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978-1-4799-0367-2/13 / $ 31.00 © 2013 IEEE
Performance of WDM–PON System Based on Optical
Frequency Comb Generation
Gabriel D. Villarreal, Ana M. Cárdenas, and Javier F. Botía
Electronic and Telecommunications Engineering Department.
University of Antioquia
Medellín, Colombia.
[email protected], [email protected], [email protected]
Abstract—This paper refers to a research about 5-channel
wavelength division multiplexed passive optical network (WDM-
PON) based on optical frequency combs generation (OFCG),
using Mach-Zehnder modulator (MZM). The simulations were
applied in standard single-mode fiber (SSMF) like an alternative
for next generation-PON (NG–PON). OFCG using MZM has
been widely studied due to the fact that the spectral spacing is
directly related with the RF frequency, and the number of
sideband components depends on the RF power. These
parameters affect the bit-error rate (BER). Therefore, OFCG
shows that suitable channel spacing is required for WDM-PON
systems with high data-rate at 10 Gbps. In addition, WDM-PON
based on OFCG using MZM at 2,488 Gbps can reach lengths up
to 37 Km; this system also provides high performance over the
link length up to 22 Km with BER <10-12, 10 Gpbs and flat
optical spectrum.
Keywords—OFCG; MZM; NG-PON; WDM-PON; SSMF
G.652D.
I. INTRODUCTION
At the end of 2016, the data growth might exceed the threshold of 1.3 zettabytes (10
21 Bytes) [1]. In fact, the
increase of traffic demand is presented when transmission of multimedia information at different video formats, combined with mobility (smartphones, tablets, etc.), generate a data traffic that could exceed the capacity of current networks [2].
Despite optical fiber networks can be used for high transmission rates, the fixed and rigid allocation of bandwidth in these networks based on conventional schemes like Optical Orthogonal Frequency Division Multiplexing (OOFDM) and Wavelength Division Multiplexing (WDM), are not well used and, consequently, some GHz are lost [3], [4].
To overcome this drawback, the Spectrum-sliced Elastic Optical Path Network (SLICE) architecture provides a flexible network that can assign the most appropriate bandwidth size, according to traffic volume, user’s demand, and hop distance, providing a sub and super-wavelength [5]. The term sub-wavelength refers to traffic where an over-provisioning may be generated due to the course granularity of the wavelength. A super-wavelength appears when traffic requires multiple wavelengths, guard-band frequencies (or spectrum gap) among multiple wavelengths which can lead to underutilization of the available spectrum resources [6], [7].
The carriers can be generated by using laser sources or Optical Frequency Comb Generation techniques (OFCG) [8]. The use of multiple laser sources may be costly. Therefore, OFCG could reduce the number of devices and costs; OFCG can be applied to systems requiring multiple carriers (WDM and OFDM) instead of the conventional way in which one laser source is used by each carrier.
Multi–carrier schemes use the spectrum efficiently in order to avoid bottlenecks at different levels, such as: long-haul networks, metropolitan networks, and access networks. In access networks, Passive Optical Network (PON) has been established as an alternative that meets the demand of bandwidth, offering a low cost [9].
Standards like GPON (ITU G.984) and EPON (IEEE 802.3ah) allow downstream at 2.488 Gbps and 1 Gbps, respectively; however, the demand growth for bandwidth seeks to reach 10 Gbps in NG–PON (Next Generation PON) [10]. WDM–PON architecture is proposed as an alternative for NG–PON, being able to offer a higher bandwidth (10 Gbps). WDM-PON assigns a wavelength to each Optical Networking Unit (ONU), providing a large bandwidth. Additionally, each ONU operates in a single transmission rate, instead of the total bit rate of WDM [11].
This research proposes an OFCG technique using Mach-Zehnder Modulator (MZM) as a source of multiple wavelengths, aimed to analyze the feasibility of considering this technique on a network under WDM–PON architecture. The above allows satisfying the scope and capacity of PON. This technique can be proposed as a likely scheme for NG–
PON. This paper collects some discussions about the management of main characteristics of the carriers, as in SLICE, to provide flexibility to the network and to make an efficient use of the spectrum. In addition, it seeks to evaluate its performance in a standard optical fiber (SSMF) ITU G.652D.
