Improved Channel Estimation for TDS-OFDM Based on ... - IEEE Xplore

418

IEEE TRANSACTIONS ON BROADCASTING, VOL. 56, NO. 3, SEPTEMBER 2010

Improved Channel Estimation for TDS-OFDM Based on Flexible Frequency-Binary Padding Zhixing Yang, Xiaoqing Wang, Zhaocheng Wang, Jintao Wang, and Jun Wang

Abstract—By adopting the pseudo-random noise (PN) sequences as the guard interval (GI) as well as the training sequence, time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM) outperforms the conventional OFDM using the cyclic prefix (CP) in the spectral efficiency at the cost of the iterative padding subtraction (IPS) in channel estimation (CE). To avoid the high computational complexity of the IPS, an improved frequency domain CE scheme based on a newly designed frame structure is proposed. To further improve the CE accuracy, the time domain training sequences, which are frequency binary (FB) after the fast Fourier transform (FFT), are flexibly padded as the frame header instead of the PN sequences. Theoretical analyses and computer simulations show that the proposed scheme can improve the system performance under the time frequency doubly selective channels, even in the presence of large channel delays in the single frequency networks. Index Terms—Channel estimation (CE), frequency binary (FB) sequence, time domain synchronous orthogonal frequency division multiplexing (TDS-OFDM), time-frequency doubly selective channel.

I. INTRODUCTION O OVERCOME the frequency selective fading under multipath channels, orthogonal frequency division multiplexing (OFDM), by dividing the entire channel into numbers of essentially flat subchannels, is widely exploited in many broadband wireless environments, such as IEEE 802.11 and DVB-T standards. To further improve the performance of OFDM systems, many techniques to combat with the intercarrier interference (ICI) have been studied [1]–[5]. Moreover, the inter-symbol interference (ISI) caused by the multipath propagation, can be efficiently eliminated with the insertion of the guard interval (GI) between adjacent OFDM symbols. Time-domain synchronous OFDM (TDS-OFDM) has been further developed, by using the known time-domain pseudo-random noise (PN) training sequences instead of the cyclic prefix (CP) as the GI [6], [7]. Moreover, by using the PN training sequences rather than pilots for synchronization and channel estimation (CE) [8], [9], TDS-OFDM has gained some

T

Manuscript received April 03, 2010; revised June 09, 2010; accepted June 11, 2010. Date of publication July 12, 2010; date of current version August 20, 2010. This work was supported in part by Standardization Administration of the People’s Republic of China (SAC) with AQSIQ Project 200910244 and in part by Tsinghua University Initiative Scientific Research Program 20091081280. The authors are with the State Key Laboratory on Microwave and Digital Communications, Tsinghua National Laboratory for Information Science and Technology (TNList), Department of Electronics Engineering, Tsinghua University, Beijing 100084, China (e-mail: [email protected]; [email protected]; [email protected]; wangjintao@ tsinghua.edu.cn; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TBC.2010.2053970

spectral efficiency improvement and provided relatively better system performance over CP-OFDM [9]. The Chinese digital television terrestrial broadcasting (DTTB) standard, namely digital television/terrestrial multimedia broadcasting (DTMB), has adopted TDS-OFDM as its key technique [6], [7]. For digital television broadcasting, it is better to use the same network to support both terrestrial and mobile services. However, when the iterative padding subtraction (IPS) is carried out in CE for traditional TDS-OFDM systems, the high processing complexity restricts the practical implementation in mobile or handheld reception [9]. Recently, a dual PN padding (DPN) method was studied for simplifying the CE without the iteration effort [10], which is similar to the pilot cyclic prefixed single carrier (PCP-SC) scheme in [11]. However, in [10], the PN sequences are always doubled, which results in an inherent decrease of spectral efficiency. In addition, in both [10] and the traditional TDS-OFDM, since the PN sequences are binary in time domain, their frequency responses after the fast Fourier transform (FFT) are not flat. It would lead to significant noise enhancement in the frequency-domain CE [11]. Furthermore, in [10], the identical channel impulse response (CIR) between adjacent OFDM frames is assumed during the equalization, which degrades the system performance under fast fading channels. To address the above issues, a newly-designed frame structure together with an improved frequency-domain CE method under time-frequency doubly selective channels is proposed for TDS-OFDM systems. In the proposed frame structure, one or two of the so-called time-domain padded frequency-binary (TPFB) sequences, which are binary all pass in FFT domain, are flexibly padded. At the cost of small spectral efficiency decrease, it greatly improves the CE accuracy and the symbol error rate (SER) performance under the time-frequency doubly selective fading channels, especially when the Doppler spread is large. Due to its good performance, the proposed frame structure might become a potential candidate for future Chinese digital television terrestrial broadcasting (DTTB) standard. The outline of this paper is as follows. In Section II, the TDS-OFDM system model is presented. The new frame structure with the flexibly padded TPFB sequences is introduced in Section III. The proposed frequency-domain CE method, together with two different equalization schemes having low complexity, is proposed in Section IV. Simulation results are shown in Section V, and conclusions are drawn in Section VI. II. TDS-OFDM SYSTEM MODEL A typical uncoded TDS-OFDM system model with data subcarriers is illustrated in Fig. 1. At the transmitter side, after the modulation, the symbols are operated by the -point inverse fast Fourier transform (IFFT) as the OFDM frame body. Meanwhile, a frequency-domain padded frequency-binary

