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Block Turbo Code and Its ~Application to OFDM SYSTEMDesign For Cognitive Radio

Vivek Kr Srivastava

Deptt. of Electronics andCommunication Engineering,Noida Institute of Engineering &Technology Gr Noida, Indiasrivastava. vivek050@gmai/.colll

INTRODUCTIONAbstractAbstract- Recently, Cognitive Radio has beenproposed as a promising technology to improvespectrum utilization.A highly flexible OFOMsystem isconsidered to be a good candidate for the CognitiveRadiobase band processing where individual carrierscan be switched off for frequencies occupied by alicensed user. In order to support such an adaptiveOFOMsystem, we propose a Multiprocessor System-on-Chip (MPSoC) architecture which can bedynamically reconfigured. However, the complexityand flexibility of the base band processing makes theMPSoC design a difficult task. Thispaper presents adesign technology for mapping flexible OFOM baseband for Cognitive Radio on a multiprocessor System-on-Chip (MPSoC) .

To overcome multi path fading and Inter

symbol Interference (ISI), in convolution Singlecarrier systems equalizers are used. But itincreases the system complexity. Anotherapproach is to use a multi carrier modulationtechnique such as OFDM, where the data streamto be transmitted is divided into several lowerrate data streams each being modulated on asub carrier. To avoid ISI, a small interval, knownas the guard time interval, is inserted into OFDMsymbols. The length of the guard time interval ischosen to exceed the channel delay spread.Therefore, OFDM can combat the multi pathfading and eliminate ISI almost completely. Theother problem is the reduction of the error rate intransmitting digital data. For that we use errorcorrecting Codes in the design of digitaltransmission systems. Turbo Codes have beenwidely considered to be the most powerful errorcontrol code of practical importance. Turbocodes can be achieved by serial or parallelconcatenation of two (or more) codes called theconstituent codes [16]. The constituent codescan be either block codes or convolution codes.Currently, most of the work on turbo codes haveessentially focused on Convolution Turbo Code(CTC) and Block Turbo Code (BTC) have beenpartially neglected. Yet, the BTC solution is moreattractive for a wide range of applications.

In this paper simple explanation of BTCOFDM

theory is given. The BER performance is.,,,,,.---:--,

Keywords: Keywords: OFOM, MPSoC, Multiprocessorp, Cognitive Radio

evaluated for the Block Turbo coded BPSK andQPSK OFDM system, under both AWGNchannel and Rayleigh fading channel. It alsocompares the BER performance of Block Turbocoded OFDM with the uncoded OFDM. In thispaper the BTCOFDM system with 4 iterations issufficient to provide a good BER performance.Additional number of iterations does not shownoticeable difference The simulation resultsshows that the BTCOFDM system achieves largecoding gain with lower BER performance andreduced decoding iterations, therefore offeringhigher data rate in wireless mobile.

The concept of using parallel data transmissionby means of frequency division multiplexing(FDM) was published in mid 60's [1]. Some earlydevelopment with this can be traced back to the50s [1]. A U.S. patent was filled and issued inJanuary 1970 [2]. The idea was to use paralleldata streams and FDM with overlapping subchannels to avoid the use of high speedequalization and to combat impulsive noise, andmulti path distortion as well as to fully use theavailable bandwidth. The initial applicationswere in the military communications. Weinsteinand Ebert [3] applied the discrete Fouriertransform (DFT) to parallel data transmissionsystem as part of the modulation anddemodulation process. In the 1980s, OFDM hasbeen studied for high speed modems [4], digitalmobile communications [5] and high-densityrecording [6]. In 1990s, OFDM has found itsapplications in wideband data communicationsover mobile radio FM channels [9], wireless LAN[8], wireless multimedia communication [10],high-bit-rate digital subscriber lines ( HDSL )[11], asymmetric digital subscriber lines (ADSL)[7], digital audio broadcasting ( DAB ) [12],digital video broadcasting ( DVB ) [17]. OFDMhas been chosen as the modulation techniquefor the new 5 GHz IEEE802.lIa [13] standard aswell as High-Performance LAN (HIPERLAN) [14],[15]. 4 For the reduction of the error rate intransmitting digital data we use error correctingCodes in the design of digital transmissionsystems. Turbo Codes proposed by Berrou in

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1993 [16] have been widely considered to be themost powerful error control code of practicalimportance. Turbo codes have error correctingcapability very close to the theoreticalperformance limits.

2. Cognitive Radio

Due to the explosive growth of wirelesscommunication, the demands for radiospectrum are rapidly increasing. It is very difficultto accommodate new wireless services underthe current spectrum allocation scheme. On theother hand, the allocated spectrum is notefficiently utilized. [16]

Cognitive radio is a promising technology toimprove today's spectrum utilization. CognitiveRadio is proposed as a technology to solve theimbalance between spectrums Scarcity andspectrum under-utilization. Spectrum utilizationcan be improved by making it possible for a userwho does not have the license for spectrum(secondary user) to access the spectrum whichis not occupied by the licensed user (primaryuser). This secondary user has the awareness ofthe spectrum and adapts its transmissionaccordingly on a non-interference basis. Thisspectrum access and awareness scheme isreferred to as Cognitive Radio. The idea is alsoknown as Dynamic Spectrum Access (DSA) orOpen Spectrum Access (OSA). Cognitive Radiois seen as the final point of software definedradio (SDR) platform evolution. A fully flexibleand efficient software defined radio platform willbe the enabling technology for Cognitive Radio.Cognitive Radio imposes a number ofrequirements on the processing platform suchas flexibility, energy efficiency and guaranteedthroughput/latency. The trend in theimplementation of SDR is moving towardsMultiprocessor System-on-Chip (MPSoC)platforms.

This paper mainly focuses on the design of theadaptive physical layer (base bandprocessing).The physical layer considered in this workmainly consists of two parts: transmission andspectrum sensing. A recognizable MPSoC

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platform is used to support the adaptivebaseband processing of Cognitive Radio. [18]

Although MPSoCs offer many advantages, it is achallenging task to map applications ontoMPSoCs, especially highly dynamicapplications such as Cognitive Radio. There is agap between the application models used forthe specification of such applications and anoptimized implementation of the application onan MPSoC. To close the gap, we propose to usea task transaction level (TTL) interface approachboth for developing the Cognitive Radioapplication at system level and for the platforminterface between the application and theproposed MPSoC platform. The TTL approachis used throughout the work as the system-leveldesign methodology and its advantages areelaborated by mapping adaptive physical layeralgorithms for Cognitive Radio onto the MPSoCplatform [16]. The TTL model allows verifying thesystem's functional behavior and providesprofile information for complexity analysis.

The transmission of Cognitive Radio strictlydepends on the reliable detection of the primaryuser through spectrum sensing [19]. As a result,spectrum sensing is an essential part ofCognitive Radio. Spectrum sensing should alsobe considered as a part of the physical layer.The major task of the physical layer spectrumsensing is to detect the licensed signal byemploying various signal processingtechniques. This paper reviews different signalprocessing schemes for sensing and focuseson so-called energy detection. An energy basedmulti-resolution spectrum sensing scheme isproposed in this work. The sparse FFTproposed for OFDM based Cognitive Radio alsosuits this multi-resolution sensing scheme quitewell. The filter bank spectrum sensing techniqueis also considered due to its easy integrationwith a filter bank multi carrier system.

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