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
Dr. R. M. Buehrer	Committee Chair
Dr. B. D. Woerner	Committee Member
Dr. I. Jacobs	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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