An Efficient Secured Turbo Codes for Long Term Evolution System Enhancement

Transcription

1 Internatonal ournal of Networks and Communcatons 2016, 6(3): OI: /j.jnc An Effcent Secured Turbo Codes for Long Term Evoluton System Enhancement M. E. Abd El-Hameed 1, Mohsen A. M. El-Bendary 2, I. O. Bekhet 3, H. M. Abd El-Kader 3 1 Worker Unversty, Egypt 2 epartment of Communcaton Technology, Faculty of Industral Educaton, Helwan Unversty, Egypt 3 epartment of Electoncs and Communcaton, Faculty of Engneerng, Banha Unversty, Egypt Abstract In ths paper an effcent secured nterleaver s utlzed n Turbo codes for Long Term Evoluton (LTE) system. The engne of the presented technque s the Baker formula; t works based on secret key. The proposed technque system mproves the Bt Error Rate (BER) and the throughput as well as the securty also. Ths scheme depends on chaotc Baker map encrypton. A comparson study between the proposed chaotc nterleavng scheme and the tradtonal block and convolutonal nterleavng schemes for LTE system transmsson over an Addtve Whte Gaussan Nose (AWGN) channel s presented. The smulaton results show the superorty of the proposed scheme over the tradtonal schemes. It reveals also, the turbo codes perform better wth the proposed technque compared to the nternal block nterleaver used n LTE system. The data orderng n the proposed scenaro based on the determned key length, ths key controls the power of data randomzng. It can be used as a secret key also, to enhance the LTE securty, packet-by-packet protecton. Keywords LTE, Turbo codes, Interleaver, Baker map, Packet protecton 1. Introducton The company Long Term Evoluton [1] has long been seen as the frst advancement towards stronger, faster and more effcent 4G data networks. LTE has been developed to meet the requrements of ths era and to realze the am of achevng global broadband moble communcatons. The objectves of ths evolved system ncludes hgher rado access data rates, mproved system capacty, coverage, flexble bandwdth operatons, mproved spectral effcency, low latency, reduced operatng costs and seamless ntegraton wth the Internet and exstng moble communcaton systems [2]. The technology under LTE can currently reach downlnk peak rates of 100Mbps and uplnk speeds of 50Mbt/s. The LTE technology s also a scalable bandwdth technology for carrers operatng anywhere from 20 MHz to 1.4 MHz. Long Term Evoluton offers some excellent advantages over current 3G systems ncludng hgher throughput, plug and play compatblty, Frequency vson uplexng (F) and Tme vson uplexng (T), low latency and lower operatng expendtures. It also offers legacy modes to support devces operatng on General Packet Rado Servce (GPRS) systems, whle supportng seamless pass- through of technologes operatng on other older cellular towers. The technologes put forth by LTE wll not only be mplemented over tme, they are desgned to be * Correspondng author: Publshed onlne at Copyrght 2016 Scentfc & Academc Publshng. All Rghts Reserved scalable. Ths scalablty means can slowly ntroduce LTE technologes over tme. 2. LTE Physcal Layer In the base staton and one n the moble staton wth a channel bandwdth s 20 MHz. One key feature of LTE systems s the support of a scalable channel bandwdth that ranges from 1.4 MHz up to 20 MHz, whch makes ts mplementaton more feasble to the servce provders. As we see n Fg. 1 the LTE block dagram consst of: A. CRC A CRC (cyclc redundancy check) s used for error detecton n transport blocks. The entre transport block s used to calculate the CRC party bts. The transport block s dvded by a cyclc generator polynomal to generate 24 party bts. These party bts are then appended to the end of transport block. The polynomal s as follows: Gx ( ) = x + x + x + x + x + x + x x + x + x + + x + x+ Segmentaton and 2nd CRC: If the nput block sze s greater than 6144 bts, t s splt n to smaller blocks. Agan CRC s performed and redundant party bts are appended to each resultng smaller block. Also, fller bts are added so the code block szes match a set of vald block szes nput to turbo code. 1 (1)

