comparison and performance evaluation of orthogonal frequency and multi-carrier code division...

8/13/2019 Comparison and Performance Evaluation of Orthogonal Frequency and Multi-Carrier Code Division Multiple Access

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Comparison and Performance Evaluation ofOrthogonal Frequency and Multi-Carier Code

Division Multiple Access Systems

Ahmed Hassan Mansour

Manufacturing Engineering Dept.rab Orgization for Industrialization

Cairo, [email protected]

btct-Fuure moile ommuniaion sysems aim o provideexremely high speed daa ransmission, espeially in hedownlink. Broadand orhogonal frequeny ode division

muliple aess (OFCDMA) wih wodimensional (2D) ime andfrequeny domain spreading is eoming a very promisingehnique for high speed wireless ommuniaions due o isadvanages over muliarrier ode division muliple aess (MCCDM), dire sequene CDMA (DSCDMA) and orhogonalfrequeny division muliplexing (OFDM). This paper presens a

omparison, hrough simulaion, eween he performane ofOFCDMA and MCCDMA sysems operaing under he sameondiion. The paper also explains he asi sruure of muliarrier dire sequene CDMA (MCDSCDM) ehnique, is

advanages and OFCDMA sysem sruure. I is shown ha

OFCDMA is supeio o he aove menioned sysems.

Keywords: FM; FMA; -MA; M-MA; M-MA; to dimensional spreading

1. RODUCO

Fourh generation (4G) systems will suppor multmediaservices like high speed inteet access and broadcast servicesom infoation sites. Due to the natre of these multimediaservices, the data trafc in the downl is expected to besignicantly higher than the upli [1]. Various wireless accessschemes have been proposed for the broadbad downlitransmission in 4G systems. In single-cier direct sequencecode division multiple access (SC-DS-CDM), the symbols ofeach user are spread by a user-specic code, which expands the

bandwidth compared to the data rate and decreases chipduration [2], [3] which makes it suitable over a broadband

chael due to multipath interference (MPT) which results omtransmission on a sngle wideband carier [4]. On the otherhand, a multi-carier approach, orhogonal equency divisionmultiplexng (OFDM), has awn a lot of attention in highspeed wireless communications. OFDM employs a large

number of orhogonal subcarriers to transmit symbols nparallel, so the symbol duration is ncreased substantially andthe system can combat inter-symbol interference (SI) whichresults om MPT. Since the modulatio/demodulation of alarge number of subcaiers can be realized by inverse fastFourier transfo (FF)/FF, OFDM is easy to implementand is cost effective. Orhogonal equency doman multiple

978-1-4577-0128-3/11/$2600 2011 IEEE 161

Salwa H. Elramly, Mirete Sadek

Electronics and Communications Dept.Facult of Engineering, An Shams University

Cairo, [email protected], [email protected]

access (OFDM) can provide multiple access by adessing asubset of subcarriers to individual receivers. Moreover, it can

use multiple atea teciques to eance receiver eciency.

Also MC-CDM has become a ver atractive tecnique nbroadband wireless communication as it transmits on morethan one carier, i.e.; it enables equency diversit. In additionsince MC-CDM uses equency doman spreading, this factorlowers system performance because the orhogonalit beteen

used codes is lost over equency selective fading chaels.Although OFDM is active for high speed wirelesscommunications, it does not have coherent equency diversit.Moreover, in mobile cellular systems, OFDM suffers omadjacent cell interference unless the same subcariers are notused among adjacent cells hus, spreading has beenintroduced to OFDM to provide equency diversit andfacilitate one cell equency reuse n a cellula enviroent.Combinng OFDM with 2D spreadng (time and equency

domain spreading), an OFCDM system has been proposedfor the donli transmission in tre 4G networks [4]. Basedon OFDM, OFCDM provides not only all advantages ofOFDM, but also additional benets by means of 2D spreadng.For example, equency diversit gan can be achieved toughequency doman despreading due to different fadngexperienced by subcaiers n a broadband chael.Furheore, with the introduction of time domain (D)spreading, the system can provide exible transmission rates.he time and equency domain spreading factors NT and Ncan be changed adaptively to provide vaiable spreading factor(VSF) in order for the system to work in different cellenvroents and chanel conditions.

he rest of this paper is organized as follows: Section ITinoduces multi-c ier direct sequence CDMA (MC-DSCDM), ncludng 2D spreading. Section TIT is devoted to thestudy of OFCDM system sucte. Section IV presents

numerical results. Finally, Section V draws conclusions.

