTRANSMITTING

STATIONS INTAIWAN. (NTSCE = 54 dB  V/m; DTV E = 41 dB V/m,  = 90)

nance burden and the lack of additional UHF channels, it is not tech-nically feasible or economically attractive for TV broadcasters to con-sider relaying DTV signals at all of the 129 relay stations. However, at the eleven high-power NTSC relay sites with transmission power over 100 W, relaying DTV is still the most economical way to bring DTV signals to a large number of households. The challenge is to as-sign the additional four TV channels at each of the eleven sites without causing interference to either the NTSC or DTV broadcasting. For the rest low-power relay stations, alternative technologies were sought to extend DTV coverage.

Because the four commercial TV broadcasters have used the 12 UHF channels for NTSC relaying at each of the 129 sites, only 5 out of the 11 high-power relay sites can find 4 UHF channels available to relay DTV signals. All of the 11 high-power relay sites were considered as potential candidates to use VHF taboo channels for DTV relaying. The 2-D wave propagation computer model was used to predict the DTV

Fig. 2. Coverage map of the nine DTV relay stations in Taiwan. ( = 90, E = 41 dB V/m).

and NTSC field strengths in order to check if the NTSC-INTO-DTV and DTV-INTO-NTSC adjacent channel interference constraints [4] are violated. This exercise led to the finding that 7 out of the 11 sites could use VHF taboo channels to relay DTV signals. Unfortunately, only 9 out of the 11 high-power relay sites can have either UHF chan-nels or VHF taboo chanchan-nels for DTV relaying. The coverage areas of the 9 DTV relay stations are shown in Fig. 2. The cost of investing in DTV relaying at these 9 sites certainly justifies the benefit of extending DTV coverage to additional 700 000 households in 18% of Taiwan’s land area.

B. MMDS

MMDS has long been used for TV distribution in rural areas in some countries. Even though the capital investment is low, due to the demand for radio frequency in the 2 GHz band is very crowded; the Taiwan DGT has no plan of allocating MMDS frequency for the purpose of TV relaying.

C. LMDS

LMDS is a new technology designed to offer broadband wireless telecommunications services. Because the current LMDS receiver equipment cost is still prohibitively high for consumers, its initial target customers are urban business offices with broadband commu-nications needs rather than households in remote villages looking to receive TV broadcasts.

Taiwan has liberalized the market of fixed telecommunications network service. 200 MHz has been allocated to each of three fixed telecommunications network operators, specifically for LMDS broad-band wireless access service. At least 50 LMDS hubs are expected to go into effect sometime in the year 2001. This timing matches with Taiwan DTV island-wide broadcast schedule. Because LMDS can carry multiple digital video channels, it can relay DTV signals to its recipients. From this point of view, LMDS is certainly a viable alternative to extend DTV coverage. However, it is very unlikely that LMDS deployment will reach the remote mountain villages beyond the coverage of terrestrial TV broadcasting.

Fig. 3. Layout of the field strength measurement locations along paths P1 (denoted by “ ”), P2 (denoted by “+”), and P3 (denoted by “X”).

Fig. 4. Illustration of measured and predicted DTV signal strengths on channel 35 along path P1.

D. Cable TV System

Taiwan has about 6 million households on an island of 36 006 square kilometers. The low average cable span per household passed has led to over 90% cable household coverage and over 80% cable penetration in Taiwan. The majority of Taiwan households now receive TV broadcast via cable. Therefore, Taiwan cable TV systems play an important role in delivering terrestrial TV programs to households. However, for some of those really hard-to-reach households, even cable TV operators find it uneconomical to offer service.

Most cable systems might not have additional channels or spectrum space to add DTV channels. There will either have to be enough in-centives to squeeze out some of their existing channels to make rooms for DTV channels; or cables have to expand capacity by upgrading to digital.

E. Direct-To-Home

Taiwan’s satellite broadcasting regulation was passed in January of 1999. By now, there are four licensed DTH operators. All of them have obtained re-transmission consents from the four commercial TV broad-casters. Households throughout Taiwan can receive terrestrial TV pro-grams in the DTH basic service package via a satellite antenna and a

Fig. 5. Illustration of measured and predicted DTV signal strengths on channel 35 along path P2.

