This work experimentally demonstrates a free-space optical (FSO) communication system using a low-cost commercial optical transmitter. The system supports data transmission with QPSK and QPSK with frequency division multiplexing (QPSK-FDM), aiming to evaluate bit error rate (BER) performance under stable lab conditions. MATLAB-based algorithms generate QPSK and QPSK-FDM waveforms, which are uploaded to an arbitrary function generator for optical modulation. At the receiver, matched filtering and threshold detection are used for signal decoding and BER calculation across varying optical power levels. Results show that QPSK-FDM outperforms standard QPSK in terms of BER performance, though BER degrades with higher data rates. This study demonstrates the feasibility of low-cost FSO systems and provides practical algorithms for signal processing, highlighting their potential for research, education, and future optical communications. Read More

This work advances rate-splitting multiple access (RSMA) to address interference, fairness, and spectral efficiency challenges in future wireless networks. It introduces a novel quadrature stream-splitting integrated SIC-driven multiple access (Q-SSMA) scheme, combining RSMA, Q-RSMA, and NOMA to enable interference-free common stream detection, simplified SIC for private streams, and simultaneous decoding, improving sum-rate, reliability, and robustness under imperfect SIC. Additionally, Q-RSMA eliminates SIC by using quadrature carriers, achieving zero-SIC detection, decoupled decoding, lower complexity, and superior spectral and energy efficiency. To accommodate unequal rate demands, closed-form solutions for optimal rate and power allocation are derived, ensuring fairness without sacrificing sum-rate. A constellation-based framework is also developed to derive symbol error rates and resolve constellation ambiguity for arbitrary modulation orders. Together, these contributions establish an efficient, adaptable RSMA framework that enhances performance and simplifies system design. Read More

This work explores the limitations of a conventional reconfigurable intelligent surface-assisted spatial modulation (RIS-SM) system by analyzing the signal-spatial constellation diagram. These limitations include closely packed signal-spatial points and a lack of flexibility in adjusting their positions, negatively affecting system performance. To address these challenges, this work proposes a unified RIS-SM (U-RIS-SM) system that utilizes optimal RIS reflector phases and a modified modulation scheme with a tuning parameter. The proposed U-RIS-SM system facilitates the constellation redesigning of signal-spatial points by adjusting the tuning parameter. Additionally, the optimal tuning parameter value is determined to achieve the best possible performance of U-RIS-SM. Closed-form expressions for the symbol error rate (SER), average SER (ASER), and average mutual information (MI) of the proposed U-RIS-SM system under the Nakagami-m fading channel are derived and validated through Monte-Carlo simulations. Moreover, since the proposed U-RIS-SM system can be transformed into existing RIS-SM and RIS-assisted space shift keying (RIS-SSK) systems by varying the tuning parameters and RIS reflector phases, the performance metrics for existing RIS-SM/SSK systems are derived from expressions for U-RIS-SM. The U-RIS-SM system is compared with existing RIS-SSK and RIS-SM systems across various parameters, demonstrating its superior performance. Furthermore, to assess its effectiveness under practical conditions, its performance is examined using a low-complexity two-stage maximum likelihood detector, as well as under RIS phase errors and discrete RIS phase shifts Read More

This work analyzes a terrestrial-based free-space quantum key distribution system employing quadrature-phase shift keying signaling (QKD-QPSK) and octa-phase shift keying QKD (8PSK-QKD), which is based on the BB84 protocol. The QKD-QPSK (shown in Fig. 1) is readily transformed into QKD-based binary phase shift keying (QKD-BPSK). Moreover, the QKD protocol to design and implement 8PSK-QKD is proposed for the first time, which shares two key bits simultaneously and consequently exhibits increased spectral efficiency compared to existing modulation signaling-based QKD protocols (shown in Fig. 2). A novel analytical framework is proposed for QKD-QPSK and 8PSK-QKD to determine various performance metrics like the probability of sifted keys P_sift, quantum bit error rate (QBER), etc., under the gamma-gamma turbulence channel model. Comparative results of QKD-QPSK and QKD-BPSK (shown in Fig. 3) indicate that QKD-BPSK is superior to QKD-QPSK for P_sift and QBER. In addition to this, QKD-BPSK based on the proposed framework perfectly matches the Monte-Carlo simulation. In contrast, QKD-BPSK curves, based on the previously proposed Gaussian–Laguerre (GL) method, do not accurately match the simulation. Furthermore, the numerical result of 8PSK-QKD (shown in Fig. 4) reveals that the 8PSK-QKD system follows the criteria of an acceptable QKD system, even with transmitting two key bits at a time. Additionally, the 8PSK-QKD system outperforms the SIM/BPSK-QKD and QPSK-QKD by offering better P_sift Read More