Comparative Effect of Rake Receiver Technology on CDMA Rayleigh Multipath Transmission.

Transcription

1 Comparative Effet of Rake Reeiver Tehnoogy on CDMA Rayeigh Mutipath Transmission. Nnebedum V.I & Nwobu E.J 2,2 Eetria and Eetroni Engineering Department University of Port Harourt, Rivers State, Nigeria Abstrat: Code Division Mutipe Aess (CDMA) aows mutipe data signas to be ombined for simutaneous transmission over a ommon frequeny band and it aows mutipe users to share singe ommuniations hanne effetivey we. Nonetheess, the presene of refetors in the environment auses signas that trave from transmitter to reeiver to transverse over mutipe paths. This mutipart propagation resuts in interferene, frequenty referred to as fading. This has been a big haenge in wireess ommuniation word. Deep fading often auses temporary faiure in ommuniation, resuting in severe drop in the hanne signa-to-noise ratio (SNR) and inrease in Bit Error rate (BER). In this artie, Rake Reeiver tehnoogy is pratiay used to improve some of the desirabe system harateristis suh as BER and interferene suppression. With the mutipe orreators of Rake Reeiver, the reeived signa is mutipied by timeshifted versions of a oay generated ode sequene. In this artie the CDMA system and the transmission hanne are modeed firsty, before deriving a Matab modes to pratiay simuate (varying number of users, Rake fingers, and other parameters) to demonstrate omparativey the effet of the Rake Reeiver in SNR and BER of DS-CDMA transmission signa.. Introdution One of the areas of teeommuniations that has attrated muh researh interest in the ast few years is outdoor wireess ommuniations, partiuary wireess euar networks. This has been due to the arge growth of subsriber base. Current generation mobie systems are designed for high speed data transmission and wi mainy operate or are urrenty operating high frequeny bands. At these bands, spread spetrum (SS) methods in form of either diret sequene spread spetrum (DSSS) or frequeny hopping spread spetrum (FHSS) an be used to improve system performane. There is, however, signifiant bit error rate (BER) degradation when the data rate is of severa Mbps, whih is typia of asynhronous transfer mode (ATM). BER degradation is aused by the hanne dispersions due to mutipath propagation. Mutipath effets ause serious distortions to the reeived signa. RAKE reeiver, espeiay in Code Division Mutipe aess (CDMA) euar systems, is used to ombine mutipath omponents, whih are timedeayed versions of the origina signa transmission. This ombination is done in order to improve the signa to noise ratio (SNR) at the reeiver end. RAKE reeiver attempts to oet the time-shifted versions of the origina signa by providing separate orreation reeiver for eah of the mutipath signas. 2. Mutipath Propagation, Fading and Moduation Basiay, wireess ommuniations are affeted by the phenomena of refetion, refration, diffration, and sattering, as they propagate. Fading is one of the most haenging probems faing signa propagation system. Fading is the time variation of the reeived signa power aused by the hanges in the transmission medium or paths. In ase of the fixed environments, fading is affeted by hanges in the atmospheri onditions, for exampe by the presene of moisture in atmosphere, or rainfa [].Many physia fators in the radio hanne auses fading. These inude: 2.Mutipath propagation The presene of refeting objets and satterers in the hanne reates a onstanty dissipation of the signa energy in ampitude, phase and time. This is aed mutipath propagation. Mutipath propagation auses mutipe versions of the transmitted signas to arrive at the reeiving antenna, dispaing one another in time and orientation in spae. Mutipath propagation aso engthens the time required for the baseband portion of the signa to reah the reeiver, whih auses smearing due to inter-symbo interferene [2]. Imperia Journa of Interdisipinary Researh (IJIR) Page 258

