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Digital all-optical Physical-layer Network Coding for 2Gbaud DQPSK signals in mm-wave radio-over-fiber networks
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Digital all-optical Physical-layer Network Coding for 2Gbaud DQPSK signals in mm-wave radio-over-fiber networks ⁎
Charoula Mitsolidoua,b, , Nikos Plerosa,b, Amalia Milioua,b a b
Department of Informatics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Center for Interdisciplinary Research and Innovation, Aristotle University of Thessaloniki, Thessaloniki 57001, Greece
A R T I C L E I N F O
A BS T RAC T
Keywords: All-optical encoding Optical logic devices Radio-over-Fiber Phase regeneration Semiconductor optical amplifier-Mach Zehnder interferometers
We propose the first digital All-Optical Physical-layer Network Coding (AOPNC) scheme for up to 2Gbaud Differential Quadrature Phase Shift Keyed – Sub-Carrier Modulated (DQPSK-SCM) signals in future Radioover-Fiber (RoF) networks. The proposed encoding unit employs Delay Interferometers (DIs) for the DQPSKto-OOK conversion, a first stage of Semiconductor Optical Amplifier-Mach Zehnder Interferometers (SOAMZIs) as optical OOK-XOR and OOK-NXOR logic gates, and a second stage of SOA-MZIs as phase regenerators of the final four-level phase-formatted encoded signal. The proposed encoding operation is performed on-the-fly at the Central Office (CO) and the resulting packet is broadcasted at the end-users, where the electrical decoding takes place. The proof of concept scheme is demonstrated for 4 Gb/s Non-Return-to-Zero (NRZ) DQPSK-SCM signals modulated on a 60 GHz Sub-Carrier (SC). The encoding and decoding operation are investigated and evaluated, achieving error free operation for both synchronous and asynchronous data.
1. Introduction The insatiable data traffic growth along with the exponential proliferation of smart mobile devices with advanced computing and multimedia capabilities have created increasing demand for higher capacity wireless access networks at Gb/s scale [1,2]. In this regime, Radio over Fiber (RoF) networks have been proposed as the means to satisfy this need by seamlessly combining the ubiquity and mobility of wireless access infrastructures with the high bandwidth of backhaul optical networks [3,4]. RoF technology has managed so far to incorporate in its portfolio a variety of signal generation and modulation schemes [5,6] as well as advanced functionalities related to enduser mobility, hand-off schemes [7] and recently to Network coding [8,9]. Network Coding (NC) has attracted intense research focus for its potential to provide network throughput enhancements, security and reduced network congestions, improving in this way the overall network performance without requiring additional resources [10]. Up to now, NC has been implemented separately for purely wireless networks [11] and purely optical infrastructures based on Passive Optical Networks (PONs) [12–14]. Particularly, in wireless networks, NC is performed at the relay by using conventional electronic processing of the wireless data-packets. Similarly, most of the NC schemes in optical networks rely on OEO conversion with electronic buffering and
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processing at each node, resulting on further complexity at the Central Office (CO) and additional latencies to the bidirectional communication [13,15]. To avoid opto-electronic conversion, all-optical PHY-layer NC (AOPNC) schemes have been recently proposed for the encoding of baseband optical signals transmitted in wired Fiber-to-the-Home (FTTH) links. Particularly, the Cross Gain Modulation (XGM) and Cross Phase Modulation (XPM) phenomena in Semiconductor Optical Amplifiers (SOAs) and SOA-Mach Zehnder Interferometers (SOAMZIs) have been exploited to perform the XOR NC-encoding operation between baseband On-Off Keyed (OOK) data signals [16,17], while the Four Wave Mixing (FWM) effect in SOA-based XOR gates [18] and highly-nonlinear fiber (HNLF) –based XOR gates [19], has been deployed for AOPNC encoding of Differential Phase Shift Keyed (DPSK) signals. Although all the above schemes provide performance enhancements when they are used in wired FTTH optical links, they are not capable to deal with the Sub-Carrier Modulated (SCM) RoF signals and the asynchrony that might exist between the data transmitted by different transmitters, making them unsuitable for RoF networks. This additional challenge associated with the Sub-Carrier Modulated (SCM) signal processing has allowed only for a rather limited number of RoF-compatible OPNC demonstrations [8,9,20]. These have mainly relied on orthogonal polarization multiplexing and optical power addition techniques, favoring only analog physical layer
Corresponding author at: Department of Informatics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece. E-mail address: [email protected] (C. Mitsolidou).
http://dx.doi.org/10.1016/j.osn.2017.10.002 Received 27 May 2017; Received in revised form 2 September 2017; Accepted 6 October 2017 1573-4277/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Mitsolidou, C., Optical Switching and Networking (2017), http://dx.doi.org/10.1016/j.osn.2017.10.002
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approaches in order to cope with the increased coding complexity of the SCM formats and data asynchrony. Recently, we demonstrated the first digital AOPNC for RoF networks, where a bitwise XOR function between OOK-SCM data is performed by exploiting a logical gate based on the mature and high-speed technology of SOA-MZIs [21]. Moreover, the current trend in RoF networks is moving towards the use of phase modulation formats and high-order modulation [22,23]. aiming to the improvement of the spectrum efficiency and transmission capacity. Therefore, the employment of a digital AOPNC scheme compatible with multi-level phase formatted SCM signals is necessary, but so far has not been demonstrated. In this paper, we propose for the first time an AOPNC scheme for high symbol rate 60 GHz SCM RoF Non-Return-to-Zero (NRZ) DQPSK signals. The all-optical encoding unit resides in the Central Office (CO) and employs a Delay Interferometer (DI) stage for the DQPSK-to-OOK conversion, a stage of SOA-MZI based OOK-XOR gates followed by the SOA-MZI based phase regenerator [24,25] that forms the 4-level phase encoded signal. Finally the remote 60 GHz Oscillator wavelength feeder generates an additional wavelength spaced by 60 GHz to the encoded signal wavelength for allowing up-conversion through optical beating at the receiver site [26]. For validation purposes, the optical encoding operation is further evaluated by deploying XOR operators at the user's receiver to decode the signal and retrieve the original information. The proposed AOPNC scheme was evaluated in a complete full-duplex link for up to 4 Gb/s (2 Gbaud) synchronous and asynchronous data, phase modulated over a mm-wave Sub-Carrier of 60 GHz. Error free operation was reported for both synchronous and asynchronous operation. The rest of the paper is organized as follows: Section 2 presents the proposed AOPNC-RoF based conceptual scheme and Section 3 describes the setup employed to evaluate the physical layer AOPNC process. Sub-Section 4.1 summarizes the results obtained during the encoding of synchronous data, while sub-Section 4.2 shows the results after the AOPNC operation between asynchronous data streams. Finally, conclusions are addressed in Section 5.
Fig. 2. Frame scheduling for RoF network: (a) without Network Coding and (b) with the proposed AOPNC scheme.
