Single photodiode direct detection system of 100Gb/s OFDM/OQAM-64QAM over 80-km SSMF
within a 50-GHz optical grid
Chao Li,1,2 Haibo Li,2,* Qi Yang,2 Ming Luo,2 Xuebing Zhang,3 Rong Hu,2 Zhaohui Li,3
Wei Li,1,4 and Shaohua Yu2
1
Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Info., Huazhong University of
Science and Technology, Wuhan, Hubei, 430074, China
2
State Key Lab. of Optical Comm. Technologies and Networks, Wuhan, Hubei, 430074, China
3
Institute of Photonics Technology, Jinan University, Guangzhou, Guangdong, 510632, China
4
weilee@hust.edu.cn
*hbli@wri.com.cn
Abstract: We propose a novel guard-band-shared direct-detection (GBSDD) scheme to improve the receiver spectrum efficiency (SE). The 100Gb/s signal is modulated by 2 sub-bands, which are assigned onto two
orthogonal polarizations. The central wavelengths of the two sub-bands are
set as 10.84-GHz frequency space. The two sub-bands are then received
simultaneously using a single conventional photodiode (PD) of 40-GHz
bandwidth. Only one optical pilot carrier is inserted to beat with the 2 subbands on the two polarizations. When the 2 sub-band signal entering into
the receiver, the signal-to-signal beat interference (SSBI) terms fall and
overlap in the same guard band. As a consequence, the bandwidth usage of
the PD is enhanced from 1/2 to 2/3. The 100-Gb/s signal is modulated using
orthogonal frequency-division multiplexing based on offset quadratureamplitude-modulation
of
64-quadrature
amplitude
modulation
(OFDM/OQAM-64QAM), and transmitted over 80-km standard single
mode fiber (SSMF) within a 50-GHz optical grid. It is shown that the
proposed GBS-DD scheme can be implemented by the current commercial
optical/electrical devices.
©2014 Optical Society of America
OCIS codes: (060.2330) Fiber optics communications; (040.1880) Detection;(060.4080)
Modulation.
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#217223 - $15.00 USD Received 17 Jul 2014; revised 27 Aug 2014; accepted 31 Aug 2014; published 10 Sep 2014
(C) 2014 OSA
22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.022490 | OPTICS EXPRESS 22490
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1. Introduction
With the emergence of internet social networking sites, mobile phones with internet access
and expansion of voice and video communication service, the internet has become an
indispensable part of people’s daily life. The excessive growth of Internet traffic is pushing
optical communication systems towards higher speed and higher capacity per wavelength
channel. In recent years due to its high spectral efficiency (SE), dispersion resilience, and
flexibility in frequency multiplexing, coherent optical orthogonal frequency division
multiplexing (CO-OFDM) has witnessed a feasible solution to realize multi-Terabit capacity
and thousands of kilometers reach transmission [1–3]. A number of sophisticated
optical/electrical devices and components are involved, such as four digital-to-analog
converters (DACs), a dual-polarization I/Q modulator, four radio-frequency (RF) amplifiers
and several Erbium Doped Fiber Amplifiers (EDFAs) at the transmitter side; two polarization
beam splitters (PBSs), a local oscillator (LO) laser, two 2 × 4 90° optical hybrids, four
balanced photodiodes (PDs) with four trans-impedance amplifiers and four analog-to-digital
converters (ADCs) at the receiver side.
Recently, cost-effective short and medium reach networks such as data centre, access and
metro need to increase the data rate per wavelength to 40-Gb/s or even 100-Gb/s to meet the
ever-increasing data traffic demand [4–8]. Direct detection (DD) systems could provide a
more cost-effective solution than coherent detention systems for such networks due to the
simple system configuration and the lower the number of optical and electrical components
required. By employing the traditional intensity modulation/ direct detection (IM/DD) system,
W. Yan et al. have demonstrated a 100-Gb/s optical IM/DD transmission with 10G-class
devices using 65 GS/s Fujitsu high-speed ADC [4]. However, such IM/DD scheme highly
relies on the development of electronics devices/components such as ultra-high-speed ADCs
[9] and the transmission distance is highly limited by the fiber chromatic dispersion. To
overcome these two drawbacks, another alternative DD scheme, a host of self-coherent
system [5–8] is proposed for short-reach applications since it can significantly lower the
expense compared with coherent counterpart while achieving both high data rate and
moderate reach. In our previous work, we has also demonstrated that 6 sub-bands 100-Gb/s
signal is modulated by one single polarization I/Q modulator, transmitted over 320-km
standard single mode fiber (SSMF) and received simultaneously with only one conventional
40-GHz PD [10]. However, the required optical bandwidth is more than 600-GHz.
