Bidirectional hybrid OFDM based Wireless-over-fiber transport system using reflective semiconductor amplifier and polarization multiplexing technique

Accepted Manuscript Regular paper Bidirectional Hybrid OFDM based Wireless-over-fiber Transport system using Reflective Semiconductor Amplifier and Polarization Multiplexing Technique Khaleda Mallick, Rahul Mukherjee, Binoy Das, Gour Chandra Mandal, Ardhendu Sekhar Patra PII: DOI: Reference:

S1434-8411(18)32154-X https://doi.org/10.1016/j.aeue.2018.09.041 AEUE 52520

To appear in:

International Journal of Electronics and Communications

Received Date: Revised Date: Accepted Date:

10 August 2018 25 September 2018 27 September 2018

Please cite this article as: K. Mallick, R. Mukherjee, B. Das, G. Chandra Mandal, A. Sekhar Patra, Bidirectional Hybrid OFDM based Wireless-over-fiber Transport system using Reflective Semiconductor Amplifier and Polarization Multiplexing Technique, International Journal of Electronics and Communications (2018), doi: https:// doi.org/10.1016/j.aeue.2018.09.041

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Bidirectional Hybrid OFDM based Wireless-over-fiber Transport system using Reflective Semiconductor Amplifier and Polarization Multiplexing Technique Khaleda Mallick1, Rahul Mukherjee1, Binoy Das1, 2, Gour Chandra Mandal1 and Ardhendu Sekhar Patra1* 1

Sidho-Kanho-Birsha University, Dept. of Physics, Purulia, West Bengal, India- 723104 2

Dept. of Physics, J. K. College, Purulia, West Bengal, India

Abstract In this paper, a bidirectional hybrid OFDM based Wireless-over-fiber architecture has been investigated and demonstrated to transmit 10 Gbps as well as 6.25 Gbps OFDM data for downlink transmission and 5 Gbps as well as 2.5 Gbps OFDM data for uplink transmission over 50-km single mode fiber (SMF) employing polarization multiplexing technique (POLMUX) at optical line terminal (OLT) and optical network unit (ONU). The POLMUX technique is exercised by polarization beam splitters and polarization beam combiners. Machzehnder modulator and RSOA have been used for modulation at OLT and ONU respectively. Transmission performances are observed by constellation diagrams, EVM and BER values. For 10 Gbps, 6.25 Gbps downlink signal and 5 Gbps, 2.5 Gbps up-link signal the power penalty of 3 dBm, 2.3 dBm and 4 dBm, 3.2 dBm at a BER of 10-9 between back-to-back and over 50-km SMF plus 10-m and 5-m wireless link, are observed respectively. For 32-QAM<10.5% EVM and for 16-QAM<13% EVM are recorded. Our architecture is a prominent alternative, not only due to its have potential of both optical and wireless technology but also it is offers a powerful platform to communicate high data rates and support various type of future unforeseen applications and services.

Keywords: OFDM, WoF, RSOA, Polarization multiplexing, Remodulation Address all correspondence to: Ardhendu Sekhar Patra, Dept. of Physics, Sidho-Kanho-Birsha University, Purulia, West Bengal, India -723104; E-mail: [email protected]; Telephone: +919433963530

Introduction With the swift development, very effective application on multimedia, and also for easy maintenance, the need of optical access network in the industry of communication becomes mandatory and also attracted global attention during the last few years. Integration of wireless and optical fiber network is a key technology which creating a new landscape in optical access network [1, 2]. During the last few years a lots of work have been proposed and experimentally demonstrated to realize the integration of wire and wireless link, such as, dual-mode colorless laser diode-based millimetre Wireless-over-fiber (MMWoF) system, which perform as a helping hand to combining wired and wireless link and also provide great support to the next generation 5G WoF network [3,4]. WoF system is a potential solution to

