Design and Usage of Fiber Optic Return Path

20 January, 2014 (09:35) | Applications & Solutions | By: fiberopticis

The CATV industry has responded to the demand of interactive, real-time, two-way television programming with return path technology. Return path management allows the viewer to send information from a transmitter located within the set-top-box (STB) located in the viewer’s home to the headend with the touch of a few buttons on the television remote. This advancement came from the design of the hybrid fiber coax (HFC) networks that meet the demands of increased transmission distances required for today’s cable television. Return path management is prevalent in digital broadcast systems (DBS), such as Directv and Dish Network. Typical HFC networks, illustrated in Figure 1, use coaxial cable for shorter transmission lengths between video equipment and the transmitter, or the receiver and the cable customer’s television, while the path between the transmitter and the receiver uses single-mode (SM) fiber to extend transmission distances. Even in limited distance applications, this combination allows the system designer to use the lower cost solution for each portion of the network. In return path HFC networks, illustrated in Figure 2, the same system design principle applies, but now there is bidirectional transmission between the viewer location and the headend allowing for the interactive return path management. The signals from the headend are transported over SM fiber using either a 1310 nm or 1550 nm distributed feedback laser (DFB) or Fabry Perot (FP) transmitters. The receiver contains a return path laser that sends the viewer-sent signals back to the headend. Currently, wavelength-division multiplexing technologies, such as DWDM and CWDM, increase the transmission distance and system reliability.

Figure 1 – Typical HFC Network

Figure 2 – HFC Network with Return Path

Return Path Management Interactive Exchanges
Return path management supports the following interactive exchanges:

* Order and Payment Transactions: these orders could consist of purchasing a pay-per-view movie, merchandise from a home shopping network, or items from any store that may be available to the viewer on television channels.
* Obtaining Data from a Centralized Database: this action pertains to inquiring about local weather, television guides, and pay-per-view movie selections.
* Answer and survey information: the viewer can play along with interactive game shows, and vote in viewer integrated polls and surveys.
* Stand-alone games: the viewer can play games that relate to television shows. This exchange may consist of pay-per-play, high-score competitions, and pay-per-skill level.
* Enhanced Programming: the viewer can obtain more information on the subject of the television show. This interactivity especially pertains to nature, history, and technology based programming.
* Customizable Financial Reports: the viewer can customize a stock ticker to be displayed on the screen to watch the stock market.
* Email and chat: a viewer can chat with another viewer watching the same program, or send emails.
* Interactive Sports Channels: allows the viewers to watch two games simultaneously, choose camera angles, obtain game statistics, and score updates.
* Information Services: the viewer may get information on travel, sports, or education. These channels may be used for vacation promotions, and sporting events.
* Music Choices: the viewer may select a channel that plays streaming music of any genre. These channels may be free, pay-per-listen, or a pay channel. Also, included in these channels are information about the artist, album, and occasionally, how to purchase the album.

Return Path Management Signal Path
The return path signal, which typically ranges from five to 42 MHz, originates in the home and flows through the fiber plant toward the headend. The signal level in the plant is determined by the RF level produced by the transmitter contained within the STB, plug-in PC card, stand-alone modem, or a side-of-the-house box. However, when the signal leaves the viewer’s home it experiences losses from either the in-house cable, splitters, ground blocks, drop cables, tap ports, or feeder cables before it reaches the amplifier station port. Furthermore, each box has a different loss, but all signals coming from viewers’ homes should arrive at the amplifier at the same level. The design of the return path determines the signal level, and this requires a system designed to produce the correct level. Once the signal reaches the amplifier, it is transported through the span of cable with the correct unity gain so that the return path gain of every amplifier station matches the loss of the following cable. When the signal reaches the node station it is then transported over optical fiber to the headend. Once at the headend, the signals are converted to RF signals by the fiber optic receiver and fed to the demodulator designated for the service requested by the viewer.

Wavelength-division Multiplexed Schemes and Digitized Return Paths

Multiplexed Return Paths
As the popularity of return path management schemes increases, the ability to reliably transmit high-speed data in the five to 42 MHz bandwidth becomes more difficult. Currently, upgrades to HFC networks rely on dense wavelength-division multiplexing (DWDM) utilizing a frequency-stacking scheme. These upgrades, like most in the fiber optic industry, come from the demand for more bandwidth and better, more reliable transmission. This transmission scheme works by placing an uncooled distributed feedback laser (DFB) or Fabry-Perot (FP) laser transmitter operation at either 1310 nm or 1550 nm at the fiber node to transmit data to the secondary hub. At the secondary hub the data drives a directly modulated DWDM laser transmitter using time-division multiplexing (TDM) and frequency-stacking techniques. Figure 3 illustrates a return path HFC using DWDM and a frequency-stacking system (FSS). A reference pilot tone is generated at the frequency of 370 MHz in the block up-converter to synthesize the down-conversion; this technique removes frequency-offset errors. The composite RF signal drives each of the DWDM upstream laser transmitters at the secondary hub. The optical signal, using a 1 x 4 DWDM multiplexer, is transmitted over SM fiber to the headend. When the signal reaches the headend, an in-line erbium-doped fiber amplifier (EDFA) amplifies the optical signal. Once amplified, a demultiplexer demultiplexes the signal, and transmits the signal to four receivers. The composite RF signal gets transmitted from each receiver to the block-down converter unit, which extracts the four 5 to 42 MHz bands. The signals can then use different return path receivers to return to the viewer location.

Figure 3 – HFC using DWDM

Digitized Return Path
Using an analog-to-digital (A/D) converter changes an analog return path into a digital return path. The analog-to-digital converter operating at 100 MHz with eight to 12 bits of resolution. The digitized signals are converted to serial bit stream with the appropriate synchronization at the fiber node to recover the signal at the optical receiver output in the local headend. At the node or secondary hub, several signals can be combined using TDM. Currently, cable television broadcasters use two 12-bit A/Ds to modulate the laser transmitter and produce approximately a 2.5 Gb/s TDM data stream. The data stream is then multiplexed at the hub. Once the data stream reaching the headend, it is demultiplexed and deserialized. In the final step, the data stream is transmitted to a digital-to-analog (D/A) converter and returned to the viewer location. The digitized return path offers a couple of advantages over an analog return path. The TDM digitized signals are transparent in a DWDM network, meaning that the signals can be transmitted throughout the network without degradation. Also, the techniques of processing digital signals reduces degradation before the A/D.

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