We propose a novel polarization demultiplexing technique for complex modulated signals using the Stokes space polarization representation of the signal samples. Polarization demultiplexing of an endless time-varying SOP signal is achieved through the adaptive estimation of the inverse rotation matrix, without the calculation of a best fitting plane. Simulation results show the effectiveness of the proposed technique, with a high convergence speed, and a good response to time-varying SOP rotations. The performance of the method is assessed through EVM calculations for frequency rotations up to 15 μrad/symbol, showing a maximum penalty of 1 dB. Moreover, the technique can incorporate an additional stage for PDL compensation without penalties in terms of EVM results and algorithm complexity.

There has been a growing interconnection across the world owing to various multimedia applications and services. Fixed wireless access (FWA) is an attractive wireless solution for delivering multimedia services to different homes. With the fifth-generation (5G) and beyond mobile networks, the FWA performance can be enhanced significantly. However, their implementation will present different challenges on the transport network due to the incessant increase in the number of required cell-sites and the subsequent increase in the per-site requirements. This paper presents a comprehensive tutorial on the enabling technologies, design considerations, requirements, and prospects of broadband schemes. Furthermore, the related technical challenges of FWA are reviewed, and we proffer potential solutions to address them. Besides, we review various transport network options that can be employed for FWA deployment. In this regard, we offer an in-depth discussion on their related requirements for different use cases. Moreover, we give an insight into the 3GPP RAN functional split implementations and implications on the 5G FWA transport network solutions. The concepts of virtualized RANs for attending flexibly to the dynamic nature of different use cases are also presented.

A Stokes space based technique for digital polarization-dependent losses (PDL) monitoring and compensation is proposed. The channel PDL monitoring is carried out with a resolution of 0.15 dB by computing the mean point of the signal samples represented in the three-dimensional Stokes space. The presented compensation technique is based on rotations and translations of the signal samples in the Stokes space, and provides a PDL compensation up to 2.5 dB. Error vector magnitude results show a gain of about 3 dB (assuming a PDL value equal to 1.5 dB) to a Stokes space based polarization demultiplexing technique without PDL compensation.

Discrete modulation continuous variable quantum key distribution (DM-CV-QKD) is highly considered in real implementations to avoid the complexity of Gaussian modulation (GM), which is optimum in terms of the key rate. DM-CV-QKD systems usually consider M-symbol phase shift keying (M-PSK) constellations. However, this type of constellation cannot reach transmission distances and key rates as high as GM, limiting the practical implementation of CV-QKD systems. Here, by considering M-symbol amplitude and phase shift keying (M-APSK) constellations, we can approximate GM. Indeed, considering finite-size effects, 4 ring 64-APSK can reach 52.0 km, only 7.2 km less than GM and 282% the maximum achievable transmission distance for 8-PSK.

A model for quantum BER estimation in polarization encoded quantum key distribution systems is presented. Both TDM and WDM based polarization control schemes are analyzed. It is shown that TDM presents some important advantages when compared with the WDM control scheme. In WDM, the polarization decorrelation between the reference and data signals is an intrinsic and very limitative impairment. This effect has a contribution to quantum BER that increases with the propagation distance, and is highly dependent on the fiber polarization mode dispersion. In the TDM control scheme, the polarization decorrelation is less critical and other issues, like the single photon detector and feedback polarization control system performance tend to dominate. We show that for long distances the fiber losses represent the main contribution to the total quantum BER. Nevertheless, for distances shorter than 70 km and frequencies higher than 5 MHz the after pulse detections provide an important contribution to the total quantum BER.

We experimentally evaluate the performance of Kramers-Kronig (KK) receivers for free-space optical transmission. We transmit a dual-carrier signal, each carrier modulated as a 25 Gbaud 16/32 quadrature-amplitude modulation (QAM) signal. This dual-carrier signal, with overall symbol rate of 50 Gbaud, is detected in a single receiver with about 35 GHz bandwidth. Key parameters such as the carrier-to-signal power ratio, the frequency allocation of the pilot tone and the frequency separation between both modulated carriers are optimized. In addition, we also study the effect of the receiver bandwidth limitations on the KK technique. We perform long-term measurements, beyond one hour, for both 16QAM and 32QAM signals in a 55 m outdoor free-space link during daylight with sunny weather conditions. Our measurements demonstrate that the link operates below the BER threshold assuming 20% forward-error correction (FEC) for the 32QAM signal and 7% FEC overhead for the 16QAM signal. The resulting net data rates are 208 Gbit/s and 184 Gbit/s for the 32QAM and 16QAM signals, respectively.