# Multi-Access Channel Based on Quantum Detection in Wireless Optical Communication
## 1 Introduction  
Wireless optical communication (WOC), a high-speed wireless communication technology, has been widely studied in recent years [1, 2, 3]. It has the advantages of fast deployment, high bandwidth quality, and high security [4, 5]. More recent works delve into the system level by deploying cells of multi-point-to-point or multi-point-to-multi-point communication. The related multiple-input multiple-output (MIMO) principle has been investigated [6, 7, 8]. Usually, multiple access techniques include orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA). OMA is widely used and analyzed, such as for time division multiple access (TDMA), frequency division multiple access (FDMA) and code division multiple access (CDMA). In the system of [9], the authors proposed an optical CDMA (OCDMA) network. For OCDMA, optical orthogonal codes(OCCs) can be used to implement [10]. In the work of [11], TDMA of WOC is analyzed. Moreover, [12] suggests that NOMA is more suitable for WOC, especially in strong turbulence channels. Some recent work, such as the filter-enhanced scheme proposed in [13], shows promising solutions to providing high-speed spatially modulated optical wireless communication for multiple users. On the other hand, as we know, the birth of quantum information science at the end of the last century is called a revolution in the world of science and technology. Since the beginning of the 21 st century, quantum information science has been developing rapidly, and scholars from physics, computer science, communication electronics and other industries have joined the wave of research in the field of quantum information science, producing many remarkable research results in areas such as quantum communication, quantum computing, quantum control and so on. Quantum measurement is the cornerstone of the whole quantum information science building, and the detection of different quantum-state signals involves the basic principles of quantum measurement, especially in the WOC based on the coherent state, and the value of applying the latest quantum measurement research results cannot be overstated. There are two complementary approaches to differentiating between non-orthogonal quantum states. The first one is minimum error discrimination (MED). This method aims to find the measurement of minimum error probability to identify different quantum states [14]. The second approach is called unambiguous state discrimination (USD). It introduces a perfect quantum-state-discrimination using uncertain results [15; 16], which aims to maximize the probability of obtaining a definite result. However, the imperfection of reality makes the ideal error-free USD impossible. Therefore, real-world USD becomes a measurement strategy between MED and ideal USD, which contains some errors in these deterministic results even though the deterministic results of ideal USD can be obtained. Although traditional techniques for measuring coherent states based on post-selection exist, of course they obtain results with errors and uncertainties, and they do not improve the performance of standard quantum bounds [14; 17]. However, compared to the many important advances made in the field of MED [14; 18; 19; 20; 21; 22; 23; 24], the work on the study of non-orthogonal coherent-state USD is more limited [25; 26; 27]. Research on the experimental side has also been limited to performing USD for two coherent states [28], and the [29] introduced optimal USD for two matter quantum bits achieved using generalized measurements and showed that they were able to outperform standard projection measurements. The work in [30] gives a physical implementation of USD measurements of quadratic coherent states in reality. The feasibility of implementing USD measurements of multiple non-orthogonal coherent states and whether any ideal measurement that reaches the standard quantum bounds can be performed using the existing imperfect realistic conditions is an open question.