Improved Preamble Scheme Utilizing Hybrid ...

2015 IEEE INTERNATIONAL SYMPOSIUM ON BROADBAND MULTI MEDIA SYSTEMS AND BROADCASTING (BMSB)

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Improved Preamble Scheme Utilizing Hybrid Differential Modulation in Polarized DVB-T2 MISO System Zifeng Kui, Jae-Shin Han, Jeong-Min Choi, Sungho Jeon, Youngho Oh, and Jong-Soo Seo

Abstract—In this paper, we propose an improved preamble scheme utilizing a novel hybrid differential modulation, which can provide both the spatial multiplexing and the transmit diversity gain. The proposed preamble scheme has demonstrated a significantly improved signaling error rate (SER) performance in various channel conditions as compared to the conventional P1 symbol scheme in DVB-T2 standard. Specifically, as much as 4dB SNR gains are observed in static TU-6 and DVB-NGH outdoor channels with and without 20 Hz Doppler shift presented, respectively. Index Terms—differential modulation, preamble, P1 symbol of DVB-T2, space-time block code, transmit diversity

I.

INTRODUCTION

He digital video broadcasting - second generation terrestrial (DVB-T2) standard has adopted a preamble called P1 symbol, in order to provide a fast time and frequency synchronization. The P1 symbol also gives the fast initial scanning for the T2 frame and carries basic transmission parameters, such as fast Fourier transform (FFT) size, guard interval (GI) size and transmission type - single input single output (SISO) or multiple input single output (MISO), etc. For that purpose, the P1 symbol required a robust synchronization performance and extremely strict detection probability even under the negative signal-to-noise ratio (SNR) environment. In [1]-[3], a modified coarse synchronization and an offset estimation with a triangle correlation is presented to provide the improved performance in DVB-T2 system. In [4], authors presented a preamble based distance detection inserting training sequences in the frequency domain, which can enhance the signaling performance and decrease the computational complexity. Authors in [5] proposed an improved transmission parameter signaling (TPS) scheme utilizing the phase information obtained from the cross-correlation between discrete Fourier transform (DFT)-spread Chu sequences. Most of the previous researches mentioned above, are based on the

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research was supported by Samsung Electronics Co., Ltd, under the outsourcing development plan of “Physical layer framework for the nextgeneration broadcasting standardization.” 0 Z. F. Kui, J. S. Han, J. M. Choi, and J. S. Seo are with Department of Electrical and Electronic Engineering, Yonsei University, Seoul 120-749, Korea.(email: [email protected], [email protected], choi [email protected], [email protected]) 0 S. Jeon is with Technical Research Institute, Korean Broadcasting System (KBS), Seoul, 150-179, Korea (e-mail: [email protected]). 0 Y. H. Oh is with Standard and Technology Enabling Team, DMC R&D Center, Samsung Electronics Co., LTD, Suwon, Korea (e-mail: [email protected]).

synchronization techniques and TPS schemes for DVB-T2 P1 symbol. However, the modulation technology of P1 symbol was relatively paid little attention in the academia. That reason motivated this work. Unlike to previous research interests, in this paper, we propose an improved preamble scheme utilizing a hybrid differential modulation to achieve both spatial multiplexing and transmit diversity in multiple antenna environments. First, we consider the dual-polarized 2×1 multiple-input single-output (MISO) mode due to the advantages of simple implementation [7]. Specifically, we propose a new concept of hybrid differential modulation, consisting of two dimensional differential encoding, space time block code (STBC), and differential space frequency block code (DSFBC) decoding. It is shown that spatially superposed signals projecting both real and imaginary axis can be perfectly decoupled by exploiting quaternomic form at the destination. For the comparison purpose, we also compare the performance between coherent BPSK, differential BPSK and hybrid differential BPSK for the transmit diversity MISO system. By applying hybrid differential modulation technology to P1 symbol, the main transmitter can transmit P1 streams without the knowledge of channel state information (CSI). Finally, we accomplish the performance evaluations in practical broadcasting channels. The remainder of the paper is organized as follow. Section II will briefly review the P1 symbol in the DVB-T2 system. In Section III, the transmit and receive structures of our proposed preamble scheme as well as the novel hybrid differential modulation will be explained. The performance evaluation results will be analyzed in Section IV. Finally, Section V will conclude this paper. II. P1 SYMBOL REVIEW IN DVB-T2 SYSTEM In this section, we briefly overview the P1 symbol in DVBT2 system before introducing our proposed preamble. A. Overview of P1 Symbol Preamble P1 has been placed at the beginning of every T2 frame. According to the DVB-T2 standard specification [8], P1 symbol has four main purposes. • During the initial received signal scanning, it is used to recognize the T2 signal much faster instead of the whole T2 frame. • It can be differently discriminated from the other formats used in the Future Extension Frame (FEF) part to identify itself as a preamble of T2 frame.

