Title page for ETD etd-12122002-180628


Type of Document Master's Thesis
Author Jain, Payal
Author's Email Address pajain@vt.edu
URN etd-12122002-180628
Title On the Impact of Channel and Channel Quality Estimation on Adaptive Modulation
Degree Master of Science
Department Electrical and Computer Engineering
Advisory Committee
Advisor Name Title
Buehrer, Richard Michael Committee Chair
Jacobs, Ira Committee Member
Woerner, Brain D. Committee Member
Keywords
  • Pilot Symbol Assisted Modulation
  • Adaptive Modulation
  • Quadrature Amplitude Modulation
  • Rayleigh Fading
  • Channel Estimation
  • Short term SNR estimation
  • Channel Quality Estimation
  • Long term SNR estimation
Date of Defense 2002-12-02
Availability unrestricted
Abstract
The rapid growth in wireless communications has given rise to an increasing demand for channel capacity using limited bandwidth. Wireless channels vary over time due to fading and changing interference conditions. Typical wireless systems are designed by choosing a modulation scheme to meet worst case conditions and thus rely on power control to adapt to changing channel conditions. Adaptive modulation, however, exploits these channel variations to improve the spectral efficiency of wireless communications by intelligently changing the modulation scheme based on channel conditions. Necessarily, among the modulation schemes used are spectrally efficient modulation schemes such as quadrature amplitude modulation (QAM) techniques.

QAM yields the high spectral efficiency due to its use of amplitude as well as phase modulation and therefore is an effective technique for achieving high channel capacity. The main drawbacks of QAM modulation are its reduced energy efficiency (as compared to standard QPSK) and its sensitivity to channel amplitude variations. Adaptive modulation attempts to address the first drawback by using more energy efficient schemes in low SNR conditions are reserving the use of QAM for high SNR conditions. The second drawback leads to a requirement of high quality channel estimation. Many researchers have studied pilot symbol assisted modulation for compensating the effects of fading at the receiver. A main contribution of this thesis is the investigation of different channel estimation techniques (along with the effect of pilot symbol spacing and Doppler spread) on the performance of adaptive modulation.

Another important parameter affecting adaptive modulation is the signal-to-noise ratio. In order to adapt modulation efficiently, it is essential to have accurate knowledge of the channel signal-to-noise ratio. The performance of adaptive modulation depends directly on how well the channel SNR is estimated. The more accurate the estimation of the channel SNR is, the better the choice of modulation scheme becomes, and the better the ability to exploit the variations in the wireless channel is. The second main contribution

of this thesis is the investigation of the impact of SNR estimation techniques on the performance and spectral efficiency of adaptive modulation. Further, we investigate the impact of various channel conditions on SNR estimation and the resulting impact on the performance of adaptive modulation. Finally, we investigate long term SNR estimation, its use in adaptive modulation and present a comparison between the two approaches

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