Digital Domain Power Division Multiplexed Dual
Polarization Coherent Optical OFDM Transmission
Qiong Wu1, Zhenhua Feng1, Ming Tang1*, Xiang Li2, Ming Luo2, Huibin Zhou1, Songnian
Fu1, and Deming Liu1
1

Wuhan National Lab for Optoelectronics (WNLO) & National Engineering Laboratory for Next Generation
Internet Access System, School of Optical and Electronic Information, Huazhong University of Science and
Technology, Wuhan, 430074, China
2
State Key Laboratory of Optical Communication Technologies and Networks, Wuhan Research Institute of Post
and Telecommunication, Wuhan 430074, Hubei, China
* tangming@mail.hust.edu.cn

ABSTRACT
Capacity is the eternal pursuit for communication systems due to the overwhelming demand of bandwidth
hungry applications. As the backbone infrastructure of modern communication networks, the optical fiber
transmission system undergoes a significant capacity growth over decades by exploiting available physical
dimensions (time, frequency, quadrature, polarization and space) of the optical carrier for multiplexing. For each
dimension, stringent orthogonality must be guaranteed for perfect separation of independent multiplexed signals.
To catch up with the ever-increasing capacity requirement, it is therefore interesting and important to develop
new multiplexing methodologies relaxing the orthogonal constraint thus achieving better spectral efficiency and
more flexibility of frequency reuse. Inspired by the idea of non-orthogonal multiple access (NOMA) scheme, here
we propose a digital domain power division multiplexed (PDM) transmission technology which is fully compatible
with current dual polarization (DP) coherent optical communication system. The coherent optical orthogonal
frequency division multiplexing (CO-OFDM) modulation has been employed owing to its great superiority on high
spectral efficiency, flexible coding, ease of channel estimation and robustness against fiber dispersion. And a
PDM-DP-CO-OFDM has been theoretically and experimentally demonstrated with 100Gb/s wavelength division
multiplexing (WDM) transmission over 1440km standard single mode fibers (SSMFs). Two baseband quadrature
phase shift keying (QPSK) OFDM signals are overlaid together with different power levels. After IQ modulation,
polarization multiplexing and long distance fiber transmission, the PDM-DP-CO-OFDM signal has been
successfully recovered in the typical polarization diversity coherent receiver by successive interference
cancellation (SIC) algorithm. Non-orthogonal overlaid signals different in power double the system spectral
efficiency and it enables flexible provisioning of quality of service (QoS) by properly adjusting power ratios of
non-orthogonal multiplexed branches.

Introduction
To meet the increasing demand of high capacity optical fiber transmission network, five available physical
dimensions including time, frequency, quadrature, polarization and space have been utilized for modulation and
multiplexing in optical communications1. In these schemes, stringent orthogonality must be satisfied to avoid
inference or crosstalk from other channels so that signals can be separated individually without degrading each
other’s detection performance. However, one of the major problems in these orthogonal multiplexing systems
such as orthogonal frequency division multiplexing (OFDM) or Nyquist WDM is that they do not allow frequency
reuse of two independent signals within the same physical dimension. Recently, the non-orthogonal multiple
access (NOMA), also known as power domain multiple access, is proposed as a potential candidate for the
upcoming 5G wireless communication standard due to its superior spectral efficiency2. In NOMA, multiple users
are multiplexed with different power levels using superposition coding at the transmitter side and successive
interference cancellation (SIC) based multi-user detection algorithms at the receivers. The non-orthogonal
feature significantly improves the capacity and throughput in both wireless and visible light communication
systems by allocating the entire bandwidth to different users simultaneously3,4. Inspired by the idea of NOMA,
we have recently proved the feasibility of using a new multiplexing dimension to enhance the capacity of direct
detection optical OFDM (DDO-OFDM) system by multiplexing the spectrally overlaid signals with different power
levels in digital domain5,6.

In this paper, we further develop the idea of digital domain power division multiplexing (PDM) into dual
polarization coherent optical OFDM (DP-CO-OFDM) transmission systems, which is promising for ultra-large
capacity long-haul coherent optical fiber communications. After theoretical analysis and numerical simulations,
we experimentally demonstrated a 100 Gb/s WDM transmission over a standard single mode fiber (SSMF) link
of 1440km. The results show that system capacity can be nearly doubled when two baseband quadrature phase
shift keying (QPSK) OFDM signals are overlaid together with proper power ratio before optical modulation
process and decoded after phase recovery process using carefully designed SIC algorithm, which is proved to
show better performance than conventional hierarchical de-mapping approach in our experiment. Besides the
capacity upgrade, the PDM scheme is also flexible to allocate customized quality of service (QoS) for different
subscribers by properly setting suitable power levels of non-orthogonal multiplexed branches.