both approaches, each RAU transmits data packets modulated on different wavelengths (λ1, λ2) while the CO uses only one wavelength for the downlink transmission (λ3). As shown in Fig. 2(a), RAUs transmit data A and B to the CO during the first timeslot, while the CO receives both packets and forwards them in a serial fashion over the downlink wavelength, using two successive time slots. On the other hand, in the AOPNC-based scheduling scheme that is shown in Fig. 2(b), the CO broadcasts the NC-encoded packet to both Users in a single timeslot, achieving end-to-end full-duplex communication in only two timeslots. This concept exploits the pre-amble and post-amble that are included in the leading and trailing part of the frames [27] for the bit-level synchronization of the locally stored and the NC-encoded data during the decoding operation. 3. Setup Fig. 3(a) shows the setup employed in order to evaluate the proposed AOPNC scheme in a RoF bilateral communication system. The 4 Gb/s DQPSK-RF data A, B signals are generated in the Users’ Transmitter (Tx) circuit and sent to the respective RAUs, where the electrical-to-optical conversion takes place. The Continuous Waves (CWs) at λ1 = 1551 nm (RAU A) and λ2 = 1553 nm (RAU B) are emitted by the LDs and modulated in the corresponding optical MachZehnder Modulators (MZMs) by the incoming data streams, forming in this way the optical DQPSK-SCM signals. These signals are transmitted through 4 km spools of Single Mode Fibers (SMFs), amplified by Erbium Doped Fiber Amplifiers (EDFAs) and multiplexed by an Arrayed Waveguide Grating (AWG) before they reach the encoding unit at the CO. At the CO, the DQPSK signals are converted to the OOK-u and -v complementary components by using two Delay Interferometers (DIs), each with a delay equal to the symbol period of τ = 0.5 ns. The differential optical phase between DI1-u and DI2-v arms is set to 45° and −45° so as to recover the u- and v-constituents of data A, B, respectively. Note that since the DIs perform the inverse operation of the (electronic) differential encoder at the Users’ Tx, the u and v streams corresponds to the original PRBS sequences before the differential encoding to I and Q bits [24]. The upper output port of each DI is connected with an AWG that de-multiplexes data A and B signals, while the output of the DI's lower port is filtered by Band Pass Filter (BPF) centered at λ2 in order to isolate the OOK-SCM inverted data B. Those signals are injected into the control ports of the OOK-XOR and OOK-NXOR SOA-MZI gates, while a CW at λt (temp) = 1555 nm was inserted in their probe input. The u constituent of data A and B are inserted into the first SOA-MZI so as the OOK-XORu signal to be formed at the output port by exploiting the cross-phase modulation (XPM) phenomenon. The data A and inverted data B u constituents are applied at the second SOA-MZI forming the OOK-NXORu at the output of the switching port. Similarly, the OOK-XORv and OOK-NXORv are obtained at the output ports of the third and fourth MZIs, respectively. All the OOK encoded signals are then filtered and driven to the phase regeneration stage where the amplitude-to-phase conversion is performed by means of two SOAMZIs. Particularly, the OOK-XORu and OOK-NXORu are fed as control
2. Concept overview Fig. 1 shows the schematic layout of the proposed AOPNC-based RoF network, where phase-formatted SCM traffic is exchanged over both fiber and wireless media. The network comprises the AOPNC unit at the CO and two wireless users connected with the respective RAUs, which in turn connect to the CO via a fiber. Users A, B transmit their DQPSK data signals to the RAUs, where they are converted to optical streams through the modulation of the Laser Diodes (LDs) and forwarded to the CO through spools of fiber. The encoding unit at the CO performs on-the-fly the all-optical XOR operation between the two incoming DQPSK-SCM streams, resulting in a 4-Level phaseformatted NC-encoded signal. The resultant signal is broadcasted back to both RAUs, converted to electrical stream by using a Photodiode (PD) and gets transmitted to the respective wireless User. Each User recovers the bit sequence originating from the other User by means of a second XOR function between its own locally stored data and the incoming NC-encoded data. Fig. 2(a) and (b) compare the traditional frame scheduling without NC and the proposed AOPNC-based scheduling in RoF networks. In
Fig. 1. All-optical Network Coding (NC) scheme at the Central Office of RoF networks.
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Fig. 3. a) Simulated setup of the proposed AOPNC scheme comprising: the two users, the respective Remote Antenna Units (RAUs) and the all-optical Encoding unit at the Central Office. b) Setup of the users’ Transmitter generating the DQPSK-RF data. c) Setup of the users’ Receiver performing the decoding operation.
signals to the upper SOA-MZI, while a CW at λ3 = 1557.36 nm was launched in the probe input port. The relative phase in the SOA arms was controlled by a high level bit either in the OOK-XORu or the OOKNXORu arm, resulting in the phase-formatted XOR stream between the u constituent of data A and B [24,25]. Equivalently, the OOK-XORv and OOK-NXORv streams are injected in the control arms of the lower SOA-MZI, forming the phase- and wavelength (λ3)- converted XORv signal. The phase formatted XORu and XORv output streams are filtered and recombined with a relative Phase Shift (PS) of 90° so as to form the 4-level (4-L) phase formatted XOR signal. Finally, the phase regenerator output at λ3 is coupled with a coherent CW at λ3 + 60 GHz generated by the remote 60 GHz Oscillator wavelength feeder, in order to form the final 4-L phase-formatted SCM-XOR stream. The NCencoded signal is transmitted through SMFs of 4 km length, to the RAUs A and B receivers (Rxs) where it was converted to RF data by utilizing the beating at the PD [26]. The output is filtered by a BPF centered at 60 GHz and transmitted through an assumed wireless link to the User's Rx, where the electrical decoding is performed. Fig. 3(b) and (c) provide a more detailed illustration of the end-user transmitter (Tx) and receiver (Rx), respectively. Each User's Tx comprises a Programmable Pattern Generator (PPG) loaded with a 4 Gb/s NRZ 27−1 Pseudo Random Bit Sequence (PRBS), so as to form the electrical data. A serial-to-parallel distributor is fed with the output stream of the PPG and synchronously splits it into the two output streams (u and v), each having a data rate of 2 Gb/s, equal to the symbol rate of 2Gbaud. Those signals are modified properly to I and Q streams by the DQPSK differential encoder and launched to the electrical Phase Modulators (PMs) in order to modulate a 60 GHz Local Oscillator (LO). RF signals coming from the LO have a relative phase difference of 90°, so as the DQPSK-RF signal to be generated after the combination of the phase-formatted I and Q streams. Fig. 3(c) presents the User's Rx which receives the 4-L phase encoded XOR signal from the RAU and splits it into two identical signals. Those signals are multiplied with the respective in-phase signals originating from the LO and having a relative phase difference of 90°. Phase Shifters (PSs) are used for the phase adjustment of the LO. The multiplication function results to the simultaneous phase-to-
OOK and down-conversion of u and v components, whose power levels are adjusted by adding a constant-amplitude electrical signal (DC source). The encoded signals are filtered by Low Pass Filters (LPFs) and inserted in the electrical XOR gates where the decoding process is performed by the XOR operation between the NC-encoding signals and a local copy of user's data. In this way, User A extracts the data B constituents, while User B decodes the data Au and Av streams. A time delay (Δτ) is used to synchronize the data and encoding streams before the decoding function. The PHY-layer setup has been simulated with the aid of VPI Photonics software [28] and has employed a custom-made SOA-MZI model described in [29], following closely the experimental behaviour of a commercially available 1600 µm long hybridly integrated SOA. The input power levels that were used, are −1 dBm for the control signals and −4 dBm for the probe light, for all SOA-MZIs of the OOK-XOR (NXOR) stage. Both SOAs of the OOK-XOR and OOK-NXOR MZI gates were driven by current values of 250 mA and had a recovery time of 100 ps, significantly longer than the 16.67 ps period of the 60 GHz SC. In this way, the SOA-MZI response of all gates neglects the 60 GHz SC of the control signals resembling the response of a low-pass filtering element, while still allowing the SOA gain and phase characteristics to successfully respond to the changes induced by the 2 Gbaud data envelope [30]. The SOAs of the phase-regeneration XOR gates are driven by a 300 mA dc current and had an 80 ps recovery time. 4. Results 4.1. Synchronous operation Fig. 4 illustrates the phase state-to-digital I,Q bit pair correspondence which has been obtained at the output of RAUs’ transmitters (Txs) and corresponds to either the optical DQPSK-SCM data A or B signal phase states. As can be seen, the four different bit pair (symbol) combinations [“00″, “01″, “10″, “11″] are represented by the four possible phase states of [−135°,135°,−45°,45°] respectively. Those optical phase shifts are occurred at the beginning of each symbol's SC, a SC with a period equal to τsc=0.0167 ns (τsc=1/60 GHz). 3
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NRZ-OOK streams by the multiplication with the respective in-phase 60 GHz LO at the users’ receiver. Fig. 5(f) and (g) depict the successfully decoded data Bu and Au after the electrical XOR operation between the encoded XORu trace and a copy of the user's data Au and Bu patterns, respectively. Similarly, Fig. 5(h) and (i) show the decoded data Bv and Av traces, each generated by the bitwise XOR between the NC-encoded XORv trace of Fig. 5(e) and either a copy of data Av or Bv pattern, showing that the initial patterns were correctly retrieved after the decoding operation. Fig. 5(j),(k) depict the eye diagrams of the OOK-XORu and OOKXORv signals at the outputs of the SOA-MZI1 and SOA-MZI3 after the removal of the 60 GHz SC achieved by exploiting the low pass filtering response of the SOAs. Both eye diagrams exhibit an Extinction Ratio (ER) of 11.7 dB, an Amplitude Modulation (AM) of 1.1 dB, a Pulse Overshoot (PO) of 1.4 dB and a jitter of 23 ps, revealing a limited impact of pattern effects, which would normally be expected in the signal processing with SOAs. Fig. 5(l) illustrates the intensity eye diagram as well as the phase “eye diagram” of the 4-Level phase formatted stream at the output of the phase regeneration stage B. The intensity eye diagram reveals a constant power level with an AM=1.8 dB, PO=2.1 dB, while the respective phase eye diagram shows the four different phase levels of the encoded signal, revealing a small phase fluctuation of 1.5° from the ideal values of: [−135°,135°,−45°,45°] and a jitter of 25 ps. In both diagrams, small duration dips at the beginning of the symbol pulse are observed, without yielding, however, any data loss as can be confirmed by the successful decoding traces shown in Fig. 5(f)-(i). Finally, Fig. 5(m) and (n) show the eye diagrams of the electrical down- and OOK-converted complementary XORu and XORv signals exiting the LPF at the User's receiver. Both signals exhibit an ER, AM, PO and jitter equal to 9 dB, 0.7 dB, 0.9 dB and 32 ps respectively, with the dips appearing at the beginning of the pulses. Fig. 5(o) illustrates the optical spectrum of the phase formatted SCM-XOR stream exiting the AOPNC unit at the CO after the coupling of the encoded signal with the CW beam generated by the 60 GHz OSC wavelength feeder. The 2 Gbaud phase formatted signal at the wavelength of λ3 = 1557.36 nm (192.5 THz) and the CW light beam at the wavelength of 1556.88 nm (192.560 THz) were clearly observed with frequency spacing equal to 60 GHz. The respective time trace is illustrated in Fig. 5(p), where it is shown that the SCM output stream is formed by a constant power envelope modulated by a Sub-carrier with a frequency equal to 60 GHz. Fig. 5(q) presents the electrical spectrum of the 2Gbaud phase formatted SCM-XOR beating signal at 60 GHz, generated by the PD and transmitted to the wireless user's Rx. Fig. 5(r) corresponds to the respective time trace, showing the oscillations of the 60 GHz electrical signal below the constant power envelope of the 4-L phase formatted encoded signal.
