Adaptive Subband Allocation in FH-OFDMA with Channel Aware Frequency Hopping Algorithm

Transcription

1 Internatonal Journal on Communcatons Antenna and Propagaton (I.Re.C.A.P.), Vol. 2,. ISS February 202 Adaptve Subband Allocaton n FH-OFDMA wth Channel Aware Frequency Hoppng Algorthm Ardalan Alzadeh, Seyed Mohammad-Sajad Sadough Abstract Ths paper presents a new subband allocaton scheme for multuser OFDMA systems. Adaptve OFDMA systems have focused on adaptng the allocaton of subcarrers and power to the nstantaneous channel condtons of all users. Usng frequency hoppng pattern n OFDMA system allow users to mnmze ntercell and ntracell nterference, and perform frequency dversty. In contrast to conventonal FH-OFDMA, whch uses a channel state ndependent hoppng sequence, the transmtter n the channel aware scheme hops to the avalable frequency subband whch has the largest transmsson gan. Smulaton results demonstrate that n power constrants assumpton, usng our proposed channel aware frequency hoppng (CAFH) along wth OFDMA scheme outperforms multuser OFDMA system wth fxed subband assgnment. Copyrght 202 Prase Worthy Prze S.r.l. - All rghts reserved. Keywords: OFDMA, Channel Aware Frequency Hoppng Algorthm, FH-OFDMA omenclature ( ) T Transpose operaton F IFFT matrx r umber of rounds Total number of subcarrers C Set of subcarrers assgned to the -th user umber of subcarrers assgned to the -th user n(t) AWG nose L umber of subbands T Duraton of one OFDM symbol I. Introducton Orthogonal frequency dvson multplexng (OFDM) has been presented as a new technology for next generaton wreless communcaton systems. In OFDM systems, hgh-rate nformaton can be dvded nto a number of parallel lower-rate streams wth the advantage of avodng the requrements of complex equalzaton []. These systems also provde the multple access schemes termed as orthogonal frequency dvson multple access (OFDMA). In OFDMA, a fracton of avalable subcarrers s assgned to each user based on the demand for bandwdth. Three advantages of OFDMA nclude () the flexblty n subcarrers' allocaton; (2) the absence of multuser nterference due to subcarrers' orthogonalty; (3) the smplcty of the recever desgn []. To mprove the system throughput and spectral effcency, frequency hoppng (FH) technque s generally used n OFDMA cellular systems. As mentoned n [2], t s desrable for FH patterns to satsfy the followng condtons: () mnmze ntracell nterference; () average ntercell nterference; () avod ambguty whle dentfyng users; (v) explot frequency dversty by forcng hops to span a large bandwdth. The frst aspect s relatvely easy to acheve by usng orthogonal hoppng patterns wthn a cell. To average ntercell nterferences, hoppng patterns are constructed n a way that two users n dfferent cells nterfere wth each other only durng a small fracton of all hops. The thrd condton requres base statons to have the capablty of dstngushng dfferent user effcency accordng to ther unque FH sgnatures. Fnally, the last requrement not only ensures the securty of the transmsson, but also mtgates the effect of fadng by explotng frequency dversty [2]. On the other hand, frequency hoppng pattern scheme has acheved consderable attenton n both mltary and commercal communcaton systems. There has been much research on desgnng FH-OFDMA systems. For nstance n [3], concepts of fast frequency hoppng along wth OFDM are provded. Orthogonal Latn squares (LSs) are presented as FH patterns n TCM/BICM coded OFDMA n [4]. In LS-aded FH-OFDMA systems, there s wde varablty n performance of users wthn dfferent cells. Therefore, t s not a useful scheme when the farness consderaton s mportant. Therefore, although users of each cell experences sgnfcant performance mprovement, the cell may not occupy all of the avalable bandwdth to receve full frequency dversty. Other aspects on preventng hostle jammng and plot-asssted channel estmaton n FH-OFDMA are provded n [5], [6]. Manuscrpt receved and revsed January 202, accepted February 202 Copyrght 202 Prase Worthy Prze S.r.l. - All rghts reserved

