Comparatve Analyss of FBMC and OFDM Multcarrer Technques for Wreless Communcaton Networks Bdyalaxm Dev Tensubam ost Graduate cholar Nongmathem Lallema Chanu ost Graduate cholar onka ngh, h.d Assocate rofessor ABTRACT Multcarrer (MC) technques are hghly attractve for the development of modern wreless communcaton systems such as 4 th Generaton (4G) and Long Term Evoluton (LTE) systems. o far, Orthogonal Frequency Dvson Multplexng (OFDM) s the most popular and hghly recommended modulaton technque for Dynamc spectrum access (DA) and Cogntve Rado (CR) applcatons. Despte ther numerous advantages, OFDM system suffers from few drawbacks. Flter bank Multcarrer (FBMC), another modulaton technque evolved from OFDM whch overcomes these drawbacks s examned. FBMC apples flter banks and omts the severe out of band leakage of OFDM. Ths paper presents the comparatve performance analyss of conventonal OFDM system and FBMC system n terms of gnal to nose rato (NR), Bt error rate (BER), acket delvery rato(dr) and number of packets dropped wth the help of computer smulatons performed usng NETWORK IMULATOR N-2.34. Keywords OFDM, FBMC, DA, OFDM-OQAM, N2, MAC 1. INTRODUCTION The growth of nternet and the ncreasng demand of hgh data rate users have gven Dynamc spectrum access networks a wdespread attenton n the recent years. In DA networks, multple nodes compete to utlze a shared frequency spectrum. OFDM wth cyclc prefx (C-OFDM), whch uses an orthogonal set of subcarrers s by far the most popular case of multcarrer systems that has been proposed for the purpose of sharng the dfferent subsets of these subcarrers among nodes [1],[2] deployed for DA. However, the tght tmng and synchronzaton requrement n OFDM s dffcult to acheve n practce when the nodes belong to dfferent admnstratve enttes. Therefore, mutual nterference among the sgnals of dfferent users are resulted n case of an lack of synchronzaton. Moreover, the C employed s purely redundant n terms of nformaton and consderably reduces the bandwdth effcency. The lmtatons of OFDM were overcome by a multcarrer communcaton system known as FBMC frst proposed by altzberg [3] whch provdes a better spectral shapng of subcarrers than OFDM systems. Ths s accomplshed by the careful desgnng of the prototype flter, whch smplfes equalzaton n the absence of C and also promses a more effcent spectral utlzaton by mnmzng nterference across subcarrers [4]. By employng Offset Quadrature Ampltude Modulaton (OQAM), the full capacty of the transmsson bandwdth can be acheved n FBMC systems. Usng NETWORK IMULATOR (N-2.34), the most commonly used smulator for networkng and wreless studes, both the modulaton technques.e, FBMC and OFDM are smulated. Through extensve smulatons, performance of both the systems are analyzed and compared n terms of NR, BER, DR and number of packets dropped. 2. FBMC Vs OFDM FBMC s an advancement of OFDM. The modulators of OFDM and FBMC are shown n Fg.1. The only fundamental change s the replacement of the OFDM wth a multcarrer system based on flter banks, wheren the IFFT plus Cn s replaced by the synthess flter bank (FB) whle FFT plus Cout s replaced by the analyss flter bank (AFB).The basc prncples of OFDM and FBMC are explaned beow: OFDM: In OFDM system, the frequency spectrum of the subcarrers are overlapped wth mnmum frequency spacng and the orthogonalty s acheved between the dfferent subcarrers.. In fg.1, the nput s splt nto parallel data s usng the seral to parallel () converter, whch are then passed nto an nverse fast Fourer transformaton (IFFT) block to generate tme sequence of the s. ubsequently, by addng a cyclc prefx (C), the OFDM symbol tme sequences are extended. The C s a copy of the last part of the symbol that s added n the begnnng of the sequence and should be larger than the network delay spread norder to mtgate the ntersymbol nterference (II) generated by the arrval of dfferent OFDM symbols wth dfferent delay. The resultng dgtal sgnal s converted nto analog form and transmtted over the channel. 27
