Fiber Optics Microwave Transmission

Optical fiber is the medium of choice for high capacity digital transmission systems and high speed local area network. Besides these applications, optical fiber also can be used to transmit microwave signals for cable television, cellular radio, WLAN and microwave antenna remoting. To transmit microwave over optical fiber, the microwave signal is converted into optical form at the input of the fiber and at the output of the fiber, it is converted back to electrical signal. The main advantage of fiber transmission of microwave is reduced losses relative to metallic media (e.g. copper coaxial cable). This results in longer transmission distance without signal amplification or use of repeaters.

There are two approaches to optical signal modulation and recovery. The first type is IMDD (Intensity Modulation Direct Detection) and the second type is Coherent Detection. In IMDD, the optical source intensity is modulated by the microwave signal and the resulting intensity modulated signal passes through the optical fiber to a photodiode where the modulation microwave signal is converted back to electrical domain. In Coherent Detection, the optical source is modulated in intensity, frequency or phase by the microwave signal. The modulated signal passes through the optical fiber to the receiver where it is mixed with the output of a local oscillator (LO) laser. The combined signal is converted to electrical domain using a photodiode. This produces an electrical signal centered on the difference frequency between the optical source and the LO laser (i.e. intermediate frequency). This signal is further processed to recover the analog microwave signal.

RFoG (Radio Frequency over Glass) is the cable operators’ implementation of microwave transmission over optical fiber in which the coax portion of the HFC (Hybrid Fiber Coax) is replaced by a single fiber, passive optical network architecture (PON). RFoG allows cable operators to deploy fiber connectivity to customer premises (FTTP) while keeping its existing HFC and DOCSIS infrastructure. Like the HFC architecture, video controllers and data networking services are fed through a CMTS/edge router.

These electrical signals are then converted to optical and transported via a 1550 nm wavelength through a wavelength division multiplexer (WDM) and a passive splitter to a R-ONU (RFoG Optical Network Unit) located at the customer premises. R-ONUs terminate the fiber connection and convert the traffic to RF for delivery over the in-home network. Video traffic can be fed over coax to a set-top box, while voice and data traffic can be delivered to an embedded multimedia terminal adapter (eMTA), The return path for voice, data, and video traffic is over a 1310 nm or 1590 nm wavelength to a return path receiver, which converts the optical signal to RF and feeds it back into the CMTS and video controller.

The advantage of radio-over-fiber technology is that it centralizes most of the transceiver functionality by transmitting the microwave signals in their modulated format over fiber. This reduces the number of access points to antennas with amplifiers and frequency converters. In-building passive picocell for GSM or UMTS is implemented using radio-over-fiber. Wireless base stations are located in a central communications room and their outputs/inputs fed through RF multiplexers to lasers/photodiodes contained within the optical transceiver hub. The modulated optical signals are linked to/from the remote antenna units (AUs) in the building using single-mode optical fiber. The base station uses a combined detector/optical modulator, which is directly coupled to the antenna, so that no electrical amplification or other processing is required.