Performance of WCDMA Downlink FDD Mode at 10 MHz Bandwidth

Transcription

1 Performance of WCDA Downln FDD ode at 0 Hz Bandwdth Suyeb Ahmed han *, Jasvr Sngh **, ahmood an *** * Research Scholar,**Deptt. of Electroncs echnology, GND Unversty, Amrtsar, Inda ***Deptt. of Appled Physcs 3, GND Unversty, Amrtsar, Inda ABSRAC -Wdeband code dvson multple Accesses s the system favored by most operators able to obtan new spectrum and to provde a global moblty wth wde range of servces ncludng telephony, pagng, messagng, nternet and broadband data.. In WCDA, each user transmts a data sequences spread by a code commonly called spreadng code. hs code s Unque to the moble staton (S) to base staton (BS) connecton on both upln and downln. hs paper deals wth analytcal treatment and computer aded performance analyss of downln FDD mode of WCDA under the varable strategc condtons of processng gan, sgnal to nose as well as number of nterference at 0 Hz bandwdth.. INRODUCION Emergng requrements for hgher rate data servces and better spectrum effcency are the drvers for the thrd generaton moble rado system. IU thrd generaton networ (I 2000) and Europe (US) have proposed man objectves for the thrd generaton as follows: Full coverage and moblty for 44 bps, Preferably 384 bps, Lmted coverage and moblty for 2 bps, Hgh spectrum effcency compared to exstng system. Hgh flexblty to ntroduce new servces WCDA has an edge over the exstng technques n terms of capacty, voce qualty, coverage area, power requrement, securty and bandwdth etc. Algorthm for computer aded smulaton has been developed. he study s useful n the Ln level smulaton of WCDA for moble communcaton. Computer Aded system level smulaton of WCDA FDD mode has been attempted whch s useful n the followng Scenaro. In order to ncrease the accuracy and performance of WCDA networ Capacty. In the plannng of WCDA networ. o acheve the flexblty n use data rates n dfferent envronment. In the reducton of multple Access Interference (AI) whch s the domnate factor n system capacty and qualty of communcaton at mnmum power level. Better use of avalable rado frequency bandwdth. Desgn of future Cellular oble Communcaton Networ. Useful to enhance voce qualty, Coverage area, Securty etc. Wdeband CDA s desgned to flexbly offer support for hgher bt rates, hgher spectrum effcency, hgher qualty of servces wdeband servces, such as wreless Internet servces (.e. pea rate of 384 b/s to download nformaton web) and vdeo transmsson (data rate up to 2b/s). Wdeband s essental about the data rate []. he physcal lmtatons and mparments to rado channels such as bandwdth constrants, multpath fadng, nose and nterference present fundamental techncal challenges to the goal of relable hgh data rate communcatons [2]. In WCDA system, the access scheme s Drect Sequence Code Dvson ultple Access (DS-CDA). he Drect Spectrum s the most commonly used technque among the dfferent Spectrum technques. In ths technque the transmsson system that combnes the sendng data sgnals wth Pseudo-nose code (PN code), ndependent of the nformaton data s employed as a modulaton waveform to spread the sgnal energy over a bandwdth much hgher than the sgnal nformaton bandwdth (0 Hz). At the recever the sgnal s despread usng a synchronzed replca of the PN code. FDD WCDA uses spreadng factors 4-52 to spread the base band data over ~5Hz band [3,4]. Fgure,2 shows the spreadng process and spreaded waveform for WCDA at bandwdth 5 Hz. ransmtter converts an ncomng data (bt) stream nto a symbol stream where each symbol represents a group of one or more bts hs technque s relable and hghly resstance to nterference and gve the opportunty to multple users can communcate through one channel [5]. 5 Hz BW Dat a rat 0 Hz BW Sprea d Spread ed Seque odu lator XOR and trans mtte External Nose Intercell Interfere FIG. SPREADING PROCESS DeSpr eaded Demod ulator and Recev Intra cell here are two dfferent modes of operaton namely:- 5 Hz XO Data Sprea d Seque 0 Hz