OFDM: FBMC: r nput e e channel output output IFFT FFT p f x n Transmtter r f x o u t Recever ynthess Flter Bank Analyss Flter Bank nput channel output IFFT N p N FFT Transmtter Recever Fg 1: Modulators of OFDM and FBMC[5] Table 1: Major dfferences between OFDM and FBMC systems roperty OFDM FBMC C Extenson C extenson requred and therefore reduces Bandwdth (BW) effecency Not requred and hence conserves BW delobes Large sdelobes Low sdelobes ynchronzaton Doppler effect MIMO ystems pectrum sensng For correct detecton, multple access nterference (MAI) cancellaton should be performed at the recever Hghly senstve to the carrer frequency offset Hgh flexblty whle adoptng MIMO technques. Degraded spectrum sensng performance due to the spectral leakage n OFDM sgnals MAI s suppressed due to the excellent frequency localzaton of the subcarrers Less senstve and hence performs sgnfcantly wth the ncrease of the user moblty Lmted flexblty Hgh spectrum sensng resoluton Computatonal Complexty Less complex More complex At the recever, the sgnal s reconverted nto dgtal form and the fast Fourer transform (FFT) s performed n the receved s after removng the C. Lastly, the parallel s are gathered nto a sngle as the orgnal transmtted one. ome of the drawbacks of OFDM are lsted below: Reduced spectral effcency due to the C employed Hgh spectral leakage due to the rectangular wndowng Interference between the unsysnchronzed sgnal n the adjacent bands FBMC: FBMC overcomes the shortcomngs of OFDM by addng generalzed pulse shapng flters whch produces a well localzed subchannel n both tme and frequency doman. Accordngly, FBMC systems have more spectral contanment sgnals and provde more effectve use of the rado resources where no C s needed. 28
Flterbanks can be defned as an array of N flters that processes N nput sgnals to produce N outputs. If the nputs of these N flters are connected together, the system n analogous manner can be consdered as an analyzer to the nput sgnal based on each flter characterstcs. Hence, ths type of flter bank s called analyss flter bank (AFB). Whle on the other hand, by addng the outputs of the flter array, a new sgnal s syntheszed and hence ths type of flter bank s called synthess flter bank (FB)[6]. The synthess-analyss confguraton s called transmultplexer and s appled n the MC communcaton systems[7]. handlng of the data traffc, the wdely used TC protocol s used. Here, f a partcular node s congested then the network coordnator dentfes the congested node by applyng ts algorthm and delvers the packet to any neghbourng node whch s less congested. The process to determne f a packet s receved correctly comprses of the followng steps: 1. Estmaton of the sgnal to nose rato (NRrx) at the recever, based on the power receved and the nose fgure of the receptor. 2. Calculaton of the bt error rate (BER) of the packet based on the NRrx. 4. IMULATION ETU AND REULT The NETWORK IMULATOR N-2.34 s used for the analyss. A snapshot of the smulaton topology s shown n Fg.3. Fg 2: Frequency response of OFDM and FBMC [8]. The frequency response of OFDM and FBMC shown n Fg.2 reveals the mpact of the transmsson of data due to ths exchange. Here, OFDM exhbts strong sdelobes due to the rectangular wndowng, whle on the other hand energy s concentrated wthn the frequency range of a sngle subcarrer n case of FBMC. Table 1 shows the major dfferences between OFDM and FBMC systems. 