overcome the increased demands of data in the field of communication [5, 6]. Recently, several hybrid WoF systems are demonstrated to mitigate the increasing demand of data traffic [7, 8]. WoF provide advantages of both optical and wireless technologies and hold great potential to make our daily life easy and comfort such as to listening song and news in radio, mobile phone, TV with remote and internet facilities in home, hospital, hotels, shopping malls, etc., where people spend their significant amount of time [9-17]. As symbol period and data rate is inversely proportional to each other, high data rate transmission over wireless channel has always been a limiting factor. Different type of obstacles like inter symbol interference (ISI), dispersion, multipath fading, inter carrier interference (ICI), etc. are occur in case of high data rate transmission and degrade the overall performance of the wireless system. After a large number of investigations a strong solution to overcome the barrier of wireless system is found, named orthogonal frequency division multiplexing (OFDM) [18-20]. OFDM is considered as an important part of many standard high rate data transmission systems such as Long Term Evolution (LTE), IEEE 802.11x wireless local area network (LAN), digital video broadcasting (DVB) and IEEE 802.16e-WiMAX, 4G long-term evolution (LTE)-based cellular systems etc [21-25]. OFDM is a multiple sub carrier transmission, where the high data rate is divided into many orthogonal subcarriers. As the guard interval called cyclic prefix (CP) is introduced in every OFDM symbol, the duration of symbol period is very longer to that of the channel impulse response which leads to eliminate dispersion and ISI effect [26,27]. On the other hand, some effective power fading compensation scheme for an OFDM system based on colourless laser diode was illustrated previously [28]. By combining the capacity of the OFDM technology with the mobility of wireless network, the increasing demand for high data rate wireless communication is satisfied and also the overall system performance of the wireless network can be significantly increased [29-31]. Reflective semiconductor optical amplifier (RSOA) is used not only that it

has the capability of reusing the downstream signal and transmitting it in upstream but also it have lots of advantages such as easy integration, small form factor, low noise, large gain etc [32-34]. POLMUX techniques enhance the spectral efficiency and transmission capacity of the system. In POLMUX technique the optical domain is divided into two parts and both of them is used as an optical carrier with same wavelength, as a result the cost and complication of the multi-service provider systems is minimize [35, 36]. Chongfu Zhang et al. [37] transmits 10 Gbps 16-QAM OFDM signal and 10 GHz RF signal using OFDM-passive optical network (OFDM-PON) system and POLMUX technique over 20-km SMF in downlink and 8Gbps 16-QAM OFDM signal in uplink. However, the system had a quite high average BER value, resulting into limited transmission distance 20-km only. In this paper, we have proposed and demonstrated a successful application of hybrid OFDM based WoF transport system using RSOA and POLMUX technique to support nextgeneration bidirectional wireless network. The downlink signal is modulated by 10 Gbps as well as 6.25 Gbps OFDM data stream mixed with 200 MHz radio frequency and communicated over 50-km SMF plus 10-m wireless link with the assistance of horn antenna (HA). By properly utilizing the reusing capability of RSOA, the uplink signal is remodulated by 5 Gbps as well as 2.5 Gbps OFDM data stream and also transmitted over 50-km SMF plus 5-m wireless link employing HA. The use of POLMUX technique in downlink as well as uplink transmission makes the system cost-effective. In our presented architecture, we have used mixed modulation format such as 32/16 QAM for down link transmission and 16-QAM format for uplink transmission. The spectral efficiency of the system become increased about 3.4 bits/s/Hz and also the transmission BER of the system is enhanced of the order of 10-9 due to the employment of advanced mixed modulation format. As we have used very low RF signal of the order of MHz, our proposed system is provide wireless network in those places which is RF sensitive such as hospital, health care centre etc.

Experimental setup

Fig.1. Schematic diagram of our proposed full duplex bidirectional OFDM based WoF system.