2015 IEEE INTERNATIONAL SYMPOSIUM ON BROADBAND MULTI MEDIA SYSTEMS AND BROADCASTING (BMSB)

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The preamble sequences can help initialization process to determine some basic transmit information such as FFT size, GI size and transmission type (SISO/MISO), etc. Finally, P1 symbol enable the receiver to make a fine time and frequency synchronization.

B. Generation of P1 Symbol The P1 symbol in DVB-T2 consists of two signaling fields, that is 3-bit signaling S1 and 4-bit signaling S2. The extended S1 and S2 sequences are generated based on eight orthogonal complementary sequences of length 64 and sixteen orthogonal complementary sequences of length 256, corresponding to the reference table of S1 and S2 (CSS table) [8]. Two 64-length S1 sequences are attached at both sides of 256-length S2 sequence to compose a signaling sequence of length 384. After the differential binary phase shift keying (DBPSK) modulation and scrambling, signaling sequences are mapped onto 384 active subcarriers according to the carrier distribution sequence (CDS) pattern [8]. As shown in Fig. 1 (a), the number of available carrier of P1 symbol is 853, where only 384 carriers are activated within 6.83 MHz in the middle of the nominal bandwidth 7.61 MHz, and the other carriers are nullified. This carrier allocation strategy makes P1 symbol robust to large carrier frequency offset (CFO). For example, P1 symbol can be detected even the CFO is up to 500 kHz since the active carriers are still stay within the nominal bandwidth. After inverse fast Fourier transform (IFFT), the P1 symbol in time domain of size 1024 (part “A” in Fig. 1 (b)) is encapsulated as a 2K OFDM symbol with two guard interval portions (part “C” and “B”). Mathematically, encoded P1 symbol P k can be expressed as  k j2πf k SH , 0 ≤ k < 542  PA e PAk−542 , 542 ≤ k < 1566 Pk = (1)  k−1024 j2πfSH k PA e , 1566 ≤ k < 2048 where PAk is the baseband signal of part “A”, and fSH = 1/1024 indicates the frequency shift to improve robustness of P1 symbol applied to part “C” and part “B”.

Fig. 2. Double correlation structure in DVB-T2 system.

C. Detection of P1 Symbol At the receiver, the symbol time offset (STO) and fractional CFO (fCFO) can be estimated and compensated by using the double correlation scheme, as illustrated in Fig. 2. Here, the received part “A” will be transformed to the frequency domain by 1K FFT. In frequency domain, the integer CFO (iCFO) estimation can be exploited by correlating the received signal and CDS pattern, then the estimated sequences are extracted from active carriers. Finally, the S1 and S2 bits can be decoded by checking the cross-correlation between received sequences and S1, S2 CSS table. III. P ROPOSED P REAMBLE S CHEME A. Transmission of Proposed Preamble Signal The key technology of our proposed preamble are based on the hybrid differential modulation as illustrated in Fig. 3. To clarify, we restrict our system model to 2×1 co-located dualpolarized MISO system since an extension of MIMO system can be straight forward. Similar to the preamble transmission in DVB-T2 system, 3-bit signaling S1 and 4-bit signaling S2 are generated to 384-length modulation signaling sequence (MSS) by using the same reference table (CSS table). Then the conventional DBPSK modulation for the ith (i = 0, 1, ..., 383) index is encoded by i−1 i = DR · M ssiR DR

