CODE COMBINING ARQ SCHEME

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1 CHAPTER 2 CODE COMBINING ARQ SCHEME 2.1 INTRODUCTION In a WATM network, services are extended over shared radio channels through the incorporation of suitable MAC and LLC layers using the standard ATM protocols. An LLC layer is required to provide link-to-link error control in order to insulate the ATM services from fugh error rate. In a wireless environment, the purpose of an ARQ protocol is to provide error free data transmission and to reduce the burden of retransmission of packets in enor. In this chapter, one of the LLC techniques called Code combining ARQ is proposed for WATM networks to reduce the effect of errors and to minimize the number of retransmissions for the AWGN and Rayleigh fading channels. Even though Code combining ARQ is available, it has not been applied to all the wireless channels especially Rayleigh fading channel. The present work improves the technique by extending the application of Code combining for delay constraint communication and applied this to both AWGN and Rayleigh fading channels. The performance analysis for Code combining ARQ is carried out and compared with that of conventional ARQ. In a WATM environment, the unit of transmission over the wireless link is a wireless ATM eel. Once the communication link is established, the Mobile Switching Centre (MSC) accepts the ATM cells fiom users. An ATM cell payload which consists of 48 bytes or a suitable integer sub multiple of it, e.g., 24 or 16 bytes, could become the basic unit of data [39]. In addition, it may be appropriate to compress essential ATM header information into a compressed header to avoid a high overhead and: add a wireless medium specific logic link header and tail to form a wireless ATM cell.

2 The WATM format used in the system is as show in Figurn 2.1. To improve efficiency, the payload length should not be less than 16 bytes and should be 48/11 bytes, where 'nn" is an integer. So, the conversion from one format to the other can be easily made. The cell is named as a standand cell when n=l. The division of cells results in additional overhead. The system performance is different for different cell lengths. Wireless packet header A TM senice data Figure 2.1 Logical link cell format in WATM The functions of logical link layer may include virtual channel identification, error control and segmentation and reassembly. As shown in Figure 2.1, a packet sequence number (PSN) is required to keep the order of the cells when retransmission takes place. Virtual Channel Identifier field is used for addressing. Header Error Control (HEC) is used to protect the data in the header. CRC bits pereonn cyclic redundancy check. 2.2 CODE COMBINING ARQ SCBE=ME Two techniques called conventional ARQ and Code combining ARQ techniques are investigated in this work. Convolution coding is usually considered as a viable candidate for error correction. The decoding is based on a soft decision Viterbi algorithm. The present work considers the convolutional code only with two different decoding approaches. The fust is the conventional decoding technique in which cells are decoded when they anive and discarded if there is an enor. As an alternative, Code combining can be used as a meam of achieving adaptive operation.

3 Code combining represents a technique for combining the minimum number of retransmitted packets encoded with codes of rate nz/;k to obtain a lower code rate and thus a more powerhl enor correcting code. Here 'm' is information bits and 'k' is output encoded bits. The process of Code combining is illustrated in Figure 2.2. Encoder 1 Encoder 2 Figure 23 Block diagram of Code combining ARQ The Code combining algorithm is discussed as follows. At the first transmission, the cell is encoded with a code rate of m/k by Encoder 1. At the receiver, after Decoder1 decodes the received cell at the rate of m/k, the cyclic redundancy check (CRC) bits are used to determine whether the cell has been decoded correctly. If emm are: detected, this cell is stared in the Code combining unit instead of being discarded and ARQ is performed. The retransmitted cell is encoded by Encoder 2 with a code of the same rate and decoded by Decoder 2. If the received cdl contains errors, it is interleaved with the cell stored in the Code combiner symbol by symbol and then the combined cell is decoded by Decoder 3 at the rate of d2k. The flowchart of the Code combining algorithm is illustrated in Fip 2.3. Retrammission stops when the CRC bits indicate an error free cell. In this process, Encoder 1 and Encoder 2 hnction alternatively and the cells in the air interface are encoded at a code rate of m/k. The Code combiner combines every two retraasmitted cells and the cells stored in the combiner are replaced every time by a new received cell which is retransmitted.