Fig.1. shows the block diagram of our proposed bidirectional OFDM based WoF system. A carrier wave of central wavelength 1550.92 nm, coming from distributed feedback laser diode (DFBLD) is transformed in right circularly polarized light with the help of circular polarizer. The light wave is divided into a horizontal and a vertical polarized light by polarizing beam splitter (PBS) PBS1. The horizontal polarized light wave is fed into Mach-Zehnder modulator (MZM) MZM1 for modulation with 10 Gbps OFDM data stream which is mixed with 200 MHz sinusoidal wave (radio frequency) and vertical polarized light wave is fed into MZM2 and also modulated by 6.25 Gbps OFDM data rate mixed with 200 MHz radio frequency (RF) signal. The OFDM data stream is generated by an arbitrary wave form generator (AWG) with 8 bit resolution, 1/32 CP, 512-point FFT, 12 GS/s and 32/16 QAM mixed modulation format. In our presented architecture, 32-QAM and 16-QAM modulation format

is used respectively; enabling total 10 Gbps and 6.25 Gbps downlink transmission. In both of the cases (32-QAM and 16-QAM), the size of FFT is 512 and all the subcarriers bearing 200 data. 1/32 CP is added in each 16/32 QAM modulation for 50-km SMF as well as10-m wireless link transmission. As the signal gets distorted and various types of noise is added to the signal while goes through the SMF as well as wireless link, a training frame is introduced in each 64 data frames for channel estimation and frame synchronization. Then the two modulated signals, coming from MZM1 and MZM2 have been combined by polarization beam combiner (PBC1) and then fed into erbium-doped fibre amplifier (EDFA) and variable optical attenuator (VOA) respectively. Then the light waves passes through the optical circulator (OC1) and send over 50-km SMF. After SMF transmission the received optical signal is divided into two parts in the ONU section by an optical splitter. One of them is fed into PBS2 and split again according to their polarization state to observe the transmission performance of 10 Gbps and 6.25 Gbps data rate individually over 10-m wireless link. Both of the signals are detected by 10 GHz photo diodes (PD) and amplified by 10 GHz power amplifiers (PA). Now, the amplified signals are wirelessly communicated over 10-m wireless link with the assistance of HA (frequency range, 200 MHz–2 GHz). At the end of wireless link the signals are received by another HA and filtered by a 10-GHZ band pass filter (BPF). Finally the signals are lunched into a real-time scope (DSA) and the down link data is recovered by offline digital signal processing (DSP). The remaining signal again divided into two orthogonal parts with the help of PBS3 and both of the signals are send into RSOA for further modulation with the 5 Gbps and 2.5 Gbps 16-QAM OFDM data signal respectively. The operating current of RSOA is 65 mA and also the backscattering effect is reduced as the RSOA is operated in the saturation region. For uplink signal the 16-QAM OFDM data streams are also generated by AWG with 8 bit resolution, 1/32 CP, 512-point FFT. Therefore, the two signals are combined by PBC2 and fed into EDFA and then VOA respectively.

EDFA perform as a booster amplifier to improve the BER performance of the signal. After that, the signal is send over another 50-km SMF. The uplink signals then received by PBS4 and both are detected by 10-GHz PD and amplified by 10-GHz PA. After that the uplink signals are transmitted wirelessly over 5-m, employing HA at the transmitter and receiver end of the wireless link. Over 5-m wireless transmission both the signals are filtered by BPF to remove the unwanted noise which arises due to wireless communication. Therefore, finally the signals are fed into OFDM analyzer to analyzing the transmission performance. Through DSP the uplink data is recovered and constellation diagrams as well as BER value are evaluated. In bandwidth limited RSOA the uplink signals are modulated by ubiquitous advantages of OFDM data stream with advanced modulation format which leads to increase the spectral efficiency and transmission bit rate of the system. The detailed system parameters of our experimental setup are shown in table I. Table I system parameters

Parameter Arbitrary wave form generator (AWG)  Sampling frequency  Resolution Real-time scope (DSA)  Sampling frequency Photo detector (PD)  RF bandwidth  Gain  jitter tolerance  Sensitivity  High Responsivity Horn antenna (HA)  frequency range  Maximum Continuous Power  VSWR (average)  Mounting Base Mach zehnder modulator (MZM)  Data rates  Modulator bandwidth  Optical Insertion Loss

Value  12Gs/s  Up to 10 bits  50 Gs/s     

~ 10 GHz 400 V/W 0.6 UIp-p -19 dBm at 10 Gb/s

   

