International Journal of Electronics and Electrical Engineering Vol. 1, No. 3, September, 2013 MC-DS-CDMA

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1 International Journal of Electronics and Electrical Engineering Vol. 1, No. 3, Septeber, 2013 Analysis of Carrier Frequency Selective Offset Estiation - Using Zero-IF and ZCZ In MC-DS-CDMA Venkatachala. Karthikeyan Departent of ECE, SVSCE, Coibatore, India karthick77keyan@gail.co Jeganathan. Vijayalakshi.V Departent of EEE, SKCET, Coibatore, India vijik810@gail.co Abstract A new ethod for frequency synchronization based upon Zero-Interediate Frequency (Zero-IF) receiver and characteristics of the received signal s power spectru for MC-DSCDMA-Uplink syste is proposed in this paper. In addition to this, eploying Zero Correlation Zone (ZCZ) sequences, designed specifically for quasisynchronous uplink transissions, is proposed to exploit frequency and teporal diversity in frequency-selective block-fading channels. The variance for Carrier Frequency Offset (CFO) estiators of MC-DS-CDMA Uplink is copared with that of an OFDM syste to estiate the CFO. Our study and results show that the MC-DS-CDMA syste is outperforing the OFDM ethod. the signal is orthogonal to one another there will absence of utual interference. The ajor advantage of OFDM signal is to avoid the effect of reflection and ISI over noisy channel. The carrier spacing is equal to the reciprocal of the sybol period. To study and perfor coparative analysis of carrier frequency offset estiation of two types of ultiple access signals such as OFDM and MCDS-CDMA. Furtherore, the proposed ethod is sipler to ipleent than the forer ethods for the reason that the correlation with the CP, the identical parts such as pilot carriers and the null carriers are not used. II. ANALYSIS of CARRIER FREQUENCY OFFSET Index Ters Carrier Frequency Offset (CFO), ZeroInterediate Frequency (Zero-IF), Multi-Carrier Code Division Multiple Access I. The MC-DS-CDMA Ref. [14] is ultiple access techniques for future networks physical layer due to its ability to support ultiple access capability, robustness to frequency selective channels, high spectral efficiency and narrow band interference rejection. Our contribution is to suggest a coarse offset estiation schee for MC- DSCDMA syste based on Zero-IF and on ZCZ sequence spreading codes Ref. [13] INTRODUCTION Technological advances over the past two decades have led to the rapid evolution of the telecounications industry. No longer liited to narrow-band voice signals, odern counications integrate voice, iages, data, and video on a level that was once considered to be ipossible. Basically, in cellular systes there are four ain ethods of ultiple access schees such as FDMA, TDMA, CDMA, and OFDM. The ain purpose of ultiple access schees is used to achieve the following nuber of goals such as to handle several nubers of users in the sae channel without any utual interference proble in it. Also, to axiize the range of the spectral efficiency and in addition to this its robustness, that is enabling the ease of handover/handoff between the cells. This OFDMA is based around OFDM which is a for of transission use large nuber of close spaced carriers that are odulated with low rate data. Though A. MC-CDMA Multi-Carrier Code Division Multiple Access (MCCDMA) is a ultiple access schee used in OFDMbased telecounication systes, allowing the syste to support ultiple users at the sae tie. In addition to this the table.1 clearly depicts the coparative analysis of various paraeters like spreading, spectral efficiency, PAPR and application as shown in Table I. B. MC-DS-CDMA One way of interpreting MC-CDMA is to regard it as a direct-sequence CDMA signal (DS-CDMA) which is transitted after it has been fed through an inverse FFT (Fast Fourier Transfor) Ref.[11] Multi-Carrier Code Division Multiple Access (MC DS-CDMA) is a ultiple access schee used in OFDM-based telecounication systes, allowing the syste to support ultiple users at the sae tie. In the MC-DS-CDMA technique, the seri- Manuscript received April 16, 2013; revised June 25, 2013 doi: /ijeee

