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Title of Thesis

Hybrid QAM-FSK(HQFM) OFDM Transceiver with Low PAPR

Author (s)
Asma Latif
Institute/University/Department Details
Ghulam Ishaq Khan Institute of Engineering Sciences and Technology
Session
2009
Subject
Electronic Engineering
Number of Pages
177
Keywords (Extracted from title, table of contents and abstract of thesis)
 Partial Transmit Sequence , inter modulation distortion, High Peak to Average Power Ratio

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is an attractive multicarrier technique for mitigating the effects of multipath delay spread of radio channel, and hence accepted for several wireless standards as well as number of mobile multimedia applications. Alongside its advantages such as robustness against multipath fading, spectral efficiency and simple receiver design, OFDM has two major limitations. One of these is its sensitivity to carrier frequency offsets (CFO) caused by frequency differences between the local oscillators in the transmitter and the receiver and the other is high peak to average power ratio (PAPR). This high PAPR is due to the summation of sinc-pulses and non-constant envelope. Therefore, RF power amplifiers (PA) have to be operated in a very large linear region. Otherwise, the signal peaks get distorted, leading to intermodulation distortion (IMD) among the subcarriers and out-of-band radiation. A simple way to avoid is to use PA of large dynamic range but this makes the transmitter  costly. Thus, it is highly desirable to reduce the PAPR. In order to reduce the PAPR, several techniques have been proposed such as clipping, coding, peak windowing, Tone Reservation (TR), Tone Injection (TI), Selected Mapping (SLM) and Partial Transmit Sequence (PTS). After studying these schemes, it was found that most of these methods are unable to achieve simultaneously a large reduction in PAPR with low complexity, low coding overhead and without performance degradation and transmitter/ receiver symbol handshake. In this study, an OFDM transceiver is proposed which makes use of hybrid modulation scheme instead of conventional modulator like QAM or PSK. In addition to improved BER performance both in AWGN and frequency selective fading channel, it exhibits low PAPR. The modified OFDM transceiver makes use of multilevel QAM constellations, where the level of QAM is decided by specific number of bits chosen arbitrarily from a group of bits to be encoded in the QAM symbol. The simulated results show that PAPR is considerably reduced, though at the cost of a slight increase in detection complexity. Like PTS or SLM, it works with arbitrary number of subcarriers but needs no side information to be transmitted. It is also shown that PAPR reduction capability of the proposed system is comparable to PTS. However, to further reduce the PAPR, one has to alter this hybrid MQAM/LFSK (HQFM) signal sets like in PTS, but there is no need of transmitting any additional side information. At the receiver, these deformations can be removed in one or two iterations, thus, original data retrieved but with a little increase in the receiver complexity.

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1639 KB

S. No. Chapter Title of the Chapters Page Size (KB)
  0    
1 1 Introduction 1
  1.1 Orthogonal Frequency Division Multiplexing 3
  1.2 Pros and Cons of OFDM 4
  1.3 Peak-to-Average Power Ratio 5
370 KB
  1.4 Motivation 6
  1.5 Dissertation Outline and Contributions to Field 7
 
2 2 Fundamentals of OFDM 12
  2.1 Historical Background of OFDM 13

420 KB

 

  2.2 Qualitative Description of OFDM 15
  2.3 OFDM Generation 16
  2.4 Mathematical Description of OFDM 18
  2.5 Research Challenges 22
  2.6 Different PAPR Reduction Schemes 31
  2.7

Hybrid FSK-QAM Modulation (HQFM): A Novel Technique with Low PAPR

35
3 3 Hybrid MQAM-LFSK (HQFM) Signaling 44
  3.1 Brief Review of MPSK, MQAM and MFSK 45
  3.2 Hybrid Modulation: Literature Review 49


550 KB

  3.3 HQFM Signals 50
  3.4 Power Spectral Density (PSD) 53
  3.5 Bandwidth Efficiency 61
  3.6 Spectral Properties of HQFM-OFDM 68
4 4 PAPR issues in HQFM-OFDM 72
306 KB
  4.1 Hybrid MQAM-LFSK (HQFM) OFDM 73
  4.2 PAPR as a function of N (Number of Subcarriers) 75
  4.3 PAPR as a function of L (Number of FSK Tones) 75
  4.4 Modified HQFM-OFDM 81
  4.5 System’s Complexity 84
       
5 5 Performance in AWGN 88

351 KB

  5.1 HQFM Demodulation 89
  5.2 BER and SER Relationship 94
  5.3 BER Performance of HQFM-OFDM in AWGN 109
         


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6 6 Performance in Fading multipath channels 112
  6.1 Preliminary Discussion: Characterization of Fading Multipath Channels 112
  6.2 Performance of HQFM in Rayleigh Fading Channel 117
  6.3 Frequency Selective Mobile Channels 122
  6.4 HQFM-OFDM over Rayleigh Slow Fading Channel 124
  6.5 Performance of HQFM-OFDM, HQFM-I and HQFM-II 132

 

 

 

 

 

235 KB

 

  6.6 Performance in Frequency Selective Channels 134
7 7 Conclusion 142
  7.1 Future Extension 145
       
    Appendix A Power Spectral Density of MFSK 147
    Phase Acquisition Algorithm   154