The receiver section is made up of Positive Intrinsic Negative diode (PIN), Avalanche photo detector diode (APD) or any diode that produces an electrical signal proportional to the received power level. In some cases, an amplifier is added to take care of attenuation effects by boasting the signal level. Sometimes, regenerators could be used instead of amplifiers, and the job of the regenerator is to boast the signal before being fed to an analyser. Low pass filter separates the noise, also separates wavelengths outside the preferred range. Finally, an integrated signal processing circuit like Bit error rate (BER) analyser is added to analyse the delivered signal. Figure 3.35 shows a block diagram of a typical optical receiver section.
Figure 3.35: Block diagram of optical receiver section
Figure 3.37 shows Optisystem diagram from the receiver section of the work. Low pass filter is used to allow only the desired frequency components at each case. Optical receivers were used. The receiver sensitivity is -29dBm as obtained from the system downstream power budget of table 3.10. Amplifier or regenerators were not used since all loses were accounted
Photo-Detector//Optical receivers
Amplifier/
Regenerator
Low Pass Filter (LPF)
Bit Error Rate (BER) Analyser
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for in the power budget. Figures 3.36 and 3.37 show the frequency allocation in low pass filter and receiver sensitivity of the optical receiver respectively.
Figure 3.36: Low pass filter showing frequency of 193.1THz.
Figure 3.37: Receiver sensitivity of the optical receiver.
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Integration of Transmitter, Channel/fibre link, WDM/splitter and receiver sections make up the network. Figures 3.38 and 3.39 gave a block view of all the sections put together in both segments A and B respectively. The exchange/switch houses the OLT (eNB0) and the OLT in turn accommodates the transmitter section, the splitter is seen to represent the spot after the shared fibre, it could be WDM or optical power splitter that distributes/multicasts the signal down to each ONU in the downstream direction. Each ONU represents eNB and BER analyser indicates among other things the bit error rate, Q factor per OLT-ONU pair. Block diagram showing segments A and B of the network is shown in figure 3.38 and 3.39 respectively. It consists of one OLT, a splitter and thirty two ONUs. The parameters of each of the components used in this design is as contained in the downstream power budget of table 3.10 and the system/ network architecture showing full network implementation in Optisystem for segment A and segment B is shown in the figure 3.40 and 3.41 respectively.
However, full network diagrams of segment A and B in downstream direction is shown in A3 print out in appendix D and E respectively.
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Figure 3.38: Block diagram of segment A of the network
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Figure 3.39: Block diagram of segment B of the network
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111 Figure 3.40: Full Network implementation for Segment A
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Figure 3.41: Full Network implementation for Segment B
114 3.7.4 UPSTREAM TRANSMISSION SECTION
In upstream direction, ONUs are transmitting to the OLT whereas the OLT receives the signals from various ONUs. WDM transmitters are used in ONU, each ONU is assigned different wavelength, different frequency. From upstream power budget calculations of table 3.11, ONU transmit power is 2dBm. Figure 3.42 shows different ONU transmitters with different frequencies but same power
Figure 3.42: Different WDM transmitters with different frequencies but same power
Combiners were used in upstream direction instead of splitters, here; the combiners combine signals transmitted by four ONUs. Each of the combiners combines separate signals from different ONUs using time division principles. 4x1Combiner is used as shown in figure 3.43.
Figure 3.43: 4x 1 Combiner in Optisystem environment
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Also shown in figure 3.43 are Bessel filter, time delay, optical attenuator and optical fibres of varying lengths. The filter ensures that only frequency assigned to the transmitter is allowed.
Time delay assigns delay to the signal to avoid possible collision in the combiner. The combiner employs time division principles. Hence, signals from the four ONUs are combined as shown in figure 3.43. Just like in the downstream transmission, the upstream is divided into two segments (segments A and B)
Eight number of 4 x 1 combiners were used to combine signals from thirty two (32) ONUs, the output from the 8 combiners constitute the inputs to 8x1 WDM mux as seen in figure 3.44.
The output of the WDM mux became input to DCF and in turn feeds the shared fibre of length 4.8km. Output of fibre feeds WDM demux. The outputs of WDM demux are connected to receiver sections.
Figure 3.44: DWM mux, WDM demux, Fibre and DCF in optisystem environment
In upstream, 0.3dB attenuation coefficient was used to determine the attenuation per fibre and is measured in dB/km. For DCF, 0.6dB/km was used. Figure 3.45 is an optisystem image showing attenuation in both fibre and DCF, attenuation, also is lengths of fibre and DCF
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Figure 3.45: Fibre and DCF lenghts and attenuation
Figure 3.46: WDM demux and Output Port connections
The 1x8 WDM demux has eight ports as output. Each of the eight output ports connects to the receiver section as shown in figure 3.46. The receiver section is made up Bessel filter, optical receiver and BER analyser. The Bessel filter filters unwanted frequencies, hence allows only
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assigned frequencies. The optical receiver has sensitivity of -30dBm as provided by upstream power budget of table 3.11. The BER analyser displays the eye diagrams. Figures 3.47 and 3.48 show the complete network diagrams of segment A and B of upstream transmission respectively. However, full network diagrams of segment A and B in upstream direction is shown in A3 print out in appendix F and G respectively
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Figure 3.47: Full Network implementation for Upstream Segment A
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Figure 3.48: Full Network implementation for Upstream Segment B
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CHAPTER FOUR RESULTS AND DISCUSSION
This chapter discusses various results obtained from each method used in chapter 3. Results from such methods like optimal Splitter location determination, Power budget analysis for both Downstream and Upstream transmissions, Ranging delay determination, Implementation/validation of the designed network on Optisystem simulation environments, etc were extensively discussed in this chapter.