Fig. 4. The phase state - to - digital I, Q data correspondence for the DQPSK-SCM data A or B optical signal transmitted by the respective RAUs to the CO.
Fig. 5 shows the time traces, eye diagrams and spectra obtained at various stages of the proposed AOPNC-based RoF communication between bit-level synchronized data A and B signals, both with a symbol rate equal to 2Gbaud (bit rate = 4 Gb/s). The parts of the input patterns used for the evaluation of the system, are “1011111001″ and “1010100110″ for the u and v components of data A, while “1011010110″ and “0100001100″ were used for the data B constituents (u and v), respectively. Fig. 5(a), (b) illustrate the OOK-XORu (Au,Bu) and OOK-XORv (Av,Bv) traces at the output of the first and third SOA-MZI. The encoded signal featured a pulse, only when the respective component of data A and B featured different logical values, while it gets a power level equal to 0 when the XORed data represents the same logical value. Fig. 5(c) shows both intensity and phase time traces of the 4-Level phase formatted signal exiting the regenerator stage after the recombination of the Phase-XORu and Phase-XORv signals (Fig. 3(a)). As can be seen, the power envelope of the stream is almost constantly at a "high" level featuring small duration sub-bit dips generated by the transition of the differential phase between the SOAMZI5 and SOA-MZI6 branches from +90° to −90°. The phase trace confirms the OOK to 4-L phase conversion, since the information is encoded in the optical phase φ of the signal, where φ may take one of the four values: [−135°, 135°,−45°, 45°]. The grey markers highlight the present encoding rules, where the XORv pulses are imprinted as 135°, the absence of the pulses as −135°, the existence of both XORu, XORv pulses as 45° and the XORu pulses as −45°. The Truth Table 1(a) and (b) summarize the encoding rules of the proposed AOPNC system. Table 1(a) presents the binary encoding scheme at stage A, where the XORu or XORv output is true (“1”) when the input data Au (or v) and Bu (or v) constituents have different logical values, while the output is false (“0”) when both data inputs are both true (“1”) or both false (“0”). Truth Table 1(b) shows how the 4-Level phase-formatted XOR waveform is encoded by the NC unit at both stages B and C. The phase can get the four values of [−135°,135°,−45°, 45°] depending on the logical values of the binary encoded XORu and XORv pair combinations of stage A as shown in the truth table. Fig. 5(d),(e) illustrates the electrical OOK- and down-converted XORu and XORv components respectively, exiting the LPF at the Users’ receiver as shown in Fig. 3(c). Those traces confirm the conversion of the 4-L phase formatted XOR signal to two binary
4.2. Asynchronous operation Considering that in a real network scenario, the mobile end-users may reside at different distances from their respective RAUs, yielding a sub-bit mismatch between the arrival times of the two packets reaching the encoding unit at the CO, it is critical for the AOPNC concept to operate also between non-synchronized DQPSK-SCM data. In this section, the asynchronous operation was investigated by using sub-bit temporal offsets equal to 125 ps (0.25 of the symbol duration) and 250 ps (0.5 of the symbol duration) between the two data signals, introduced as time delay to data B. Fig. 6(a)–(i) summarize the time traces of the asynchronous encoding and decoding process for a time offset equal to 0.25 of the symbol time duration. Fig. 6(a) illustrates the optically down-converted OOK-XORu performed between the data Au and the delayed by 125 ps data Bu constituents at the output of the SOA-MZI1, while Fig. 6(b) shows the resultant OOK-XORv signal recorded at the output of the SOA-MZI3 after the XOR operation between the data Av and the 4
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Fig. 5. (a)-(i) Time traces of the AOPNC scheme for synchronous data. (j)-(n) Eye diagrams for the synchronous encoding operation. (o), (p) Spectrum and time trace of the SCM encoding signal at the CO's output. (q), (r) Spectrum and time trace of the SCM RF encoding signal at the output of the RAU's Rx. 500 ps/div (traces) and 100 ps/div (eyes).
levels: [−135°, 135°, −45°, 45°] of the encoded signal representing the respective logical values of: [XORu, XORv] = [“00″, “01″, “10″, “11″]. Since the 4-L phase formatted signal originated from the two “interrupted” OOK-XOR components, “parasitic” phase pulses and dips with a duration of 125 ps are also recorded on the phase trace of Fig. 6(c). Fig. 6(d) and (e) depict the filtered by LPFs “interrupted” XORu and XORv signals that were simultaneously OOK- and down- converted by the multiplication of the 4-L phase formatted XOR signal with the respective in-phase signals coming from the 60 GHz LO at stage E. Fig. 6(f) and (g) illustrate the decoded data Bu and Au components after the digital XOR operation between the encoded pattern of XORu
delayed data Bv input streams. As it is highlighted by the green markers, the data asynchrony generates short "parasitic" pulses or dips with a time duration equal to the offset at both encoded XOR streams, which are said to be “interrupted”. The intensity and phase time traces of NC-encoding signal at the output of the phase regenerator are presented in Fig. 6(c), confirming the conversion of the two binary OOK-XOR signals to one 4-Level phase formatted XOR signal. Particularly, that signal shows a constant “high” power envelope with small duration sub-bit dips generated from the transitions of the differential phase from +90° to −90° between the SOA-MZI5 and/or SOA-MZI6 branches. The phase trace reveals the four possible phase 5
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diagrams after a time delay equal to 125 ps from the beginning of the pulse. The intersection seen after the time offset is formed by both “parasitic” pulse-falls and dip-risings during the asynchronous XOR function and has a jitter equal to 21 ps. The intensity and phase eye diagrams of the NC-phase encoding signal leaving the regeneration stage are illustrated in Fig. 7(c). A constant “high” intensity envelope with an AM and PO equal to 1.8 dB and 2.1 dB respectively is observed, exhibiting sub-bit dips when the relative phase of SOA-MZI5 and/or SOA-MZI6 is changed from 90° to −90° and vice versa. The only difference observed with respect to the corresponding eye diagram of the synchronous operation shown in Fig. 5(l) is that the dip appears after time equal to the 0.25 offset due to asynchrony between data A and B. The respective phase eye diagram shows the four different phase levels of the encoded signal, revealing a small fluctuation of 1.5° around the nominal levels of [135°,45°,−45°,−135°]. Small duration phase transitions can be seen at the beginning of the symbol as well as after the time offset of 125 ps. Finally, Fig. 7(d) and (e) depict the electrical eye diagrams of the binary OOK- and down-converted XORu and XORv encoded components at the Users’ receiver, exhibiting an eye opening with an ER of 9 dB, an AM of 0.7 dB, a PO of 0.9 dB and a jitter of 32 ps. Despite the “parasitic” sub-bit intersection observed after the time offset of 125 ps, the other pulse characteristics, like ER, AM, PO, jitter and noise are not affected. This fact indicates that the system may show similar performance when the decoding XOR operation is performed between the XOR and the properly time adjusted local copies of the data, operation that was confirmed also in Fig. 6(f)–(i). Similar results were obtained for the time offset equal to 0.5 of the symbol duration, as shown in Fig. 7(f)–(j). Particularly, Fig. 7(f) and (g) depict the OOK-XORu and OOK-XORv eye diagrams with an ER of 11.7 dB, an AM of 1.1 dB, a PO of 1.4 dB and a jitter of 25 ps, while Fig. 7(h) shows the intensity (AM = 1.8 dB and PO = 2.1 dB) and the respective phase eye diagrams (phase fluctuation=1.5°, jitter = 25 ps) at the encoding unit residing in the CO. The electrical eye diagrams of the down-converted OOK-XORu and XORv signals at the Users’ Rx are illustrated in Fig. 7(i) and (j), revealing an ER, AM, PO and a jitter equal to 9 dB, 0.7 dB, 0.9 dB and 32 ps respectively. In this time-offset scenario the data asynchrony generates “parasitic” intersections after a time delay of 250 ps from the beginning of the pulse, yet without changing the other metrics of the signal quality such as the ER, AM, PO, jitter and noise. The successful asynchronous decoding operation was evaluated with the aid of eye diagrams and Bit-Error-Rate (BER) measurements for the u and v constituents of both data A and B. Fig. 8(a)–(c) depict the eye diagrams of the decoded data Au constituent for sub-bit time offsets equal to 0.25 (125 ps), 0.5 (250 ps) and 0.75 (375 ps) of the symbol duration, respectively. All these eye diagrams exhibit an ER of 8.7 dB, an AM of 1.1 dB and jitter of 40 ps, with short duration dips and spikes appearing after time equal to 125 ps, 250 ps and 375 ps from the beginning of the symbol, respectively. Eye diagrams with similar quality (ER = 8.7 dB, AM = 1.1 dB, jitter = 40 ps) were obtained for the decoded Bu component as shown in Fig. 8(d)–(f) for time offsets equal to 0.25, 0.5 and 0.75 of the symbol duration. The only difference observed for the decoded Bu is that the dips and spikes appear after time equal to the time offset starting from the end of the symbol. Fig. 8(g) illustrates the BER curves as function of the received RF power, for the final decoded signals Au and Bu, for various sub-symbol time offset values. All BER curves reveal error free operation below the required BER threshold of 10−9, as it has been set by the International Telecommunication Union (ITU) for RoF networks. The inset depicts the eye diagram of the decoded data A or B during the synchronous operation, reporting an open eye with an ER, AM and jitter equal to 8.7, 1.1 and 40 ps, respectively, with the dips and spikes appearing at the beginning of the symbol transaction. No additional power penalty was observed when comparing the curves of the asynchronous and synchronous operation. This owes to the dips and spikes that were
