2011 International Conference on Advanced Technologies for Communications (ATC 2011) A Scheme of Dual Carrier Modulation with Soft-Decoding for MB-OFDM MIMO Systems Tien Hoa Nguyen1 , Nguyen Thanh Hieu1 , Tran Van Tuyen1 , Truong Vu Bang Giang1 , and Van Duc Nguyen2 Faculty of Electronics and Telecommunications University of Engineering and Technology Vietnam National University, Hanoi; Email: tienhoa@vnu.edu.vn Institute of Electronics and Telecommunications, Hanoi University of Science and Technology Abstract—The basic structure of the Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) Ultra-Wide Band (UWB) System is discussed in this paper The fixed-point simulation platform using the WiMedia Alliance proposed by ECMA 368 standards has been implemented An extension of the physical layer using the Alamouti method to a MultipleInput Multiple-Output (MIMO) system is also presented Based on the theoretical Dual Carrier Modulation (DCM) supported by Media Alliance, we propose a scheme of the DCM-Modulation and DCM-Demodulation with Soft-decoding for a MB-OFDM MIMO system The system performance is analyzed using the Saleh-Valenzuela channel model in terms of the bits error rate and the transmission range for indoor environments Keywords: UWB, MB-OFDM, BER, Saleh-Valenzuela (S-V) channel model, STBC, fixed-point simulation, Soft-decoding CM4 models an extreme RMS delay spread of 25 ns In this paper, as a solution for higher rate and reliable transmission, a MB-OFDM MIMO system is proposed A combination of space-time block codes (STBC) and hopping multiband UWB transmission is presented to exploit the spatial and frequency gain of the system The implemented system is available for further performance evaluation with more transceiver antennas Simulation results show that the system performance with DCM provides the robustness of the UWB system to fading channel and has a gain such as frequency diversity By spreading, each OFDM symbol is sent twice over each sub-band, therefor the system also achieves a frequency diversity gain of 3dB I I NTRODUCTION II MB-OFDM UWB SYSTEM DESCRIPTION Multiband OFDM (MB-OFDM) UWB is a technique that transfers data at very high data rate [1] By using OFDM on several frequency bands, MB-OFDM systems can support high speed data over multipath fading with severe long delay spread In order to avoid interference to other systems, Federal Communications Commission (FCC) has given spectrum mask for this technique In this order, the FCC allocated the spectrum from 3.1 to 10.6 GHz for unlicensed use by UWB transmitters operated at a limited transmission power of -41.25 dBm/MHz or less In order to transmit at very high data speeds, two modes of operation are required For data rate 53.3−200 Mbps the system uses QSPK Modulation and time domain spreading (TDS), where the first data symbol is used to form the second symbol For data rates of 320 Mbps and higher, data subcarriers of an OFDM symbol are modulated, using dual carrier modulation (DCM) DCM modulates the same four bits in two different subcarriers of the same OFDM symbol, separated by 50 subcarriers (approximately 206 MHz), DCM adds frequency diversity and thereby reduces the impact of frequency-selective fading The MB-OFDM signal is transmitted over multipath fading channels Several channels have been proposed for high speed UWB systems in IEEE 802.11.3a standard These channels are CM1, CM2, CM3 and CM4 that have been built based on Saleh Valenzuela model CM1 and CM2 cover − meters with line-of-sight and non-line-of-sight CM3 and CM4 are non-line-of-sight models CM3 is used for − 10 meters and The WiMedia Alliance has already published its ECMA International Standard for UWB Systems In December 2007 the latest version ECMA 368 has been proposed The ECMA defines a standard, that divides the entire unlicensed 7.5 GHz bandwidth from 3.1 GHz to 10.6 GHz into 14 sub-bands of each 528 MHz bandwidth, with the first twelve sub-bands contains four groups and the last two sub-bands are combined to the fifth group The data transmission is modulated with OFDM techniques in each sub-band In the conventional OFDM systems the transmitted signal is sent over one spectral allocation The ECMA 368 rules that the frequency band is changed with each recently generated OFDM symbol These frequency changing of