5.1 Introduction
5.2 The key areas to protect
    5.2.1 Practical networks
        5.2.1.1 The studio
        5.2.1.2 The ensemble multiplexer
        5.2.1.3 Distribution for ensemble multiplex to transmitters
        5.2.1.4 The transmitter station
5.3 Radio Authority requirements
    5.3.1 Data rates
    5.3.2 Spectrum mask
    5.3.3 Analogue and digital service areas
    5.3.4 Technical plans
        5.3.4.1 Co-siting of transmitters
        5.3.4.2 Software tools
        5.3.4.3 Multiplex FIC content and adherence to ETS 300 401
5.4 Summary

5.1 Introduction

As experience in the delivery of digital radio services increases, the boundary between cost and reliability/availability will be drawn. At the time of writing, there are currently few receivers in the market. So, cost of entry into digital radio delivery must be kept down. ntl’s experiments so far have shown that the technology employed does not, on the whole, exhibit graceful failure. A digital radio system which is not working properly either produces no audio or data services at all or is so errored that the final product is unusable. So the question is how to configure a digital radio transmission network for the optimum cost/availability trade off.

back to top

5.2 The key areas to protect

The list below shows the most vulnerable parts of a digital radio network.

• The studio Musicam codec plus the service multiplexer.
• The ensemble multiplexer.
• Distribution – studio to ensemble multiplex and multiplex to transmitter.
• COFDM modulator.
• Time and frequency reference when operating in a single frequency network.
• Output power.

The way in which these elements are protected is dependent on the physical location of the studios, the ensemble multiplex and transmitting sites, the telecommunications connectivity used between them and the investment case which will be driven by the number of receivers in the market. The higher the required availability, the more duplication required.

back to top

5.2.1 Practical networks
The design of the studio hardware for digital radio must assume four sources: audio, programme associated data (PAD), non PAD data and service information (SI). At the start of the service it may not be deemed necessary to build duplication into the studio or to design for multiplex reconfiguration, but the “hooks” must be there to allow these functions to be added when the service provider deems fit.

ntl recommends the use of a service multiplexer at the studio using the service transport interface (STI) which will be, by late 1998, the ETSI standard for studio to ensemble multiplexer interconnection. The STI will allow interconnection between the studios and the ensemble multiplex over standard telecommunications links. Using the STI will allow, in the future, multiplexer reconfiguration to be used and, using a service multiplexer, will allow new data and audio services to be added at the studio as service delivery concepts develop.

The complete Musicam/PAD inserter/data formatter/service multiplexer system can be duplicated, monitored and switched between in the case of failure. Plus, the addition of ISDN back up can be employed for protection of the link between the studio and the ensemble multiplexer.

back to top

5.2.1.2 The ensemble multiplexer
The ensemble multiplexer is the heart of the network and is where all data is brought together for multiplexing and sending on to the transmitters. The multiplexer will take inputs from all the service providers in the form of Musicam plus PAD, SI and non PAD data.

The Musicam audio services have to be multiplexed together with any data sub-channels. The non PAD data can be added into the ensemble in two ways: as a stand alone data channel or it can be multiplexed with other data services in packet mode. The resulting data stream is added to the ensemble as a single data channel.

All the SIs must be added together from each of the service providers, to form part of the fast information channel (FIC). From the ensemble multiplex onwards the failure of a component now results in the loss of all services. This could result in six or so (depending on the number of services) advertising revenues being effected. Protection of the ensemble multiplexer is critical to network protection. So the output of the ensemble multiplexer, be it NI or NA frames, must be closely monitored, and on the detection of an error in the frames, the reserve system switched into operation.

back to top

5.2.1.3 Distribution for ensemble multiplex to transmitters
The output of the ensemble multiplex is a data stream called network independent (NI). This is a 2Mbits data stream which is not G704 compliant. The major problem with distribution over any telecommunications network for a digital radio SFN is guaranteeing that the data is transmitted at the same time at all the transmitters. No telecoms network, either terrestrial or satellite, has constant delay. Figure 32 shows a typical distribution system. The output of the ensemble multiplex at NI is converted to a G704 compliant frame structure (NA) plus the inclusion of a time stamp and Reed Solomon error protection. The time stamp = UTC+t with t = maximum conceivable delay across the network. UTC is taken from the GPS receiver at the ensemble multiplexer.

The data arrives at the transmitter site and the time stamp is read. The NA to NI conversion takes place and the data is held in the buffer until the time stamp equals UTC at the transmitting station. At this point the data is transmitted at all the transmitting stations synchronously no matter what the network delay was.

back to top

5.2.1.4 The transmitter station
Figure 33 shows the topology of a fully duplicated transmitter system. A full monitoring system also can be included comprising a telemetry unit, monitor receiver and system controller. At the start of service a single GPS, network adapter and COFDM modulator can be fitted and a full reserve system can be added when receivers become available in greater numbers.

Figure 34 shows a two module and a four module power amplifier. If each amplifier module contributes V₁ volts at the output.