2 A. Alzadeh, S. M.-S. Sadough A channel-aware frequency hoppng (CAFH) multple-access scheme was proposed n [7]. CAFH algorthm montors the channel status at the base staton (BS). The BS then determnes the subband gans, for each MS, and uses ths nowledge to assgn, to each MS, the subband that enjoys the hghest gan, whle ensurng that no more than one MS s assgned to the same subband. The resultng channel assgnment algorthm s teratve n nature and has, n general, r rounds. The BER of CAFH s analyzed assumng ndependently Raylegh faded subbands. In ths paper, we propose a novel strategy based on frequency hoppng pattern by channel aware frequency hoppng algorthm for practcal OFDMA cellular systems. Several channel allocaton schemes are currently offerng approprate performance n consderatons of power and bt constrants. Our smulaton results show that CAFH-OFDMA system can offer sgnfcant performance mprovement over fxed subband assgnment. II. System Model In ths secton, frst we descrbe the FH-OFDMA transmtter for each ndvdual user. Then, we ntroduce the channel aware frequency hoppng scheme under one subband per user assumpton. II.. FH-OFDMA System The bloc dagram of a smple FH-OFDMA system wth FEC s shown n Fg.. In ths model, data bts of every user are channel coded and then mapped to complex constellaton ponts. K j2 /T t S t x e, 0 t T () where s (t) represents the tme-doman sgnal, and T denotes one OFDM symbol duraton. In OFDMA systems, dfferent set of subcarrers s assgned to every user. Ths subband allocaton s dynamc n the case of frequency hoppng OFDMA. In ths scheme, the frequency assgnment follows a predetermned FH pattern by the IFFT module. If subcarrers are not assgned to users, zeros are transmtted. For convenence, we note C as the subcarrer that s assgned to user. Hence, nformaton symbols vector of user can be wrtten as: x 0, C x, C (2) The dscrete form of the transmtted sgnal s (t) s then gven as: s Fx (3) where F s the IFFT matrx defned as: 00 0 W W F (4) W W where W pq =e j2 pq/. 0 II.2. Channel Aware Frequency Hoppng System Descrpton: We adopt the slowly tmevaryng, frequency-selectve Raylegh fadng channel model whch s commonly used for wdeband systems [8]. The nstantaneous sgnal-to nose rato (SR), n each subband s exponentally dstrbuted accordng to: 0 P e (5) 0 Fg.. FH-OFDMA system wth FEC [2] We assume that there are M users n the system, utlzng a total of OFDM subcarrers. Each user s assgned a specfc set of subcarrers out of the total avalable subcarrers accordng to hs/her data rates. Let be the number of subcarrers allocated to user. Then, user transmts the nformaton symbols x = (x, x 2,, x K, ) T on the assgned K subcarrers where ( ) T represents the transpose operaton. Therefore, the baseband transmtted sgnal of user can be expressed as: where 0 s the average value of. The slowly varyng nature of the channel mples that t can be treated as tme-nvarant over the duraton of a frame or an OFDM symbol. Consder the upln of a cellular system n whch K MSs transmt over L, L K, subbands to the BS. The sgnal receved (at the BS) durng the -th sgnalng nterval at the BS s: K r t g S t n t (6) Copyrght 202 Prase Worthy Prze S.r.l. - All rghts reserved Int. Journal on Communcatons Antenna and Propagaton, Vol. 2,. 2

3 A. Alzadeh, S. M.