## 3 System Model  
The system model proposed in this paper is developed based on the actual scenario requirements of QKD networks of China Telecom. As shown in Figure 1, the base station acts as the legitimate communication receiver (Alice) and implements the embedded FPLA mechanism to authenticate the UE attempting to access the network. The legitimate quantum UE ([PERSON]) and an unspecified number of malicious UEs ([PERSON]), as well as an unspecified number of regular UEs, attempt to connect to the base station. Figure 1: System model of a 5G network with FPLA mechanism. The base station (Alice) uses an embedded FPLA mechanism to authenticate the UE trying to access the network. The legitimate quantum UE ([PERSON]) and the malicious UEs ([PERSON]) are also shown. [PERSON] impersonates the identity of a legitimate quantum UE and attempts to access the network for malicious attacks. Meanwhile, the regular UEs are treated as malicious UEs in the FPLA process, in order to prevent them from accessing the quantum communication process. Assuming that [PERSON] and Bob are in fixed positions, although the locations of malicious UEs vary randomly, the distance between Eve and Bob is always greater than half a wavelength to ensure wireless physical layer independence. The system model proposed in this paper operates in a TDD scenario. Based on the SRSs transmitted by users, Alice estimates the channel conditions for each user. These channel estimates are then utilized to authenticate users and calculate the parameters of Physical Down-link Shared Channel (PDSCH) for legitimate users. This transmission method of this TDD system is commonly referred to as reciprocity-based transmission. Assuming that Alice has \(Na\) antenna ports and the maximum limit of antenna ports for UEs is \(P_{max}\), which is randomly generated in \(2^{i}\) (Assume \(Na=32\), \(P_{max}=4\) and \(i\in\{1,2,4\}\)). (The randomness is due to the inherent characteristics of different UEs, as they may not all support the same number of antennas. To avoid confusion, in this paper, CSI refers specifically to the Channel State Information obtained from channel estimation based on SRS transmitted by UEs in reciprocity-based transmission, and it records the channel estimation and noise. Alice will require the UE to use the same ports as used for transmitting SRS in subsequent transmission processes. The proposed scheme in this paper is applied during the UL transmission process of the UE. Alice obtains the channel estimation \(H_{i}\) from the CSI transmitted by the UE \(i\) as training samples or authentication basis. The mathematical description of the system model is given below. First, according to the model of a forward link for a multi-user system [27], the transmission model can be simply expressed as:  
\[\mathbf{y}=\sqrt{\rho_{i}}\mathbf{Hx}+\mathbf{N} \tag{1}\]  
where \(\mathbf{y}\) represents the received signal of Alice, \(\mathbf{y}\in\mathbb{C}^{N_{a}\times 1}\); \(\rho_{i}\) denotes the power level of SRS transmitted by the UE \(i\); \(\mathbf{H}\) is the channel response matrix, \(H\in\mathbb{C}^{N_{a}\times N_{b}}\); \(\mathbf{x}\) represents the signal transmitted from the antenna ports \(P_{i}\), \(\mathbf{x}\in\mathbb{C}^{N_{b}\times 1}\); and \(\mathbf{N}\) denotes the Additive White Gaussian Noise (AWGN), \(\mathbf{N}\in\mathbb{C}^{N_{a}\times 1}\), assumed to be independently and identically distributed with a covariance matrix \(N_{0}\mathbf{I}\). In addition, \(N_{rb}\) represents number of resource blocks and \(N_{a}\) represents number of [PERSON]'s antennas. Let vector \(\mathbf{N_{P}}(t)\) represents the number of ports of all UEs which are in the system at time \(t\), it can be expressed as:  
\[\mathbf{N_{P}}(t)=[N_{P_{1}},N_{P_{2}},\ldots,N_{P_{n}}]^{T} \tag{2}\]  
where \(\mathbf{N_{P}}\) denotes the number of antenna ports for device \(i\). It is important to note that the ZF (Zero Forcing) pre-coding method is employed in this model, thus the number of UE's SRS ports is equal to the number of its physical antenna ports.