This paper is organized as follows: in section I, the methods and techniques for generating combs with flat spectrum and efficiently, as well as a description of some PON standards are described. In section II, the simulations scheme using SSMF, combs, and the bit rate used in PON are shown. The analysis and results of performance for the proposed system are presented in section III. Finally, the conclusions are mentioned.
II. METHODS AND TECHNIQUES.
A. Comb Optimal Generation Using MZM.
Many techniques have been developed for OFCG; some are based on Mode-Locked lasers, optical cavities, nonlinear effects, and electro-optic modulation [12]. This paper describes the technique with electro–optic Mach–Zhender (MZ) modulation.
The principle of OFCG using a MZM is to have a source of continuous wavelength (CW) which is modulated with high amplitude radio frequency signals (RF). High–order sidebands components are generated. These components are considered as combs due to constant frequency spacing. The intensity of each comb depends on the harmonic order and, therefore, it is possible to find a flat spectrum, if appropriated parameters are set to MZM control, such as the phase, the bias voltage, amplitude, and frequency of RFs [13].
The phase shift is induced by driving the MZM with two
RF signals, written in the form 1 1
sin ωs t A t and
2 2
sin ωs t A t . The spectral spacing is directly linked to
the frequency of RFs, and the number of components depends on RFs power. In fact, the bandwidth of the optical comb is defined in terms of the power and frequency of the RFs [14]. In Eq. (1), the expression for the optical field is shown; it is governed by Bessel function and depends on the magnitude of each component (harmonic) of the combs:
1 1
2 2
exp ω θ2
exp ω θ
inout k
k
k
E tE t J A j k t
J A j k t
(1)
where Ein(t) is the optical field of the CW source; θ1 and θ2 are phases in each arm. For large amplitudes in the RF signal, Ai, the Bessel function can be asymptotically approximated to
2 2 1 π
cosπ 4k i i i
kJ A A A
. Thus, the efficiency in
power conversion, for each harmonic, respect to the CW is defined by:
2
2η
out,k
k
in
E
E
or
1η 1 cos 2Δθ cos 2Δ
2π
2 1 πcos 2Δθ cos 2Δ cos
2
kA
A
k + A A2
(2)
where 1 2
2
A AA
, 1 2Δ
2
A AA
, and 1 2
θ θΔθ
2
.
Hence, 2ΔA means a peak–to–peak phase difference induced in each arm; 2Δθ is a DC bias difference between the two arms.
Flat spectrum condition: the conversion efficiency is highly dependent on the order of harmonic, k. It indicates that comb generation using an MZM does not have a flat spectrum. To make the comb flat on the frequency domain, the intensity should be independent of k. In [13]; it is shown that from (2) it is possible to have the condition:
πΔ Δθ
2A (3)
in which frequency components of the generated optical frequency comb should have the same intensity.
Maximum efficiency condition: under this condition, the efficiency of conversion, excluding the insertion loss, is theoretically derived from (2) and (3), obtaining:
1 cos 4Δθη
4πk
A
(4)
when ΔA = Δθ = π/4, the efficiency can be maximized up to:
1η
2πk,max
A (5)
Eq. (5) indicates that the combs generated have a flat spectrum and maximum efficiency.
B. Migration from G(E)PON to NG-PON
Services requiring high bandwidth -HDTV, High Speed Internet, etc.- can currently be supported by PON. However, the increase in demand for the future requires a new PON scheme which supports a higher bit rate, NG–PON (Next Generation PON). There is not a strict definition of NG-PON, but it is known that the transmission rate can reach 10 Gbps or more [15]. Table I shows the difference among the main features of the PON schemes.
