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Page 1: [IEE International Broadcasting Conference IBC '95 - Amsterdam, Netherlands (14-18 Sept. 1995)] International Broadcasting Conference IBC '95 - Transmission of digital signals over

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TRANSMISSION OF DIGITAL SIGNALS OVER MMDS

S . O’Leary & K. Cleary

North West Labs, Republic of Ireland

ABSTRACT

This paper focuses on the work being undertaken with regard to the testing of the suitability and applicability of 64-QAM modulated signals over MMDS. This work is part funded under the accompanying measures of “Digital Image Transmission” of the EU RACE II programme as the project DIMMP.

The paper outlines briefly the market penetration and service characteristics of MMDS as an analogue transmission medium and the related functionality of such a system. Service comparisons with competing delivery media are described. The required equipment functionality at the transmit chain for introduction of digital services is then analysed.

There follows a description of the equipment configuration for undertaking a series of trials involving the transmission of a 64-QAM signal in an 8 MHz channel at 2.5 GHz. The trials will assess the threshold C/N for acceptable BER rates, the associated power efficiency, the impact of phase noise, the appropriate channel coding and the permitted rolloff when used in channels adjacent to analogue PAL signals. The related testing methodology together with some results will then be outlined.

Finally, the demonstration scenario for the broadcasting of a four programme MPEG2 multiplex over a single MMDS channel is developed.

MARKET PENETRATION OF MMDS

At present, the interest in wireless cable is growing significantly due to its ability to deliver unutilised bandwidth to broadcasters and also due to the economic attractiveness of this technology, especially in areas without an established infrastructure for the delivery of TV, e.g. emerging countries and rural areas. It is also considered to be attractive in the promotion of competition amongst operators within urban areas and is seen to extend existing networks to fringe areas.

The swift start-up of MMDS systems has been one of the factors engendering the proliferation of wireless cable systems throughout the world. In some cases, once the desired frequencies have been allocated, systems can be deployed in as little as 90 days. Implicit in this figure is the fact that wireless cable systems do not, by definition, require an extensive and expensive installed plant, unlike hardwired systems such as CATV.

Impressive growth figures (I) have resulted in many countries, such as Mexico, where one operator, MVS Multivision, had 300,000 MMDS subscribers by mid- 1994 having started the service in 1989. The company expects to have 420,000 subscribers by the end of 1995. Hundreds of MMDS licenses could become available in Brazil during 1995. Venezuela has two systems with a total of 100,000 subscribers, and, among Latin American countries, there are also installations in Chile, Ecuador and Panama. Middle Eastern countries such as Saudi Arabia, Kuwait, Jordan and Dubai all see wireless cable as a means of countering the access to Western programming made possible by direct satellite broadcasting.

In Hong Kong. MMDS has been chosen by the cable operator, Wharf Cable, to link some 1,000 hub sites. From each hub, coaxial cable takes TV signals to an average of 1,500 homes. Optical fibre will eventually supplant MMDS as a means of linking the hubs. Wharf Cable’s decision to start with MMDS and subsequently replace it with optical fibre was based on the desire for greater rollout speed and less initial outlay and also for the creation of a revenue stream and the ability to build first in the more profitable areas.

Hence we have seen that MMDS is attractive and has proliferated where:

it is impractical and expensive to cable a swift return on a relatively small outlay is required.

THE SERVICE AND SYSTEM REQUIREMENTS OF MMDS

This section seeks to articulate the demands and needs of the end-users of MMDS services. From such needs and requirements, the technical aspects of an MMDS

International Broadcasting Convention, 14-18 September 1995 Conference Publication No. 413,O IEE 1995.

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system are developed in response to the end-user services. Finally the economics of MMDS regarding its impact on the end-user is detailed.

Service Requirements

The needs of end-users of MMDS services are expanded from the perspective of the market. In short end-users want greater diversity of programming, expanded programme capacity and alternative services. First of all it is necessary to classify the term “end-user” into “domestic” and “professional”. Whereas domestic end-users generally implies the delivery of entertainment based programming to individual households, professional end-users require altemative services over the MMDS infrastructure.

Domestic End Users. To determine the service content desired by domestic end-users, a survey was issued to a sample of typical domestic users. This survey concemed itself with the content of programming required for a rural community and is used only as an indication of some of the requirements of domestic end- users. The survey indicated that the majority of users are interested in entertainment services, followed by local news and educational programming services. Regarding entertainment services most people expressed an interest in music and sports events, while social news, appointments and local weather were of preference in regard to news services.

