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  1. 1. ISSN: 2278 – 1323 International Journal of Advanced Research in Computer Engineering & Technology Volume 1, Issue 5, July 2012 Interference Mitigation by CDMA RAKE Receiver With Walsh-Hadamard Sequence Braj Bhooshan Pandey Prof. Abhishek Choubey Research Scholar, M.E. Head, Department of Electronics and comm. R.K.D.F. Institute of Science & R.K.D.F. Institute of Science & Technology, Bhopal Technology, Bhopal Bhopal, INDIA Bhopal, INDIApandey_023brajbhooshan@yahoo.com abhishekchoubey84@gmail.comAbstract- Currently, a global third generation cellular system flexible voice and multi data service. In America thebased on CDMA is being developed with a wider bandwidth second generation system (2G) DS-CDMA standard IS-than existing second generation system. The RAKE Receiver 95 is being used as a basis for third generation systeminstead of completely eliminating multipath uses techniques (3G) with wider bandwidth.to effectively utilize multipath to produce the desired signal. The purpose of this paper to design and implementationThis paper proposes interference mitigation by CDMA of CDMA RAKE Receiver over the conventionalRAKE receiver with Walsh- Hadamard Sequence for DS-CDMA systems. In CDMA RAKE receiver multiple CDMA Receiver in wireless communication and tocorrelator are used to dispread the multipath signals and achieve a better Bit Error Ratio (BER). CDMA RAKEbefore making a bit decision alignment and combination of Receiver consists of many correlators that correlate tothose signal are necessary. Proposing aim of the study are- spread signals. The simple RAKE Receiver structure is1)Design and Implementation of CDMA RAKE Receiver 2) used in which despread values produced by RAKEImplementation of Walsh-Hadamard sequence 3) Receiver “finger” and then combined to generate a decisionstructure is that it requires multiple correlators and to achieve statistics. In order to mitigate the multipath fading abetter Bit Error Ratio(BER). diversity receiver is used in the DS-CDMA systems.Index Terms – Code Division Multiple Access (CDMA), The RAKE receiver uses the maximum ratio combiningRAKE Receiver, Interference Suppression, Walsh-Hadamard and multiuser interference suppression to get aSequence, Correlators, Maximum Ration Combining (MRC), considerable increase in performance and analysis of DS-CDMA system. Multipath fading is one of the major practical concerns in wireless communications. I. INTRODUCTION The RAKE Receiver has been used to reduce theWireless Communication has made a huge leap since its multipath fading in DS-CDMA system. In order tofirst commercial service. In the wireless mitigate the multipath fading a diversity receiver iscommunication, transmitted signals arrive at the used in the DS-CDMA systems in which considerablereceiver via a direct, unobstructed path or via multiple gain is achieves by the combination of multipath signalpaths from the reflection, diffraction and scattering of with the different delays and that is known as RAKEsurrounding objects such as buildings and trees. This “finger” Receiver. Finally we will see that in ourmultipath propagation causes the signal at the receiver proposing work CDMA RAKE Receiver is betterto distort and fade significantly leading to inter-symbol performer over the Conventional CDMA Receiver andinterference (ISI). A Conventional CDMA Receiver achieve better Bit Error Ratio (BER).ignores the multipath component and concentrateson the direct line of sights components. As CustomerDemand for wireless communication continues to grow.We got third generation (3G) cellular mobilecommunication systems are being capable to provide 60 All Rights Reserved © 2012 IJARCET