2 Frequency Dvson Duplex (FDD): - he upln and downln transmsson employ two separated frequency bands for ths duplex method (transmtter/recevng). A par of frequency bands wth specfed separaton for a connecton. me Dvson Duplex (DD):- upln and downln transmssons are carred over the same frequency band by usng synchronzed tme ntervals thus tme slots n a physcal channel are dvded nto transmsson and recepton part.[6] 2. SYSE ODEL In Cellular oble Communcaton systems, base staton transmt sgnal to all the users present n a cell ndependently, snce ther relatve tme delays are randomly dstrbuted. ndependent user uses the same carrer frequency and may transmt smultaneously [7-0]. he th bnary source generates a bnary sequence b (m), where m s the tme nstant. he spreaded data s gven by X. X ( t) EcbI( mc ) I( tm ) j EcbQ( mc ) Q( tm ) m () E c = th transmtted energy per chp. = me shft of the th User. C I = Pseudorandom Code Sequence of I channel C Q = Pseudorandom Code Sequence of Q channel. In an asynchronous system, transmtted sgnals have dfferent tme shft but the symbol nterval () for the transmtters are assumed to be equal and C I, C Q s the PN codes assgned to I and Q channel. Suppose the data at the I channel s represented by the sgnal d I, whle the data at Q channel s d Q n eqn, C I = C I t m ) ( ( C Q = C Q t m ) d I = Ec bi (m) d Q = Ec bq (m) he fgure 2 shows the transmtter and sgnals at dfferent ponts X = d I C I + jd Q C Q. (2) DPDCH DPCCH SERIAL O PARALLEL C I C Q Fg.2 Sgnal ransmtter X C SC S S ( t) X ( t). C ( t) (3) where C SC s the complex Downln scramblng code. S Fgure 3 ransmsson through Channel and Recepton at the Rae Front End R = R e j = {R + n} e j = R e j + n e j Suppose n = n e j R = R e j + n sc Each transmtted sgnal s passed through a multpath channel []. he channel s modeled by the zero mean Addtve Whte Gaussan Nose (AWGN) n wth varance 2 n, and there s no other dstorton n the channel apart from constant lnear scalng of sgnal ampltudes and multple access nterference caused by the presence of other actve users as shown n fg 3. R s the receved sgnal, h(t, ) s the complex channel response due to multpath, n s the complex Gaussan nose at the front end of the recever, than R' ( t) h( t, )S (t- ) A (4) A s the attenuaton of the th sgnal, due to propagaton; we assume that we have N multpath component n the channel. Each of these h (t, ) s complex.e. h = h (t, ).e j (t, ) he receved sgnal s gven by, R = R + n he fgure 4 represents the sgnals at the th fnger of the rae recever. s the tme shft of the th user. R e -j h(t,) R R C sc * (t- ) n fg.4 Descramblng at Rae fnger R R he spreaded data s than coded wth complex downln scramblng code. 2 Internatonal Symposum on Antennas and Propagaton ISAP 2006

3 e - R = h(t- )S(t- ) e j e j + n (5) Descramblng R = R C * SC (t- ) + n C * SC (t- ) = h (t )S(t- ) C * sc (t- ) A + n = h (t - )X (t- ) C sc (t- ) C * sc (t- ) A + n R = h (t - )X (t- )A + n (6) he recever conssts of number of rae fnger for smultaneous demodulaton of user sgnals followed by a decson bloc as shown n fg 5. he out put of ntegrate rae fnger bloc s sampled at the of the mth symbol nterval. It s represented by Y (m)= ( m) R' ( t) C ( t m ) dt m - m (7) he fnal processng operaton n the demodulator adds the receved samples Y (m) for all samplng nstants wthn one bt and forms the decson varable [Z (m)] represented as Z Rae fnger Resampl Descrambl Despr m Y From other fngers Fg 5 Rae Recever where s the number of chps per bt, whch s assumed to be equal to the code sequence length (N). he th decson devce estmates the mth symbol of the th user by examnng the sgn of the decson varable (Z ).[2] R C Integrate all the rae Decson Bloc Channel Estmat A Phase correcto Y (m fz 0 b (m) = sgn[z ] = fz 0 where m s the samplng nstant, -- m, Substtutng R n Eqn. 7 Y (m)= ( m ) h t X( t ) AC ( t m ) dt n ''( t) m (8) X contan the two channel parts (I & Q). But here we tae only real parts at the front end of the recever as shown n fg.6. R Y (m) = ( m ) h () t Ecb ( ma ) h ( t ) A Ec b () Ct ( ) C ( tm ) dt n m (9) n (m) s the Gaussan nose sample at the samplng nstant m. he frst term represents the desred sgnal, whle the second term represents multple Access nterference. he Eqn 9 s further wrtten as Y (m) = h ( t) E b A h ( ) E A b ( ) ( l) n c (0) E ( l) j ( m) m c C ( t ) C ( t m ) dt E (l) s the cross correlaton of code sequences n whch l = (-m). he eqn 0 shows, the nterference term s lnear n ampltude of the nterferng users. Y (m)= h( t) E b A h( ) E Ab( ) ( l) n c Re( ) C (t- - ) Fg 6 Despreadng n Rae fnger j c Y (m) Y 2 (m) Y 3 (m) he frst term n above equaton s the desred sgnal Y (m). he Second term s multple access nterference Decson Bloc Z Internatonal Symposum on Antennas and Propagaton ISAP

