OFDM based RF and optical wireless systems

Open Access
Author:
Ranjha, Bilal Aslam
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 03, 2014
Committee Members:
  • Mohsen Kavehrad, Dissertation Advisor
  • Mohsen Kavehrad, Committee Chair
  • William Kenneth Jenkins, Committee Member
  • Julio Urbina, Committee Member
  • Jian Xu, Committee Member
Keywords:
  • OFDM
  • ACO-OFDM
  • DC-Bias
  • PAPR
  • PAM-DMT
  • Timing synchronization
Abstract:
Orthogonal frequency division multiplexing (OFDM) is currently being used predominantly in radio frequency (RF) mobile broadband communication systems because of its ability to combat inter-symbol interference (ISI) and robustness against frequency selective fading caused by multipath wireless channel. Wireless mobile standards like 3G and 4G long term evolution (LTE) use orthogonal frequency division multiple access (OFDMA) as a multiplexing/modulation scheme. Despite its many advantages like single tap frequency domain equalization and fast discrete time implementation, OFDM suffers from certain disadvantages like high peak-to-average power ratio (PAPR) and high sensitivity to carrier frequency offset (CFO). Although OFDM has solved problems like multipath fading but it cannot solve the emerging problems like scarcity of RF spectrum for mobile wireless broadband applications. Optical wireless (OW) communication has recently gained a lot of attention as a candidate to complement RF communication. It offers advantages like virtually infinite bandwidth, data security and use of low cost transmitters and receivers like solid state light emitting diodes (LEDs) and optical detectors. OFDM is also being considered as a candidate for visible light communication (VLC) as it offers robustness against multipath caused by diffuse indoor OW channel. One way to realize VLC is intensity modulation direct detection (IM/DD). Although the major difference between RF and OW based OFDM lies in the front end of transmitter and receiver, but due to the unipolar nature of optical intensity in IM/DD system, methods of generating baseband OFDM signal, techniques to reduce PAPR and timing synchronization schemes for RF cannot be directly applied to optical OFDM systems and therefore must be revisited. Therefore, in this thesis, we will first look into the interference caused by CFO in RF based OFDMA system and will analyze the characteristics of this interference for two mapping subcarrier strategies. We will explicitly calculate SINR expression for OFDMA based systems and analyze two types of symbol mapping strategies and characterize interferences due to CFO for each scheme. We will also develop some techniques to reduce high PAPR in OFDM based OW systems since the non-linear characteristics of LED transmitters can severely affect system performance. We will look into various precoding based PAPR reduction techniques. We will then analyze performance of various OFDM based OW schemes in multipath diffuse indoor wireless channel. We will compare performance of conventional schemes with a precoded version. We will then describe in detail our newly proposed power and spectrally efficient hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (HACO-OFDM) system and compare its performance with previously proposed schemes. Finally, we will present details of our newly proposed timing synchronization scheme for power efficient asymmetrically clipped (AC) OW OFDM systems. Detailed performance analysis will be presented and a comparison will be developed. Simulation results show that our proposed scheme outperforms all other timing synchronization techniques and exhibits perfect accuracy even at very low signal-to-noise ratio (SNR). Besides performance, our scheme works perfectly for multiple AC OW which proves its high versatility