Chapter 9
Frequency Synchronization in LTE
The Long-Term Evolution (LTE) standard1 employs an Orthogonal Frequency-Division Multiplexing (OFDM) physical layer in the Downlink (DL), making an accurate frequency synchronization become a crucial issue. The mismatch between the two local oscillators at the transmitter and the receiver introduces a Carrier Frequency Offset (CFO). This CFO destroys the orthogonality between subcarriers and degrades the system performance severely. While in simulations the CFO can readily be set to zero, this is never the case in reality. Owing to cost limitations, local oscillators at the User Equipment (UE) side have a typical frequency stability tolerance of ±10 ppm. Consider an oscillator for LTE at 2.5 GHz, ±10 ppm results in an offset of ±25 kHz. With the fixed subcarrier spacing of 15 kHz defined in LTE, a CFO of ±1.67 subcarrier spacing has to be handled at the receiver.
The literature on CFO estimation falls into two categories: data-aided methods and non-data-aided methods. Data-aided methods estimate the CFO by periodically transmitted reference symbols, where usually a repetitive pattern is required [10, 13, 15, 16, 20, 22]. These methods can be applied to burst transmission protocols such as IEEE 802.11 and Worldwide Inter-operability for Microwave Access (WiMAX). On the other hand, blind or non-data-aided methods [3, 4, 11, 12, 14, 25, 26, 28] do not rely on dedicated ...
