{Đồ Án} Nghiên Cứu Công Nghệ OFDM và Ứng Dụng, {Đồ Án} Nghiên Cứu Công Nghệ OFDM và Ứng Dụng,{Đồ Án} Nghiên Cứu Công Nghệ OFDM và Ứng Dụng{Đồ Án} Nghiên Cứu Công Nghệ OFDM và Ứng Dụng{Đồ Án} Nghiên Cứu Công Nghệ OFDM và Ứng Dụng{Đồ Án} Nghiên Cứu Công Nghệ OFDM và Ứng Dụng
LIST OF ACRONYMS AWGN Additive White Gaussian Noise BER Bit Error Rate CSI Channel State Information FDM Frequency Division Multiplexing ICI Inter-Carrier Interference ISI Inter-Symbol Interference MLD Maximum Likelihood Decoding M-PSK M-ary Phase-Shift Keying MMSE Minimum Mean Square Error MRC Maximum Ratio Combining MRT Maximum Ratio Transmit OFDM Orthogonal Frequency Division Multiplexing PDF Probability Density Function SISO Single Input Single Output SNR Signal Noise Ratio STBC Space-Time Block Code TABLE OF CONTENT LIST OF ACRONYMS i TABLE OF CONTENT i ACKNOWLEDGEMENTS iii LIST OF FIGURES iv i vi LIST OF TABLES vi ABSTRACT 1 CHAPTER 1 2 MOBILE RADIO CHANNEL CHARACTERISTICS 2 1.1 Introduction 2 1.2 AWGN 3 1.3 Path loss 5 1.4 Delay spread 6 1.5 Doppler shift 7 1.6 Fading 9 1.6.1 Flat fading versus frequency selective fading 9 1.6.2 Slow fading versus fast fading 11 1.7 Conclusion 12 CHAPTER 2 12 DIVERSITY TECHNIQUES 12 2.1 Introduction 13 2.2 Diversity 13 2.2.1 Frequency diversity 13 2.2.2 Time diversity 14 2.2.3 Space diversity 14 2.3 Diversity combining methods 15 2.3.1 Selection combining 15 2.3.2 Switched Combining 16 2.3.3 Maximal ratio combining method 17 2.3.4 Equal Gain Combining 18 2.4 Transmit diversity 19 2.4.1 Maximal ratio transmission 21 2.4.2 Delay transmit diversity 22 2.4.3 Alamouti Space-Time Coding 23 23 2.5 Conclusion 27 CHAPTER 3 28 ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING 28 3.1 Introduction 28 3.2 Block diagram of OFDM 30 3.3 Signal OFDM 32 3.4 Orthogonality condition 33 3.5 ISI in OFDM system 34 3.6 ICI in OFDM system 38 3.7 PAPR in OFDM system 41 ii 3.7.1 Clipping 43 3.7.2 Selected mapping 44 3.7.3 Partial Transmit Sequences 45 3.8 Conclusion 46 CHAPTER 4 46 COMBINED OFDM AND TRANSMIT DIVERSITY SYSTEMS 46 4.1 Introduction 47 4.2 OFDM combined with transmitter diversity 47 4.2.1 Delay approach 47 4.2.2 Permutation approach 49 4.2.3 Space-time coding approach 51 4.2.3.1 System description 51 4.2.3.2 Maximum likelihood detection 54 4.3 Conclusion 62 CONCLUSION 63 APPENDIX 63 REFERENCES 71 ACKNOWLEDGEMENTS First of all, I would sincerely like to thank my supervisor, Doctor Tran Xuan Nam for many discussion hours, valuable advice, and his continuous iii guidance. I would also like to acknowledgement Associate Professor Nguyen Quoc Binh for many useful and interesting information about wireless communication. Thanks to lecturers in Military Technical Academy providing me with full knowledge during 5 years. Most of all, I am especially grateful to my parents for their sacrifices and extreme love to help me complete this thesis. LIST OF FIGURES Figure 1.1: An example of multi-path propagation in a wireless channel. 3 Figure 1.2: AWGN noise characteristics 4 Figure 1.3: An illustration of power density on sphere 5 Figure 1.4: Delay spread 7 Figure 1.5: Doppler shift 8 iv Figure 1.6: An illustration of multi-path signal 9 Figure 1.7: Frequency selective fading and flat fading 10 Figure 1.8: An illustration of slow fading and fast fading 12 Figure 2.1: Frequency diversity 13 Figure 2.2: Time diversity 14 Figure 2.3: Space diversity 15 Figure 2.4: Selection combining 16 Figure 2.5: Switched combining 17 Figure 2.6: Maximal combining 18 Figure 2.7: Equal Gain Combining 19 Figure 2.8: Transmit diversity systems 20 Figure 2.9: Maximal ratio transmission 21 Figure 2.10: Delay transmit