Non-orthogonal multiple access (NOMA) systems allowing multiple users sharing the same resource block offer significant gains in spectral efficiency which can enable the required massive access in future wireless systems. However, they face several challenges due to their sensitivity to power allocation coefficients, fading effects, and imperfect channel state information (CSI). To address these limitations, this paper proposes Hadamard-NOMA, an approach leveraging the Hadamard Transform (HT) at the source level prior to modulation. By introducing HT, the system mitigates the adverse impact of fading and CSI imperfections, reducing bit error rates (BER) and enhancing overall system reliability. Theoretical analysis and Monte Carlo simulations validate the effectiveness of this technique, demonstrating robust NOMA transmission in dynamic wireless environments. The proposed method offers a promising solution for next-generation wireless networks, ensuring more reliable performance under diverse transmission conditions. Simulation results confirm analytical predictions, demonstrating significant performance improvements over state-of-the-art T-NOMA and Usman-NOMA schemes. Specifically, for the Near user, a gain of 15 dB is achieved at a Bit Error Rate (BER) of $10^{-2}$, while the Far user benefits from a 10 dB gain at a BER of $10^{-1}$. Compared to Usman-NOMA, the proposed method provides an improvement of 15 dB for the Far user at BER $10^{-1}$. Additionally, in a two-user scenario with imperfect Successive Interference Cancelation (SIC), user 1 requires an SNR at least 14 dB lower than user 2 to achieve a BER of $10^{-3}$. These findings highlight the effectiveness of applying HT at the source stage, significantly mitigating CSI errors and making NOMA more resilient for next-generation wireless networks.

翻译：允许多用户共享同一资源块的非正交多址接入（NOMA）系统在频谱效率方面具有显著增益，能够满足未来无线系统所需的大规模接入。然而，由于其对功率分配系数、衰落效应以及不完善信道状态信息（CSI）的敏感性，此类系统面临若干挑战。为应对这些局限，本文提出Hadamard-NOMA方法，该方法在调制前的信源级利用哈达玛变换（HT）。通过引入HT，系统减轻了衰落和CSI不完善的不利影响，降低了误码率（BER）并提升了整体系统可靠性。理论分析与蒙特卡洛仿真验证了该技术的有效性，证明了其在动态无线环境中的鲁棒NOMA传输能力。所提方法为下一代无线网络提供了一种有前景的解决方案，确保在多样化传输条件下实现更可靠的性能。仿真结果证实了理论预测，相较于先进的T-NOMA与Usman-NOMA方案，本方法展现出显著的性能提升。具体而言，对于近端用户，在误码率（BER）为$10^{-2}$时获得了15 dB的增益；而对于远端用户，在BER为$10^{-1}$时获得了10 dB的增益。与Usman-NOMA相比，所提方法在BER为$10^{-1}$时为远端用户提供了15 dB的改进。此外，在不完善连续干扰消除（SIC）的双用户场景中，用户1需要比用户2至少低14 dB的信噪比（SNR）以实现$10^{-3}$的BER。这些发现凸显了在信源阶段应用HT的有效性，其显著缓解了CSI误差，使NOMA对下一代无线网络更具适应性。