Decision-directed Channel Estimation Method for OFDM systems with high velocities

7/28/2019 Decision-directed Channel Estimation Method for OFDM systems with high velocities

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Decision-directed Channel Estimation M ethodfor OFDM systems with high velocities

J ianjun Ran, Rainer Griinheid, Hermann RohlingTechnical University Hamburg-HarburgDepartment of TelecommunicationsHamburg, Germanyb.ran, gruenheid, [email protected]}

Abstract-The preamble-based channel estimation schemestypically applied in both the HIPERLANIZ and IEEE 802.11asystems cannot fulfill the requirements in scenarios with highvelocities. Therefore, this paper deals with the Decision-DirectedEstimation (DDE) method for a blind channel estimation proce-dure. With the help of the output decision symbols from the de-modulator at the receiver side, the statistical characteristics of thetransmitted data via the noisy fading channel are applied fo rtracking the fast changing channel transfer function. As a result,DDE can improve the performance of OFDM systems in highvelocity applications with a very low computation complexity.

Keywordv-hnnnel estimation; OFDM; HI PE RUND; DDE

1. INTRODUCTIONChannel estimation procedures are in general of great im-portance for modem radio communication systems, since theyallow the receiver to approximate the channel transfer factorsfor processing steps such as adaptive equalization, diversitycombining, maximum likelihood detection, link adaptation andcoherent demodulation. There are basically two categories ofchannel estimation techniques: one is the training-sequencebased method that can be simply implemented at the cost oflower bandwidth efficiency; the other is the blind estimationtechnique, which utilizes certain underlying statistical proper-ties of the transmitted data.The most recent WLAN standards HIPERL ANR [I ] andIEEE 802.1 l a make useof the Orthogonal Frequency DivisionMultiplexing (OFDM ) transmission technique at the physicallayer. In this case, a pilot structure of inserting preambles forchannel estimation is periodically embedded at the beginningof a transmission f rame. The channel information is thenmeasured from the pilot sequence and applied within the re-maining frame for the equalization process for all the informa-tion symbols.However, the WL AN preamble structure works well onlyfor systems with slowly time-variant channels. When it comesto new scenarios with higher terminal velocities, it is necessaryto embed more frequent pilot signals within the whole frame totrack the time-variant behaviour in the radio channel. In [2],the pilot-based concept of mid-amble approach is proposed forthe application cases where the bandwidth efficiency and some

Edgar Bolinth, Ralf K ernSiemens AGICM MP P TI 2Bocholt, Germany{edgar.bolinth, [email protected])

system overhead are not so crucial. On the other hand, blindchannel estimation methods are more bandwidth efficient, butneed a slightly higher computation complexity. In [3], the lin-ear channel prediction information is also used for fadingchannel tracking with a decision directed form.In this paper, a semi-blind channel estimation technique i sproposed which is based on a decision-directed estimation(DDE) approach for high velocity scenarios, which tries tocombine the benefits from both the preamble pilots and statis-tic characteristics of the transmitted data from a finite modula-tion alphabet due to the noisy fading channels. Similarly asdescribed in [2], a recursive filter i s used to provide the reli-able feedback information at the output of the symbol demodu-lator. I n this way, with a low complexity, the channel transferfunction could be updated from symbol to symbol in the timedirection only with the help of the DDE.

11. SYSTEM DESCRIPTIONA. OF DM SysfemwithDDE

1 1j w M =J Ii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J

Figure I : Block diagramfar an OFDM system nith DDEA typical block diagram of an OFDM transmission systemis shown in Figure 1. A t the transmitter side, the input datasequence is first operated by a serial-to-parallel operation fol-

0-7803-7757-51031317W 02W 3 IEEE 2358


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lowed by a symbol modulator. After IFFT processing the se-quence is again transferred by a parallel-to-serial operation. Aguard inteival is inserted for each OFDM symbol in order toavoid InterA symbol-interference ( I S ) caused by multi-pathchannels. In the receiver, after FFT processing each subcarriersignal is fi rst equalized by a simple Zero-Forcing (ZF) [4 ]scheme to remove the channel distortion. Therefore the chan-nel transfer function is continuously estimated based on a DDEprocedure. In this case the detected symbols at the output ofthe demodulator are considered as an input sequence for theDDE processing.A guard interval of sufficient length is assumed that canavoid IS1 completely. The additive noise in the channel is as-sumed to have the characteristics o f a white Gaussian process.

LtOFDM Symbol

t t t

Updm by DnlrbnO l r r u e d EItlmlPm (OOEJFigure2 FrameDataSfmcture with DDE in OFDM Systems

B. F rame Structure withDDEThe frame data structure of DDE isshown in Figure 2. Af-ter the preamble, a number of L OFDM symbols are transmit-ted inside one frame. In addition to the typical purely pream-ble-based estimation used in HIPERLANR, a DDE is appliedduring the channel estimation procedure to provide an OFDMsymbol-wise update for the time-variant radio channels. Thetechnical details of a DDE procedure will be explained in sec-

tion 111I f the frame length in Figure 2 is too long compared to thecoherence time of the channel, further pilots such as midam-bles [2] should periodically be introduced into a single frameto re-initialize the DDE in order to achieve a good estimationperformance. I n this paper the focus is put on the analysis andperformance investigations of the DDE according to Figure 2.

