A beginner’s guide to optimal data transmission using RF over Fibre
Author: Andy Haslam, Director of Business Development, PPM Systems
Radio Frequency (RF) Transmit (Tx) and Receive (Rx)
An RF system generally consists of an antenna to receive or transmit signals, RF conditioning such as amplifiers and filters, and a radio to generate or process the RF signals. Typically, these elements are coupled via copper coaxial (coax) cable, but this results in substantial losses – fundamentally reducing the effectiveness of the RF system.
The losses can attenuate (reduce) the strength of received signals such that they are lost in the noise level, or are simply too weak to be processed; this results in a reduced signal capture, which is a key function of an Rx RF system. The same attenuation significantly reduces the strength of the transmitted RF signal from the antenna, which reduces transmission range – a key function of a Tx RF system.
The coax cable losses impacting the effectiveness of typical RF systems are significant for higher frequencies e.g. 6 GHz and above, with notable impacts from 1 GHz. These losses are also significant over long cable lengths, with shorter lengths of 5 to 10 m having a notable impact at higher frequencies.
What is RF over Fibre technology?
RF over fibre (RFoF) is a highly effective method of distributing data, For an Rx application, it converts RF into light that is sent down a fibre optic cable to be converted back into the electrical domain for processing. The same process applies in reverse for Tx applications.
This technology is hardware agnostic (it works with any antenna, receiver or transmitter); can be utilised for both analogue and or digital data; and is effectively loss-less over hundreds of metres (we recently provided a 250km link). It has been adopted by many industries and related applications, including:
| Sector | Application |
| Satcom ground segment equipment | Teleports, broadcast facilities, VSAT gateways |
| Broadcast production | Wireless cameras, microphones, IFBs, Talkback |
| GPS for timing | Timeservers, base stations, re-radiating |
| Maritime | GPS, Satcom |
| Government and Defence | Secure Comms, Satcom, EWSI, ESM, ECM, ELINT |
| Cellular and timing | GPS, re-broadcasting, distribution |
| Mining and Public Safety | Timing, communications |
| Telemetry | Satellite, missiles, spacecraft |
How does RFoF work?
Simply, fibre optic cable replaces the coax cable link between the radio and the antenna, transmitting pulsed light through its hundreds of glass threads.
Using a receive application as an example, a laser diode converts the RF signal to light (referred to as Electrical-Optical (EO) conversion), which is transmitted along the fibre cable to the end point where it is re-converted to RF by a photo diode for the receiver (referred to as Optical-Electrical (OE) conversion).
Why is RFoF technology better than coax?
Unimpaired signal
Since RFoF is effectively lossless, RF signals can be transported over substantial distances without impairing the signal quality (eg attenuating / reducing the signal strength). For example, using a 100 m coax cable will give about 25 dB of loss, which is more than 50x the loss from RFoF technology at 30,000 m!
Resilience against interference
Fibre Optic cable is non-conductive and therefore immune to Electro-magnetic interference (EMI) including from lightning strikes and static, it also helps protect sensitive receiver or transmitter equipment from Radio Frequency Interference (RFI) sources such as Bluetooth, Wi-Fi, GPS and jamming devices.
Resilience against interception
Signals transmitted over fibre cannot be intercepted without physically compromising the cable. In the event of an interception attempt, monitoring equipment can easily identify this and cease data distribution if required. Conversely, coax cable can be intercepted without the receiver or operator being aware, potentially affecting strategic / tactical information advantage, national security, or public safety.
Practicality and scalability
Fibre optic cable is a fraction of the diameter of coax cable, making it lightweight, flexible, portable, and easier to install. This can either be used as a space save from the typical coax cable conduit, or to build in future capability growth opportunities. In comparison, coax cable needs to increase in size and weight as distances increase (noting the cable length limitations due to losses), or requires significant power amplification at a notable cost and size impact to partially mitigate these losses.
Could RFoF improve my organisation’s data transmission practices?
For organisations that rely on secure, lossless data transmission, or have longer distances between antennas and radio equipment; RFoF would be transformational. We have demonstrated increased Rx detection ranges of 5x compared to equivalent coax solutions, and an 86% improvement in un-amplified Tx power across a range of use cases.