2 f f d to f d f +, where d f is the Dopper spetrum, whih depends on the reative veoity of the mobie, and the ange θ between the diretion of motion of the mobie and diretion of arriva of the sattered waves. 2.6 Coherene Time T C Figure Mutipath Propagation Mehanism 2.2 Movement of Surrounding Objets When objets in the radio hanne are in motion, they indue a time varying Dopper shift (effet) on mutipath omponents. Dopper effet is the hange in frequeny of a wave for an observer moving reative to the soure of the wave. 2.3 Transmission Bandwidth of the Signa It is estabished that when the transmitted signa bandwidth is greater than that of the mutipath hanne, the reeived signa wi be distorted, but wi not fade muh over a oa area. Fading in wireess hanne is aso haraterized by other important parameters ike:- 2.4 Coherene Bandwidth, B It is defined as the statistia measure of the range of frequenies over whih the hanne an be onsidered fat (i.e., a hanne whih passes a spetra omponents with approximatey equa gain and inear phase). Two frequenies within the oherene bandwidth have strong potentia for ampitude orreation. Signa separated by more than B are affeted quite differenty by the hanne. If the oherene bandwidth is defined as the bandwidth over whih the frequeny orreation funtion is above 0.9, then oherene bandwidth is approximatey [3]. 50δ τ B Where δ τ = rms deay spread. 2.5 Dopper Spread Dopper spread B D is a measure of the spetra broadening aused by the time rate of hange of the radio hanne and is defined as the range of frequenies over whih the reeived Dopper spetrum is essentiay non-zero. When a pure sinusoida tone of frequeny f is transmitted, the reeived signa spetrum, aed the Dopper spetrum, wi have omponents in the range It is the time domain dua of the Dopper spread and is used to haraterize the time varying nature of the frequeny dispersiveness of the hanne in the time domain. The Dopper spread and oherene time are inversey proportiona to one another. Coherene time is atuay a statistiay measure of the time duration over whih the hannes impuse response is essentiay invariant, and quantifies the simiarity of the hanne response at different times. If the reiproa bandwidth of the baseband signa is greater than the oherene time of the hanne, then the hanne wi hange during the transmission of the baseband message, thus ausing distortion at the reeiver. If the oherene time is defined as the time over whih the time orreation funtion is above 0.5, then the oherene time is approximatey [4]. 9 TC 6πf Where m f m is the maximum Dopper shift. 3. Spread Spetrum Moduation Spread spetrum is a moduation tehnique whih inreases the transmit signa bandwidth. That is the transmission bandwidth is severa orders of magnitude greater than the minimum required signa bandwidth. Whie the system is bandwidth ineffiient for a singe user, the advantage of spread spetrum is that many users an simutaneousy use the same bandwidth without signifiant interferene with one another. In a mutipe user, mutipe aess interferene (MAI) environments, spread spetrum systems beome very bandwidth effiient. Aside oupying a very arge bandwidth, SS signas are pseudorandom and have a noise-ike property when ompared to digita information data. The spreading waveform is ontroed by a pseudorandom-noise (PN) sequene or ode. The PN ode is a binary sequene that appears random but an be reprodued in a deterministi manner by the intended reeivers. Spread spetrum signas are demoduated at the reeiver through rossorreation with a oay generated version of the PN ode. Cross-orreation with the orret PN Imperia Journa of Interdisipinary Researh (IJIR) Page 259

3 sequene dispreads the SS signa and restore the moduated message in the same narrow band as the origina signa, whereas ross-orreating the signa from an undesired user resuts in a very sma amount of wideband noise at the reeiver output [5]. Cod e Division Mutipe Aess (CDMA) is a form of mutipe aess tehnique that makes use of the orthogonaity of the eah users spreading ode. 4. The Rake Reeiver A Rake reeiver uses the autoorreation properties of the ode to oherenty ombine a mutipath omponents. The Rake reeiver is shown beow. Figure 2 Diagram of a M branh Rake Reeiver It is essentiay another form of diversity ombining, where the diversity is provided by the fat that the mutipath omponents are pratiay unorreated from one another when their reative propagation deays exeed a hip period. A RAKE reeiver utiizes mutipe orreators to separatey detet the M strongest mutipath omponents. The outputs of eah orreator are then weighted to provide a better estimate of the transmitted signa than is provided by a singe omponent. Demoduation and bit deisions are then based on the weighted outputs of the M orreators [7]. 5. System Mode The entire system is here modeed on the Diret Sequene spread spetrum transmission. In Diret Sequene Code Division Mutipe Aess (DS- CDMA), the user data signa is mutipied by a ode sequene, mosty, binary sequenes. Considering a binary DS-CDMA transmission with K u independent users, sharing a hanne simutaneousy, eah transmitting with power P at a w ommon arrier frequeny f = 2 Π, using a data rate Rb = T b and a hip rate RC = T.The K th C user, k=, 2.,, K u, is assigned a unique ode sequene {a k,j } of hip eements(+,-), so that its ode waveform is given by: + a k (t) = a k, j j= P T ( t jt ) P T (.) denotes the hip puse Where the funtion of duration T. In the singe-arrier ase we assume that P T (.) is a unit retanguar puse, whereas in the mutiarrier ase we wi onsider Nyquist puses. The ode sequene{a k,j } is assumed to be periodi, with T period equa to the proessing gain, PG = b TC. The data signa waveform b k (t) given by bk + t ) = b p ( t jt ) ( k, j j= Tb b Is binary-phase shifted keyed (BPSK) onto the arrier at f, whih is then spread by that user s ode sequene and transmitted over the hanne. The resuting K th user s transmitted signa s k (t) is thus given by: s k (t) = 2 pa k ( t) bk ( t)os( wt) The omposite transmitted signa s(t) at the input of the hanne an be expressed as: s( t) k = u 2 pak k = 6. Channe Mode ( t) b ( t)os( w t) k DS-CDMA systems invove a spreading proess whih resuts in a transmitted signa whose bandwidth is muh wider than the hanne oherene bandwidth, and therefore undergoes frequeny-seetive fading. This type of fading is typiay modeed by a inear fiter whih for the k th user is haraterized by a ompex vaued owpass equivaent impuse response represented as: Lp jθk, h( t) = ak, e ( t τ k, ) = Where (.) is the Dira deta funtion, the propagation path index, and { a k, }, { } θ = k,,{ τ = k, }, are the random = path ampitudes, phases and deays respetivey. We assume that the sets{ a k, }, { } θ = k,,{ τ = k, } are mutuay = independent. In the anayses, we assume a sow fading, so that L is onstant over time, P { k, } = and a, { } θ k,, { } τ = k, are a = onstant over a symbo. 7. Matab/Simuink Mode Matab/Simuink is we-known CDMA simuator and simuation software and is used in this researh Imperia Journa of Interdisipinary Researh (IJIR) Page 260

