Implementation of acquisition & tracking algorithms of spread spectrum signals in high dynamic scenario

Sandip Sudani., Dhaval Upadhyay and Pravin Patidar

This paper presents the Acquisition and tracking algorithms of a CDMA (Code Division Multiple Access) receiver. A CDMA receiver, which is placed on a satellite, experiences high dynamic scenario. High dynamic scenario means, where the doppler shift may vary in the range of ±60kHz and Doppler rate may be up-to ±66Hz/s and there can be jerk of 10g/s due to orbital and attitude control systems. This paper deals with the implementation of algorithms to acquire and track signals under high dynamics at C/No (carrier to noise density ratio) of the order of 40dBHz. In order to decode the information from the received signal, Acquisition and Tracking algorithms must be used to synchronize the locally generated carrier and code with the incoming signal. Acquisition algorithm firstly detects the presence of visible satellite and then after, finds coarse values of Doppler shift and code phase delay, Paper presents FFT (Fast Fourier transform) based acquisition algorithm, because it is less time consuming than the serial search. These coarse values are then passed to the tracking module for fine tuning. There are two types of tracking loop: code tracking loop and carrier tracking loop. The paper presents DLL (Delay locked loop) with early minus late discriminator and 1st order filter for code tracking loop, and for carrier tracking loop initial frequency estimation is performed by 2nd order FLL (frequency locked loop) tracking loop, this value is passed to the 3rd order PLL tracking loop for finding exact carrier replica. Here first FLL reduces the locking time and allows PLL (Phase locked loop) to lock on dynamically changing phase of the signal with high accuracy. The proposed algorithms were implemented using MATLAB. The results show that the proposed algorithms improves the performance in high dynamic scenario.

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