Domestic end-users can thus be said to be interested in:

more programming: In numerous markets, the number of programmes available to the domestic viewer is not sufficient. Due to the propaganda potential of television, most countries throughout the world originally restricted their populations to the television services of the state-controlled or public service broadcaster. Such services are usually limited to one or two programmes. In Europe and North America, throughout the seventies and eighties, cable and satellite television driven by commercial operators set out to increase the available programming. MMDS in the nineties is another medium capable of delivering multichannel TV programming to viewers, who hitherto a limited range of programme content. The availability of MMDS delivered programming in a competitive environment involving satellite and cable increases the power of the consumer, ie the domestic end-user, in regard to choice of programming.

increased diversity in programnie content: The delivery of multiple programmes of similar programming content is not of interest to the consumer,

restricts his choice of viewing and is therefore not in the delivery operator’s best interests. This reasoning is valid for satellite, cable and MMDS operators. The development of programming packages such as that offered by British Sky Broadcasting for its DTH delivered programming from ASTRA is indicative of meeting the demand for theme based programme services, if one is delivering multiple programmes to the same viewers. The offering of movie based, sport based, music based, education based programmes etc in a multi-programme package for delivery over a common medium meets the increased diversity in programming requirements of the domestic viewer.

Furthermore this attribute can be expanded to include increased service diversity. As the transmission of digital television will enable the delivery of not just tens but potentially hundreds and even thousands of different programmes, diversity of programme service will become a requirement from the domestic viewers. Clearly not satisfied to switch between programme number 67 and programme number 123, viewers will require some distinction related to specific programme options in order to be attracted to use the available programming. The development of video-on-demand and near-video-on-demand services is indicative of this process and will need to be accommodated over MMDS as over other transmission media

locally based programming services: The desire for locally based programming, as realised in the reported survey, by the end-user is a valid attribute in that their is little point in delivering multiple programming packages if all of the programming content is irrelevant to the end-users’ environment. Delivery of programmes by satellite is global in that exactly the same content is distributed to all viewers located in the footprint, regional and local “optouts” are not technically possible. There is a demand among end-users for programmes with content related to their own particular environments. MMDS as a delivery medium can clearly respond to such demand in both urban and rural environments, in a manner which no other delivery medium can.

Professional End Users. Professional end-users can be described as those users who wish to use the existing MMDS infrastructure in aiding their business. Todate the market requirements of professional end-users has been perceived as ancillary to that of domestic end-user needs. Professional user needs usually are application specific, require interconnection between geographically remote locations and in some cases require some combinative element. Presently data broadcasting is a service required by professional end- users, other services tend to involve business and educational programming. Indeed distance education

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involving the Instructional Television Fixed Service (ITFS) in the US is a well established application using MMDS technology.

Professional end-users can thus be said to be interested in:

distributive service availability: The availability of bandwidth for specialist applications involving distribution of data is an important requirement for professional users. With present analogue TV standards, data broadcasting has utilised the teletext and NICAM technologies, in order to distribute data from one to many. This has been achieved without impinging on the bandwidth available to broadcast TV services. The development of digital multi-programme TV delivery per channel, should enable a more flexible accommodation of data programme services in the future. Such services will be transparent to the delivery medium employed.

service interactivity: As most professional users will need to interact reactively rather than passively with applications distributed by MMDS, provision for a return path will be essential. This attribute though not presently available with existing MMDS systems will become increasingly important in the future. Unlike most other end-user needs, this need is not transparent to the delivery medium.

System Requirements

The above service requirements translate into the following system requirements.

Quality of Service: The service delivered over MMDS, (eg HDTV, LDTV etc,) will need to remain transparent, ie no perceptible degradation in service quality will be permitted over the MMDS transport medium.

Service Availability: The absence of discontinuities or disruptions in the service due to the MMDS transport link expressed as a percentage of time in terms of worst month. The quantifiable values obtained are dependant on the RF frequency used and the atmospheric conditions within the permitted service area.

Service Interoperability wiih Alternaiive Media: The services delivered over MMDS will need to be transparent in order to enable service transportation over other delivery or communications media. The television standards used should not be specific to MMDS, rather MMDS should employ existing and

emergent digital TV standards for effecting digital broadcasting.

