Fiber Optic Transmission

Optical fiber may be the medium of choice for high capacity digital transmission systems and speed local area network. Besides these applications, optical fiber is also used to transmit microwave signals for cable tv, cellular radio, WLAN and microwave antenna remoting. To deliver microwave over optical fiber, the microwave signal is converted into optical form in the input from the fiber and at the creation of the fiber, it’s converted back to electrical signal. The benefit of fiber transmission of microwave is reduced losses in accordance with metallic media (e.g. copper coaxial cable). This leads to longer transmission distance without signal amplification or utilization of repeaters.

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

RFoG (Radio Frequency over Glass) is the cable operators’ implementation of microwave transmission over optical fiber where the coax portion of the HFC (Hybrid Fiber Coax) is substituted with 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. Such as the HFC architecture, video controllers and knowledge networking services are fed through a CMTS/edge router.

These electrical signals are then converted to optical and transported via a 1550 nm wavelength via a wavelength division multiplexer (WDM) and a passive optical 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 could be fed over coax to a set-top box, while voice and knowledge traffic could be delivered to an embedded multimedia terminal adapter (eMTA), The return path for voice, data, and video visitors are on the 1310 nm or 1590 nm wavelength to some return path receiver, which converts the optical signal to RF and feeds it back into the CMTS and video controller.

The benefit of radio-over-fiber technologies are that it centralizes the majority of the transceiver functionality by transmitting the microwave signals within their modulated format over fiber. This reduces the number of access suggests 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 as well as 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) within the building using single-mode optical fiber. The bottom station utilizes a combined detector/optical modulator, that is directly coupled to the antenna, to ensure that no electrical amplification or any other processing is needed.


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