A Novel Carrier Loop Algorithm Based on Maximum Likelihood Estimation (MLE) and Kalman Filter (KF) for Weak TC-OFDM Signals  
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3 School of Electronic Engineering, Beijing University of Posts & Telecommunications, No. 10 Xitucheng Road, Haidian District, Beijing 100876, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.D. ); [EMAIL_ADDRESS] (J.J.); [EMAIL_ADDRESS] (B.J.) [PERSON]  
4 Correspondence: [EMAIL_ADDRESS]; Tel. : +86-010-6119-85094  
detection techniques. Soft range information-based localization can be found in [5] for high accuracy network localization. Meanwhile, precise localization is of great importance in many applications such as vehicular networks [6]. A state of the art vehicular communication system for the cooperative awareness of connected vehicles is proposed in [7]. Studies show that people spend more than 80% of their time in indoors environments [8]. With the increasing number of tall buildings, indoor location-based services have emerging applications in commercial applications and public safety. As the main positioning technology, GNSS has many significant advantages outdoors, such as a large coverage area, high positioning accuracy, and superior navigation performance [9]. However, buildings obstruct, reflect, and diffract the GNSS signals, the positioning performance is limited in indoor environments and cities. Recently, terrestrial radio positioning systems and their enhancements to the GNSS have drawn increasing attention, and the digital broadcasting signal is a promising positioning signal for indoors uses [10]. As for DVB-T/H signals, a different low-complexity solution to perform the coupling channel estimation in an on-channel repeater can be found in [11; 12; 13]. This paper mainly studies a novel terrestrial radio positioning system called Time & Code Division-Orthogonal Frequency Division Multiplexing (TC-OFDM), which is based on the China Mobile Multimedia Broadcasting (CMMB) system. The TC-OFDM system multiplexes the CMMB signal and pseudorange noise (PRN) codes in the same frequency band, and the positioning performance can be achieved by adding some simple modifications to the deployed CMMB facilities. The positioning part of the TC-OFDM system is a direct-sequence spread spectrum code division multiple access (DSSS-CDMA) system employing binary phase shift keying (BPSK) modulation, and [10] describes the TC-OFDM system in detail. Compared with the GNSS, the TC-OFDM system has many potential advantages: the TC-OFDM signal transmission power is stronger, the frequency band is the U band. The TC-OFDM receiver demodulates PRN codes for high accuracy positioning, which can overcome the positioning limitations caused by the CMMB single network coverage. Weak signal strength is the main factor that limits the performance of the receiver tracking loop to improve tracking sensitivity. Meanwhile, the indoor environment is more complex than outdoors and the signal strength is significantly attenuated, which restricts the tracking performance of the existing receivers for weak TC-OFDM signals. The main reason is that the accuracy of the residual carrier estimation by the existing carrier loop algorithm is not ideal under weak signal conditions. Since the carrier loop is the weakest link in the receiver tracking loop, the superior carrier loop performance can improve the tracking sensitivity of the receiver for weak TC-OFDM signals to a certain degree [14]. The carrier loop is usually divided into a frequency-locked loop (FLL) and phase-locked loop (PLL) [15; 16]. The FLL has good dynamic performance, but the carrier phase measurement accuracy is lower due to the wide noise bandwidth [17]. The PLL tracks the received signal more closely and has the high carrier phase measurement accuracy, but it has poor tolerance of dynamic stress [18]. Due to the complexity of the indoor environment, the received signal strength is greatly attenuated.