(2)

i where DR represents the real part of the differentially encoded symbol, M ssiR denotes the 384-length MSS sequence where M ssiR ∈ {−1, 1}. It is significantly noted that the data information in (2) stays on the In-phase axis (I axis). Since the performance of differential modulation is rapidly deteriorated as increasing the modulation order [16], the only DBPSK transmission is considered as an acceptable scheme for practical scenario. Fig. 4 shows the constellation of the real encoding of DBPSK, imaginary encoding of DBPSK, and differential quadrature phase shift keying (DQPSK), respectively. Assuming that high order differential encoding such as DQPSK is exploited as illustrated in Fig. 4 (c), 8 constellation points in the complex plane are observed. Due to the inherent characteristics of imaginary DPSK, only binary bits can be conveyed even though 4 constellation points are transmitted. Our proposed hybrid differential modulation can be exploited

2015 IEEE INTERNATIONAL SYMPOSIUM ON BROADBAND MULTI MEDIA SYSTEMS AND BROADCASTING (BMSB)

CDS Table

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Fig. 3. Transmit block diagram of the proposed preamble scheme.

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where z1k and z2k denote zero mean Additive White Gaussian Noise (AWGN) with variance N0 /2 per dimension. Thus, h1 and h2 represent the channel coefficients from from dualpolarized directional antennas.

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slots n and n + 1 for the kth sample can be represent as1 [ ] ] [ k ] PRk −PIk∗ [ k [ ] k k yn yn+1 = h1 h2 + z1k z2k (5) PIk PRk∗

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Fig. 4. Signal constellations of (a) DBPSK real part, (b) DBPSK imaginary part, and (C) DQPSK.

B. Detection of Proposed Preamble Signal by combining differential modulation for imaginary part with differential space-time block (DSTBC) decoding, which will be discussed in the next subsection. Specifically, independent 384-length MSS sequences with another 7 bits signaling are parallel encoded as following DIi = j · DIi−1 · M ssiI

(3)

√ where j = −1. Then the constellation of encoded sequence DIi can be mapped into Fig. 4 (b). The information stays in both I axis and Quadrature-phase axis (Q axis). Here, the 384i length MSS sequence M ssiR is encoded to DR in real numbers i i and the other M ssI is encoded into DI in imaginary numbers. Then, both of this two differentially encoded sequences perform a same generation structure followed by scrambling, active carriers padding, IFFT and C-A-B structure referring to the Fig. 3. After C-A-B construction, the time domain baseband signal for the real and imaginary numbers (for k = 0, 1, ..., 2047) can be represented by PRk and PIk , respectively. With two transmit antennas, according to the STBC encoding rule [10], all the real and imaginary symbols group into pairs [PRk , PIk ]T (where [·]T denotes the transpose), which are processed by space-time block encoder as: [

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At the receiver, the destination transforms the received k∗ k , then (5) can be equivalently treated into −yn+1 symbol yn+1 as ] [ k] [ k ] [ k ] [ k PR z1 hk2 h1 yn (6) + = k∗ hk∗ PIk −z2k∗ −hk∗ −yn+1 1 2 Note that we once again transform (6) into a quaternionic form [11] to perform a differential decoding of SFBC signal. Modified received signal matrix can be rewritten as ][ k ] ] [ k [ k ] [ k k hk2 z2k yn+1 h1 z1 yn PR −PIk∗ = + k∗ hk∗ ynk∗ −hk∗ −z2k∗ z1k∗ −yn+1 PIk PRk∗ 1 2 | {z } | {z }| {z } | {z } Yk

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(7) In order to estimate the STO and fCFO and also to decode S1 and S2 sequences, the double correlation scheme in DVBT2 (Fig. 2) can be used. Since that the whole received symbols suffer from the same STO and CFO effects during the same transmission time, thus we consider only one of the received symbol ynk (k = 0, 1, ..., 2047) as the input of the correlator, instead of the whole received matrix Yk for the simplicity of the analysis. Fig.5 shows the absolute value of each correlator’s output according to time samples. It is noted that the total output of the correlator gives a clear indication to estimate the timing offset. In additional, the fractional frequency offset can be estimated by considering the angle of time domain correlation. The C and B branches of the correlator can be seen as frequency shift compensating guard interval correlators. Both of them produce correlation peaks whose angel depends on the fractional frequency offset.