2 50 M. E. Abd El-Hameed et al.: An Effcent Secured Turbo Codes for Long Term Evoluton System Enhancement Input CRC EXTERNAL INTERLEAVER TURBO ENCOER Addtve Scrambler OFM Transmtter (a) Transmtter Output e- CRC EXTERNAL E INTERLEAVER TURBO ecoder e -Scrambler OFM Recever (b) Recever Fgure 1. Block agram of LTE System B. Turbo codng and decodng Channel codng s used n most dgtal communcaton and especally n moble communcaton to mprove the error correctng capablty. The LTE standards have adopted turbo codng. The scheme of the turbo encoder s a parallel-concatenated convolutonal code wth two 8-state convolutonal encoders and one nternal turbo code nterleaver and a bt reorderng block to reorder the coded bts. The performance of the turbo encoder depends crtcally on the nterleaver structure [3], where the turbo codng nterleaver vector (X) n LTE s set as follows: x = f + f K (2) 2 (.. ) mod 1 2 where f 2 and f 2 are chosen from the LTE specfcaton dependng on the frame sze k. C. Scrambler Scramblng produces a block of scrambled bts from the nput bts accordng to the relaton gven by the equaton: ^ B = b+ cmod 2 (3) The B ^ symbol denotes the scrambled bts, b denotes the nput bts, c denotes the scramblng sequence.. OFM System Block As we see n Fg. 2 Orthogonal Frequency-vson Multplexng (OFM) [4] s a multcarrer transmsson technque that s used as the LTE downlnk transmsson scheme. In OFM, the wde band frequency carrer s dvded nto narrow band subcarrers orthogonal to each other as n Fg. 3. Bascally splts a hgh-rate data stream nto a set of low-rate sub-streams that are transmtted smultaneously over a number of sub-carrers. Thereby, the bandwdth of the sub-carrers become small compared wth the coherence channel bandwdth as n Fg.4. Ths orthogonalty n combnaton wth an approprate choce of subcarrer spacng (Δf) and the Cyclc Prefx (CP) length makes the OFM system a robust transmsson technque for frequency selectve channels. The wde band frequency selectve channel s converted nto a group of narrow band flat fadng channels at each subcarrer. Each subcarrer s modulated usng one of Bnary Phase-Shft Keyng (BPSK). The Quadrature Phase Shft Keyng (QPSK) and Quadrature Ampltude Modulaton (QAM) schemes suggested by the LTE standards [6]. The OFM modulator at the transmtter sde s mplemented usng an N-pont Inverse Fast Fourer Transform (IFFT) operaton, where N denotes the total number of subcarrers. Usng an IFFT reduces the mplementaton complexty sgnfcantly compared to usng a bank of modulators for each sub-carrer. Each OFM symbol n an LTE transmsson frame conssts of N subcarrers n the frequency doman wth a frequency spacng Δf between each consecutve subcarrer. The choce of the proper Δf depends on the frequency selectvty of the channel and the maxmum rate of channel varaton, and the choce of the number of subcarrers depends on the assumed overall transmsson bandwdth [7]. Not mantanng cyclc convoluton for the OFM subcarrers may lead to a loss of the subcarrers orthogonalty, whch results n nterference between adjacent subcarrers. To avod that stuaton, an approprate CP length s used. CP samples are chosen from the last part of the OFM symbol, where a number of samples are coped and nserted at the begnnng of the OFM symbol. In LTE-OFM based systems, the CP has two types: normal and long. The CP s length vares dependng on the channel bandwdth used and the number of OFM symbols. At the recever sde, the OFM demodulator s mplemented usng an N-pont Fast Fourer Transform (FFT) operaton to convert the sgnal back to the frequency doman after removng the CP.