2 International Journal of Electronics and Electrical Engineering Vol. 1, No. 3, Septeber, 2013 al-to-parallel converted data strea is ultiplied with the spreading sequence, and then the chips belonging to the sae sybol odulate the sae subcarrier. Here the spreading is done in the tie doain. Thus this MC-DS- CDMA technique plays vital role in uplink transission of wireless counication than any other access. The Transitter section of this MC-DS-CDMA is given by the generalized block diagra shown in Figure.1 In TFdoain spreading MC-DS-CDMA, assue that there are J ZCZ tie-doain spreading sequences available of length Q chips, i.e., a j (t) ( j = 0,.., J-1). Each tiedoain spreading sequence is associated with P frequency-doain spreading sequences of length M B S, i.e., c j,p ( j = 0,., P-1), where S and M B are the nuber of frequency tones and fading blocks over which frequency-doain spreading is done, respectively Ref. [10] Each user is assigned one tie-doain spreading sequence a j (t) and one frequency-doain sequence c j,p. The axiu nuber of users supported by TF-doain spreading MC-DS-CDMA is given by N = JP. Figure 1. TF-doain spreading for MC-DS-CDMA Fig.1 shows the transitter scheatic for a single-user in each fading block. At the transitter side, a sybol strea b j, p (t) with sybol period T sy is serial to parallel converted into U parallel branches in each fading block. The above assuption iplies that the coherence tie ( t) c of the channel is higher than but close to the sybol duration of UT sy (i.e. ( t) c UT sy ). In this paper, we refer to the sybol duration of UT sy as a fading block. Note that, in practice, if the fading blocks are subject to correlated fading then interleaving ay be eployed over fading blocks at the transitter, in order to guarantee the independent fading of the sub-carrier signals in each fading block. [10]Following tie-doain spreading, the spread signal in each branch u is repeated on S parallel branches in each of MB fading blocks, where each parallel branch is ultiplied by the corresponding chip value of a user-specific frequency-doain spreading sequence c j,p = [c j,p [1], c j,p [2],., c j,p [SM B ]] τ. In this analysis it is assued that the sae set of frequency-doain spreading sequences are used for each branch u. The U.S X 1 transission vector after tie-doain and frequency-doain spreading for each user, in fading block can be expressed in (1) Z j,p, = c c c () t b j, p j j, p,1 () t b j, p j j, p,2 () t b j, p j j, p, U =1,,M B (1) Where c j,p = [c j,p [(-1)S + 1], c j,p [(-1)S + 2],.., c j,p [(-1)S + S ]] τ, = 1,, M B. The cyclic prefix insertion atrix is expressed as T CP = [I τ CP I τ U.S] τ, where CP is the cyclic prefix length. The cyclic prefix reoval atrix at the receiver can be expressed as R CP = [0 U.SXCP I U.S ]. The atrix γ is defined as a U.S X U.S perutation atrix that axially separates the data of each branch u in the frequency-doain, i.e., the (x,y) th eleent of is equal to 1 if x = (y- 1)od(U).S + [(y-1)/u]+1 and equal to 0 otherwise. [9]The transitter output for each user in each fading block, fro Fig.1, the transitted signal can be expressed in (2) S (t) = T CP F H U.S γ z j, p,, = 1 MB. (2) Where the above equation (2) represents the diagonal atrix of the channel frequency response in fading block C. Zero-IF Receiver The zero-if receiver, also known as a hoodyne, synchrodyne or direct conversion receiver, is a special case of the super heterodyne receiver that uses an LO with the sae frequency as the carrier. In Ref. [13] order for the detector to differentiate between signal coponents both above and below the LO frequency, zero IF receivers generate both In-Phase and Quadrature (IQ) signals. The following Fig. 2 which shows generalized block diagra of ZERO-IF receiver architecture. Further the ain advantages of preferring this receiver is nothing but, Lower coplexity and power consuption (no IF aplifier, no IF band-pass filter, or no IF local oscillator), which has the potential to reach the one chip goal and No iage frequency. D. ZCZ Sequences Figure 2. Zero-IF Receiver Architecture 172

