2005 IEEE Intemational Symposium on Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications Proceedings

IMPROVED CHANNEL ESTIMATION ALGORITHM FOR OFDM OVER LEOCHANNELS

FanglingPU , Jianya GONG*b, Liangcai GAN a

aSchool of Electronic Information, Wuhan University, 129 Luoyu Road, Wuhan, 430079, P.R.China.Email: flpu67@yahoo.com.cn

bNational Laboratory for Information Engineering in Surveying, Mapping and remote Sensing, WuhanUniversity, 129 Luoyu Road, Wuhan, 430079, P.R.China.

ABSTRACT for high rate transmission due to its favorableproperties like high spectral efficiency and

For the study of OFDM (Orthogonal Frequency robustness to channel fading. But some distortions,Division Multiplexing) system for LEO (Low Earth such as multipath delay, canier frequency offset, fastOrbits) satellite communication, a channel estimation fading and Doppler frequency, cause thealgorithm by Kalman filtering based on comb-type orthogonality loss and introduce inter canerpilot subcariers is proposed in thisqaper. Due to the interference (ICI), consequently, affect thestatistical characteristics ofLEO channel, an OFDM performance of an OFDM system. Inserting a guardsystem model on the frequency domain is derived interval can eliminate the inter symbol interferenceand the correlation of the process noises in Kalman (ISI) caused by time invanrant multipath delays.filtering between two consecutive identical signals at However, a dynamic estimation of channel isthe same pilot subcarrier are modeled as the first necessary before the demodulation ofOFDM signalsMarkov chain. The performance of the proposed for LEO channel with both time valying andmethod is compared with LS (Least Square) channel frequency selective for wideband communication [1].estimator by measuring Bit Error Rate (BER) with The channel estimation can be performed by eitherQPSK, BPSK, 8-PSK and 16QAM as modulation inserting pilot tones into all of the subcarriers ofschemes. The simulation results illustrate the OFDM symbols with a specific period or insertingeffectiveness of the proposed algorithm. pilot tones into each OFDM symbol [4][5] as shown

in Figure 1. The first one, block type pilot estimation,Keywords: LEO, channel esfimation, Kalman, has been developed under the assumption of slowOFDM. fading channel. The later, the comb-type pilot

1. INTRODUCTION channel estimation, has been introduced to channelequalizing when the channel changes in one OFDM

The large majority of remote sensing satellites block.operational since 1991 and most of those planned In this paper, an estimation and tracking approachthrough 2005 are under 3500kg and will be deployed by Kalman filtering for the fast fading LEO satelliteto Low Earth Orbits (LEO), mostly at 400 to 1500 channel is proposed. This method is under thekm altitudes [3]. The advantages result from that, assumption tat time and frequency synchronizationfom LEO, remote sensors will look down to provide are not required. The paper is organized as follows.more earth detail than can be obtained from The OFDM system model for LEO channel isgeostationary earth orbit (GEO) satellite. The introduced in Section 2. Section 3 presents andquantity of observation data by one LEO satellite has analyzes the proposed channel estimation scheme.been highly increased. Therefore, we show special Section 4 provides the simulation result of theinterests in the high rate data downlink from LEO proposed method. Section 5 concludes he paper.remote sensing satellite to the ground station. 2. SYSTEM MODELOFDM (Orthogonal Frequency Division

Multiplexing) is known to be a promising technique

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The LEO satellite communication channels cause Yk* = Hk Xk,m + k,m (significant fading [1]. The transmitted signal is both kmLkm m(Doppler and delay spread by the channel. Due to the where okA-m = Crk, +WkMm and Hk,m = EHoj,ey2'E1relatively high altitude, the satellite moves much 1=0faster than any motion of observer on the earth, and Assuming that the LEO satellite channel is atherefore it exhibits large Doppler shift [2]. At carrier Gaussian wide-sense stationary uncorrelatedfrequency of 2GHz, this Doppler shift can be as high scattering (GWSSUS) channel with uniformlyas 40kHz. The delay spread typically range from 250 distributed angle of arrval [1], according to theto 800 ns [2]. This range depends on whether there Jake's model [6], the autocorrelation function of theare line-of-sight (LOS) or non-LOS (NLOS) i-th channel fading coefficients can be given byconditions between the LEO satellite and the earth E h { ih(fl>)O(2dS) =I (observer. When a LOS path exists, the received (n,i) m 1,(n,j)J i0 (signal amplitude is Ricean distribution, while in the ycase of NLOS, the received signal amplitude follows where n is the n-th subcarrier of m-th data symbol, ia Rayleigh distribution. Here we consider the bad and j are paths. Th is the power of the channelstate, Rayleigh fading process.Assumt,R ling thatinthoesmultipath fadingchcoefficients, fd is the maximum Doppler frequency,Assuming that the multipat fading channelconsists of L resolvable paths, the impulse response T, is the OFDM symbol interval, and J0 (e) is theof the channel at time n can be modeled as a zero-th order Bessel function of the first kind.sequence ofL discrete time taps. Assuming that the energy of channel impulse

