Infact Tata-Sky, Star’s joint venture with Tatas for DTH services plans to use this system for achieving faster penetration of their DTH service in big cities.
Collective reception systems are installed for a number of reasons. A relatively simple system could be intended just to provide multiple reception points within a home, allowing independent programme selection at each reception point. More extensive systems could serve a block of apartments, where limited line of sight, local regulations or physical limitations on the number of antennas that can be installed preclude the installation of multiple individual reception systems. The cost to the viewer is potentially lower with collective reception systems, since the cost of equipment is shared amongst all users. Service availability will also be enhanced with a single, relatively large antenna for collective reception with respect to a typical DTH installation, where for aesthetic and practical reasons the antenna is often kept as small as possible.
Collective reception systems can be implemented in a number of different ways. These include:
a) FM/QPSK to AM re-modulation.
b) Switched IF systems.
c) IF-IF conversion.
d) IF-RF-IF conversion.
e) Remote controlled head-ends.
f) QPSK to QAM re-modulation.
Each of these technical solutions is discussed briefly in the following sections. DVB-S IRDs could be employed in the many of these architectures.
FM/QPSK to AM Re-Modulation
This is a traditional technique for distributing clear (unscrambled) analogue channels. The FM (analogue) or QPSK (digital) satellite signal is demodulated and the recovered baseband signal is used to amplitude modulate a new carrier in the frequency bands used for terrestrial television services (i.e. the VHF/UHF bands). The recovered baseband signal is not processed in any way prior to re-modulation. Signal distribution is via a single cable to individual TV sets equipped with a conventional VHF/UHF tuner. Such distribution systems are limited to number of channels, depending upon the system bandwidth. This total includes all terrestrial channels received by conventional means.
Figure 1 is an illustration of the spectrum of the broadband signal that is distributed to the individual TV sets. The “hyperband” that is located between the UHF and VHF frequency bands, is not usually used in distribution networks of this type.
Figure 2 is an illustration of the distribution system architecture. Networks of this type require one satellite receiver and one AM modulator per (satellite) channel at the “head-end” of the distribution network. These two functions are often combined in a “transmodulator”. A “quad” LNB is employed to simultaneously provide signals received on both polarisations (horizontal and vertical) and in both satellite frequency bands (the 10.7 - 11.7 GHz “low” band and the 11.7 - 12.75 GHz “high” band).
Existing distribution networks could be adapted to accommodate digital television transmissions by replacing individual FM TV receivers with digital IRDs at the head-end. However, given the potentially large number of services that can be delivered by digital means, the capacity of the distribution system would soon be exceeded. Conventional means (i.e. teletext) would also need to be relied upon for data services and some service information may be irrelevant to the end user. For example, tuning information for the satellite delivery network would have no meaning in the context of collective reception in the VHF/UHF bands. example).
Switched IF Systems
Switched IF systems distribute the satellite Intermediate Frequency (IF) signal supplied by the LNB directly to each user. They require multiple cables each having sufficient bandwidth to deliver both the full satellite IF range (950 – 2150 MHz) and the terrestrial frequency range (47 – 860 MHz). The number of cables depends upon the number of orbital positions, polarisations and frequency bands to be supported.
The system architecture for a single user is illustrated in Figure 3. In this example, reception is fr0m a single orbital position. As before, a quad LNB is used to deliver signals received on both polarisations (horizontal and vertical) and in both frequency bands (lower and upper).
Each user is equipped with a “multi-switch” that provides a number of outlets. The purpose of the switch is to select the correct polarisation and frequency band for the programme of interest. In other words, it switches between the signals delivered by the four outputs of the quad LNB. Polarisation/band switching for one outlet is independent of the switching for other outlets. Terrestrial signals are added to the IF signal delivered by the satellite and are distributed via the same cable network.
The domestic distribution network is flexible in that it supports a variety of different equipment (satellite FM TV receivers, digital satellite IRDs and conventional television sets), each having the capability for independent programme selection. As far as the user is concerned, the system is equivalent to having a dedicated DTH reception system. Switched IF distribution systems thus create a “virtual” antenna for each user. One difference with DTH installations is that the satellite IRD may require an equaliser to compensate for distortions introduced by the distribution network. The equaliser compensates for the fact that the attenuation of the network is not constant across the frequency band used to distribute the TV signals. This variation in attenuation (gain) is often referred to as the “gain slope”. The equaliser is sometimes built into the cable distribution amplifiers.
This distribution method has the important advantage that it conveys all the digital information that is available in the DVB-S multiplex directly to each user’s IRD. The IRD is identical to that used in DTH systems and the user is provided with all the services that a DTH viewer would enjoy. Furthermore, the system is transparent to any system used for conditional access (pay TV).
Figure 4 shows the general architecture of a switched IF distribution system, again for reception fr0m a single orbital position. A backbone comprising five cables, four carrying the satellite IF signals and one carrying the terrestrial signals, feeds each of the multi-switches connected in cascade. Each switch provides a through path for the input signals to feed the following switch in the cascade.
This architecture can be extended to support more than one orbital position, as indicated in Figure 5.The backbone now contains additional cables, the total number depending upon the number of orbital positions to be supported. The multi-switches must also provide a greater number of inputs to provide full flexibility to each user.
IF-to-IF conversion IF-to-IF conversion systems distribute a limited number of satellite transmissions chosen fr0m all those delivered by the satellite antenna. They “re-build” the satellite IF spectrum so that it is suitable for distribution via a single cable.