TABLE I. MAIN PROTOCOLS PON
Down
[Gbps]
Upstream
[Gbps]
Standar Distance
[Km]
Year
BPON 0.155
0.622
0.155
0.622
ITU-T
G.933.x 20 1996
EPON 1 1 IEEE 802.3ah 20 2004
GPON 1.244 2.488
0.622
1.244
2.488
ITU-T
G.984.x
(FSAN)
20 2004
NG
EPON 10
10
1 IEEE - 2010
NG
GPON 10 10 2.5
ITU-T (FSAN)
- 2011
In addition, there are two formats of NG-PON:
NG-PON1 refers to an update for mid-term that includes four types: 10/1 GEPON, 10/10 EPON, GEPON, and 10/10GPON 10/2.5.
NG–PON2 is a long-term solution because there is not a standard and cost efficient technology yet. The used technologies include WDM, OFDM, among others.
NG–PON2 has the WDM–PON alternative which can also be combined with Time–Domain Multiple Access (TDMA) techniques already used in GPON and EPON standards. This leads to hybrid WDM–TDMA that can improve scalability and allows splitting ratios of up to 1:1000. PONs based on WDM techniques, particularly broadband amplification, can also support distances in the range of 100 Km. The above defines the concept of active PONs, which could play an important role in future metro access and backhaul convergence scenarios [11].
In [16], the implementation of WDM–PON system was proposed by means of an Amplified Spontaneous Emission (ASE) source, as a source of multiple carriers, in order to get them, and they are separated by Arrayed Waveguide Gratings (AWGs) filter. Nevertheless, due to the high intensity of ASE source noise, WDM–PON based on ASE source can reach bit rates of 2.5 Gbps. A possible improvement for an increased bandwidth is the use of a Fabry-Perot laser as sources for the optical system [17].
Looking for a better performance in WDM–TDM PON system [18], each carrier is modulated using Adaptively Modulated Optical OFDM (AMOOFDM). This system uses the performance information of each electrical sub-carrier, and control is established to ensure a similar Bit Error Rate (BER) among them.
III. SIMULATION
This research studies WDM-PON network employing multiple source of wavelengths generated by OFCG technique, using MZM. In order to make proper use of the spectrum, as discussed in SLICE, this research attempts to analyze the effect of the power and the spacing among the carriers (combs). In addition, an algorithm to adjust the transmission rate of each carrier will be used in order to analyze the channel state through the BER estimation, looking for a particular BER among carriers. As a first approximation, the combs’ behavior is analyzed in function of the spacing and power through the BER estimation in a SSMF.
There are several configurations for generating combs with MZM. In Table II, the parameters of devices are mentioned to carry out the simulation.
TABLE II. PARAMETERS FOR THE SIMULATION
Device Parameter Magnitude
Laser
Power 30 mW
Line width 3 MHz
Wavelength 1552 nm
MZM
Insertion loss 6 dB
RF Driving 6 Vpp
DC Bias ± 15 V
5 V
Mux/Demux Channel Spacing 25 GHz
Insertion loss 6 dB
SSMF G.652D Attenuation 0.2 dB/Km
Dispersion 18.0 ps/nm.km
Receptor Sensibility -25 dBm
A. OFCG with maximun efficiency condition.
Based on the previous section, the condition ΔA = Δθ = π/4 must be satisfied for a flat spectrum with maximum efficiency. In Table III, the magnitudes applied to the MZM to control the combs generation is shown.
TABLE III. MZM CONTROL PARAMETERS
Parameter RFa RFb
Amplitude [ 6 4.43
Voltage DC [ 5 3.43
Frequency [GHz] 25 25
B. Simulation scheme
The proposed method in this paper is simulated in Virtual Photonics ® (VPI). Fig. 1 shows the general scheme. CW source is employed for generating optical carriers based on OFCG technique using MZM. Each comb is separated with a demultiplexer. The wavelengths are modulated with on–off keying (OOK) format. Then, by using a multiplexer, the carriers are recombined, creating a WDM system. The multiplexed signal is transmitted through a SSMF G.652D. The wavelengths are separated by another demultiplexer in the receiver side. Finally, the BER is estimated to establish the status information corresponding to each channel to the comb.