4 due to other user (AI) denoted by Y 2 (m). he last term s nose factor Y 3 (m). Y (m) = Y (m) + Y 2 (m) + Y 3 (m) he decson Varable, Z can be than represent as, Z = [Y(m) + Y 2 (m) + Y 3 (m)] m he average Probablty of Bt Error, P b can be expressed as P b = ½ P r { Z > 0, b = -} + ½ P r {Z < 0 when b = + } Assumng that the probabltes of transmttng symbols - and + are equal. he Bt Probablty can then be wrtten as P b =P r { Z > 0, b = -} = P r {Z < 0 when b = + } Assumng that the number of chps per bt, G p s large, the decson varable Z can be approxmated accordng to the Central lmt theorem by a Gaussan random varable. he Bt Error probablty s gven by [ E( Z )] 2 Q Pb Var ( Z ) E[Z ] s the mean and Var[Z ] s the varance of the decson varable Z. he mean value of Z s gven by E[Z ] = E Y b Y 2 Y 3 m E[Z ] = E m Y E E b Y 2 Y 3( m m m Snce E Y 2 0 and m ) E Y 3 0 m E[ Z ] EY( m) b Ec m the varance var(z ) s gven by var(z ) = var[ Y (m)]+ var[y 2 (m)] + var[y 3 (m)] he desred sgnal varance s var[y (m)] = 0 he varance due to thermal Gaussan nose s var[y (m)] = No/2 Where No s the one sded thermal nose power spectral densty. he varance of the nterferng sgnals can be computed assumng that the nterferng sgnal s modeled as whte nose wth the two sded power spectral densty of Ec/c. ang nto account the relatve phase dfference between the desred sgnal and nterferng g sgnals and averagng over them.[3] E c ( ) var[y 3 (m)] 2 hen the bt Error Probablty s lower bounded by 2E c Pb Q No Ec he term 2E c s the double bt energy 2E b and the denomnator represents the total power spectral densty comng for the thermal nose and multple access nterference. If we denote by I o, than I o N o j j he bt error Probablty lower bound can be wrtten as 2E b Pb Q I o he bt error probablty P b on a Gaussan Channel can be approxmated by E cj 2 No P b Q 3G p 2Eb where No s the Gaussan nose one sded power spectral densty. hs expresson for the Bt Error Probablty s obtaned assumng Perfect power Control [4]. he degradaton depends on operatng, the number of User () and the spreadng factor. ore users mply greater degradaton, as one mght expect. 5. PERFORANCE EVALUAION he Flow chart for Downln FDD mode of WCDA for computng BER was developed for computer aded performance analyss under followng varyng condton. he flow chart for above computaton s gven n Appendx. Fgure 7, 8 and 9 shows the varaton of Bt Error Rate at dfferent data rates, when s changed from (2-0). It can be observed from the fg. 7 that system becomes nterference lmted. As the Number of nterference s ncreased at the fxed value of processng gan wth the bt rate s 2.2 bps wth the varyng of Sgnal to nose rato 4 Internatonal Symposum on Antennas and Propagaton ISAP 2006

