A Robust Frequency Tracking Loop at the eNodeB for LTE Uplink

Abstract

4G systems like LTE (Long Term Evolution) provide an uplink data rate up to 80 Mbps, utilize higher order modulation (64-QAM) and introduce space division multiple access (SDMA) to improve spectral efficiency. These features pose stringent frequency and time synchronization requirements for the modem to guarantee satisfactory performance. This paper presents a simple yet robust scheme to estimate the frequency offset on a per user-basis at the eNodeB. The algorithm proposed utilizes a non-coherent method to provide frequency error estimation, hence decoupled from the channel estimator. This estimation can be used by the eNodeB in a "open loop" fashion to correct the user's local oscillator frequency via downlink (DL) timing adjustment messages or in a "closed loop" scheme such as frequency tracking loop (FTL) to iteratively reduce the measurement error fluctuations. The performance of the proposed estimator have been investigated in open loop configuration through simulations and found within 5% of Cramer-Rao bound (CRB) in AWGN channel. For closed loop operation mode, the presented FTL exploits multipath diversity in time and space to mitigate the effects of fading, thus showing less than 0.3 dB of performance gap with respect to perfect frequency correction in TU-30 kmh channel. Analysis is presented for both open loop and closed loop to provide further insights in the algorithm and to allow parameter optimization study, thus facilitating the design process.