Fig. 1. General scheme for a WDM-PON system
C. Simulation scenarios.
To evaluate the performance of the combs, as carriers for WDM-PON system, this system is compared with the current GPON standard at 2.488 Gbps. This system is also evaluated as an alternative for NG-PON at 10 Gbps. The system is evaluated to use a standard SSMF ITU G.652D, and the following scenarios are considered:
When combs were used with maximum efficiency condition (flat spectrum).
Applying a non-flat spectrum for the combs (each carrier has a different power).
By changing the spacing among the combs.
IV. RESULTS AND DISCUSSION
The impact of five carriers generated with OFCG technique using MZM in a WDM–PON system is assessed. This proves that the shape of the optical combs spectrum can be modified by controlling the parameters of the RF, generating spectra as shown in Fig. 2a, for a flat spectrum (with maximum power difference among carriers at 0.11 dBm). In Fig. 2b, a non-flat spectrum is shown which the power changes in some carriers, generated by changing some parameters of MZM control such as RFs amplitudes.
(a)
(b)
(a)
(b)
(b)
(a)
Fig. 2. OFCG spectrum using a MZM for: a) Flat combs and b) Non-flat combs.
In Table IV, the principal features of the used carriers in each scenario are shown.
TABLE IV. PRINCIPAL FEATURES FOR COMBS
Combs with flat spectrum (25 GHz)
Carrier Frequency [THz] Wavelength [nm] Power [dBm]
1 193.050 1552.92649 -2.44
2 193.075 1552.72541 -2.53
3 193.1 1552.52438 -2.48
4 193.125 1552.32341 -2.55
5 193.150 1552.12249 -2.43
Combs with non-flat spectrum (25 GHz)
1 193.050 1552.92649 -3.84
2 193.075 1552.72541 -2.80
3 193.1 1552.52438 0.31
4 193.125 1552.32341 -2.81
5 193.150 1552.12249 -3.83
Combs with flat spectrum (15 GHz)
1 193,07 1552,76562 -2.44
2 193,085 1552,64499 -2.53
3 193,1 1552,52438 -2.48
4 193,115 1552,40379 -2.55
5 193,13 1552,28322 -2.43
Combs with non-flat spectrum (4 GHz)
1 193,092 1552,5887 -3.84
2 193,096 1552,55654 -2.80
3 193,1 1552,52438 0.31
4 193,104 1552,49222 -2.81
5 193,108 1552,46006 -3.83
In Table IV, the central wavelength variations (a few nm) of the CW source were observed, where there are shifts in the frequency of each carrier. Meanwhile, the same characteristics as power and spectral shape are maintained.
A. Combs spaced 25 GHz.
In order to analyze flat combs behavior, having a spacing at 25 GHz among each carrier, the performance for each carrier is estimated through BER, depending on the distance of the SSMF fiber. In Fig. 3a, the BER vs. distance is illustrated for a transmission rate at 2.488 Gbps, where an error less than 10
-12 is possible to reach for a distance of around 37 km. In
Fig. 3b, the results show that by using a transmission rate at 10 Gbps, it manages to transmit about 22 km. It is also shown that each comb has a similar bit error rate due to little differences in the power.
Fig. 3. BER estimated in flat combs at a bit rate at a) 2.488 Gbps and b) 10
Gbps
Fig. 4 illustrates the BER corresponding to a spectrum in which the power is different in some carriers. It is emphasized that the carriers with more power can transmit information at a longer distance. Because of the above, these carriers may be modulated with a higher transmission rate, or those in which have a lower power being modulated with a lower rate, in order to maintain an overall similar BER among the carriers.
Fig. 4. BER estimated for combs with variations in power, for a bit rate of a)
2.488 Gbps and b) 10 Gbps.
B. Combs changing their spacing.
The change of the spacing among carriers affects the system performance. In this case, the spacing of combs is varied between 5 GHz and 15 GHz. In Fig 5, the spacing among carriers is exposed where affects directly the BER,
(a)
(b)
(a)
(b)
showing poor performance at short distances and very large variations in its measure, regardless that carriers have similar power (flat optical spectrum). By using a bit rate at 10 Gbps, the system has a BER greater than 10
-12 without reaching 5
Km.