4 Receive cell of initial transmission and decade it by Viterbi decoder 1 of rate m.4 Error detected Receive repeated cell and decode it by Viterbi decoder 2 of rate m/lk No error I Error detected Combine two successive cells and decode it by Viterbi 1 decoder 3 of rate mrk I Figure 2 3 Rawchart of Crwfe combining AlRQ

5 23 PERFOMANCE ANALYSIS The simplest application of Code combining is used in an ARQ with a two-way communication system. The data are transmitted in blocks. Each block consists of a number of WATM cells. Cells are repeated if a block is received with emrs. A feedback link is used to signify an error free block (by an ACK signal) or a request for a repeat (by an NACK signat) when they ase detected. In this work, the performance analysis of the Code combining ARQ is studied as well as their impact on the wireless systems in terms of throughput, cell loss rate, frame emr rate and deiay me evaluated for direrent cell lengths. The analysis is canied out in AWGN channel and Rayleigh fading environment. Let padenote the probability of successfully decoding a cell in exactly one transmission and P, denote the probability of successfully decoding a cell in one transmission under Code combining. It can be shown that for the conventional decoding technique, the average number of transmission times for success cell is given by [39], The average number of transmission times for cell success under Code combining is, where, L, - Average Transmission Times Psi - Probability of successfully decoding a cell in one transmission Ps2 - Probability of successfully decoding a cell in one transmission under Code combining

6 ps, and Ps, are determind by the channel coding. As already mentioned, convolution coding is used for channel coding and Viterbi algorithm is used with soft decision at the receiver. y is the signal to noise ntio and it is assumed that y is the same for transmission as well as retransmission. The error probability 4 is given by, where, p(di is given by, PV,) =M J-1 i = 3,2 R = m/k is the code rate of WATM cell and d, is the distance. ax) denotes the probability density function of x. For different cell lengths, the Code combining ARQ technique is employed. The packet length considered in this work is 48/n. For a cell length ~(n), P, is bounded by, Pa = [l-elb where, i= 1,2 (2-51 The variances of the number of transmission times are, D; = E(L:) - L: and G; = E(L:) - L:

7 The required numkr of transmissions is distributed around the average value more ciosefy because both the avenge value and the variance are smaller with a Code combining ARQ technique. The cell length parameter B is calculated by varying the value of n = 1, 2 and 3. As shown in Figure 2.1, the payload is 48/n bytes. The analysis is canied out for different measures by changing the SNR. As GLR is one of the important performance measures, it is evaluated for conventional decoding technique. Let the ailowed delay of a cell be K, when a standard cell is transmitted. An ATM cell is divided into 'n' cells. By considering the conventional decoding technique, the probability af CLR can be calculated as follo~vs: For n = I, For n=2, On the other hand, when the payload length is reduced, the overhead is increased and the radio resource cannot be utilized efficientiy. The impact of the throughput of the channel for different payload length is another performance measure. The throughput is defined as the ratio of the average number of information bits successfully accepted by the receiver per unit time to the total number of bits that could be transmitted per unit time. The throughput is then, The Frame Error Rate FER) is the basic performance measure which has great impact on other perfcrmce parameters. The throughput measure is evaluated for AWGN channel and Rayleigh fading channel. For a standard cell, the CLR is compared for conventional ARQ and Code combining ARQ.

8 The simulation results for conventional ARQ and Code combining ARQ in AWGN and Rayleigh fading environment are obtained for a Coherent QPSK system with a typical data rate of 1 Adb/s and their performance in terns of throughput, Cell Loss Rate (CLR) and Frame Error Rate (FER) are analysed. By varying SNR, all the parameters are measured. 4 n=l no code combining -9- n=2 no code combining n=3 no code combining n=3 code combining - Q n=2 code combining - f n=3 code combining - SNR (db) Figure 2.4 SNR Vs Average transmission times for different 'n' values On experimenting upon the cell length B(n), n=l, 2 and 3 by varying the SNR value, average transmission times for cells are found. Figure 2.4 shows the relationship between the upper bound of average transmission times and the signal to noise ratio for dieerent values of k'. From Figue 2.4, it can be observed that shorter cells need fewer transmissions because they have a high probability of correct transmission. When the Code combining technique is used, the required number of retransmissions is abruptly reduced.

