How to Calculate the Power Budget for GPON

What is GPON?

GPON (Gigabit-Capable Passive Optical Network) is a crucial branch of the Passive Optical Network (PON) family.

Compared to the earlier EPON, GPON supports more users and can cover distances up to 20 kilometers. Utilizing fiber as the transmission medium, it features a distributed architecture capable of supporting 32, 64, 128, or even more end users.

Defined by the Optical Distribution Network (ODN), GPON employs WDM technology and mandates single-fiber bidirectional transmission. Implementation scenarios include Fiber to the Home (FTTH), Fiber to the Curb (FTTC), and Fiber to the Node (FTTN). The ITU-T G.902 standard provides a framework for the architecture and functions of access networks.

Based on the ITU-T G.984.x series standards, GPON typically allows for 64 to 128 optical splitter branches, supports high bandwidth, long-distance transmission, and offers triple-play services at low costs. Due to its passive nature, GPON is easy to maintain as the network includes no active devices.

Relationship Between Transmission Distance and Optical Splitters

A critical factor in GPON design is the transmission distance between the Optical Line Terminal (OLT) and users, which relates to the system’s maximum allowable optical budget. The optical budget encompasses losses from connectors, splices, fibers, and optical splitters, with splitter loss being the most significant.

For instance, a typical 1×32 optical splitter may have an insertion loss between 17dB and 18 dB. Despite this relatively high loss, splitters are indispensable, making GPON an economical and simplified network solution.

The loss value of splitters varies by type (e.g., PLC or FBT). As passive devices, splitters are easy to install and use. However, the greater the number of splitter branches, the higher the optical power loss. Therefore, GPON designers must carefully consider the rationality of choosing splitters with specific branch numbers, as more branches increase splitter loss, reducing the maximum achievable distance between OLT and Optical Network Units (ONUs).

Principles of Power Budgeting

A power budget in optical communication systems concerns the transmission of optical signals via fiber rather than voltage and current in electrical communication. It accounts for all system gains and losses, with a reasonable power budget reducing network complexity and maintenance issues.

GPON comprises OLTs, ONUs, and the connecting fibers and joints. Hence, losses from single-mode fiber, connectors, passive optical splitters, passive optical attenuators, and joints must be considered. Inaccurate power budget calculations could result in issues for network receivers. Excessive power can damage detectors, while insufficient power may prevent detectors from correctly sensing optical signals or result in high error rates during data detection.

The power of the transmitter and the sensitivity of the receiver are two critical parameters that define the potential coverage area of a network. By calculating the power budget under worst-case conditions, one can determine the total network loss and thus the maximum coverage area.

Insertion loss refers to the power difference between the input and output of a device (such as a splitter). For instance, if a splitter receives an input power of 20 dBm and outputs at 3 dBm, the insertion loss is 17 dB.

The formula for power budget calculation is:

Transmitter Power = Receiver Sensitivity + Total Loss

In this formula, the total loss is expressed in dB. Fiber attenuation is measured in dB/km, and other losses, including splitter loss, splice loss, and connector loss, are also expressed in dB.

Common types of receivers include Avalanche Photodiode (APD) and PIN Photodiode (PIN), with typical sensitivity ranges as follows:

• PIN: Sensitivity ranges from -18 dBm to 0 dBm, with an optimal value of -7 dBm.
• APD: Sensitivity ranges from -28 dBm to -8 dBm, with an optimal value of -14 dBm.

Typical values for laser-based transmitter output power are:

• Short Distance: -3 dBm
• Medium Distance: 0 dBm
• Long Distance: +3 dBm

Note that these values are typical specific transmitter output power and receiver sensitivity should be checked in the manufacturer’s product specifications.

Power Budget Calculation Example

To simplify the learning process, we can use the following parameters to calculate the fiber length of a simple network:

• Single-mode fiber with attenuation of 0.35 dB/km.
• Splitter loss of 14 dB as per the datasheet.
• Two mechanical splices and two connectors in the fiber.

Other parameters include:

• Transmitter Power: 5 dBm
• Receiver Sensitivity: -14 dBm
• Fiber Attenuation: 0.35 dB/km

The calculation formula is:

5 = −14 + (Fiber Attenuation per km × Distance D (km) + Splitter Loss + Additional Loss)

Assuming two splices (each with 0.1 dB loss) and two connectors (each with 0.75 dB loss), the total additional loss is: 0.2 dB+1.5 dB=1.7 dB

Substituting into the formula:

19=0.35×D+14+1.7

Solving this equation:

0.35×D=19−15.7

D=10 km

Thus, the distance between the OLT and ONU is approximately 10 kilometers.