A marriage of the right performance parameters with technology advances, particularly in RFIC amplifiers, can create economic benefits for MSOs eyeing RFoG-based fiber access networks.
Radio frequency over glass (RFoG) deployment has become increasingly popular for greenfield construction and network upgrades because it uses the current cable-TV infrastructure so well and offers a smooth, incremental upgrade path in particularly competitive markets. To maximize the economic viability of RFoG, network operators apply careful attention to the RF optical network unit (R-ONU), as it is the most expensive piece of equipment in the system on a cost-per-subscriber basis. Critical to making the overall premises economics attractive is a means of ensuring that the R-ONU provides the horsepower to support a wide variety of possible subscriber scenarios, hence eliminating the need for excess drop amplifiers in both directions.
The R-ONU performs the basic functions of a node but has the cost challenge of not being a shared device. However, with the application of recent advances in optics and RFIC amplifiers, the cost of the R-ONU can be optimized to enable RFoG as an inexpensive, capability-enhancing, economic network approach.
Cost and performance
To optimize the cost performance of the R-ONU, we need to look at the constraints placed on carrier and home network architectures. Subscribers increasingly demand more televisions and premises boxes, including embedded multimedia terminal adapters (EMTAs) for phone service, multiple cable modems for more high-speed data, additional advanced television sets driven by high-definition (HD) TV, digital set-top boxes, and “media gateways.” The services demanded now and in the future need to be supported effectively at the premises, where the cost to make changes to the network are the most expensive, so it's instructive to first look closely at requirements at the demarcation point into the home (see Fig. 1).
For as long as cable-TV systems have been deployed, there has been a careful balance between centralized versus distributed RF gain, taking into account cascades of line amplifiers, taps, and splitters to the home. Cascades forced stringent distortion specifications, and gain was centralized as much as feasible for cost sharing. RF-hungry homes (requiring many splits), traditionally a small percentage of the total, were provisioned with drop amplifiers to ensure that the network was not over-engineered.
FTTH architectures such as RFoG have changed this model significantly. Now with fiber all the way to the home and no cascading, power levels can be high enough to support tough subscriber demands optimized by reduced distortion requirements. With the variety of cable lengths, splits, and potentially the addition of technologies such as multimedia over coax that can attenuate signals in the home, approaches that minimize the need for an excessive percentage of drop amplifiers are a compelling objective of the RFoG network architecture.
The highest reasonable output level, in the worst case where a TV or set-top box is connected right at the R-ONU, is in the range of 22 dBmV. A fair target with margin is 20 dBmV, but the challenge then is cost-effectiveness. In order to output at this level economically, the gain of the amplification often requires at least two stages, particularly if the optical input is low from being optimized for maximum link budget.
A key consideration on the cost of the receiver is the optical input range. It is important that the receive side is as low as possible to accommodate more fiber-sharing network-side splits, lower-cost EDFAs, and generally longer link budgets. Governing this low end is the noise and gain of RFIC amplifiers. With recent advances in GaAs technology using pHEMT processes, and a focus on integration of cable-TV ICs, significant noise reduction and gain improvement can now be achieved in a single cost-effective chip (see Fig. 2).
Another consideration is the demand placed on the range of the automated gain control (AGC) circuit. A typical approach to AGC attenuation is the use of pin diode attenuators. With current GPON standards requiring approximately ‚Äì8 to +2 dBm of optical input, a number of devices are necessary to provide the needed AGC range (25+ dB). With a slightly reduced range of ‚Äì6 to +1 dBm, the necessary AGC range drops to below 20 dB and a simpler circuit can be used to reduce costs (see Fig. 3). Considering the RF output level, if the level is reduced to 15 from 20 dBmV, the AGC range becomes closer to 20 dB and the single diode device can be achieved with engineering margin.
Output of 15 dBmV at the side of the house is still a significant improvement on what is typically engineered today (~10 dBmV for some systems). With new ICs, this requirement can be effectively attained by integrating two stages of amplification as well as AGC functionality. If the output level is reduced to less than 15 dBmV, then the per-premises costs escalate again with the need for more drop amplifiers. If the optical input requirement is relaxed to ‚Äì5 dBm, significant link budget is lost, equivalent to 4 km of reach, an additional split (64- versus 32-way PON), or a large step increase in EDFA cost.
Setting the target
To confront these cost and performance challenges, the SCTE RF over Glass committee and cable system operators are evaluating the targets for downstream (as discussed here) as well as return path and extended reach (over 20 km). The ideal RF output signal to support residential subscribers appears to be in the 17-dBmV range, being fed from the network with a ‚Äì6- to +1-dBm optical input. These levels provide an ideal balance of network and premises amplification and can be achieved with recent advances in low-cost, highly integrated amplifier ICs. RF video signals at these optimized levels have the potential to fulfill current and growing FTTH video subscriber demands at a more economic overall cost throughout DOCSIS, GEPON, GPON, and other network deployments.
Brian Bauer is vice president of marketing at TriAccess Technologies (www.triaccesstech.com), a semiconductor provider of 75-Œ© cable-TV amplifier integrated circuits.
FIGURE 2. Curves of low-noise RFIC versus older technology with higher noise. New GaAs processes optimal for noise and gain enable more than a decibel advantage in the generally required 46- to 48-dB CNR range. This translates to an extra split, lower-cost EDFA, and/or longer reach.
FIGURE 3. Block diagram of the receive side of a typical FTTH/RFoG circuit. Gain adjust is now available on-chip. Given the idealized allowable input range, the AGC can be simplified.
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