In this paper, we propose a novel DD scheme for cost-efficient short reach networks,
named “guard-band-shared direct detection (GBS-DD)”, to receive 100-Gb/s optical signal
using only one conventional 40-GHz PD within a 50-GHz optical grid. Orthogonal frequency
division multiplexing/offset quadrature–amplitude-modulation (OFDM/OQAM) is selected as
the modulation format to provide the signal spectrum with high side-lobe suppression ratio,
#217223 - $15.00 USD Received 17 Jul 2014; revised 27 Aug 2014; accepted 31 Aug 2014; published 10 Sep 2014
(C) 2014 OSA
22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.022490 | OPTICS EXPRESS 22491
which can effectively reduce the electrical sub-band frequency interference in the receiver
[11]. Only one optical pilot carrier is inserted to beat with the 2 sub-bands in the two
polarizations. When the 2 sub-band signal entering into the receiver, the signal-to-signal beat
interference (SSBI) terms fall and overlap in the same guard band. As a consequence, the
bandwidth usage of the PD is enhanced from 1/2 to 2/3. The distance of the 100-Gb/s signal
transmission over SSMF is up to 80-km.
2. Principle of the novel guard-band-shared DD-OFDM/OQAM system
Optical Spectrum
Electrical Spectrum
Shared guard band
x-pol.
Guard interval
band1
40-GHz
PD
1
y-pol.
BS
2
band2
1 2
50-GHz
≥BS BS
Fig. 1. Proposed guard-band-shared direct-detection scheme.
In the traditional one sub-band pilot-assisted DD optical system, only less than half of the PD
bandwidth can be used [10, 12]. Thus, it is practically impossible to transmit and detect 100Gb/s signal simultaneously by employing the current electrical/optical components. In order
to promote the PD bandwidth utilization and overcome the PD bandwidth limitation, a guardband-shared scheme based on multi-band modulation to carry out 100-Gb/s signal is proposed
and demonstrated [10]. In [10], the signal is first split into 6 sub-bands with equal electrical
bandwidth. Each sub-band is assigned with an individual pilot carrier. At the receiver side,
the 6 sub-bands are sent into one PD and detected at the same time. However, the optical
spectrum efficiency (SE) is quite low, because each optical sub-band has to be > 100-GHz in
order to eliminate the cross sub-band beating terms. Figure 1 gives our proposed method that
the 2 sub-bands optical signals are allocated in x-/y- polarizations. Only one optical pilot
carrier is required to support both x- and y- polarizations. The process of pilot carrier beating
with the signal is as follows: one guard band BS is needed. After signal entering into the
single PD, two sorts of beating terms are generated, inner sub-band and cross sub-band
beating terms. For the inner sub-band beating terms, each sub-band will generate a SSBI,
which falls into the same guard band with the frequency from 0 to BS. Because the 2 subbands are individually modulated onto two orthogonal polarizations, the sub-band signal only
beat with the corresponding carrier when 2 sub-band optical signal entering into the PD. No
cross sub-band beating terms will be generated. In such way, only a small guard interval is
needed between the 2 sub-bands. Compared with the scheme in [10], the proposed scheme not
only enhances the usage efficiency of PD bandwidth to 2/3, but also can be implemented
within a 50-GHz optical grid. Moreover, the proposed scheme can be applied onto
wavelength division multiplexing (WDM) transmission. 50-GHz optical multiplexers/demultiplexers are required to combine/separate the channel signal. Compared to [10], much
more optical channels can be supported in the WDM system using the proposed method.
#217223 - $15.00 USD Received 17 Jul 2014; revised 27 Aug 2014; accepted 31 Aug 2014; published 10 Sep 2014
(C) 2014 OSA
22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.022490 | OPTICS EXPRESS 22492
ECL1
IQ mod.1
ECL3
ECL2
(a)
: PMOC
: PM-EDFA
: EDFA
: PBS/PBC
80km SMF
(b)
100
GSample/s
ADC
IQ mod.2
AWG2
VOA
OFDM/OQAM-64QAM
PD
Tektronix
DPO73304D
Offline
DSP
5
-15
(a)
-35
-55
-75
1549.6
40
0
Power(dBm)
OFDM/OQAM-64QAM
AWG1
Optical Power(dBm)
3. Experimental setup
30
1550
Wavelength(nm)
1550.4
(b)
0
20
0
10
0
0
0
0
Band 1 Band 2
SSBI
10
20
30
Frequency(GHz)
40
Fig. 2. Experimental setup of 100-Gb/s GBS-DD OFDM/OQAM-64QAM system. (a) Optical
spectrum of the 2 sub-bands and pilot carrier together at the transmitter; (b) Electrical spectrum
of the 100-Gb/s 2 sub-bands signal at the receiver. ECL: external-cavity laser; AWG: arbitrary
waveform generator; VOA: variable optical attenuator; PMOC: polarization maintaining
optical coupler; PBS/PBC: polarization beam splitter/combiner; PD: photodiode.
