JOURNAL OF TELECOMMUNICATIONS, VOLUME 28, ISSUE 1, NOVEMBER 2014 1

Wide range CFO estimation for TDS-OFDM Hamada Esmaiel and Danchi Jiang

Abstract An improved carrier frequency offset (CFO) estimation scheme is proposed in this short paper, for time domain synchronization orthogonal frequency division multiplexing (TDS-OFDM) system over multi-path fading channel with significant tap delay. Equalized processing method is proposed based on the frequency-domain pseudorandom noise (FDPN) of TDS-OFDM to estimate the carrier frequency offset. Time reversal is proposed to improve the estimation accuracy.

Index TermsCFO, OFDM, underwater acoustic communication, time reversal.

u

1 INTRODUCTIONDS-OFDM is the based technique for digital televi-

sion terrestrial broadcasting (DTMB), proposed for spectral and energy efficiency improvement, where

no pilots signal is used as in others OFDM scheme such as cyclic prefix OFDM (CP-OFDM) and zero padding OFDM (ZP-OFDM) [1]. TDS-OFDM as any other OFDM scheme is very sensitive to carrier frequency offset (CFO). CFO severity is destroyed orthogonally among OFDM sub-carrier, bringing on inter-carrier interference (ICI). Accu-rate estimation of CFO is one of the most important tech-nical challenges for OFDM transmission [2, 3].

Frequency-domain pseudorandom noise (FDPN) se-quence is adapted to TDS-OFDM system as a training sequence for channel estimation as well as frame syn-chronization. Many literatures have focused on the CFO estimation TDS-OFDM system [3]. CFO normally esti-mated based on the received PN sequence and local PN sequence cross-correlation. CFO based on the cross corre-lation only works well under a small CFO range [3].

Time reversal (TR) technique is a promising technique for signal and multicarrier modulation. Thanks to its capa-bility for multi-path fading channel focussing. In the same line, TR service on equalizer simplification and multiuser interference reduction [4].

In this short paper, the equalized processing method [5] is proposed based on the TDS-OFDM training sequence for wide range CFO estimation over multipath channel. The time reversal is suggested to reduce the multi-path channel effect and improve the CFO estimation accuracy.

2 SIGNAL MODEL The TDS-OFDM signal frame can be expressed as:

( )( )

( ) ( ),1,,,1

1,,1,0,1

21

0

1

0

2

+==

==

=

=

=

MNNnencM

nb

NneXN

nans

MknjM

k

N

k

Nknjk

(1)

where, N is inverse fast Fourier transform (IFFT) size, kX is the modulated data block in the TDS-OFDM k-th

sub-carrier and ( )nc is the PN sequence with M length. The received OFDM frame over multi-path channel given by: ( ) ( ) ( ) ( ),2 nwnvenynr Nnj ++= (2)

where, the ( )nv is interlock interference (IBI) between the TDS-OFDM data blocks and ( )nw is the additive white Gaussian noise. is the normalized carrier fre-quency offset (CFO). ( )ny the signal component in (2) is given by:

( ) ( )

=

=1

0,

L

kk knshny (3)

kh is the k-th channel impulse response.

3 ESTIMATION METHOD The envelop equalized processing (EEP) factor method is proposed for wide range of CFO estimation for OFDM system using a training sequence [5]. In this paper, the FDPN sequence of the TDS-OFDM is used as a training sequence for CFO estimation. The EEP factor nf defined as [5]: ( ) ( )

( ),2

*

nbnbnf = (4)

where * denote the complex conjugate and . is the Euclidean norm. At the receiver side, the received FDPN TDS-OFDM after equalized by the EEP factor over

1,, += MNNn is represented as: ( ) ( ) ( )

( ) ( ) ( ) ( ) ( ) ( )

( ) ( ) ( ),

,

20

1

1

220

nwnfenbh

nfnwnveknbhnfenbh

nfnrnd

Nnj

L

k

Nnjk

Nnj

+=

+++=

=

=

(5)

where,

( ) ( ) ( ) ( ) ( ) ( ) ( ).1

1

220 nfnwnveknbhnfenbhnw

L

k

Nnjk

Nnj

+++=

=

The normalized CFO to the sub-carrier spacing is given by:

,FI += (6) Based on the EEP estimation algorithm [5] the CFO es-

timation is based on a three steps. First, based on a peri-odogram of the received TDS-OFDM FDPN sequence

Hamada Esmaiel and Danchi Jiang are with School of Engineering and

ICT, University of Tasmania, Hobart, Australia.

