Low-Complexity Signaling-Embedded Preamble Design ... - IEEE Xplore

IEEE COMMUNICATIONS LETTERS, VOL. 18, NO. 9, SEPTEMBER 2014

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Low-Complexity Signaling-Embedded Preamble Design Based on Relative Subcarrier Position Bo Hao, Jun Wang, and Zhaocheng Wang, Senior Member, IEEE

Abstract—Preamble is widely used for initial synchronization and signaling transmission in wireless broadcasting systems. The second-generation terrestrial digital video broadcasting standard (DVB-T2) defines the preamble using several orthogonal sequences to convey the signaling, which yields high complexity due to the large number of required sequence correlations at the receiver. To solve this problem, this letter proposes a low-complexity preamble design that embeds the signaling by relative subcarrier position of two power-boosted subcarriers. At the receiver, the signaling can be simply extracted by power detection; thus, the complicated sequence correlation is not required any more, which leads to a significant reduction in the computational complexity. Meanwhile, the proposed preamble can be also used for reliable timing/frequency synchronization and channel estimation as well. Simulation results verify the robustness of the proposed preamble over typical multipath fading channels. Index Terms—Preamble, signaling, subcarrier position, power– boosted subcarrier, power detection.

I. I NTRODUCTION

W

ITH the rapidly increased requirement of multiservice applications, broadcasting standards are expected to offer a variety of services such as high definition television (HDTV), mobile TV and data casting to meet different quality of service (QoS) requirements. The second-generation terrestrial digital video television broadcasting standard (DVB-T2) can support seven guard interval (GI) modes, six fast Fourier transform (FFT) sizes as well as four kinds of modulations to provide different services in different environments [1]. To identify the specific system configuration at the receiver, a special preamble at the beginning of each super-frame is defined for signaling transmission and initial synchronization. This preamble selects two sequences from two orthogonal sets to carry several signaling bits, so the receiver requires multiple sequence correlations for signaling demodulation, which yields a large amount of computation [2], [3]. To reduce the complexity, an improved preamble design was proposed in [4], where the signaling is carried by the distance of two special sequences in the frequency domain, and the signaling can be demodulated by distance detection at the receiver. This preamble reduces Manuscript received March 20, 2014; accepted July 5, 2014. Date of publication July 16, 2014; date of current version September 8, 2014. This work was supported by the National Nature Science Foundation of China under Grant 61271266, by the National Key Basic Research Program of China under Grant 2013CB329203, by the National High Technology Research and Development Program of China under Grant 2014AA01A704, and by the ZTE Fund Project under Grant CON1307250001. The associate editor coordinating the review of this paper and approving it for publication was Y. Li. The authors are with Tsinghua National Laboratory for Information Science and Technology (TNList), Department of Electronic Engineering, Tsinghua University, Beijing 100084, China (e-mail: [email protected]; [email protected]; [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/LCOMM.2014.2337298

the required number of sequence correlations due to the used special sequences are deterministic. This preamble has been adopted by Chinese next-generation digital terrestrial multimedia broadcast-advanced (DTMB-A) standard [5], but its complexity is still high as it requires two sequence correlations of long length to generate the corresponding two correlation peaks for distance detection. In addition, both of those two preambles use a large number of consecutive zero subcarriers, which prohibits them from reliable channel estimation, and they also require time-domain cyclic extensions for timing synchronization. Low-complexity receiver is quite favorable in broadcasting systems due to the number of receivers is huge. This letter proposes a low-complexity preamble design by exploiting the relative subcarrier position (RSP) of two power-boosted subcarriers (PBSs) to carry the signaling. The idea is inspired by the MIMO spatial modulation technique, which relies on the on/off status of one or several specific antennas in MIMO systems to transmit extra bits [6]. The main contributions of this letter can be summarized as follows. First, unlike the conventional preambles, which adopt some specific sequences to convey the signaling, the proposed preamble maps the signaling to the RSP of PBSs, and significantly reduces the complexity of signaling extraction by power detection at the receiver. Second, different from a large number of consecutive zero subcarriers in the conventional preambles, the proposed one adopts uniformly spaced active subcarriers in the frequency domain, which enables robust channel estimation over multipath fading channels. Third, the conventional preambles commonly require cyclic extensions in the time domain to build a suitable signal structure for timing synchronization, while the proposed preamble naturally forms a [A −A] symmetrical structure for synchronization by adopting only even subcarriers in the frequency domain, so no cyclic extension is needed and the total symbol length can be shortened. The rest of the letter is organized as follows. Section II briefly introduces two conventional preambles for typical broadcasting standards. The proposed preamble design is addressed in Section III. Section IV presents the performance analysis, and Section V shows the simulation results. Finally, Section VI concludes this letter. II. C ONVENTIONAL P REAMBLE FOR B ROADCASTING S YSTEMS This section introduces two conventional preambles for typical broadcasting standards DVB-T2 and DTMB-A, respectively. Their shortcomings are also pointed out. A. Preamble for DVB-T2 As shown in Fig. 1, the so-called P1 preamble for DVB-T2 adopts only 384 of total 1024 subcarriers to compose one s2 and

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Fig. 1. Structure of P1 preamble for DVB-T2 [2].