II. MC-DS-CDM USG2D SPREADING

MC-DS-CDM transmits D DS-spread signals usingmultiple subcarriers but the MC-DS-CDM schemeconsidered here is more general which employs equencydomain (FD) spreading in addition to D spreading (i.e.,


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employs TFD spreading) [5]. The corresponding MC-DSCDM transmitter scheme, as shown in Fig. 1, includes aserial to parallel (SIP) converter, which reduces the subcarrierdata rate by mapping the serial data to a number of reducedrate parallel streams. n MC-DS-CDM systems DS-basedTD subcarrier spreading is invoked to increase the achievable

processing gain associated with each subcarrier signal, whileFD spreading across several subcarriers is employed to rher

increase the total attainable processing gain. The totalprocessing gain is usually determined by the product of theTD and FD spreading factors, and respectively.

According to Fig. 1, the transmitted MC-DS-CDM signalfor kthuser using BPSK modulation can be expressed as:

'n)=

,% ;[U],[h]PT' (I -i -h f [s]co[2nC +1")] (

where P and Ie represent the transmitted power and carrier

equency, respectively and Pr represents the chip waveform

deed over the interval [0, ]. ct [h] represents the htchip of the TD spreading code, while Ct [s] represents thest chip of the FD spreading code.

The M S x U nber of subcarrier equencies arerepresented by {Fu +} for = 0, 1, ... , U 1; = 0, 1, ... S 1. As in (, formulated in the context of MC-DS-CDM, Snumber of sub carriers is used for FD spreading of the samedata bit, and we have the maximum possible equencyspacing beteen any two of the S subcariers. Furtheore, in(1) U represents the number of bits that are SIP converted,

where each transmitted symbol contains U data bits.

fqyTe d Do

.pre


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diversit gain as well as the required equencydiversit gain without havng to accept any ther

tradeoffs (i.e., without decreasing the total number ofusers supported by the system). This can be achievedby using both TD and FD (i.e., TFD) spreading nMC-DS-CDM systems. Specically, as suggested

by Fig. 1 and 1 each user n the system employs anFD spreading code in addition to a TD spreadingcode. Consequently, the total number of userssupported by the TFD spread broadband MC-DSCDM scheme is determined by the product of theTD spreading factor,Nr and the FD spreading factor,S , that is, by NT X S = UbI x M/ =Mi. Explicitly, the number of users suppored byTF domain spread MC-DS-CDM is independent ofthe equency diversit order S

B Dsadvanags f2D sadng The Doppler equency shi of the lowest and

highest subcarriers may be substantially different.

m. OFCDM SYSTEM STRUCTUEn VSF-OFCDM systems, users access the systems with

orthogonal spreading factor codes using VSF-OFCDM. nVSF-OFCDM systems, for each user code with a spreadingfactor ofN = NTXNp the transmitter performs 2D spreading

by using a TD spreading code with length (spreading factor)Nl and a FD spreading code with lengthNF Both TD and FDspreading codes are generated om orthogonal VSF (OVSF)codes [4]. More details about OFCDM system structure can befound in [6], [7], and [8].

Spreading factors for time and equency domain spreadingare varied according to chanel conditions to achieve highsystem performance. Snce chanels with different TDspreading codes are orthogonal to each other, the multicodeinterference (MCI) om code chnels with different TDspreading codes approaches zero in slow fading and A WGNchanels. Frequency diversit is provided due to FDspreading. Similarly, equency domain spreading codes areorthogonal to each other. n Gaussian chnels, there is noMC among equency domain spreading codes. However,

because fading parameters on subcarriers bearing the sameinformation are not the same, orthogonalit in the equencydomain no longer maintains among code chnels at the

receiver. Thus, MCI results .Consider the /h user data streamas in Fig. 2. The symbol sequence is rst serial to parallelconvered to suppose M/NF is an integer and M is the

total number of subcaiers) parallel sequences and then

spread by a TD spreading code CHi . Each TD spreadingT signal is duplicated intoNF parallel copies forNF subcarriers.Each copy is multiplied by a chip of the equency domain

spreading code, which is the combination of a shortchanelization code CCH} (n) and a cell-specic longp

. SC ) C

(CH)scramblg code C . n . eN" (n) and NJ are

real valued binar chaelization codes taking the value of

,whereas (C) n is a real valued binar scrambling

code that is the same for all code chanels in a cell. K is thetotal number of parallel users code with differentcombinations of time and equency domain chanelization

d {c (CH) C (CH) k - } h co es N k ' N k n , - 0, . . . ,K- . T erelore , F,VSF-OFCDM, each data symbol is impressed over NFsubcarriers by NT OFCDM symbols (chips) in eachsubcarrier. To aid chnel estimation at the receiver, acommon pilot chael with spreading factor of Not isemployed. Note that En is the chip energy of the transmittedsymbol on data chael, is the power ratio of pilot chanel

to one code data chanel, and for the mt (m = 1 . . . , )subcarrier the pilot symbol dm is known to the receiver and

the spreading code for pilot chael is an all- sequence. Inorder to exploit equency diversit, a equency interleaver isemployed before OFDM modulation Therefore the largest