  2. 2. ISSN: 2278 – 1323 International Journal of Advanced Research in Computer Engineering & Technology Volume 1, Issue 5, July 2012 𝑵−𝟏 c(t) = 𝒌=𝟎 𝒄 𝒌 p(t-kTC ), (1)II SYSTEM DESCRIPTION ALONG WITH 0≤ 𝒕 < 𝑻 𝒃 = NTcCONVENTIONAL CDMA RECEIVER With ck = +1,-1 and p(t) is a rectangular pulse of duration Tc and unit amplitudeIn the previous time single user detection methods havebeen developed which model interference in a similar The transmitted signal of user k can be expressed as,way after that proposed a CDMA receiver. In CDMAsystem many user transmit information simultaneously. ∞ xk(t) = 𝑬𝑲 𝒊=−∞ 𝒔 𝑲 (i) 𝒂 𝒌 ,i(t-IT) (2)In CDMA system each user has a unique code becauseof it is known as code division multiple accesstechniques. User sends many information in the form of Where Ek is the average symbol energy and T is symbolimages, data message, audio, video etc. The duration.communication system as in fig.1 The spreading waveform for the kth user, 𝟏 𝑵−𝟏 ak, i(t) = 𝑱=𝟎 𝒄 𝒌,𝒊 (j) p(t-jTC) (3) 𝑵 The received signal can be obtained as, 𝑳−𝟏 𝑲−𝟏 𝑳−𝟏 r(t) = 𝒊=𝟎 𝒈𝒍𝒙 𝟎 (t- 𝑻 𝒍 ) + 𝒈𝒍𝒙 𝒌 (t-Tl)+ n(t) 𝒌=𝟏 𝒍=𝟎 (4) Transmitted signals propagated through a radio channel are affected by fading. Two major physical factors that influence the fading are multipath propagation and the Two major physical factors that influence the fading are multipath propagation and the a constantly changing environment that dissipates the signal energy in amplitude, phase, and time. The relative motion between the base station and the mobile results in a frequency modulation due to different Doppler shifts on Fig no.1 System Model each of the multipath components [Rap96].At the receiver front end, the additive white Gaussian noiseIn DS-Spread Spectrum, each user is assigned a unique (AWGN) is introduced by the thermal noise of theWalsh code and transmitted data is multiplied by its electric components of the receiver.corresponding spreading code. Modulated data throughthe spread code occupies more bandwidth thantransmitted data. One of the most important limitations in the field ofdigital communications is channel capacity. Cause of this,many methods has been designed to goal the usage of achannel in the most efficient way. For cellular mobilecommunications the most relevant method is multipleaccesses techniques. This method allows us to transmit andreceive information from different sources through the samechannel. So, many users can communicate using the samechannel. Some of the multiple access methods are Time-Division Multiple Access (TDMA), Frequency-DivisionMultiple Access (FDMA), and Code-Division MultipleAccess (CDMA). There are two types of CDMA. The first isFrequency-Hopping (FH-CDMA), and the second is directsequence (DS-CDMA).Each user’s data sequence is first modulated using QPSKmodulation. Each user within a cell is assigned a unique codesignal c(t) called spreading or Walsh-Hadamard code, which Fig no.2 Channel model for simulationis then used to spread the QPSK-modulated signal usingdirect sequence CDMA (DS-CDMA).After mapping the Walsh-Hadamard code takes theform 61 All Rights Reserved © 2012 IJARCET
  3. 3. ISSN: 2278 – 1323 International Journal of Advanced Research in Computer Engineering & Technology Volume 1, Issue 5, July 2012Figure no.2 shows the channel model with fourmultipaths, which is used for simulation of our rakereceiver. The transmitted signal propagates throughfour different time variant channels and produces four The most well-known approach to deal with multipathmultipaths. The rake receiver receives the multipathsignals added with AWGN. wireless channels is to use RAKE receivers, as implemented successfully in the “traditional” wideband III. Proposed model CDMA systems. Therefore, it is straightforward to A. Receiver Structure apply this concept for channel estimation in IR-UWB.The receiver design should aim the recovery of data Basically, a RAKE receiver consists of multiplefrom a multipath environment. The most commonly correlators (also called RAKE fingers). Each fingeradopted receiver architecture a multipath environment matches (correlates) the received pulse sequenceis a rake receiver shown in Figure3. (spread by the multipath channel) with a delayed version of a template pulse g(t-τn).The correlator’s output is an estimate of the amplitude an of the corresponding channel tap (from eq.5). transmitted over a multipath channel then the received signal is, r(t) = ∈ ∞ 𝒔 𝒊 𝑵 𝒇 𝒄 𝒊 h(t-ItF) + n(t) 𝒊=𝟎 (6) Where n(t) is additive noise , h(t) is the composite channel response h(t)= hp(t)*g(t). Conventional RAKE Receiver ∞ h(t) = 𝒏=𝟎 𝒂 𝒏 g(t- 𝝉 𝒏 ) (7)A rake receiver has multiple fingers to resolvemultipath component signals that have different path Matching the received signal with a template pulse,delays. A finger receives a signal