The experimental setup of the proposed 100-Gb/s GBS-DD OFDM/OQAM system is shown
in Fig. 2. At the transmitter, three external-cavity lasers (ECLs) with linewidth of 100 kHz are
employed as the light sources. ECL1 (at 1549.87 nm) and ECL2 (at 1549.789nm) are fed into
two single polarization I/Q modulators to carry the sub-band signal. Two arbitrary waveform
generators (AWGs) running at 12 GS/s sampling rate produce OFDM/OQAM-64QAM RF
signal for each modulator. The ECL3 (at 1550 nm) is evenly split into two orthogonal
polarizations using a PBS. The upper polarization branch is coupled with the corresponding
sub-band signal using a polarization maintaining optical coupler (PMOC), and then combined
with the lower branch by a polarization beam combiner (PBC). The combined optical
spectrum is shown in Fig. 2(a). A 390-MHz guard band between the 2 sub-bands is reserved
to prevent the wavelength variations of the two source lasers. Each sub-band is comprised of
222 subcarriers with OFDM/OQAM-64QAM loading, 4 of which are selected as the pilots to
estimate the phase noise. The central subcarrier is unloaded to avoid the direct current (DC)
influence. The modulated signal is converted to time domain via an IFFT of size 256. Thus
the baseband bandwidth of each sub-band is 223(numbers of subcarriers including the central
one)/256(FFT size) × 12(sampling rate) = 10.45-GHz. For the channel estimation, we employ
several pairs of training symbols (TSs) in a fashion of [A 0; 0 A], where ’A’ denotes an
independent known OFDM/OQAM symbol. In this experiment, 10 TSs are periodically
inserted in the front of each OFDM/OQAM frame, which is then followed by 500 payload
symbols. The OFDM symbol time duration is 0.46 ps. It is worth noting that, no cyclic prefix
is used in OFDM/OQAM modulated signal. The specially designed digital prototype filter in
the transmitter/receiver signal processing is designed to combat the inter symbol interference
and inter carrier interference [13], which is a square-root raised cosine filter with roll-off
factor of 0.5. The net rate of OFDM/OQAM signal of the 2 sub-bands can be calculated as
follows: (222-4)(numbers of information subcarriers)/256 × 12(sampling rate) × 500(payload
symbols)/510(symbols of payloads and trainings) × 6(bits per sample of 64-QAM) × 2(dual
polarization) × 0.833(20% FEC limit) = 100.18-Gb/s after 20% FEC limit. The transmission
link is constructed by a span of 80-km SSMF with EDFA amplification support. A single 40GHz bandwidth PD is used to detect the entire 2 sub-bands OFDM/OQAM signal. The
received electrical spectrum is shown in Fig. 2(b). The signal is sampled by a Tektronix realtime oscillator scope operating at 100-GS/s with 33-GHz electrical bandwidth. The off-line
digital signal processing is done with MATLAB program orderly, which includes: 1) carrier
frequency offset estimation and OFDM window synchronization; 2) digital filter designed
(for OFDM/OQAM only); 3) fast Fourier transform (FFT); 4) channel estimation and phase
noise estimation; 5) constellation decision and bit-error-rate (BER) calculation.
#217223 - $15.00 USD Received 17 Jul 2014; revised 27 Aug 2014; accepted 31 Aug 2014; published 10 Sep 2014
(C) 2014 OSA
22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.022490 | OPTICS EXPRESS 22493
4. Results and Discussions
Relative power(dBm)
0
(a)
(b)
Band 1
-10
Band 2
Band 1
Band 2
-20
Overlapped
spectrum
-30
-40
-12.5
-7.5
-2.5
2.5
Overlapped
spectrum
7.5
12.5 -12.5
-7.5
Frequency(GHz)
-2.5
2.5
7.5
12.5
200
250
Frequency(GHz)
(c)
(d)
20
SNR(dB)
18
16
14
OFDM/OQAM 1st band
OFDM/OQAM 2nd band
Conventional OFDM 1st band
Conventional OFDM 2nd band
12
0
50
100
150
Subcarriers
200
250
0
50
100
150
Subcarriers
Fig. 3. (a) electrical spectrum of conventional OFDM for the entire 2 sub-bands; (b) electrical
spectrum of OFDM/OQAM for the entire 2 sub-bands; (c) electrical signal to noise ratio versus
signal subcarriers for the 1st band; (d) electrical signal to noise ratio versus signal subcarriers
for the 2nd band.