possible equency separation between subcarriers caing thesame information is achieved. Aer equency interleaving

with FFT, spread signals occupy all M subcarriers. Similar toOFDM, in the transmitter, a guard interval is used betweenever OFCDM symbol to avoid the SI caused by multipath

propagation. In general, whenK


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Thus, the maximum number of codes available is - 1) Nwhich must be equal to or greater than K When K NT whereK is assumed to be integer times of Nr 1, the same NT -codes have to be assigned repeatedly with the other differentFD spreading codes. Then,MCI may result due to differentfading gains on subcarriers. As previously mentioned, K is thetotal number of code chanels. For the kth code chanel, the

code set of other K- code chaels can be divided into twosubsets: one set, D with the same TD spreading code as theklh code but different equency domain spreading codes andthe other set, Dr with different TD spreading codes, givenrespectively by (2) and (3).

k F = O ..... ,NF but k F * k J- -

n = {C (k]) C (k J ) l N ' Nk T = O ..... ,N

T but k r * k

r

(2)

3

*0

According to the orthogonalit of codes in FD or TDspreading, one obtains:

NF-lC (k F)C kF) = L kF *k 4NFI NFI Fi=OandNT -1L C (kr c (r

) = kT *k 5NTI NT ,f Ti=O

The Code chanels in different subsets (D or D havedifferent contribution to the MC on the klh code chael. nhighly equency selective chaels, code chaels om cause severe MC to the klh code chanel because theirorthogonalit in equency domain is distorted by equencyselective fading on subcarriers. On the other hand, in a slowfadng chanel or in a short packet, the orthogonalit in TDbetween any code chael om and the klh code chnel ismaintained. Thus, there is no MCI om . Therefore, onlycode chnels om Dr cause MC to the klh code chael. Thenumber of effective interference code chaels is equal to thenumber of code chanels in , which is dened as K. Asthere is a total of N r - 1 TD orthogonal codes for data

chaels, when K is less than or equal to Nr 1, orthogonalitbetween any two code chaels can be maintained byassignng chanels with different TD orthogonal codes. n thiscase, Dr does not exist. However, when K > Nr - 1, reuse ofthe same TD spreading codes is unavoidable and mustexist, so that MCI results. To keep MCI small for each codechanel, TD spreading codes should be assigned rst and thecode assigment will be carried out as mentioned previously.For example, assume Nr= 4,N= 2 and K= 6 then;

14

Ct) for TD spreading for pilot chanel. 1 , O} 2D spreading code for k= . 2 , O} 2D spreading code for k= 2. 3 , O} 2D spreading code for k= 3. p , l} 2D spreading code for k= 4. 2 , l}, 2D spreading code for k= 5. 3 ,l} 2D spreading code for k= 6.

Then, for k = 1 example, it has K-1 interferences which are 5can be classied as following

Dr group (the same TD spreading code as the k = 1code but different equency domain spreadingcodes)= one interference due to k= 4.

DT group (different TD spreading codes) 4interferences due to k= 2,3,5 and 6.

IV. SIMULATIONS

n this section, we test OFCDM via computersimulations and compare the performance with MCCDMunder the same chanel conditions.

The simulation is performed rst for OFCDM system in afading chnel with 2 simultaneous users, Nr = 8 and Nr=4.Then, the simulation is performed for MCCDM systemfor 2 simultaneous users under the same chanel conditions ofOFCDM system. The simulation investigates the effect of anumber of factors, e.g. the number of active users and the used

spreading factor (S.F). Finally, we compare beteenOFCDM and MCCDM systems performances. Theconclusions obtained om the results are presented. Table summarizes the system conguration, which is used in

OFCDM system simulation.

TABLE I OFCMA SYSTEM CONFIGURATION USE INSIMULATION

Paamete Vaue

TD spreadng factor Nr 8

Frequency domain spreadng4

factor

Number ofOFDM subcariers8

(M

Number of SP branches =2


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BER Vs Eb/No for dierent systems

heoretical BER for BPSK on AWGN- heoretical BER r BPSK on Rayleigh Channel- I bit error rate for OFCMA system,fading,two users,NT=8, N=2. bit error rate r OFCMA system,fading,two users,NT=8,N=4

10- .

o 5 20Eb/No, dB

Figure 3 OFCDMA simulation results for AWGN+quency selective fading,NF = 2 and Nr 4 assuming two users and NT= 8

In Fig. , the performance of OFCDM system for multi-usercase is assessed in AWO and equency selective fadingchael der the application of system congation in table Ibut with exceptions; the used NF= 2, M= 4. It can be seen that,

there is performance degradation n terms of BER n case of Np= 2 over NF= 4. This is because increasing FD spreading factorleads to ncreasng the diversit order, which n improvesBER perfoance in case of fading chnel.

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