the sub-bands depends on the Time Frequency Code TFC, that defines the frequency allocation of the transmitter signal Figure shows an example with TFC = [1, 3, 2] It means in this cace that, the first OFDM symbol is sent on band 1, the second OFDM symbol is sent on band and the information of the third OFDM symbol is sent on band The transmitter signal of MB-OFDM can be illustrated mathematically as follows 978-1-4577-1207-4/11/$26.00 ©2011 IEEE N −1 rn (t − nτSym )ej2πfn t s(t) = Re , (1) n=0 where rn (t) denotes the complex baseband signal of the nth OFDM symbol, which is in the interval from to τSym not equal to zero N is the number of the OFDM Symbols, τSym is the symbol interval and fn is the center frequency of the 220 f(MHz) where is the floor function, mod(a, b) is the modulus operator and NTDS = 2, if QPSK is used If DCM is used NTDS = We denote as (j) and aT (j), where j = 0, , NCBPS − 1, represent the input and output bits of the symbol block interleaver The output of the tone interleaver is given as j + 10 × mod(j, NTint ) , aT (j) = aS NTint 4572 BAND BAND 4224 BAND BAND 3696 BAND 3168 BAND 312.5 625 937.5 1250 1562.5 t(ns) Fig Time-frequency representation of multiband UWB symbols with TFC = [1, 3, 2] nth sub-band For carrier frequency is fn = (2904 + 528 · n)MHz with n = 14 (2) The MB-OFDM system have 128 subcarriers on each OFDM symbol (−64 ∼ −63, is DC) 12 pilot carriers are used for synchronization and located on -55, -45, -35, -25, -5, and 5, 15, 25, 35, 45, 55 Only 100 of 128 subcarriers for data transmission are needed Ten subcarriers are guard interval and the six remaining subcarriers are inserted with zero OFDM signal processes a bandwidth of 528 MHz and a duration of 312.5 ns The parameters for an OFDM symbol in time domain are represented on Table I The subcarriers are modulated with QPSK or DCM depending on the data rate QPSK modulation is used for data rates 200 Mbit/sec and lower DCM is used for data rates 320 Mbit/sec and higher The current WiMedia Alliance standard for modulation is introduced on Table II A MB-OFDM baseband transceiver simulation model has been implemented using fixed point Matlab/Simulink based on the specifications of the ECMA 368 Standard The implemented system includes scrambler, Reed-Solomon and punctured convolutional coder, interleaver, QPSK or DCM modulator, OFDM transmitter, frequency hopping and filter On the receiver side consists of de-scrambler, Reed-Solomon decoder, de-punctured Viterbi decoder, de-interleaver, QPSK or DCM de-modulator, OFDM receiver, frequency de-hopping and filter A frame-based processing is used in this simulation model A Interleaver und De-interleaver The motivation of interleaving is to provide robustness against burst errors The bit interleaving operation is performed in two distinct stages Symbol interleaving, which permutes the bits across consecutive OFDM symbols, enables the PHY to exploit frequency diversity within a band group Tone interleaving, which permutes the bits across the data subcarriers within an OFDM symbol, exploits frequency diversity across subcarriers and provides robustness against narrow-band interferers The symbol interleaving operation is performed by first grouping the coded bits into blocks of NCBP6S bits, corresponding to six OFDM symbols [1] We denote a( i) and as (i), where i = 0, , NCBP6S − 1, to represent the input and output bits of the symbol block interleaver i + × mod(i, NCBPS ) , aS (i) = a NCBPS NTDS where NTint = NCBPS /NTDS The output of the tone interleaver is then modulate with QPSK or DCM B Proposal a mapping method for DCM Modulation After bit interleaving, the 1200 interleaved and coded bits aT (i) are the input data b[i] to QPSK or DCM modulation, where i = 0, 1, 2, b[i] shall be divided into groups of 200 bits and converted into 100 complex numbers using DCM technique The 200 coded bits are grouped into 50 groups of bits Each group is represented as (bg(k) , bg(k)+1 , bg(k)+50 , bg(k)+51 ), where k ∈ [0 49] and g(k) = 2k 2k + 50 k ∈ [0 24] k ∈ [25 49] We denote (3) (4) xg(k) = 2bg(k) − The two resulting DCM symbols (A[k], A[k+50]) are mapped into two 16-QAM like constellations [1], H= A[k] A[k + 50] =H 1 −2 xg(k) xg(k)+1 (5) jxg(k)+50 jxg(k)+51 (6) and then allocated into two individual OFDM data subcarriers with 50 subcarriers