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(IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 6, September 2010 A.R.Q. techniques using Sub-block retransmission for wireless networks A.N.Kemkar, Member, ISTE and Dr. T.R.Sontakke Member,ISTE Abstract—In this paper we mainly focus our In an attempt to improve throughput investigation on the throughput performance in performance, we have included an analysis using conjugation with sub-block transmission scheme. forward error correcting (FEC) block codes (used The throughput of a wireless data in Hybrid ARQ). The optimum amount of FEC communications system depends on a number of coding was found to be dependent upon the Block variables, one of it is length of the message length. As the Block length increases, the number blocks. Over a noisy communication medium like of correctable errors to optimize the throughput wireless medium used for mobile ad-hoc network, also increases, mathematical expression is shown our propose scheme performs effectively. In in 2.2. propose scheme random length of the message is The paper is organized as follows. In Section 2 divided in to fixed length blocks and applying .Summary on the Related work and basic concept. ARQ techniques if the error occurs. A threshold In section 3.scheme description and system model is used for fading channel, estimation and model. We consider the performance analysis of CRC detection codes are used. Comparison of the proposed scheme for simulation study in transmission efficiency of proposed scheme with section 4. followed by conclusion in Section 5 varying channel condition is shown. . 2.RELATED WORK : .Index Terms— FEC, Hybrid ARQ, BER. 2.1: Related Work- The efficiency of HARQ scheme is compared with GBN schemes using 1. INTRODUCTION: different lengths of IP Blocks. Further show that Wireless channels are highly affected by usage of smaller Blocks and hybrid schemes leads unpredictable factors such as co-channel to an improved throughput. Differences between interference, adjacent channel interference, pure and hybrid GBN schemes are also discussed. propagation path loss, shadowing and multi path  fading. The unreliability of media degrades the When the channel is quiet the sub-block transmission quality seriously. Automatic Repeat retransmission scheme behaves like a ReQuest (ARQ) and Forward Error Correction conventional ARQ or hybrid ARQ scheme. As (FEC) schemes are frequently used in wireless the channel becomes increasingly noisy, the data environments to reduce the high bit error rate of block is divided into smaller sub-blocks for the channel. transmission. Each sub-block is encoded for error As we have seen, the throughput efficiencies of control by an appropriate shortened code of all the basic ARQ schemes are functions of the which the code length is adapted to the packet size n. ,,,,. Our main result corresponding channel BER. Further optimum is a mathematical technique for determining the block size in accordance with the channel block size as a function of the other variables like conditions  A single code HARQ scheme was BER, signal-to-noise ratio. proposed in which transmitter is operating in any -------------------------------------------------------- one mode with the degree of errors encounter. A.N.Kemkar1 ,S.R.T.M.U,Nanded. The operating state is selected based on the +91-9819150392,email@example.com channel BER. Data bits are divided in blocks and Dr.T.R.Sontakke2 are encoded with shortened codes. During the retransmission new coded blocks are combined Ex.Director – S.G.G.S.I.T.E.- Nanded and at the receiver end proper decoding Principal,Sidhant college of Engineering techniques are used to separate retransmitted Pune.+91- blocks from the new blocks. 9822392766,firstname.lastname@example.org 2.1. BASIC CONCEPT - 234 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 6, September 2010 Analytical expression how Throughput L-C R( 1− P e( γ ) ) L T= (5) performance of the system varies with the size of L the block length and FEC. :Consider the following two cases to verify the Throughput Case 2.: Now instead of transmitting those L bits performance of the system. with no error correction capability, we will now Case 1. Blocks are transmitted without FEC. add B error correcting bits and transmit a total of Case 2. Blocks are transmitted with FEC. bits L+B .Using a block code forward error Our analysis includes the following simplifying correction scheme, the minimum number of B assumptions:1. The CRC decoder detects all bits required to correct t errors is given by  errors in the output of the FEC decoder.2. t L +B Transmission of acknowledgments from the B ≥ log 2 ∑ (6) n =0 n receiver to the transmitter is error free and instantaneous. Now that we can correct‘ t ’ errors, our block System throughput ( T ) is the number of payload success rate, f (γ ) should be larger than its bits per second received correctly: previous value with no error correction. Recall that f (γ ) with t = 0 is given by: K T= Rf (γ ) (1) L L f (γ ) = ( 1 − P e ( γ ) ) where Pe ( γ ) is the probability KR where b/s is the payload transmission rate of a bit error as a function of the SNR. Now, with L error correction capability, the Block success rate Where f (γ ) = Block success rate defined as for some arbitrary value of t is  probability of receiving block correctly. t L +B L +B -n Probability is a function of signal to noise ratio. f t(γ ) = ∑ P n ( γ ) ( 1 − Pe ( γ ) ) (7) n e E n =0 γ= b (2) No Our new equation for the throughput as a function P of the signal to noise ratio is: In which E b = joules in received energy per bit. P R where R = Transmission rate. Probability is a L-C N o T(γ ) = f t(γ ) (8) function of signal to noise ratio. L +B γ Eb P γ = = (3) N o N oR From (5) and (8) it is clear that throughput of the Each Block, of length L bits, is a combination system is a function of message block length. of a payload ( K ) and overhead ( L-K ) . Because Further (5) and (8) are used for pure and hybrid the Block success rate, f (γ ) is a decreasing ARQ techniques. function of, L