Abstract
In this paper, we demonstrate experimentally the uplink of a 7 $\times$ 622 Mb/s incoherent spectral
amplitude coded optical code-division multiple access (SAC-OCDMA) passive
optical network (PON) with burst-mode reception. We consider two network architectures:
local sources (LS) at each optical network unit (ONU) versus a single source
located at the central office. We examine both architectures over a 20-km
optical link, as well as a reference back-to-back configuration. Our architectures
can adopt two-feeder and single-feeder topologies; however, we only test the
two-feeder topology and therefore the effect of Rayleigh backscattering is
neglected. We also study the relative merits (cost and performance) of local
sources versus centralized architectures. A penalty of less than 2 dB between
the LS and the centralized light sources (CLS) architectures was measured
at a bit error rate (BER) of $10^{-9}$ under certain assumptions on the relative power of the sources.
The power budget in the CLS architectures is more critical than in the LS
architectures; extra splitting and propagation losses exist as the uplink
travels through the network back and forth. Doubling the number of users while
maintaining the same distance and source power in LS architectures imposes
3-dB additional losses, whereas for CLS architectures, there are 6-dB extra
losses. CLS architectures can overcome these penalties using amplification
at the central office. Alternately, central office amplification can be used
to more than double the number of users in LS SAC-OCDMA PONs. A standalone
(no global clock) burst-mode receiver with clock and data recovery (CDR),
clock and phase alignment (CPA), and Reed–Solomon RS(255,239) forward-error
correction (FEC) decoder is demonstrated. A penalty of less than 0.25 dB due
to the nonideal sampling of the CDR is reported. The receiver also provides
an instantaneous phase acquisition time for any phase step between consecutive
packets, and a good immunity to silence periods. A coding gain of more than
2.5 dB was reported for a single-user system, and BER floors were completely
eliminated. Error-free transmission (BER $<10^{-9}$) for a fully loaded PON was achieved for the LS architecture as
well as the CLS architecture. Continuous and bursty upstream traffic were
tested. Due to the CPA algorithm, even with zero preamble bits we report a
zero packet loss ratio (PLR) for up to four simultaneous users in case of
bursty traffic, and more than two orders of magnitude improvement in the PLR
for fully loaded PON systems.
© 2008 IEEE
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