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In this chapter, simulations results will be presented along with the underlying evaluate assumptions. The aim is to evaluate the performance of HPON by comparing it with ITU-T G.984 GPON. The performance is evaluated in terms of minimum BER and Q-Factor. Where Q-Factor replaces SNR in WDM networks.
We first present the simulation scenario and the system parameters that were used during the simulation and then proceed to simulation results, where minimum BER, Q-factor and eye diagrams for both GPON and HPON are discussed and then compared.
5.1 SIMULATION METHODOLOGY
The simulation is based on the scenario developed in Optisystem Optical Communication System Design software, where by using BER analyzer and eye diagram analyzer of visualizer library we obtained the plots of minimum BER, Q-factor and eye diagrams.
5.2 SIMULATION PARAMETERS
Frequency = 1550 nm
Frequency spacing = 100 GHz
Bandwidth = 10 GHz
Extinction ratio = 30 dB
Amplifier gain = 17 dB
Amplifier Noise Figure = 6 dB
Filter type = Gaussian
Filter order = 2
Return loss = 65 dB
Unicast transmission wavelength = between 1546.92 nm and 1560.1 nm
Multicast transmission wavelength = 1561.42 nm to 1575.37 nm
Single mode fiber length = 26 Km
Attenuation = 0.24 dB/Km
BER analyzer algorithm = Gaussian
BER time window = 1.5 bit
5.3.1 OLT section of HPON:
Figure 5.1: OLT section of HPON
5.3.2 RN Section of HPON:
Figure 5.2: RN section of HPON
5.3.3 ONU Section of HPON:
Figure 5.3: ONU Section of HPON
The above architecture is based on two cascaded AWG's. The first stage is an 8x8 AWG which is used as a central router that interconnects the optical transmission equipment with remote nodes. These remote nodes are 1x8 AWG's that route wavelengths to each of the output ports. Both devices have channel spacing of 100GHz, Insertion loss of the 8x8 AWG are of 6dBs, while for 1x8 AWG losses are in between 4 and 5dBs. The wavelengths used for unicast transmission are in range between 1546.92 nm and 1560.1 nm. For multicast transmission 1561.42 nm to 1575.37 nm are used. The ONU has two receivers. The multicast receiver is a simple photo detector. The unicast receiver modulates the upstream data. A device that can remotely modulate the carrier sent from the OLT is used because there is no light generation pattern at the ONU.
The OLT optical sources are a tunable laser stack and a fixed laser stack. The tunable laser stack performs unicast transmission and is shared on TDM basis among ONU's, which are connected to one of the remote nodes. This architecture allows dynamic bandwidth allocation as time slots can be dynamically assigned depending on the transmission requirements. The fixed laser stack is composed of fixed lasers with wavelengths compatible with the AWG routing table. The optical sources are coupled and modulated with a single modulator and then split to the input ports of the central AWG. Optical Amplification after the modulator is performed. This architecture shares each laser by a factor of N. Each tunable laser serves N users on TDM basis while the N lasers from the multicast laser stack serve the entire network, to which NxN users can be connected.
Wavelength and time multiplexing techniques are used on the proposed topology. Wavelength multiplexing is used for routing purposes and to combine unicast and multicast traffic.
BIT ERROR RATE (BER)
In telecommunication, an error ratio is the ratio of the number of bits, elements, characters or blocks incorrectly received to the total number of bits, elements, characters or blocks sent during a specified time interval. Examples of Bit Error Rate are:
Transmission BER, i.e., the number of erroneous bits received divided by the total number of bits transmitted.
Information BER, i.e., the number of erroneous decoded (corrected) bits divided by the total number of decoded (corrected) bits.
People usually plot the BER curves to describe the functionality of a digital communication system. In optical communication, BER (dB) vs. received power (dBm) is usually used; while in wireless communication, BER 9dB0 vs. SNR (dB) is used.
Bit errors can be caused in serial communication systems due to improper design or due to random events. Under these circumstances the BER is determined by the circuit design and by probability. Bit errors can also be caused in digital serial communication systems due to intrinsic or extrinsic factors. In optical links the errors occur primarily because of the physical components used to make the link (optical driver, optical receiver, connectors, optical fiber, etc). Errors are also caused by optical attenuation and optical dispersion .
With the introduction of high quality optical transmission circuits centered mainly on long distance trunk lines and undersea cables, transmission path quality requirements have become extremely severe. For example, when requiring a bit error rate of 1.0E-15, a minimum measurement time of 27 hours is required for bit error measurement even for a 10 Gbps signal. In these circumstances, instead of measuring bit error rate, Q-factor measurement has become the new quality evaluation parameter.
