4-SAT System
I encountered the problem of transporting Satellite TV signals over FTTH.
Typically Satellite transponders transmit to earth in the Ku band (around 12 Ghz).
At the satellite receive antenna, this carrier frequency is down-converted by a local oscillator in the LNB to the L band (around 1 Ghz). The signal is then carried over low-loss coaxial cable in the house to the STB (set top box) decoder which then sends the channel selected to the TV set.
The stream of many channels is transmitted in QPSK modulation (Quadrature Phase Shift Keying) and encrypted so as to stop hackers from watching Pay TV for free. Obviously the Satellite Pay TV companies are very keen to do nothing which would possibly increase the risk of hacking their programs.
In some countries, like Australia, they have used a hybrid cable system called QAM (Quadrature Amplitude Modulation) which requires the L -band signal to be converted down into the CATV UHF frequency region.
When I asked one satellite carrier if they would accept that option, they flatly refused, saying "you can carry our signal any way you want, as long as it stays exactly the way it emerges from the receive antenna".
This meant modulating the native QPSK stream at over 1 Ghz, something that most FTTH vendors don't offer. So I hunted around the world and found only two companies who could offer equipment to carry the Satellite native Intermediate frequency without modification.
I have since installed this system here in New Zealand and it works perfectly. The headend antenna sends the full QPSK stream over the fibre and into the STB without any modification.
But then the plot thickened! This operator started talking about enabling a second transponder polarity from the same satellite. This effectively doubles the potential number of channels they can offer from the same satellite transponder.
The subscriber chooses the new channel, the STB knows which QPSK stream it is on, and automatically changes the DC supply voltage sent up the coaxial cable to the LNB on the antenna.
The change in supply voltage is sensed by the LNB which changes its local oscillator frequency so as to switch polarities and thus QPSK streams.
This is easy enough when everybody has their own antenna on the roof. But imagine 1.000 or more subscribers all receiving their satellite TV from a single antenna! How do you switch?
Then the plot thickened again. A satellite operator in Australia now told me that they have FOUR QPSK streams to switch: two polarities on two different satellites.
So I went back to my supplier friends in Germany (www.bktel.com) and discussed how we could provide a solution, particularly since several other countries had exactly the same problem.
Thus was born the 4-SAT system which was just announced at the FTTH Council Conference in Melbourne recently. It will also feature at CommunicAsia 2009 in Singapore shortly.
Here's how it works:
Special LNBs have been developed which will output both vertical and horizontal polarities at the same time, stacked one above the other. A second LNB on the same dish, (or on a second dish, depending on the relative position of the satellites) will further stack the streams so that we end up with the four QPSK streams stacked one on top of the other.
The good thing is that the stacking is done right at the antenna without need for special equipment in the headend. We need however over 5 Ghz of total RF bandwidth to carry the four streams simultaneously on the 1550 nm channel.
Bktel have stretched the bandwidth performance of their light transmitter from 2.6 Ghz to 5.45 Ghz.
This modulated signal is then amplified by an EDFA (Erbium doped fibre amplifier) so as to provide correct signal levels throughout our network. It can then be multiplexed on to the same fibre as the data wavelengths and sent over the network, be it P2P or PON of either type.
At the home end, a simple but special ONU (optical network unit) will demultiplex and separate the four QPSK streams for presentation to the standard STB, without any reprogramming or other subterfuge. This ONU exists in two models, one for typical home use with outputs for up to four TV receivers or STBs The second is for MDUs (Multiple dwelling units) where each of the four streams are available separately for distribution throughout say a high-rise building.
The 4SAT system can also carry DAB/DTT signals simultaneously.
I have just received the documentation and specifications that we will be using at the Singapore show. These are available here as follows:
| 4SAT Presentation.ppt | A Powerpoint presentation of the system |
| Fiber_4SAT-1.pdf | A flyer with block diagram of the system |
| LNB.4SAT | Spec. sheet of the special new LNB |
| FX - DS55 Transmitter | Spec. sheet |
| XON80-4SAT-M | ONU with destacker and multiswitch |
| XON80-4SAT-T | ONU with destacker |
The system is simple, elegant and inexpensive.
Quotations and delivery will be available after CommunicAsia mid-June 2009.