Fig. 3. Proposed preamble with one fixed PBS and one variable PBS, and the relative subcarrier position is used to carry signaling.

cannot be used for reliable channel estimation due to a large number of consecutive zero subcarriers are used, and the timedomain cyclic extensions also reduce the spectral efficiency. III. P ROPOSED P REAMBLE D ESIGN

Fig. 2. Structure of the preamble based on distance detection.

two s1 sequences to carry the signaling, whereas the remained 640 subcarriers are all set to zero. s2 is retrieved from a set of 16 orthogonal sequences of length 256 to carry the 4-bit signaling, while s1 is selected from 8 orthogonal sequences of length 64 to carry the 3-bit signaling [2]. Thus the preamble can carry total 7-bit signaling. The P1 preamble adopts [C A B] time-domain structure for timing synchronization, where the central part A is an OFDM symbol of length 1024 Ts (where Ts denotes the sampling period), part C is the cyclic extension of the first 542 Ts symbols of A, and part B is the last 482 Ts symbols of A. After timing synchronization facilitated by the [C A B] timedomain structure [7], the receiver has to decide one of 16 × 8 = 128 possible combinations of sequences s2 and s1 . Hence, the P1 preamble requires a large number of sequence correlations to extract the transmitted 7-bit signaling. B. Preamble for DTMB-A As shown in Fig. 2, unlike the P1 preamble using different sequences to carry the signaling for DVB-T2, the preamble for DTMB-A dynamically selects the distance of two identical sequences c to convey the signaling. Each of these two sequences occupies the consecutive 255 subcarriers (L = 255Ts ), and their variable distance ΔL (ranging from 0 to 127) is used to map to the 7-bit signaling [4], [5]. In the time domain, the structure [B A B −B] also needs cyclic extensions for timing synchronization, as shown by B and −B located at the left and right sides of the original OFDM symbol [A B] of length 1024 Ts , respectively. Such time-domain structure contributes a shaper correlation peak for timing synchronization than the [C A B] structure of the P1 preamble for DVB-T2. However, their signaling extraction methods at the receiver are essentially similar, since the preamble for DTMB-A still requires two sequence correlations of long length to generate the corresponding two correlation peaks for distance detection. Thus, the complexity is still high, which is not preferable for broadcasting systems with tens of thousands of receivers. In addition, the preamble for DTMB-A

To further reduce the complexity of the signaling demodulation, which is especially useful for enormous receivers in broadcasting systems, this section presents a low-complexity preamble design based on RSP, which will be discussed from both the transmitter and the receiver sides, respectively. A. Signaling-Embedded Preamble Based on RSP As illustrated in Fig. 3, all the available N subcarriers of the proposed signaling-embedded preamble based on RSP can be divided into three types: normal subcarriers (NSs) with normal power, PBSs with boosted power, and zero subcarriers with zero power. More specifically, the kth subcarrier Xk (0 ≤ Xk ≤ N − 1) of the proposed preamble can be denoted as ⎧ k is odd, ⎨ 0, Xk = βck , k = N2 or N2 − Δk, (1) ⎩ ck , others, where ck is known for non-zero subcarriers, and β denotes the power boost factor for the fixed PBS located at the fixed subcarrier position k = N/2 and the variable PBS located at the variable subcarrier position k = N/2 − Δk. The RSP between the fixed PBS and the variable PBS is Δk, which is used to carry signaling embedded in the preamble. The boosted power of PBSs will enable simple signaling extraction based on power detection at the receiver. It is worth noting that the spatial modulation technique [6] relies on the on/off status of one specific antenna in MIMO systems to transmit extra bits, which implies that we can also adopt only one PBS with variable subcarrier position to carry signaling, but why the other fixed PBS is used in the proposed preamble? The reason is that large carrier frequency offset (CFO) will cause the cyclic shift of the subcarriers of the preamble [4], so the fixed PBS at the fixed subcarrier position k = N/2 is adopted for CFO detection. Consequently, the other PBS with the variable subcarrier position at k = N/2 − Δk can be used to carry the specific signaling according to the RSP Δk between the fixed PBS and the variable PBS. Assume that a zero subcarriers around the fixed PBS are reserved for CFO detection, the possible position set for the fixed PBS, Ωf , and position set for the variable PBS, Ωv , can be denoted as Ωf = [N/2 − a, N/2 + a], (2) Ωv = [a, N/2 − a) ∪ (N/2 + a, N − a − 1],

