COL 9(10), 100606(2011) CHINESE OPTICS LETTERS Octob er 10, 2011
SAC-OCDM/TDM system for upgrading the TDM PON
He Chen (陈陈陈 荷荷荷)
1,2∗
, Shilin Xiao (肖肖肖石石石林林林)
1
, Lilin Yi (义义义理理理林林林)
1
,
Yi Dong (董董董 毅毅毅)
1
, and Weisheng Hu (胡胡胡卫卫卫生生生)
1
State Key Laboratory of Advanced Optical Communication Systems and Networks,
Shanghai Jiaotong University, Shanghai 200240, China
School of Information Science and Engineering, Xinjiang University, Urumqi 830046, China
∗
Corresp onding author: gravelhe@gmail.com
Received March 7, 2011; accepted April 21, 2011; posted online July 29, 2011
A spectral amplitude coded-optical code division multiplexing time division multiplexing (SAC-
OCDM/TDM) passive optical network (PON) for upgrading the traditional TDM PON is proposed. To
the best of our knowledge, our work is the first to report on the use of both spectral and orthogonal code
domains, which are transparent to optical network unit (ONU) for hybrid PON, in order to upgrade TDM
PON seamlessly. The fewer encoder/decoders and cheaper optical source under the conditions of high
bite rate and large ONU accommodation make the system cost-effective. A downstream experiment is
demonstrated, and the results demonstrate that the proposed system is feasible.
OCIS codes: 060.2330, 060.4230, 060.4254.
doi: 10.3788/COL201109.100606.
The time division multiplexing (TDM) technique is used
in the Ethernet passive optical network (EPON) and
Gigabit passive optical network (GPON), which is cur-
rently used commercially and performs optimally. Next-
generation broadband access networks are a new topic.
Therefore, whether there is a need to upgrade directly
the bit rates, of EPON and GPON or to introduce new
techniques in passive optical network (PON) remains to
be elucidated.
Several hybrid PON systems have been proposed
to upgrade the traditional TDM PON, such as wave-
length division multiplexing (WDM)/TDM PON, op-
tical code division multiplexing (OCDM)/TDM PON,
and OCDM/WDM/TDM PON
[1−4]
. These systems pro-
vide more optical network units (ONUs) and a larger
traffic capacity for PON. However, they present more
challenges to the ONU because every ONU must own
two-dimentional (2D) or three-dimentional (3D) address
information, which will b e expensive for subscribers.
In Ref. [5], we have proposed a hybrid wavelength di-
vision multiple access (WDMA)/OCDM system which is
capable of encoding multiple wavelength channels by em-
ploying one encoder and one corresponding optical code.
It makes the OCDM transparent to the ONU and so
we can upgrade easily the existing WDM PON without
redesigning the optical line terminal (OLT) and ONU.
Then a full duplex 10G-TDM-OCDM-PON system us-
ing only a pair of encoder/decoder was proposed
[6]
. It
arranges the multi-port optical encoder/decoder at the
remote node (RN) and decreases substantially the num-
ber of encoders/decoders. However, the chip rate is
200 Gchip/s for a 16-chip-code, so it presents a challenge
to the encoder/decoder and optical source.
In this letter, we propose a novel spectral amplitude
coded (SAC)-OCDM/TDM PON. The SAC-OCDM sys-
tem is based on coding the signal in the spectral domain;
the coding is independent of the signal bit rate and is
transparent to the ONU. Therefore, the proposed system
takes full advantages of the spectral domain, orthogonal
code domain, and time domain, and it can be equipped
with lower cost devices. Meanwhile, the traditional ONU
and OLT in TDM PON do not need to be redesigned in
the hybrid PON.
The SAC-OCDM system is considered as one achieve-
ment in the optical domain of the CDMA technique
[7−9]
.
The code chips are described by spectral slices, where 0
or 1 corresponds to the on or off mode of the spectral
slices, respectively. It is based on coding the signal in
the optical spectral domain, so the spread spectral gain
is independent of the bit and chip rates.
The coding/decoding techniques in SAC-OCDM al-
most have an identical principle of selecting the spectral
chip automatically. When optical pulses arrive, their car-
rier wave will be split and their spectra will be selected
automatically regardless of the time the pulse arrives;
the spectral slices are then coupled together. Therefore,
we neither need to know the time the optical pulse goes
into the optical encoder nor the bit rate of the signals.
When the optical signal arrives at the receiver end, the
decoder runs autocorrelation, extracts the destined sig-
nal, and excludes the interference signals. The optical
coding/decoding scheme for SAC-OCDM is independent
of the bit rate of the signal from all ONUs and does not
change the pulse shape in the time domain if the optical
source is a non-coherent broadband source. In compari-
son, the traditional CDMA in the electric field does not
have such property. The new SAC-OCDM/TDM PON
is proposed by taking advantage of such unique feature,
and its architecture is shown in Fig. 1.
In the proposed system, the hybrid PON is considered
to have two layers. The first layer is the traditional TDM
PON, and the second layer is based on the SAC-OCDM
technique which makes the coding/decoding transparent
to the ONU of TDM-PON. In downstream, the data
with a high bit rate modulate the amplified spontaneous
emission (ASE) source light and then are directed to
the encoder. The data after coding can be coupled to-
gether with the other M –1 data streams in the same
or partly overlapped time slot. At the ONU side, the
data streams are first selected by the M correspond-
1671-7694/2011/100606(4) 100606-1
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° 2011 Chinese Optics Letters