Performance Analysis and Allocation Strategy for the WCDMA Second Scrambling Code

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1 Performane Analysis and Alloation Stratey for the WCDMA Seond Sramblin Code Frank Brouwer Twente Institute Wireless & Mobile Communiations, Institutenwe 30, 752 PK Enshede, The Netherlands Abstrat. The WCDMA downlink is limited by Node-B ell ower and the number of hannelization odes. With uomin aaity enhanement tehniques, like MIMO and HSDPA, the otential limitation in hannelization odes beomes visible more frequently. The WCDMA standard allows for alloation of traffi on a seondary sramblin ode, whih is non-orthoonal to the rimary sramblin ode. This aer analyzes the aaity imat of usin the seondary sramblin ode. In addition this aer introdues an alloation stratey for traffi to the seondary sramblin ode. The analysis shows a otential ain in seondary sramblin ode usae for this stratey. Introdution The air-interfae is a very exensive resoure in ellular systems. In order to utilize this resoure to its full extend, WCDMA based systems will be enhaned with tehniques like MIMO and HSDPA that imrove the aaity of the system. When inreasin the aaity of a WCDMA based system, an imortant limit to onsider is the number of hannelization odes in the downlink, whih reate the individual hannels. The Orthoonal Variable Sreadin Fator (OVSF tehnique is used for eneratin hannelization odes. The OVSF tehnique use of the Hadamard matrix, 2 = Hn Hn H and = H H H n n [] n, where eah row of H reresents a sinle OVSF ode word, and n is the Sreadin Fator (SF. Fiure shows the way this roess works. Both the matrix notation and the fiure show that the number of OVSF odes is equal to the SF. When the aaity inreases the risk that the required number of odes exeeds the number of available odes is realisti. The reason for suh limitation an be various: Caaity enhanement tehniques aim at inreasin the overall aaity, whih tyially result in more odes to be alloated. Mobile systems are used indoor muh. Suh environments have little time disersion, ausin little intra-ell interferene, and therefore rovide very hih aaity. The ode tree an have a sattered alloation. A seifi ode is only available for alloation when its hildren are available. (E.. ode C 4 ( is a hild of C 2 (. ( I. Niemeeers and S. Heemstra de Groot (Eds.: PWC 2004, LNCS 3260, , IFIP International Federation for Information Proessin 2004

2 390 F. Brouwer Some tehniques, like SSTD, alloate more odes that they atually use. Hih aaity area Node-Bs will use the HS-DSCH, whih ouies semistatially between 5/6 and 5/6 [7] of a omlete ode tree. When little HS- DSCH traffi is arried the alloated odes are under-utilised. C ( = C 2 ( = C 2 (2 = - C 4 ( = C 4 (2 = - - C 4 (3 = - - C 4 (4 = - - SF = SF = 2 SF = 4 Fi.. OVSF hannelization ode tree [], [2] When more hannelization odes are required than fit into a sinle OVSF ode tree, WCDMA offers the ossibility to alloate a seondary ode tree. Surlus traffi is alloated to a seondary sramblin ode. This seondary ode has its own OVSF ode tree. Doin suh, the number of hannelization odes is doubled. As sramblin odes are non-orthoonal, addin this seondary sramblin ode will inrease the intra-ell interferene. This aer analyzes the aaity imat of addin this seondary sramblin ode, and analyzes the erformane of a roosed alloation stratey. 2 Performane Analysis The number of available hannelization odes and the available ower determine the downlink aaity of the system. To find the downlink aaity of a W-CDMA system requires the ombined analysis of both. 2. Available Channelization Codes Common and shared hannels ouy a art of the first ode tree. The downlink hannelization ode aaity for dediated hannels is the art that is not alloated to the ommon and shared hannels. For a servie with sreadin fator SF this aaity in terms of OVSF hannelization odes an be alulated as M SF = SF k SF ommon SF shared, (2