Table 1 Encoding rules of the AOPNC scheme represented by truth tables of: (a) the binary XOR u or v encoding output, (b) the 4-Level phase encoding output. (a) Binary u(or v) constituents
Binary encoding
Data A u (or v)
Data B u (or v)
XOR u (or v)
0 0 1 1
0 1 0 1
0 1 1 0
(b) Binary encoding
4-Level encoding
XOR_u
XOR_v
XOR phase
0 0 1 1
0 1 0 1
−135° +135° −45° +45°
and the initial pattern either of data Au or data Bu at the Users A and B receiver. Similarly, Fig. 6(h) and (i) show the decoded data Bv and Av at the User A and B receiver, respectively. In that operation, it is shown that despite the interruptions appeared as “parasitic” pulses or dips at the NC-encoded signals, both the components of data A and B were at the end correctly recovered with the decoded data B components having a delay equal to the time offset. Similar results were obtained for the asynchronous AOPNC operation between data A and delayed by 250 ps (0.5 of the symbol duration) data B, as shown in Fig. 6(j)-(r). Fig. 6(j) and (k) show the encoded OOK-XORu (Au, Bu) and OOK-XORv (Av, Bv) complementary signals at the output of the SOA-MZI1 and SOA-MZI3, revealing “parasitic” pulses or dips with a duration equal to the offset of 250 ps. Fig. 6(l) depicts both the intensity and phase time traces of the 4-Level phase encoded stream at the output of the regenerator after the recombination of the binary Phase-XORu and Phase-XORv signals. The power trace shows an almost constantly "high" level power envelope featuring small duration sub-bit dips originated by the phase transitions at the SOA-MZIs arms from 90° to −90°, while the phase trace shows the “parasitic” phase pulses and dips resulted due to the asynchrony between data A and B. The electrical OOK-XORu and OOK-XORv signal constituents that are OOK- and down –converted at the Users’ Rx, are presented in Fig. 6(m) and (n). The highlighted part of the time traces illustrates the interruptions appearing due to the asynchrony at the encoded signals for both time offset scenarios, revealing that the “parasitic” pulse increases as the time offset increases. Despite those occurrences, by performing a second XOR operation between the respective encoded XOR constituents and the data A components, the traces of the decoded data Bu and Bv patterns can be successfully retrieved yet after a time delay equal to the time offset of 250 ps, as shown in Fig. 6(o) and (q). Similarly, in Fig. 6(p) and (r) data Au and Av components were also correctly decoded by the XOR operation between the encoded stream and delayed by Δτ=250 ps data B pattern at the User B receiver. The respective eye diagrams of the asynchronous encoding operation for offsets equal to 0.25τsymbol and 0.5τsymbol are presented in Fig. 7. Fig. 7(a) and (b) depict the eye diagram of the OOK-XORu and OOK-XORv constituents for 0.25 time offset, revealing an ER of 11.7 dB, an AM of 1.1 dB, a PO of 1.4 dB and a jitter of 23 ps. The existence of the interruptions can be easily observed at both eye 6
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Fig. 6. (a)–(i) Time traces at various steps of the proposed Network Coding configuration for asynchronous data signals, with a sub-bit time offset equal to 0.25 bit. (j)-(r) Time traces at various steps of the proposed Network Coding configuration for asynchronous data signals, with a sub-bit time offset equal to 0.5 bit. Time scale 500 ps/div.
the symbol. Fig. 8(k)–(m) illustrates the respective eye diagrams of decoded data Bv for the same offset, reporting similar eye characteristics (ER = 8.7 dB and AM = 1.1 dB, jitter = 40 ps), with the dips and spikes obtained after time equal to the offset starting from the end of the symbol. Fig. 8(n) shows the BER curves of the decoded components Av and Bv for time offset equal to 0τsymbol, 0.25τsymbol, 0,5τsymbol and 0.75τsymbol, all achieving error free operation with negligible power penalty between the different BER curves. This is an evident that the performance of the proposed AOPNC scheme remains similar even in the case of asynchronous packets arriving at the encoding unit. The
present at the edge of the pulse during synchronous operation being shifted within the duration of the pulses, however without affecting the other pulse characteristics, such as ER, AM, PO, jitter and noise. Therefore, it should be noted that the asynchronous decoded operation shows similar performance with the synchronous case. Similarly, Fig. 8(h)–(j) depicts the eye diagrams of decoded data Av constituent when relative offsets of 0.25τsymbol, 0,5τsymbol and 0.75τsymbol between the Data A and B were considered, exhibiting an ER, AM and jitter equal to 8.7 dB, 1.1 dB and 40 ps, with interruption appearing after a time equal to the offset, starting from the beginning of 7
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Fig. 7. Eye diagrams of the encoding operation for asynchronous data signals, (a)-(e) with a sub-bit time offset equal to 0.25 bit and (f)-(j) with a sub-bit time offset equal to 0.5 bit. Time scale 100 ps/div.
Fig. 8. Eye diagrams of the asynchronous decoding operation: (a)-(c) decoded Au for sub-bit time offsets equal to 0.25, 0.5 and 0.75 bit, respectively. (d)-(f) decoded Bu for offsets equal to 0.25, 0.5 and 0.75 bit, respectively. (g) BER curves of: decoded data Au and Bu for various time offsets (inset of the eye diagram for the synchronous decoded data). Eye diagrams of: (h)-(j) decoded Av for time offsets of 0.25 bit, 0.5 bit and 0.75 bit and (k)-(m) decoded Bv for the same time offsets. (n) BER curves of the decoded Av and Bv components in asynchronous operation (inset of the eye diagram for the synchronous decoded data). Time scale of the eye diagrams: 100 ps/div.
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inset of eye diagram in Fig. 8(n) corresponds to the decoded data A or B signal during synchronous operation, showing an ER of 8.7 dB, an AM of 1.1 dB and a jitter of 40 ps.
[11]
[12]
5. Conclusions We proposed for the first time a method to implement all-optical digital PHY-layer Network Coding between four-level phase-formatted Sub-Carrier Modulated (SCM) signals employed in millimetre-wave RoF networks. The all-optical encoding unit resides at the CO and is capable of handling two DQPSK data streams modulated on a 60 GHz Sub-carrier. The encoding process consists of four stages: the DQPSKto-OOK conversion by the DIs, the OOK-XOR, NXOR operations as well as the phase regeneration with the aid of SOA-MZI gates and the remote 60 GHz oscillator wavelength feeder stage for the generation of the final phase-formatted SCM XOR signal. All-optical encoding at the CO unit, as well as the successful electrical decoding process at the enduser's receiver was successfully confirmed for a data rate equal to 4 Gb/ s (2Gbaud), reporting error free performance for both synchronous and asynchronous data. The proposed scheme may operate in higher speeds than 2Gbaud as long as the employed symbol rate does not exceed the unlicensed bandwidth of 7 GHz, being available in the 60 GHz mmwave band [31]. It should be noted, that our AOPNC scheme may in principle be applied also in RoF networks employing other phase modulation formats, such as DPSK-SCM and Dual Polarization (DP)– DQPSK-SCM modulation techniques.
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
Acknowledgements
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This work has been supported by the European FP7-PEOPLE2013-IAPP project COMANDER (contract no. 612257) and the H2020ITN-2016 project 5 G-STEP-FWD (contract no. 722429).
