A Novel Carrier Loop Algorithm Based on Maximum Likelihood Estimation (MLE) and Kalman Filter (KF) for Weak TC-OFDM Signals

Abstract

Digital broadcasting signals represent a promising positioning signal for indoors applications. A novel positioning technology named Time & Code Division-Orthogonal Frequency Division Multiplexing (TC-OFDM) is mainly discussed in this paper, which is based on China mobile multimedia broadcasting (CMMB). Signal strength is an important factor that affects the carrier loop performance of the TC-OFDM receiver. In the case of weak TC-OFDM signals, the current carrier loop algorithm has large residual carrier errors, which limit the tracking sensitivity of the existing carrier loop in complex indoor environments. This paper proposes a novel carrier loop algorithm based on Maximum Likelihood Estimation (MLE) and Kalman Filter (KF) to solve the above problem. The discriminator of the current carrier loop is replaced by the MLE discriminator function in the proposed algorithm. The Levenberg-Marquardt (LM) algorithm is utilized to obtain the MLE cost function consisting of signal amplitude, residual carrier frequency and carrier phase, and the MLE discriminator function is derived from the corresponding MLE cost function. The KF is used to smooth the MLE discriminator function results, which takes the carrier phase estimation, the angular frequency estimation and the angular frequency rate as the state vector. Theoretical analysis and simulation results show that the proposed algorithm can improve the tracking sensitivity of the TC-OFDM receiver by taking full advantage of the characteristics of the carrier loop parameters. Compared with the current carrier loop algorithms, the tracking sensitivity is effectively improved by 2-4 dB, and the better performance of the proposed algorithm is verified in the real environment.

A Novel Carrier Loop Based on Adaptive LM-QN Method in GNSS Receivers

“…This LM method is widely used in relative researches [10][19] [20][21] [22]. But as we can see, the Hessian matrix is extremely complicated and brings about lots of computation cost.…”

“…Besides, Zhang proposed an LM-KF method in which a Kalman filter is used to reduce the errors of MLE [21]. This LM based on Newton's method is also utilized further in the carrier loop for weak TC-OFDM signals [22]. In these researches, iteration is now the most common approach to finding the solution of MLE for the estimation of Doppler frequency, because the parameters cannot be solved analytically by conducting the differential of cost function.…”

“…Among the iteration methods for solving MLE and finding the Doppler estimation, LM is one of the most popular and effective one [23]. But for state of art, the LM method used in references are all based on Newton's frame which linearizes cost function directly [10] [19] [20][21] [22], but not taking full advantage of the NLS form of cost function. The problem comes with this direct linearization is the extremely complicated Hessian matrix.…”