T

2

symbol the integer frequency offset (IFO) can be estimat-ed.

( ) ( ){ },1maxarg ++= kkfI fIfIk (7)

where

+ 12

.,,.........12

,2

MMMfk and ( )kfI is the

( )nd periodogram:

( ) ( ) .21

0

2

=

=M

i

Mifjk

keidfI (8)

Secondly, the fractional frequency offset (FFO) based on I value is computed.

( )( ) ( )

.1

1++

+=

II

IF II

I

(9)

In the third step, the residual frequency error estima-tion between the true CFO and the sum of I and F is estimated for more accuracy by:

( ) ( )( ) ( )

.5.05.05.05.0

21

++++

+++=

FIFI

FIFIR

IIII

(10)

Then, the estimated CFO is obtained by . RFI ++= (11)

This algorithm is working perfect in case of not signifi-cant multipath channels.

4 MULTIPATH FADING CHANNELS WITH SIGNIFICANT TAP DELAYS

Algorithm is used eq. (5) for CFO estimation; the algo-rithm can use and estimate CFO correctly in additive white Gaussian noise (AWGN) channels and multipath fading channels when the multipath fading is insignifi-cant. But in significant multipath channel the algorithm cant estimate the CFO perfectly where eq. (5) cant con-sider multipath signal as noise.

Time reversal is a feedback wave focusing technique that can be used to transparently compensate for multi-path dispersion in digital communications over several types of physical propagation media, such as radio or acoustic channels [6]. In TR process, a known probe signal transmitted to receiver used in channel estimation, and retransmitted at receiver. This technique is used in chan-nel focussing where a matched filter based on estimated channel impulse response is used. Thus, signal focusing can be achieved at the transmitter if the channel does not change significantly [7]. Here we consider a passive time reversal system in the long multipath fading channels such as underwater channels, which consists of a single input single output system. Time reversal technique is used to convert the UWA channel to be an impulse re-sponse channel [8-10].

5 SIMULATION RESULTS The proposed method performance was evaluated and tested by simulations based on two different types of channels. One is the simulated channel where one chan-nel taps are generated based on a multipath fading chan-

nel with maximum channel tap delay L= 32. A measured UWA is the other channel. The UWA channel is adopted from an experimental data collection in the ASCOT01 [1, 11].

(a)

(b) Fig. 1. Average CFO estimate versus normalized CFO. (a) Simulated Channel; (b) UWA channel.

Experiment is conducted off the coast of New England in June 2001. UWA channel is reported and truncated to have a channel order L=128. TDS-OFDM subcarrier num-ber 512=N is used with FDPN sequence length 32=M and 128=M in the simulated and UWA channels respec-tively as reported in [1, 12].

As shown in Fig. 1, the CFO estimation based on the proposed method estimate the CFO with wide range over the differential correlation method [13]. Time reversal technique with multi-path channel focusing improves the CFO estimation accuracy in the equalized processing method as well as CFO estimation based on the differen-tial correlation method.

6 CONCLUSION In this paper, a new frequency offset estimation meth-od for TDS-OFDM system is proposed. The FDPN se-quence of the TDS-OFDM is used based on an enve-

3

lope equalized processing method. The proposed method improves the CFO of the TDS-OFDM estima-tion range. Time reversal technique is proposed to re-duce the multipath delay spread effect on the accuracy of CFO estimation. Time reversal provides significant improvement in the CFO estimated accuracy in the proposed and differential correlation method. Pro-posed method is carefully tested using simulated channels as well as real channels measured from one sea-going experiment.

