4.1 Digital radio receivers
4.2 Data service reception
    4.2.1 Programme associated data
    4.2.2 Sub channel data services
    4.2.3 Using the receiver data interface (RDI)
4.3 Data transport methods
4.4 Data application
    4.4.1 Interactivity
    4.4.2 Speed of access
4.5 Summary

There is an inextricable interrelationship between data services and digital radio receivers. The data service must be coded in such a way that it can be presented to the user on the receiver. So the data service provider must have intimate knowledge of the platform on which the data is to be received. Figure 29 shows a conceptual model of data service provision.

4.1 digital radio receivers

Digital radio receivers are now starting to appear in many forms, including:

• Car receivers with an optional LCD screen and a 16 x 2 textual display.
• PC cards - A receiver card which uses the PC infrastructure for power audio output, control and data display.
• Hi-fi sets – Units are available for integration into stacks with LCD screens and textual displays.
• Portables – Battery power is a problem. Bosch have demonstrated a Walkman-like receiver but the battery autonomy is poor. However, chip sets are getting less power thirsty so it is only a matter of time before the digital radio Walkman is available.

Effective, low cost digital radio receivers must become available if there is to be a world wide acceptance of digital radio. There are some very interesting products now being demonstrated including a digital radio receiver and GSM receiver combined. This demonstrates the multipoint broadcast advantage of digital radio with the return path via the point to point GSM channel.

Figure 30 shows a block diagram of a receiver. The down converter mixes the RF signal from band III or L band down to a base band signal. The base band signal is then analogue to digital converted and the time domain samples are fed to a fast fourier transform (FFT). Out of the FFT comes the 1536 phase states (2 bit word per carrier, DQPSK) every 1.246ms in mode I.

The FFT is the heart of the reception process. A simple model of an FFT is shown in Figure 31 and the following description explains how the FFT can remove the data pairs from each single carrier in the ensemble.

Mixing a 1kHz sine carrier with 1536 sinusoidal carriers, all at 1kHz spacing, gives a DC component due to the 1kHz/1kHz mix and a large number of sinusoids as multiples of 1kHz. The integration time is 1ms and the integration of any sinusoid over 1ms, as long as it is a multiple of 1kHz, is zero. So out of the 1kHz tap comes a DC voltage which represents the phase state of the 1kHz carrier. Repeat the process for all 1536 carriers with 1536 mixers and 1536 oscillators and the 1536 phase states are known. This process happens on every symbol with 3072 bits decoded per symbol.

Referring to Figure 25 in section 3.5.1, the first symbol of a transmission frame is the null symbol where all the carriers are switched off, except for the TII. This tells the receiver where to start looking for the synchronisation information. The sync symbol is next and contains a Casac sequence. This is an auto correlation function that allows fine frequency lock to be gained, plus the symbol is used as a phase reference for the next symbol in the transmission frame. The frequency information from the sync symbol is used to drive the down conversion process and the clocking rate of the analogue to digital converters and the FFT.

The FIC is then decoded and, as this is not time interleaved, this happens quickly. This is used by the receiver to select the wanted sub-channel. The main service channel is then time de-interleaved and depending on the receiver implementation various processes happen. The best receivers convolutionally decode the co#mplete main service channel (MSC) but some only decode the wanted sub-channel. So depending on the implementation, the blocks marked with an asterix in Figure 30 may be reversed. In the case of an audio service being the wanted sub-channel, the Musicam data comes out of the convolutional decoder and is fed to a Musicam decoder and left and right audio are generated.

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4.2 data service reception

For a data service there are three options for delivery to the user using a dedicated receiver.

4.2.1 programme associated data
A PAD data service will be extracted from the Musicam frames and if it is to be presented on the receiver man machine interface (MMI) it will have to be decoded and presented in a form that the MMI can display. To do this, a protocol has been agreed by Eureka 147, called the multimedia object transport (MOT) protocol. Dynamic label (DLS) and dynamic range control (DRC) can also be carried in PAD and the agreed method for coding DLS and DRC is defined in the ETS (Ref. 1).

4.2.2 sub channel data services
This is handled in exactly the same way as PAD. If it is to use the MMI in the receiver, the data must be coded using MOT so the receiver can decode it and present the data via the receiver MMI.

4.2.3 using the receiver data interface (RDI)
The data handled by the RDI comprises one or more decoded sub-channels, the TII and the FIC. This gives the user access to the sub-channel so an external audio decoder can be used, or a PC can be used as the data service MMI. Using the PC as the data handling device means that the MOT protocol does not have to be used. The digital radio system can be used as a pipe and the encoding system used by the service provider can be any bespoke algorithm as long as:

• The data rate does not exceed the capacity of the data channel.
• The receive PC has the software to identify the way the data files have been formatted by the service provider.

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4.3 data transport methods

Inside the digital radio specification (Ref. 1) there are three ways of sending non audio data:

• Using PAD
• Using stream mode in a dedicated sub-channel
• Using packet mode in a dedicated sub-channel

Packet mode has many advantages because a number of data applications can be multiplexed together using a data multiplexer, (a statistical multiplexer) which can get, for example, four 16 kbits/s data services down a single 16 kbits/s sub-channel. This is due to the burst nature of data applications and their mean data rate being quite low. This makes for very efficient use of data capacity in the digital radio system.

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4.4 data application

For a data application to be a success the end user must find the service entertaining or useful, find it easy to interact with and the data services must be delivered to the user when they want it and at a reasonable speed. Examples of data applications could include:

• Using PAD
• Electronic radio times.
• Quizzes.
• Adverts.
• Shopping – for CDs, hotel rooms, concert tickets and so on.
• Information delivery – such as stocks and shares, sports results etc.
• Weather.
• Traffic – route finding facilities, highlight traffic congestion and warnings.

4.4.1 interactivity
For any of the above services to be a success they have to be easy to access, display and interact with. This requires that the delivery platform and software are well designed and the required end user input is simple and intuitive. This tends to lead towards web technology, HTML pages and web browsers.

4.4.2 speed of access
The data rates for the data services are fairly low so if there are many web pages to be accessed, it is important that they can be stored inside the receiver because the repetition rate of any specific file or page will be slow. The size of the receiver memory is important because the more files that can be stored, the faster the access to any page, i.e. a mini web site could be built up in the receiver. Finally, any file corrupted due to transmission, will be discarded by the receiver. Therefore the service provider must repeat the files on a regular basis to make the service reliable and so any user can join the service at any time.

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4.5 summary
1. Receivers and data services are inextricably linked.

2. If a data service is to be presented on a receiver an agreed protocol between service provider and receiver manufacturer has to be used.

3. If a PC receiver is used there is greater freedom in data services protocol.

4. Digital radio supports three data transport mechanisms, including programme associated data (PAD), stream mode and packet mode.

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