The satellite IF signal can be distributed along with signals occupying the terrestrial frequency bands. The latter could comprise any mixture of traditional AM signals, transmodulated satellite signals or even digital terrestrial signals.
The distribution network architecture is illustrated in Figure 6. Each user is equipped with a satellite receiver and can use a conventional television set to receive the terrestrial signals. The distribution network must operate up to a frequency of 2150 MHz. Once again, digital IRDs may benefit fr0m the use of an equaliser to compensate for distortions introduced by the distribution network.
As is the case for switched IF distribution systems, IF-IF conversion systems are essentially transparent to the signals that they carry. However, there is one important difference for digital satellite television signals. Because the IF signals are simply shifted in frequency, some of the service information delivered to the user will be inappropriate for the delivery network. In particular, tuning information, which is provided to allow IRDs to tune to the correct intermediate frequency for a particular multiplex when receiving directly fr0m the satellite, will be incorrect. Digital IRDs connected to IF-IF distribution networks may therefore not be able to tune automatically and may require a degree of manual intervention by the user.
As the name implies, distribution networks employing IF-RF-IF conversion are based on the IF-IF conversion approach with an additional frequency conversion to the “RF” band for distribution via the cable network. In this context, the term “RF” refers to the VHF/UHF frequencies used for terrestrial signal distribution.
The desired satellite transmissions are selected fr0m those supplied by the satellite delivery network and are converted to a frequency in the VHF band, the UHF band or the so-called “hyperband”. The selected satellite signals are distributed to the wall outlets at this frequency, along with the terrestrial signals that occupy the UHF and VHF bands.
As shown in Figure 7 the selected satellite transmissions are converted to a new intermediate frequency and are then downconverted to a frequency in the hyperband (or even in the UHF or VHF band). At the wall outlet the signal is fed to a block upconverter which shifts the (satellite) signals back into the satellite IF range so that they can be received using a standard satellite receiver. The user thus requires a block upconverter in addition to a satellite receiver. Again, the satellite IRD may benefit fr0m the use of an equaliser to compensate for distortions introduced by the distribution network.
The advantage of this system is that it is compatible with terrestrial distribution systems that do not provide sufficient bandwidth to support satellite IF distribution (i.e. systems that were designed and specified for frequencies up to 860 MHz only). Its principal disadvantage is that it can distribute only a very limited number of satellite signals in the available bandwidth. Furthermore, since it is unlikely that the satellite signals will be restored to their original IF frequencies, digital IRD tuning problems may also be encountered with this type of distribution system.
In the future, there is the possibility to have a remotely tuned transmodulator at the head end. The user would have a fixed frequency in the VHF/Hyperband/UHF band, all types of digital signals (satellite QPSK, terrestrial COFDM & CATV QAM) would be converted to a QAM signal in the local network. Each user would occupy approximately 8 MHz, so if all the bandwidth were free (50-850 MHz) 100 users could be supported on one cable with no restriction on the source of the digital programming. Of course with this combined transmodulation and frequency conversion, there are complex issues regarding the SI which.
The matrix switches should provide the possibility of combining the satellite IF signals with signals received on the terrestrial frequencies (47 – 862 MHz).
Optimum Network Configuration
The optimum network architecture for collective reception systems depends on many factors. Among these are:
i The type of services that are required.
ii The type of building that will accommodate the system (old or new, large or small).
iii The number of homes to be connected.
iv Whether or not there is an existing distribution network.
v The condition and capability of such networks.
Detailed consideration of such issues will ultimately determine the optimum architecture. However, it is possible to make some general observations.
Firstly, distribution systems should be transparent to Conditional Access (CA) systems that are used to implement, for example, pay-TV services. They should also be transparent, as far as possible, to the service information that is transmitted in the DVB-S multiplexes (such as, for example, tuning information). This is not possible with QPSK-QAM transmodulation, for example, unless the signals fr0m the satellite are de-multiplexed to allow the service information to be adapted to the new distribution network..
Secondly, low cost is important if users are to obtain cost savings or at least cost parity with DTH reception systems. Systems that require signals to be de-multiplexed are generally very expensive and consequently are often unsuitable for SMATV applications. Transmodulation fr0m QPSK to QAM also requires the availability of QAM terminals at a reasonable price.
Based on these observations, only architectures that rely on IF distribution of QPSK signals appear satisfactory today. Of these, switched IF architectures, which employ a multi-cable “backbone”, are usually only suitable for new buildings because of problems of physical access in existing buildings. Single cable solutions are more attractive in the latter, although there is a significant cost associated with upgrading a network designed for the distribution of terrestrial signals only to provide the bandwidth necessary to accommodate satellite IF signals (fr0m 47 MHz up to 2150 MHz).
Systems of this type should utilise the entire terrestrial frequency band (i.e. up to 850 MHz) before they are extended to 2150 MHz. Use of this band can be maximised by distributing clear AM analogue channels in the VHF/UHF bands and a limited number (5 or 6) of (satellite) QPSK signals in the hyperband. This is the “IF-RF-IF conversion” architecture.
Further QPSK signals can be accommodated by extending the network to 2150 MHz and using IF-IF conversion techniques.
Finally, if more QPSK signals are required in future, then the network could be reconfigured by allocating a frequency to each user and performing remote tuning at the head-end. This is the “remote controlled head-end” architecture which is limited to around. 30 users for a single cable.