3 Internatonal ournal of Networks and Communcatons 2016, 6(3): OFM modulator Input ata S/P Mappng IFFT Cp nnserton P/S /A OFM demodulator Channel l Output ata P/S emappng IFFT S/P Cp removal A/ Fgure 2. OFM Transmtter and Recever Fgure 3. Spectra of (a) an OFM sub-channel and (b) and OFM sgnal (a) Conventonal multcarrer technque (b) Orthogonal multcarrer modulaton technque. Fgure 4. Representaton of OFM sgnal [5]

4 52 M. E. Abd El-Hameed et al.: An Effcent Secured Turbo Codes for Long Term Evoluton System Enhancement 3. Turbo Codng n LTE Turbo codes were frst ntroduced by Berrou, Glaveux and Thtmajshma n 1993 and have Performances wthn a few tenth of a db from the Shannon lmt. The Turbo encoder n LTE [9] s a code Parallel Concatenated Systematc Convolutonal (PCCC) wth two 8-state consttuent encoders the same as n Unversal Moble Telecommuncatons Servce (UMTS) and one Turbo code contentons free nternal nterleaver (dfferent from UMTS). The encoders are based on RSC (Recursve Systematc Convoluton) codes and ther generator polynomal s gven by G=[1, g 0 /g 1 ], where g 0 =[1011] (Feedback) and g 1 =[1101] (feed forward). The structure of the Turbo encoder used n LTE s shown n Fg.5, the output of the LTE Turbo encoder conssts of three parts, a systematc bt and two party bts. The systematc bt (X k ) s the untouched nput bt. The frst party bt (Z k ) s the output of the frst convolutonal encoder wth the orgnal nput (C k ) nput and the second party bt (Z0 k ) s the output of the second convolutonal encoder after nterleavng (by the Turbo code nternal nterleaver) of the nput bt (C0 k ) as ts nput For trells termnaton the tal-bts X0 k are nserted [8] [9]. A. Interleaver role n turbo code Interleaver sze and structure consderably affect turbo code error performance [10]. Turbo codes consst of a parallel concatenaton of two Recursve Systematc Convolutonal (RSC) coders n conjuncton wth an nterleaver and assocated decoder as shown n Fg.4. Interleavers n Turbo code can be categorzed as tradtonal and random types. Tradtonal nterleavers consst manly of a block nterleaver and a convolutonal nterleaver [1]. Currently, the random nterleaver attract much attenton snce t can contrbute not only to correct abrupt errors occurred n sgnal transmsson but also to enhance the randomness of code sequences resultng n lowerng the BER and frame error rate (FER) of the communcaton system. However, t s dffcult n practce to realze the pure randomness n nterleaver s desgn based on current technques. As regard to the mplementaton of the Turbo codes, there are two man obstacles ncludng algorthm complexty and system delay for the Turbo codes beng employed nto dgtal communcaton system n the near future. The man contrbuton for performance on Turbo codes n system s nterleavng and decodng. X k Z k C k Internal nterlever Z k C k Fgure 5. Structure of the LTE Turbo Encoder X k