(4)

where (·)∗ denotes the complex-conjugate operation. In the followings, the received signals over two consecutive time

1 √ In this paper, all of the power normalization should be considered as Es , where Es is the average transmit power for each transmit antenna, Mt Mt indicates the number of transmit antennas. Since we have assumed that our proposed model is restricted to 2×1 MISO model, we can define Es = 1 and Mt = 2.

2015 IEEE INTERNATIONAL SYMPOSIUM ON BROADBAND MULTI MEDIA SYSTEMS AND BROADCASTING (BMSB)

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While the receiver performs STO and fCFO estimations and compensations, the guard interval parts “C” and “B” are discarded and only the received part “A” transforms back into frequency domain, where the integer CFO can be estimated by observing the correlation between the received signal and CDS pattern [6]. Upon completing the integer CFO compensation, the received symbols can be unpadded from the 1K subcarriers and then descrambled. Note that all of those processes must be done in the block unit in order to enable the differential STBC decoding. Let us define the received preamble matrix in frequency domain after descrambling as YiD (i = 0, 1, ..., 383). Then, the equivalent system model can be represented as YiD = Hi Di + Zi , in which ] [ ] [ i ] [ i b i −D b i∗ i z2i D z1 hi2 h1 i i R I Z = D = H = bi b i∗ −z2i∗ z1i∗ hi∗ −hi∗ D D 1 2 I R (8) Assuming that the channel states are constant over two consecutive transmission block times, i.e., Hi−1 = Hi , then the correlation between Hi−1 and Hi can be perfectly decoupled owing to the orthogonal properties as ∥Hi ∥2 = H(i−1)† Hi = ∥hi ∥2 I2 = (|hi1 |2 + |hi2 |2 )I2 . Here, (·)† denotes the Hermitian operation, and I2 is the identity matrix of size 2. Then, the DSFBC decoding can be exploited as (we ignore the noise terms for mathematical convenience): (i−1)†

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Fig. 5. Absolute value of outputs for (a) C branch correlator, (b) B branch correlator, and (c) total correlator.

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= D(i−1)† H(i−1)† Hi Di = ∥Hi ∥2 D(i−1)† Di [ b (i−1)∗ D bi + D b (i−1)∗ D bi ) (|hi1 |2 + |hi2 |2 )(D R I R I = i−1 b i i b i−1 i 2 i 2 b b (|h1 | + |h2 | )(−DR DI + DR DI )

]

(9)

α β

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Substituting equation (2) and (3) into ri and after some manipulating, we obtain i

i

[ [ ssI ) ri = (|hi1 |2 + |hi2 |2 )(M ssR + j · M

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Fig. 6. BER performances of hybrid differential BPSK, coherent detection BPSK[10] and differential BPSK[14] for MISO system over Rayleigh fading channel.

As a result, we can separately decode the 384-length MSS sequences M ssiR and M ssiI by taking the real and imaginary parts of ri (i = 0, 1, ..., 383), i.e., i

[ M ssR = ℜ(ri ) i

[ M ssI = ℑ(ri )

(12)

where ℜ(·) and ℑ(·) indicate real and imaginary part of the signal, respectively. Finally, the S1 and S2 bits are indepeni [ dently found by observing the correlation of M ssR,I and the CSS table.

IV.

PERFORMANCE EVALUATIONS

In this section, we accomplish the performance evaluations to reveal the gain of our proposed hybrid differential modulation and preamble scheme.