3 Performane Analysis and Alloation Stratey 39 M SF SF SF ommon SF shared k DL OVSF ode aaity for DCHs with sreadin fator SF Sreadin fator SF for ommon hannels SF for shared hannels Number of ode trees. 2.2 Available Node-B Transmit Power The Node-B transmit ower is a shared resoure. A fixed ortion of the ower is assined to ommon hannels. The ower assinment of shared hannels an be fixed or variable. In the latter ase a stratey is to alloate the remainder of the Node-B transmit-ower to the shared hannel, with a re-determined minimum and maximum General Exressions for Downlink Power The Node-B dediated hannel transmit ower is ower-ontrolled. The UE omares the quality of the reeived sinal with a servie deendent taret SIR, and requests an inrease or derease of the ower to remain at the taret as lose as ossible. Many studies reort E b /N o instead of SIR. The SIR is the ratio of E b /N o and roessin ain (PG. The PG is the ratio of the hi rate and bit rate inludin sinallin overhead. E / N E / N b o b o γ = =, PG Rhi / RDCH (3 γ E b /N o PG R hi R DCH SIR Enery er bit over noise density Proessin Gain Chi rate DCH bit rate (inludin sinallin overhead. The reeived SIR results from the transmit ower and the interferene. This interferene onsists of reeived non-orthoonal ower from the own and other Node-Bs. Sinals transmitted with the same sramblin ode are in rinile orthoonal. The roaation hannel between Node-B and UE introdues time disersion, reduin orthoonality between transmissions usin the same sramblin ode. This effet is environment deendent, and an vary over a ell. For ease of analysis it is assumed that an averae orthoonality fator an be derived in a ell. The synhronization hannel (SCH is a seial ase. The SCH is not modulated with any sramblin ode, and is therefore by definition non-orthoonal. The SCH ower determines the quality of the synhronization, and is therefore of lare imortane as system arameter. Only extensive analysis an rovide a ood estimate for its settin. Settin the SCH ower to % of the maximum ower of a Node-B is enerally assumed to be a realisti assumtion, and is used in this analysis. The SIR is the ratio between the wanted and the unwanted sinals. The wanted sinal is the transmitted sinal times the ath-ain. The unwanted sinal is the sum of all non-orthoonal ower and thermal noise. The SIR of a reeived sinal at UE i from Node-B an be exressed as

4 392 F. Brouwer γ =, (α + N orth non orth + k k + k (4 γ SIR of sinal at UE i of Node-B ath-ain between UE i and Node-B ower of the sinal for UE i from Node-B orth other ower transmitted from the same sramblin ode non-orth non-orthoonal transmit ower (SCH and other sramblin ode(s k total transmit ower of Node-B k α non-orthoonality between UE i and Node-B. When r t is the reeived ower of a sinal that is transmitted orthoonal, and r n is the reeived non-orthoonal ower of the sinal, α is the ratio of r n over r t. α deends on the environment and the reeiver tehnoloy. It does not deend on the sinal. Loaded ells surround the ell under onsideration. For a onstant transmit ower of the surroundin Node-Bs the reeived ower will vary between db for free sae and 5 db for dense urban, disreardin effets of shadowin. Bein small in relation to the variation in reeived ower from the own Node-B, it is of seondary imortane to the analysis in this aer. The total interferene is modelled as a nonorthoonal ell transmittin at full ower max at distane β times the site-to-site distane. β may vary er environment between 0.38 and In this analysis is fixed to 0.4. Thermal noise is small omared to the inter-ell interferene in most ases. It is therefore neleted here. In addition the orthoonality is assumed onstant over the ell. Takin the above assumtions the formula for SIR redues to γ = = ( αorth + non orth + β max β αorth + non orth + max The SIR γ is inut to ower ontrol, whih will kee the effetive SIR as lose as ossible to a taret. Instead of havin a SIR er onnetion, the analysis uses the taret SIR γ as inut and the Node-B transmit ower as result. The SIR taret is set equal for all DCHs. The ower at whih the Node-B transmits the sinal for UE i equals γ ( α + + = orth non orth The quality of reetion at the ell boundary determines the ower for ommon and shared hannels. This imlies that equals the ain at the ell ede, bein at 0.5 times the site-to-site distane. Common and shared hannels transmit at a different bit rate. Assumin that the same E b /N o alies to these hannels the SIR γ for the DCH an be orreted with the ratio of the bit rates. As a result the ombined ommon and shared hannel transmit ower is exressed as R γ ( α + + max k with R = orth non orth R max β + R ommon shared = and k RDCH. =, β 0.5 (5 (6 (7

5 Performane Analysis and Alloation Stratey 393 R ommon R shared R DCH ombined bit rate on all ommon hannels (inludin overhead ombined bit rate on all shared hannels (inludin overhead bit rate on a DCH (inludin overhead. The ath-ain for the DCH varies over the ell. With a uniform user distribution over the ell area, it is ossible to exhane this varyin ath-ain by an averae athain, where the averae must onserve the sum of transmit owers of all onnetions. This is valid usin the followin alulation: max k DCH = max av = ellarea β max dos k DCH = os ellarea β So the averae transmit-ower er DCH equals ell DCH = ( orth non orth max DCH ell β os dos. (8 γ α + + k. (9 The slit in orthoonal and non-orthoonal ower for the DCH deends on its alloation to rimary or seondary sramblin ode, and the amount of DCHs on eah ode. For N DCHs on the rimary sramblin ode and N 2 DCHs on the seondary sramblin ode the sum of owers for all DCHs on rimary and seondary sramblin ode and the ower for the ommon and shared hannel resetively are = N γ ( α ( + + SCH max k DCH 2 = N 2 γ ( α 2 + SCH max kdch = R γ α ( k. ( SCH 2 max The maximum transmit ower of a Node-B max and the ower for ommon and shared hannels are fixed. So dediated hannels an be hosted as lon as SCH max +. ( As all values are saled in max the analysis simlifies by exressin all owers as a fration f of max, resultin in the followin set of exressions. f = N γ ( α ( f + f + fsch + f2 + k f = N γ α f + f + f + f + k DCH 2 2 ( 2 SCH DCH f = R γ ( α ( f + f + f SCH + f2 + k f + f 2 + f + f SCH. (0 ( Caaity for Only the Primary Sramblin Code When the ower related downlink aaity is less than the number of hannelization odes for DCHs M SF (or when disreardin this limit all DCHs are alloated to the rimary sramblin ode. In the above formulas N 2 and effetively f 2 equal zero. The aaity than equals the maximum number N that fulfils the set of equations: f = N γ α ( f + f + f + k ( SCH DCH f = R γ ( α ( f + f + f SCH + k f + f + fsch, (3