In this paper, OFDM/OQAM-64QAM is selected as the modulation format for the proposed
100-Gb/s signal transmission system. We compare the performance of OFDM/OQAM with
conventional OFDM. For conventional OFDM multi-band scheme, a guard band is required
to avoid inter symbol interference and crosstalk. Figure 3(a) and 3(b) shows the electrical
spectra of conventional OFDM and OFDM/OQAM for 2 sub-bands power loading scheme.
Due to the inherent property of high side lobe suppression ratio, the OFDM/OQAM signal
provides nearly rectangular spectrum, which is able to effectively reduce the channel
crosstalk. We also conduct an experiment to investigate the crosstalk influence. As shown in
Fig. 3(c) and 3(d), the signal-to-noise ratio (SNR) for OFDM/OQAM scheme is stabilized at
the average level of ~18.45/18.68 dB for all the measured subcarriers of the 2 sub-bands,
which suggests that negligible interference from the other band is introduced. Then we
change the modulation format to conventional OFDM. While for conventional OFDM the
average SNR is ~17.28 dB for the 1st band and ~17.46 dB for the 2nd band. The worst SNRs
for the 1st band of OFDM/OQAM and conventional OFDM are 16.13 dB and 12.34 dB,
respectively. Meanwhile, 15.92 dB and 13.39 dB are observed for the 2nd band. Obvious
crosstalk is observed for conventional OFDM scheme (seen in Fig. 3(c) and 3(d)) at the edges
of the 2 sub-bands. It is proved that compared with conventional OFDM, a much smaller
guard band is needed for OFDM/OQAM modulated multi-band scheme. Our previous work
also has shown that OFDM/OQAM has superiority to load high order QAM format for multiband power loading transmission system without distortions induced by the electrical low
pass filters at the transmitter [14].
#217223 - $15.00 USD Received 17 Jul 2014; revised 27 Aug 2014; accepted 31 Aug 2014; published 10 Sep 2014
(C) 2014 OSA
22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.022490 | OPTICS EXPRESS 22494
7×10-2
OFDM/OQAM
conventional OFDM
6×10-2
BER
5×10-2
4×10-2
3×10-2
20% FEC limit
2×10-2
1×10-2
2
3
4
5
6
7
Received Power (dBm)
8
9
Fig. 4. Received power versus BER for 100 Gb/s OFDM/OQAM and conventional OFDM at
back-to-back.
We measure the BER performance as a function of the received power sensitivity for 100
Gb/s OFDM/OQAM and conventional OFDM at back-to-back, as shown in Fig. 4. Under the
received power of 8.2 dB, the BER of OFDM/OQAM is 1.84 × 10−2, below 20% FEC limit
(BER = 2 × 10−2), while the measured BER of conventional OFDM cannot achieve the 20%
FEC limit, as shown in Fig. 4. Compared with conventional OFDM, about 1 dB power
improvement is observed for OFDM/OQAM at the BER level of 3 × 10−2. The price paid for
OFDM/OQAM is the induced computational complexity.
BER
4×10-2
Only 1st sub-band loading
Only 2nd sub-band loading
Entire 2 sub-bands loading
20% FEC limit
2×10-2
1×10-2
2
4
6
8
10 12
14
16
18
20
CSPR
Fig. 5. BER versus CSPR at back-to-back.
Optical carrier to signal power ratio (CSPR) is one of the most important issues in optical
DD system [15]. We first investigate the BER performance versus CSPR for only one subband power loading at back-to-back. As shown in Fig. 5, the individual sub-band 1 and 2 has
almost the same optimized CSPR of 10.8 dB. The corresponding BER performance is 1.1 ×
10−2. Then, we test the BER performance of the entire 2 sub-bands. Compared to the single
band detection, the optimized averaged CSPR for all the 2 sub-bands is slightly reduction to
9.7 dB due to less signal power in the 2-band detection case. The corresponding averaged
BER is 1.84 × 10−2.
#217223 - $15.00 USD Received 17 Jul 2014; revised 27 Aug 2014; accepted 31 Aug 2014; published 10 Sep 2014
(C) 2014 OSA
22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.022490 | OPTICS EXPRESS 22495
8×10-2
BER
4×10-2
2×10-2
CSPR=6.3 dB
CSPR=8.3 dB
CSPR=10.3 dB
1×10-2
0
5
10
20
15
Launch Power (dBm)
Fig. 6. BER versus launch power for entire 2 sub-bands over 80-km SSMF with various CSPR
values.
