# Interference Mitigation for Metro Train Location Based on Karhunen-Loeve Decomposition
## 1 Introduction  
Train positioning technology is the core on-board technology in train control systems, and is the basic guarantee for the safe operation of urban rail transit [1; 2; 3]. Traditional train positioning methods include track circuits, transponders, and cross loops. Track circuits can only achieve occlusion zone positioning [4]. The positioning of the transponder can realize the positioning in the point mode, and the real-time positioning of the train can be realized in combination with the inertial navigation system (INS) [5]. Cross-loop lines can realize real-real-time sub-meter-level positioning of trains, but the cost is relatively high [6]. [PERSON] et al. and [PERSON] et al. [7; 8] proposed a metro train positioning system based on the GSM network. By collecting the cell parameters of the train location, and using the received power of the base station signal to estimate the distance between the train and the base station, the positioning of the train can be achieved, but the positioning accuracy is affected by the propagation environment. [PERSON] et al. [9] introduced a train positioning method composed of a Beidou satellite navigation system and INS, and achieved a positioning accuracy higher than five meters in the simulation environment. Compared with traditional positioning methods, the positioning method based on sensor fusion improves the positioning accuracy, but cannot achieve sub-meter-level positioning [10; 11]. In addition to the wide coverage of the communication capability, the mobile communication network also has a positioning function. The 3 GPP TR 22.872 standard requires the positioning accuracy of transportation. Compared with the traditional train positioning method, the mobile communication network based on 5G has the advantage of convenient maintenance without adding positioning equipment. [PERSON] in [12] proposed a network-side positioning method for high-speed trains in the 5G NR scenario, using the time difference of arrival observed by the 5G uplink detection reference signal and the extended Kalman filter algorithm to estimate the position and speed of the train at the same time, and carried outa 100 km simulation experiment in the simulation scenario meeting the requirements of the 3 GPP Release 15 standard, achieving sub-meter-level positioning in 90% of cases. We propose a communication and positioning fusion system based on a 5G mobile communication network to achieve high-precision positioning based on Time Difference of Arrival (TDOA) [13; 14]. The communication positioning integrated signal (CPIS) is based on the NOMA principle, in which the positioning signal is superimposed on the communication signal and broadcast in the form of low power. CPIS has the following characteristics: (1) The positioning signal does not interfere with the communication. i.e., The positioning signal reuses the communication time-frequency resources and does not affect the quality of service (QoS) of the communication; (2) The positioning signal power of different users can be configured, i.e., the interference from users can be reduced by configuring different positioning signal power; (3) The positioning signal is continuous, i.e., the positioning signal is based on Direct Sequence Spread Spectrum-Code Division Multiple Access (DSSS-CDMA), and the positioning terminal tracks continuous signals to achieve the sub-meter level. This technology of superimposing positioning and communication signals has been popularized and used in shopping malls and airports, and has achieved high-precision wide-area indoor and outdoor positioning [13]. We verified the application of communication and navigation fusion positioning technology in subway positioning based on a leaky coaxial cable, and the laboratory has verified that the positioning method based on time difference of arrival can achieve sub-meter positioning accuracy [14].