A. Performance of Hybrid Differential Modulation

where the equality of D(i−1)† H(i−1)† Hi Di = ∥Hi ∥2 D(i−1)† Di is met if and only if ∥Hi ∥2 denotes a diagonal matrix. b (i−1)∗ D b i∗ + In (9), we define α = (|hi1 |2 + |hi2 |2 )(−D I R i−1 b i∗ i∗ b (i−1)∗ i 2 i 2 b i−1 b i∗ b b DR DI ) and β = (|h1 | + |h2 | )(DR DR + DI DI ) to represent the entries (1,2) and (2,2), respectively. It is firstly observed that all entries perform the same differential decoding for each preamble sequence. Therefore, we define the first meaningful entry in (9) as ri , i.e., i b i−1∗ D bR b i−1∗ D b Ii ) ri = (|hi1 |2 + |hi2 |2 )(D +D R I

Bit Error Rate

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(11)

Firstly, we compare the performance of various modulations in order to verify the robustness of our proposed hybrid differential modulation. Fig. 6 depicts the BER performances of hybrid differential modulation, coherent BPSK [10] and differential BPSK [14] for MISO system over Rayleigh fading channel. It is clearly shown that the differential detection has 3 dB performance degradation compared to the coherent detection as in [14]. However, the hybrid differential BPSK achieves the same BER performance as coherent BPSK. It is noticed that 3 dB performance degradation of the differential detection can be eliminated by exploiting our proposed hybrid differential BPSK, and low computational complexity can be also achieved since the receiver does not require channel estimations.

2015 IEEE INTERNATIONAL SYMPOSIUM ON BROADBAND MULTI MEDIA SYSTEMS AND BROADCASTING (BMSB)

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Fig. 7. SER performances of the P1 symbol in DVB-T2 and the proposed preamble over static TU-6 and DVB-NGH outdoor channel.

B. Performance of Proposed Preamble Scheme We analyze the signaling error rate (SER) performances of DVB-T2 P1 and proposed preamble signaling over multi-path fading channels. Specifically, a typical urban 6-path channel model (TU-6) [9] and DVB-NGH 8-tap outdoor channel model [15] are generated with a combination of Jakes spectrum. To actualize the intra-tap correlation and cross-polarization impairment of the antennas, two co-located transmit antennas have been considered. The antenna correlation coefficient is set to 0.12, and the line-of-sight (LOS) factor K is considered. It is assumed that the same transmit configurations of both P1 and proposed preamble are applied. That is, the durations of both preambles are 224µs in 8MHz nominal bandwidth. Both of the transmitted signals consist of two 2K OFDM random symbols (the same length with the preamble) before and after the baseband preamble. We set the STO to δ = −60, the normalized CFO as ε = 5.5, and the iteration number as 15000 values/SNR. Fig. 7 shows the SER performances of the P1 symbol in DVB-T2 and proposed preamble scheme in static TU-6 and DVB-NGH outdoor channels, whereas Fig. 8 compares the corresponding performances in the presence of 20Hz Doppler shift. It is clearly observed that the proposed preamble scheme can provide up to 4dB SNR gain compared to the conventional P1 symbol in DVB-T2 . In the presence of 20 Hz Doppler shift, the proposed preamble scheme provides around 3dB and 4dB SNR gains in DVB-NGH and TU-6 channels, respectively. V. C ONCLUSION In this paper, we have presented an improved preamble scheme, which is modulated by a hybrid differential modulation to provide both spatial multiplexing gain and transmit diversity gain for the dual polarized DVB-T2 MISO system. Simulation results have clearly demonstrated that our proposed preamble scheme has a significant improved SER performances compared to the conventional P1 symbol in

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DVB-T2 P1, fd=20Hz, NGH outdoor Proposed, fd=20Hz, NGH outdoor DVB-T2 P1, fd=20Hz, TU-6 Proposed, fd=20Hz, TU-6 -10

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Fig. 8. SER performance of the P1 symbol in DVB-T2 and the proposed preamble over TU-6 and DVB-NGH outdoor channel in the presence of 20Hz Doppler shift.