there is an optimum Block length,. L • . When 3. SCHEME DISCRIPTION AND SYSTEM L p L∗ excessive overhead in each Block limits MODEL : This paper presents a sub-block retransmission the throughput. When L > L ∗ Block errors limits scheme for ARQ . The data block is divided into the throughput. smaller sub-blocks for transmission. Each sub- For case 1. When there is no forward error block is encoded by an appropriate error detection correction coding. In this case codes. The encoded block is then transmitted. The L f (γ ) = ( 1 − P e ( γ ) ) (4) received block is checked for errors sub-block by where f (γ ) = block success rate defined as sub-block. The proposed scheme provides probability of receiving block correctly. improved throughput by retransmitting only the P e ( γ ) is block error rate. sub-blocks in the occurrence of errors. Therefore, in a system without FEC, the 3.1 SYSTEM MODEL : throughput as a function of L ,from (1) 235 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 6, September 2010 We consider an ad-hoc network with V nodes SIMULATION RESULTS: We evaluate the and assume that each node is equipped with only performance of the proposed scheme one antenna. A Point to Point protocol is used at implemented with Matlab. We run the simulation the medium access control layer. A Selective for two schemes i.e. with sub block transmission repeat request ARQ mechanism is used. and without sub block transmission. The Particularly, the source node transmits a data simulation parameters are shown in the table packet with a C-bit CRC attached. The 1.Simulation run for 5000 total blocks. Result is destination node detects CRC and then sends an the average of independent experiments where acknowledgement that is either positive (ACK) or each experiment uses different randomly negative (NACK) back to the source node. If the generated uniform parameters. We use mean packet is correctly detected by the destination values which are obtained independent node (with ACK feedback), the source node experiments as a basic data to get the result. continues to transmit a new data packet and the Simulation results are shown Table 2 above process is repeated. Otherwise, Table 1: System Parameters: retransmission will start. A threshold model for Parameters Notaion Values channel characterization is used for fading Signal to Noise Ratio γ Varied channel. Total number of blocks ------- 5000 4. PERFORMANCE ANALYSIS OF THE Total sub block M 32 PROPOSE SCHEME: Information bits in a block K 16 Packet length n 5000*32*16 The performance analysis of the scheme is Max. number of -------- 3 measured in terms of throughput of the proposed Retransmissions scheme. Further we show the comparison of Number of sub blocks L Varied throughput with sub block and without sub block retransmitted transmission schemes. Cyclic Redundancy Check CRC Varied Expression of throughput for ARQ for present Bit error rate BER Varied Packet error rate PER Varied scheme: η Throughput efficiency Varied K η= (9) E[T] Table 2 : Simulation Results: Following where K =information bits in a block. E [ .] Simulation results shows the comparison of =Expectation of number of transmitted bits in a Throughput efficiency verses varied block size given block. verses changing channel condition in terms of ∞ PER. T = Mn + ∑ T i (10) Table 2: i =1 where M =number of sub blocks, n =number of Block length Packet error rate Throughput bits in a sub block, T i =number of transmitted bits Without With Without With Without With sub block sub sub sub sub sub for i th transmission. block block block block block ∞ E [ T ] = Mn + ∑ E [ T i ] (11) i =1 Whole Whole 0.1 0.1 0.9 0.99 where E [ T ] =Average number of transmitted block is transmitted block is 0.3 0.3 0.86 0.96 0.5 0.5 0.66 0.94 bits. with out divided 0.7 0.7 0.59 0.93 Out of M sub blocks if L sub blocks are sub into 0.9 0.9 0.57 0.9 division. sub 4 1 1 0.5 0.89 transmitted at the i th retransmission ,then random blocks variable, T i takes the value Ln ,if L out of M sub-blocks are retransmitted at the i th retransmission. 236 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 8, No. 6, September 2010 5. CONCLUSION: From the Fig.1 shown below performance of the 6. REFERENCES: proposed scheme. This paper has presented a sub- block retransmission scheme for  W.W. Chu. Optimalmessage block size 1. With micro block ARQ for computer communications with error 2. Without micro block ARQ detection and retransmission strategies. IEEE Transactions on Communications, Proposed sub-block retransmission schemes COM-22:1516– 1525, October 1974. showed better overall performance compared to  J.S. DaSilva, H.M. Hafez, and S.A. the other competitive schemes by retransmitting Mahmoud. Optimal packet length for without sub-blocks in the occurrence of errors. fading land mobile data channels. In From Table 2 and the Fig.1 it is clear that as the Proceedings of ICC 1980, pages 61.3.1– packet error rate increases i.e. channel condition 61.3.5, June 1980. gets deteriorated the throughput performance goes down by more than 40% as compare to  R.L. Kirlin. Variable block length and proposed scheme. There fore we proposed sub- transmission efficiency. IEEE block retransmission scheme with ARQ is more Transactions on Communication reliable than existing i.e. without sub block Technology, COM-17:350–355, June transmission scheme. 1969. Fig.1: Performance of proposed scheme vs.  E. Modiano. An adaptive algorithm for existing schemes. optimizing the packet size used in T h ro u g h p u t V s . P a c k e t E rro r R a te wireless ARQ protocols. Wireless 1 Networks, 5:279–286, July 1999. 0 .9 5 0 .9  J.M.Morris. Optimal block lengths for ARQ error control schemes. IEEE 0 .8 5 Transactions on Communications, COM- 0 .8 W it h M ic ro b lo c k 27:488–493, February 1979 W it h o u t M ic ro b lo c k T hroughput 0 .7 5 0 .7 0 .6 5 0 .6 0 .5 5 0 .5 0 0 .1 0 .2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1 P a c k e t E rro r R a te 237 http://sites.google.com/site/ijcsis/ ISSN 1947-5500
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