The Q-factor theory was announced in 1993 and was subsequently standardized as ITU-T G.976 (1997) Q factor measurement recommendation. In N. America, it was adopted in 1999 as Rec. OFSTP-9 TIA/EIA-526-9. Currently, quality evaluation using the Q-factor is increasingly becoming the method of choice for research and design, manufacturing, installation, acceptance, inspection and maintenance of high quality optical circuits such as undersea optical circuits and long distance trunk circuits.
The Q-factor is a measurement of the digital signal eye aperture; it adopts the concept of S/N ratio in a digital signal and is an evaluation method that assumes a normal noise distribution .
The Q-factor is used to predict the minimum Bit Error Rate (BER). The Q-factor from BER is calculated numerically by:
BER = Ã-
This visualizer allows the user to calculate and display the Bit Error Rate (BER) of an electrical signal automatically. It can estimate the BER using different algorithms such as Gaussian and Chi-Squared and derive different metrics.
It records when and where errors occur relative to each other in the data stream. By using exact bit location information, the bit analyzer can separately measure bit errors, burst errors, total errors and error rates, where user can define the burst criteria. Existence of burst errors is the first indicator that errors are not random. After running the simulation, the visualizer generates graphs and results based on the signal input. The graphs and results can be accessed from the Project Browser, from the Component Viewer, or by double clicking a visualizer in the Main Layout .
5.8 EYE DIAGRAM
You have a signal that is varying extremely quickly - so quickly that we need sophisticated receiver circuitry to detect its changes of state. Yet we expect to be able to measure and display the signal very accurately - much more accurately than we could ever possibly receive it. The secret is that we receive the signal many times (indeed millions of times) and display the aggregate. Signals when they carry information vary and therefore we can never get a good solid picture of a particular state or change of state.
However we can get an excellent idea of the aggregate. The eye-diagram has over the years become the recognized way of looking at an electronic signal and determining its "goodness" as a carrier of Information. It consists of many (from hundreds to millions) of instances of the signal displayed over the top of one another. In extremely fast equipment you might get only one or two points on a trace at a single sweep. But displaying them together allows us to assess the quality of the received signal very well indeed.
Figure 5.4: Eye diagram
The following aspects of the eye are important:
1. The vertical eye opening indicates the amount of difference in signal level that is present to indicate the difference between one-bits and zero-bits. The bigger the difference the easier it is to discriminate between one and zero. Of course this is affected significantly by noise in the system.
2. The horizontal eye opening indicates the amount of jitter present in the signal. The wider the eye opening is on this axis the less possibility that we are likely to have jitter.
3. The thickness of the band of signals at the zero-crossing point is also a good measure of jitter in the signal.
4. The best indication of signal "goodness" is just the size of the eye opening itself. The larger it is, the easier it will be to detect the signal and the lower will be the error rate. When the eye is nearly closed it will be very difficult or impossible to derive meaningful data from the signal .
5.9.1 BER Comparison between GPON and HPON
126.96.36.199 Minimum BER of GPON
Figure 5.5: Minimum BER of GPON
Above figure shows the relationship between minimum BER and bit period for GPON, in which we can observe that for a bit period of 0-1 we have minimum BER of 1.765 exp (-013)
188.8.131.52 BER of HPON
Figure 5.6: Minimum BER of HPON
Above figure shows the relationship between minimum BER and bit period for HPON, in which we can observe that for a bit period of 0-1 we have minimum BER that is 6.16717 exp (-069)
Q-factor comparison between GPON AND HPON
184.108.40.206 Q-factor of GPON
Figure 5.7: Q-factor of GPON
Above figure shows the relationship between Q-factor and bit period for GPON, in which we can observe that for a bit period of 0-1 we have maximum Q-factor value i.e. 7.2747.
220.127.116.11 Q-factor of HPON
Figure 5.8: Q-factor of HPON
Above figure shows the relationship between Q-factor and bit period for HPON, in which we can observe that for a bit period of 0-1 we have maximum Q-factor i.e. 17.5081.
18.104.22.168 Total Q-factor gain
By comparing the above figures we observe that Q-factor of GPON for a bit period of 0-1 is 7.27247, similarly Q-factor of HPON for the same bit period is 17.5081 so there is a big difference between the Q-factor of the both.
Total Q-factor Gain = Q (HPON) - Q (GPON)
= 17.5081 - 7.2724
The above value of Q-factor gain shows that HPON has greater performance than GPON.
