Burst-mode receiver

A burst-mode receiver (BMR) is an optical receiver designed to handle a time-division-multiplexed signal in which sequential bursts arrive from different optical transmitters, with each burst potentially having different amplitude, phase, and arrival time. This contrasts with a continuous-mode receiver, which receives a single steady signal of approximately constant amplitude.

Standard application: passive optical networks (PON). In a typical fiber-to-the-home PON architecture:

• A central optical line terminal (OLT) at the central office serves up to 64 or 128 optical network units (ONUs) at customer premises
• Downstream traffic (OLT → ONUs) is broadcast: all ONUs see all data; each ONU keeps only data addressed to it
• Upstream traffic (ONUs → OLT) is TDMA: each ONU transmits during its scheduled time slot
• The OLT receiver sees the upstream signal as a sequence of bursts, one per scheduled ONU

The bursts vary in optical power by typically 5 – 15 dB depending on the distance to each ONU and the optical losses along its specific path. Each burst also arrives with an arbitrary bit phase relative to the previous one: the ONUs derive their transmit clocks from the downstream signal, so the frequency matches, but their different fiber lengths leave the phase unknown.

Key challenges and design responses.

Wide dynamic range. A continuous-mode receiver can be designed with adjustable gain or a tightly-controlled input power; a burst-mode receiver must accept the widest input power range without saturation or below-noise-floor operation. Standard approach: AC-coupled amplifier with very fast settling time, or DC-coupled with automatic gain control (AGC) that re-acquires within a few bits of each new burst.

Fast clock recovery. A continuous-mode receiver uses a phase-locked loop (PLL) with long lock acquisition time (kHz – MHz bandwidth); a burst-mode receiver must find the bit phase within the first bits of each burst. Standard approaches are oversampling clock and data recovery, which samples each bit several times and picks the best phase, and gated-oscillator designs that start in phase with the first data transition.

Burst preamble. Each transmitted burst begins with a known synchronization pattern (the preamble) that the receiver uses to:

• Detect that a burst has arrived (energy detection)
• Set the receiver's gain and decision threshold for this burst
• Lock the clock recovery to the bit-rate
• Confirm bit-phase alignment

The OLT tells each ONU which preamble and delimiter to send, within limits the standard defines; longer preambles give the receiver more settling time at the cost of upstream capacity. In GPON at 1.244 Gb/s, the recommended upstream burst overhead, guard time, preamble and delimiter together, is 96 bits.

PON-specific standards.

A GPON class B+ OLT receiver must reach $-28$ dBm at 1.244 Gb/s. Burst-mode operation costs sensitivity relative to a continuous-mode receiver at the same rate, because:

• the decision threshold is estimated from a short preamble, so it is noisier than a threshold averaged over a continuous signal
• the fast-settling gain-control and offset loops admit more noise than slow ones

Implementation technologies.

• TIA + LA + CDR (transimpedance amplifier + limiting amplifier + clock-data recovery): the standard analog receiver architecture, with burst-mode adaptations
• DC-coupled BMR: explicit AGC with feedback control of the TIA gain at burst level; better sensitivity but higher complexity
• AC-coupled BMR: capacitive coupling with a short time constant, so the offset settles within the preamble; simpler, but the short time constant makes the baseline wander during long runs of identical bits
• Digital-burst-mode receiver: high-speed ADC followed by all-digital processing; highest performance but highest power

Optical components. The burst-mode receiver photodiode is typically an avalanche photodiode (APD) to achieve the necessary sensitivity at the link budget. PON OLTs use:

• InGaAs APD for receiver
• 1490 nm (GPON) or 1577 nm (XG-PON and XGS-PON) DFB or EML for the downstream transmitter
• Wavelength-division multiplexer to combine TX and RX wavelengths on the same fiber

Comparison to coherent reception. Some proposals for 100G-PON and beyond consider coherent reception, which would replace direct-detection burst-mode receivers with coherent burst-mode receivers. Coherent reception is more sensitive, which allows larger split ratios or longer reach, but its DSP must estimate the frequency offset, polarization and equalizer settings from each burst's preamble, and the optics and DSP cost more per port.

References: ITU-T G.984 series (GPON), G.987 series (XG-PON), G.9807.1 (XGS-PON), G.989 series (NG-PON2), G.9804 series (50G-PON); F. J. Effenberger, "The XG-PON system: cost effective 10 Gb/s access," J. Lightwave Technol. 29, 403 (2011).