Chapter5Demonstration of 25 Gbit/s PAM4 Hybrid Photonic-Wireless TransmissionIntroductionThe most encouraging way to achieve acceptabledelivery of wireless and baseband signals was Optical-Wireless accesstechnologies.

The transmission distance and bandwidth increase while the powerbudget decrease at the same time.  A gigabitpassive optical network (GPON) is a solution to simplify the architecture toapply to the ROF system. Through that we will able to afford higher bandwidthat a longer connection distance 59.While Radio-over-fiber(RoF) systems and Passive Optical Network (PON) are promising candidates inwireless and wired access networks, the primary concern is to transmit bothradio-frequency (RF) and baseband signals on a single wavelength over a singlefiber in a cost-effective way with acceptable performance.

A most straightforwardsolution is to build the RoF link based on the existing PON system with minimalmodification, which will be seamlessly integrated intothe existing optical distribution network. Recently, a high loss-budgetamplifier-less 100G-PON is demonstrated 60, onwhich our proposed architecture is based. To achieve a low cost and complexitylink, 10G-class transmitter is promising, which requires advanced modulationwith high spectrum efficiency like four-level pulse amplitude modulation(PAM4).In this chapter, weexperimentally demonstrate the transmission of a 25 Gbit/s PAM4 signal with theproposed architecture integrating a Ka-band hybrid photonic-wireless link andan amplifier-less wired link. In our wireless link, the generated PAM4 signalis delivered over 40-km SMF before up-converting to 25GHz Ka-band signal,achieving a BER less than the HD-FEC threshold of 3.8×10-3 afterdigital coherent down-conversion with the help of volterra equalization.

In addition,we also demonstrate an amplifier-less wired link, which is similar to thetypical Passive Optical Network (PON), to prove the feasibility of theintegration architecture of PON and RoF link, achieving receiver sensitivity at-21.5 dBm and about 26dB loss budget with simple linear feed-forwardequalization.A 4-level pulse amplitude modulation(PAM4) scheme take placed Non-return to-zero (NRZ) signaling, because PAM4 usedhalf the bandwidth to transmit the same load compare with NRZ signal.

While PAM4 agitates signal integrity,it increased complexity as well. This means that we face with new problems andissues. More accurate measurements and design verification before generate aresome requirement at the beginning as principal. 5.1 High Data Rates beyond NRZ andPAM4 Bandwidth Demands With the aim of increasing the bit rate within the minimum bandwidth toincrease the performance of NG-PON, different modulation schemes are apparentlyneeded. As illustrated infigure 3, PAM4 signal uses four separate levels to encode two bits (00, 01, 10,11) in each symbol, while  NRZ encodesone bit, either 0 or 1.

 Figure 3: Comparison between NRZ and PAM4. 5.1.1 PAM4 ComplexityWe face potential nonlinearities byusing PAM4 and it brings an incremental leap in signal complexity.

 “Eye compression” is differences in the eyeheights of the three eye openings, “timing skew” happen when the centers of thethree eyes are not regulated in the same line.5.1.2 PAM4 ISI and EqualizationThe frequency content of both NRZ and PAM4signals consist of square wave harmonics and sub-harmonics from sequences ofidentical symbols. The frequency components have precisely coordinated amplitudes and phases. The amplitudes and phases of eachdifferent frequency component has influenced by the channel.  By de-correlating its frequency and phase configuration,the channel causes ISI. We can get rid of ISI by inverting the channel responsethrough Equalization.

The problem is equalization in PAM4 systems is morecomplicated than in NRZ systems, since PAM4 has 4 levels. 5.2 System Description5.2.

1 MethodologyExperiments and simulations are different methods to carry out researchin the field of optical systems and wireless. The idea of diving into two paths is rooted in theproblem of accessing to users and the need to use wireless system is inevitablewithout modifying CO or transmission line. If we cannot access an end-to-end fibre connectionand t is not economical to investigate a new fibre link, as might be the case in difficult-to-access terrainsor if an already existing fibre connection fails, a permanent wirelessconnection could help. There are severaloptical techniques for generating and transporting signals over fiber andwireless connection. Therefore, depending onthe transmission method used, the RAU could be simple or in a complicated structure. Thus, heterodynephotonic is configured because it is cost efficient and easy toimplement. While we can transfer data in typical PON, wireless transmission inother path is aggregated.

So it is reasonable tointroduce the low cost and less complexity RAU and CO. And also, the downconverting methods could let us flexibly rearrange the spectrum when theantenna receives signals. 5.2.1.1 Ka band The frequency range between 26.5 GHz and 40 GHz , which is the loweredge of the mmW range, two bands around 28 GHz and 36 GHz have been assignedfor usage in mobile communication networks , that introduced as Ka-band in theIEEE with an additional allocation in the adjacent K-band at 24 GHz 61.