5 Internatonal ournal of Networks and Communcatons 2016, 6(3): Proposed Modfcatons We study the feasblty of data nterleavng pror to transmsson over LTE system. The paper presents a new chaotc nterleaver and compares to the tradtonal block and convolutonal nterleavers. A. Block Interleaver Scheme The block nterleavng s used wth turbo decoder n LTE system, the data s rearranged nto a matrx n a row-by-row manner, and then read from the matrx n a column-by-column manner. B. Convolutonal Interleaver Scheme A convolutonal nterleaver [4] [11] conssts of N rows of shft regsters, wth dfferent delay n each row. In general, each successve row has a delay whch s symbols duraton hgher than the prevous row as shown n Fg.6. The code word symbol from the encoder s fed nto the array of shft regsters, one code symbol to each row. Wth each new code word symbol the commutator swtches to a new regster and the new code symbol s shfted out to the channel. The -th (1 N-1) shft regster has a length of (-1) stages where = M/N and the last row has M-1 numbers of delay elements. C. Chaotc Interleaver Scheme As mentoned n the prevous subsecton, the block nterleaver s not effcent wth 2- error bursts. As a result, there s a need for an advanced nterleaver for ths task. As we see n Fg.7 the 2- chaotc Baker map [12] [13] n ts dscretzed verson s a good canddate for ths purpose. After rearrangement of bts n to a 2-format, the chaotc Baker map s used to randomze the bts. The dscretzed Baker map s an effcent tool to randomze the tems n a square matrx. Let B(n 1,..., n k ), denote the dscretzed map, where the vector, [n 1,... n k ], represents the secret key, S key. efnng N as the number of data tems n one row, the secret key s chosen such that each nteger n dvdes N, and n ,+n k = N. Let N =n ,+n 1. The data tem at the ndces (r, s), s moved to the ndces: N N Brs (, ) = [ ( r N) + smod( ), n n (4) n N ( s smod( )) + N ] N n Where N r N + n, 0 s N, and N = 0. I In steps, the chaotc permutaton s performed as follows: 1. An N N square matrx s dvded nto N rectangles of wdth n and number of elements N. 2. The elements n each rectangle are rearranged to a row n the permuted rectangle. Rectangles are taken from left to rght begnnng wth upper rectangles then lower ones. 3. Insde each rectangle, the scan begns from the bottom left corner towards upper elements. elay Element From Encoder N-1 To Channel Fgure 6. Convolutonal Interleaver

6 54 M. E. Abd El-Hameed et al.: An Effcent Secured Turbo Codes for Long Term Evoluton System Enhancement B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 B 9 B 10 B 11 B 12 B 13 B 14 B 15 B 16 B 17 B 18 B 19 B 20 B 21 B 22 B 23 B 24 B 25 B 26 B 27 B 28 B 29 B 30 B 31 B 32 B 33 B 34 B 35 B 36 B 37 B 38 B 39 B 40 B 41 B 42 B 43 B 44 B 45 B 46 B 47 B 48 B 49 B 50 B 51 B 52 B 53 B 54 B 55 B 56 B 57 B 58 B 59 B 60 B 61 B 62 B 63 B 64 Fgure 7. Mechansm operaton of the proposed nterleaver wth 8 8 matrx data sze Fg. 7 shows an example for chaotc nterleavng of an 8 8 square matrx (.e., N = 8). The secret key, S key = [n 1, n 2, n 3 ] = [2, 4, 2]. Note that, the chaotc nter-leavng mechansm has a better treatment to both 1- and2- error bursts than the block nterleavng mechansm. Errors are better dstrbuted to bts after de-nterleavng n the proposed chaotc nterleavng scheme. Ths S key controls the matrx segmentaton and the nterleaved data arrangng. Ths key can be changeable and long to enhance the securty. So, the Turbo code performance wll be developed wth securty bonus [14]. 5. Smulaton Results After the Baker map chaotc nterleavng mplemented by the step 1 to step 3 and as can be seen from the algorthm steps, t s easy to desgn the nterleaver n practce. And the

7 Internatonal ournal of Networks and Communcatons 2016, 6(3): overall performance for Turbo codes wll be enhanced sgnfcantly Another advantage of the proposed nterleaver s that durng the chaotc mappng only a few parameters are needed beng transferred from transmtter to recever, thus not only we can save channel volume for transmttng more source data on the one hand, but also decrease the probablty of data error occurred n the sgnal transmsson on the another. To verfy the effectveness of the proposed nterleaver, BER performance compared wth other nterleavers from the smulaton results are gven as curves shown n Fgs. 8 and 9 respectvely. As can be clearly seen from the curves shown n the fgure, wth reducng the computaton complexty the proposed nterleaver exhbts a hgh BER performance than other random nterleavers. From the Fgs 8 and 9, we found that the throughput also mproved from the other nterleavers teraton block nterleaver conv nterleaver chaotc nterleaver Fgure 8. BER versus SNR for LTE system wth chaotc nterleaver on AWGN Channel technques teraton4 block nterleaver conv nterleaver chaotc nterleaver Fgure 9. Throughput of LTE System wth Chaotc Interleaver on AWGN Channel Technques