separation Because each OFDM subcarrier occupies a bandwidth of 4.125 MHz, the bandwidth between the two individual OFDM data subcarriers is at about 200 MHz, which offers frequency diversity gain against channel deep fading C DCM Soft bit Demapping Two equalized complex numbers, which are transmitted on different subcarriers with 50 subcarriers separation, can be combined at the receiver side to demodulate the signal The receiver signals of the k th and (k + 50)th subcarriers can be given as: Ak = (2xg(k) + xg(k)+1 ) + j(2xg(k)+50 + xg(k)+51 ) Ak+50 = (xg(k) − xg(k)+1 ) + j(xg(k)+50 − xg(k)+51 ) (7) we can obtain the received symbol as: (2Ak + Ak+50 ) = 5xg(k) (2Ak + Ak+50 ) = 5xg(k)+50 (Ak − 2Ak+50 ) = 5xg(k)+1 (Ak − 2Ak+50 ) = 5xg(k)+51 (8) The decoded bit stream can be calculated with soft decoding as follows: sign(xg(k) ) + (9) bg(k) = 221 III MIMO MB-OFDM SCHEME The matrix H is defined as: The Alamouti method has been published in several publications [2] In general, the Alamouti code is implemented, when the space coded symbols are sent over one spectral allocation As mentioned above, the frequency sub-band depends on TFC So that there are three channels, in which the transmitted signal is hopped and sent, (see Figure 1) To avoid this requirement, a new process for STBC has been developed Modulation STBC Encoder b[i] QPSK/DCM Am [i] A0,m.[0] A0,m [ND−1 ] A3,m.[0] A3,m [ND−1 ] Fig Distribute the subcarriers on the Antenna where ND is the number of data-subcarriers, m = 0, 1, 2, is the OFDM symbol index OFDM Symbols [Am , , Am+5 ] X0 HH H = = (13) Hi∗ Hi2 i∗ H2 −Hi1 |Hi1 |2 + |Hi2 |2 Hi1 Hi∗ Hi2 −Hi∗ |Hi1 |2 + |Hi2 |2 (14) = (|Hi1 |2 + |Hi2 |2 )I2 , where I2 is a (2 × 2) identity matrix The superscript H denotes the Hermitian (transpose conjugate) of a matrix Hi1 and Hi2 represent the transfer function from transmit antenna one and two to receiver antenna in the ith transmitted subband The receiver signal after demodulated can be written in matrix form as: X0 X3 IFFT Add Pilots, CP, GI = HH H = HH H exp(jnωs t) X3 Hi2 −Hi∗ We obtain A STBC MB-OFDM UWB transmitter The coded data stream is first modulated using QPSK or DCM modulator then the modulated symbols are changed from serial to parallel The N parallel symbol vectors are given as: Am = [Am [0], Am [1], , Am [ND − 1]]T , (10) Hi1 Hi∗ H= = IFFT Add Pilots, CP, GI X0 X3 −1 −1 HH Y0 Y3∗ HH H + HH H X0 X3 −1 HH + N0 N3∗ N0 N3∗ IV SIMULATION RESULTS STBC Transmitter are firstly grouped, then coded with the encoder matrix g2 as follows: transmitted antennas ✲ X0 X3 X1 X4 X2 X5 (11) STC g2 = ∗ ∗ time slots −X X −X4∗ X1∗ −X5∗ X2∗ ❄ With the proposed coding the repeated couple signals A0 , A3 and −A∗3 , A∗0 are sent in the 1st and 4th time-slots in the same spectral allocation The designed scheme with Matlab/Simulink is showed in Figure Because of three subbands are used to transmit Data, so that the channel matrix H = H1 H3 H2 if TFC = [1 2], where H1 denotes the transfer function of the sub-band 1, H2 denotes the transfer function of the sub-band 2, and the transfer function of the sub-band is denoted by H3 To support the theoretical analysis given previously and implement the physical layer of a MB-OFDM system proposed by ECMA 368, we present our simulation with employing STBC The number of antennas used in this simulation is NTx = and NRx = A convolutional encoder (Rc = 5/8, 1/2) and Viterbi decoder are used for channel coding and decoding The results are represented for BER versus the transmitted bit energy to the noise power spectral density Eb /N0 The BER for 50 channel realization results for each measurement has been averaged Figure shows the results for different data rates (160, 200 and 400 Mbps), Rc = (5/8, 1/2) and different mode (SISO, MIMO) First we note that in both B Receiver description During the first transmission, the symbols X0 and X3 are transmitted simultaneously from antenna one and antenna two respectively In the fourth transmission period, the symbol −X3∗ is transmitted from antenna one and the symbol X0∗ from transmit antenna two Then we write: Y0 Y3∗ = Hi1 Hi∗ Hi2 −Hi∗ X0 X3 + (15) N0 N3∗ (12) 222 Fig Bit error rate Performance of UWB MB-OFDM Systems SISO and MIMO mode the performance of system with lower code rate Rc = 1/2 is better than higher code rate Rc = 5/8 The reason for this