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In quantum information science, it is very important to solve the eigenvalue problem of the Gram matrix for quantum signals. This allows various quantities to be calculated, such as the error probability, mutual information, channel capacity, and the upper and lower bounds of the reliability function. Solving the eigenvalue problem also provides a matrix representation of quantum signals, which is useful for simulating quantum systems. In the case of symmetric signals, analytic solutions to the eigenvalue problem of the Gram matrix have been obtained, and efficient computations are possible. However, for asymmetric signals, there is no analytic solution and universal numerical algorithms that must be used, rendering the computations inefficient. Recently, we have shown that, for asymmetric signals such as amplitude-shift keying coherent-state signals, the Gram matrix eigenvalue problem can be simplified by exploiting its partial symmetry. In this paper, we clarify a method for simplifying the eigenvalue problem of the Gram matrix for quadrature amplitude modulation (QAM) signals, which are extremely important for applications in quantum communication and quantum ciphers. The results presented in this paper are applicable to ordinary QAM signals as well as modified QAM signals, which enhance the security of quantum cryptography.

Internet of Things (IoT) enables the inter-connectivity of different “things” using which wide range of items and devices can communicate with each other and their external environment. 5G technology offers enhanced quality of service with high-data transmission rates, which necessitates the implementation of IoT in 5G architecture. Free space optics (FSO) is considered as a promising technology that can offer high-speed information transmission links and therefore is an optimal choice for wireless networks to satisfy the full potential of 5G technology offering 100 Gbit/s or more speed. By implementing 5G features in IoT, the coverage area and performance of IoT will be enhanced using high-speed FSO links. This work proposes the development of high-speed long-reach FSO link for the implementation of 5G and IoT. We investigate a long-haul, single-channel polarization division multiplexed 16-level quadrature amplitude modulation (PDM-16-QAM) based FSO link at 160 Gbit/s incorporating digital signal processing with coherent detection at the receiver terminal. The results show that the proposed system demonstrates a good bit error rate performance under different weather conditions. The proposed system can be deployed for high-speed, long-haul, spectral efficient, robust information transmission links in future 5G wireless networks under dynamic weather conditions.

In this paper, the theoretical bit error rate (BER) of N -level pulse amplitude modulation (PAM- N ) and M -ary quadrature amplitude modulation ( M -QAM) have been studied and compared under different scenarios, including (i) PAM with intensity modulation with direct detection (IM/DD) and field modulation with detection (FMD) (including coherent detection and single-sideband modulation with direct detection (SSB-DD)), and (ii) QAM with coherent detection and SSB-DD. Considering the relationship between the symbol spacing and signal-to-noise ratio (SNR), we provide the mathematical BER equations, including the optical signal-to-noise ratio (OSNR) and carrier-to-signal power ratio (CSPR), especially for PAM signals. To verify the validity of our theoretical expressions for SSB systems, transmissions with 224-Gb/s SSB-PAM4/16QAM signals using the Kramers-Kronig (KK) receiver were implemented on a unified optical system platform. The simulation results agreed well with theoretical calculations both in back-to-back (BtB) and 120-km transmission scenarios, which showed that the BER evaluation methods can serve as a theoretical guidance and system assessment criteria for SSB scenarios.

A blind discrete-cosine-transform-based phase noise compensation (BD-PNC) is proposed to compensate the inter-carrier-interference (ICI) in the coherent optical offset-quadrature amplitude modulation (OQAM)-based filter-bank multicarrier (CO-FBMC/OQAM) transmission system. Since the phase noise sample can be approximated by an expansion of the discrete cosine transform (DCT) in the time-domain, a time-domain compensation model is built for the transmission system. According to the model, phase noise compensation (PNC) depends only on its DCT coefficients. The common phase error (CPE) compensation is firstly performed for the received signal. After that, a pre-decision is made on a part of compensated signals with low decision error probability, and the pre-decision results are used as the estimated values of transmitted signals to calculate the DCT coefficients. Such a partial pre-decision process reduces not only decision error but also the complexity of the BD-PNC method while keeping almost the same performance as in the case of the pre-decision of all compensated signals. Numerical simulations are performed to evaluate the performance of the proposed scheme for a 30 GBaud CO-FBMC/OQAM system. The simulation results show that its bit error rate (BER) performance is improved by more than one order of magnitude through the mitigation of the ICI in comparison with the traditional blind PNC scheme only aiming for CPE compensation.

In this paper, a subset-optimized eight-dimensional trellis-coded quadrature amplitude modulation (SO-8DTCM-16QAM) format for higher-order constellations in high-speed optical communications is proposed. This scheme increases the number of subsets of base 2D constellation divisions. On this basis, it is further optimized by using 2D subsets for Cartesian product combinations to obtain 4D subsets and eliminate the combinations with small Euclidean distances. Finally, the 4D subsets are utilized to construct interrelated 8D subsets for trellis coding modulation and signal transmission. The proposed scheme can effectively reduce the decoding complexity and outperforms the conventional scheme at a high signal-to-noise ratio (SNR). Simulation verification of the proposed scheme is carried out, and the results show that the SO-8DTCM-16QAM achieves signal-to-noise ratio (SNR) gains of 1.60 dB, 1.56 dB, 1.51 dB, and 1.33 dB, respectively, compared with the conventional 8D-16QAM signals when BTB and 5/20/30 km optical signal transmission are performed. The SO-8DTCM-16QAM also achieves an SNR gain of 1.86 dB, 1.75 dB, and 1.22 dB at a net transmission rate of 14/21/28 GBaud. In addition, the SO-8DTCM-16/32/64QAM achieves an SNR gain of 1.27 dB, 0.80 dB, and 1.24 dB, respectively, when compared with the unoptimized 8DTCM-16/32/64QAM. Meanwhile, the proposed eight-subset SO-8DTCM-QAM scheme reduces the complexity of the decoding computation in the subset selection part and the constellation point selection part by 93.75% and 50%, respectively, compared with the unoptimized eight-subset and four-subset 8DTCM-QAM schemes. It can be seen that the proposed scheme simultaneously optimizes the transmission performance and complexity of high-speed optical communication systems and has practical application value.