REFERENCES [1] H. Esmaiel and D. Jiang, "Zero-pseudorandom noise training

OFDM," Electronics Letters, vol. 50, pp. 650-652, 2014. [2] H. Esmaiel and D. Jiang, "OFDM Inter-Carrier Interference

Reduction Using Pulse Shaping Function for Underwater Acoustic Communications Systems," in IEEE Second International Japan-Egypt Conference on Electronics, Communications and Computers, Cairo, 2013.

[3] W. Jianming, Y. Chen, Z. Xiaoyang, and M. Hao, "Robust Timing and Frequency Synchronization Scheme for DTMB System," IEEE Transactions on Consumer Electronics, vol. 53, pp. 1348-1352, 2007.

[4] N. Hung Tuan, J. B. Andersen, and G. F. Pedersen, "The potential use of time reversal techniques in multiple element antenna systems," IEEE Communications Letters, vol. 9, pp. 40-42, 2005.

[5] G. Ren, Y. Chang, H. Zhang, and H. Zhang, "An efficient frequency offset estimation method with a large range for wireless OFDM systems," IEEE Transactions on Vehicular Technology, vol. 56, pp. 1892-1895, 2007.

[6] J. Gomes and V. Barroso, "Time-reversed OFDM communication in underwater channels," in IEEE 5th Workshop on Signal Processing Advances in Wireless Communications, 2004, pp. 626-630.

[7] S. Aijun, M. Badiey, A. E. Newhall, J. F. Lynch, H. A. DeFerrari, and B. G. Katsnelson, "Passive Time Reversal Acoustic Communications Through Shallow-Water Internal Waves," IEEE Journal of Oceanic Engineering, vol. 35, pp. 756-765, 2010.

[8] L. Zhiqiang and T. C. Yang, "On the Design of Cyclic Prefix Length for Time-Reversed OFDM," IEEE Transactions on Wireless Communications, vol. 11, pp. 3723-3733, 2012.

[9] L. Zhiqiang and T. C. Yang, "Time reversal multicarrier communications over long multipath fading channels," in Military Communications Conference, 2012, pp. 1-6.

[10] Z. Liu and T. C. Yang, "On Overhead Reduction in Time-Reversed OFDM Underwater Acoustic Communications," IEEE Journal of Oceanic Engineering, vol. PP, pp. 1-13, 2013.

[11] T. C. Yang, "Temporal resolutions of time-reversal and passive-phase conjugation for underwater acoustic communications," IEEE Journal of Oceanic Engineering, vol. 28, pp. 229-245, 2003.

[12] H. Esmaiel and D. Jiang, "Time reversal time-domain synchronisation orthogonal frequency division multiplexing over multipath fading channels with significant tap delays," The Journal of Engineering, vol. 1, 2014.

[13] H. Lifeng, Y. Fang, Z. Chao, and W. Zhaocheng, "Synchronization for TDS-OFDM over multipath fading

channels," IEEE Transactions on Consumer Electronics, vol. 56, pp. 2141-2147, 2010.

Hamada Esmaiel received the B.S. degree, and

the M.S. degree in Electrical Engineering from South Valley University, Aswan, Egypt, in 2005, 2010 respectively, where he is currently working towards the Ph.D. degree, School of Engineering, University of Tasmania, Hobart,

Australia. During 2007-2010 he was a Demonstrator with South Valley University. In 2011 he was a Research Engineer in Wire-less Communication Lab. Wonkwang University, Iksan, South Korea. In 2011, he was Assistant Research & Lecturer with As-wan University, Aswan, Egypt. His current research focusses on communications theory and signal processing, with an em-phasis on multi-carrier for underwater communication systems, channel estimation, and iterative processing.

Dr. Danchi Jiang is a senior lecturer in tele-communication engineering and chair of computer systems engineering stream at the School of Engi-neering, University of Tasmania. He has obtained his PhD in Systems Engineering from Australian National University and conducted research in the area of intelligent system and telecommunication at

Chinese University and Hong Kong and National ICT Australia (NIC-TA). His main research interests are advanced multidimensional signal processing, intelligent system learning and control with appli-cations in telecommunication, biomedical engineering and micro-grid power systems, and dynamical systems on manifold and recurrent neural networks for intelligent optimization.