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Freescale Semiconductor Application Note

AN3059 Rev. 0, 1/2006

Channel Estimation in OFDM Systems by Yushi Shen and Ed Martinez

CONTENTS

1 OFDM Background .....................................................2 2 Baseband Model ..........................................................2 3 Block-Type Pilot Channel Estimation .........................4 4 Comb-Type Pilot Channel Estimation.........................7 5 Other Pilot-Aided Channel Estimations ......................9 6 Performance Evaluation.............................................10 7 Conclusions................................................................14 8 References..................................................................15

This application note gives an overview of the channel estimation strategies used in orthogonal frequency division multiplexing (OFDM) systems. Section 1 describes the protocols associated with OFDM systems and the problems posed by such systems. Section 2 through Section 5 describe the various types of channel estimation methods for use in such systems. The implementation complexity and system performance of the methods are studied and compared in Section 6, measuring performance in terms of symbol error rate (SER).

© Freescale Semiconductor, Inc., 2006. All rights reserved.

OFDM Background

1 OFDM Background OFDM is becoming widely applied in wireless communications systems due to its high rate transmission capability with high bandwidth efficiency and its robustness with regard to multi-path fading and delay [1]. It has been used in digital audio broadcasting (DAB) systems, digital video broadcasting (DVB) systems, digital subscriber line (DSL) standards, and wireless LAN standards such as the American IEEE® Std. 802.11™ (WiFi) and its European equivalent HIPRLAN/2. It has also been proposed for wireless broadband access standards such as IEEE Std. 802.16™ (WiMAX) and as the core technique for the fourth-generation (4G) wireless mobile communications.

The use of differential phase-shift keying (DPSK) in OFDM systems avoids need to track a time varying channel; however, it limits the number of bits per symbol and results in a 3 dB loss in signal-to-noise ratio (SNR). Coherent modulation allows arbitrary signal constellations, but efficient channel estimation strategies are required for coherent detection and decoding.

There are two main problems in designing channel estimators for wireless OFDM systems. The first problem is the arrangement of pilot information, where pilot means the reference signal used by both transmitters and receivers. The second problem is the design of an estimator with both low complexity and good channel tracking ability. The two problems are interconnected. In general, the fading channel of OFDM systems can be viewed as a two- dimensional (2D) signal (time and frequency). The optimal channel estimator in terms of mean-square error is based on 2D Wiener filter interpolation. Unfortunately, such a 2D estimator structure is too complex for practical implementation. The combination of high data rates and low bit error rates in OFDM systems necessitates the use of estimators that have both low complexity and high accuracy, where the two constraints work against each other and a good trade-off is needed. The one-dimensional (1D) channel estimations are usually adopted in OFDM systems to accomplish the trade-off between complexity and accuracy [1–7]. The two basic 1D channel estimations are block-type pilot channel estimation and comb-type pilot channel estimation, in which the pilots are inserted in the frequency direction and in the time direction, respectively. The estimations for the block-type pilot arrangement can be based on least square (LS), minimum mean-square error (MMSE), and modified MMSE. The estimations for the comb-type pilot arrangement includes the LS estimator with 1D interpolation, the maximum likelihood (ML) estimator, and the parametric channel modeling-based (PCMB) estimator. Other channel estimation strategies were also studied [8–12], such as the estimators based on simplified 2D interpolations, the estimators based on iterative filtering and decoding, estimators for the OFDM systems with multiple transmit-and-receive antennas, and so on.

2 Baseband Model The basic idea underlying OFDM systems is the division of the available frequency spectrum into several subcarriers. To obtain a high spectral efficiency, the frequency responses of the subcarriers are overlapping and orthogonal, hence the name OFDM. This orthogonality can be completely maintained with a small price in a loss in SNR, even though the signal passes through a time dispersive fading channel, by introducing a cyclic prefix (CP). A block diagram of a baseband OFDM system is shown in Figure 1.