In this paper, a nonbinary (NB) LDPC-coded four-dimensional (4-D) rotated quadrature amplitude modulation (QAM) constellation scheme in Rayleigh fading channel is proposed. The optimal rotated angle of 16QAM constellation is obtained by exhaustive search. Compared with the conventional 16QAM constellation without rotation in Rayleigh fading channel, the proposed 4-D rotated 16QAM constellation scheme can obtain diversity gain. When spectral efficiency (SE) is 3.20 bits/s/Hz, compared with the regular 16QAM constellation system, the proposed 4-D rotated 16QAM constellation scheme can obtain 1.00 dB theoretical gain, while the simulated error performance gain of the proposed GF(256) LDPC-coded 4-D rotated 16QAM constellation scheme is 1.08 dB. When SE is 3.333 bits/s/Hz, compared with the regular 16QAM constellation system, the proposed 4-D rotated 16QAM constellation scheme can get 1.28 dB theoretical gain, while the simulated error performance gain of the proposed GF(256) LDPC-coded 4-D rotated 16QAM constellation scheme is 1.28 dB. Both the theoretical average mutual information analysis and simulated error performance show that the proposed scheme is efficient and reliable, which is suitable for the future ultra-high-reliable communications.

In this paper, our first attempt at visible light communication system, based on software defined radio (SDR) and implemented in LabVIEW is introduced. This paper mainly focuses on two most commonly used types of LED lights, ceiling lights and LED car lamps/tail-lights. The primary focus of this study is to determine the basic parameters of real implementation of visible light communication (VLC) system, such as transmit speed, communication errors (bit-error ratio, error vector magnitude, energy per bit to noise power spectral density ratio) and highest reachable distance. This work focuses on testing various multistate quadrature amplitude modulation (M-QAM). We have used Skoda Octavia III tail-light and Phillips indoor ceiling light as transmitters and SI PIN Thorlabs photodetector as receiver. Testing method for each light was different. When testing ceiling light, we have focused on reachable distance for each M-QAM variant. On the other side, Octavia tail-light was tested in variable nature conditions (such as thermal turbulence, rain, fog) simulated in special testing box. This work will present our solution, measured parameters and possible weak spots, which will be adjusted in the future.

Inter-satellite communication is a free-space optical technology which is used to establish communication between satellites in space. This work is focused on the transmission of 10 Gbps data over 4,000 km inter-satellite communication link by incorporating orthogonal frequency division multiplexing scheme. Moreover, a comparison of 4 quadrature amplitude modulation and 4 phase shift key encoding scheme is also presented in this work. The performance of proposed system is evaluated in terms of signal-to-noise ratio, total received power, radio-frequency spectrum and constellation diagrams.

The 5G system-based cognitive radio network is promised to meet the requirements of huge data applications with spectrum. However, the M-ary effect on the detection has not been thoroughly investigated. In this paper, an M-ary of quadrature amplitude modulation detection system is studied. Many rates are used in this study 4, 16, 64, and 256 constellation points. The detection system is applied to cooperative spectrum sensing to enhance the performance of detection for various rates of M-ary with low signal-to-noise ratio (SNR). Further, three kinds of signals based 5G system are sensed: filtered-orthogonal frequency division multiplexing (F-OFDM), filter bank multi-carrier (FBMC), and universal filtered multi-carrier (UFMC). The best detection performance is obtained when the M-ary=4 and number of SUs=50 user, whereas the worst detection performance is obtained when the M-ary=256 and number of SUs=10 user, as revealed in the simulation results. In addition, the detection performance for the F-OFDM signal is better than that of UFMC and FBMC signals for SNR <0 dB.

We demonstrate in this work effective linearization on a millimeter-wave (mm-Wave) broadband monolithic gallium nitride (GaN) power amplifier (PA) using digital predistortion (DPD). The PA used is a two-stage common-source (CS)/2-stack PA that operates in the mm-Wave 5G FR2 band, and it is linearized with the generalized memory polynomial (GMP) DPD and tested using 4G (4th generation) long-term-evolution (LTE) 64-QAM (quadrature amplitude modulation) modulated signals with a PAPR (peak-to-average power ratio) of 8 dB. Measurement results after implementing GMP DPD indicate considerable broadband improvement in the adjacent channel leakage power ratio (ACLR) of 16.9 dB/17.3 dB/16.5 dB/15.1 dB at 24 GHz/28 GHz/37 GHz/39 GHz, respectively, with a common average POUT of 15 dBm using a 100 MHz LTE 64-QAM input signal. At a fixed frequency of 28 GHz, the GaN PA after GMP DPD achieved signal bandwidth-dependent ACLR improvement and root-mean-square (rms) EVM (error vector magnitude) reduction using 20 MHz/40 MHz/80 MHz/100 MHz LTE 64-QAM waveforms with a common average POUT of 15 dBm. The GaN PA thus achieved very good linearization results compared to that in other state-of-the-art mm-Wave PA DPD studies in the literature, suggesting that GMP DPD should be rather effective for linearizing mm-Wave 5G broadband GaN PAs to improve POUT, Linear.