Design and performance analysis of absolute polar-duty cycle division multiplexing system with different data rates.
Multiplexing is the mandatory means for impressive utilization of bandwidth in today's Digital Communication. It permits numerous users to transmit data simultaneously over a single optical fiber link by sharing the bandwidth of the transmission medium. There are a variety of multiplexing techniques such as Time Division Multiplexing(TDM), Sub-Carrier Multiplexing (SCM), Code- Division Multiplexing (CDM), Wavelength Division Multiplexing (WDM), Dense- Wavelength Division Multiplexing (DWDM), Coarse0Wavelength Division Multiplexing (CWDM) etc., each having its own advantages on spectrum utilization. A comparative analysis has been done for different optical digital modulation formats and multiplexing techniques within and beyond 400 Gbps in .
In optical communication, a modern multiplexing technique named Duty Cycle Division Multiplexing (DCDM) uses different duty cycle values for differentiating multiple users in a single wavelength channel. The Parameters such as number of users, data rate, bit error rate (BER), Q-factor, optical signal to noise ratio(OSNR), width of eye opening, spectral efficiency, dispersion tolerance and transmission capacity evaluate performance of the system. A multiplexing technique which uses the bipolar RZ signal to realize DCDM is called as Absolute Polar- Duty Cycle Division Multiplexing (AP-DCDM), where the signal energy shows APDCDM progress which carries more users than DCDM .
The differentiation of AP-DCDM system with and without Guard Band (GB) in symbol duration has been demonstrated for 40Gbps system in , where the design with GB holds the spectral width of 100GHz and the design without GB occupies lesser Spectral width of 80 GHz. Here, the allocation of Guard Band reduces the Inter Symbol Interference. The performance of AP-DCDM is enhanced by modelling a system with Dual-Drive Mach-Zehnder Modulator (DD-MZM) in , where the optimization of bias voltage in DD-MZM provides improved receiver sensitivity for 1.28 Tbps (32 x 40 Gbps) AP-DCDM-WDM system over 320km fiber and results in larger eye opening.
In terms of transmission capacity, five user AP-DCDM system each with 20Gbps data rate transmits data up to 75km in  and six user AP-DCDM system each with 10 Gbps data rate propagates over 150 km in  using standard single mode fiber (SSMF).It has been proved in  that AP-DCDM has more spectral efficiency than RZ-WDM and NRZ-WDM systems and also observed when attenuation parameter is increased the BER of the AP-DCDM system gets worse.
In this paper, 80(4x20) Gbps, 60(6x10) Gbps and 120(6x20) Gbps AP-DCDM systems have been modelled in Optisystem and the performance analysis based on spectral width utilization of the system, eye pattern, effect of launched input power, receiver sensitivity and transmission capacity is done. It is found that by optimizing the bias voltages of Dual-Drive Mach-Zehnder-Modulator (DD-MZM), the sensitivity of the system can be enhanced. The spectral width increases from 90 GHz to 200 GHz when the data rate of AP-DCDM system is increased from 60(6x10) Gbps to 120(6x20) Gbps. The minimal received power required to achieve Minimum BER increases by 5.2dBm between 80(4x20) Gbps & 120(6x20) Gbps systems.
The paper is organized as follows. Section II describes the working principle of AP-DCDM system and the simulation setup of intended AP-DCDM designs. Section III provides simulation results of designed systems for performance analysis and discussion based on analysed factors. The article is concluded in section IV with the discussion of future scope.
Working Principle And Simulation Setup:
Absolute Polar Duty Cycle Division Multiplexing multiplexes number of channels differentiated on the bases of Duty Cycle. The combo of User Defined Bit Sequence Generator (BSG) and Return-to- Zero pulse Generator produces a digital pulse spectrum depending upon their inputs.
In RZ Pulse generator, every user transmits bit '0' with zero volts and bit '1' with +A volts (odd numbered users), and-A volts (even numbered users) which results in bipolar RZ signal. The Gaussian pulse shape used is described as in equation (1),
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)
where [c.sub.r] is the rise time coefficient and cf is the fall time coefficient. [t.sub.1] and [t.sub.2], together with [c.sub.r] and [c.sub.f], are numerically determinate to generate pulses with the exact values of the parameters rise time and fall time. [t.sub.c] is the duty cycle duration, and [T.sub.s] is the bit period. For providing different duty cycles to n number of users, the bit period [T.sub.s] is divided into n+1 slots. First n slots are assigned to individual ith users, whereby the pulse duration [T.sub.i] for each user is defined as in equation (2),
[T.sub.i] = I x [T.sub.s]/n+1 (2)
The remaining one slot is considered as Guard Band which eliminates Inter symbol interference.
The bit sequence for each BSG is allocated based on the number of users ,where 2n possible combinations of bits for 'n' users produces 64 possible bit combinations for 6 users which is represented in Fig. 1. In this paper, Six UDBS each with 20 Gbps bit rate are used to design a 120(6x20) Gbps AP-DCDM system and this simulation setup is shown in Fig. 2.
Since bit period Ts is divided into n+1 slots, the 6 user AP-DCDM system possesses the duty cycles of [T.sub.s]/7, 2[T.sub.s]/7, 3[T.sub.s]/7, 4[T.sub.s]/7, 5[T.sub.s]/7 and 6[T.sub.s]/7 respectively for different users. The positive and negative signals results from the odd and even users are electrically added and passed through Electrical Abs to find the absolute value of the multiplexed input signal which is called as AP-DCDM signal.