Bandwidth Ej'jkiency: given that one of the service requirements given above is an increased diversity of programme content, future MMDS systems must be bandwidth efficient. The advent of digital compression techniques and bandwidth-efficiend modulation techniques will alleviate the traditional achilles' heel of MMDS - the low number of available channels.

Return Channel Capability: a highly important service requirement for new digital services is that of interactivity. Future digital MMDS systems must incorporate a return channel, preferably hertzian, from the subscriber to the headend.

INTRODUCTION OF DIGITAL TRANSMISSION TECHNOLOGY ON MMDS

MMDS networks are characterised by the limited number of channels available in low RF frequency bands. In the cases of MMDS at 2.5 Ghz and 3.6 Ghz there is only 200 Mhz of spectrum available in the entire MMDS band. This coupled with frequency and network planning constraints reduces the effective number of channels in one MMDS system/cell to 12. Analogue transmission systems involving the encoding of black and white (luminance) information and colour (chrominance) information using PAL, SECAM, NTSC using VSB AM (ITU-R 624), only allow one programme per channel to be transported. As analogue formats do not involve any compression it is not possible to increase the spectrum efficiency.

In order to the meet the basic need of the end-users ie that of more programme capacity, it is necessary to introduce digital compression and transmission technology. The deployment of MPEG-2 technology (2) in broadcasting for compressing and then multiplexing programme components and entire TV programmes in one transport stream is set to become a reality. ISO/IEC 13818 is now an intemationally approved standard for the compression of video and audio and for the multiplexing of video, audio and data components (packetised elementary streams) into programme and transport streams. It is expected that this standard will be employed for source coding and multiplexing for all applications involving the transport of broadcast quality video over distributive and communitive media. Furthermore the success of the European DVB project in realising an ETSI draft standard for channel coding and modulation technology over satellite(3), CATV (4)

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and SMATV delivery media will impact on the digitial transmission technologies to be deployed over MMDS.

As the draft ETSI standards can accommodate a useful bit rate before channel coding of approx 30 Mbit/s in an 8 Mhz channel and as MPEG-2 can realise compression factors approaching 100:1, up to approx I O programmes per channel of LDTV quality could be realised.

Thus it can be said in all earnest that increased spectral efficiency is afforded by the developing digital transmission technologies for distributive applications. The data rate capacity of an 8 MHz channel is usually considered as a data container ( 5 ) into which any combination of programmes with same or different programme quality can be mapped. Thus if it is assumed that 30 Mbit/s of useful data could be accommodated in one 8 Mhz channel then eg two 15 Mbith HDTV programmes or two 10 Mbitis EDTV programmes and two 5 Mbitis SDTV programmes could be loaded into this container.

In can be thus extrapolated that in addition to the increased spectral efficiency afforded by multiple TV programmes per channel, there is enhanced programme flexibility ie the programme quality could reflected in the programme content. One might chose one LDTV programme to depict news services, while an EDTV programme could show sports material, for example.

TYPICAL DIGITAL MMDS TRANSMISSION CONFIGURATION

A typical MMDS transmitter consists of the reception of audio, video, data signals from a variety of sources including satellite, terrestrial TV, local programming and direct cable feeds. The received RF signals are processed, using baseband processing techniques to FM demodulate and subsequently decodeidescramble the satellite source signals, while heterodyne processing can be employed with terrestrial and cable feed sources. However, it is advisable to use baseband processing techniques over heterodyne processing where possible to ensure optimum signal transmission.

The baseband video audio and data signals can be applied to suitable gainkqualisation processing circuitry prior to being encodedscrambled. Scrambling and encoding of these signals is not necessary.

The baseband signals (video, audio, NlCAM data) are passed into a modulator unit. Here the video is modulated according to AM-VSB or FM techniques, the audio, the data signals are FM or QPSK modulated accordingly. The modulated signals at a suitable IF frequency are subsequently upconverted to RF

microwave frequencies. Each programming source contains a modulator-transmitter unit to effect microwave transmission.

High frequency stability is required of most transmitters. This is provided by a crystal oscillator in the upconverter mixer with frequency stability specifications of approximately +/-IO kHz.