Fig. 6. Illustration of measured and predicted DTV signal strengths on channel 35 along path P3.

Fig. 7. Illustration of measured and predicted NTSC signal strengths on channel 11 along path P1.

digital IRD (Integrated Receiver Device). Therefore, it is a technically feasible approach to extend DTV coverage to any of the hard-to-reach locations in Taiwan. On the economic side, DTH operators are willing to spare satellite transponders to carry terrestrial TV programs because they believe it is essential to the success of their DTH business. In-stead of subsidizing on building UHF relay stations to extend DTV erage, the government could subsidize households outside DTV cov-erage areas toward the purchase of satellite IRD.

Fig. 8. Illustration of measured and predicted NTSC signal strengths on channel 11 along path P2.

Fig. 9. Illustration of measured and predicted NTSC signal strengths on channel 11 along path P3.

IV. CONCLUSION A. NTSC Relaying at the 11 High-Power Relay Sites

For the 11 high-power relay sites, the existing NTSC relaying using UHF frequencies should continue because the number of households served by these relay stations is fairly large.

B. DTV Relaying at the 9 High-Power Relay Sites

For the 9 high-power relay sites, which have either 4 UHF chan-nels or 4 VHF taboo chanchan-nels available, relaying the 4 commercial TV broadcasters’ DTV signals is recommended. This can make additional 700 000 households accessible to DTV broadcasting.

C. NTSC Relaying at the 118 Low-Power Relay Sites

For the 118 low-power relay sites with transmission power below 100 W, the existing relaying of NTSC signal using UHF frequency is recommended to continue till the systems break down. Because NTSC is expected to shutdown, continued investment in NTSC relaying may not be as economical as compared with using DTH or cable TV to provide households with access to NTSC programs.

D. DTV Relaying at the 120 Relay Sites

For the 118 low-power relay sites with transmitter power below 100 W and the 2 high-power relay sites, relaying DTV signal is not recommended. One reason is because there are just not enough channels available to relay DTV and NTSC simultaneously during the NTSC-to-DTV transition period. The other reason is because Taiwan commercial TV broadcasters do not think it is economical to invest in

TABLE III

COVERAGEAREAS OFDTVANDNTSCATDIFFERENTLEVELS OFERPFROM

CENTRALBROADCASTINGSITE

TABLE IV

COVERAGEAREAS OFDTVANDNTSCATDIFFERENTLEVELS OFERPFROM

SOUTHERNBROADCASTINGSITE

DTV relay equipment at so many sites, especially because the potential number of viewers in such areas may be very limited. DTH or cable TV could be a convenient means to allow households in those areas to have access to terrestrial DTV programs. This recommendation is for the initial stage of DTV deployment in Taiwan. Once the TV broadcasters see the great potential of DTV in the future, they may want to invest in terrestrial relaying as a means to extend terrestrial DTV coverage.

E. Government Subsidy Policy

1) Means of Subsidy: It is deemed every taxpayer’s right to have access to free terrestrial TV programs. The government could con-sider subsidizing households residing outside of terrestrial TV cov-erage areas. Similar to the concept of the “Universal Service Fund” in telecommunications arena, a “Television Universal Service Fund” can be organized to pursue the goal of universal access to terrestrial TV programs by all residences. The subsidy can go to those households to-ward the purchasing of receiver boxes in order to receive the free TV programs from DTH or from cable TV service offerings.

The subsidy process is recommended to be administered by local township governments, which have detailed information about the DTV reception conditions within their jurisdictions. The DTH opera-tors and the local cable TV operaopera-tors are welcome to submit proposals for providing re-transmission of terrestrial DTV programs to their customers in an attempt to compete, in an open bidding process, for government subsidy.

2) Priority of Subsidy: Households, which are neither within the six major NTSC broadcasting areas, nor within any of the 129 NTSC relay areas, have never been able to receive terrestrial TV programs, should be subsidized at the highest priority.