The signal multiplexing process for the bit combination cases of 44, 60 & 64 in Fig. 1 whose bit stream 110101, 110111 and 111111 are illustrated in AP-DCDM with guard band as in the Fig. 3. Consequently the multiplexed signal is modulated with the laser source operating at 1550nm by the use of Dual Drive MachZehnder Modulator. The bias and modulation voltages of DD-MZM have to be optimized to obtain better receiver sensitivity. The modulated optical signal is transmitted over a Standard Single Mode Fiber (SSMF) and which is detected by the PIN diode kept at receiver section. Later the electrical signal is passed through the Low Pass Gaussian filter with cut of frequency 0.75xbit rate which eliminates the unwanted higher frequency noise produced by the optical fiber.
At the Clock and recovery unit, the received signal is sampled and then fed to the regeneration and decision making circuit. The same simulation setup is constructed for 80(4x20) Gbps and 60(6 x 10) Gbps AP-DCDM systems and results have been measured.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
BER analyser results Bit Error Rate (BER), Q- Factor and Eye Pattern of the system. The 6 users and 4 users APDCDM eye pattern is shown in the Fig. 4(a) & (b). Visualizers such as power meter, oscilloscope can also be used to analyse the performance of the system.
[FIGURE 4a OMITTED]
[FIGURE 4b OMITTED]
RESULTS AND DISCUSSIONS
The performance of the system is evaluated by simulating the designs in accordance with various parameters which have been compared with each other. Fig. 5 compares Modulation spectra of six user APDCDM system each with 10 Gbps and 20Gbps. It has been analysed that the modulation bandwidth increases as bit rate increases. This is because AP-DCDM with Guard Band requires a null-to-null modulated spectrum bandwidth of 2* [(n+1) x single channel bit rate]. This feature enhances the spectral efficiency of WDM systems.
[FIGURE 5 OMITTED]
The input power dependence of BER and OSNR for the proposed schemes are depicted in Fig. 6 (a), (b). To transmit the data over long distance, high input power is required. The 4*10 Gbps AP-DCDM system has high
BER for the given input power in . The Proposed 80(4x20), 60(6x10) & 120(6 x 20) Gbps AP-DCDM systems has minimum BER with linear increment depending on the input power. Due to the degradation in system performance with raise in number of users and bit rate, the 80(4x20) Gbps system has high performance than 120(6x20) Gbps AP-DCDM system.
[FIGURE 6a OMITTED]
[FIGURE 6b OMITTED]
Generally Optical signal to noise ratio increases with the increase in input power. At this juncture, each design has linear increment in OSNR with input power increased from 1dBm to 5 dBm. Simultaneously, OSNR gets decreased in the comparative figure due to the degradation caused by increased number of users and Bit rate in various designs.
Receiver sensitivity for 4 user and 6 users AP-DCDM systems each with 20Gbps Bit rate are shown in Fig. 7(a) & (b).
[FIGURE 7a OMITTED]
[FIGURE 7b OMITTED]
Where 4 user system has -28.4dBm and 6 user system has -23.2 dBm. It is observed that when the bit rate of the system increases from 80(4x20) Gbps to 120(6x20) Gbps, the minimal received power required to achieve Minimum BER increases about 5.2dBm. This performance of the system can be reduced linearly by increasing attenuation of the system and it can be increased by employing preamplifier with appropriate gain.
The variation of BER and Q factor with respect to fiber transmission length is shown in Fig. 8(a)&(b).
[FIGURE 8a OMITTED]
[FIGURE 8b OMITTED]
Usually BER increases and Q Factor decreases when fiber length is increased for the same input. From both the BER and Q-Factor analysis, it is clear that the proposed system provides better transmission capacity. The 80(4x20) Gbps system transmits data up to 110 km , 60(6x10) Gbps system transmits data up to 95 Km and 120(6x20) Gbps system transmits data up to 70 Km. Systems with low data rate would provide high transmission capacity by this proposed technique.
The performance of 80(4x20), 60(6x10) & 120(6x20) Gbps AP-DCDM designs are presented. By optimizing the Bias voltages of Dual Drive- Mach Zehnder Modulator performance of the system can be strengthened. The spectral width increases when data rate gets increased. The minimal received power required to achieve Minimum BER increases when number of users and bit rate are increased. In the effect of transmission length, systems with low data rate would provide high transmission capacity. These results are highly noticeable in optical domain to be used in Long-haul communication systems.
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(1) Savari Manonmani T. and (2) Helena Margaret D.
(1) PG Scholar, Dept of Electronics and Communication Engineering, Alagappa Chettiar College of Engineering and Technology, Karaikudi
(2) Assistant Professor, Dept. of Electronics and Communication Engineering, Alagappa Chettiar College of Engineering and Technology, Karaikudi
Received 25 January 2016; Accepted 28 March 2016; Available 10 April 2016
Address For Correspondence:
Savari Manonmani T., PG Scholar, Dept. of Electronics and Communication Engineering, Alagappa Chettiar College of Engineering and Technology, Karaikudi.
This work is licensed under the Creative Commons Attribution International License (CC BY). http:// creativecommons.org/licenses /by/4.0/
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|Author:||Manonmani T., Savari; Margaret D., Helena|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Mar 1, 2016|
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