3 International Journal of Electronics and Electrical Engineering Vol. 1, No. 3, Septeber, 2013 III. Traditional orthogonal spreading sequences, such as orthogonal Gold sequences and Walsh-Hadaard sequences, exhibit non-zero off-peak cross-correlations, which liits the achievable perforance in asynchronous or quasi-synchronous scenarios. However, ZCZ sequences exhibit an interference free window over which both the cross-correlation and autocorrelation function are zero Ref. [13]. Consequently, ZCZ sequences are able to suppress MUI in the quasi-synchronous uplink channel. We denote a set of ZCZ sequences as (Lseq, Mseq, Z0) ZCZ, where Lseq is the sequence length, Mseq is the sequence faily size and Z0 is the one-sided ZCZ length in chips. For a sequence set {sq(q=0)}mseq with faily size Mseq, and sequence length Lseq, the periodic correlation characteristics of ZCZ sequences are defined in (3) and (4) ] s, r ( ) Lseq 1 i 0 Figure 3 copares the variance theory of the CFO estiator with siulations for MC-DS-CDMA and OFDM systes for a sybol of 256 saples, according to the length of the cyclic prefix NCP. Specifically, these coponents are pattern-dependent self-noise and AWG channel noise. Figure 4 and Figure 5 copare the siulated and theoretical variances as a function of SNR. At high SNRs, there is again a difference of approxiately 10% between the siulated and the theoretical variances for the ajority of the siulations. At very low signal to noise ratios, this difference increases until the siulated variance is approxiately 70% larger than the theoretical variance at an SNR. As the SNR increases, less iproveent in variance perforance is observed in Figure 4 and Figure 5 which is consistent with the atheatical Variance expression as the contribution fro the AWG coponent of the noise becoes overshadowed by the estiator self-noise. Figure 4 shows the effects of nuber of sybols used to estiate the CFO at an SNR of 10dB. At all Nsy values, the variance of the CFO in MCDS-CDMA syste is roughly 70% saller than the OFDM variance and roughly 10% larger than the theoretical variance. Fig 5 shows the effects of the AWGN coponent in variance of the CFO as a function of SNR. At very low SNR (< 10 db), the variance is approxiately larger than the theoretical variance, but is saller than the OFDM variance, while for SNRs above 10dB, the theoretical variance predicts the observed siulated. Figure 5 shows the perforance results using the optial ML detector and the MMSE block linear detector at the receiver. One observes fro Figure 6 that there is an approxiate 2.7 db perforance iproveent for the optial ML detector at a BER of 10-4 to Sq[l ]od ( Lseq) (3) = And Η = (4) Here each sequence eleent sq[l] is a coplex nuber and Z0 is the ZCZ length in chips. The paraeter η 1, is only equal to one when every sequence eleent sq[l] has unit aplitude, otherwise η < 1.. For a given ZCZ length, the theoretical upper bound is given in (5) Z0 Lseq 1 Mseq (5) TABLE I. COMPARISONS OF MC-CDMA AND MC-DS-CDMA The above expression suggests that to obtain a large ZCZ length, the sequence length Lseq needs to be considerably larger than the faily size Mseq. Thus, the desirable properties of these sequences coe at the cost of supporting only a sall nuber of users, copared to other orthogonal sequences, such as Walsh-Hadaard sequences Ref. [9] we will focus on the power spectru of the signal Xlb (k) to estiating the blind carrier frequency offset without any identical parts and without addition of any redundant data. The power spectru, ρl (k), is given in (6) ρl (k) = E [Xl (k) Xl *(k)] Frequency Spectral Efficiency High Application High in Uplink Synchronous Uplink and Downlink IV. (6) φ Spreading PAPR Tie Direction Low when ulti-carrier odulation other than OFDM is used Low in Uplink (Appropriate for uplink in ulti-user syste ) Asynchronous uplink and downlink SIMULATION RESULTS Siulations are carried out to investigate the perforance of the syste by choosing BER as a figure of erit. In the siulations, the channel coefficients are constant during one sybol block duration (i.e. UTsy), but change fro one block to another one. One observes only a slight perforance iproveent, when the MMSE detector is eployed at the receiver. Thus, the proposed syste out-perfors the syste proposed in literature. Where Xl* (k) is the coplex conjugate of Xl (k). In the MC-DS-CDMA, the odulation of each sybol is constant over Nf = N + NCP saples, and e j(φi(n) φi()) is a stationary process. Therefore, its expected value is calculated as given in (7) E [ej (φi (n) φi ()] = RESULTS AND DISCUSSIONS (7) 173