0018-9316/$26.00 © 2010 IEEE


419

while the former one has larger spectral efficiency and can be used for the fixed broadcasting. According to the Sampling Theorem [12], as long as the channel is sampled at sufficiently high rate, there is little degradation due to CE errors. In the data-aided CE, time-domain training sequences and frequency-domain pilots are two effective methods. Therefore, the CE error is minimized when the aiding-data insertion frequency satisfies [12], (1) Fig. 1. Uncoded TDS-OFDM system model.

(FPFB) sequence with the length of is generated ( is any integer greater than 1), and then transformed into the time-domain TPFB sequence by the -point IFFT as the frame header. By dynamically padding single or dual TPFB sequences, the signal frame is formed, which will be discussed in detail in Section III. After that, signals go serially through the wireless multipath channels with the additive white Gaussian noise (AWGN). At the receiver side, the received data are processed by the corresponding inverse operations. After the channel state information is obtained via the frequency-domain channel estimation using the FPFB/TPFB sequences, which will be introduced in Section IV-A, two simple equalization methods are then proposed in Section IV-B.

is the theoretical maximum Doppler spread, is where and is the frame interval of the known aiding data is relethe time interval per frame. It is worth noting that, vant to depending on the DTPFB allocation pattern. With the observation that only DTPFB sequences are used as the aiding data, (1) can be rewritten as (2) that is, (3) is the frame interval of the DTPFB padding, and where is the symbol interval in time domain. As compared to the conventional TDS-OFDM, the slight spectral efficiency penalty is

III. PROPOSED TPFB PADDING SCHEME

(4)

A. Frame Structure With Flexibly Padded TPFB Sequence Fig. 2 depicts the TDS-OFDM frame structure, which is composed of the IFFT data block and the frame header, i.e., a single TPFB sequence (STDFB) or two identical TPFB sequences, namely dual TPFB (DTPFB). Denote as the -th transmitted and is any integer greater than frame, where 1. Every frames form a group called a super-frame. As shown in Fig. 2, periodically among the so-called super-frame, the first frame,

, is padded with the DTPFB sequences,

frames, , can choose eiwhile the last ther STPFB or DTPFB. For the DTPFB padding, the first TPFB sequence can be viewed as the CP of the second TPFB sequence. On the contrary, the STPFB padding is only inserted as the GI. Moreover, the STPFB/DTPFB sequences are binary all pass in frequency domain. By using the sign operation instead of actual frequency domain complex division, the proposed scheme can reduce the CE complexity, which will be further addressed in Section IV. Based on various spectral efficiency requirements, different modes of DTPFB allocation can be used. Moreover, the specific parameters to indicate DTPFB allocation patterns, can be not only predefined by both transmitters and receivers, as shown in Fig. 2(a), but also be dynamically transmitted via the system information header (SIH) per super-frame, as shown in Fig. 2(b). It is clear that, the latter case offers more flexibility and can be applied to the realization of point-to-point communications,

, the Doppler spread tolerance It is clear that, when is preferred to be level reaches the maximum, and smaller used for severe multipath fading with large Doppler spread and vice versa. In this way, the flexible padding scheme provides an inherent tradeoff between the spectral efficiency penalty and the maximum Doppler spread tolerance level. B. Criterion to Search TPFB Sequence It has been demonstrated in [11] that, when the training sequences have constant modulus in frequency domain, the frequency-domain CE is considered to be the most accurate. However, the time binary training sequences such as the PN sequences, would greatly enhance the noise effect as their frequency responses are not flat. For example, the minimum frequency amplitude of the PN420 sequence in DTMB is only 0.08 of the root-mean-square value (RMS) after 3780-point FFT [6], where through the frequency division, the noise at the subcarrier with the minimum amplitude would be amplified as much as 1/0.08, i.e., 12.5. as the time-domain training sequence Denote and as its FFT, based on the observation that the noise is i.i.d. AWGN, [11] has demonstrated that, when the frequency-domain CE is carried out, the minimum mean square error (MMSE) can be achieved when the training sequence satisfies (5)

