臺灣博碩士論文加值系統

The OFDM system has been a widely adopted technique for both wired and wireless transmission. It is well-known that OFDM systems suffer from the noise enhancement problem due to channel spectral nulls and the PAPR problem. Recently, another new system called SC-CP (single carrier with cyclic prefix) system is receiving growing attention as another wireless transmission technique. This is because the SC-CP system can mitigate the noise enhancement problem and it does not suffer from PAPR problem.

Both of OFDM and SC-CP systems are ISI- and IBI-free if the following two conditions hold. First, the channel is quasi-static, which means the channel does not change during a data block transmission. Second, the length of cyclic prefix (CP) is larger than the channel order. When any of these two conditions is violated, the BER performance will degrade significantly.

In the first part of this thesis, we will consider a special doubly selective channel model. This type of channel is time-varying and hence no longer satisfies the quasi-static condition. We will analyze ISI effect and BER performance of the OFDM and SC-CP systems for conventional ZF FEQ and conventional MMSE FEQ respectively. Then, we derive the exact MMSE FEQ for given channel model and analyze the BER performance.

In the second part of this thesis, we will study the effect of the OFDM and SC-CP system when the CP length is smaller than the channel order. We will derive new MMSE receivers for both systems to improve the BER performance.

The simulation results will validate our performance analysis and we can find that new MMSE FEQ will outperform conventional ZF FEQ and conventional MMSE FEQ.

List of Tables viii
List of Figures x
Abstract xiii
Acknowledgements xv

1 Introduction 1

2 OFDM, SC-CP and Precoded OFDM Systems 5
2.1 Introduction to OFDM Systems 6
2.1.1 Basic System Structure 6
2.1.2 Performance of OFDM Systems 10
2.2 Introduction to SC-CP Systems 11
2.2.1 Basic System Structure 11
2.2.2 Performance of SC-CP Systems with ZF FEQ 12
2.2.3 SC-CP Systems with MMSE Receiver 13
2.3 Precoded OFDM Systems 15
2.3.1 Basic System Structure 15
2.3.2 Performance of Precoded OFDM Systems with ZF FEQ 17
2.3.3 Precoded OFDM Systems with MMSE Receiver 18
2.4 BER Comparison 20
2.4.1 Using ZF FEQ 20
2.4.2 Using MMSE FEQ 23

3 OFDM, SC-CP and Precoded OFDM Systems in Time-Varying Transmission Environment 27
3.1 Channel Model 28
3.2 OFDM Systems 34
3.2.1 Conventional ZF FEQ 34
3.2.2 Conventional MMSE FEQ 38
3.2.3 MMSE FEQ 40
3.3 SC-CP Systems 45
3.3.1 Conventional ZF FEQ 45
3.3.2 Conventional MMSE FEQ 48
3.3.3 MMSE FEQ 51
3.4 Precoded OFDM Systems 56
3.4.1 Conventional ZF FEQ 56
3.4.2 MMSE FEQ 58
3.5 BER Comparison 61
3.5.1 Using Conventional ZF FEQ 61
3.5.2 Using MMSE FEQ 64

4 OFDM and SC-CP Systems with Insuffcient CP 71
4.1 Channel Model 72
4.2 OFDM Systems with Insuffcient CP 74
4.2.1 Unbiased FEQ 76
4.2.2 MMSE FEQ 77
4.3 SC-CP Systems with Insuffcient CP 78
4.3.1 Unbiased FEQ 78
4.3.2 MMSE FEQ 80
4.4 Simulation Result 81

5 Conclusion 85

A Channel Estimation Error for Given Time-Varying Channel Model 87

B Pilot-Tone Estimation for Transmission with Insuffcient CP 91

Bibliography 94

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