8 56 M. E. Abd El-Hameed et al.: An Effcent Secured Turbo Codes for Long Term Evoluton System Enhancement 6. Conclusons Ths paper presented a smple and effcent novel chaotc nterleaver for the transmsson of data over LTE system. A comparson study between the proposed nterleaver and the conventonal nterleavers has been presented. The computer smulaton results have revealed the effectveness of the proposed nterleaver at medum and hgh SNR values. Also, the proposed nterleaver enhanced the securty level as t s based on chaotc map encrypton. REFERENCES [1] A Chaotc Interleaver Used n Turbo Codes Hongyu Zhang Ln Wang Qngsheng Yuan, Hongxa Wang uebang Yu (School of Electronc Engneerng UESTC, Chengdu, , Chna. ksha [2] uggal and yoteesh Malhotra Performance analyss of Long Term Evoluton Physcal Channels Internatonal ournal of Future Generaton Communcaton and Networkng Vol. 9, No. 3 (2016), pp [3] W. C. akes, Mcrowave Moble Communcaton, Wley, New York, 1974 [4]. Yuan, B. Vucetc, W. Feng, Combned turbo codes and nterleaver desgn IEEE Trans. Comm., vol.47, No.4, pp , Apr [5] H. Schulze and C. Luders, Theory and Applcaton of OFM and CMA Wdeband Wreless communcaton, ohn Wley, 2005 [6] Manjunatha K N, Kran B, Prasanna Kumar. C/ esgn and ASIC Implementaton of a 3GPP LTE Advance Turbo Encoder and Turbo ecoder Internatonal ournal of Engneerng Research and Applcatons (IERA) ISSN: Vol.2, Issue 4, uly-august 2012, pp [7] R. V. Nee and R. Prasad, OFM for Wreless Multmeda Communcatons, Artech House, Terrestral Rado Access (EUTRA) Multplexng and Channel Codng, an [8] Emla Käsper, Turbo Codes, ys/turbo.pdf, cted 15 une, [9] Patel Sneha Bhanubha, Mary Grace Shajan, Upena. alal Performance of Turbo Encoder and Turbo ecoder for LTE IEIT Vol2, Issue6, Oct 2012 [10] R.V. BANGAR, S.N. WAR, A.K, S. SHETH, Performance Analyss of Turbo Coded OFM Over Uncoded & Convolutonal Coded OFM, Internatonal ournal of Industral Electroncs and Electrcal Engneerng, Volume-3, uly-2015 [11] Prabhavat. Bahrgond, Shantanu k. xt Ber Analyss of turbo code nterleaver Internatonal ournal of Computer Applcatons ( )Vol.126-No.14,September2015 [12] V. Murugan,. Svakumar, Non-Coherent Bt Interleavng Coded Scheme for PAPR Reducton of OFM Sgnal", 4th Natonal Conference on Advanced Computng, Applcatons & Technologes, May 2014 [13] An Effcent Chaotc Interleaverfor Image Transmssonover IEEE Zgbee Network Mohsen A. M. M. El-Bendarya, Atef Abou El-Azmb, Nawal El-Fshawyb, Fard S. M. Al-Hosareyb, Mostafa A. R. Eltokhya, Fath E. Abd El-Sameb, and H. B. Kazemanc [14] M.A.M ElBendary Moblty Effects Combactng through Effcent Low Complexty Technque CT, Vol5, No12, ecember GPP TS v10.4.0: Evolved Unversal [15] ohn G. Proaks, gtal Communcatons, Mc-Graw-Hll: Internatonal Edtons, 4th ed. [16] Ausgewählte Kaptel der Nachrchtentechnk, WS 2009/2010LTE: er Moblfunk der Zukunft Channel Codng and Lnk Adaptaton Shahram Zare 16. ecember 2009