9 SmaIIer variance to the required transmission time is distributed around the average value more closely because both the average value and variance are smaller with the Code combining AFtQ technique. Due to less number of retransmissions, the buffer size needed at the receiver can be reduced. The relationship between CLR and SNR ratio for different 'n' and allowed delay are shown in Figures 2.5 and 2.6. The delay (K) = 1,3 and 5 corresponds to I rns, 3 ms and 5 ms respectively. From these figures, it is inferred that when no retransmission is allowed using standard cell, it produces a lower CLR because dividing cells result in additional overhead and the cell error rate is increased. However, when the service is not sensitive to delay, lower CLR can be obtained for the shorter packet. This is because a shorter cell can be transmitted correctly with greater probability, while a longer cell may get more chances to be affected. On the other hand, when the payload length is reduced, the overhead is increased and radio resources cmot be utilised efficiently. Figures 2.5 and 2.6 are drawn for n=l and n=2 respectively. SNR (db) Figore 2.5 SNR Vs CLR for n=l with different deiays in AWGN channel

10 SNR (db) Figure 2.6 SNR Vs CLR for n=2 with different delays in AWGN channel [ --8- w3 code combining o.xb/ --f7 rl=3 na code cornhining + n=l code combining ik n=l no code combining -8 n=2 no code combining n=2 cade combining 0 1 I 2 I I 7 I 8 I 9 10 SNR (db) Figure 2.7 SNR Vs Throughput for different 'ny values in AWGN channel

11 Figure 2.7 shows the throughput for different 'nhalues in an AWGN channel. For conventional ARQ when the SNR is lower than 6 db, shorter cells are more efficient. As SMi increases more than 6 db, the usage of standard cells become advantageous. Hence it can be observed that when Code combining is used, the throughput is increased greatly in the low SNR range and the use of a standard cell is more efficient for large SNR values SNR (db) Figure 2.8 SNR Vs Throughput for different Cn' values in Rayleigh fading channel?he simulation results for the throughput of conventional and Code combining ARQ in Rayleigh fading channel for different cell lengths are shown in Figure 2.8. When conventional ARQ is performed, the usage of shorter cells is more efficient at lower SNR vaiue. Otherwise the usage of shorter cells results in a lower throughput.

12 The kune error rate is the basic performance measure which has great impact on other performance parameters. The results obtained for IFER when n=l, 2 and 3 are shorn in Figure 2.9. From the graph it is inferred that lower SNR results in larger FER, which reveals that larger number of transmissions are required to deliver a cell. As expected, the usage of shorter cells gives less frame error rate. The frame error rate ixl Rayleigh fading channel and AWGN channel are show in Figure 2.9. SNR (db) Figure 2.9 SMR Vs F'ER in Rayleigh fading and AWGN channels Figure 2.10 shows the CLR of conventional and Code combining ARQ methods for a Rayleigh Fading channel. For obtaining curves in Figure 2.10, only standard cells are cansidered and it is assumed that three transmission attempts may be allowed. From the graph, it can be clearly observed that the Code combining ARQ has much better performance than the conventional method. Therefore by using Code combining ARQ, cell loss ratio can be greatly reduced.

13 SNR (db1 Figure 2.10 Comparison of schemes in Rayleigh fading channel 2.5 SUMMARY In this chapter, the performance of the Code combining ARQ is analysed for different cell lengths, Fxom the simulation results, it can be concluded that the number of retransmissions is much reduced in Code combining ARQ. When retransmissions are allowed, shorter cells reduce the CLR. The usage of standard cells gives a better pdomance for larger SNR (greater than 6 db) values. In general, for real time service, it is not preferable to use shorter cells as retransmission is not allowed. For other services, when delay can be tolerated, re~mission is allowed. No Code combining with standard cell can be used if SNX is greatw than 6 di3, but Code combining with shorter cells can be used for SNR lower than 6 db.

14 When conventional ARQ is used, shorter cell can get larger throughput in a low SNR, but when Code combining ARQ is used, there is a little difference in throughput for variable cell length. On the other hand, for large SNR, the two ARQ methods can give almost the same throughput. It is also concluded that when Code combining ARQ is used, standard cell is more efficient than the shorter cell and it is unnecessary to divide the cell into a short cell. At the same time, the use of a Code combining ARQ can provide a much lower CLR. The main conclusion here is that Code combining ARQ is a preferred technique due to its simplicity and robustness under fluctuating conditions especially for Rayleigh fading environment.

Payload Length and Rate Adaptation for Throughput Optimization in Wireless LANs

Payload Length and Rate Adaptation for Throughput Optimization in Wireless LANs Sayantan Choudhury and Jerry D. Gibson Department of Electrical and Computer Engineering University of Califonia, Santa Barbara