ThJ2
Real-time Coherent Optical MIMO-OFDM
Reception up to 6.67 Gbps
Simin Chen1, Qi Yang2, Yiran Ma2 and William Shieh1
ARC Special Research Centre for Ultra-Broadband Information Networks
Department of Electrical and Electronics Engineering, the University of Melbourne, Melbourne, VIC 3010, Australia
Phone: +(61) 0383440369, Fax: +(61) 0383446678, Email: siminc@unimelb.edu.au
(2 National ICT Australia, Department of Electrical and Electronics Engineering
The University of Melbourne, Melbourne, VIC 3010, Australia)
1
Abstract
A real-time dual-polarization CO-OFDM receiver in a
2×2
multiple-input
multiple-output
(MIMO)
configuration is demonstrated. The signal streams are
processed in real-time mode, and the data rates of 3.33
Gbps and 6.67 Gbps are realized for 4-QAM and 16QAM respectively.
Introduction
Coherent optical OFDM (CO-OFDM) combines
coherent optical detection and orthogonal frequencydivision multiplexing (OFDM), and has been shown to
be effective to combat chromatic dispersion [1,2,3] and
polarization-mode dispersion [ 4 , 5 ]. Recently, we
reported the first multi-gigabit real-time CO-OFDM
experiment with single-polarization [6]. The single mode
fiber can be represented as a 2×2 multiple-input
multiple-output (MIMO) channel, and some sort of
polarization diversity can be employed for practical
implementation to double the data rate. On the other
hand, the high-speed analog-to-digital converter (ADC)
at multi-gigahertz sampling rate and high-speed largevolume field programmable gate array (FPGA) are now
commercially available. One of the advantages of the
state-of-the-art FPGA is that it has sufficient gate
resource, embedded memory and DSP blocks to process
the digital signal in multiple parallel channels, allowing
for high-speed signal processed at a relatively lower
clock rate. In this paper, we will show the first multigigabit real-time CO-OFDM receiver for dual
polarizations in a 2×2 MIMO-OFDM configuration. The
receiver consists of four 1.5 Giga samples per second
(Gsps) high-speed analog-to-digital converters (ADC)
and one Stratix III field programmable gate array
(FPGA). Data rates of 3.33 Gbps and 6.67 Gbps are
realized respectively for 4-QAM and 16-QAM
modulations. To the best of our knowledge, this is the
record real-time data rate for coherent OFDM reception,
in either RF domain or optical domain.
Experimental setup
Figure 1 shows the experimental configuration with
dual polarizations in a 2×2 MIMO-OFDM mode. The
procedure of generating optical OFDM signal with direct
up-conversion is the same as that reported in [6]. At the
output of I/Q modulator the optical signal is evenly split
into two polarization branches with a polarizing beam
splitter (PBS), with one branch delayed by one OFDM
symbol period, i.e., 48 ns in this experiment. The two
polarization branches are subsequently combined
together, emulating two independent transmitters, one on
each polarization, resulting in a composite data rate of
3.33 Gbps (4-QAM) and 6.67 Gbps (16-QAM).
Fig. 1. Experimental setup for real-time coherent optical MIMO-OFDM
reception.
At the receiver side, direct optical-to-RF downconversion is employed. Optical signal is fed into a PBS
for the polarization diversity coherent receiver. Each
branch of the splitter is mixed with a local laser with an
optical 90° hybrid, and I and Q ports from the optical
90° hybrid are used for direct down-conversion. Two
balanced receivers at each branch are used to detect I and
Q components. The RF signals from the four balanced
detectors are firstly passed through the anti-aliasing lowpass filters with a bandwidth of 575 MHz, and then
sampled with four high-speed ADCs at 1.5 Gsps. Then
the signals are transmitted into FPGA through 1:2
demultiplexed outputs, which lower the rate down to 750
Msps. The multiple inputs are received and demultiplexed into 8 channels at 187.5 Msps in the FPGA
for further signal processing. After all the OFDM signal
978-1-4244-4103-7/09/$25.0 © 2009 IEEE
processing, the recovered data are compared with
transmitted ones in FPGA and errors are counted. This
error count, together with transmitted OFDM symbol
numbers, is then sampled by SignalTap II debugging
module and transported via JTAG cable to PC for BER
collection.
Signal processing algorithms
The CO-MIMO-OFDM receiver signal processing
procedure is divided into nine stages: (1) timing
synchronization, (2) frequency synchronization, (3) CP
removal to recover OFDM block, (4) FFT to recover the
frequency-domain symbols, (5) phase estimation for
training symbols, (6) channel estimation, (7) Jones
Matrix inversion to recover two polarization signals, (8)
phase estimation for payload symbols, (9) symbol
decision, error accumulation and BER computation. The
timing and frequency synchronization methods are the
same as those discussed in signal-polarization real-time
experiment [6]. The algorithms of phase estimation for
training symbols, channel estimation and phase
estimation for payload symbols can be found in [1].
The training symbols structure is shown in Figure 2.
The odd symbols are filled, while leaving the even
symbols blank. After the polarization multiplexing
emulator, the training symbols form a pattern of
alternative polarization launch for channel estimation
stage to estimate channel transfer function H.
OFDM signal at back-to-back transmission. Each point
in this figure is obtained by averaging over 50 OFDM
transmission blocks each containing 288 OFDM data
symbols. The combined laser linewidth is about 100
kHz. A BER of 10-3 can be observed at an OSNR of 0.6
dB (ASE noise bandwidth of 0.1 nm) for 4-QAM signal
with the polarization dependence less than 0.3 dB. The
data rate is limited to 3.33 Gbps due to the 1.5 GHz
maximum sampling rate of ADCs. The choice of only
filling 46 subcarriers out of 64 is due to 575 MHz antialias filter used. The data rate can be further improved
by employing higher sampling rate ADCs. We also show
the performance of 6.67 Gbps 16-QAM in the figure.
The OSNR sensitivity for the BER of 10-3 is about 10
dB. The relative large penalty compared to the 4-QAM is
due to the BER floor at 10-4. This BER floor for 16QAM is attributed to the limited 7-bit resolution, and
large phase drift due to long OFDM symbol length by
using relatively low sampling rate of 1.5 Gsps. This can
be avoided to use ADCs with higher sampling rate.
Nevertheless, our demonstration has achieved the record
data rate of 6.67 Gbps for real-time reception of coherent
OFDM signal, in either RF domain or optical domain.
Conclusions
A multi-gigabit real-time dual-polarization COOFDM receiver in a 2×2 multiple-input multiple-output
(MIMO) configuration is demonstrated and the record
data rate of 6.67 Gbps is realized for coherent OFDM
systems either in RF or optical domain.
References
ig. 2. The time-domain representation of the dual-polarization OFDM
lock including training symbols for timing and frequency
ynchronization, channel estimation, and payload. ‘x’ and ‘y’ represent
wo polarization components.
ig. 3: The real-time BER performance for CO-OFDM signal at back-toack transmission. The 4-QAM and 16-QAM CO-OFDM signals carry
ata rates of 3.33 Gbps and 6.67 Gbps respectively.
Measurement results and discussion
Figure 3 shows the BER performance of 3.33 Gbps 4QAM and 6.67 Gbps 16-QAM Coherent Optical MIMO-
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