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HomeArticlesDTHA Shortcut To VIewers Direct To Home Television
Wednesday, 21 June 2006 00:00

A Shortcut To VIewers Direct To Home Television

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This article dicusess, the histroical background in the DBS (DTH), freq. Allocations, channel coding, source coding, modulations suitable for satellite-based digital TV, and finally the standards for a Digital TV for India.

As in terrestrial broadcasting service, transmission in the broadcasting satellie.

Services (BSS) are intended for direct reception by the general public are characterized by high effective isotropic radiated power (E.I.R.P.) to permit the use of simple and inexpensive home receiving system. It is only during the last few years that high transmit powers from the satellites have become possible through the development of high power travelling wave tube amplifiers (TWTA), larger solar panels and the narrow and shaped antenna beams. The size of the receiving earth terminal antennas has been reducing. A complete 4 GHz TV signal cost 4 lakh rupees whereas 4 GHz direct reception system (DRS) for home viewers costs less than Rs 40,00/- today.

High power from Ku-band satellites has permitted the use of an antenna lesser than 1-m in diameter. Broadcasting satellites presently under development in Ku-Band, will have enough radiated power to enable the use of receive antenna as small as 0.5 m with DRS costing around Rs 8,000/- only. The trend is towards higher frequency bands. The users want to use smaller & smaller antennas, more number of programme channels and cost-effective systems.

Technology Trends and Evolution of SAT TV. Satellite TV is developing in Europe, America, Japan and also in India in response to the emergence of new technologies and the creation of a favourable commercial environment through the deregulation of the communication and broadcasting sectors.

As mentioned above in the new allocation of BSS frequency band 21.4-2 GHz, for Wide-HDTV in Regions 1 and 3 , several European organizations are encouraged to join a project called HD-SAT. This began in 1992 for a duration of 4 years. Technical feasibility of bandwidth efficient coding and digital modulation systems with a high picture are to be proven.

SATELLITE DIGITAL TV Before HDTV comes, digital TV is to be developed. The significant advantages are:

High and constant quality and service reliability Ruggedness against noise & interference. Spectrum efficiency & planning flexibility

Flexibility of the multiplex for different service configuration e.g. Multi-programme TV or HDTV.

The Satellite channel, in contrast to terrestrial broadcast and cable channels, is basically non-linear and wide-band.

The non-linearity is due to the amplitude and phase characteristics of the on-board TWTA, which is operated close to saturation in order to optimize the power of efficiency.

Consideration of the following technical factors is necessitated in optimizing digital satellite TV and HDTV.

Minimum bit rate per programme required to provide various levels of picture quality. Requirements in terms of C/N, C/I and B.E.R.

Suitable modulation and channel coding techniques and usable transmission capacity.

Interference compatibility with analogue FM/TV systems in a hybrid digital/analogue scenario.

Constraints due to the need for commonality with terrestrial digital TV/HDTV services on broadcast channels and cable network.

PROGRESS IN VIDEO COMPRESSION TECHNIQUES The remarkable achievements of video compression systems based on the use of hybrid DCT (discrete cosine transform) motion compensation and entropy coding have enabled the technical community to accomplish the standardisation of codecs for the transmission of conventional definition TV at 34 and 45 MBPS for contribution purposes.

The European project EU-256 of EBU has done a pioneering work in Digital TV and HDTV.

Recent results at various laboratories involved in this activity seem to indicate that a subjective quality virtually transparent to the studio-standard (CCIR Rec.601) can be achieved with a bit rate of about 0.9 bit/pel, while visible impairments are expected for some programme material at 0.4/bit/pel. These compression ratios, correspond to CCIR Rec.601 and to conventional composite TV signals. With the addition of some capacity for high quality sound, data services and error correction by Reed Solomon RS (255,239) code, gross bit rates of about 11 MBPS and 5.5 MBPS are therefore necessary for enhanced-definition TV (EDTV) and standard definition (SDTV) respectively.