4 to evauate the performane of RAKE Reeiver performane on the CDMA hanne [9]. Here we measured the performane of a Rake Reeiver in Rayeigh fading pus A White Gaussian Noise (AWGN) hanne in omparison to a simpe onventiona reeiver (without a Rake) by varying the SNR of the AWGN hanne. Tabe Parameters for the Simuink mode 7. Simuation In this mode a Rayeigh fading path with 3 different deays are used to mode the mutipath with an AWGN bok to mode the hannes SNR. SNR(dB) BER Without rake reeiver With rake reeiver Simuation 2 Here we measured of the performane of the rake reeiver of different number of orreator or rake fingers for a fixed number of mutipath under varying Signa to Noise ratio. In the mode a Rayeigh fading hanne of four path is modeed and a noisy hanne represented by the AWGN bok. The aim is to investigate the performane of a rake reeiver with different number of fingers/orreators under fixed number (4) of signifiant mutipath. Figure 4 Bit Error Rate of Rake reeiver and non rake reeiver As an be seen from tabe 2 and figure 4, the Bit error rate performane is better for the rake reeiver in DS-CDMA when ompared to that of a onventiona (non-rake reeiver) for varying Signa to Noise ratio. This is beause the rake reeiver is abe to harness a the transmitted signa energy in the various mutipath diretions than the onventiona reeiver. Imperia Journa of Interdisipinary Researh (IJIR) Page 26

5 Figure 5 Simuink mode of a 4 path Rayeigh mutipath with Rake reeiver of different rake fingers SNR(dB) BER finger finger finger finger Tabe 3 SNR (db) Vs BER for rake reeiver with different rake finger for 4 mutipath diretions Figure 6 BER Vs SNR of Rake reeiver with different rake reeiver for 4 mutipath hannes From Tabe 3.0 and fig.6.0 it is seen that as the number of fingers/orreator of the rake reeiver approahes the number of signifiant mutipath in the hannes, the BER improves for varying signa-to-noise ratio. 7.3 Simuation 3 Here we measured the performane of a rake Reeiver in a mutiuser environment. Figure 8 Performane of Rake Reeiver for mutiuser senario It is observed (shown in fig.8) that as the number of users in a system inreases, the bit error rate inreases as we. Moduation sequenes are assigned to eah user by taking one PN sequene with a ong period and assigning users deayed versions of the sequene. Inreasing users wi ause eah user's sequene to be ess orthogona with the rest of the users, resuting in more errors at the reeiving end. When the number of users inrease up to a point the a high SNR beomes of itte importane as the high number of user is seen as a noise by other users, and hene the genera performane of the system degrades Figure 7 Simuink set-up to simuate the performane of a rake Reeiver in a muti user environment 8. Resut and Conusion As seen in fig 4 the use of Rake reeiver in CDMA redues the BER in a mutipath environment. This is ahieved by using severa "sub-reeivers" aed fingers, that is, severa orreators eah assigned to a different mutipath omponent. Eah finger independenty deodes a singe mutipath omponent; at a ater stage the ontribution of a fingers are ombined in order to make the most use of the different transmission harateristis of eah transmission path. This therefore resuted in higher signa-to-noise ratio (or E b /N 0 ) in a mutipath environment than in a "ean" environment. Our simuations as shown in Fig 6.0, aso Imperia Journa of Interdisipinary Researh (IJIR) Page 262

6 demonstrate that the performane of a rake reeiver in term of BER worsens with inrease in the number of user. 9. Referenes [] Wiiam Staings, Wireess Communiations and Networks Seond Edition,200 [2] Proakis, J.G. Digita Communiations. (3rd ed).engewood Ciffs, New Jersey: Prentie-Ha,995. [3] Lee,W.C.Y.Mobie Ceuar teeommuniations Systems,MGraw Hi Pubiations,New York.989. [4] Steee, R.ed.994.Mobie Radio Communiation, IEEE Press. [5] Viterbi, A.J. CDMA Prinipes of Spread Spetrum Communiations. Addison-Wesey,995 [6] Couh, L.W. Digita and Anaog Communiation,2000 [7] Prie,R, and Green,P,E.. A ommuniation Tehnique for Mutipath Channe, proeeding of the IRE, pp Systems. New York: Mamian Pubishing Company,958. [8] Theodore S. Rappaport (2002). Wireess Communiations: Prinipes and Pratie [9] Vaibhav Khairnar eta, Design and performane anaysis of DS-CDMA with Rake and without Rake Reeiver Communiation system Internationa Journa of Emerging Tehnoogy and Advaned Engineering Website: (ISSN , ISO 900:2008 Certified Journa, Voume 3, Issue 2, Deember 203) Imperia Journa of Interdisipinary Researh (IJIR) Page 263