| dc.description.abstract | The use of OVSF codes in the WCDMA system can provide variable data rates to flexibly
support applications with different bandwidth requirements. However, there are some
constraints when using the OVSF codes, such as code blocking and exponentially quantized
data rates. Code blocking, which is defined as the condition that a new call is rejected even
though the system has enough bandwidth, induces external fragmentation of an OVSF code
tree. Exponentially quantized data rates, resulting from the exponentially decreased spreading
factors, induces internal fragmentations for requests. Both external and internal fragmentations
waste the precious wireless bandwidth. In this dissertation, the effects of these
constraints are investigated and several strategies are provided to eliminate such limitations.
Two important issues on such an environment are the code assignment problem and code
reassignment problem. The former may have significant impact on code utilization and thus
code blocking probability, while the latter may affect the code reassignment cost if dynamic
code assignment is to be conducted. The general objective is to make the OVSF code tree
as compact as possible so as to support more new calls by incurring less blocking probability
and less reassignment costs. Earlier studies about these two problems either do not
consider the structure of the OVSF code tree or cannot utilize the OVSF codes efficiently.
Two code assignment and reassignment strategies, leftmost and crowded-first, are proposed
in this dissertation to solve these problems. Simulation results show that the crowded-first
scheme increases the OVSF code tree utilization significantly.
To reduce internal fragmentation, it is suggested to use multiple codes to support a call.We show how using multiple codes can reduce internal fragmentation of a OVSF code tree.
The tradeoff between bandwidth utilization and hardware complexity of a multi-code system
is analyzed. The result shows that using 2 or 3 codes will be quite cost-effective. Several
multi-code assignment and reassignement strategies, namely random, leftmost, crowded-
first-space, and crowded-first-code, are also proposed based on such environment.
In order to further increase the bandwidth utilization, strategies that utilize time-shared
OVSF codes are proposed to enhance statistical multiplexing. In particular, we propose to
allow a user to simultaneously use multiple OVSF codes in a time-sharing manner, which
we call a multi-code, shared model. Using multiple codes allows us to compensate those
users suffering from communication interferences or even errors. The proposed schemes
can tolerate a multi-state link condition (compared to the typically assumed two-state, or
good-or-bad, link condition) by adjusting the spreading factors of OVSF codes. Through
theoretical analyses and computer simulations, the proposed strategies are verified to be
efficient and cost-effective. It is expected that the capacity of WCDMA systems can be
effectively utilized when the strategies proposed in this dissertation are applied. | en_US |