DVB-T2 system over practical mobile broadcasting channels. Specifically, as much as 4dB SNR gains are observed in static TU-6 and DVB-NGH outdoor channels with and without 20 Hz Doppler shift presented, respectively. Our proposed preamble signaling scheme shall be considered as a very promising technology for use in the next generation broadcasting systems. R EFERENCES [1] J. G. Doblado, V. Baena, A. C. Oria, D. Perez-Calderon, and P. Lopes, “Coarse time synchronization for DVB-T2,” Electron. Letts., vol. 46, no. 11, pp. 797-798, May 2010 [2] M. Rotoloni, S. Tomasin and L. Vangelista, “On correlation-based synchronization for DVB-T2,” IEEE Commun. Lett., vol. 14, no. 3, pp. 248-250, Mar. 2010. [3] A. Vieβann, A. Waadt, C. Spiegel, C. Kocks, A. Burnic, P. Jung, G.H. Bruck, J. Kim, J. Lim and H.W. Lee, “Implementation-friendly synchronization algorithm for DVB-T2,” Electron. Lett., vol. 46, no. 4, pp. 248-250, Feb. 2010. [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] J. M. Choi, H. J. Kim, S. R. Yun, Y. H. Oh, Z. F. Kui, and J. S. Seo, “Improved transmission parameter signaling scheme utilizing crosscorrelation properties of DFT-spread Chu sequence,” in Proc. IEEE BMSB 2014, pp. 1-4, June 2014 [6] T. Jokela, M. Tupala, and J. Paavola, “Analysis of physical layer signaling transmission in DVB-T2 Systems,” IEEE Trans. Broadcast., vol. 56, no. 3, pp. 410-417, Sept. 2010 [7] J. S. Han, J. S. Baek, J. S. Seo, “MIMO-OFDM Transceivers With Dual-Polarized Division Multiplexing and Diversity for Multimedia Broadcasting Services,” IEEE Trans. Broadcast., vol. 59, no. 1, pp. 174,182, Mar. 2013 [8] Digital Video Broadcasting (DVB); Frame structure, channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2), ETSI Standard, EN 302 755, V1.3.1, Apr. 2012. [9] Digital Video Broadcasting (DVB); Implementation guidelines for a second generation digital terrestrial television broadcasting system (DVBT2), ETSI Standard, TS 102 831, V1.2.1, Aug. 2012. [10] S. M. Alamouti, “A simple transmitter diversity scheme for wireless communications,” IEEE J. Select. Areas Commun., vol. 16, pp. 14511458, Oct. 1998.

2015 IEEE INTERNATIONAL SYMPOSIUM ON BROADBAND MULTI MEDIA SYSTEMS AND BROADCASTING (BMSB)

[11] S. Diggavi, N. Al-Dhahir, A. Stamoulis, and A. R. Calderbank, “Differential space-time coding for frequency-selective channels,” IEEE Commun. Lett., vol. 6, pp. 253-255, June 2002. [12] N. Al-Dhahir, “A new high-rate differential space-time block coding scheme,” IEEE Commun. Lett. vol. 7, no. 11, pp. 540-542, Nov. 2003 [13] P. Tarasak , H. Minn, and V. K. Bhargava “Differential modulation for two-user cooperative diversity systems,” IEEE J. Select. Areas Commun., vol. 23, no. 9, pp. 1891-1900, Sept. 2005 [14] V. Tarokh, and H. Jafarkhani, “A differential detection scheme for transmit diversity,” IEEE J. Select. Areas Commun., vol. 18, pp. 11691174, July 2000 [15] P. Moss, DVB-NGH channel models, TM-NGH063, Nov. 2010. [16] S. Haykin, M. Moher, Introduction to Analog and Digital Communications, John Wiley and Sons Inc., 2007. [17] Y. S. Cho, J. W. Kim, W. Y. Yang, C. G. Kang, MIMO-OFDM Wireless Communications with Matlab, John Wiley and Sons (Asia) Pte Ltd, 2010

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