In a possible digital MMDS headend configuration, programme sources could include existing analogue TV signals from satellite and terrestrial networks and local sources. Such signals form the input to an MPEG-2 compatible coder as baseband analogue video and audio components. If it is assumed that the MPEG-2 compatible coder can take as inputs in parallel four programme groupings i.e. four groups of one analogue video signal and one pair of stereo audio channels, then AID conversion, source coding and programme level multiplexing can be performed. Finally the transport multiplex layer of the MPEG-2 coder will enable the transport multiplexing of the four programme multiplexes into one serial bit stream.

Furthermore, if the diverse programme sources include digital multiplexed TV channels in accordance with MPEG-2 principles, then after channel tuning and downconversion to the second satellite IF, only QPSK demodulation and the related FEC decoding strategy need be employed prior to FEC coding and QAM modulation for MMDS. However, it must be stressed that this technique is dependent on the transport multiplexed bit stream output from the satellite QPSK demodulator is compatible with the input requirements of the MMDS QAM modulator. Such a transmodulation approach eliminates the need for MPEG-2 compatible coders and so is economical on equipment requirements for the operator.

This serial bit stream is the fed to a 64 QAM modulator incorporating channel coding for FEC. However, in order to correct for burst errors at the receiver it will be necessary to implement a two level FEC strategy of an outer Reed Solomon Code for an inner FEC Convolutional code. This strategy should enable the realisation of a Viterbi decoder for inner FEC decoding at the receiver.

The 64 QAM modulated signal can then be fed into an MMDS transmitter incorporating a channel upconvertor and RF amplifier. For 2.5 GHz MMDS, the amplifier can be implemented using GaAsFET technology, and in the case of 12 GHz systems, using TWT amplifiers (see section 3.1.1 below).

The RF signal is then passed into a combiner waveguide which filters and matches the RF signal in the 8 MHz channel passband onto the broadband waveguide. The

. .

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multi-channel multiplex of analogue and digital signals is then passed through low loss heliax cable to the transmitting antenna.

DIGITAL MMDS FIELD TRIAL

Digital television is recognised as one of the best means to deliver an improved quality of service, multimedia compatibility, interactivity and security (6). However, uncompressed digital TV transmission is impractical due to the enormous bandwidths necessary, a typical uncompressed digital video signal has a bit rate of approximately 270 Mb/s (7). The Digital Video Broadcasting group (DVB) has to-date agreed standards for the transmission of digital TV based on MPEG-2 coding. The standard for satellite systems uses QPSK with bit rates from 18.4 Mb/s - 48.4 Mbis. The cable standard uses 64 QAM with rates from 9.6 Mb/s - 38.4 Mb/s (8). Work has been carried out elsewhere (9) into the use of COFDM for digital TV transmission over MMDS resulting in successful field trials and the first HDTV transmission over MMDS . This paper will examine single carrier systems employing QPSK, 16- QAM, 32-QAM and 64-QAM modulation formats as transmitted over an existing MMDS system.

Wireless Cable Site Description. In Ireland the frequency allocation for wireless cable is from 2.5 GHz - 2.676 GHz. There are 22 possible 8 MHz channels, of which 11 are allowed to be used per cell. The cells are divided into eight groups depending on channel allocation, polarisation and line offset. The ce!l used in this experimental set-up has a fifteen mile assigned radius and consists of a standard IO metre vertically polarised cardioid antenna with a nominal 16 dBi gain, mounted at the top of a 67 metre high aerial which is located 238 metres above sea level.

The transmitter used is a standard AM-VSB type MMDS transmitter with baseband audio and video inputs, RF aural and visual outputs with a maximum visual peak power of 13 dBW. The RF outputs are extemally diplexed and subsequently combined with remaining channels in a broadband waveguide and the resulting bouquet i s fed to the transmit aerial. A description of similar commercial MMDS systems is available elsewhere( IO).

Experimental Setup. The experimental setup involved the modification of standard MMDS transmit equipment to accept a digitally compressed signal at an IF frequency of 35.57 MHz. The modulation and demodulation hardware was developed by Thompson Broadband Systems and successfully tested in a

laboratory environment. The details of the modem are available elsewhere ( I 1).