3. YTEM MODEL The model s based on the operaton of the HY and MAC layer. The channel model and the dfferent process nsde them are assumed accordng to the IEEE 802.11 standard. We assumed the channel to be the default wreless channel class developed n N2. The HY enttes are nserted n the lst wth the approprate command n the TCL envronment. The model mplementaton n N2 has the basc lnk budget parameters for Wreless LAN technology stored n a header n the transmtted packet. For the node confguraton, we assumed the two ray propagaton model. For effcent Fg 3: napshot of the NAM smulator Here, the smulaton scenaros are defned n the TCL envronment to make use of the avalable enttes wth ther correspondng parameters. For each of the enttes, we had to defne the proper TCL command to buld and confgure them. Ths enables moble nodes to communcate among each other through wreless network nterfaces, ncludng the ablty to smulate multhop wreless networks. We utlze 802.11 OFDM rado parameters at 5GHZ for packet data transfer among the 40 (0-39) nodes deployed. The default wreless channel consdered s of 20MHZ bandwdth. HY data rates are upto 54Mbps maxmum and transmts power vares from 0-20dBm.Other physcal layer parameters lke path loss, fadng, nterference and nose computatons are usually not taken nto account n wreless smulatons nspte of ther mportant effects n smulaton results [9]. 29
Fg 4: Comparson of gnal to Nose Rato (NR) Fg 5: Comparson of bt error rate (BER) Fg 6: Comparson of acket delvery rato(dr) 30
Also, the propagaton model [10] s assumed to be the two ray propagaton mode for the smulaton. Further detals of ths smulaton envronment are avalable n [10]. Here, the default N2 physcal layer modulaton comprsng of OFDM-QAM s modfed to the FBMC (OFDM-OQAM) system. Fg 4 shows the comparson of sgnal to nose rato of both the systems wth respect to number of nodes. FBMC gves better NR performance than the OFDM system. BER of OFDM s greater than FBMC and s shown n Fg 5. acket delvery rato (DR) may be defned as the rato of number of delvered packets to the total number of packets transmtted. FBMC has hgher DR than OFDM and s shown n Fg 6. Fg 7 shows the comparson of dropped packets for each system. The dropped packets are the ones whch have not reached the destnaton, even though a lnk s beng establshed between the communcaton nodes. FBMC performs better than OFDM. 5. CONCLUION In ths paper, the performance comparson between the most popular multcarrer modulaton technque OFDM and a lesser known technque FBMC s performed n terms of Throughput, NR, BER, packet delvery rato and dropped packets. mulaton results show that FBMC gves overall performance mprovement compared to conventonal OFDM for all the parameters consdered, provng FBMC as an deal canddate for future development n wreless communcatons. 6. REFERENCE [1] T. A. Wess and F. K. Jondral, pectrum oolng: An nnovatve strategy for the enhancement of spectrum Fg 7: Comparson of dropped packets effcency, IEEE Communcaton Mag., vol. 42, no. 3, pp. 8-14, Mar. 2004. [2] D.J.chafer, Wde area adaptve spectrum applcatons, n roc. IEEE MILCOM, 2001, vol. 1, pp. 1-5. [3] B.altzberg, erformance of an effcent parallel data transmsson system, IEEE Trans. Com. Tech, vol. 15, COM-6, pp.805-811, Dec. 1967. [4] B. Farhang Boroujeny, A square root Nyqust (M) flter desgn for dgtal communcaton systems, IEEE Trans. gnal rocess.,vol. 56.no.5,pp. 2127-2132,May 2008. [5] Frank chach, Flterbank based multcarrer transmsson (FBMC)-evolvng OFDM, Bell Labs Alcatel-Lucent, 2010. [6] T.Hdalgo ttz, Flterbank technques for the physcal layer n wreless communcatons, Tampere Unversty of Technology publcatons:919,2010. [7] Vadyanathan, Multrate systems and flterbanks.1993. [8] HYDYA- hyscal layer for dynamc spectrum access and cogntve rado, roject webste: www.ctphydyas.org. [9] M. Taka, J.Martn, and R.Bargoda, Effects of Wreless hyscal Layer modelng n Moble Ad Hoc Networks, In rocessng of MobHoc 2001. [10] G. Holland, N. Vadya and.bahl, A Rate Adaptve MAC rotocol for Mult-Hop Wreless Networks, MOBICOM, Rome, July 2001. IJCA TM : www.jcaonlne.org 31