RF Over Fiber | FAQ

An RF signal which is received through an antenna needs to be processed by a receiver.  Typically, antennas are connected to nearby receivers using coax cables.  Even though coax cables weaken the strength of the RF signal, RF power stays within the acceptable input power range of the receivers over short distances. However, in some applications using a coax cable becomes impractical or impossible due to the longer distance and RF over Fiber must be used.

RF over Fiber is a technology used to transmit Radio Frequency (RF) signals over long distances over Fiber.  It is also called RF over Glass, RF Optical Transceiver, and Coax Replacement solution. With RF over Fiber technology the RF signal is converted to an Optical Signal by the laser diode and converted again to an RF signal via a photo diode.

The decision to switch to RFoF is mainly based on the RF link budget calculation.  Since there is a coax cable loss for distances longer than 150 ft, RFoF must be used especially if high frequency above 10 GHz is used. RFoF is also used where coax cable deployment is difficult or costly due to its size and weight or where fiber already exists.

RFoF solutions are made of 2 modules.  The Tx module takes in the RF signal and via a modulation and laser converts it to an optical signal which can have a wavelength of 1310 nanometers or 1550 nanometers depending on the laser type used.  At the far end, the Rx module takes in the optical signal and converts it back to RF signal which then can be connected to a receiver. This is known as the direct modulation method.

Another method is the indirect modulation, where the RF signal is modulated by an external Mach Zehnder modulator and is then converted back again. This method is used for high frequency applications from 8 GHz up to 40 GHz.

In designing an RFoF link, one needs to calculate the optical link budget as well as the RF link budget.  The optical link budget is used to calculate the loss introduced by the fiber cable.  Even though the fiber has substantially less loss than the coax cable, there is still some signal degradation. Fiber cable typically introduces 0.25 dBo (dB optical loss) per kilometer (km).  There is a 1:2 ratio relationship between optical loss and RF loss.  0.25 dBo relates to 0.5 dB RF loss and the condition of the fiber cable. It could be a huge difference between the theoretical calculation and the real loss due to many parameters. It is therefore recommended to measure the optical power end to end. This can be achieved easily with RFOptic’s programmable RFoF series.

Once the optical link budget is calculated, now the end to end RF link budget has to be calculated.  For this, it becomes important to know the specs of the RFoF solution that will be used.  RFoF units based on the frequency range they support can introduce varying degrees of loss or gain.  RFOptic also offers RFoF solutions with nominal gain which means 0 dB S21 system gain.  The gain can be changed easily by using the Tx and Rx 30 dB RF attenuator so in a case of low loss adding gain is done easily and the link budget can be adjusted to the required value.

Example:  When using an RFoF solution that has a nominal gain (S21=0dB) if the input signal, S1, is -10 dBm, then the output signal, S2, will still be -10 dBm.   In this scenario if the Tx and Rx modules are connected with a 10 km single mode fiber, the fiber will introduce 10 x 0.5 dB = 5 dB loss, S2 will be -15 dBm.  The system gain, S21, will be -5 dB.

When calculating link budget, one should also consider if there are any fiber patch panels and/or connectors on the fiber line.  Typically, each connector will introduce about 0.5 dB loss.

The KPI for system gain is S21 and is measured in dB. It denotes the dB level difference between the output and input of the system.  If the input signal is -10 dBm and the output signal is -5dBm, then the S21 is 5 dB. Even when there is a loss, it is called system gain but noted as negative.  For example, if the input signal is -15 dBm and the output signal is -25 dBm, the system gain is -10 dB.  Typically the gain of the programmable RFoF is 10dB and 40 dB with LNA and 15 dB without LNA. RFOptic offers units with adjustable system gain. The GUI user can turn on the LNA in the Tx unit and adjust the attenuator to get to the system gain levels needed within the link budget.

RFoF system gain depends on the RF frequency signal. If the RFoF system gain doesn’t change much for the whole bandwidth that the unit supports, it is said to have a flat response. System gain is important for deployments where the input signal frequency changes. RFOptic units are very flat, meaning there is not much variation in the system gain. For example, RFOptic’s 3 GHz RFoF solution has a flatness of ±1.5 dB between 5 MHz to 3 GHz, and ±2.5dB between 5 MHz to 6 GHz.

This parameter is especially important to determine the maximum linear RF input power for an RFoF solution. A signal that is stronger than the P1db point of the RFoF system, it will be compressed and distorted. This causes the RFoF system not to be able to produce the same RF signal at the far end.

RFOptic provides an option to its customers to adjust the P1dB compression point to fit their needs.  In the GUI configurable units, P1dB point can be adjusted by modifying the S21 gain levels. In higher frequency units (8 Ghz – 20 Ghz), RFOptic can still adjust the parameter by incorporating a pre- and/or post LNA (low noise amplifier) based on the customer needs.