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[23]
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Contents lists available at ScienceDirect
Optical Switching and Networking journal homepage: www.elsevier.com/locate/osn
Digital all-optical Physical-layer Network Coding for 2Gbaud DQPSK signals in mm-wave radio-over-fiber networks ⁎
Charoula Mitsolidoua,b, , Nikos Plerosa,b, Amalia Milioua,b a b
Department of Informatics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece Center for Interdisciplinary Research and Innovation, Aristotle University of Thessaloniki, Thessaloniki 57001, Greece
A R T I C L E I N F O
A BS T RAC T
Keywords: All-optical encoding Optical logic devices Radio-over-Fiber Phase regeneration Semiconductor optical amplifier-Mach Zehnder interferometers
We propose the first digital All-Optical Physical-layer Network Coding (AOPNC) scheme for up to 2Gbaud Differential Quadrature Phase Shift Keyed – Sub-Carrier Modulated (DQPSK-SCM) signals in future Radioover-Fiber (RoF) networks. The proposed encoding unit employs Delay Interferometers (DIs) for the DQPSKto-OOK conversion, a first stage of Semiconductor Optical Amplifier-Mach Zehnder Interferometers (SOAMZIs) as optical OOK-XOR and OOK-NXOR logic gates, and a second stage of SOA-MZIs as phase regenerators of the final four-level phase-formatted encoded signal. The proposed encoding operation is performed on-the-fly at the Central Office (CO) and the resulting packet is broadcasted at the end-users, where the electrical decoding takes place. The proof of concept scheme is demonstrated for 4 Gb/s Non-Return-to-Zero (NRZ) DQPSK-SCM signals modulated on a 60 GHz Sub-Carrier (SC). The encoding and decoding operation are investigated and evaluated, achieving error free operation for both synchronous and asynchronous data.
1. Introduction The insatiable data traffic growth along with the exponential proliferation of smart mobile devices with advanced computing and multimedia capabilities have created increasing demand for higher capacity wireless access networks at Gb/s scale [1,2]. In this regime, Radio over Fiber (RoF) networks have been proposed as the means to satisfy this need by seamlessly combining the ubiquity and mobility of wireless access infrastructures with the high bandwidth of backhaul optical networks [3,4]. RoF technology has managed so far to incorporate in its portfolio a variety of signal generation and modulation schemes [5,6] as well as advanced functionalities related to enduser mobility, hand-off schemes [7] and recently to Network coding [8,9]. Network Coding (NC) has attracted intense research focus for its potential to provide network throughput enhancements, security and reduced network congestions, improving in this way the overall network performance without requiring additional resources [10]. Up to now, NC has been implemented separately for purely wireless networks [11] and purely optical infrastructures based on Passive Optical Networks (PONs) [12–14]. Particularly, in wireless networks, NC is performed at the relay by using conventional electronic processing of the wireless data-packets. Similarly, most of the NC schemes in optical networks rely on OEO conversion with electronic buffering and
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processing at each node, resulting on further complexity at the Central Office (CO) and additional latencies to the bidirectional communication [13,15]. To avoid opto-electronic conversion, all-optical PHY-layer NC (AOPNC) schemes have been recently proposed for the encoding of baseband optical signals transmitted in wired Fiber-to-the-Home (FTTH) links. Particularly, the Cross Gain Modulation (XGM) and Cross Phase Modulation (XPM) phenomena in Semiconductor Optical Amplifiers (SOAs) and SOA-Mach Zehnder Interferometers (SOAMZIs) have been exploited to perform the XOR NC-encoding operation between baseband On-Off Keyed (OOK) data signals [16,17], while the Four Wave Mixing (FWM) effect in SOA-based XOR gates [18] and highly-nonlinear fiber (HNLF) –based XOR gates [19], has been deployed for AOPNC encoding of Differential Phase Shift Keyed (DPSK) signals. Although all the above schemes provide performance enhancements when they are used in wired FTTH optical links, they are not capable to deal with the Sub-Carrier Modulated (SCM) RoF signals and the asynchrony that might exist between the data transmitted by different transmitters, making them unsuitable for RoF networks. This additional challenge associated with the Sub-Carrier Modulated (SCM) signal processing has allowed only for a rather limited number of RoF-compatible OPNC demonstrations [8,9,20]. These have mainly relied on orthogonal polarization multiplexing and optical power addition techniques, favoring only analog physical layer
Corresponding author at: Department of Informatics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece. E-mail address: [email protected] (C. Mitsolidou).
http://dx.doi.org/10.1016/j.osn.2017.10.002 Received 27 May 2017; Received in revised form 2 September 2017; Accepted 6 October 2017 1573-4277/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Mitsolidou, C., Optical Switching and Networking (2017), http://dx.doi.org/10.1016/j.osn.2017.10.002
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approaches in order to cope with the increased coding complexity of the SCM formats and data asynchrony. Recently, we demonstrated the first digital AOPNC for RoF networks, where a bitwise XOR function between OOK-SCM data is performed by exploiting a logical gate based on the mature and high-speed technology of SOA-MZIs [21]. Moreover, the current trend in RoF networks is moving towards the use of phase modulation formats and high-order modulation [22,23]. aiming to the improvement of the spectrum efficiency and transmission capacity. Therefore, the employment of a digital AOPNC scheme compatible with multi-level phase formatted SCM signals is necessary, but so far has not been demonstrated. In this paper, we propose for the first time an AOPNC scheme for high symbol rate 60 GHz SCM RoF Non-Return-to-Zero (NRZ) DQPSK signals. The all-optical encoding unit resides in the Central Office (CO) and employs a Delay Interferometer (DI) stage for the DQPSK-to-OOK conversion, a stage of SOA-MZI based OOK-XOR gates followed by the SOA-MZI based phase regenerator [24,25] that forms the 4-level phase encoded signal. Finally the remote 60 GHz Oscillator wavelength feeder generates an additional wavelength spaced by 60 GHz to the encoded signal wavelength for allowing up-conversion through optical beating at the receiver site [26]. For validation purposes, the optical encoding operation is further evaluated by deploying XOR operators at the user's receiver to decode the signal and retrieve the original information. The proposed AOPNC scheme was evaluated in a complete full-duplex link for up to 4 Gb/s (2 Gbaud) synchronous and asynchronous data, phase modulated over a mm-wave Sub-Carrier of 60 GHz. Error free operation was reported for both synchronous and asynchronous operation. The rest of the paper is organized as follows: Section 2 presents the proposed AOPNC-RoF based conceptual scheme and Section 3 describes the setup employed to evaluate the physical layer AOPNC process. Sub-Section 4.1 summarizes the results obtained during the encoding of synchronous data, while sub-Section 4.2 shows the results after the AOPNC operation between asynchronous data streams. Finally, conclusions are addressed in Section 5.
Fig. 2. Frame scheduling for RoF network: (a) without Network Coding and (b) with the proposed AOPNC scheme.
