PCB Design for Radio-Over-Fiber Technology
It’s possible that radio-over-fiber technology is one of the least discussed aspects of fiber telecom networks. However, it stands to be one of the many portions of telecom infrastructure for bringing broadband wireless access to remote areas. This approach is likely to play a major role for future 5G infrastructure in rural areas where a standard wireless backhaul is not possible or cost-prohibitive.
Just like any other electronic system, radio-over-fiber links require PCBs. Radio-over-fiber is designed for passive optical networking, where the extracted RF signal is steered to its destination without active electronic components. This provides an extremely fast solution for faster data transfer in telecom networks. It is also scalable up to THz frequencies. Many engineers in the research community are focusing on light sources and fundamental infrastructure for this technology, but board designers also have a role to play in ensuring accurate signal extraction and routing at the Tx/Rx sides of a radio-over-fiber link.
What is Radio-over-fiber?
Radio-over-fiber is a method for transmitting RF signals to a distant receiver in passive optical networks. In radio-over-fiber, a radio signal is used to apply amplitude modulation to an optical signal from an infrared laser diode. This optical signal can then be sent through a standard single-mode fiber, providing reach up to ~20 km.
The primary advantage of radio-over-fiber technology is that it does not suffer from the same issues of attenuation as coax cable, allowing it to reach longer distances that cannot be reached with a traditional wireless backhaul. This technology offers much greater bandwidth over existing optical solutions without digital-to-analog conversion (DAC). This eliminates the latency problems that arise in active components as latency is effectively determined by the propagation delay for a radio signal on the PCB.
At the receive side, a photodiode is used to receive the optical signal, and an all-optical or GHz envelope detector is used to extract the encoded microwave signal. This allows extraction of a GHz analog signal, which can be used with a variety of amplitude modulation schemes (e.g., up to digitized QAM for LTE, WiFi, or other RF protocols). An overview of the architecture used in radio-over-fiber transmission is shown below.
The architecture shown above can also be used with DWDM in a BBU (Tx end). An arrayed wavelength grating unit can be used to separate different carriers and route them to different Rx ends. This provides a simple way to route analog radio-over-fiber signals to multiple receivers in a passive optical network.
Tx Side Layout Challenges
The layout challenges on the Tx side of a radio-over-fiber link depend on the transmission method. Laser diodes were used in a number of papers in the past, but mode combs are becoming a promising method for radio-over-fiber transmission. In earlier radio-over-fiber research and development, the output from a laser diode was modulated with the desired RF signal, and the output from the diode was coupled to a fiber using a standard transceiver. This method was limited in terms of the frequencies that could be transmitted were limited near WiFi frequencies due to the turn-on and recovery time of most laser diodes and photodiode receivers.
The layout challenges in upcoming applications for lower RF frequencies lies in routing the RF signal to the modulator. On the Tx side, the critical challenges consist of properly designing the board with sufficient isolation between the RF and optical sections. Transmission line design will still be confined around WiFi frequencies, which is greatly aided by routing tools with integrated electromagnetic field solvers. The real layout challenges begin at the Rx side, especially when we look into the high GHz and THz regimes.
Rx Side Layout Challenges
If the single laser diode is modulated, the same technique will likely be used on the Rx side. A better solution on the Tx side uses two free running lasers, which allows long-distance transmitted RF frequencies to reach GHz and even THz levels. The output from these lasers is made to beat through heterodyning, which produces an amplitude modulated output. Tunable vertical cavity surface emitting lasers (VCSEL) are currently being used to probe anywhere from 5G frequencies up to THz frequencies.
As this can bring signal behavior up to THz frequencies, this region is still uncharted territory for the vast majority of designers in the industry. Academia has been working on THz interconnect design and modeling for PCBs for some time, and there is plenty of innovation to be seen in this area. Substrate integrated waveguide routing is proposed as a better alternative to coplanar waveguide routing on PCBs, but dispersion in the dielectric will limit the bandwidth of the system. These systems require impeccable impedance control throughout the desired bandwidth, which can be complicated by dispersion in the dielectric substrate.
The matching network and amplifier on the Rx side also require careful design, especially in the high GHz and THz regimes. Matching in these systems can be quite complex and careful signal chain design is required to prevent signal loss, reflections, and power transfer through the device.
Looking to the Future: Heterodyning and Mode Combs
In both types of radio-over-fiber systems discussed above, transmission is a relatively easy process with standard optical components. This shifts the system complexity to the demodulator and routing at the Rx side. Envelope detection and square law detection circuits for receiving and demodulating signals are currently being studied. There is still plenty of innovation to be seen in this area.
One important application area for radio-over-fiber technology, specifically for mode combs generated at the Tx end of a link, is for optical data transmission while interfacing with photonic integrated circuits. Mode combs can be used to produced a very high frequency beat signal (e.g., several GHz), which is then used as a clock, for modulation, or interfacing with other RF circuits. The power of radio-over-fiber is the ability for the optical carrier signal to transport this GHz beat signal over long distances. This is just one area of emerging microwave techniques that will enable unique applications in the near future.
If you’re working on new designs for radio-over-fiber technology, you’ll need the right PCB design software to create your board layout. The routing features and unique design environment in Altium Designer® is ideal for building cutting-edge boards for high frequency RF systems. Altium Designer also gives you access to a complete set of post-layout simulation tools for signal integrity analysis.