“We need you to add significant satellite downlink capacity, fix the legacy issues, oh, and do it within 3 months, ok Mark?”
I love a challenge, and this was just one of several major roles I was tasked with during the Babcock to Encompass building move and 200+ service migration.
Where do you start?
Well, experience helps! I have had to upgrade several systems over the years. One of which was to maintain all 24/7 downlinks while clearing out one half of a large flat roof of around 10 ageing dishes and antennas of various sizes so that roofers could renew the leaking roof beneath. As if this wasn’t a big enough challenge, I also had to install a completely new dish farm of some 25 dishes on the other half of the roof, which had already been renewed. It was tricky technically, but regular and sustained communication with all stakeholders was the key to a smooth changeover.
In this case the first task was a comprehensive internal audit, list the dishes and downlinks that Encompass already had worldwide, what downlinks had spare ports available, which racks were the IRDs going in, which IRD types did they already have and could we continue adding those for simplicity sake?
Then it was a case of identifying those services that needed migrating and their sources. As we had to maintain all those services 24/7 while we built them into Encompass we couldn’t just transport the IRDs between sites so the changeover would require careful planning on a service by service basis.
What issues did the audit reveal?
The assessment stage identified a number of challenges, from the location and the physical restrictions that prevented us from adding more dishes to the Chiswick building roof and the existing Chiswick coax L-band routing and distribution being mostly coaxial and at its technical limit.
The first issue was fairly straightforward to resolve, we had no choice but to bring the desired services in over circuits from alternative downlink sites that Encompass now owned, however experience shows new circuits never come quickly especially from greenfield sites, so always allow a very long lead time for these.
The second issue took just one look to realise the existing RF distribution was at its capacity. It consisted of rooftop dishes with a mix of LNB types with coax RF outputs. The good news was that these connected to rooftop RF to optical modulators and then onto a 24 fibre backbone from the roof to the central apparatus room (CAR). Once in the CAR these fibres fed into Optical to RF demodulators. The coax RF output from these then connected into a large ETL RF Matrix. The nature of these Matrix is that once cabled up you really want to avoid trying to disconnect as they are difficult to access and you could well cause more outages than you fix.
The other problem was the distance to a second CAR for further splitting and distribution. This was the location where our additional services were to be added too. This large distance over coax had caused equalisation problems where there were reports of intermittency, especially during bad weather.
How can the cable equalisation set-up cause faults?
As an engineer, it is a very satisfying feeling to replace a conventional coax satellite distribution system with a fibre optic system. It is also greatly appreciated by customers and control room staff alike as the system performance and reliability can soar. The majority of those frustrating intermittent freezing and blocking downlink issues can disappear overnight.
This is because when you are sending signals that range from 1Ghz to around 2GHz (the satellite L-Band) over a coax cable they all reduce (attenuation, measured in dB) at different amounts. So, a transponder that has a frequency of 1Ghz at the LNB Output will reduce by around 20dB if you measure it at the end of 100m of RG6 cable, but a transponder that has a frequency of 2GHz will reduce by around 30dB over the same cable, which is 10dB (or 10x) more.
This is caused by the skin effect. So, if the amplifiers and equalisers between the LNB and the IRD aren’t aligned perfectly to compensate for this difference at each step in the chain you will easily experience signal degradation. This can occur with legacy systems that just get continuously built onto with haste.
However, thankfully those same signals can now be sent over many kilometres of fibre and they will arrive exactly as they are at the LNB i.e. level with excellent performance and reliability.
So how do you ensure that you achieve a robust design that delivers high availability?
The best approach in this instance was to overlay and distribute by fibre, thus leaving the current system untouched. So, I designed a method to split the existing fibres just before they fed the demodulators, then run a “24 fibre cable” to the second CAR where a new set of demodulators would provide the best signal quality to a series of new local amp/splitter networks.
We could then systematically replace the current unreliable signals that fed the live kit as well as expand the available outputs dramatically. Any split in a fibre causes a 3dB loss in optical power to the existing demodulators so this needed compensating for independently as each fibre was connected in order to maintain current signal levels.
It is essential with RF services, even with fibre, that signal levels are accurately matched to the components through each step in the reception and distribution chain. For instance, too much light into an optical to RF demodulator will cause the device to distort signals for the entire downstream chain causing odd intermittent problems that may only occur on clear days. Too little light, perhaps due to a poor connection, will increase the noise floor dramatically such that blocking and freezing may be constant, especially on cloudy days.
If you would like to discuss significantly improving your RF distribution and service reliability, then visit our website at www.HawkMediaPartnership.com or contact us at connect@HawkMediaPartnership.com
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