Two experimental configurations of a hybrid K-band (25 GHz) microwave photonic link (MPL) are investigated for seamless broadband wireless access networks. Experimental configurations consist of optical fiber, free-space optics (FSO) and radio frequency (RF) wireless channels. We analyze in detail the effects of channel impairments, namely fiber chromatic dispersion, atmospheric turbulence and multipath-induced fading on the transmission performance. In the first configuration, transmission of the 64-quadrature amplitude modulation (QAM) signal with 5, 20 and 50 MHz bandwidths over 5 km standard single-mode fiber (SSMF), 2 m turbulent FSO and 3 m RF wireless channels is investigated. We show that, for QAM with a high bandwidth, the link performance is being affected more by atmospheric turbulence. In the second configuration, the 20 MHz 4/16/64-QAM signals over a 50 km SSMF and 40 m FSO/RF wireless links are successfully transmitted with the measured error vector magnitude (EVM) values of 12, 9 and 7.9%, respectively. It is shown that, for all transmitted microwave vector signals, the bit error rate is lower than the hard-decision forward-error-correction limit of 3.8×10 −3. Moreover, an extended FSO link span of 500 m for 25 GHz hybrid MPL with 16-QAM at 10 Gb/s under the weak and strong turbulence regimes is evaluated via simulation analysis to mimic a practical outdoor system.

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The performance assessment of a novel multilevel modulation format based on partial-response signalling called absolute added correlative coding (AACC) by numerical simulations is delineated, targeting short-range and interconnect high-speed optical networks. The spectral efficiency, chromatic dispersion tolerance and receiver sensitivity of 4-instensity level AACC are discussed and compared against other well-known existing modulation formats, namely, non-return-to-zero on-off keying and 4-ary pulse-amplitude modulation (4-PAM). The robustness of the higher order of AACC over 8-PAM and 16-PAM is also discussed.Introduction: The development of multilevel modulation formats and detection techniques is essential in enabling efficient high-speed and high-bit rate optical fibre links, such as the 40 G synchronous optical networking/synchronous digital hierarchy and 100 G Ethernet [1]. The goal of utilising higher-order modulation formats with coherent detection and digital signal processing is to set up a high-capacity optical communications system that combines high spectral efficiency and long transmission distance capability [1]. In contrast, intensitymodulated direct-detection (IM/DD) is more suitable for short-range applications, e.g. optical interconnects, storage area networks and data centres. Recently, on-off keying (OOK) and 4-pulse-mplitude modulation (PAM) are receiving mounting attention in short-range optical interconnects because of their outstanding performances and low cost [2]. However, M-PAM has an inherent back-to-back receiver sensitivity penalty in comparison with OOK.Partial-response signalling is an effective candidate in achieving narrow spectral width and high resilience to fibre dispersion; it was fruitfully demonstrated as binary signals, namely duobinary or phase-shaped binary transmission, and has received much attention recently for the long-haul 40 Gbit/s system [3]. However, the concept of the partialresponse format has never been applied in multilevel-intensity signalling.We therefore suggest in this Letter a novel approach of generating a 4-intensity level signal based on partial-response signalling, called absolute added correlative coding (AACC) in a 20 GBaud (40 Gbit/s) lightwave system.

Local and remote photonic millimeter wave (mmW) signal generation schemes are theoretically and experimentally evaluated in order to compare both approaches for practical deployment in a cloud-radio access network (C-RAN) fronthaul network. The paper presents a full comprehensive formulation of the frequency response of a system based on a directly modulated laser transmitting data over 40 GHz signal generated by external carrier suppressed modulation and optical frequency multiplication. Theoretical and experimental characterization of the system response at baseband and mmW band for local and remote generation setups show very good agreement. The remote configuration leads to higher electrical output power (i.e. 15 dB higherin 25 km fiber links) due to the combined effect of chirp and fiber dispersion than the local generation setup in the mmW band, although intermodulation distortion is higher in the former case. Transmission experiments using quadrature phase-shift keying (QPSK) signal with 250 MHz bandwidth centered at 0.5 GHz over 10 and 25 km fiber links also confirm the superior performance of the remote setup, whereas the local setup leads to similar results to optical back-to-back (OB2B) measurements, which is also validated with data signal centered at different frequencies within the laser bandwidth frequency range. Finally, experimental results show the quality of the recovered signals in terms of error vector magnitude (EVM) as a function of the received electrical power and demonstrate that no further penalties are introduced by photonic mmW signal generation with respect to electrical back-to-back (EB2B) levels.

Microwave photonics is a promising solution to transmit millimeter wave (mmW) signals for the 5th generation (5G) mobile communications as part of a centralized radio access network (C-RAN). In this paper, we experimentally evaluate the impact of turbulent free space optics links on photonically generated mmW signals in the frequency range of 26−40 GHz. We analyze the remote generation of mmW signals over hybrid links based on free-space optics (FSO) and standard single mode optical fiber (SSMF) with −39.97 dBm received electrical power and phase noise level at 100 kHz as low as −95.92 dBc Hz at 26 GHz. Different thermal distributions along the FSO link have been implemented and Gamma-Gamma model has been employed to estimate the thermally induced turbulence. The results show high electrical power decrease and fluctuation of the generated mmW signal according to the particular level of the turbulence in terms of refractive index structure parameter and thermal distribution along the FSO link. 8 Gb/s 16-quadrature amplitude modulation (QAM) data transmission at 42 GHz has been demonstrated over the hybrid link with minimal error vector magnitude (EVM) value of 5% whereas turbulent FSO link introduced up to 5 dB power penalty.

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