4 International Journal of Electronics and Electrical Engineering Vol. 1, No. 3, Septeber, 2013 Owing to the additional teporal diversity exploited in block fading conditions. Figure 6. Perforance of BER Vs Nuber of sybols Figure 3. NCP for a sybol of 256 saples V. CONCLUSION Thus the objective of this project work was to develop and to analyze an algorith for blind CFO recovery suitable for use with a practical zero-if OFDM telecounications syste [14]. MC-DS-CDMA is ore sensitive to carrier frequency offsets than other odulation techniques like QAM. CFOs significantly degrade the SNR at the output of the receiver. REFERENCES S. Suwa, H. Atarashi, and M. Sawahashi, Perforance coparison between MC/DS-CDMA and MC-CDMA for reverse link broadband packet wireless access, IEEE Transactions on Signal Processing, vol. 03, pp , 2002 [2] C. Langton, Intuitive guide to principles of counication, IEEE Transactions of Co. Dec [3] M. Li and W. Zhang, A novel ethod of carrier frequency offset estiation for OFDM systes, IEEE Transactions on Consuer Electronics, vol. 49, pp , Nov [4] H. Wei, L. L. Yang, and L. Hanzo, Tie- and frequencydoain spreading assisted MC DS-CDMA using interference rejection spreading codes for quasi-synchronous counications, in Proc. of IEEE VTC, vol. 1, no , Sep. 2004, pp [5] H. Steenda and L. Hanzo, Perforance of broadband ulticarrier DS-CDMA using space-tie spreading-assisted transit diversity, IEEE Trans. on Wireless Co., vol. 4, no. 3, pp , May [6] L. Liu and X. Dai, Pilot aided carrier frequency offset estiation in MC-DS-CDMA systes, IEEE Trans. on Wireless Co., 2008 [7] X. Wang, Linear Zero-IF direct conversion receiver, IEEE Trans. on Consuer Electronics, Dec [8] S. Philip, Multi-carrier systes, WITS lab, NSYSU, [9] Jean-Paul M.G. Linnartz, The basics of code division ultiple access, IEEE the Global Telecounications Conference, [10] S. A. B. Mustafa and S. N. Qadir, Perforance coparison of OFDMA and MC-CDMA over wireless channel. 5th International Advanced Technologies Syposiu (IATS 09), May 2009, Karabuk, Turkey. [11] F. Classen and H. Meyr, Frequency synchronization algoriths for OFDM systes suitable for counication over frequency selective are fading channels, in Proceedings IEEE the Global Telecounications Conference, vol.3, 1994, pp [12] Y. J. Huang and S. W. Wei, Modified guard band power detection ethods for OFDM frequency offset recovery, in proc. Ve- [1] Figure 4. Effects of Nuber of sybols Figure 5. Effects of AWGN 174

5 International Journal of Electronics and Electrical Engineering Vol. 1, No. 3, Septeber, 2013 hicular Technology Conference volue 4, Oct 2003, pp [13] M. Mitzel and M. Salt, Carrier frequency offset recovery for a zero-if OFDM receiver, IEEE CCECE/CCGEI, Saskatoon, May [14] A.B. Djebbar, K. Abed-Merai, and A. Djebbari, Blind channel equalization and carrier frequency offset estiation for MCCDMA systes using guard interval redundancy and excess codes, Int. J. Electron. Coun. pp , 2009 College of Engineering and Technology, Coibatore. He has about 7 years of teaching Experience. Jeganathan. Vijayalakshi has copleted her Bachelor s Degree in Electrical & Electronics Engineering fro Sri Raakrishna Engineering College, Coibatore, India She finished her Masters Degree in Power Systes Engineering fro Anna University of Technology, Coibatore. She is currently working as Assistant Professor in Sri Krishna college of Engineering and Technology, Coibatore. She has about 5 years of teaching Experience. Venkatachala. Karthikeyan has received his Bachelor s Degree in Electronics and Counication Engineering fro PGP college of Engineering and technology in 2003 Naakkal, India. He received Masters Degree in Applied Electronics fro KSR college of Technology, Erode in He is currently working as Assistant Professor in SVS 175