5 (), the requred qualty of servce (BER) s decreased. It should be noted that the processng gan of the desred user remans constant and all users transmt at the same power. he effectve value of BER s 0-3 s acheved at s 6 only when one nterference wth the desred user s present. But as the nterference users are ncreased from 2 to 5 the achevable target.e BER 0-3 s acheved at s 0. he same study was carred out at dfferent data rates 64 bps and 44 bps. he fgure 8 & 9 clearly shows that there s degradaton of Bt Error rate at fxed bandwdth 0 Hz as well as data rates of 64 bps & 44 bps respectvely, when changed from 2 to 5. At data rate 2.2 bps, more energy s requred to get the achevable target. he other users are not algned n tme therefore the code do not algn n an orthogonalty way that s retan n the recever. So these users causes the multple access nterference to be nonzero and the performance of the system s deterorates as the number of users s ncreased. BER.00E+00.00E-0.00E-02.00E-03.00E-04.00E Interfernce 3 Interference 5 Interference CONCLUSION In ths Paper the performance of the FDD downln of WCDA s analyzed n term of BER and Number of nterference wth desred user n the varyng condtons for the WCDA system. Wth a wdeband sgnal, the dfferent propagaton path of a wreless rado sgnal can be resolved at hgher accuracy than wth sgnals at a lower bandwdth. hs result n hgher dversty content aganst fadng and thus mproves the performance. In WCDA nterface dfferent users can smultaneously transmt at dfferent data rates and data rates can vary n tme. he processng gan, together wth the wdeband nature, suggests a frequency reuse between dfferent cells of a wreless system (.e. a frequency s reused n every cell/sector). hs feature can be used to obtan hgh spectral effcency..00e+00.00e-0 2 nt 3 nt 5 nt BER.00E+00.00E-0.00E-02.00E-03.00E-04 Fg.8. BER Vs at 0 hz bandwdth wth 64 bps Bt rate, varaton of nterference from 2 to 5 2 Interference 3 Interference 5 Interference BER.00E-02.00E-03.00E E-04 Fg.9. BER Vs at 0 hz bandwdth wth 44 bps Bt rate, varaton of nterference from 2 to 5..00E Fg.7. BER Vs at 0 hz bandwdth wth 2.2 bps Bt rate, varaton of nterference from 2 to 5 Internatonal Symposum on Antennas and Propagaton ISAP

6 REFERENCES:. Dungan, R Fran, " Electronc Communcaton " hrd Edton, Dalmar Publshers, London. 2. C.yung, Samuel " CDA RF SYSE ENGINEERING " Artech House, Boston. 3. Prasad, Ramjee "CDA FOR WIRELESS PERSONAL COUNICAION. Artech House,New Yor. 4. Prasad Ramjee,ero Ojenpera, Wdeband CDA for thrd generaton oble Communcaton, Artech House, Boston London. 5. US 3G WCDA ln budget.htm. 6. Shanmugan,. Sam, "Dgtal and Analog Communcaton Systems", John Wlley & Sons, USA. 7. hrd Generaton Partnershp Project echncal Specfcaton Group Rado access Networ Worng Group, ultplexng and Channel Codng (FDD), S V2.0. (999-08). 8. Lee, Edward A, esser Schmtt, Davd, G "Dgtal Communcaton" luwer Publsher, Boston. 9. hrd Generaton Partnershp Project echncal Specfcaton Group Rado Access Networ Group, Spreadng and odulaton, S V2..2 (999-4). 0. Sarar, N. " Dgtal Communcaton ", hanna Publcaton, New Delh.. Vjay. Garg, enneth F. Smol & Joseph E. Wles, Applcaton of CDA n Wreless/Personal Communcatons, Prentce Hall PR, NJ. 2. hrd Generaton Partnershp Project echncal Specfcaton Group Rado Access Networ Worng Group, Physcal Channels and appng of transport Channels on to physcal Channels (FDD), S 25.2 V 2.2. (999-08). 3. Savo Glsc, Spread Spectrum CDA Systems for Wreless Communcatons, Artech House, Inc, Norwood, A S. Hayn and. ohar, odern Wreless Communcatons, Pearson Educaton New Delh, Lee.W.C.Y Overvew of cellular CDA, IEEE ransacton 6. on vehcular echnology, VOL. 40. ay 99,pp Journal of selected Area of Communcaton, vol. 8, No. 8 August Varyng the number of Interference wth desred user Start Generate Data for all Actve U Generate Code for Actve users Set Bandwdth 0 Hz Set Processng Gan accordng to Data Rate Shft the nterference spreaded data to mae frame delay Set Number of Interference n cell Buld ultple Access Interference and add Gaussan whte nose wth (AI) Varyng (2-0) Compute Bt Error Rate (BER) o change the Data Rate No Yes END Appendx- Computatonal flow chart of BER wth Number of nterference n cell for performance Evaluaton 6 Internatonal Symposum on Antennas and Propagaton ISAP 2006