HAO et al.: SIGNALING-EMBEDDED PREAMBLE DESIGN BASED ON RELATIVE SUBCARRIER POSITION

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so the signaling bits mx carried by the RSP-based preamble is m = log2 ((N − 4a)/2 − 1) ,

(3)

where · is the floor operator. If N = 1024 and a = 50, the proposed preamble can carry m = 8 bits signaling, which is slightly larger than that of two conventional preambles. In addition, the zero subcarriers at odd subcarriers positions as indicated in (1) leads to the symmetrical time-domain structure [A −A] of the proposed preamble, which can be used for the initial timing synchronization without cyclic extensions [8], so the length of proposed preamble can be reduced by half. Consequently, although the proposed preamble boosts the power of two special subcarriers (i.e., the PBSs), the overall preamble power can be reduced.

Fig. 4. Probability density function of the modulus value of the received PBS for different power boost factors.

B. Signaling Extraction Based on Power Detection At the receiver, the received signal Yk over the kth subcarrier of the preamble after FFT can be represented as Y k = Hk X k + V k ,

(4)

where Hk and Vk denote the channel frequency response (CFR) and the additive white Gaussian noise (AWGN) over the kth subcarrier, respectively. The integer CFO εi can be detected by the position shift of the fixed PBS located at k = N/2 using the following metric |Yk |2 N − , εi = arg max N −1 k∈Ω 2 f k |Yk |2

Fig. 5. RSP-based signaling detection and the integer CFO estimation.

(5)

k=0

while the fractional CFO εf can be estimated by using the timedomain structure [A −A] of the preamble with the popular method used in [9]. The signaling extraction can be realized by directly checking the RSP of the two PBSs based on power detection as |Yk |2 |Yk |2 Δkˆ = arg max N −1 − arg max N −1 , (6) k k∈Ωv k k∈Ωf 2 2 |Yk | |Yk | k=0

k=0

where Δkˆ yields an estimate of the signaling-embedded RSP, which can be demapped to the corresponding signaling. It can be observed from (6) that an obvious advantage of the proposed preamble is that it only requires the simple power detection for signaling extraction, and the complicated sequence correlation is not required anymore, so the complexity can be considerably reduced at the receiver. Another important merit of the proposed preamble is its ability for channel estimation. Due to the uniform insertion of frequency-domain NSs across the whole signal bandwidth, a reliable CFR estimate over all subcarriers can be obtained as ˆ k = Yk , k is even, H Xk (7) 1 ˆ , k is odd, Hk = 2 Hˆ +Hˆ ( k−1 k+1 ) where the CFRs over odd subcarriers are linear interpolated by their two neighbors. IV. P ERFORMANCE A NALYSIS This section evaluates the theoretical signaling detection probability of the proposed preamble.

Assume that all the ck ’s in (1) are allocated with the same energy Es, then the√received NSs and PBSs √ follows the Gaussian distribution CN ( Es, σn2 ) and CN (β Es, σn2 ), respectively, where σn2 is the variance of the AWGN. Under the frequency-selective fading channels, signals coming from different paths will superimpose at the receiver, which makes the modulus value y = |Yk | follow the Rician distribution as below [10] √ yβ Es I0 , σn2

2 yEs y − y +Es fNS (y) = 2 e 2σn2 I0 , σn σn2

y −y fPBS (y) = 2 e σn

2 +β 2 Es 2 2σn

(8) (9)

where fPBS (y) and fNS (y) denote the probability density functions (pdf) of the PBS and the NS, respectively, and I0 (·) is the zero-order modified Bessel function of the first kind. Fig. 4 depicts fPBS (y) in (8) against different power boost factors β, where the signal-to-noise ratio (SNR) Es/σn2 is set as 0 dB. The value of β varies from 1 to more than 10 according to the channel environment in practical applications. It can be found out that when no extra energy is allocated (i.e., β = 1) to the PBS, the PBS follows the same distribution as the NS, which is shown by the reference curve on the most left in Fig. 4. The pdf curve will move toward right as the power boost factor β increases, resulting in less intersection area between fPBS (y) and fNS (y), thus false detection probability can be reduced. Note that higher SNRs will generate sharper curves due to Rician properties [10], which can further decrease false detection probability under the same power boost factor β.

