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Preamble-based SNR Estimation in Frequency Selective Channels for Wireless OFDM Systems
Synopsis
Introduction
There are two general categories of average SNR estimators. Data-aided (DA) estimators are based on either perfect or estimated knowledge of the transmitted data. However, a certain portion of data is needed for estimation purposes, which reduces bandwidth efficiency. Blind or in-service estimators derive SNR estimate from an unknown information-bearing portion of the received signal preserving efficiency at the cost of decreased performance. For packet based communications, block of information data is usually preceded by several training symbols (preambles) of known data used for synchronization and equalization purposes. Therefore, DA SNR estimators can utilize preambles without additional throughput reduction.
Back ground
OFDM is a multicarrier modulation scheme that provides strong robustness against intersymbol interference (ISI) by dividing the broadband channel into many narrowband sub- channels in such a way that attenuation across each subchannel stays flat. Orthogonalization of subchannels is performed with low complexity by using the fast Fourier transform (FFT). The serial high-rate data stream is converted into multiple parallel low-rate streams, each modulated on a different subcarrier.
Proposed methodology
We propose an efficient algorithm for the average SNR estimation in wireless OFDM systems. The SNR per subcarrier can be additionally estimated using channel estimates and the estimated average SNR. The proposed estimator utilizes preamble structure it allows synchronization over a wider frequency offset range with only one preamble, hence reducing the training symbol overhead. Since the proposed estimation algorithm relies on the signal samples at the output of the FFT, its performance depends strongly on the given preamble structure. The performance of the proposed algorithm is compared with the MMSE algorithm and two preamble-based algorithms .It is shown that the proposed algorithm is robust against frequency selectivity and may therefore be used for subchannel SNR

Preamble-based SNR Estimation in Frequency Selective Channels for Wireless OFDM System

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I. INTRODUCTION
OFDM is a multicarrier modulation scheme that provides
strong robustness against intersymbol interference (ISI) by
dividing the broadband channel into many narrowband subchannels
in such a way that attenuation across each subchannel
stays flat. Orthogonalization of subchannels is performed with
low complexity by using the fast Fourier transform (FFT). The
serial high-rate data stream is converted into multiple parallel
low-rate streams, each modulated on a different subcarrier.



V. SIMULATION RESULTS
The performance of PS estimator is evaluated and compared
with the performance of MMSE, Boumard’s and Ren’s estimator
using Monte-Carlo simulation. OFDM system parameters
used in the simulation are taken from WiMAX specifications
giving N = 256 subcarriers and cyclic prefix length of
32 samples [8].


CONCLUSION
In this paper, a novel preamble-based SNR estimator for
wireless OFDM systems has been proposed. Reuse of the
synchronization preamble for the SNR estimation purposes by
exploiting its time domain periodic structure puts no additional
overhead on transmitted OFDM frame. Increasing the number
of repeated parts by nulling the subcarriers on specified
positions improves the performance of considered estimator,
but also increases its sensitivity to frequency selectivity. Low
complexity and robustness to frequency selectivity combined
with the bandwidth efficiency favors the proposed estimator
compared to the considered preamble-based estimators given
in the literature.