Abstract: APD or FEC in the ONT? That is the question on the minds of many GPON system vendors. Gigabit passive optical networks (GPON) are being heavily considered to deliver Triple-Play services (Video, Voice, Data) to households across North America and in other locations throughout the world. There is currently a lot of debate if avalanche photodiode (APD) receivers or standard receivers with forward error correction (FEC) are the best solutions for meeting the link budgets and cost targets at the user location (ONT – Optical Network Terminal). Given the large volumes expected for GPON deployment this truly is a million dollar question as the implementation used could result in millions of dollars of gained or lost revenue due to material costs, testing cost or lateness to market.
Receivers implementing APDs can easily meet the sensitivity requirements but at a high cost. Avalanche photodiodes are expensive, require high-voltage biasing, and they also need to be temperature compensated. FEC adds some cost and complexity to implement the encoding and decoding but is generally believed to be much less expensive than an APD solution. However, some design challenges and consequently increased complexity and cost are often overlooked when designing a GPON compatible FEC receiver.
This article describes briefly how FEC works and then illustrates the impact of jitter on a receiver's sensitivity when using FEC. The article also presents actual test data to show the expected performance when using two different types of receiver circuits. This highlights important issues that we should consider when selecting the ONT receiver components of a GPON system that uses FEC.
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