TABLE V

COVERAGEAREAS OFDTVANDNTSCATDIFFERENTLEVELS OFERPFROM

YI-LANGBROADCASTINGSITE

TABLE VI

COVERAGEAREAS OFDTVANDNTSCATDIFFERENTLEVELS OFEREP

FROMHWA-LIANGBROADCASTINGSITE

TABLE VII

COVERAGEAREAS OFDTVANDNTSCATDIFFERENTLEVELS OFERPFROM

TAI-DONGBROADCASTINGSITE

Households, which are neither within the six major DTV broad-casting areas, nor within any of the 9 DTV relay areas, cannot receive terrestrial DTV programs but can receive terrestrial NTSC programs, should be subsidized at the second highest priority.

F. DTV Allotment Plan After the Shutdown of NTSC

Taiwan’s four commercial TV stations currently use VHF frequency band (Chan. 5/7/9/11) for NTSC broadcasting. This research recom-mended using VHF taboo channels (Chan. 6/8/10/12) for DTV relaying at 7 relay sites. After the shutdown of NTSC, channels 5/7/9/11 will be returned to DGT. Because some UHF channels currently used for NTSC relaying shall also be returned to DGT, these vacated UHF chan-nels can substitute for chanchan-nels 6/8/10/12 suggested for DTV relaying. The entire VHF band will become available by then.

APPENDIX A

VALIDATION OF THE2-D WAVEPROPAGATIONMODEL In order to verify the accuracy of the 2-D wave propagation computer model, a group of predicted signal strength data is compared with the actual measured signal strength in the field. The validation exercise in-volves DTV transmission at channel 35 in the UHF band, and NTSC transmission at channel 11 in the VHF band from the Northern Sta-tion. Field strength data are measured along three paths in the northern part of Taiwan. Fig. 3 illustrates the 45 measurement locations along the three paths P1, P2, and P3. Figs. 4–6 illustrate the measured and predicted DTV signal strengths along paths P1, P2 and P3 respec-tively. Figs. 7–9 illustrate the measured and predicted NTSC signal strengths along paths P1, P2 and P3 respectively. From these figures, it is found that the means of the estimation error(") range between 0.2 dB and07.2 dB, and the standard deviations (") range between

6 dB and 9 dB. The relatively close match between the measured and predicted field strength data shows that the 2-D wave propagation com-puter model is an acceptable tool to estimate radio coverage.

APPENDIX B

DETERMINATION OFDTV EFFECTIVERADIATIONPOWER(ERP) The ERP of DTV station is analyzed in order to find a proper value that could provide similar radio coverage as that of the NTSC station located at the same site. An effective radio coverage contour can be ex-pressed byC(Eth; ). It means the field strength at any location within

the contour should be higher than the threshold valueEthwith prob-ability a. In this study, is set at 0.9. In other words, 90% of the area enclosed by the contourC should have field strength above E_th. Note that, in Taiwan, NTSC broadcast is in the VHF band, while DTV broad-cast is in the UHF band. Based on the guidelines from FCC, theE_thfor NTSC and DTV are set at 54 dBv/m and 41 dB v/m respectively.

A preliminary study of the radio coverage by DTV transmitting at different ERP’s has been conducted. Tables II–VII list the DTV cov-erage areas at different ERP’s from the six major broadcasting sites. The results suggest that, in order for DTV to have similar coverage as that of NTSC, the ERP of a DTV station operating in the UHF band should be 7–9 dB lower than that of the corresponding NTSC station operating in the VHF band. This study chose 7 dB below as the ERP for DTV in an attempt to be on a more conservative side to estimate DTV coverage.

REFERENCES

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IEEE Trans. Antenans Propagat., vol. 40, no. 10, pp. 1121–1131, 1992.

[3] , ATSC, ATSC Digital Television Standards, Sept. 1995.

[4] “Fifth Report and Order, DTV Allocation,”, FCC 97-115, Apr. 22 1997. [5] “Sixth Report and Order, DTV Rules,”, FCC 97-116, Apr. 22 1997. [6] W. Y. Zou, Y. Wu, and M. Guillet, “Analysis of ATV transmission

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