both approaches, each RAU transmits data packets modulated on different wavelengths (λ1, λ2) while the CO uses only one wavelength for the downlink transmission (λ3). As shown in Fig. 2(a), RAUs transmit data A and B to the CO during the first timeslot, while the CO receives both packets and forwards them in a serial fashion over the downlink wavelength, using two successive time slots. On the other hand, in the AOPNC-based scheduling scheme that is shown in Fig. 2(b), the CO broadcasts the NC-encoded packet to both Users in a single timeslot, achieving end-to-end full-duplex communication in only two timeslots. This concept exploits the pre-amble and post-amble that are included in the leading and trailing part of the frames [27] for the bit-level synchronization of the locally stored and the NC-encoded data during the decoding operation. 3. Setup Fig. 3(a) shows the setup employed in order to evaluate the proposed AOPNC scheme in a RoF bilateral communication system. The 4 Gb/s DQPSK-RF data A, B signals are generated in the Users’ Transmitter (Tx) circuit and sent to the respective RAUs, where the electrical-to-optical conversion takes place. The Continuous Waves (CWs) at λ1 = 1551 nm (RAU A) and λ2 = 1553 nm (RAU B) are emitted by the LDs and modulated in the corresponding optical MachZehnder Modulators (MZMs) by the incoming data streams, forming in this way the optical DQPSK-SCM signals. These signals are transmitted through 4 km spools of Single Mode Fibers (SMFs), amplified by Erbium Doped Fiber Amplifiers (EDFAs) and multiplexed by an Arrayed Waveguide Grating (AWG) before they reach the encoding unit at the CO. At the CO, the DQPSK signals are converted to the OOK-u and -v complementary components by using two Delay Interferometers (DIs), each with a delay equal to the symbol period of τ = 0.5 ns. The differential optical phase between DI1-u and DI2-v arms is set to 45° and −45° so as to recover the u- and v-constituents of data A, B, respectively. Note that since the DIs perform the inverse operation of the (electronic) differential encoder at the Users’ Tx, the u and v streams corresponds to the original PRBS sequences before the differential encoding to I and Q bits [24]. The upper output port of each DI is connected with an AWG that de-multiplexes data A and B signals, while the output of the DI's lower port is filtered by Band Pass Filter (BPF) centered at λ2 in order to isolate the OOK-SCM inverted data B. Those signals are injected into the control ports of the OOK-XOR and OOK-NXOR SOA-MZI gates, while a CW at λt (temp) = 1555 nm was inserted in their probe input. The u constituent of data A and B are inserted into the first SOA-MZI so as the OOK-XORu signal to be formed at the output port by exploiting the cross-phase modulation (XPM) phenomenon. The data A and inverted data B u constituents are applied at the second SOA-MZI forming the OOK-NXORu at the output of the switching port. Similarly, the OOK-XORv and OOK-NXORv are obtained at the output ports of the third and fourth MZIs, respectively. All the OOK encoded signals are then filtered and driven to the phase regeneration stage where the amplitude-to-phase conversion is performed by means of two SOAMZIs. Particularly, the OOK-XORu and OOK-NXORu are fed as control
2. Concept overview Fig. 1 shows the schematic layout of the proposed AOPNC-based RoF network, where phase-formatted SCM traffic is exchanged over both fiber and wireless media. The network comprises the AOPNC unit at the CO and two wireless users connected with the respective RAUs, which in turn connect to the CO via a fiber. Users A, B transmit their DQPSK data signals to the RAUs, where they are converted to optical streams through the modulation of the Laser Diodes (LDs) and forwarded to the CO through spools of fiber. The encoding unit at the CO performs on-the-fly the all-optical XOR operation between the two incoming DQPSK-SCM streams, resulting in a 4-Level phaseformatted NC-encoded signal. The resultant signal is broadcasted back to both RAUs, converted to electrical stream by using a Photodiode (PD) and gets transmitted to the respective wireless User. Each User recovers the bit sequence originating from the other User by means of a second XOR function between its own locally stored data and the incoming NC-encoded data. Fig. 2(a) and (b) compare the traditional frame scheduling without NC and the proposed AOPNC-based scheduling in RoF networks. In
Fig. 1. All-optical Network Coding (NC) scheme at the Central Office of RoF networks.
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Fig. 3. a) Simulated setup of the proposed AOPNC scheme comprising: the two users, the respective Remote Antenna Units (RAUs) and the all-optical Encoding unit at the Central Office. b) Setup of the users’ Transmitter generating the DQPSK-RF data. c) Setup of the users’ Receiver performing the decoding operation.
signals to the upper SOA-MZI, while a CW at λ3 = 1557.36 nm was launched in the probe input port. The relative phase in the SOA arms was controlled by a high level bit either in the OOK-XORu or the OOKNXORu arm, resulting in the phase-formatted XOR stream between the u constituent of data A and B [24,25]. Equivalently, the OOK-XORv and OOK-NXORv streams are injected in the control arms of the lower SOA-MZI, forming the phase- and wavelength (λ3)- converted XORv signal. The phase formatted XORu and XORv output streams are filtered and recombined with a relative Phase Shift (PS) of 90° so as to form the 4-level (4-L) phase formatted XOR signal. Finally, the phase regenerator output at λ3 is coupled with a coherent CW at λ3 + 60 GHz generated by the remote 60 GHz Oscillator wavelength feeder, in order to form the final 4-L phase-formatted SCM-XOR stream. The NCencoded signal is transmitted through SMFs of 4 km length, to the RAUs A and B receivers (Rxs) where it was converted to RF data by utilizing the beating at the PD [26]. The output is filtered by a BPF centered at 60 GHz and transmitted through an assumed wireless link to the User's Rx, where the electrical decoding is performed. Fig. 3(b) and (c) provide a more detailed illustration of the end-user transmitter (Tx) and receiver (Rx), respectively. Each User's Tx comprises a Programmable Pattern Generator (PPG) loaded with a 4 Gb/s NRZ 27−1 Pseudo Random Bit Sequence (PRBS), so as to form the electrical data. A serial-to-parallel distributor is fed with the output stream of the PPG and synchronously splits it into the two output streams (u and v), each having a data rate of 2 Gb/s, equal to the symbol rate of 2Gbaud. Those signals are modified properly to I and Q streams by the DQPSK differential encoder and launched to the electrical Phase Modulators (PMs) in order to modulate a 60 GHz Local Oscillator (LO). RF signals coming from the LO have a relative phase difference of 90°, so as the DQPSK-RF signal to be generated after the combination of the phase-formatted I and Q streams. Fig. 3(c) presents the User's Rx which receives the 4-L phase encoded XOR signal from the RAU and splits it into two identical signals. Those signals are multiplied with the respective in-phase signals originating from the LO and having a relative phase difference of 90°. Phase Shifters (PSs) are used for the phase adjustment of the LO. The multiplication function results to the simultaneous phase-to-
OOK and down-conversion of u and v components, whose power levels are adjusted by adding a constant-amplitude electrical signal (DC source). The encoded signals are filtered by Low Pass Filters (LPFs) and inserted in the electrical XOR gates where the decoding process is performed by the XOR operation between the NC-encoding signals and a local copy of user's data. In this way, User A extracts the data B constituents, while User B decodes the data Au and Av streams. A time delay (Δτ) is used to synchronize the data and encoding streams before the decoding function. The PHY-layer setup has been simulated with the aid of VPI Photonics software [28] and has employed a custom-made SOA-MZI model described in [29], following closely the experimental behaviour of a commercially available 1600 µm long hybridly integrated SOA. The input power levels that were used, are −1 dBm for the control signals and −4 dBm for the probe light, for all SOA-MZIs of the OOK-XOR (NXOR) stage. Both SOAs of the OOK-XOR and OOK-NXOR MZI gates were driven by current values of 250 mA and had a recovery time of 100 ps, significantly longer than the 16.67 ps period of the 60 GHz SC. In this way, the SOA-MZI response of all gates neglects the 60 GHz SC of the control signals resembling the response of a low-pass filtering element, while still allowing the SOA gain and phase characteristics to successfully respond to the changes induced by the 2 Gbaud data envelope [30]. The SOAs of the phase-regeneration XOR gates are driven by a 300 mA dc current and had an 80 ps recovery time. 4. Results 4.1. Synchronous operation Fig. 4 illustrates the phase state-to-digital I,Q bit pair correspondence which has been obtained at the output of RAUs’ transmitters (Txs) and corresponds to either the optical DQPSK-SCM data A or B signal phase states. As can be seen, the four different bit pair (symbol) combinations [“00″, “01″, “10″, “11″] are represented by the four possible phase states of [−135°,135°,−45°,45°] respectively. Those optical phase shifts are occurred at the beginning of each symbol's SC, a SC with a period equal to τsc=0.0167 ns (τsc=1/60 GHz). 3
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NRZ-OOK streams by the multiplication with the respective in-phase 60 GHz LO at the users’ receiver. Fig. 5(f) and (g) depict the successfully decoded data Bu and Au after the electrical XOR operation between the encoded XORu trace and a copy of the user's data Au and Bu patterns, respectively. Similarly, Fig. 5(h) and (i) show the decoded data Bv and Av traces, each generated by the bitwise XOR between the NC-encoded XORv trace of Fig. 5(e) and either a copy of data Av or Bv pattern, showing that the initial patterns were correctly retrieved after the decoding operation. Fig. 5(j),(k) depict the eye diagrams of the OOK-XORu and OOKXORv signals at the outputs of the SOA-MZI1 and SOA-MZI3 after the removal of the 60 GHz SC achieved by exploiting the low pass filtering response of the SOAs. Both eye diagrams exhibit an Extinction Ratio (ER) of 11.7 dB, an Amplitude Modulation (AM) of 1.1 dB, a Pulse Overshoot (PO) of 1.4 dB and a jitter of 23 ps, revealing a limited impact of pattern effects, which would normally be expected in the signal processing with SOAs. Fig. 5(l) illustrates the intensity eye diagram as well as the phase “eye diagram” of the 4-Level phase formatted stream at the output of the phase regeneration stage B. The intensity eye diagram reveals a constant power level with an AM=1.8 dB, PO=2.1 dB, while the respective phase eye diagram shows the four different phase levels of the encoded signal, revealing a small phase fluctuation of 1.5° from the ideal values of: [−135°,135°,−45°,45°] and a jitter of 25 ps. In both diagrams, small duration dips at the beginning of the symbol pulse are observed, without yielding, however, any data loss as can be confirmed by the successful decoding traces shown in Fig. 5(f)-(i). Finally, Fig. 5(m) and (n) show the eye diagrams of the electrical down- and OOK-converted complementary XORu and XORv signals exiting the LPF at the User's receiver. Both signals exhibit an ER, AM, PO and jitter equal to 9 dB, 0.7 dB, 0.9 dB and 32 ps respectively, with the dips appearing at the beginning of the pulses. Fig. 5(o) illustrates the optical spectrum of the phase formatted SCM-XOR stream exiting the AOPNC unit at the CO after the coupling of the encoded signal with the CW beam generated by the 60 GHz OSC wavelength feeder. The 2 Gbaud phase formatted signal at the wavelength of λ3 = 1557.36 nm (192.5 THz) and the CW light beam at the wavelength of 1556.88 nm (192.560 THz) were clearly observed with frequency spacing equal to 60 GHz. The respective time trace is illustrated in Fig. 5(p), where it is shown that the SCM output stream is formed by a constant power envelope modulated by a Sub-carrier with a frequency equal to 60 GHz. Fig. 5(q) presents the electrical spectrum of the 2Gbaud phase formatted SCM-XOR beating signal at 60 GHz, generated by the PD and transmitted to the wireless user's Rx. Fig. 5(r) corresponds to the respective time trace, showing the oscillations of the 60 GHz electrical signal below the constant power envelope of the 4-L phase formatted encoded signal.