	(2V1)2
The two module amplifier produces	-------------- watts
	50
	(4V1)2
The four module amplifier produces	-------------- watts
	50

If one module fails in each system the output power falls to:

	(V1)2
Two module	-------------- watts
	50

	(3V1)2
Four module	-------------- watts
	50

This equates to a loss in power of:

In a two module amplifier	6 dB
In a four module amplifier	2.5 dB

If there was a loss of one module in an eight module system the output power would fall by one 1 dB. So having a multi module design PA is a great advantage in reducing loss of coverage when a module fails. But there is a cost penalty in going to a multi module amplifier because of the increased complexity and component count in the amplifier.

The critical design criteria is looking at the module failure in the power amplifier. Figures 35, 36, 37 and 38 show the service area from Croydon transmitter site serving London. Coverage is being judged as 99% of location, and protected for 99% of the time in a 1 km square, having a signal strength of greater than 35 dB µV, at 1.5m receive antenna height.

      

The plots are calculated at 10m receive antenna height but measured results have shown that there is a 15 dB correlation between 10m and 1.5m receive antenna height and this factor has been applied. The table shows the importance of ensuring that a single component fault does not cause a large reduction in output power in the PA.

The important factor to be understood is that digital radio does not show graceful failure. The service is either there or not and the window of change from a usable service to a non-usable service is only a few dB in carrier to noise performance in the receiver front end. This is completely different to FM where the background audio noise rises as the signal strength reduces. In digital radio the service will be totally lost or become unusable to listeners on the edge of the service area when a PA module fails.

The decision on PA design, and therefore cost, has to be made on a site by site basis between the transmission provider and the multiplex operator looking at the effect of module failure on loss of population served and loss of key commuter routes.

Referring back to figure 33, after the PA a multi cavity filter is used, which reduces the intermodulation products down to a reasonable level, to allow the adjacentchannel to be used in the same geographical area. The intermodulation performance is known as the critical mask and is shown as in figure 39. The antenna will be duplicated but an antenna fault will reduce the ERP by 3 dB. With this in mind it may be worth considering additional head room in the amplifier or running the amplifier in a more non linear region when the antenna is on half stack (3 dB down).

back to top

5.3 Radio Authority requirements

It is recommended that the reader is fully acquainted with the Radio Authority documents. See Ref. 3.

5.3.1 Data rates
The Radio Authority will apply a bit rate minimum to a service depending on the type of programme being transmitted and will stipulate a period over which the bit rate cannot be changed (15 minutes).

5.3.2 Spectrum mask
The Radio Authority will define a spectrum mask for the output of the transmitter and it is envisaged that the mask will be the Eureka 147 critical mask. See figure 39.

5.3.3 Analogue and digital service areas
An analogue licence area is usually based on a coverage prediction from a specific site; in digital radio it will be different. The Radio Authority will specify the licensed area in the form of a polygon which will be defined by a series of points or the ground. Each point will be given a National Grid reference. The Radio Authority will also specify the frequency of transmission and define a minimum coverage in the polygon called a qualifying area. This will include a minimum population, certain settlements and certain sections of road. The Radio Authority will also define interference levels which will be co-channel into the polygon plus outgoing interference levels into other polygons which cannot be exceeded.

With this information the licence applicant can build a technical file or plan.

5.3.4 Technical plans
The applicant for a multiplex licence must submit a technical plan to prove to the Radio Authority that the applicant fully understands the technical issues of delivering a digital radio service.

The plan will include:
• The coverage the licensee is planning to achieve.
• The timescale for achievement giving detail of when each transmitter assignment will be implemented

The means of achievement:
• Location (National Grid reference).
• Aerial height agl.
• Maximum radiated power.
• A horizontal radiated pattern/template.
• A description of how the timing of each of the transmitters in the single frequency network will be set up and controlled.
• Assurance that such a plan can be implemented which will need to be in the form of a written undertaking from a transmission provision organisation.
• Adequate restrain of outgoing interference to other ensembles serving different areas, but on the same frequency.
• Recognition of the need to avoid causing adjacent channel interference to and from other ensembles serving the same area.

5.3.4.2 Software tools
To produce the technical plan frequency planning software is needed to take field strengths from multiple transmitters and form a composite field strength plot, plus be able to add in interference from co-channel transmitters radiating a different multiplex many kilometres away. Coverage plots, and population counts need to be produced giving 99% of time and 99% of location coverage predictions at 1.5m antenna height. This percentage for time and location has been chosen because of the non graceful failure of digital radio and the fact that small holes in coverage are very annoying. A 1.5m antenna height is chosen because of the requirement for mobile reception. Figure 40 shows a coverage prediction of a two station SFN covering Cardiff. Figure 41 gives the antenna pattern used on both sites.

5.3.4.3 Multiplex FIC content and adherence to ETS 300 401
The Radio Authority will need to be shown how the multiplex configuration information (MCI), which the receiver uses to select services in the ensemble, is not anti-competitive and is accurate. The way the service is implemented must adhere to the European Telecommunication Standard (ETS 300 401) and the Eureka 147 implementation guidelines which are assumed by the receiver manufacturers.

5.4 Summary

As the Radio Authority has put the onus on the multiplex licence applicants to provide a technical plan to prove that their implementation and role out plans are achievable, the planning of digital radio services is going to require a very close relationship between the multiplex licence applicants and the transmission service providers.