Results and discussion
Theory

Fig 1. Schematic of our proposed digital domain power division multiplexed dual polarization coherent optical
OFDM transmission system: (a) system model, (b) digital domain power division multiplexing module, (c) OFDM
frame structure, (d) SIC algorithm based power division de-multiplexing module.
Fig 1(a) gives a global view of the system model of our proposed digital domain power division multiplexed dual
polarization coherent optical OFDM (PDM-DP-CO-OFDM) transmission system. The digital signal processing
(DSP) module at the transmitter side of conventional OFDM modulation includes serial-to-parallel (S/P)
conversion, symbol mapping, discrete Fourier transform (DFT) spreading, inverse fast Fourier transform (IFFT),
and the parallel-to-serial (P/S) conversion. Two branches of OFDM baseband signal are overlaid together with
different power levels before dual polarization modulation process. After transmission along the optical fiber
channel, at the receiver side, the synchronization including frame symbol aliasing and frequency offset
estimation/compensation is conducted first. Then, serial-to-parallel (S/P) conversion is completed before the fast
Fourier transform (FFT), after which multiple input multiple output (MIMO) processing is performed to separate
signals with different polarization states. Finally, inverse DFT spreading and phase recovery are carried out
before de-multiplexing overlaid signals with different power levels.
Suppose D and F are the DFT spreading matrix and FFT matrix respectively,（X i）and ( x i ) are the
baseband QAM symbols and OFDM symbols respectively, then the DFT-spread OFDM modulation process
could be expressed as

xi  F 1 Z P DX i (i  1, 2,..., n)

(1)

Where Z P is the zero-padding matrix used to pad the data-absent subcarriers with zeros. As shown in Fig
1(b), multiple baseband OFDM signals（ x i）are linearly combined together after coding and modulation using
power division multiplexing in digital domain to form one new baseband version

x   i 1 pi xi
n

  i 1 pi F 1 Z P DX i
n

 F 1 Z P D i 1 pi X i
n

where pi is the power of

(2)

i th signal, and the total power is normalized, namely



n
i 1

pi  1 . The idea of

our non-orthogonal scheme is to utilize power division multiplexing in terms of using the same frequency but
exploiting different power levels. Therefore, the multiplexed signals should have different power levels. Without
loss of generality, we assume that the power levels ( pi ) are arranged in descending order, namely

p1  p2      pn . The OFDM frame structure is depicted in Fig 1(c) to show how two baseband OFDM
signals are overlaid together. After transmission through the communication channel, suppose h and H are
the temporal and frequency response of the baseband signal respectively,  is the phase rotation matrix caused
by phase noise, then the received power multiplexed signal can be expressed as

r  h * x  N
 F 1 HFx  N

 F 1 HZ P D i 1 pi X i   N
n

(3)

where N is the additive white Gaussian noise (AWGN). Then the received signal after OFDM demodulation,
can be expressed as
T
1
T
1
y  D1 Z（
R H H ) H Fr 
n

T
1
T
1
 D 1 Z（
pi X i   N ) 1
R H H ) H F ( F HZ P D 
i 1

n

T
1
T
1
  pi X i  D 1 Z（
R H H ) H FN 
i 1
n

  H i X i  kN  1

(4)

i 1

Where Z R is the zero-removing matrix used to remove the data-absent subcarriers, H i is the channel
th

response of the i multiplexed baseband QAM symbols, k is a constant noise coefficient. Eventually, the
detected signal will be demodulated and decoded one by one according to the descending order of power levels
using SIC algorithm, whose flow chart is shown in Fig 1(d). It is worth mentioning that the overlaid signals can
also be separated by hierarchical de-mapping7.
Simulation

Fig 2. BER of both QPSK branches versus power division ratio

Fig 3. Constellation diagram: (a) 2:1, (b) 4:1, (c) 6:1, (d) 8:1
Without loss of generality, we study the digital domain power division multiplexing scheme in the scenario of two
branches of QPSK-OFDM signals in this work. To investigate the relationship between the power division ratio
(PDR, defined as P1/P2) and BER performance of each power multiplexed branch, we firstly conduct Matlab
based simulation under the condition of AWGN channel with SNR= 8 dB. Two baseband QPSK-OFDM signals
are generated from PRBS 215 using 512 points IFFT among which 232 subcarriers are used to carry data
information. From Fig 2, we can conclude that the optimized PDR to minimize the BER of the second branch
with lower power is 4:1, in which case the two superposed QPSK could be emulated by a 16QAM constellation.
We also show the evolution of the constellation diagram vs different PDR in Fig 3.
Experiment
OOFDM Transmitter
Offline DSP
AWG
I

