Bit Error Rate Performance Evaluation of Various Modulation Techniques with Forward Error Correction Coding of WiMAX

Transcription

1 Bit Error Rate Performance Evaluation of Various Modulation Techniques with Forward Error Correction Coding of WiMAX Amr Shehab Amin Abdelrahman Taha Yahia Mobasher Mohamed Yasser Mahmoud Gouda German University in Cairo April 30, 2018

2 2 Modulation&CodingFinall.nb OUTLINE Abstract Introduction WIMAX Orthogonal Frequency Division Multiplexing (OFDM) Convolutional Coding Reed Solomon Coding WIMAX System Simulation Results Adaptive Modulation Conclusion

3 Modulation&CodingFinall.nb 3 Abstract Physical layer simulator for WiMAX corresponding to the IEEE e specification is simulated Bit error rate performance evaluation of a WiMAX system using Reed-Solomon (RS) coding with Convolutional coding with 1/2, 2 3 and 3/4 rated codes

4 4 Modulation&CodingFinall.nb Introduction The family standard is called Wireless Metropolitan Area Network (MAN) commercially known as WiMAX (Worldwide interoperability for Microwave Access) Performance is investigated using the modulation schemes BPSK, QPSK and QAM with 1/2, 2/3 and 3/4 rated Convolutional coding using Reed Solomon coding as the forward error correction code.

5 Modulation&CodingFinall.nb 5 WIMAX WiMAX (Worldwide Interoperability for Microwave Access) is a telecommunications protocol that provides fixed and fully mobile internet access TABLE I PARAMETERS OF WIMAX [10] FFT symbol period 3.2 μs Number of fft points 256 Number of null sub carriers 56 Number of data sub - carriers 192 Number of pilot sub - carriers 8 Subcarrier frequency spacing MHZ Cyclic prefix 0.8 μs

6 6 Modulation&CodingFinall.nb ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM) OFDM is a combination of modulation and multiplexing In OFDM the signal is split into independent signals OFDM is a special case of FDMA

7 Modulation&CodingFinall.nb 7 OFDM is a Multicarrier Modulation Technique that divides the available spectrum into many carriers OFDM uses spectrum more efficient by making the carrier orthogonal to each other

8 8 Modulation&CodingFinall.nb Coded OFDM uses forward error correction before signal transmission OFDM over comes frequency selective fading and inter symbol interference

9 Modulation&CodingFinall.nb 9 CONVOLUTIONAL CODING Convolutional Code is a type of error correcting codes Convolutional Code transform m-info bits into n-bits (n>=m) The transformation is a function of constraint length

10 10 Modulation&CodingFinall.nb Punctured convolutional code is a high rate code Punctured convolutional code is obtained by periodic elimination of specific code symbols We can obtain high rate codes from low rate codes using puncture codes

11 Modulation&CodingFinall.nb 11 If puncture pattern is (111001) The ones in position 1,2,3,6 means bits in position 1,2,3,6 are transmitted The zeros in position 4,5 means bits in position 4,5 are removed from transmitted bits Thus it transformers ½ code rate into ¾ code rate

12 12 Modulation&CodingFinall.nb High rate codes can be decoded using low rate decoder which need less number of computations Puncture increase code rate without increasing complexity

13 Modulation&CodingFinall.nb 13 REED SOLOMON CODING They are linear block codes which means any linear combination of codewords is also a codeword The Reed-Solomon encoder takes a block of digital data and adds extra redundant bits.when an Error occur during transmission for any reason. The Reed-Solomon decoder processes each block and attempts to correct errors and recover the original data. A Reed-Solomon code is specified as RS (n, k) with m-bit symbols,the encoder takes k data symbols of m bits each and adds parity symbols to make an n symbol codeword

14 14 Modulation&CodingFinall.nb A popular Reed-Solomon code is RS(255,223) with 8-bit symbols. Each codeword contains 255 code word bytes, of which 223 bytes are data and 32 bytes are parity The number of errors that can be corrected depends on the characteristics of the Reed-Solomon code. it can be calculated by (n-k/2)

