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.
Highlights
With the development of mobile Internet and mobile smart devices, location-based services have become the focus in many wireless network applications
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
After the receiver radio frequency (RF) front-end, down-conversion, low-pass filter and analog-digital conversion module (ADC), the digital intermediate frequency (IF) signal is expressed as: r IF (i) = A IF s(i)e j2π ( f IF + f d,i )nTs + φ0,i + ω (n) where Ts is the sampling duration, AIF is the IF signal amplitude, τ i is the incoming signal delay, fIF is the IF frequency, fd,i is the Doppler shift, φ0,i is the initial carrier phase, ω(n) is the additive Gaussian white noise (AWGN) with zero mean and variance σn 2
Summary
With the development of mobile Internet and mobile smart devices, location-based services have become the focus in many wireless network applications. 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 existing carrier loop performance needs to be improved for better performance in indoor weak signal environment. The EKF and UKF based on linear minimum mean square error estimation criterion represent a sub-optimal carrier loop scheme, which still produces a large parameter estimation error for weak received signals. Methods based on FLL-assisted PLL can ensure the dynamic of the carrier loop but the estimation accuracy is limited in a weak signal environment [24]. Reference [25] uses the MLE to adjust the Doppler frequency in a high dynamics receiver carrier loop, but it does not make a detailed analysis for weak received signals.
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