containing all the which is a delayed copy of the transmitted pulse g(t),multiple paths then processes one of the multipath the correlator’s output for a RAKE fingercomponent signals assigned to it. Each multipath has a corresponding to the multipath component at τn delaydifferent distortion, diffraction, scattering, reflection etc in the i-th frame will be,that is mainly caused by the Rayleigh fading, AWGN, x i, n = 𝒓 𝒕 𝒈(𝒕 − 𝒊𝑻 𝒇− 𝝉 𝒏 ) 𝒅𝒕 (8)and the frequency offset between the source and thedestination site. Therefore, the received signal of afinger should undergo phase alignment and frequency = ∈ 𝒔𝒊 𝑵𝒇 𝒄 𝒊 𝒉 𝒕 − 𝒊𝑻 𝒇 𝒈 𝒕 − 𝒊𝑻 𝒇 − 𝝉 𝒏 𝒅𝒕 + 𝒏,𝒊𝒏offset alignment specific to the multipath as well as the (9)despreading process. A power channel estimator ∞measures the signal power of the multipath to determine h(t) = 𝒏=𝟎 𝒂 𝒏 g(t- 𝝉 𝒏 ) (10)if it is strong enough to be considerable. The deskewercompensates individual multipath delays. After theseprocesses the multipathpath signals are combined to = ∈ 𝒔𝒊 𝑵𝒇 𝒄 𝒊 𝒂, 𝒏 + 𝒏, (t) (11)recover the desired data .Let hp(t) be the (physical)multipath channel impulse response. Channel models The RAKE receiver is a matched filter (the received pulse is matched with a template that has the sameproposed by IEEE 802.15.3a Task Group [28, 52] are waveform) and therefore (with known channelbased on the famous multipath Saleh-Valenzuela coefficients) optimum with respect to the BERmodel [58], in which the multipath components arrive performance, and it also benefits from the fact thatat the receiver in clusters, many results in existing literature on RAKE receivers for wireless communication systems e.g. WCDMA can still apply. However, there are some serious practical x i, n = 𝒓 𝒕 𝒈(𝒕 − 𝒊𝑻 𝒇− 𝝉 𝒏 ) 𝒅𝒕 (5) issues in this kind of receiver. 62 All Rights Reserved © 2012 IJARCET
  4. 4. ISSN: 2278 – 1323 International Journal of Advanced Research in Computer Engineering & Technology Volume 1, Issue 5, July 2012 𝑴B.Walsh-Hadmard sequence- x/= 𝒌=𝟏 𝜷𝒌 ∗ 𝒙𝒌 (13)Basically for spreading data we used different types ofsequence so we have P-N sequence and Walsh – IV PERFOMANCE ANALYSISHadmard sequence but in this paper for spreading data The standard Gaussian approximation regarding fading isand also for better orthogonally we used Walsh- used with a signal to noise ratio analysis to derive the BitHadamard sequence so that sequence also called error ratio for QPSK modulation and we have specificorthogonal code. For improving better efficiency we weighted vector ω then the signal to noise ratio at the outputused orthogonal function or Walsh-HadmardOrthogonal sequence. For generating Walsh-Hadmard of the maximum ratio combiner can be easily shown asequence we have NxN square matrix. To get the SNR = ωK ggk ω / ωK kω (14)desired length of Hadamard matrix can be generated bythe following manner- Power spectral density of total noise after that Bit Error Ratio can be written as, 𝟏 𝟏𝑯𝟏 = Pe(d) =½erfc(rootof(β SNR(C)) (15) 𝟏 −𝟏 𝑯𝑵 𝑯𝑵 𝑯 𝑯 𝟐 𝟐𝑯𝑵 = 𝑯𝟏 = 𝑯 −𝑯 𝑯𝑵 −𝑯 𝑵 𝟐 𝟐 V. RESULT AND DISCUSSIONSpreading sequence used for DS-CDMA system can be QPSK modulation is a technique that will modulate therepresented by JxK matrices BJK. Where J is the transnitted signal and used for the simulations or spreadnumber of sequence and K is sequence code length. with the spreading code sequence. Impulse responseThe code sequence is known as orthogonalSequence if, channel measurement is also utilized in whichand only if matrix BJK is orthogonal – multipath components are generated through Rayleigh BJK BJKT = k IM (12) fading channel characteristics. Channel noise is generated and added to the multipath components. SoWhere k is the constant and BJKT is transposition of all the multipath components are added together formatrix BJK of order N and IN is the NxN unity matrix. In simulation at the receiver site. Integration process canthe DS-CDMA system the number of chips per data take place after the multipath components is detectedsymbol is known as spreading factor (SF). after that Maximum ratio combining can be used forC Maximal Ratio Combining (Combining weight storing the integration results here combining result ofcoefficient and finger delay) multipath components to send for the decision bit device after all