We further evaluate the transmission performance of the entire 2 sub-bands over 80-km
SSMF link. The signal powers of the 2 sub-bands are maintained as a constant in the
transmitter, and the averaged CSPR is adjusted by changing the carrier power. We also tune
the variable optical attenuator to control the launch power. Figure 6 shows the averaged BER
versus launch power with several CSPR values. Under the optimized CSPR of 8.3 dB, the
BER is ~1.96 × 10−2, when the launch power is 9.5 dBm.
Tab. 1: BER performance for each sub-band at back-to-back and over 80-km
Back-to-Back (Con. OFDM)
Band 1
2.58 × 10−2
Band 2
2.84 × 10−2
Back-to-Back (OFDM/OQAM)
1.81 × 10−2
1.86 × 10−2
80-km (OFDM/OQAM)
1.95 × 10−2
1.98 × 10−2
9
9
7
7
5
5
3
3
1
1
-1
-1
-3
-3
-5
-5
-7
-7
-9
-9
-7
-5
-3
-1
1
3
5
back-to-back
7
9
-9
-9
-7
-5
-3
-1
1
3
5
7
9
80-km
Fig. 7. Constellations of the recovered OFDM/OQAM-64QAM signals at back-to-back and
over 80-km SSMF.
Under the optimized averaged CSPR, the BER performances of the both 2 sub-bands are
listed in Table 1. At back-to-back, for OFDM/OQAM the BERs of the 2 sub-bands are 1.81 ×
10−2 and 1.86 × 10−2, while for conventional OFDM are 2.58 × 10−2 and 2.84 × 10−2. We also
transmit the signal through 80-km SSMF. The BERs of both 2 sub-bands are 1.95 × 10−2 and
1.98 × 10−2 respectively, which are still under the 20% FEC threshold (BER = 2 × 10−2). The
constellations of the recovered OFDM/OQAM-64QAM signal at back-to-back and after 80km transmission are also shown in Fig. 7. It is expected to further extend the transmission
#217223 - $15.00 USD Received 17 Jul 2014; revised 27 Aug 2014; accepted 31 Aug 2014; published 10 Sep 2014
(C) 2014 OSA
22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.022490 | OPTICS EXPRESS 22496
reach when applying lower order modulation format (such as 32-QAM and 16-QAM) and
occupying larger optical/electrical bandwidth.
1×10-1 (a)
(b)
83.48 Gb/s OFDM/OQAM 32QAM
BER
20% FEC limit
20% FEC limit
6
1×10-2
4
4
2
2
0
0
-2
-2
-4
1×10-3
-6
-6 -4
0
66.78 Gb/s OFDM/OQAM 16QAM
-2
0
2
80
160
Transmission Distance (km)
4
6
240 0
-4
-4
-2
0
2
4
80 160 240 320 400 480 560 640
Transmission Distance (km)
Fig. 8. BER versus transmission distance; (a) 83.48 Gb/s OFDM/OQAM-32QAM; (b) 66.78
Gb/s OFDM/OQAM-16QAM.
To evaluate the effect on net data rate and achievable transmission distance with different
modulation formats, we conduct another measurement of BER versus transmission distance
with 32-/16-QAM formats, as shown in Fig. 8. With 20% FEC loading, the net rate is 83.48
Gb/s for 32-QAM, and 66.78 Gb/s for 16-QAM. The maximum SSMF transmission reaches
for the two cases are 240-km and 640-km with EDFA amplification. The measured 32-/16QAM constellations at back-to-back are also shown in the inserts.
5. Conclusion
We experimentally demonstrated a 100-Gb/s optical OFDM/OQAM-64QAM signal over 80km SSMF with a single PD within a 50-GHz optical grid based on a novel GBS-DD scheme
by employing the current commercial optical/electrical components. It has been shown that
OFDM/OQAM is an efficient modulation format to eliminate the channel crosstalk in multiband optical transmission system.
Acknowledgments
This work is supported by the National Basic Research (973) Program of China
(2010CB328300), and 863 Program of China (2012AA011302).
#217223 - $15.00 USD Received 17 Jul 2014; revised 27 Aug 2014; accepted 31 Aug 2014; published 10 Sep 2014
(C) 2014 OSA
22 September 2014 | Vol. 22, No. 19 | DOI:10.1364/OE.22.022490 | OPTICS EXPRESS 22497