Fiber Link

Polarization MUX
RF

40km SMF

Q (12G Sa/s)

40km SMF

DELAY

PC

ECL1

π/2

PC

I/Q Modulator

Intensity
Modulator

PBS

PBC

EDFA1

Raman
Amplifier

VOA1

Raman
Amplifier

OOFDM Receiver
90⁰
Hybird

OSA

10:90

EDFA3

TOF

EDFA2

VOA2

90⁰
Hybird

DSP
Receiver
50 G sa/s DSO

PBS

PBS

ECL2
After TOF

Before TOF

Fig 4. Experimental setup. ECL: External cavity laser. AWG: arbitrary waveform generator. PC: Polarization
controller. PBS/C: Polarization beam splitter/combiner. EDFA: Erbium doped fiber amplifier. VOA: Variable
optical attenuator. TOF: Tunable optical filter. OSA: Optical spectrum analyzer.
To verify the proposed digital domain power division multiplexing scheme, we performed further experiments
using the setup shown in Fig. 4. At the transmitter, two baseband QPSK-OFDM signals are generated in Matlab
originated from PRBS 215 -1 using 512 points IFFT among which 232 subcarriers are used to carry data
information. To combat the fiber dispersion, the cyclic prefix (CP) is set to 64. Each OFDM frame has 140 OFDM

symbols and at the beginning of each OFDM frame are 15 training symbols (TSs), one of which is employed for
OFDM timing synchronization and the rest fourteen are used for polarization de-multiplexing and channel
estimation. Two branches of QPSK-OFDM signals with power division ratio 4:1 are overlaid together in digital
domain and then D/A converted by an AWG (Tektronix, 7122C) at a sampling rate of 12GSa/s. After upconversion the modulated optical signal is then duplicated to three copies by an intensity modulator in order to
ensure orthogonal band multiplexing with channel spacing of 7.03125GHz8. Therefore, the total data rate is 3×
2×2×2×12×232/576×125/140 = 103.57 Gb/s after sub-band multiplexing. Afterward, the multi-band optical signal
is polarization multiplexed by a pair of polarization beam splitter/combiner with one branch delayed by one
OFDM symbol. The transmission link contains several spans of 40km SSMF whose loss is fully compensated by
Raman amplifiers. At the receiver, a VOA followed by an EDFA is used to adjust the optical signal noise ratio
(OSNR) of the total optical link, which can be measured by optical spectrum analyzer. A tunable optical filter
(TOF) is used to select the required band before the dual polarization coherent receiver, after which two IQ
components are acquired by an oscilloscope at 50GSa/s and the spectrally overlaid OFDM signal is power demultiplexed by a SCI receiver after phase recovery of both polarization tributaries during the offline DSP.
-1

10

Branch1 W/ SIC
Branch2 W/ SIC
Branch1 W/O SIC
Branch2 W/O SIC

-2

BER

10

-3

10

-4

10

-5

10

-10

-8

-6
-4
-2
Launching Power(dBm)

0

2

Fig 5. BER versus launch power with 480km transmission in both power division multiplexed branches using SIC
algorithm and hierarchical de-mapping.
0

10

-1

10

-2

BER

10

FEC Limit 3.8e-3

-3

10

Branch1 W/ SIC
Branch2 W/ SIC
Branch1 W/O SIC
Branch2 W/O SIC

-4

10

-5

10

12

14

16

18

20
22
OSNR(dB)

24

26

28

Fig 6. BER versus OSNR with 480km transmission in both power division multiplexed branches using SIC
algorithm and hierarchical de-mapping.
In our experiment, hierarchical de-mapping is also employed to demodulate the two multiplexed QPSK
branches and its performance is compared with that demodulated by SIC algorithm. In convenience, W/O SIC is
used to represent hierarchical de-mapping in the following figures.
We first investigated the transmission performance of our proposed system with fiber link of 480km. As
shown in Fig 5, the optimal launching power is found to be -7dBm, regardless of the power division de-