420


Fig. 2. Proposed OFDM frame structures with flexible TPFB padding: (a) OFDM frame structure without SIH; (b) OFDM frame structure with SIH.

at the constraint of , where is the . According to the Parseval’s Theorem and power of the Cauchy Inequality, the constraint in (5) is equivalent to (6) Thus, one choice of the ideal training sequence for the frequency-domain CE is given by (7) that is, the TPFB sequences in this paper. By taking advantage of the TPFB sequences, the CE accuracy can be improved and the additional complexity introduced by the frequency division can be reduced. Moreover, according to [11], the TPFB sequences has also good periodical correlation property, which facilitates the synchronization. However, the time-domain peak to average power ratio (PAPR) of many TPFB sequences are higher than the PN sequences, which results in the degradation of the power amplifier efficiency. To find the actual TPFB sequence with low time-domain PAPR, an alternative searching algorithm [13] instead of the Monte Carlo simulation is used in this paper, which is more realistic for practical application. The details will not be presented here due to the limitation of the paper length. for example, an asymptotical optimal Again taking is shown in (8), TPFB sequence with

Fig. 3. Transmitted and received OFDM frame structures: (a) transmitted signal frames; (b) time-domain decomposition for received signal frames.

IV. CHANNEL ESTIMATION A. Frequency-Domain CE Fig. 3 illustrates both the transmitted and the corresponding received OFDM signals. For the simplicity purpose, we focus on analyzing the case that , that is, the DTPFB padding, , is adopted in all TDS-OFDM frames. As shown in Fig. 3(b), after passing through multipath channels, the -th received signal frame excluding the noise, , comprises three overlapping parts, i.e., , , and , which are given by

(9)

(8) where “1” and “0” denote “ 1” and “ 1”, respectively.

(10)


421

respectively, where “ ” is the linear convolution and is the transmitted IFFT data in the -th frame. By applying (9) with the and (10), the received -th signal frame AWGN is thus expressed as (11) that is,

, (12) is the interference of where in -th OFDM frame, -th frame data on the first TPFB sequence, is the interference of the first TPFB sequence on the second is the interference of the TPFB sequence, and second TPFB on the OFDM data. can be When considering the DTPFB padding, viewed as the CP of , for which the relationship that holds. As a result, the second received TPFB signal is represented as (13) Since the actual CIR length is unknown to the receiver, that is considered for the worst case. According to the least square (LS) algorithm [14], the estimated CIR of the -th signal frame is derived as

Fig. 4. Equalization using CP-OFDM signal reconstruction: (a) equalization method I; (b) equalization method II.

the accurate CIR can be estimated with the aid of the DTPFB sequences, and then reused for next several frames with the STPFB sequences. Thus, the proposed flexible padding can provide an inherent tradeoff between the CE complexity and the small spectral efficiency penalty. B. CP-OFDM Signal Reconstruction From the point of the CP-OFDM signal reconstruction, the received discrete-time domain signal in the -th frame can be constructed by (16) where “ ” denotes the cyclic convolution operation, and in the -th frame, is the transmitted data, and is the independent identically distributed (i.i.d.) AWGN. Accordingly, the -point FFT-domain signal is (17)

(14) is the -point FFT and is the where -point IFFT. Since the TPFB sequence is binary in frequency domain, (14) is rewritten as

(15) where is the sign function. Taking advantage of (15), the frequency-domain CE can be simplified and by using variations of signs instead of the actual frequency division. Moreis flat in over, as mentioned in Section III, since frequency domain, the noise enhancement caused by the LS algorithm could be avoided. It has been pointed out that, according to the dynamic wireless s, i.e., different DTPFB propagation environment, different allocation patterns, can be used. To improve the spectral efficiency, when the Doppler spread in multipath channels is small,

where is the estimated channel frequency response (CFR), is the frequency-domain AWGN. As a result, after the estimated CIR is obtained by (15), two non-iterative CP-OFDM equalization methods are proposed. From one side, as illustrated in Fig. 4(a), when the multipath channels vary so slow that leads to the assumption that , the reconstructed CP-OFDM signal of the -th frame is derived by simple addition and subtraction operations as follows (defined as Method I), which is similar to [10]

. (18)

422


However, from other side, when the time variation between two adjacent OFDM frames caused by larger Doppler spread cannot be ignored, (18) will cause performance degradation. As a result, a more common equalization method (defined as Method II), is further proposed. Since the DTPFB is circularly protected, according to (9), the CP-OFDM reconstruction for the -th frame