hk=[hn , hn,X , h ll (1) response is normalized,In OFDM system, when a block of M OFDM L1 (7symbols are transmitted, the output m-th OFDM On )symbol at the n-th time is given by the N point somodulation sequence: E(HkmHMl)= Jo(2nfdTsj) (8)

xn=1/..TNmZXkeJ2=kfN (2) can be gained. Under the assuming that channelk=O impulse response does not change within one OFDM

Xk is the k-th data of m-th symbol. The data symbol duration [7], the fast fading is characterized

y,, arriving at the receiver can be expressed as by fd ' =0.1 and slow fading is characterizedL-1 by fdTS= 0.01 or more less. Thus, fading factor

Yn = Zkxn-i + wn (3) J0(2nQd T), which changes from 0.9 to 1, can be

w, is an additive white Gaussian noise with zeros chosen to be close to 1.mean and variance ~92We define equispacedly inserted Np pilot signals,

mean and valiance cr

When ISI is not considered, the demultiplexed and assume kp,m kp =k0, k,, kN,I as thedata Yk can be obtained by means of the N point frequency response of channel at pilot subcarriersdiscrete Fourier transform (DFT) demodulation. shown in Figure 1. (b) [8]. According to (5) and (8),

L-1 the system model defined in the frequency domain atYk = XN'H0)e-j2vdkN +ak +Wk (4) pilot subcarriers can be expressed as

1=0 Hk m = Hkp,m l + Vkp,m (9)

where Hk= 1/ nJ2nkIN YkPm= XkpMHkpm + Cokp,m (10)n=O where v and C are Gaussian noise with zeros

=zk-v X Hk e-J2yd1pN, and Wk = DF1T(w k,m p,mr0-= PPHkkpn mean and variance Qk mand R respectively.pOk

ak is the ICI. The m-th received symbol can be 3. CHANNEL ESTIMATIONsimply expressed in the frequency domain as

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3.1 Channel estimation and track A A)r- + HkmThrough the conventional Kalman filter described Hko S s (21)

in [9], when the pilot frequency observations m =Hk,i+4!Tkm-Hkm) .<S

are sampled from Yekm ' the channel estimate at pilotscan be obtained by a set of recursions: 4. SIMULATION RESULT

PkmPM=k,,m-l1 k,,m(I1Pkp,m = Skp,m-l + Qk*,m (11) The proposed method was evaluated by simulation.Gk,m - k,,m, k ,m +X2 (12) The parameters of the simulated OFDM system arek. PM kp.,tpkpPM PM k rnPkpPMchosen as follows.

kp m = - Xkp,mGkp,m )kp,m (13) The bandwidth ofsystem B = 10MHz.H,

H= m-1 + Gk, frp,m -Xk,mHkp,mI (14) Number of subcarniers N = 128.

In the recursions, Qk (E(V2 )) and FFTlength NEn. = 12812 k,m vp Subcarrierspacing fs =78.125kHz

R E\ )) are unknown to the receiver. It can FFT period Tfi = 12.8usbe assumed that channels at pilots to have equal Guard length TG = 3.2uspower.

Qkp,m =Qm kp =ko, k1,-*, kN -1 (15) Samplingrate t =O.lusRkm=Rk =k k k Symbolinterval T =16usRk*,mv0'pO 1' ' Np- (16) The modulation for data in OFDM system are

Under the condition that two consecutive pilot BPSK, QPSK, 8-PSK, and 16-QAM respectively.signals at one subcarrier are identical, Qmn and Since the set guard length is four times of the

Rm are modeled as the first order Markov chain maximal delay spread (800 ns), the 1S1 can beignored and the frequency nonselectivity of eachrespectively. subcarrier for the considered OFDM system is

Qm = Qmi ++71m (17) guaranteed. Rayleigh fading channel was simulatedRm = Rmnl + Ym (18) by Jake's model [7], and a mutli-path channel model

consists of 6 paths. The received signals are77 and ym are zero mean Gaussian random variable, otmntdb h aditv WieGusaNoem m ............ ~~~~~contaminated by the additive White Gaussian NoiseBy (9) and (10), we estimate Qm and Rm as follows. (WGN) as well as.

Qrn = abs [Var(p-Ik j>- Var(p~rnI (19)t" To test the proposed method, the channels at pilotsQm = abs Ykp,m-l Varfkp,m-2 (I,n(20) are also estimated by LS estimator [5], thenRm= abs V ,-Ha7(t-| (20) interpolation for data subcarriers are the same as the

Var(IOI) means the variance of..sequence 101 andproposed method. Figure 2. (a) and (b) give the BERVar>means the vanance of sequence b and .(bit error rate) of channel estimation over 10,000

abs[e] denotes the absolute value. By (19) and (20), random OFDM frames.