multiplexing method we use. Then we vary OSNR of the received optical signal to check the BER performance
of both QPSK branches with launching power fixed at -7dBm. As can be seen from Fig 6, the QPSK Branch1
allocated with higher power enjoys an OSNR redundancy of about 2dB than the QPSK Branch2 to reach the
FEC limit of BER=3.8e-3, while SIC algorithm shows an OSNR improvement of about 0.5dB compared with
hierarchical de-mapping.
We also confirmed the robustness of our proposed system with extended fiber links in Fig 7, which proves
the feasibility of digital domain power division multiplexing in long-reach optical coherent transmission systems
compatible with current wavelength division multiplexing (WDM) and space division multiplexing (SDM)
technologies. Obviously, SIC algorithm can improve the transmission distance of both multiplexed QPSK
branches, while the QPSK Branch1 with higher power level enjoys a transmission redundancy of about 250km
than QPSK Branch2 with lower power level to reach the same FEC limit of BER=3.8e-3. To ensure identical
BER performance of the two power multiplexed signals, stronger FEC coding such as soft-decision FEC can be
used for weaker signals. The constellation diagrams after 480km SSMF transmission are depicted in Fig 8,
which agrees well with the simulation predictions.
-1

10

-2

10

BER

FEC Limit 3.8e-3
-3

10

Branch1 W/ SIC
Branch2 W/ SIC
Branch1 W/O SIC
Branch2 W/O SIC

-4

10

-5

10

200

400

600
800
1000
Fiber Length(km)

1200

1400

Fig 7. BER versus fiber length in both power division multiplexed branches using SIC algorithm and hierarchical
de-mapping.

Fig 8. Constellation diagrams at length=480km. (a) Branch1 in Pol-X, (b) Branch2 in Pol-X, (c) Branch1 in Pol-Y,
(d) Branch2 in Pol-Y.

Conclusion
While chasing high capacity to meet the increasing traffic demands, the next generation optical network features
the flexibility. Traditional physical multiplexing dimensions like time, frequency and space have little room to
adjust multiplexed signal quality due to the physical stringent orthogonal condition thus sacrifice the flexibility. In
this paper, we propose a DP-CO-OFDM transmission scheme based on digital domain non-orthogonal power
division multiplexing, and verified it in a 100-Gb/s transmission over 1440km fiber link. Our scheme proves to be

feasible with a nearly doubled system capacity when two baseband QPSK-OFDM signals of different power
levels are overlaid together before optical modulation process and successively decoded after phase recovery
using either SIC algorithm or hierarchical de-mapping, while SIC algorithm turns to be a better choice to improve
BER performance. Besides, non-orthogonal digital domain power division multiplexing is compatible with current
dual polarization WDM and SDM systems. Moreover, classified quality of service (QoS) can also be realized
anytime by simply adjusting the power ratios and modulation formats of different subscribers with various
bandwidth requirements and transmission distances. Above all, it is a promising technique to improve the
system capacity and flexibility simultaneously for future network construction, expansion, maintenance and
operation.

Methods
Power de-multiplexing using SIC Algorithm As shown in Fig 1(c), The SIC process mainly consists of three
steps: (1) estimate the channel response and demodulate the stronger signal x i while treating all the other
signals x i  1    x n as interference noise; (2) re-modulate the estimated signal xˆ i and multiply it by the channel
response H i before subtract the product from the received signal y and then decode the weaker signal x i  1 ;
(3) continue with the aforementioned steps until all the signals are decoded. As SIC algorithm suppresses
interference from stronger (earlier decoded) signals for relatively weaker (yet to be demodulated) signals, even
the signals with lower power can be correctly recovered.
Hierarchical de-mapping of two overlaid QPSK branches Fig 9 draws the mapping constellation of single
QPSK branch and two overlaid QPSK branches. When demodulate with SIC algorithm, two overlaid QPSK
branches are separated first and then both de-mapped using single QPSK mapping constellation on the left.
However, when demodulate with hierarchical de-mapping, two overlaid QPSK branches are straightly demapped using the hierarchical mapping constellation on the right, then the first two bits are allocated to the
QPSK branch with higher power level while the rest two bits are allocated to the lower one.

Fig 9. Mapping constellation of single QPSK branch and two overlaid QPSK branches

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Acknowledgements (not compulsory)
This work was supported by the National Natural Science Foundation of China (NSFC) under Grant No.
61331010, the 863 High Technology Plan of China (2013AA013402) and the Program for New Century Excellent
Talents in University (NCET-13-0235).

Author contributions statement
M.T., Q.W., and Z.F. conceived the study. X.L., M.L., H.Z. and Q.W. designed and did the experiment. Q.W., Z.F.
and M.T. developed the theory and analyzed the data. Q.W., Z.F., M.T., S.F. and D.L. prepared the manuscript.

Competing financial interests: The authors declare no competing financial interests.