200 MHz to 2 GHz 300 Watts <1.6:1 1/4 inch x 20 female threads

0.7 mA/mW at 1550 nm

 Up to 12.5 Gb/s  10 GHz  < 5 dB

 RF Amplifier Gain  RF Drive Voltage (Vπ) Polarization beam combiner /splitter (PBC/PBS)  Return Loss  Insertion Loss for combiner  Insertion Loss for splitter  Extinction Ratio Erbium-Doped Fiber Amplifier (EDFA)  Operating wavelength range  Noise Figure of  Output Power  Return Loss  Input / Output Isolation Variable Fiber Optical Attenuators (VOA)  Operating wavelength range  Attenuation Range  Attenuation Resolution  Insertion Loss  Return Loss Reflective semiconductor optical amplifier (RSOA)  signal gain  Low front facet reflectivity  electrical bandwidth Circulator (OC)  Wavelength Range  Damage Threshold

 30 dB  4.5 V    

40 dB ≤1.2 dB ≤4.2 dB ≥20 dB

    

1530 - 1560 nm <5 dB >20 dB >50 dB >30 dB

    

1550 ± 20 nm 50 dB 0.15 dB ≤1.5 dB >55 dB

 >25dB  <10-5  1.5GHz  1525 - 1610 nm  500 mW

Results and Discussions The optical spectra of the source signal of central wavelength 1550.92 nm, coming from DFBLD is depicted in fig.2. Fig.3 (a) to (e) as well as (i)-(iv) represents the optical spectra and electrical spectra of different signals at some important points [insert (a)-(e) and (i)-(iv) of Fig. 1]. The 10 Gbps and 6.25 Gbps OFDM data signal, which are measure at the output of MZM1 and MZM2, are shown in fig.3 (a) and (b). The combined light wave after PBC1 in downlink is depicted in fig.3 (c). Fig.3 (d) and (e) represents the uplink remodulated 5 Gbps and 2.5 Gbps OFDM data signals, at the output of RSOA1 and RSOA2. Fig.3 (i)-(iv) shows the electrical spectra of the downlink and uplink signals at the receiving section after detected by PD.

Fig.2. optical spectra of DFBLD

Fig.3.(a)-(e) and (i)-(iv) optical as well as electrical spectra of different signals at some points of the optical and electrical path [insert (a)-(e) and (i)-(iv) of Fig. 1]

Fig.4 (a) shows the measured BER curves of the 10 Gbps OFDM data signal at the ONU for B-t-B and over 50-km SMF as well as 10-m wireless link. The receiver sensitivity is -14.51 dBm with a BER of 8.74× 10-9 for 10 Gbps OFDM data signal at the ONU after passing through 50 km SMF and 10m wireless link. A 3 dB power penalty is observed between B-t-B and 50 km SMF plus 10m wireless transport scenarios, at a BER of 10 -9. Fig.4 (b) represents the measured BER curves of the 6.25 Gbps OFDM data signal at the ONU for B-t-B and over 50-km SMF as well as 10-m wireless link. The 6.25 Gbps OFDM signal can achieve a -15.51 dBm receiver sensitivity at BER of 6.23×10-9 and there is a penalty of about 2.3 dBm. Furthermore, the measured BER curves of the 5 Gbps and 2.5 Gbps OFDM data signal at the OLT for B-t-B and over 50-km SMF plus 5-m wireless link are shown in Fig. 5(a) and (b). For 5 Gbps and the 2.5 Gbps OFDM data signal the receiver sensitivity are -15.92 dBm and 16.5 dBm at a BER of 2.8 x 10 -8 and 1.95 x 10-8 respectively. Power penalties of 4 dBm and 3.2 dBm are recorded for 5 Gbps and the 2.5 Gbps uplink OFDM data signals at a BER of 10-9. Fig.4 (a), (b) and 5 (a), (b) also display the clear constellation diagrams for 32/16 QAM downlink signals as well as for 16-QAM uplink signals. For 32-QAM below 10.5% EVM and for 16-QAM below 13% EVM, are observed from constellation diagram. The power penalties

of the downlink OFDM signal are not so large and constellation diagrams also are very clear just because of the advantageous OFDM signal. The OFDM signal is represented as [38], (1) π