TABLE I COMPUTATIONAL COMPLEXITY COMPARISON

is expressed as

, (19) where

is the interference of the second TPFB

TABLE II PARAMETERS AND THEIR DEFINITIONS

is the linear in the -th frame on the data, and -th frame, as convolution tail of the data existing in the is illustrated in the Fig. 4(b). By substituting (12) into (19), finally expressed as

, (20) When the accurate CIR is obtained by the proposed frequency CE method using the TPFB sequences, Method II would suffer from less noise enhancement as compared to Method I. Moreover, by using two additional linear convolution operations, Method II has more accurate CP-OFDM signal reconstruction under fast fading channels. After that, the one-tap zero forcing (ZF) equalization is operated as

(21)

where

is the zero-padded -point FFT, and is the frequency-domain reconstructed signals. Together with the frequency-domain CE scheme, the complexity from some well-known references is summarized in Table I. And the simulation parameters are shown in Table II. It is shown in Table I that, the complexity of both proposals is around 10% of that of IPS with 3 iterations. When compared to the DPN scheme, the calculation effort of Method I is slightly reduced while Method II requests two additional linear convolution operations. Besides that, the CP-OFDM has additional computational complexity of the interpolation operation. To evaluate different Doppler spread tolerance levels, the MSE with different s are evaluated in Fig. 5 over two typical multipath channels, where Channel I is the fixed reception F1 version of DVB-T channel model [15], and Channel II consists of a violent long delay echo, namely the Chinese State Administration of Radio Film and Television (SARFT) 108 channel [6].

Fig. 5. Doppler spread performance comparison.

As shown in Fig. 5, due to the increase of maximal Doppler spread, the CE error increases. Therefore, both algorithms have the SER performance degradation, even though Method II suffers less and thus supports higher maximum Doppler spread, which is aligned with the above analyses. According to the above discussion, the following conclusions are drawn: 1) ISI cancellation by Method I is simple enough by ignoring the channel time variations within frames. Yet, when the channel varies fast enough, it will not be effective and cause some performance degradation. 2) At the cost of a little more processing complexity, the proposed Method II has gained the benefit of much more accurate equalization with larger Doppler spread tolerance. As


Fig. 6. Performance comparison over DVB-T F1 channel with f

= 10 Hz.

a result, Method II promises a good solution for the mobile scenario with relatively high Doppler spread. V. SIMULATION RESULTS Simulation results under two channels listed above are presented to evaluate the performance of the proposed scheme. The major parameters are listed in Table II. When both QPSK and 64QAM modulation are considered, Fig. 6 compares the SER performance of different methods . The over DVB-T F1 channel with Doppler spread legend of “CP-OFDM” means the pilot-based CP-OFDM scheme. The legend of “DPN-TDS-OFDM” means the DPN-based CE scheme in [10], which uses the dual time-binary PN sequences. And the legend of “IPS-TDS-OFDM” means the IPS method in conventional TDS-OFDM systems. Assume that the average power of both training sequences and pilots is doubled compared to OFDM data in all the schemes. As shown in Fig. 6, both our proposed methods outperform the other three schemes. It can be explained in the following, on one hand, because of the more accurate CE based on the frequency-binary sequence TPFB, Method I is slightly superior to the DPN scheme, which also verifies the superiority of the TPFB over PN. The gains of Method I over the DPN scheme and 0.27 dB @ are about 0.67 dB @ for QPSK and 64QAM modulation schemes. On the other hand, the proposed Method II offers larger SER improvement over all the other three, which comes from not only the frequency-domain CE scheme but also the CP-OFDM signal reconstruction considering the time variations within OFDM frames. Moreover, it is expected that, the gap between the two proposed methods may become less in relatively high SNR regions, for which the effect of noise enhancement in the addition and subtraction operations can be negligible in high SNR region. As shown in Fig. 6, Method II improves the DPN under the QPSK scheme by about 0.93 dB @ under the 64QAM modulation and 0.4 dB @ modulation. When compared with the IPS scheme, the gain

423

Fig. 7. Performance comparison over SARFT 108 channel with f

= 100 Hz.