These bands both are suitable for indoor and outdoor communications interms of channel characterizations 60,61.The efficient use thereof has being a considerable problem even if thesebands allow for larger channel bandwidths and we have to pay attention to thecomplexity of transmitting and receiving equipment.5.2.1.

2 Volterra filter To compensate distortions Volterra filter have been applied simultaneously.In sequence the three kernels Volterra filter, the k-th sample of the outputsignal is expressed as 62In our work, the nonlinear distortions are mainly recompensed by thedigital Volterra filter.5.2.

1.3 Equalization We can use several signal processing methods at the receiver side to easethe ISI difficulties which happened by delay spread .This technique is termedas “equalization”.

Usually equalizers are small, inexpensive, and certainly tuneable, sothey can be employed digitally afterward A/D transformation.Equalization techniques are classified in to two groups: linear andnonlinear. The linear methods are almost the simplest to use and to comprehendtheoretically. Most of the wireless applications use nonlinear equalizers, becauselinear equalization techniques faced with noise boost which is not a big dealin nonlinear category.Equalizers can also be classified assymbol-by-symbol (SBS) or sequence estimators (SE). SBS equalizers eliminate ISIfrom every figure and sense every figure separately at next step. 5.

3 Experimental setup and resultsThe following section presents the experimental setup, then discuses about the results. In the current study weinvestigate the data transmission over wireless and wired connection using10G-class device, and use PPG to generate PAM4 signal and transmitted it with40km SMF. Non return to zero (NRZ) signaling has been utilized for PON, because ofthe high price of 25G components.

We considered spectrally compact modulationformats such as four level pulse amplitude modulation (PAM4), which enable toreuse the cheaper 10G components 63. Laser chirp and fiber chromatic dispersion(CD) are limitation to the performance of DML transmitters in the C-band andKa-band.   5.3.1Direct Modulated LasersIn high-capacity links, external opticalmodulators and advanced modulation formats are more useful than DirectModulated Lasers (DML).

Because DMLs are known to have serious dispersionpenalty due to modulation induced frequency chirp.Three main factors cause signal distortions beside the DML chirp affectat a DML-based transmission systems: limited bandwidth ofdevice and fiber chromatic dispersion (CD);signal-to-signalbeating noise (SSBN); fiber nonlinearities62.5.3.2Network Architecture The proposed architecture of two-paths  fiber-wireless convergence systems based on a10G class components has shown in Figure 5.4.1.The optical signal is generated by EML and is fed apseudo-random bit sequence (PRBS15) non-return-to-zero (NRZ) signal.

Thissignal transmitted over 40km (SMF) fibre and divided into wired and hybridphotonic wireless path.  Figure 5.4.1Proposed architecture for hybrid optical fiber-wireless link in the Ka-band andwired link.

CO: central office; RAU: radio access unit In first step, we initialize the experimental set-upin the Figure 5.4.1. we investigated data transmission over wireless and wiredconnection using 10G class device to generate PAM4 signal and transmitted itthrough 40km SMF, representing a typical optical link. The output power of the10G-class EML was 4.3dBm at the operating wavelength of 1550 nm.

Tuneable Dispersion Compensator Module (TDC) is usedfor compensating the chromatic dispersion and obtains a clearer eye diagram. It is a device withlow insertion loss, low Phase Ripple, low group delay Ripple, low PDL and lowPMD. It’s colorless and in support of data transmission rates of 10Gb/s, 40Gb/sand beyond. The dispersion slopecan strongly limit the usable bandwidth, which is important in the case ofwavelength division multiplexing. The optical power after TDC is -1.

3 dBm. Thesignal was transmitted over 40km SMF and divided into wired and hybrid photonicwireless paths. Figure5.

4.2 Experimental setup The wired path contains a VOA to control the outputpower which has range of -12dB to -21dBm optical power. The signal has beenrecovered by using a 10-G APD photo-detector. (b) (a)  Figure 5.4.3 Experimental setup (a) eye diagramafter EML, (b) eye diagram after EDFA Figures 5.4.

3 (a) and (b) show the eye diagrams beforeand after transmission over fiber link. From these eye diagrams, we havenoticed after the transmission over a 40km SMF, the eyes look clearly open. Wecould observe clear eye diagram at the end of wired path and after digitalsignal processing.5.3.