111. PRINCIPLE OF DECISION DIRECTED ESTIMATION(DDE)The basic principle of DDE is depicted in Fig 3.Here n de-notes the time index and k is the index for an OFDM subcar-

rier. The subcarrier-specific received complex value symbolR,, is equalized by applying a ZF technique, se_e Eq (3.1)based on the previously estimated transfer factor Hn+ that istaken asa prediction of the current transfer factor.(3.1)

The equalized symbol is then detected by the demodu-lator. The detected symbol S, ,*will be further divided by thereceived symbol R , , in order to calculate the new channeltransfer factor Ha,:

(3.2)Simultaneously the channel estimation is updated in asimple recursive filter procedure with the time-variant factor?using the most recent prediction of the channel transfer factorH"., I and the currently measured channel transfer factor Hn, :

The procedure of calculating the variable update factory sexplained in the following part.A . a-Posterioriprobabiliry ofthe decision symbol gn,*

In the following mathematical discussion, the time index nand subcarrier index k will be skipped for brevity. On the sub-carrier level of the OFDM system, it is assumed that the re-ceived signal R is a random variable which i s interfered by thechannel transfer factor H and by additive white Gaussian noiseG (0 ,02) ,whereR = S . H +w (3.4)

the PDF of R is therefore given by1 2exp(--lR-S.HI ) (3.5)P ( R I S ) =- 1T, &U 202

R -E!+ With M-QA M modulation, S (m=1,2...M) will be takenfrom a finite alphabet set s. In the demodulator, the symbolbe assigned as the measured symbol. If all the symbols S arewith toto1probability theorem and Bqves' F ormulo [SI , the U-posterior~probability f the detected symbol is:

1 7r FIHWr i S that has the nearest distance to the equalized symbol Swill

assumed to have a uniformly distribution at the transmitter,H..

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Eq (3.6) shows that p(sI R) depends on the distances, thechannel transfer factor H and the noise influence. Also, itshows that the measured symbol has the maximum posterioriprobability in the sets. Obviously,- -


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of proper frame lengths L if DDE is applied. The results areshown in Figure 6and Figure 7. Similarly, the MSE results ofDDE well outperform that of the preamble-based case. ThePER shows acceptable performance with DDE if the framelength L is limited to L =I 2. In this c ae a gain of about 2.7 dBfor a PER of 10~'occurs compared with the purely preamblebased channel estimation technique.

10Y.71 .... - ....... ...... i ..........I....~............................ I I ,............... ~,. .. .i .~~., .. .- -LL~~~~.... .........i....j.....,.. ........ACKNOWLEDGMENT

This work has been performed in the framework of the ISTproject 1ST-2000-28584 MI ND, which was partly funded bythe European Union. The authors would l ike to acknowledgethe contributions of their col leagues from Siemens AG, BritishTelecommunications PLC, Agora Systems S.A., Ericsson Ra-dio Systems AB, France Telecom S.A., K ing's College Lon-don, Nokia Corporation, NT T DoCoMo Inc, Sony Interna-tional (Europe) GmbH, T-Systems Nova GmbH, University ofMadrid, and Infineon Technologies AG.

REFERENCES[I ] ETSl DTSIBRAN-0023003, "HIPERLAN Type 2 TechnicalSpecification; Physical (PHY ) layer," November 1999.[2] Rainer Gmunhed, Herman" Rohling, Jianjun Ran, Edgar Bolinth, RalfKem: "Robust Channel Estimation in Wireless LANs for MobileEnvironments", VTC 2W2, Vancouver, Canada, September, 2002.p] Raphael J . Lyman and William W. Edmonson: "Decision-Directed

Tracking of Fading Channels Using Linear Prediction of the FadingEnvelope ",33' Asilomar Conference on Signal Processing, Systems,andComouters. 1999.val. 2. 00.1154-1158.

$0 12 34 16 I S 20 22 21cn* dSl

Figure 7: System PER performance of purely preamble and DDE basedchannel estimation procedures for I 6 Q A M (v=1001Vnih)

..[S Y aakov Bar-Shalom, Xiao-Rang Li , Estimarron and Trockzng: Pnnciple,Techniques ondSofnuore. Mech House, 1993.

V . CONCLUSIONSThe system concept and results of a decision-directedchannel estimation technique is introduced and investigated inthis paper. The performance enhancement of the decision di-rected channel estimation (DDE) compared with the purely

preamble-based method is analyzed. On the one hand, theDDE method uses the decisions at the output of the demodula-tor, which leads to a rather low complexity and a delay of onlya single OFDM symbol. On the other hand, the statistical char-acteristics of the transmitted data are additionally applied todistribute the feedback information with the help of a recursivefi lter. Thus the time variance of the channel, especially incases with high velocities, could be properly tracked. As awhole, the DDE method shows a good performance increasewith only low computation complexity.2361

Decision-directed Channel Estimation Method for OFDM systems with high velocities

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