BER V/S Q-factor comparison
Above figure shows minimum BER graph
Above figure shows Q- factor graph
It is clear from the above graphs that as Q-factor value increases BER decreases. Also it can be observed from the plots of BER versus bit period and Q-factor versus bit period that for Q-factor value of 7.2724 we get minimum BER 1.76526 exp (-013)
Above figure shows minimum BER graph
Above figure shows Q- factor graph
Similarly it is clear from above graphs that as Q-factor value increases BER decreases drastically. Also it can be observed from the plots of BER versus bit period and Q-factor versus bit period that for Q-factor value of 17.5081 the minimum BER is 6.16717 exp (-069). These results prove that HPON has superior BER performance as compared to GPON.
Relation between minimum BER and Q-factor
Based on above discussion we have found an intuitive inverse relationship between minimum BER and Q-factor that is
Eye diagram comparison between GPON and HPON
22.214.171.124 For GPON
The above figure shows eye diagram of GPON where the vertical eye opening indicates amount of difference in signal level that is present to indicate the difference between one-bits and zero-bits. It is clear that the difference is less and it is difficult to discriminate between one and zero. So it can be concluded that the system is heavily affected by noise.
Horizontal eye opening indicates the amount of jitter present in the signal, as we can see that the eye opening is not much wider, hence there are chances of having jitter in the signal.
The thickness of the band of signals at the zero-crossing point is also a good measure of jitter in the signal. In the above plot it is observed that the band of signal at the zero crossing in GPON is much thicker hence this also indicates the more likely chances of jitter.
The best indication of signal "goodness" is just the size of the eye opening itself. In the above plot the eye opening is moderate and eye height value is 5.04855 exp (-006), so it can be difficult to detect the signal and the BER can be higher as well.
126.96.36.199 For HPON
From the above graph it is clear that the difference between one-bits and zero-bits is greater and it is much easier to discriminate between one and zero. So it can be concluded that the system is less affected by noise.
We can also observe from the graph that the eye opening is much wider, hence there are less chances of having jitter in the signal.
Also the band of signal at the zero crossing in HPON is thin, so this also indicates that there are less likely chances of jitter.
Further it can observed from the above plot that the eye opening is larger and eye height value is 1.5078 exp (-005), so it is easier to detect the signal and the BER can be lower as well.
5.9.5 Comparison between Eye diagram of GPON and HPON
Above figure shows Eye diagram of GPON
Above figure shows Eye diagram of HPON
When comparing both the systems and taking vertical eye opening as reference it can be concluded from the above eye diagrams that HPON is less affected by noise so it will be easier to detect the signal compared to GPON which is much affected by noise.
Also it can be observed from the horizontal eye opening and the zero crossing point that in HPON there are less likely chances of jitter in signal compared to GPON having more likely chances of jitter.
Finally the size of the eye opening in HPON is larger, so it is easier to detect the signal and the BER is lower as well. Whereas the eye opening of GPON is moderate so it can be difficult to detect the signal and the BER can be higher as well. Similarly the eye height value of HPON is 1.5078 exp (-005) which is greater than the eye height value of GPON i.e. 5.04855 exp (-006), so in HPON the signal detection is easier compared to GPON.
The insertion losses of AWG in HPON are smaller as compared to the passive splitter in GPON. With splitter, losses can be as large as 40-80 dB but with the AWG the losses are as small as 8dB regardless of the number of ports. This extra margin can be used to enhance the system performance, thus resulting in a high Q-factor and lower BER in HPON as shown in figure 5.8 and 5.6 respectively. Similarly the low losses in AWG help to reduce the noise to acceptable levels as observed in the figure 5.10 which shows eye diagram of HPON.
With GPON, using splitter, one wavelength is used for upstream and one wavelength is used for downstream transmission resulting the average bandwidth per user to few tens of Mbps. Whereas HPON using AWG, each ONU (customer) is given a dedicated wavelength for their downstream and upstream transmission, resulting in a higher use of bandwidth (thus efficient use of fiber bandwidth). Because of low loss AWG, the link reach (or distance) and speed is increased, thus resulting in high bandwidth.
By observing the above statement and comparing it with the achieved results we can conclude that HPON has lower BER and greater Q-factor as shown in figures 5.6 and 5.8 respectively, although having a separate wavelength for upstream and downstream for each user, compared with GPON having higher BER and smaller Q-factor as shown in figures 5.5 and 5.7 respectively, while it has only a single wavelength for upstream and downstream for all users on time shared basis.
It is also observed that in GPON each ONU has to detect the wavelength itself because only one wavelength is shared among multiple ONUs compared to HPON in which Remote Node allows the specific wavelength to reach the given ONU which results in improved signal reception in HPON as observed in the figure 5.10 that shows the eye diagram of HPON.
Further it can be analyzed that AWG in HPON is less susceptible to Crosstalk compared to GPON which can be observed from the eye diagrams of both the systems.