3 BitError Rate In our experiments, in order to evaluate the link performance, we digitized andrecorded the received data with a real-time oscilloscope for offline digital signalprocessing and signal quality evaluation.We have measured the BER curve by changing thereceived optical power with digital down-conversion performed in digital signalprocessing (DSP) part. We have measured for both wired and wireless links theoutput power along the 40-km fiber link; we got a good BER as illustrated inFig 5.4.

4 which shows theperformance of the system.At the receiver, we compared the recorded bitsequence with the originally sent bit sequence and counted the number of biterrors. This number divided by the number of compared bits is then an estimatefor the BER. However, if the signal quality is high, the BER is small; asignificant amount of time is needed to count enough errors to determine areliable BER value. (a) (b)   Figure 5.4.4.

Results: (a) BER versus input power into APD for wired path, (b) BER versusinput power into PIN for wireless pathFor fiber length thatis 40 km, the BER performance remains acceptable. For wireless path, weachieved BER under HD-FEC limit @ 3.8e-3 when the received optical power is 5dBm with volterra equalization, while stay under SD-FEC limit @ 2e-2 untilreceived power decreased to 1.5 dBm.

For wired path, we achieved receiversensitivity at -21.5 dBm at HD-FEC limit. 5.3.4 The hybrid photonicwireless pathThe hybrid photonic wireless path starts with an EDFAto amplify the signal and an Optical Tuneable Filter (OTF) to narrow thebandwidth. The optical input power to the Finisar 40GHz PIN is controlledthrough two variable optical attenuators (VOA), allowing separate adjustment ofsignal and LO power. 5.3.

4.1Opticalspectrum at Hybrid photonic up conversion To convert optical signal to RF signal, a distributedfeedback (DFB) laser was usedas tuneable local oscillator (LO) for hybrid photonic up conversion on a PIN.DFB offers high efficiency and remarkable spectral purity. The modulated datasignal at 1538.087nm and the un-modulated LO at 1538.280nm are mixed by acoupler then sent to the 40GHz PIN.

The frequency spacing between the unmodulatedoptical local oscillator and the modulated data carrier defines the generatedwireless carrier frequency after optical Figure 4.4.5 Results: Optical spectrum before PIN heterodyning, which is25 GHz, as shown in Fig. 5.4.5. While recalling the output of EML is 4.

3 dBm, weachieved a loss budget of 25.8 dB with simple linear feed-forward equalization.An ADC output signalis transformed to a part of spectrum at smaller frequencies by a digital downconverter with an equalizer.

This is followed by decimation. Sampling rate isdecreased to perform correcting operations on the signals and a specificfrequency pass band is selected on the down converter to carry out theequalization. This way, computational burden is lowered enough for the downconverter to work in real time mode. It also boosts the efficiency of the BitError Rate in digital formats.

The Fig. 4 shows the eye diagram after digitaldown-conversion before and after equalization. (a) (c) (d) (b)     Figure 4. Eye diagram (a) wired pathwithout equalization, (b) wired path with equalization, (c) wireless pathwithout equalization, (d) wireless path with equalization 5.4 ConclusionWe have analyzedand experimentally demonstrated a Ka-band 25-Gb/s PAM-4 over 40-km SMF based on10G-class EML and a hybrid photonic wireless link employing heterodyne photonicup-conversion has been demonstrated.

In order to reach a simple architecture, TDCis applied at CO to control the dispersion. Thetransmission characteristic through the single mode RoF system has beenevaluated. The experiments have also shown that signals can be transmittedthrough RoF system for remote signal distribution as the same time with wiredpath, without degradation of BER performance.

Anothercontribution of our experiment is that the proposed structure would be apractical and low-cost solution for typical wired and wireless transmissionthat is EML-based four-channel PAM-4 combined with heterodyne photonicup-conversion and Volterra filter.             Chapter 6Conclusions For integration of wireless and optical access, RoF is the mosteffective technology. Combination of fiber optics and radio, and is a way to allocateradio frequency as a broadband or baseband signal over fiber.In comparison with wireless systems, RoF has some outstanding headlinesand characteristics which make it totally preferable; RoF is less complicated and entails smallerbase stationsRoF-based networks can have centralized architectureToo many researches have been done about this topic at physical layer;but upper layer network architecture and system resource management problemsdeserve more research and investigate than it has received.

Besides, in our study we have proposed access network architecture basedon RoF and 10G-Class devices which presents seamless integration of hybrid Ka-bandradio access units into existing optical distribution system, such as PON and WDMpoint-to-point links for arrear rural and remote areas that provides anefficient bandwidth management.As a result, regarding to the system resource administration, RoF foundedwireless networks are further capable than systems which structured just on wirelessnetwork.     

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