MIT 6 111 - LOCAL DECODING OF WALSH CODES (10 pages)

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LOCAL DECODING OF WALSH CODES



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LOCAL DECODING OF WALSH CODES

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10
School:
Massachusetts Institute of Technology
Course:
6 111 - Introductory Digital Systems Laboratory
Introductory Digital Systems Laboratory Documents
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LOCAL DECODING OF WALSH CODES TO REDUCE CDMA DESPREADING COMPUTATION Matt Doherty TA Kyeong Jae Lee 6 111 Introductory Digital Systems Laboratory May 18 2006 Abstract As field programmable gate arrays FPGAs continue to become more powerful and more flexible computer scientists are looking to them to combine the flexibility of software with the performance afforded by the specialization of hardware An algorithm for local decoding of CDMA Walsh codes has been translated from software to hardware and the problems and benefits associated with the translation have been analyzed In software the algorithm saves valuable computation but in hardware this computation savings results directly in power savings as well The system uses a VGA display to show in real time the decoder s bit error rate the received signal s signal to noise ratio and an estimate of the power being consumed by the device The project is a successful proof of concept not only that complex software algorithms can be implemented in hardware but also that significant power savings can be realized in CDMA base stations today through the use of a different decoding algorithm 1 Table of Contents 1 0 Introduction 3 2 0 Module Description Implementation 4 3 0 Testing Debugging 9 4 0 Conclusion 10 5 0 Bibliography 10 List of Figures 1 Labkit block diagram 4 2 Walsh decoder block digram 4 3 Bahl s 256 chip FHT design 5 4 Bahl s single FHT stage 6 5 Screenshot of VGA display 7 2 1 0 Introduction As field programmable gate arrays FPGAs continue to become more powerful and more flexible computer scientists are looking to them to combine the flexibility of software with the performance afforded by the specialization of hardware The project described herein is one such computer scientist s implementation of a software algorithm in hardware and analysis of the problems and benefits associated with the translation Code division multiple access CDMA is a wireless standard that relies on orthogonal Walsh codes to multiplex signals transmitted simultaneously over the same frequency to the same base station To recover the bits transmitted over the reverse link base stations must correlate the received symbols with all possible Walsh codes In 2005 Chan et al invented several novel classes of algorithms to reduce computation in software implementations of the IS 95 reverse link by exploiting software s inherent flexibility over hardware The algorithms function by processing only a fraction of the despread signal to decode Walsh codewords relying on a feedback loop to choose the fraction of despread signal to process in order to maintain a target bit error rate BER Because FPGAs are more flexible than application specific integrated circuits ASICs the same algorithms can be implemented in hardware in an FPGA to reduce computation This reduction in computation results directly in power savings because FPGA power consumption is tightly correlated with its gates switching activity This report details the design and implementation of the novel and elegant Generalized Local Decoding of Walsh codewords in an FPGA and estimation of the power savings of the algorithm in hardware Chan et al 1 1 System Behavior Upon reset the FPGA steps through a test vector of Walsh chips The decoder determines the six bits that correspond to every Walsh codeword of length 64 chips The user is presented with a graph and three gauges The graph shows the decoder s bit error rate BER over time The three gauges show the instantaneous decoder BER signal to noise ratio SNR of the test vector and power usage of the device Using button 3 the user can cycle among three decoding algorithms The first is the optimal Walsh decoder which feeds all 64 symbols to a length 64 Fast Hadamard Transform FHT that decodes the Walsh codeword It uses the most power because it correlates every combination of the input symbols with every Walsh code But it also results in the lowest average BER and thus serves as a benchmark against which the performance of the other algorithms can be validated The second is the generalized local Walsh decoder Using the left and right buttons the user can choose the number of length 8 FHTs with which the algorithm decodes the Walsh codeword As few as two and as many as eight length 8 FHTs can simultaneously estimate the six transmitted bits Since not all of the combinations of symbols are used in this estimation the algorithm is suboptimal However as long as the signal is relatively free of noise as few as 16 symbols can be used to decode the Walsh codeword while maintaining a low BER Especially at the singleFHT setting this decoder s gates switch much less frequently than those of the optimal decoder and so it uses much less power Third the adaptive generalized local Walsh decoder is simply a generalized local decoder with a feedback loop wrapped around it With this feedback loop the algorithm maintains a target BER while using as few length 8 FHTs as possible to decode the Walsh codeword 3 Using the up and down buttons the user can adjust the signal to noise ratio SNR of the test vector In general a lower SNR results in a higher BER because of the larger number of symbols that are received incorrectly on average 1 2 Block Diagrams Labkit Sync Vector Select Test Vectors in Block ROM Walsh Decoder DCM Divider BER Detector Display Feedback Controller Walsh Decoder Suboptimal Algorithm num fhts symbol 8x Length 8 FHT Optimal Algorithm Length 64 FHT Figures 1 and 2 The lab kit block diagram and detailed view of the Walsh decoder block 2 0 Module Description Implementation 2 1 Walsh Decoder The three decoding algorithms are instantiated within the Walsh decoder block This module sets reset high for all but the selected algorithm and outputs the six decoded bits when the selected algorithm sets its ready flag high 2 2 Suboptimal Algorithm The suboptimal decoder is the crux of the project It was originally designed for and implemented in software Albert Chan from Vanu Inc was gracious enough to provide an Octave script that simulates the IS 95 reverse link and performs local decoding on the received symbols Using Chan et al s design and this script the multiple FHT local decoder was designed 4 During cycles 0 63 after a reset signal the decoder buffers the symbols from the ROM into a memory Then from cycle 64 to cycle 67 different combinations of symbols are fed into the eight length 8 FHTs During cycles 67 70 the outputted correlation coefficients are summed


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