Frequency Synchronization for OFDM/OFDMA Systems

Introduction

OFDM has, with substantial progress in digital signal processing, become an important part of the telecommunications arena. This is because its ability to transform a wideband frequency-selective channel to a set of parallel flat-fading narrowband channels, which substantially simplifies the channel equalization problem. Several existing standards, among them WiFi 802.11g/n (802.11n, 2009), WiMAX 802.16d/e (802.16d, 2004; 802.16e, 2005) or DVB-T (DVB-T, 2004), are based on the OFDM concept. This chapter will focus on these standards when explaining the frequency synchronization process.

CFO synchronization turns out to be a critical issue for multicarrier systems. This CFO can appear because of two reasons: due to an offset between the frequencies of the local oscillator of the transmitter and the receiver or due to the Doppler effect. The CFO causes loss of orthogonality of the multiplexed signals that produces an inter-carrier interference (ICI) noise, an amplitude reduction and, also, a constant increment in the phase of the samples. Therefore a frequency synchronization process is necessary to overcome this sensitivity to frequency offset.

Frequency synchronization is often performed in two phases: acquisition and tracking. At the start of the sequence the acquisition stage is used to perform a first estimation of the CFO of the signal (Beek, 1997; Schmidl, 1997; Speth, 2001; Moose, 1994). In a circuit-switched system the acquisition phase can be fairly long since it only represents a small percentage of the total transmitted sequence. Some systems like DVB-T and cellular systems are circuit-switched. In packet-switched systems, as 802.16d and 802.11g/n, the acquisition phase is more important since the transmission sequences are short. The most common approach in such systems is to use a preamble for acquisition. As it will be shown, the acquisition stage is a well defined task that can be easily adapted to all standards being considered. Most of the CFO is usually corrected at the acquisition stage, but there still remains the problem with the residual ICI and the increasing phase of the samples.

After acquisition, the tracking problem has to be solved. Since acquisition is never performed perfectly and real world conditions are not static, there still remains a RFO. If the RFO is not tracked and corrected, oscillator instabilities and/or Doppler effect can make the CFO estimation from the acquisition stage to become obsolete, thus increasing the residual ICI and decreasing the receiver performance. The constellation points will also suffer a rotation due to this RFO and will eventually cross the decisor boundaries, thus causing the system performance to become unacceptable. This rotation affects mostly to OFDM packet based systems as 802.11g/n or 802.16d/e. This is because, for these standards, the channel estimation is only performed at the beginning of the frame since the packet length is short enough to assume a constant channel during the length of the packet. Therefore, channel estimation can not correct the constant rotation produced by the CFO. In addition, the residual ICI will also affect the performance of the systems. It is mandatory to correct this impairment for all the technologies that use OFDM.

The tracking stage can be non data-aided (Kuang, 2005), when no extra information is included in the transmitted data (as in decision-directed methods) or data-aided (Speth, 2001; González-Bayón, 2007), when periodically transmitted training symbols and/or known pilot subcarriers are used.