Where,

(2)

represents the wave form of the kth subcarriers,

kth subcarrier,

is the frequency of the subcarriers,

is the ith symbol of the

represent the symbol period,

is

the number of subcarrier. In case of OFDM, the subcarriers are overlap with each other, but they could not interfere with each other and there is no requirement of any type of filter or wavelength separator to separate them at the receiver end because they always obey orthogonality condition. Just because of the orthogonality condition the OFDM signal is always free from inter carrier interference (ICI). The condition is [39], (3) The more important item of OFDM signal is the insertion of cyclic prefix (CP), called guard interval. When CP is introduced to the every symbol the total symbol duration become increased, such as [40], (4) Where,

is the length of the CP.

After adding CP, the signal is represent as, (5)

In our research work, we take the length of the CP is greater than the time delay ( ) of the wireless link to eliminate the ISI and dispersion effect. The signal is always maintain a certain condition such as [40], (6) Where,

is the guard interval between the symbols.

From the above equations (1-6), it is clear that the OFDM signal is highly robustness to dispersion, multipath fading, ISI, ICI etc, which leads to decrease the power penalty of the signal and provide good BER value and clear constellation diagram in wireless link. Despite of this numerous advantages of OFDM signal the power penalty of the uplink signals observed a little bit larger than downlink signal because the uplink signals after passing through 50-km SMF become very weak, noisy and also the power of the signal is reduced. This power penalty could be attributed to attenuation and the non-ideal modulation characteristics of the RSOA. The attenuation can be expressed in terms of the transmission distance as follows: P (z) = P (0) e-αtotal z

(7)

Where, P(0) is the input optical power, P(z) is the output power, z is the transmission distance and αtotal is the attenuation co-efficient and αtotal (dB/Km) =10/z log [P(0)/P(z)].

Fig.4. measured BER curves of (a) 10 Gbps OFDM downlink signal (b) 6.25 Gbps OFDM downlink signal with 16/ 32 mixed QAM format

Fig.5. measured BER curves of (a) 5 Gbps OFDM uplink signal (b) 2.5 Gbps OFDM uplink signal with 16-QAM format In our system the low power penalty is observed and the measured BER curves proves that our system is free of dispersion, ISI, ICI effect and has excellent transmission performance and also possesses potential to transmit 10 Gbps as well as 6.25 Gbps OFDM data with 32/16 mixed QAM modulation format for downlink and 5 Gbps as well as 2.5 Gbps OFDM data rate with 16-QAM format for uplink bidirectionally over 50-km SMF plus 10-m and 5-m wireless link.

Conclusion This paper has presented an optimization of most suitable hybrid bidirectional OFDM based WoF transport system using RSOA and POLMUX technique for next-generation wireless network. A power penalty about 3 dBm and 2.3 dBm for 10 Gbps and 6.25 Gbps down link signal as well as 4 dBm and 3.2 dBm for 5 Gbps and 2.5 Gbps up link signal at a BER of 10 -9 between back-to-back and over 50-km SMF plus 10-m (down link) and 5-m (up link) wireless transport scenarios, are achieved. Impressively transmission performances of BER are obtained over 50-km SMF as well as 10-m and 5-m RF wireless link, constellation diagrams all are clear and also the value of power penalties as well as EVM (<10.5% for 32-QAM and <13% for 16-QAM) are good. Our proposed system has considerable support for the future provision of required capacity of the optical access network and providing internet, telecommunication, and data communication services, not only that our presented architecture is also offers wireless network in those places which is RF sensitive. Acknowledgment The authors would like to thanks Sidho-Kanho-Birsha University, Purulia and DST, Govt. of West Bengal (Memo No; 1154(Sanc.)/ST/P/S&T/3G-1/2015 dated 01.03.2016) for financial support to carry the research work. References [1] C. Tang, X. Li, F. Li, J. Zhang, and J. Xiao. A 30 Gb/s full-duplex bi-directional

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