RF over Fiber: Time to Exploit the Potential
RF over fiber has been acknowledged for some time as superior to copper coax and is now found in many teleports, typically as L-band inter-facility links. However, benefits such as increased link lengths and isolation from EMP (e.g. lightning) are just the start. Additional functionality such as multiplexing, remote monitoring and ultra-long distance links promise improved service provision and potentially huge cost savings.
Satcom is Behind the Curve
RF over fiber (RFoF) is a mature technology and is being increasingly adopted within the satcom industry. However, arguably the satcom industry is behind the curve. The broadcast industry has been using RFoF in live outside broadcasting for years. Fiber can cover the large areas required to support the roaming of wireless cameras around a race track, sports stadium or golf course. Today, live coverage of major sporting events usually involves RF over fiber.
Copper is not a good conductor at high frequency because most of the current is carried close to the skin of the cable. Consequently, copper links are capable of supporting only a few hundred meters of RF transmission and this gets worse with increasing frequency. In contrast, RFoF links can support the physical expansion of today’s teleports without the need for additional signal processing which introduces cost and noise.
The very low losses of fiber, typically only 0.6dB per km, mean that RFoF links from most manufacturers support at least 1km, and often up to 10km as standard.
Some manufacturers are now offering link lengths of up to 100km that still do not require amplification or optical dispersion compensation. Such distances now offer architectural choices for teleport operators that would be unthinkable using copper. For example, a single control room can manage multiple teleports within a radius of 100km. However, the typical distance of a teleport IFL link is only 100m. In other words, a typical teleport will make use of only 0.1 percent of the distance capability of an RFoF link.
A long distance RFoF link can enable a teleport to maintain service provision during periods of maintenance by borrowing feeds from other teleports using leased (dark) fiber. Fiber leasing costs can be reduced by over 90 percent by multiplexing. Current DWDM technology supports up to 80 L-band links on a single fiber.
This is particularly relevant for the Ku band and the problems caused by rain-fade. Long distance RFoF links between teleports allow an operator to maintain service provision by routing to another site that is not affected by the bad weather.
IFLs over Leased Fiber
The reliability of leased fiber in a mission-critical uplink service has been proven for some time. The UK broadcasting hub at Media City in Salford is a good example. In 2012, satellite uplink provider SiS Live decided to locate their teleport on the opposite side of the River Irwell to their HQ. The two sites are connected using separate loops of leased fiber to enable redundancy. A total of 80 circuits, over 70 of which are L-band carry live broadcast traffic, yet remain entirely invisible in the application.
Because of the low losses of fiber, antennas can be placed in an optimal location without being constrained by link distance. On flat ground in the middle of nowhere, placement of an antenna isn’t going to be a big problem. However, in built up areas or mountainous terrain, being able to place an antenna in a location which optimizes the view of the sky can be critical to the level of service.
The transparency of the link means ease of use and little risk of obsolescence. RFoF simply transfers an analog signal from the electrical domain to the optical domain and back again. The link is therefore agnostic of the traffic signal and can supports any modulation format. The concept itself is rather straightforward. In fact, it is even possible to use equipment from different manufacturers for transmit and receive.
We refer to the technology as “RF over fiber”, but in reality, today’s systems can also transport low frequency timing and reference signals for equipment synchronisation, either multiplexed with traffic or on a separate link. Serial data and even gigabit Ethernet are also supported. For example a user can extend their LAN from the control room to the antenna.
Remote Monitor and Control
The use of SNMP to support remote monitor and control means links can either be integrated into an existing system or controlled remotely using a web browser from the control room. This can save a maintenance engineer a long walk or drive, depending on how inaccessible the antenna-end is, and could be a significant cost saving in itself. Plus, it isn’t only the links that can be remotely controlled. RFoF systems can also support LNB powering. The capability to remotely control the application of voltages and tones means an LNB can be controlled from anywhere on your network.
The satcom industry is failing to exploit the potential of RF over fiber. Innovative operators are enjoying the additional functionality and flexibility offered by today’s systems but half the teleports in the United States still use only copper coax. Moreover, ultra-long link distances and remote LNB powering offer the opportunity to radically reconfigure teleport infrastructure. A network of unmanned ground stations administered from a single control room hundreds of kilometers away with full redundancy and the capability to share feeds? Now that sounds like the future. VS
Joe Petrie is the marketing manager at ViaLite Communications. His 15 years of marketing experience includes European and global roles in RF communication, semiconductors and computing.