-S. Sadough where s the tme delay of MS, n(t) represents addtve whte Gaussan nose (AWG) wth two-sded power spectral densty (PSD) 0 =2, and g s the subband gan of MS durng the -th sgnalng nterval. The subband gan g depends on the scheme that s used to assgn subbands to MSs. CAFH Scheme: In the CAFH scheme wth r rounds, the BS assgns subbands to actve MSs as follows. The round number s ntally set to. In round j, j= ; 2;...; r for each MS,, whch has not yet receved ts subband assgnment, the subband j,...,l, wth the j -th j best subband gan, g,, out of L subbands s determned. If a gven subband has the j -th best gan for exactly one MS, the subband s assgned to that MS and the scheme enters round j+. If an avalable subband has the j-th best gan for several MSs, the subband s assgned to the MS wth the largest gan and the scheme enters round j+. Once a subband s assgned to a MS, t s not avalable n subsequent rounds. The subband assgnment procedure contnues untl all K MSs have been assgned subbands or round r+ s reached. In the latter event,.e. there are stll unassgned MSs after round r, the BS chooses each such MS n turn, n a random order, and assgns to t the unoccuped subband for whch ts gan s hghest. A flowchart descrbng the CAFH scheme s provded n Fg. 2. It s noted that the number of rounds requred s at most mn{r, K}. Fg. 2. Channel aware frequency hoppng flowchart [7] II.3. CAFH-OFDMA Model In the presented channel model, subband gans are assumed dstrbuted exponentally where the average value ( 0 ) s changed n each OFDM symbol nterval. Ths random channel envronment ncreases multuser nterference and bt error rate of each OFDM symbol. CAFH algorthm selects hgh gan subbands and mproves fxed subband allocaton schemes. In the next secton CAFH-OFDMA model s smulated and system performance of CAFH-OFMA and fxed subband assgnment method s compared. III. Smulaton Results We assume that the nstantaneous channel gans of user and ts subcarrer are descrbed by exponental random varables. Ths random varable s changed n each OFDM symbol ntervals and sets to new random value. Before transmsson of sgnal, BS estmates subbands gan of each user as ndependent exponental random varable. For example the gan matrx of three users and four avalable subcarrer scheme ( 0 ) s wrtten as: g Then CAFH algorthm s applcable to allocate channels wth hgh value gan to users. After that, each user can transmt modulated (BPSK) sgnal through IFFT bloc n order to ts selected subband. To prevent ntersymbol nterference (ISI) effect of OFDM symbols, the guard ntervals (GI) are added n each nterval. We assume that GI length s /4 of IFFT length. When OFDM sgnal s sent, channel mposes addtve whte Gaussan nose (AWG) to OFDM symbol. Also each subband multples by ts random gan value. At the recever, guard ntervals are removed and user s modulated data s detected. Choosng random gan model for OFDMA subband channels presents hgher bt error rate. Fg. 3 shows that random gan selecton affects the multuser nterference of system, also ncreases BER undesrably. In random channel envronment, the major approach of OFDM system (.e. absence of multuser nterference) wll be dfferent. However CAFH scheme can mprove the total bt and symbol error rate of overall system. CAFH-OFDMA scheme requres 3dB lower power n comparson wth fxed scheme and for hgher teratons ths dfferent wll be ncreased. As shown n Fg. 4, hgher MS numbers results n more multuser nterference and average BER s reduced. Also for hgher teraton numbers (or rounds), BER values are not changed reasonably. Also at same condtons, CAFH-OFDMA provdes more effectve performance over fxed assgnment. Copyrght 202 Prase Worthy Prze S.r.l. - All rghts reserved Int. Journal on Communcatons Antenna and Propagaton, Vol. 2,. 3

4 A. Alzadeh, S. M.-S. Sadough The effect of hgher teraton value s llustrated n Fg. 5. Under constant SR value assumpton, total bt error rate of users s reduced and converged. For r > 2, Fg. 5 shows that the average BER of system wll be low where CAFH algorthm requres more processng tme and calculatons. Therefore our smulaton results acqured by r= and r=2. Fg. 3. BER aganst average SR of CAFH-OFDMA system (round=,2) for 6 and 64 MSs IV. Concluson In ths paper, we consdered multuser OFDMA transmsson n a random channel gan envronment and CAFH scheme s offered to reduce bt error rate under power constrant condton. Applyng CAFH-OFDMA scheme, the overall requred transmt power can be reduced by more than 3dB from the conventonal OFDMA scheme wthout