Title page for ETD etd-332122139711101


Type of Document Dissertation
Author Davidson, Boris
Author's Email Address bdavidso@vt.edu
URN etd-332122139711101
Title Fundamentals of the Simplex communication Channel With retransmissions
Degree PhD
Department Electrical and Computer Engineering
Advisory Committee
Advisor Name Title
Davis, Nathaniel J. IV
Kohler, Werner E.
Midkiff, Scott F.
Stutzman, Warren L.
Bostian, Charles W. Committee Chair
Keywords
  • direct-sequence spread spectrum
  • simplex channel
  • message collisions
  • multiple access
Date of Defense 1997-04-14
Availability unrestricted
Abstract
The need for multiple access strategies arises whenever a number of users have to share a communication resource, since it is usually either cost prohibitive or impractical to dedicate a communication channel to a particular user. A need for such algorithms arises in many instances, particularly in applications utilizing wireless systems where all users access a common channel or medium. Such random access techniques as ALOHA and slotted ALOHA have been successfully implemented in a number of wireless applications. One of the major drawbacks of these algorithms is the necessity of a return path from the central station to each system user, which makes their use both inefficient and expensive for applications where one-way communication would suffice. For such applications, a need remained for a random access algorithm which can maximize the probability of successful message transmission in a one-way communication environment. A random access technique that addresses the above-mentioned need is developed. With this technique, each user sends an original message of predetermined length to a central receiver. The user then retransmits the message a specified number of times in a predetermined interval reserved for the retransmission process. The time interval between each successive retransmission of a given message is random. Assuming total annihilation of all colliding messages, the expression for the probability of successful transmission of a given message in terms of the major channel parameters is theoretically formulated. This technique offers a significant improvement, compared to a single transmission, in ensuring that a message is successfully received. The actual message collision dynamics in this system are experimentally studied using two different types of direct-sequence spread spectrum receivers, one employing a sliding correlator and the other using a matched filter. The spreading code in such systems offers extra protection for messages against possible interferers. The results indicate that it is often possible to properly receive a given message in the presence of co-channel interferers, thus significantly improving the overall system performance. These results are subsequently incorporated with the propagation data for several different types of microcells to arrive at a more precise theory of the link.

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