6 394 F. Brouwer whih an be shown to equal f fsch N = γ ( α ( fsch + fsch + k DCH with f = R γ ( α ( fsch + fsch + k. ( Caaity for Primary and Seondary Sramblin Code When the ower related aaity limit exeeds the aaity limit for the rimary sramblin ode, only additional traffi is alloated to the seondary sramblin ode. The number of odes on the rimary sramblin ode N equals the aaity limit M SF. Workin out the set of equations 2 leads to a aaity limit N 2 of N 2 = + k γ ( f2 with f2 = f + α DCH SCH M SF ( + kdch + R + ( α ( M γ SF ( + k. + R (5 2.3 Examle Caaity Limits with Seondary Sramblin Code Numerial examles in this setion rovide an interretation of the formulas. Fiure 2 shows the imat of the seondary sramblin ode. The fiure shows a virtual aaity. This assumes that all DCHs are alloated to the rimary ode tree, inorin limit M SF. The true aaity, takin the alloation to the seondary ode tree into onsideration is sinifiantly less. This shows an imat of the mutual non-orthoonality between sramblin odes. 2.5 Code trees in use Virtual True Eb/No (db Fi. 2. Caaity imat of seondary sramblin ode This result shows that reoverin odes on the rimary ode tree is very benefiial. There are two main otions for reoverin odes.. Unassined odes may be unavailable due to satterin of ode alloation. A ode an only be assined when all its hildren odes are unassined. The SF is servie deendent. Different servies will be resent in the same ell, so a mix of small

7 Performane Analysis and Alloation Stratey 395 and lare SF is assined. Lare SF odes, sattered in the ode tree, may be unassined, but not leavin any small SF ode assinable. Conatenatin free odes by reassinin lare SF servies to other odes, enables small SF ode alloation. Various roosals are rovided in literature. [4], [5], [6] 2. The HS-DSCH is of reat imortane in inreasin the erformane of W-CDMA. This hannel an be assined semi-statially u to 94% of the ode tree. The usae of the HS-DSCH deends on the amount of relevant traffi in the ell. As this traffi an flutuate heavily, the HS-DSCH may be underutilized durin some intervals. At the same time traffi not suitable for HS-DSCH may eak. Freein odes durin a eriod of low utilization of the HS-DSCH therefore is very imortant. So far no studies were found in literature. For a more detailed analysis of the imat of the non-orthoonality between the sramblin odes the true aaity exeedin the rimary sramblin ode is exressed as a fration U of the virtual aaity exeedin one ode tree: (6 ( kdch f U γ ( + / 2 + α = with f f SCH M SF γ ( α ( fsch + fsch + kdch M SF ( + kdch + R ( + k f2 = fsch and + ( α ( M SF + R γ f = R γ α ( f + f + k..2 ( SCH SCH True ain/otential ain (U Orthoonality fator (α Fi. 3. Usability of the aaity ain on seondary sramblin ode N v Fiure 3 shows the usability of a aaity ain when this exeeds the rimary sramblin ode. The ain is shown as funtion of the orthoonality fator and the otential loadin of the seondary ode tree N v. N v is varied between 0 (no traffi on the seondary ode tree and (the seondary ode tree ould be used to its full extend when the non-orthoonality between ode trees would not exist.