At a gross bit rate of 4.5 MBPS, HDTV can easily be supported by satellite channels. The advanced stage of the several organizations world over into developing products very rapidly using this standard indicate interest in Higher end services. Particularly interesting in this context is the satellite delivery to home of multiprogramme digital TV, what we know as DTH or Direct to Home delivery.

THE TRANSPORT MULTIPLEX For broadcasting applications, the multiplex must be rugged against errors, in order to allow reliable system performance under critical receiving conditions. This can be achieved using fixed length information units(packets), which allow robust synchronization in the receiver.

Advanced modulation and channel coding systems proposed for satellite broadcasting often make use of a two-level error protection scheme, based on convolutional code (inner code) and a Reed Solomon Code RS(Outer Code). The two protection levels are separated by a suitable interleaving process to randomize the errors after viterbi decoding. The RS decoder should be associated with the demodulator i.e. before the demulitplexer, improving the error rate by several orders of magnitude.

To cope with the different propagation conditions on broadcasting channels, particularly in the 22 GHz freq. Range, hierarchical modulations and channel coding techniques have been proposed.

 

CHANNEL CODING & MODULATION

The Successful introduction of satellite digital TV requires the adoption of advanced transmission systems in order to minimize the satellite power requirements while permitting the use of small QPSK(2 bits/ Hz) and 8 PSK (3 bits/Hz) which allow the TWTA to operate close to saturation i.e. at its max power.

The following channel coding schemes, offering a wide range of spectrum and power efficiencies have been considered.

1. System A: QPSK rate 3/4

2. System B: TC-QPSK rate 7/8

3. System C TC-8 PSK 2/3

4. System D- TC –8 PSK rate 5/6

In all these cases, error protection is provided by the Reed-Solomon RS (255,239) “outer” code with a convolutional or Trellis code (TC) “inner” associated with the digital modem. The same “industry standard”, rate 1/2 Viterbi decoder already available commercially, can be used in all solutions. A typical BSS link including the digital modulator the satellite TWTA, the QMUX filter with 38.4 MHz bandwidth at –3 dB and an ideal demodulator has been simulated by computer. The optimised TWTA operating point was found/ Output back off(OBO) =0dB for QPSK and OBO=0.3 dB for 8PSK.

Systems based on QPSK modulation are easier to implement than systems based on 8-PSK which in addition require a larger implementation margin.

VLSI single-chip soft –decision viterbi decoders for rate 1 /2 convolutional code are already available commercially for a maximum. Clock rate of 45 MHz.

Single –chip RS (255.239) codecs could lead to the development of fully digital modems for TV/HDTV applications based on the advanced solutions currently available on communication satellites.

 

USE OF OFDM MODULATION IN SATELLITE BROADCASTING.

OFDM (Orthogonal freq. Division multiplex) is a multi-carrier modulation method which is particularly suitable for terrestrial broadcasting and cable distribution because of its inherent ruggedness against linear distortions caused by multipath propagation and by mis-matching in cable networks.

This modulation method shall be adopted for digital audio broadcasting (DAB) and is currently being proposed in Europe for terrestrial digital TV in the 7-8 MHz channels of the VHF/UHF bands. Its possible adoption in satellite channels will allow max. Receiver commonality on the various transmission media.

The digital systems based on OFDM allow siginificantly lower power efficiency with respect to single carrier (SC) systems with the same useful bit rate but at the cost of doubling the OFDM demodulator complexity which is already at the limit of today’s technology .

The use of FM modulation combined with OFDM/TC-16 QAM, would provide an interesting solution for satellite transmission, allowing operation with the TWTA at saturation. A common receiver may be used for terrestrial, cable and satellite receivers, the latter with an additional FM demodulator. It seems therefore that the OFDM approach high power onboard the satellite is necessary.