Fig. 4. The phase state - to - digital I, Q data correspondence for the DQPSK-SCM data A or B optical signal transmitted by the respective RAUs to the CO.
Fig. 5 shows the time traces, eye diagrams and spectra obtained at various stages of the proposed AOPNC-based RoF communication between bit-level synchronized data A and B signals, both with a symbol rate equal to 2Gbaud (bit rate = 4 Gb/s). The parts of the input patterns used for the evaluation of the system, are “1011111001″ and “1010100110″ for the u and v components of data A, while “1011010110″ and “0100001100″ were used for the data B constituents (u and v), respectively. Fig. 5(a), (b) illustrate the OOK-XORu (Au,Bu) and OOK-XORv (Av,Bv) traces at the output of the first and third SOA-MZI. The encoded signal featured a pulse, only when the respective component of data A and B featured different logical values, while it gets a power level equal to 0 when the XORed data represents the same logical value. Fig. 5(c) shows both intensity and phase time traces of the 4-Level phase formatted signal exiting the regenerator stage after the recombination of the Phase-XORu and Phase-XORv signals (Fig. 3(a)). As can be seen, the power envelope of the stream is almost constantly at a "high" level featuring small duration sub-bit dips generated by the transition of the differential phase between the SOAMZI5 and SOA-MZI6 branches from +90° to −90°. The phase trace confirms the OOK to 4-L phase conversion, since the information is encoded in the optical phase φ of the signal, where φ may take one of the four values: [−135°, 135°,−45°, 45°]. The grey markers highlight the present encoding rules, where the XORv pulses are imprinted as 135°, the absence of the pulses as −135°, the existence of both XORu, XORv pulses as 45° and the XORu pulses as −45°. The Truth Table 1(a) and (b) summarize the encoding rules of the proposed AOPNC system. Table 1(a) presents the binary encoding scheme at stage A, where the XORu or XORv output is true (“1”) when the input data Au (or v) and Bu (or v) constituents have different logical values, while the output is false (“0”) when both data inputs are both true (“1”) or both false (“0”). Truth Table 1(b) shows how the 4-Level phase-formatted XOR waveform is encoded by the NC unit at both stages B and C. The phase can get the four values of [−135°,135°,−45°, 45°] depending on the logical values of the binary encoded XORu and XORv pair combinations of stage A as shown in the truth table. Fig. 5(d),(e) illustrates the electrical OOK- and down-converted XORu and XORv components respectively, exiting the LPF at the Users’ receiver as shown in Fig. 3(c). Those traces confirm the conversion of the 4-L phase formatted XOR signal to two binary
4.2. Asynchronous operation Considering that in a real network scenario, the mobile end-users may reside at different distances from their respective RAUs, yielding a sub-bit mismatch between the arrival times of the two packets reaching the encoding unit at the CO, it is critical for the AOPNC concept to operate also between non-synchronized DQPSK-SCM data. In this section, the asynchronous operation was investigated by using sub-bit temporal offsets equal to 125 ps (0.25 of the symbol duration) and 250 ps (0.5 of the symbol duration) between the two data signals, introduced as time delay to data B. Fig. 6(a)–(i) summarize the time traces of the asynchronous encoding and decoding process for a time offset equal to 0.25 of the symbol time duration. Fig. 6(a) illustrates the optically down-converted OOK-XORu performed between the data Au and the delayed by 125 ps data Bu constituents at the output of the SOA-MZI1, while Fig. 6(b) shows the resultant OOK-XORv signal recorded at the output of the SOA-MZI3 after the XOR operation between the data Av and the 4
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Fig. 5. (a)-(i) Time traces of the AOPNC scheme for synchronous data. (j)-(n) Eye diagrams for the synchronous encoding operation. (o), (p) Spectrum and time trace of the SCM encoding signal at the CO's output. (q), (r) Spectrum and time trace of the SCM RF encoding signal at the output of the RAU's Rx. 500 ps/div (traces) and 100 ps/div (eyes).
levels: [−135°, 135°, −45°, 45°] of the encoded signal representing the respective logical values of: [XORu, XORv] = [“00″, “01″, “10″, “11″]. Since the 4-L phase formatted signal originated from the two “interrupted” OOK-XOR components, “parasitic” phase pulses and dips with a duration of 125 ps are also recorded on the phase trace of Fig. 6(c). Fig. 6(d) and (e) depict the filtered by LPFs “interrupted” XORu and XORv signals that were simultaneously OOK- and down- converted by the multiplication of the 4-L phase formatted XOR signal with the respective in-phase signals coming from the 60 GHz LO at stage E. Fig. 6(f) and (g) illustrate the decoded data Bu and Au components after the digital XOR operation between the encoded pattern of XORu
delayed data Bv input streams. As it is highlighted by the green markers, the data asynchrony generates short "parasitic" pulses or dips with a time duration equal to the offset at both encoded XOR streams, which are said to be “interrupted”. The intensity and phase time traces of NC-encoding signal at the output of the phase regenerator are presented in Fig. 6(c), confirming the conversion of the two binary OOK-XOR signals to one 4-Level phase formatted XOR signal. Particularly, that signal shows a constant “high” power envelope with small duration sub-bit dips generated from the transitions of the differential phase from +90° to −90° between the SOA-MZI5 and/or SOA-MZI6 branches. The phase trace reveals the four possible phase 5
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diagrams after a time delay equal to 125 ps from the beginning of the pulse. The intersection seen after the time offset is formed by both “parasitic” pulse-falls and dip-risings during the asynchronous XOR function and has a jitter equal to 21 ps. The intensity and phase eye diagrams of the NC-phase encoding signal leaving the regeneration stage are illustrated in Fig. 7(c). A constant “high” intensity envelope with an AM and PO equal to 1.8 dB and 2.1 dB respectively is observed, exhibiting sub-bit dips when the relative phase of SOA-MZI5 and/or SOA-MZI6 is changed from 90° to −90° and vice versa. The only difference observed with respect to the corresponding eye diagram of the synchronous operation shown in Fig. 5(l) is that the dip appears after time equal to the 0.25 offset due to asynchrony between data A and B. The respective phase eye diagram shows the four different phase levels of the encoded signal, revealing a small fluctuation of 1.5° around the nominal levels of [135°,45°,−45°,−135°]. Small duration phase transitions can be seen at the beginning of the symbol as well as after the time offset of 125 ps. Finally, Fig. 7(d) and (e) depict the electrical eye diagrams of the binary OOK- and down-converted XORu and XORv encoded components at the Users’ receiver, exhibiting an eye opening with an ER of 9 dB, an AM of 0.7 dB, a PO of 0.9 dB and a jitter of 32 ps. Despite the “parasitic” sub-bit intersection observed after the time offset of 125 ps, the other pulse characteristics, like ER, AM, PO, jitter and noise are not affected. This fact indicates that the system may show similar performance when the decoding XOR operation is performed between the XOR and the properly time adjusted local copies of the data, operation that was confirmed also in Fig. 6(f)–(i). Similar results were obtained for the time offset equal to 0.5 of the symbol duration, as shown in Fig. 7(f)–(j). Particularly, Fig. 7(f) and (g) depict the OOK-XORu and OOK-XORv eye diagrams with an ER of 11.7 dB, an AM of 1.1 dB, a PO of 1.4 dB and a jitter of 25 ps, while Fig. 7(h) shows the intensity (AM = 1.8 dB and PO = 2.1 dB) and the respective phase eye diagrams (phase fluctuation=1.5°, jitter = 25 ps) at the encoding unit residing in the CO. The electrical eye diagrams of the down-converted OOK-XORu and XORv signals at the Users’ Rx are illustrated in Fig. 7(i) and (j), revealing an ER, AM, PO and a jitter equal to 9 dB, 0.7 dB, 0.9 dB and 32 ps respectively. In this time-offset scenario the data asynchrony generates “parasitic” intersections after a time delay of 250 ps from the beginning of the pulse, yet without changing the other metrics of the signal quality such as the ER, AM, PO, jitter and noise. The successful asynchronous decoding operation was evaluated with the aid of eye diagrams and Bit-Error-Rate (BER) measurements for the u and v constituents of both data A and B. Fig. 8(a)–(c) depict the eye diagrams of the decoded data Au constituent for sub-bit time offsets equal to 0.25 (125 ps), 0.5 (250 ps) and 0.75 (375 ps) of the symbol duration, respectively. All these eye diagrams exhibit an ER of 8.7 dB, an AM of 1.1 dB and jitter of 40 ps, with short duration dips and spikes appearing after time equal to 125 ps, 250 ps and 375 ps from the beginning of the symbol, respectively. Eye diagrams with similar quality (ER = 8.7 dB, AM = 1.1 dB, jitter = 40 ps) were obtained for the decoded Bu component as shown in Fig. 8(d)–(f) for time offsets equal to 0.25, 0.5 and 0.75 of the symbol duration. The only difference observed for the decoded Bu is that the dips and spikes appear after time equal to the time offset starting from the end of the symbol. Fig. 8(g) illustrates the BER curves as function of the received RF power, for the final decoded signals Au and Bu, for various sub-symbol time offset values. All BER curves reveal error free operation below the required BER threshold of 10−9, as it has been set by the International Telecommunication Union (ITU) for RoF networks. The inset depicts the eye diagram of the decoded data A or B during the synchronous operation, reporting an open eye with an ER, AM and jitter equal to 8.7, 1.1 and 40 ps, respectively, with the dips and spikes appearing at the beginning of the symbol transaction. No additional power penalty was observed when comparing the curves of the asynchronous and synchronous operation. This owes to the dips and spikes that were
