Currently there are over 2400 satellites in orbit around the earth providing services from telecommunications, global positioning systems (GPS), disaster management and prediction, weather and military use. Majority of the satellites are privately owned and operated by various operators around the world. Some of the major operators are SES, Intelsat, Eutelsat and Telesat Canada.
Satellites have been in the forefront of delivery of telecommunications and related services around the world and is the only means of communications that has the capacity to reach the entire earth’s surface. There is currently no spot on the earth’s surface that does not have line of sight to a communications satellite.
The Satellite communication industry is set to grow in the coming years especially in the provision of DTH TV services (such as DStv), cellular back-haul and point to multi-point communications. The launch of more powerful and high-capacity Ka-band satellites will also go a long way in lowering satellite communication costs as the Ka-band satellites deliver more capacity for the same investment in launch and construction ( capacity wise, a Ka-band satellite is equal to about 100 Ku-band satellites)
With the growth of the use of satellites in communications, the problem of Radio Frequency Interference (RFI) is becoming more common in the world today. Majority of this RFI is mostly unintentional due to improper earth station set-up (configuration and dish pointing). Conditions that can cause RFI to occur when an earth station:
- Transmits at the right carrier frequency but causes cross polarity bleeding leading to the interference of the opposite pole carrier.
- Transmits on the wrong carrier frequency on the correct polarity.
- Does a combination of the above two.
- Transmit on the correct carrier frequency and polarity but at higher power overpowering other carriers from either the same satellite or adjacent ones.
For the above to be deemed as valid interference, there must be both the affecting the affected carrier. If there is no affected carrier then there is technically no Interference (e.g. if there is no carrier on the opposite polarity for condition 1 above, then there is no interference)
The problem of RFI is compounded by the use of TDMA or FDMA or a combination of both(TDMA/FDMA) or MCPC carriers where several earth station sites use the same frequency range to communicate. In such circumstances it becomes difficult in identifying a remote earth station that could be causing interference. The operator can identify the customer whose remote station is causing interference, but if the customer is transmitting from various remote stations (some times several thousand remotes spread over a large geographical location), the customer might not be in a position to single out the interfering remote station. In such situations, the operator has to shut down each remote sequentially until the interfering remote is identified, this approach is time-consuming (can take weeks or even months for many remote sites) and causes downtime to end users.
To minimize the chances of RFI, satellite operators have come up with checks to ensure that the danger of interference from a new remote earth station installation is minimized, these measures include enabling transmit on the remote only after the operator is satisfied that the installation was done to specified standards. The other option is to carry out cross-pole checks for every site using a pair spectrum analyzers, this is sometimes logistically difficult and expensive.
The Carrier ID approach
There has been a concerted effort recently by Intelsat and other operators in calling for the adoption of the ‘Carrier ID’ approach. This initiative calls for inclusion of a Carrier ID in carriers with MPEG transport streams. This therefore means that equipment manufacturers, satellite operators and customers will work together to ensure that any signal transmitted to the satellite will carry with it the following data:
- Customer name
- Contact telephone number
- Geo coordinates (latitude, longitude) of the transmitter
- Modem manufacturer name
- Modem serial number
This therefore means that if this system is adopted, it would make it easy to identify the source of interference to the exact location and contact the customer to rectify or shut down the transmission. This makes the elimination of RFI a straight forward process.
There is some opposition in the industry on the implementation of the carrier ID concept because:
- Some say that Carrier ID method will do nothing to eliminate interference, it will only alert us of who is causing it, they would rather advocate for proper training and use of certified equipment which is where the problem lies.
- Some say it will remove some anonymity of their transmission streams. This carrier ID info cannot be carried in encrypted format and this could open up some security concerns. Some operators use the modem serial number as the encryption key and transmitting it raw would render many encrypted channels vulnerable.
- The equipment manufacturers will have to redesign their equipment to be able to incorporate the carrier ID into the carrier. This they feel is not their burden and that the customer is obligated to ensure proper installation and configuration of remote equipment.
- The Carried ID portion will eat into the available user transmission capacity hence lowering the overall transponder capacity efficiency. This will lead to a longer ROI time for contracted capacity.
- The Carrier ID method is not fool proof. This is because users have the option of entering the carrier ID data. A user might enter wrong contact details for example and be unreachable to correct the RFI on his system.
I however feel that the initiative fronted by Intelsat is an idea whose time has come and satellite operators, equipment manufacturers, the ITU, IEEE and customers should work together to ensure its success. In addition to carrier ID, I would also propose more intelligent satellite systems that can identify and apply a band-stop filter to the suspected interfering carrier should the owner fail to rectify it within the stipulated time or if the owner is not reachable due to wrong contact information on the carrier ID.
The Carrier ID approach would save a lot of money spent in the identifying and eliminating sources of interference and improve satellite capacity uptime.
This approach should also be adopted by other wireless systems such as Wimax, SDH microwave systems etc.