BSS SATELLITE IN THE WARC ’77 PLAN In the case of BSS satellites at Ku-band (WARC-77), the 27 MHz channel bandwidth was defined for the receiver while the satellite OMUX to the wider 50 MHz since each country was assigned five channels, which are separated from each other by about 77 MHz. (4 x 19.18 MHz). A fundamental requirement for the introduction of digital TV is the need to comply with the WARC’77 protection ratios in order to ensure coexistence with the analogue already in operation.

Various studies show that with PSK modulations raised cosine spectrum shaping (roll off 0.4.), a max symbol rate of 30 M Baud is usable in the WARC channels. This gives the possibility of using satellite E.I.R.P’s for the digital systems which are reduced least 4 dB w.r.t. the analogue systems, which are transmitted today at the full WARC’s 77 E.I.R.P. In the digital scenario , all these E.I.R.P’s could be reduced.

STANDARDIZATION FOR D-TV & HDTV On the basis of what has been described so far, it can be recommended that QPSK

Associated with a rate 3/4 inner code can be the modulation system at a gross bit-rate of 45 MBPS, including RS (255,239) error protection. High picture quality for the various applications (SDTV,EDTV,HDTV) should be achievable for most production material at the bit rates of 5.5 MBPS, 11 MBPS & 45 MBPS respectively.

For HDTV codecs, max. BER of 2 X 10-4 at the demodulator output (after viterbi decoding) ensures high quality pictures after RS error correction. Below this service continuity is lost.

The ideal service, continuity target (outage time) for digital TV/HDTV systems at ku-band would be 99.9% of the worst month. In the digital systems the target for high quality picture availability is set at 99.7% of the worst month.

In practice a low G/T is the main limiting factor, which then drives the remaining system parameters. Studies show that a single bit stream of around 55 MBPS gross bit rate could be sustained in a 36 MHz transponder having an e.i.r.p of 50 dBW.

As an example, 55 MBPS gross bit rate, assuming a simple rate 3 /4 convolutional code, gives a net bit rate of about 42 MBPS. This could support 10 channels of video at 4 MBPS, with associated digital stereo channel for each service. On the other hand, 55 MBPS could support, say two HDTV channels at a bit rate of 20 MBPS per HDTV channel.

So, this evolutionary scenario of satellite TV stimulated by the introduction of digital techniques has made the whole world go digital. In a longer-term perspective digital W-HDTV could be introduced in the 21.4-22, GHz band. But due to the severe propagation conditions at 21.4-22 GHz, a digital HDTV system may not be able to provide on the satellite transmit power. A method has been developed to extend the service continuity without increasing the satellite power. This advanced system is based on the adoption of layered modulation in conjunction with layered picture coding and layered channel coding. It provides graceful degradation from HDTV quality, achievable for most of the time, to conventional TV quality during deep rain fades.

FUTURE PROSPECTS Encouraged by the new frequency allocation of the band 21.4-22 GHz for the BSS in ITU Regions 1 & 3, which was achieved at WARC-92, which is intended for wideband HDTV, the research work is continuing in Europe, America and Japan. The goal is for the development of coding and transmission schemes for satellite and for cable networks in order to provide studio quality HDTV as available in today’s studios, in the viewer’s homes, accompanied by multi lingual/multi channel sound and powerful new data services. There is full confidence that a total bit rate of 45 MBPS (base band signal) would suffice to meet the requirements. Due care should be taken to ensure a coordinated development with respect to other delivery media. In future, there may be a need to make digital HDTV signals available also in broadband fibre-optic networks and by terrestrial broadcast emissions. The broadcasting of studio quality HDTV by satellite must therefore fit into the newly emerging landscape of digital HDTV and digital TV broadcasting. As much commonality as possible should be sought for the signal parameter pertinent to the various means of delivery.

Mr. D.S. Kushwaha

FIETE, FE, FBES.

Scientist, (Retd.) ISRO, PAST Chmn. IETE Ahamedabad

 

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