QM and Rm are estimated by the previous variance -5. CONCLUSIONof channel and previous received data.3.2 Channel interpolation Based on the analysis of the characteristics ofThe comb-type pilot channel estimation is LEO fading channel, an OFDM system model on the

composed of algorithms to estimate the channel at frequency domain at comb-type pilots for channelpilot frequency and to interpolate the channel of data. estimation is derived. According to the denivedIn this paper, the piecewise linear interpolation system model and Kalman filtering, when twomethod is adopted due to its simplicity [10]. For data consecutive identical signals aretransmitted at thesubcarrier k, pS < k < (p + 1)S, each S = N/Np same pilot subcarrier, the correlation of the processthe estimator by piecewise-linear interpolation .......noises in Kalman filtering recursions can be modeledmethod is given by .......asthe first Markov chain. The variances of the noises

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are estimated using the previous variance estimationof channel and the previous received data The , c c00oo0 oo 00 OOO

simulation result shows that the proposed channel oo--22-o-oo :, 2:2 o:estimation algorithm outperforms the LS estimator o12:2o4 Of

under our experiment condition, although increases a 0o-0 000- 00 00 000cc0--coo--cc @0 .000- 00bit of computation complexity.

6. REFERENCES (a) Block type pilot (b) Comb type pilot[1] Linda M. Davis, Iain B.Collinga and Robin J. Evans, arrangement arrangement"Estimation of LEO Satellite Channels", International Figure 1. Pilot arrangementConference on Information, Communications and SignalsProcessing ICICS'97, Singapore, pp. 15-19, 9-12 Sept. Fading::ff=O.1 LSforOPSK1997. LS for BPSK[2] A. Papathanassiou, A. K. Salkintzis, and P. T. fproposed method fr BPSKMathlopoulos, "A comparison study of the uplink S. p fo rm htPSKperformance of W-CDMA and OFDM for mobile .. proposed method for 1QAMmultimedia communications via LEO satellite", IEEE 10

Personal Communications, pp. 35-43, June 2001. . -.=[3] http://www.hq.nasa.gov/webaccess/CommSpaceTrans 10/SpaceCommTransSec3/CommSpacTransSec3.htmlnI[4] J. J. van de Beek, 0. Edfors, M. Sandell, S. K. Wilson, 10 " -- $and P. 0. Borjesson, "On channel estimation in OFDM +

systems", in Proc. IEEE 45h Vehicular Technology Conf., 105Chicago, IL, Jul. 1995, pp. 815-819.[5] M. Hsieh, C. Wei, "Channel estimation for OFDM io%

' ' O~~~~~~~~~~~~~~~~~~5tO15 a X 3 5 4systems based on comb-type pilot arrangement in SNR(EsJNo dB)frequency selective fading channels", IEEE Tran. On (a)BERpaffanceoffirRaykhfxigdnueleslimainConsumer Electronics, vol. 44, no. 1, Feb. 1998, pp. 217- Fad10em fr=001 -O LS orQPSK

10' - proposed method for OPSK225. if- LS for8PSK

.proposed method for BPSK([6] W. C. Jakes, "Microwave mobile communications", _f sPSKJohn Wiley& Son, New York, 1974. proposrnmthodfr6PSK

--LSfotr1SAM[7] L. Wan, V. K. Dubey, "Performance of frequency and - w- proposed method for 160AMfime domain coded OFDM over fast fading LEO lo 0tschannels", IEEE EUROCOMM 2000, Information x eoSystems for Enhanced Public Safety and Security, pp. , 10' N179-183, 2000.[8] X. Cai, G. B. Giannakis, "Error probability lo' . Nminimizing pilots for OFDM with M-PSK modulation . -over Rayleigh-fading channels", IEEE Tran. Vehicular to4 iiTechnology, vol. 53, no. 1, pp. 146-155, Jan. 2004.[9] C. K. Chui, G. Chen, "Kalman Filtering and Real I LTime Applications", Berling: Springer, 1999. SNR(Es/No: dB)[10] J. Rinne and M. Renfors, "Pilot spacing in orthogonal (b)BERpesfcmaxeofsowRf ghfxingdmiel.mimnfrequency division multiplexing systems on practical Figure 2 BER performance of channel estimationchannels", IEEE Trans. Consumer Electronics, vol. 42, no.4, pp. 959-962, Nov. 1996.

7. ACKNOWLEDGMENTThis work was supported by the National Key Basic

Research and Development Program (973) (No.2003CB415205)

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