difference is due to redundancy At lower code rate Rc = 1/2, the sender adds more redundant bits and allows more efficient decoding at the receiver To compare the data rate 200 Mbps with 400 Mbps with the same code rate Rc and different modulation, the results show that the system with QPSK modulation provides around 2dB in both SISO and MIMO mode more than the DCM modulation This is due to the fact that the complex symbols on the constellation diagram of QPSK modulation are further to each other than DCM modulation by the same power transmission The results show also that the performance of the STBC MB-OFDM system is around 5dB better than the single antenna system in all simulated cases In the case of QPSK modulation, the STBC achieves theoretically a diversity gain of 3dB But in low data rate mode a significant diversity gain can be achieved by using Time Domain Spreading (TDS) Two repeated OFDM symbols are sent in different time slots and provides 3dB time diversity gain So that theoretically a maximum gain of 6dB can be achieved , and in particular the results show 5dB gain For data rate 400 Mbps, TDS is removed in all schemes The same symbols have been modulated to a pair of subcarriers It means that the frequency spreading has been used instead time domain spreading Therefore the system performance shows also around 5dB gain when comparing MIMO using STBC and single antenna system Parameters fs NFFT ND NP NG NZPS NT ∆f TFFT TSYM Description Sampling rate OFDM subcarrier Data subcarrier Pilot subcarrier Guard interval zero subcarrier used subcarrier Subcarrier frequency spacing IFFT Period Symbol duration Values 528 128 100 12 10 37 122 4.125 MHz 242.42 ns 3.125 ns V C ONCLUSION The physical layer and performance of MB-OFDM UWB system proposed by ECMA 368 has been investigated In this paper, we proposed a multiband MIMO coding using STBC and a scheme of mapping for coding and decoding DCM The performance of the system is evaluated with channel CM1 specified by the 802.15.3a, represents LOS with distances of less than m The developed simulation platform of MB-OFDM UWB baseband transceiver can be used directly to change the sub-blocks and to compare the performance variations effectively for further updated specifications VI ACKNOWLEDGMENT This work has been partly supported by the research project number QG.10.43, granted by Vietnam National University (VNU) R EFERENCES [1] ECMA, ”Standard ECMA-368: High data rate ultra wideband PHY and MAC standard”, Dec 2005 [2] S M Alamouti, ”A simple transmit diversity technique for wireless communication”, IEEE Journal on Select Areas in Communications, Vol 17, pages(s) 1451-1458, Oct.1998 [3] IEEE P802.15.3a, ”IEEE 802.15.3a high datarate alternative PHY Task Group (TG3a) for wireless personal area networks: channel modeling subcommitee report (Doc Number P802.15-02/368, SG3a”, Sep 2002 [4] W Pam Siriwongpairat and K J Ray Liu, ”Ultra-Wideband Communications Systems: Multiband OFDM Approach”, Wiley IEEE Press, December 2007 [5] Ghobad Haidari ”WiMidia UWB Technology of Choice for Wireless UWB and Bluetooth”, Wiley, September 2008 [6] Mohammad Ghavami, Lachlan Michael, and Ryuji Kohno, ”Ultra Wideband Signals and Systems in Communication Engineering”, John Wiley and Sons, May 2004 [7] A Batra et al, ”Multi-band OFDM physical layer proposal for ieee 802.15 task group 3a”, IEEE P802.15.3a, page 268r3, July TABLE I OFDM S YMBOL PARAMERTERS [1] Data rate (Mbps) 53.3 55 80 106.7 110 160 200 320 400 480 Modulation Coderate QPSK QPSK QPSK QPSK QPSK QPSK QPSK DCM DCM DCM 1/3 11/32 1/2 1/3 11/32 1/2 5/8 1/2 5/8 3/4 FSD TSD 2 1 1 1 2 2 2 1 Coded Bits/Symbol NCBPS 100 100 100 200 200 200 200 200 200 200 TABLE II ECMA 368 PARAMETERS [1] 223 ... on Table I The subcarriers are modulated with QPSK or DCM depending on the data rate QPSK modulation is used for data rates 200 Mbit/sec and lower DCM is used for data rates 320 Mbit/sec and... subcarriers with 50 subcarriers separation Because each OFDM subcarrier occupies a bandwidth of 4.125 MHz, the bandwidth between the two individual OFDM data subcarriers is at about 200 MHz, which offers... WiMedia Alliance standard for modulation is introduced on Table II A MB-OFDM baseband transceiver simulation model has been implemented using fixed point Matlab/Simulink based on the specifications