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TABLE I C OMPARISON OF D IFFERENT P REAMBLE D ESIGNS

Fig. 6. Comparison of the correct timing synchronization probability over Rayleigh fading channel.

the proposed preamble outperforms the preamble for DTMB-A and the s1 signaling for DVB-T2, and achieves very similar performance as the s2 signaling for DVB-T2 over both dispersive channels. However, the complexity of the proposed preamble to extract the signaling can be greatly reduced compared with the conventional preambles as analyzed below. As listed in Table I, the proposed preamble adopts simple power detection without complicated sequence correlation to extract the signaling, so the complexity can be reduced a lot. Such low complexity at the receiver makes the proposed preamble more favorable in the broadcasting systems, where one high-power transmitter serves a huge number of receivers. VI. C ONCLUSION

Fig. 7. SER performance comparison over Brazil-B and TU-6 channels.

V. S IMULATION R ESULTS This section evaluates the performance of the proposed preamble compared with two conventional preambles for DVBT2 [2] and DTMB-A [4], respectively. Fig. 5 illustrates the RSP-based signaling detection together with the integer CFO estimation when SNR = 0 dB and the power boost factor β is assigned as the typical value of 10. After the synchronization and fractional CFO estimation using the similar methods in [9], the integer CFO can be estimated by the position shift of the fixed PBS located at N/2. Note that 50 subcarriers around the fixed PBS are reserved for CFO estimation as addressed in (2), i.e., a = 50. As the two special PBSs have much high power than NSs, the signaling can be detected by the RSP Δk between the fixed and the variable PBSs based on simple power detection with low complexity. As the preamble also serves for the initial timing synchronization at the receiver, we compare the timing performance of the proposed preamble with existing preambles adopted by DVB-T2 and DTMB-A in Fig. 6. Note that the same energy are allocated to these three preambles mentioned above for fair comparison. It can be found that the proposed preamble performs as well as the preambles for DVB-T2 and DTMB-A, since all these three preambles adopt the similar time-domain repeated structure for timing synchronization. Fig. 7 compares the signaling error rate (SER) of the proposed preamble with that of two conventional preambles over two dispersive channels (Brazil-B [4] and typical urban TU-6 [2, pp.174–175] channel models). It can be observed that

This letter proposes a low-complexity signaling-embedded preamble design by exploiting the RSP to convey the signaling, which significantly simplifies the signaling extraction based on power detection compared with conventional preambles. Such low-complexity scheme is especially attractive for broadcasting systems, where one or several transmitters cover enormous receivers. Moreover, the proposed scheme can also assist channel estimation with high accuracy. Simulation results verify the good performance of the proposed preamble over different channels. Additionally, the proposed preamble can be also directly applied to other wireless communication systems. R EFERENCES [1] L. Vangelista et al., “Key technologies for next-generation terrestrial digital television standard DVB-T2,” IEEE Commun. Mag., vol. 47, no. 10, pp. 146–153, Oct. 2009. [2] Implemention Guidelines for a Second Generation Digital Terrestrial Television Broadcasting System (DVB-T2), DVB Document A133, ETSI Std., Feb. 2009. [3] L. Dai, Z. Wang, and Z. Yang, “Next-generation digital television terrestrial broadcasting systems: Key technologies and research trends,” IEEE Commun. Mag., vol. 50, no. 6, pp. 150–158, Jun. 2012. [4] L. He, Z. Wang, F. Yang, S. Chen, and L. Hanzo, “Preamble design using embedded signaling for OFDM broadcast systems based on reducedcomplexity distance detection,” IEEE Trans. Veh. Technol., vol. 60, no. 3, pp. 1217–1222, Mar. 2011. [5] C. Pan, J. Wang, H. Fang, and J. Song, “Field trial of advanced DTMB system DTMB-A in Hong Kong,” in Proc. IEEE Int. Symp. BMSB, London, U.K., Jun. 2013, pp. 1–4. [6] W. Liu, N. Wang, M. Jin, and H. Xu, “Denoising detection for the generalized spatial modulation system using sparse property,” IEEE Commun. Lett., vol. 18, no. 1, pp. 22–25, Jan. 2014. [7] J. Doblado, V. Baena, A. Oria, D. Perez-Calderon, and P. Lopez, “Coarse time synchronization for DVB-T2,” Electron. Lett., vol. 46, no. 11, pp. 797–799, May 2010. [8] L. Dai, C. Zhang, Z. Xu, and Z. Wang, “Spectrum-efficient coherent optical OFDM for transport networks,” IEEE J. Sel. Areas Commun., vol. 31, no. 1, pp. 62–74, Jan. 2013. [9] L. Dai, J. Fu, J. Wang, J. Song, and Z. Yang, “A multi-user uplink TDSOFDM system based on dual PN sequence padding,” IEEE Trans. Consum. Electron., vol. 55, no. 3, pp. 1098–1106, Aug. 2009. [10] , Wireless Communications: Principles and Practice. Englewood Cliffs, NJ, USA: Prentice-Hall, 1996.