diversity 22 Figure 2.11: Alamouti Space-Time Coding 23 Figure 2.12: Receiver for Alamouti scheme 25 Figure 2.13: BER performance of the Alamouti systems 27 Figure 3.1: Block diagram of a typical OFDM system 30 Figure 3.2. Performance of OFDM with M-PSK modulation 32 Figure 3.3: Basic multi-carrier transmission system 32 Figure 3.4: Illustration of OFDM signals in time and frequency domain34 Figure 3.5: Comparison of single carrier modulation and OFDM 35 Figure 3.6: OFDM symbol without cyclic prefix 36 Figure 3.7: OFDM symbol with cyclic prefix 36 Figure 3.8: OFDM-QPSK with Delay spread 37 Figure 3.9: Transmitted signal inserted guard interval 38 Figure 3.10: OFDM signal with cyclic prefix 39 Figure 3.11: Frequency offset error 40 Figure 3.12: Time error 40 Figure 3.13: PAPR in OFDM system 41 Figure 3.14: IBO and OBO 42 Figure 3.15: An example illustrates the clipped signal 43 Figure 3.16: Transmitter with clipping and filtering 44 Figure 3.17: Selected mapping 44 Figure 3.18: Partial Transmit Sequences 45 Figure 4.1: Delay transmit diversity 48 Figure 4.2: Permutation approach 50 Figure 4.3: Space time coding approach 51 Figure 4.6: STBC-OFDM over selective Rayleigh fading channel 58 Figure 4.7: The original image 58 Figure 4.8: Received images over flat fading channel using STBC-OFDM 60 v Figure 4.9: Received images over flat and frequency selective fading channel 62 LIST OF TABLES Table 2.1: Alamouti parameters with BPSK constellation 26 Table 3.1: OFDM parameters for simulation 31 Table 3.2: OFDM parameters for simulation in channel with delay spread 37 Table 4.1: Simulation parameters of STBC-OFDM system 55 vi vii ABSTRACT Due to increased demand of human, multimedia services with high rate transmission and quality are required. Wired communication is an approach which brings good performance, high rate and reliability. But it only supports fixed access services. In contrast, wireless communication is very attractive due to its mobility, portability, and accessibility. Fluctuations of radio channels in wireless communication such as the fading, the shadowing, the path loss phenomenon cause difficulties into transmission. One effective approach has been proposed to over this situation, is to combine OFDM and transmit diversity techniques. This approach not only provides high rate transmission but also improves the overall system performance, significantly due to achieving both path and space diversities. For this reason, I have chosen research topic “Combined OFDM and transmit diversity for wireless communication” for my graduation thesis. This thesis consists of 4 chapters: Chapter 1: Mobile radio channel characteristics This chapter introduces problems in transmitting signal over radio channel. Main properties of radio channels such as effect of AWGN, path loss, delay spread, Doppler shift, fading phenomenon are described. Chapter 2: Diversity techniques This chapter introduces an overview about diversity techniques. The main focuses is about transmit diversity techniques. Several approaches introduced are maximal ratio transmission, delay transmit diversity, and Alamouti space- time coding. Chapter 3: Orthogonal frequency division multiplexing This chapter introduces principles of multi-carrier transmission, OFDM and advantages and disadvantages of OFDM. Chapter 4: Combined OFDM and transmit diversity systems 1 This chapter introduces a combined approach of OFDM and transmit diversity techniques to obtain both path and transmit diversities. Matlab simulation is used to evaluate efficiency of the combined STBC-OFDM approach. CHAPTER 1 MOBILE RADIO CHANNEL CHARACTERISTICS 