under of Method II is more than 10 dB @ the QPSK modulation and 5.53 dB @ under the 64QAM modulation, respectively. Meanwhile, Method II under improves the CP-OFDM by 6 dB @ under the the QPSK modulation and 5 dB @ 64QAM modulation, respectively. In Fig. 7, the SER performance is further discussed over . SARFT 108 channel with Doppler spread Method I and Method II outperform the other three methods for the QPSK and 64QAM modulation schemes, especially Method II offers excellent SER performance. It is clear that, as increases, the error floor of the IPS method is lifted while the CP-OFDM scheme and the DPN scheme see degradation as well. From Fig. 7, as compared to the DPN scheme under SARFT 108 channel, Method I and Method II have gained under the about 0.13 dB and 2.26 dB @ 64QAM modulations, respectively. It is clear that, the proposed schemes, especially Method II, are much less sensitive to the increase of maximum Doppler spread and have better capacity to combat with the Doppler effect. VI. CONCLUSIONS In this paper, based on the flexibly padded TPFB sequences, an improved frequency-domain CE scheme together with two corresponding CP-OFDM reconstruction methods is proposed for TDS-OFDM systems. Firstly, the actual asymptotic optimal TPFB sequence having small PAPR is found, before inserted as the frame header in the newly proposed frame structure. The flexible padding scheme provides an inherent tradeoff between the spectral efficiency decrease and the maximum Doppler spread tolerance level. Secondly, by proposing two equalization methods (defined as Method I and Method II), a simplified accurate frequency-domain CE is proposed. Meanwhile, Method II outperforms Method I under high Doppler spread wireless channels. Compared with the traditional TDS-OFDM, this flexible proposal has not only realized simple and efficient CE but also offered significant performance gain over both fast and slow time-varying multipath channels.

424


REFERENCES [1] H. Wu and Y. Wu, “A new ICI matrices estimation scheme using Hadamard sequences for OFDM systems,” IEEE Trans. Broadcast., vol. 51, no. 3, pp. 305–314, Sep. 2005. [2] X. Huang and H. Wu, “Robust and efficient intercarrier interference mitigation for OFDM systems in time-varying fading channels,” IEEE Trans. Veh. Technol., vol. 56, no. 5, pp. 2517–2528, Sep. 2007. [3] H. Wu and Y. Wu, “Distributive pilot arrangement based on modified m-sequences for OFDM intercarrier interference estimation,” IEEE Trans. Wireless Commun., vol. 6, no. 5, pp. 1605–1609, May 2007. [4] H. Wu, X. Huang, and Y. Wu et al., “Theoretical studies and efficient algorithm of semi-blind ICI equalization for OFDM,” IEEE Trans. Wireless Commun., vol. 7, no. 10, pp. 3791–3798, Oct. 2008. [5] L. Zhang, Z. Hong, and L. Thibault, “Iterative ICI estimation and cancellation for mobile OFDM systems,” IEEE Trans. Broadcast., pp. 197–200, May 2010. [6] J. Song, Z. Yang, and L. Yang et al., “Technical review on Chinese digital terrestrial television broadcasting standard and measurements on some working modes,” IEEE Trans. Broadcast., vol. 53, no. 1, pp. 1–7, Mar. 2007. [7] W. Zhang, Y. Guan, and W. Liang et al., “An introduction of the Chinese DTTB standard and analysis of the PN595 working modes,” IEEE Trans. Broadcast., vol. 53, no. 1, pp. 8–13, Mar. 2007.

[8] G. Liu and S. V. Zhidkov, “A composite PN-correlation based synchronizer for TDS-OFDM receiver,” IEEE Trans. Broadcast., vol. 56, no. 1, pp. 77–85, Mar. 2010. [9] J. Wang, Z. Yang, and C. Pan et al., “Iterative padding subtraction of the PN sequence for the TDS-OFDM over broadcast channels,” IEEE Trans. Consum. Electron., vol. 51, no. 4, pp. 1148–1152, 2005. [10] J. Fu, J. Wang, and J. Song et al., “A simplified equalization method for dual PN-sequence padding TDS-OFDM systems,” IEEE Trans. Broadcast., vol. 54, no. 4, pp. 825–830, 2008. [11] Y. Zeng and T. Ng, “Pilot cyclic prefixed single carrier communication: Channel estimation and equalization,” IEEE Signal Process. Letters, vol. 12, no. 1, pp. 56–59, 2005. [12] R. G. Gallager, Principles of Digital Communication. New York, U.S.: Cambridge Univ. Press, 2008. [13] Z. Yang, K. Peng, and J. Song, “Binary all pass sequence padding for frequency-domain channel estimation,” (in Chinese) Chinese Patent, CN 101102114A, 2008. [14] J. Beek, O. Edfors, and M. Sandell et al., “On channel estimation in OFDM systems,” in Proc. IEEE VTC95, 1995, pp. 815–819. [15] “Digital video broadcasting (DVB); framing structure, channel coding and modulation for digital terrestrial television,” Tech. Rep. EN 300744 v1.1.2, ETSI Std., 1997.