15 Modulation&CodingFinall.nb 15 The generator polynomial of reed Solomon code is given by is the generator polynomial of GF (28) Reed Solomon code (n, k) Standards for different modulation schemes : QPSK 1/2 (32,24) can correct any 4 symbol errors in the code word QPSK 3/4 (40,36) can correct any 2 symbol errors in the code word 16QAM 1/2 (64,48) can correct any 8 symbol errors in the code word 16QAM 3/4 (80,72) can correct any 4 symbol errors in the code word 64QAM 2/3 (108,96) can correct any 6 symbol errors in the code word 64QAM 3/4 (120,108) can correct any 6 symbol errors in the code word

16 16 Modulation&CodingFinall.nb WIMAX SYSTEM

17 Modulation&CodingFinall.nb 17 Wimax Transmitter In the transmitter, binary input data sequence is generated Randomization is done to avoid long sequences of consecutive ones or consecutive zeros. Bits are Forward error correction encoded using Reed Solomon Encoder and Convolution Encoder

18 18 Modulation&CodingFinall.nb

19 Modulation&CodingFinall.nb 19 Bits are interleaved to randomize the occurrence of bit errors to increase performance Bits are converted into symbols by Modulation A guard band is added to each symbol to minimize the channel delay spread and inter symbol interference (ISI) (interference protection)

20 20 Modulation&CodingFinall.nb A pilot symbol is a complete OFDM symbol where the value of each subcarrier is predefined and known in transmitter and receiver, The received signal is correlated with the pilot symbol to detect the OFDM symbol start. (reference signal) Since we are transmitting large amount of digital data over a broadband wireless technology that supports up to 20MHz we will divide the BW into multiple sub carriers instead of one carrier and that to insure that the symbol time is much greater than the delay spread thus preventing (ISi) DC sub-carrier is the sub-carrier that is located at the center of the radio channel IFFT is used to convert the signal to time domain The cyclic prefix is created so that each OFDM symbol is preceded by a copy of the end part of that same symbol

21 Modulation&CodingFinall.nb 21 The ratio between the duration of the cyclic prefix and the duration of the useful OFDM symbol (1/4,1/8,1/16,1/32).

22 22 Modulation&CodingFinall.nb At the receiver Side

23 Modulation&CodingFinall.nb 23 Simulation Results Demonstrates plot of BER against SNR for various digital modulation techniques under AWGN

24 24 Modulation&CodingFinall.nb Demonstrates plot of BER against SNR for BPSK under AWGN with different cyclic prefixes.

25 Modulation&CodingFinall.nb 25. Demonstrates plot of BER against SNR comparison for various digital modulation techniques under AWGN with cyclic prefix 1/4 and 1/32.

26 26 Modulation&CodingFinall.nb Adaptive Modulation Adaptive modulation is a powerful technique for maximising the data throughput of sub carriers allocated to a user Adaptive modulation involves measuring the SNR of each subcarrier in the transmission Then selecting a modulation scheme that will maximise the spectral efficiency

27 Modulation&CodingFinall.nb 27 CONCLUSION The simulation results of estimated Bit Error Rate (BER) displays that the implementation of interleaved RS code with 2/3 rated Convolutional code of 64 QAM modulation technique gives less error as compared to other techniques. Using adaptive modulation, the remote stations can use a much higher modulation scheme when the radio channel is good. Thus as a remote station approaches the base station the modulation can be increased from 1-2 b/s/hz (BPSK-QPSK) up to 3-9 b/s/hz (8-QAM 512-QAM), significantly increasing the spectral efficiency of the overall system

28 28 Modulation&CodingFinall.nb Use of adaptive modulation can effectively control the BER of the transmission, as sub carriers that have a poor Eb/No (db) can be allocated a low modulation scheme such as BPSK rather than causing large amounts of errors with a fixed modulation scheme. For good Eb/No (db), the high modulation scheme giving priority to spectral efficiency can be considered. Eb/No of the channel is estimated before the transmission. The modulation scheme is set based on the SNR of the channel Fixing BER under good channel conditions like QAM with higher mode value gives best spectral efficiency Under worst channel conditions, we can use QPSK, BPSK. Thus, adaptive modulation should be adopted depending upon channel conditions