Table 1 Encoding rules of the AOPNC scheme represented by truth tables of: (a) the binary XOR u or v encoding output, (b) the 4-Level phase encoding output. (a) Binary u(or v) constituents
Binary encoding
Data A u (or v)
Data B u (or v)
XOR u (or v)
0 0 1 1
0 1 0 1
0 1 1 0
(b) Binary encoding
4-Level encoding
XOR_u
XOR_v
XOR phase
0 0 1 1
0 1 0 1
−135° +135° −45° +45°
and the initial pattern either of data Au or data Bu at the Users A and B receiver. Similarly, Fig. 6(h) and (i) show the decoded data Bv and Av at the User A and B receiver, respectively. In that operation, it is shown that despite the interruptions appeared as “parasitic” pulses or dips at the NC-encoded signals, both the components of data A and B were at the end correctly recovered with the decoded data B components having a delay equal to the time offset. Similar results were obtained for the asynchronous AOPNC operation between data A and delayed by 250 ps (0.5 of the symbol duration) data B, as shown in Fig. 6(j)-(r). Fig. 6(j) and (k) show the encoded OOK-XORu (Au, Bu) and OOK-XORv (Av, Bv) complementary signals at the output of the SOA-MZI1 and SOA-MZI3, revealing “parasitic” pulses or dips with a duration equal to the offset of 250 ps. Fig. 6(l) depicts both the intensity and phase time traces of the 4-Level phase encoded stream at the output of the regenerator after the recombination of the binary Phase-XORu and Phase-XORv signals. The power trace shows an almost constantly "high" level power envelope featuring small duration sub-bit dips originated by the phase transitions at the SOA-MZIs arms from 90° to −90°, while the phase trace shows the “parasitic” phase pulses and dips resulted due to the asynchrony between data A and B. The electrical OOK-XORu and OOK-XORv signal constituents that are OOK- and down –converted at the Users’ Rx, are presented in Fig. 6(m) and (n). The highlighted part of the time traces illustrates the interruptions appearing due to the asynchrony at the encoded signals for both time offset scenarios, revealing that the “parasitic” pulse increases as the time offset increases. Despite those occurrences, by performing a second XOR operation between the respective encoded XOR constituents and the data A components, the traces of the decoded data Bu and Bv patterns can be successfully retrieved yet after a time delay equal to the time offset of 250 ps, as shown in Fig. 6(o) and (q). Similarly, in Fig. 6(p) and (r) data Au and Av components were also correctly decoded by the XOR operation between the encoded stream and delayed by Δτ=250 ps data B pattern at the User B receiver. The respective eye diagrams of the asynchronous encoding operation for offsets equal to 0.25τsymbol and 0.5τsymbol are presented in Fig. 7. Fig. 7(a) and (b) depict the eye diagram of the OOK-XORu and OOK-XORv constituents for 0.25 time offset, revealing an ER of 11.7 dB, an AM of 1.1 dB, a PO of 1.4 dB and a jitter of 23 ps. The existence of the interruptions can be easily observed at both eye 6
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Fig. 6. (a)–(i) Time traces at various steps of the proposed Network Coding configuration for asynchronous data signals, with a sub-bit time offset equal to 0.25 bit. (j)-(r) Time traces at various steps of the proposed Network Coding configuration for asynchronous data signals, with a sub-bit time offset equal to 0.5 bit. Time scale 500 ps/div.
the symbol. Fig. 8(k)–(m) illustrates the respective eye diagrams of decoded data Bv for the same offset, reporting similar eye characteristics (ER = 8.7 dB and AM = 1.1 dB, jitter = 40 ps), with the dips and spikes obtained after time equal to the offset starting from the end of the symbol. Fig. 8(n) shows the BER curves of the decoded components Av and Bv for time offset equal to 0τsymbol, 0.25τsymbol, 0,5τsymbol and 0.75τsymbol, all achieving error free operation with negligible power penalty between the different BER curves. This is an evident that the performance of the proposed AOPNC scheme remains similar even in the case of asynchronous packets arriving at the encoding unit. The
present at the edge of the pulse during synchronous operation being shifted within the duration of the pulses, however without affecting the other pulse characteristics, such as ER, AM, PO, jitter and noise. Therefore, it should be noted that the asynchronous decoded operation shows similar performance with the synchronous case. Similarly, Fig. 8(h)–(j) depicts the eye diagrams of decoded data Av constituent when relative offsets of 0.25τsymbol, 0,5τsymbol and 0.75τsymbol between the Data A and B were considered, exhibiting an ER, AM and jitter equal to 8.7 dB, 1.1 dB and 40 ps, with interruption appearing after a time equal to the offset, starting from the beginning of 7
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Fig. 7. Eye diagrams of the encoding operation for asynchronous data signals, (a)-(e) with a sub-bit time offset equal to 0.25 bit and (f)-(j) with a sub-bit time offset equal to 0.5 bit. Time scale 100 ps/div.
Fig. 8. Eye diagrams of the asynchronous decoding operation: (a)-(c) decoded Au for sub-bit time offsets equal to 0.25, 0.5 and 0.75 bit, respectively. (d)-(f) decoded Bu for offsets equal to 0.25, 0.5 and 0.75 bit, respectively. (g) BER curves of: decoded data Au and Bu for various time offsets (inset of the eye diagram for the synchronous decoded data). Eye diagrams of: (h)-(j) decoded Av for time offsets of 0.25 bit, 0.5 bit and 0.75 bit and (k)-(m) decoded Bv for the same time offsets. (n) BER curves of the decoded Av and Bv components in asynchronous operation (inset of the eye diagram for the synchronous decoded data). Time scale of the eye diagrams: 100 ps/div.
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inset of eye diagram in Fig. 8(n) corresponds to the decoded data A or B signal during synchronous operation, showing an ER of 8.7 dB, an AM of 1.1 dB and a jitter of 40 ps.
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5. Conclusions We proposed for the first time a method to implement all-optical digital PHY-layer Network Coding between four-level phase-formatted Sub-Carrier Modulated (SCM) signals employed in millimetre-wave RoF networks. The all-optical encoding unit resides at the CO and is capable of handling two DQPSK data streams modulated on a 60 GHz Sub-carrier. The encoding process consists of four stages: the DQPSKto-OOK conversion by the DIs, the OOK-XOR, NXOR operations as well as the phase regeneration with the aid of SOA-MZI gates and the remote 60 GHz oscillator wavelength feeder stage for the generation of the final phase-formatted SCM XOR signal. All-optical encoding at the CO unit, as well as the successful electrical decoding process at the enduser's receiver was successfully confirmed for a data rate equal to 4 Gb/ s (2Gbaud), reporting error free performance for both synchronous and asynchronous data. The proposed scheme may operate in higher speeds than 2Gbaud as long as the employed symbol rate does not exceed the unlicensed bandwidth of 7 GHz, being available in the 60 GHz mmwave band [31]. It should be noted, that our AOPNC scheme may in principle be applied also in RoF networks employing other phase modulation formats, such as DPSK-SCM and Dual Polarization (DP)– DQPSK-SCM modulation techniques.
[13]
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Acknowledgements
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This work has been supported by the European FP7-PEOPLE2013-IAPP project COMANDER (contract no. 612257) and the H2020ITN-2016 project 5 G-STEP-FWD (contract no. 722429).
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