adaptve modulaton. Our smulaton results showed that n hgh teratons, the BER does not change reasonably. Current results acqured under smply assumpton that only one subcarrer s assgned to each user. In practce, adaptve subband allocaton selects best condton of subbands separately. Also major resource allocaton methods that optmze bt and power allocaton n OFDMA system can be merge wth CAFH-OFDMA and provde hgh performance and optmal schemes. Fg. 4. Bt Error Rate of CAFH-OFDMA System (round=2) and fxed scheme aganst number of MSs References [] S. Gault, W. Hachem, P. Cblat, Performance analyss of an OFDMA transmsson system n a multcell envronment, IEEE Transactons on Communcatons, vol. 55, no. 4, pp , [2] D. Zhu, B. atarajan, Resdue umber System Arthmetc Asssted Coded Frequency-Hopped OFDMA, EURASIP Journal on Wreless Communcatons and etworng Volume [3] T. Scholand, T. Faber, A. Seebens, et al., Fast frequency hoppng OFDM concept, Electroncs Letters, vol. 4, no. 3, pp , [4] K. Stamatou, J. G. Proas, A performance analyss of coded frequency-hopped OFDMA, n Proceedngs of IEEE Wreless Communcatons and etworng Conference (WCC '05), vol. 2, pp , ew Orleans, La, USA, March [5] T. L, Q. Lng, J. Ren, A spectrally effcent frequency hoppng system, n Proceedngs of the 50th IEEE Global Telecommuncatons Conference (GLOBECOM '07), pp , Washngton, DC, USA, ovember [6] B. M. Popovc, Y. L, Frequency-hoppng plot patterns for OFDM cellular systems, IEICE Transactons on Fundamentals of Electroncs, Communcatons and Computer Scences, vol. E89-A, no. 9, pp , [7] M. Elashlan, C. Leung, R. Schober, Performance analyss of channel aware frequency hoppng, IEE Proc.-Commun., Vol. 53, o. 6, December 2006, pp [8] B. Slar, Raylegh fadng channels n moble dgtal communcaton systems. Part II: mtgaton, IEEE Commun. Mag., 997, pp Fg. 5. Bt Error Rate of CAFH-OFDMA System aganst Rounds (SR=20dB) Copyrght 202 Prase Worthy Prze S.r.l. - All rghts reserved Int. Journal on Communcatons Antenna and Propagaton, Vol. 2,. 4

5 A. Alzadeh, S. M.-S. Sadough Authors nformaton Department of Electrcal and Computer Engneerng, Shahd Behesht Unversty G.C., Evn , Tehran, Iran. Ardalan Alzadeh was born n Rasht, Iran n 984. He receved hs B.Sc. degree n Electrcal Engneerng (electroncs) from Amrabr Unversty of Technology, Tehran, Iran n He s currently pursung hs M.Sc. degree n Electrcal Engneerng (telecommuncaton) at Shahd Behesht Unversty, G. C., Tehran, Iran. Snce May 200, he has been wth the Cogntve Rado Research Group of Shahd Behesht Unversty, Tehran, Iran. Hs current research nterests nclude cogntve rado, two-way relay networs, resource allocaton algorthms and convex optmzaton problems. Seyed Mohammad-Sajad Sadough receved hs B.Sc. degree n Electrcal Engneerng (electroncs) from Shahd Behesht Unversty, Tehran, I.R. Iran n 2002 and the M.Sc. and hs Ph.D. degrees n Electrcal Engneerng (telecommuncaton) from Pars-Sud Unversty, Orsay, France, n 2004 and 2008, respectvely. From 2004 to 2007, he held a jont appontment wth the atonal Engneerng School n Advanced Technques (ESTA), Pars, France, and the Laboratory of Sgnals and Systems (LSS), at Supélec, Gf-sur-Yvette, France. He was a lecturer n the Department of Electroncs and Computer Engneerng (UEI), ESTA, where hs research actvtes were focused on mproved recepton schemes for ultra-wdeband communcaton systems. From December 2007 to September 2008, he was a postdoctoral researcher wth the LSS, Supélec-CRS, where he was nvolved n the European research project TVMSL wth Alcatel-Lucent France. Snce October 2008, he has been a member of the Faculty of Electrcal and Computer Engneerng, Shahd Behesht Unversty, where he s currently an Assstant Professor n the Department of Telecommuncaton. Dr. Sadough's areas of research nclude sgnal processng, communcaton theory, and dgtal communcaton. Copyrght 202 Prase Worthy Prze S.r.l. - All rghts reserved Int. Journal on Communcatons Antenna and Propagaton, Vol. 2,. 5