03-09-2012, 12:32 PM
OFDM Burst Frequency Synchronization by Single Carrier Training Data
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Abstract
In this letter, we propose a burst frequency synchronization
procedure which is based on the usage of single-carrier
training data and Orthogonal Frequency Division Multiplexing
(OFDM) payload modulation. The payload modulation format is
similar to the one that is used in the DAB standard [1], whereas
training data are chosen as simple CAZAC [2] sequences. It is
shown that the resulting modulation and transmission scheme is
suitable for burst transmission and single-burst demodulation.
Performance degradation due to synchronization errors is shown
to be small.
INTRODUCTION
IN THIS LETTER, we propose a burst format for Orthogonal
Frequency Division Multiplexing (OFDM) transmission
[3], [4] that allows burst frequency synchronization of individual
data bursts. We propose algorithms for frequency
synchronization that satisfy the requirements of high phase
stability of an OFDM system. The frequency synchronization
technique is based on the use of single-carrier training data in
contrast to the techniques in [5]–[7] which use only OFDM
symbols.
SIGNAL AND BURST MODEL
Fig. 1 displays the respective burst formats. Data bursts
consist of multiplexed single carrier training data and multicarrier
OFDM payload data (transferring encoded information
on the downlink) grouped in data blocks. The training data
consists of times repeated CAZAC (Constant-Amplitude-
Zero-Autocorrelation [2]) sequence of length
The choice of the CAZAC sequence is motivated by the
fact that it is also suitable for frame synchronization [8], and,
secondly, because its flat power spectrum equally weights all
contributions in the transmission bandwidth. Furthermore, the
periodic structure of the training sequence is needed for the
frequency synchronization algorithms below.
COMPARISON AND CONCLUSION
In [5], two full OFDM symbols are used to achieve frequency
and frame synchronization leading to a larger training
overhead. The same overhead is implemented in [1]. In [7],
which treats continous transmission only, no coarse acquisition
is considered and the algorithms have a long acquisition time.
The same holds for the algorithms devised in [6].
Comparing our approach to the ones mentioned above,
fewer training symbols are needed and both spontaneous
coarse acquisition and fine frequency correction is provided.
The proposed synchronization algorithms come close to
the theoretical optimum and provide a means to devise low
complexity per-burst demodulation of digital data in wireless
networks.