data is decoded and probability of errorIn the field of wireless communication we have much is computed and simulation have run with the Walsh-option to mitigate interference such as ISI, Equalizers Hadamard sequence. The purpose of this paper toand diversity. Diversity is techniques that will reduce design and implementation of CDMA RAKE Receiveramplitude fluctuation or fading mitigation. We havemany type of diversity such as Antenna diversity, over the conventional CDMA Receiver in wirelessfeedback diversity, Scanning diversity, Maximum ratio communication and to achieve a better Bit Error Ratiocombining, Equal gain combining, etc. MRC is the (BER). CDMA RAKE Receiver consists of manyoptimist form of diversity combining cause of correlators that correlate to spread signals. MultipathMaximum ratio combining all the integration result of fading is one of the major practical concerns in wirelesscode sequence by this we achieve better bit error ratio communications.(BER) and maximum signal to noise ratio (SNR). Finally, We will see that signal to noise ratio(SNR) andIn Maximum ratio combining all integrated multipath bit error ratio (BER)graph comparison between CDMAcomponents such as x1(t), x2(t), x3(t),………. xn(t) So receiver and CDMA RAKE receiver-whole of components are multiplied withcomplex conjugate weighted β1*(t),β2*(t),β3*(t)…… βn*(t) of channel characteristics. Thenmathematical expression of the MRC can be written as- 63 All Rights Reserved © 2012 IJARCET
  5. 5. ISSN: 2278 – 1323 International Journal of Advanced Research in Computer Engineering & Technology Volume 1, Issue 5, July 2012 VI. CONCLUSION Interference Mitigation by CDMA RAKE receiver with Walsh-Hadamard sequence as the spreading code has been introduced. We found that Conventional RAKE receiver gives the better performance in comparison with the conventional CDMA receiver in respect of system efficiency, SNR and BER preformance.In the proposed model each multipath component signal correlate with each other so that is big advantages cause of multiple code generators running continuously. So that proposed model with Walsh-Hadamard sequence gives an considerable result as compared to conventional CDMA receiver. CDMA Receiver (plot SNR b/w BER) VII. REFERRENCE [1] L. Harju et. al., “A Flexible RAKE Receiver Architecture for WCDMA Mobile Terminals,” for SPAWC ’01, Taoyuan, Taiwan, Mar. 2001, pp.9-12. [2] J.G. Proakis, Digital Communication", McGRAW-HILL. [3] T.S. Rappaport, Wireless Communications Principles and Practice", Prentice Hall [4] A.J. Viterbi, CDMA Principles of Spread Spectrum Communication",ADDISONWESLEYCOMPANY . [5] B. J. Wysocki and T. A. Wysocki, “Orthogonal Binary Sequences with Wide Range of Correlation Properties,” for ISCTA’01, . Ambleside, U.K., July 2001, pp. 483-485 [6] . Cheng and C. Beaulieu, “Accurate DS-CDMA Bit-Error Probability Calculation in Rayleigh Fading,” IEEE Trans. Wireless Commun., vol.1, no. 1, pp. 3-15, Jan. 2002. CDMA RAKE Receiver (plot SNR b/w BER) [7] M. F. Madkour, S. C. Gupta and E. Wang, “Successive interference cancellation Algorithms for Downlink W-CDMA Communications,” IEEE Trans. Wireless Commu., vol. 1, no. 1, January 2002 [8] S.Mudulodu, G. Leus, and A. Paul raj, “An interference suppressing RAKE receiver for the CDMA downlink," IEEE Sig.Proc. Lett., vol.11, pp. 521-524, May 2004 [9] M. Uzzafer, “Single Correlator Receiver for Wideband MultiPath Channel, “in Proc. IEEE 6th CAS Symp., Shanghai, vol. 2, June. 2004, pp.421-424 [10] F. Adachi et. al., “Broadband CDMA Techniques,” IEEE Wireless communications, vol. 12, no. 2, pp. 8-18, Apr. 2005 [11] M. Ali Akbar, Hamza Bin Tila, M. Zulkifl Khalid and Muhammad Asim Ajaz ―Bit Error Rate Improvement using ESPRIT based Beam forming and RAKE receiver‖, 978-14244- 4873-9/09/©2009 IEEE [12] R. Fantacci et. al., “Perspectives for Present and Future CDMA- based Communications,” IEEE commun. Magazine, vol. 43, no. 2, pp. 95-100,Feb. 2005. [13] R. Fantacci et. al., “Perspectives for Present and Future CDMAbased Communications,” IEEE commun. Magazine, vol.Comparison b/w different type of combining 43, no. 2, pp. 95-100,Feb. 2005.techniques (CDMA RAKE RECEIVER) 64 All Rights Reserved © 2012 IJARCET

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