8 396 F. Brouwer The fiure learly shows that the imat on aaity ain is muh larer for the orthoonality than for the loadin of the seond sramblin ode. α deends on the environment and indiates the amount of ower that is observed as non-orthoonal. Without time disersion in the reeived sinal α is equal to 0. When α equals all orthoonal ower is reeived non-orthoonal. This orresonds to a hyothetial ondition of infinite time disersion. α deends on reeiver tehnoloy and environment. Pratial values for α in W-CDMA rane rouhly from 0. in offie environments to 0.5 in rural environments. In this rane one third to half of the otential ain remains when traffi needs to be alloated to the seondary sramblin ode. So reassinin odes from the seondary ode tree to the rimary ode tree ays bak. E.. reassinment of one onnetion from seondary to rimary sramblin ode frees suffiient ower to alloate one or two additional onnetions on the rimary ode tree. The orthoonality also has a diret imat on the aaity. This effet stronly redues when a sinifiant amount of traffi is alloated to the seondary sramblin ode. Fiure 4 shows the aaity versus the orthoonality. As soon as the seondary sramblin ode is used, the aaity ain of a smaller α diminishes. Comarin this result with the effet of a redued SIR shows that the erformane ain of reduin SIR and α are quite similar when only the rimary sramblin ode is in use. When also the seondary sramblin ode needs to be used a better erformane in SIR rovides a muh better ain than reduin α. Code trees in use Virtual True Orthoonality (α Fi. 4. Caaity imat of orthoonality 2.4 Code Alloation Stratey for a Seondary Sramblin Code So far the imliit assumtion is that traffi on both the rimary and seondary ode tree is uniformly distributed over the ell area. Traffi on the seondary ode tree uses more ower than traffi on the rimary ode tree. Traffi near the ell boundary uses more ower than traffi near the ell entre. It may be exeted that seletin traffi as near as ossible to the ell entre for alloation on the seondary ode tree is benefiial. For omarison also the oosite ase where as far as ossible traffi is seleted for alloation on the seondary ode.

9 Performane Analysis and Alloation Stratey 397 For analysis of suh alorithm the averae ath-ain is alulated searately for the two sramblin odes, slittin the ell area orresondin to the ratio between traffi usin eah sramblin ode. This ain of suh alorithm an be exressed as N 2 (with alorithm N 2 (without alorithm ain =. N (without alorithm 2 (7 Gain 2 nd ode alloation Code trees in use 30% 20% 0% 0% -0% -20% -30% border entre Code trees in use Fi. 5. Caaity ain for ode alloation strateies entre Averae Eb/ No (db Fi. 6. Effetive aaity for ode alloation strateies Fiure 5 shows the ain of alloatin traffi to the seondary ode tree as lose as ossible to the ell entre or ell border. Clearly alloatin near the ell entre is benefiial, while alloatin near the ell border osts aaity. The effet however dereases with inreasin use of the seondary sramblin ode. Fiure 6 omares the resultin aaity for the stratey of alloatin onnetions on the seondary sramblin ode as lose as ossible to the ell entre with the uniform alloation. The aaity ain is aarent, thouh not dramati.

10 398 F. Brouwer 3 Conlusions The DL aaity limit of a Node-B is determined by the available ower. The limited number of hannelization odes er sramblin ode does not diretly limit the aaity, but does imat the aaity sinifiantly. The ost for alloatin traffi to the seondary sramblin ode is two to three times as hih as for the rimary sramblin ode. This ost an be redued to some extend by referrin traffi far from the Node- B over traffi lose to the Node-B for alloation to the rimary ode tree. For maximum aaity it is imortant to manae the odes on the rimary ode tree very ood. Code realloation an free odes, suh that as little traffi needs to be alloated to the seondary ode tree. An issue for further study is the manaement of odes for the HS-DSCH. Aknowledement. This aer reflets art of the work erformed in the framework of roet TSIT025 Beyond3G. The Beyond3G roet investiates the enhanements required to 3G systems like WCDMA for interation in systems beyond the 3 rd eneration mobile ommuniation systems. Referenes. H. Holma, A. Toskala (Editors, WCDMA for UMTS - Radio Aess for Third Generation Mobile Communiations, John Wiley and Sons, Seond Edition, H.D. Shotten and Hafez Hadinead-Mahram, Analysis of a CDMA Downlink with Non- Orthoonal Sreadin Sequenes for Fadin Channels, Pro. VTC2000-Srin, aes , Tokyo, Jaan, May E. Dahlman, B. Gudmundson, M. Nilsson, and J. Sköld, UMTS/IMT based on Wideband CDMA, IEEE Communiation Maazine, vol. 36, , Se Y. Tsen, C. Chao and S. Wu, Code Plaement and Relaement Strateies for Wideband CDMA OVSF Code Tree Manaement, IEEE Trans. on Mobile Comutin, Vol., No. 4, Ot.-De. 2002, C. Chao, Y. Tsen, and L. Wan, Reduin Internal and External Framentations of OVSF Codes in WCDMA Systems with Multile Codes, Pro. WCNC, A.N. Rouskas, and D.N. Skoutas, OVSF Codes Assinment and Reassinment at the Forward Link of W-CDMA 3G Systems, Pro. PIMRC 2002, Vol. 5, , Lisbon, Portual, Setember 5-8, GPP, Sreadin and modulation (FDD, TS v5.4.0, htt://