Fig. 5. BER estimated in flat spectrum with a bit rate at a) 2.488 Gbps and
spacing at 4 GHz, and b) 10 Gbps and spacing at 15 GHz.
When the used carries at different powers and variations on spacing at 5 GHz and 15 GHz are generated, the power is a crucial factor in the system performance. This result is illustrated in Fig. 6. Other important observation is that the carriers with more power have lower BER, as expected.
Fig. 6. BER estimated with a non-flat spectrum for a bit rate at a) 2.488 Gbps
and spacing at 4 GHz b) 10 Gbps and spacing at 15 GHz.
For a bit error rate threshold of 10-3
, the coding techniques using Forward Error Correction (FEC) like block turbo code and Reed Solomon, makes possible to reduce the error rate to values of the order of 10
-12 [19], [20]. This verifies that a
distance up to 45 Km is achievable with a bit rate at 10 Gbps with carriers, having a flat spectrum.
When the central wavelength of combs is varied, in the third window (1490 – 1600 nm), there were no significant changes in system performance due to the fact that the channel characteristics such as attenuation and dispersion are similar in those wavelength for the used fiber.
PON provides a maximum transmission rate at 2.488 Gbps, a physical reach up to 20 km, a bit error rate (BER) less than 10
-10, and a power transmission between -4 and +6 dB,
with typical sensitivity in the ONU of -25 dBm. When PON is compared with the proposed WDM–PON system based on OFCG using MZM under the following conditions: power transmitted approximately -2.5 dBm, a bit rate at 2,488 Gbps and a BER of 10
-12, is possible to achieve the maximum
distance at 37 Km. Employing a bit rate at 10 Gbps, it is possible to transmit information to a distance up to 22 Km. This condition could be interesting for NG–PON2 scheme.
As expected, the highest bit rate increases the likelihood of error. Carriers with more power are more resistant to error. Therefore, this condition could be a control scheme of the transmission rate and/or modulation format for the carriers (comb) generated with MZM. Thus, the best performance is obtained according to channel conditions. In Fig. 7, a likely control scheme to obtain a suitable optical spectrum is shown, according to channel conditions for adapting the maximum transmission rate which keeps on the same BER. In the literature, there are some algorithms for adapting the bit rate, well-known as bit-loading -measurement based on Signal-to-Noise Ratio (SNR)-. Bit-loading can be divided into two categories: rate-adaptive and margin-adaptive. Rate-adaptive algorithms maximize the bit rate for a fixed bit-error ratio (BER) and given power constraint, while margin-adaptive algorithms minimize the BER for a fixed bit rate [21]. Examples of these algorithms are the Levin–Campello [22] and Chow [23].
Fig. 7. Control scheme for carriers
The control scheme is planned as an alternative for the efficient management of the optical spectrum, as SLICE proposes, in order to increase the total bit rate or for each carrier, considering the distance or fixed BER according to channel conditions and user’s requirements.
V. CONCLUSIONS
Based on the obtained results from the simulation, it can be affirmed that OFCG technique is an important solution for optical communications, especially in systems which require multiple carriers such as WDM and ODFM. The optical
combs spectrum can be designed according to the requirements of the system, due to the fact that parameters which control the MZM can be altered, making possible to adjust the spacing, the power, the location of the central carrier in the spectrum, and the quantity of carriers. These are some qualities of flexibility required for the implementation under SLICE architecture. The OFCG technique using MZM has similar performance for a bit rate than current GPON standard (2488 Mbps) in a SSMF fiber G.652D. Furthermore, this technique can be applied to support similar capabilities to the proposals on the future generation PON (NG–PON), complying with the requirement of speed (10 Gbps) and distances which can reach up to 22 km over SSMF G.652D.
ACKNOWLEDGMENT
This work was supported by CODI project at the University of Antioquia called “Analysis of Applications Environment of Optical Ultra–Short Generated through Filtered of Different Portions of the Ultra–Short Pulses Spectrum”.
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