Channel Estimation in OFDM Systems, Rev. 0

2 Freescale Semiconductor

http://www.wi-fi.org/OpenSection/index.asp http://www.wi-fi.org/OpenSection/index.asp http://www.wimaxforum.org/home

Baseband Model

The binary information is first grouped, coded, and mapped according to the modulation in a “signal mapper.” After the guard band is inserted, an N-point inverse discrete-time Fourier transform (IDFTN) block transforms the data sequence into time domain (note that N is typically 256 or larger). Following the IDFT block, a cyclic extension of time length TG, chosen to be larger than the expected delay spread, is inserted to avoid intersymbol and intercarrier interferences. The D/A converter contains low-pass filters with bandwidth 1/TS, where TS is the sampling interval. The channel is modeled as an impulse response g(t) followed by the complex additive white Gaussian noise (AWGN) n(t), where αm is a complex values and 0 ≤ τ mTS ≤ TG.

Equation 1

At the receiver, after passing through the analog-to-digital converter (ADC) and removing the CP, the DFTN is used to transform the data back to frequency domain. Lastly, the binary information data is obtained back after the demodulation and channel decoding.

Let and denote the input data of IDFT block at the transmitter and the output data of DFT block at the receiver, respectively. Let and denote the sampled channel impulse response and AWGN, respectively. Define the input matrix and the DFT-matrix,

Equation 2

where . Also define , and .

Figure 1. A Digital Implementation of a Baseband OFDM System.

s(t)

nt)

r(t)

Input Data

Output Data

Channel Coding and Modulation (Signal Mapper)

Guard Band

Insertion S/P P/S

I D F T

CP Insertion D/A

Fading Channel

g(t)

A/D CP

Deletion

Guard Band

Deletion

Channel Decoding and Demodulation P/S

D F T

S/P

X x

Y y

. . .

. . .

. . .

. . .

g t( ) αmδ t τ mTS–( ) m 1=

M

∑=

X Xk[ ] T

= Y Yk[ ] T

= k 0 ..., N 1–,=( ) g gn[ ]

T = n nn[ ]

T = n 0 ..., N 1–,=( )

X diag X( )=

⎥ ⎥ ⎥ ⎥ ⎥

⎦

⎤

⎢ ⎢ ⎢ ⎢ ⎢

⎣

⎡

=

−−−

−

)1)(1(0)1(

)1(000

NN N

N N

N NN

WW

WW

F

L

O

L

WN i,k

1 N⁄( ) j– 2Π ik N⁄( )

= H DFTN g( ) Fg= = N Fn=

Channel Estimation in OFDM Systems, Rev. 0

Freescale Semiconductor 3

Block-Type Pilot Channel Estimation

Under the assumption that the interferences are completely eliminated [1–3], you can derive:

Equation 3

This equation demonstrates that an OFDM system is equivalent to a transmission of data over a set of parallel channels.

As a result, the fading channel of the OFDM system can be viewed as a 2D lattice in a time-frequency plane, which is sampled at pilot positions and the channel characteristics between pilots are estimated by interpolation. The art in designing channel estimators is to solve this problem with a good trade-off between complexity and performance.

The two basic 1D channel estimations in OFDM systems are illustrated in Figure 2. The first one, block-type pilot channel estimation, is developed under the assumption of slow fading channel, and it is performed by inserting pilot tones into all subcarriers of OFDM symbols within a specific period. The second one, comb-type pilot channel estimation, is introduced to satisfy the need for equalizing when the channel changes even from one OFDM block to the subsequent one. It is thus performed by inserting pilot tones into certain subcarriers of each OFDM symbol, where the interpolation is needed to estimate the conditions of data subcarriers. The strategies of these two basic types are analyzed in the next sections.

3 Block-Type Pilot Channel Estimation In block-type pilot-based channel estimation, as shown in Figure 2, OFDM channel estimation symbols are transmitted periodically, and all subcarriers are used as pilots. The task here is to estimate the channel conditions (specified by or ) given the pilot signals (specified by matrix or vector ) and received signals (specified by ), with or without using certain knowledge of the channel statistics. The receiver uses the estimated channel conditions to decode the received data inside the block until the next pilot symbol arrives. The estimation can be based on least square (LS), minimum mean-square error (MMSE), and modified MMSE.

3.1 LS Estimator The LS estimator minimizes the parameter , where means the conjugate