Modulation techniques for high-speed wireless indoor systems using narrowbeam antennas

We consider high-speed wireless indoor links in which narrowbeam antennas are used to limit multipath delay spread. For this kind of link, the performances of simple 2- and 4-level modulation schemes are analyzed. The amplitudes and phases of all multipath rays are assumed to be static (no time variation), as expected for fixed links in an indoor environment. We consider coherent PSK with data-directed and pilot tone carrier recovery, differential PSK, and coherent and noncoherent forms of 2-level ASK. Our objective is to find techniques for which factors other than multipath limit the attainable bitrate. Thus for each scheme, analysis is used to identify regions of the multipath parameter set over which data eye closure occurs. Results are given for the 2- and 3-ray multipath channels which can arise in an indoor environment with narrowbeam antennas. Specifically, we assume two parallel walls approximately perpendicular to the direct ray and a narrowbeam antenna at one or both ends of the link. The 2-level schemes are found to be quite robust with respect to 2-ray multipath, and can mostly tolerate a second ray amplitude equal to that of the direct ray. The 4-level schemes are sensitive to 2-ray multipath, showing eye closure at many phases with a second ray power about 7.5 dB below the first (direct) ray. A similar sensitivity is found for 2-level schemes with 3-ray multipath. Finally, 4-level schemes with 3-ray multipath are significantly more sensitive, showing eye closure with a second ray power about 11 db below the direct ray. Thus, considerations of simplicity and robustness favor 2-level PSK schemes (coherent with pilot tone, or differential) for narrowbeam high speed transmission indoors.<
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