ABSTRACT
If most academic studies on orthogonal frequency-division multiplexins (OFDM) often consider ideal synchronization, the reader must understand that synchronization is an important task and, as such, should not be undermined. solutions such as wireless local area networks (WLAN) were already available in the early 90’s, some 15 years were needed for the first mobile OFDM systems to appear. The difficulty in OFDM is to preserve the orthogonality between subcarriers when mobile terminals are in motion and thus are subject to Doppler frequency shifts. Besides, wireless networks encourage more and more packet-switched (e.g., IEEE 802.16 WiMax [4],Third Generation Partnership Project (3GPP)-Long Term Evolution (LTE) [5]) than connected (e.g., digital video broadcasting [6] and digital audio broadcasting [7]) transmission modes: the former has the strong advantage to be highly dynamic and has coped with its past latency problem. Future communication technologies will therefore rely on short data (i.e., packet) transmissions, compelling the synchronization recovery processes to operate very fast.∗ In most OFDM technologies, the synchronization phase consists first a power detection process, meant to roughly identify a power source. The next procedure is classically
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an acquisition phase aiming to give a rough estimate of the system synchronization parameters (e.g., slot start time, a reference frequency). First reliable data exchanges are in general possible at a low rate at this point. During the rest of the communication, especially in the connected mode, the synchronization processes enter the tracking phase to regularly update the parameter estimates. In the rest of this chapter, we concentrate on the following three main synchronization parameters:
The carrier frequency offset (CFO), which corresponds to a mismatch between the transmitter and the receiver frequency references. Even small values of CFO are detrimental to the system performance; as a consequence, frequency offsets must be efficiently corrected. Particularly, in a mobile system, CFO is fast varying due to Doppler shifts. It is thus very challenging for mobile OFDM designers to ensure a continuous quality of service in high mobility conditions.