1.1 Introduction In an ideal radio channel, received signal consists of only a single direct path so it can be recovered perfectly at the receiver. In real channel, wireless communication channel suffers from many impairments such as the thermal 2 noise, often modeled as Additive White Gaussian Noise (AWGN), path loss in power, shadowing effects due to the presence of fixed obstacles in the radio path, fading due to the effect of multi-path propagation, and Doppler effect due to movement of mobile units. Consequently, signal copies undergo different attenuations, distortions, delays and phase shifts. An example of multi-path propagation in a wireless channel is illustrated in Figure 1.1. Due to these problems, the overall system performance is degraded significantly. Figure 1.1: An example of multi-path propagation in a wireless channel 1.2 AWGN In practice, transmission is always effected by noise. The appearance of noise reduces ability in detecting exact transmitted signal, so transmission efficiency is reduced, too. Noise is resulted from many different sources, such as thermal noise, noise of electronic devices, man-made noise and other sources. Superposition of many independent processes, noise can be modeled as a Gaussian distributed random process with white spectral density. The popular noise model in communication system is Additive White Gaussian 3 [...]... as follows Firstly, OFDM is an approach which has high spectrum efficiency Since OFDM sub-carriers have overlapped spectrums, thus OFDM utilizes the spectrum better than the conventional frequency division multiplexing (FDM) with non-overlapped spectrum Secondly, OFDM is robust against frequency selective fading Theoretically, a high rate data stream has narrow symbol duration With OFDM, the high rate... errors By using OFDM, the symbol duration is increased Thus, a fade may affect a fraction of OFDM symbols In a single carrier system, a single fade or interferer can cause the entire link to fail, but in OFDM system, only a small percentage of sub-carriers will be affected Error correction coding can be used to correct the erroneous subcarriers Due to the above-mentioned attractive merits, OFDM modulation... equalization at the receiver Effect of inter-symbol interference (ISI) is thus reduced Moreover, OFDM signals are inserted with a cyclic prefix to mitigate ISI If the cyclic prefix length is selected larger than the maximum delay spread of the channel τ max , then OFDM can perfectly mitigate ISI Thirdly, OFDM is robust against deep fades and narrow-band interference When the data are transmitted serially,... interference (ISI) OFDM is a special technology to mitigate ISI OFDM is a form of multi-carrier modulation (MCM) The principle of multi-carrier modulation is to transmit data by dividing input stream into several streams Each stream has a much lower rate These lower rate streams are then used to modulate a separate single carrier These carriers are referred to as subcarriers (SC) 28 The reasons to use OFDM are . Introduction 28 3.2 Block diagram of OFDM 30 3.3 Signal OFDM 32 3.4 Orthogonality condition 33 3.5 ISI in OFDM system 34 3.6 ICI in OFDM system 38 3.7 PAPR in OFDM system 41 ii 3.7.1 Clipping 43 . Comparison of single carrier modulation and OFDM 35 Figure 3.6: OFDM symbol without cyclic prefix 36 Figure 3.7: OFDM symbol with cyclic prefix 36 Figure 3.8: OFDM- QPSK with Delay spread 37 Figure. multi-carrier transmission, OFDM and advantages and disadvantages of OFDM. Chapter 4: Combined OFDM and transmit diversity systems 1 This chapter introduces a combined approach of OFDM and transmit diversity