The transmitter configuration consists of a pseudo- random binary sequence of user configurable bit-rates depending upon the chosen modulation scheme which is input to the modulator. The output signal at IF is fed into a modified MMDS transmitter which upconverts the signal to a preset frequency at 2.672 GHz or Channel 22. The modifications allowed an input signal at this IF frequency and also allowed usage of an external local oscillator with low phase noise. The output signal is fed to an RF Wattmeter to accurately measure the average power level before being diplexed with other analogue channels and fed to the transmit antenna

The receiver was mounted in a signal survey van equipped with a 15 metre mast and a directional mesh type antenna with a gain of 24 dBi. The signal was passed to a standard MMDS downconverter with a nominal gain of 30 dB and a noise figure of less than 2 dB. This was modified to allow the usage of an extemal low phase noise local oscillator at a frequency of 2.005 GHz and an RF level of 13 dBm. The mixed output signal at Channel 45 (UHF) was fed to a professional TV receiver and the output at IF subsequently sent to the demodulator. It was possible to monitor the constellations for various formats and perform bit error- rate measurements. Also, the digital carrier-to-noise ratio was measured without modulation using numerical integration over the channel bandwidth. It was possible to visualise the magnitude of the equalisation coefficients with a PC to gain information about echo magnitudes under multipath conditions.

Experimental Results Synopsis. The transmission power of the digital signal was backed off by IO dB relative to the adjacent analogue channel and from a preliminary analysis of the results, this seemed to be adequate to achieve coverage with acceptable levels of quality. A comparison of the carrier-to-noise ratios between the digital and analogue signals showed that both were equally affected by propagation impairments. The bit error rate measured for a QPSK signal was found to improve dramatically over a 5 dB range from approximately 12 dB to 17 dB and above this, no errors were detected. For the 16-QAM case, to achieve a bit error rate of similar value required a carrier-to-noise ratio of approximately 27 dB, and for the 32-QAM case, the C M ratio necessary increased to approximately 31 dB. For the 64-QAM case, it was not possible to achieve error rates in excess of 10e-7 even with CM ratios in excess of 35 dB. However, it should be noted that all of these measurements were performed without forward error correction and thus coding gain must be expected. In all cases, a measurement was performed by

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taking a sampled signal from the waveguide and feeding it through passive attenuators directly to the Digital MMDS downconverter, and then by the injection of noise it was possible to achieve a transmission baseline characteristic for performance which excluded propagation effects.

It was also possible to transmit an MPEG-2 multiplexed video sequence containing 4 digital channels by using a DI video tape player and the 64-QAM modem with FEC. In the receive van, an MPEG-2 demultiplexer allowed the selection of any of the four channels. At all receive sites, the quality of the digital signal was superior to the comparative analogue channel.

11. Monnier R., 1995, DlMMP EuroDean WorkshoD o n

CONCLUSIONS

The field trials have successfully proven the viability of transmission of digital signals over MMDS systems using single carrier modulation schemes. The best performance was shown for QPSK, however the bit rate achievable was reduced to approximately 12.6 Mb/s. For 16-QAM, the bit error rate performance was excellent, but the achievable bit rate was only 25.3 Mb/s. For 32-QAM, a reduced bit error rate performance was balanced against a higher bit rate of 3 1.6 Mbis, whereas for 64-QAM, a bit rate of 38 Mb/s was achieved but with the poorest bit error rate performance. However, when FEC was imposed upon the 64-QAM signal, the bit rate was reduced to 32 Mb/s, but the digital TV subjective quality was excellent at all sites where reception was possible and was seen to be significantly better than the analogue comparative channels. The digital signal had no effect on adjacent channels and the propagation environment was seen to affect both digital and analogue similarly.

REFERENCES

1 , 1995, World Broadcast News. I8,22-26 2.1994,.DVB Document A003-Wser Reauirements for Cable and Satellite Deliverv of DVB Services” 3. DVB Document TM1432 “Report to DVB-TM on the first meetin? of DV B ad-hoc g o u p on Digital MMDS’ 4. 1995, International Broadcasting. 18,-12-16 5. DVB Document, “Background documents on Digital Video Broadcasting” 6. Waltrich J . , 1993, Montreux International Television Symposium and Technical Exhibition Papers 7. Natarajan K., 1995, IEEE So ectrum, 66-69 8. Fox B., 1995, IEEE Soectrum, 50-53 9. O’Leary S., Caffrey J . and Kerrin F.. 1995, WCAl Technical Svmposium IO. Hope P., 1988, Cable Television Enqineering, 14