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|Title: ||Interference analysis of and dynamic channel assignment algorithms in TD–CDMA/TDD systems|
|Authors: ||Haas, Harald|
|Supervisor(s): ||McLaughlin, Stephen|
|Issue Date: ||Jul-2001|
|Publisher: ||University of Edinburgh; College of Science and Engineering; School of Engineering and Electronics|
|Abstract: ||The radio frequency spectrum for commercial wireless communications has become an expensive
commodity. Consequently, radio access techniques are required which enable the efficient
exploitation of these resources. This, however, is a difficult task due to an increasing diversity
of wireless services. Hence, in order to achieve acceptable spectrum efficiency a flexible air–
interface is required.
It has been demonstrated that code division multiple access (CDMA) provides flexibility by
enabling efficient multi user access in a cellular environment. In addition, time division duplex
(TDD) as compared to frequency division duplex (FDD) represents an appropriate method to
cater for the asymmetric use of a duplex channel. However, the TDD technique is subject to
additional interference mechanisms in particular if neighbouring cells require different rates of
asymmetry. If TDD is combined with an interference limited multiple access technique such as
CDMA, the additional interference mechanism represents an important issue. This issue poses
the question of whether a CDMA/TDD air–interface can be used in a cellular environment.
The problems are eased if a hybrid TDMA (time division multiple access) / CDMA interface
(TD–CDMA) is used. The reason for this is that the TDMA component adds another degree
of freedom which can be utilised to avoid interference. This, however, requires special channel
This thesis analyses cellular CDMA/TDD systems used in indoor environments. A key parameter
investigated is the interference in such systems. In the interference analysis a special
focus is placed on adjacent channel interference since the jamming entity and victim entity can
be in close proximity. The interference analysis shows that co–location of BS’s using adjacent
channels is not feasible for an adjacent channel protection factor that is less than 40 dB
and frame synchronisation errors of more than 10%. Furthermore, it is demonstrated that ideal
frame synchronisation does not necessarily yield the highest capacity. As a consequence, a new
technique termed ’TS–opposing’ is introduced. This method is intended to enable a cellular
TD–CDMA/TDD system to apply cell independent channel asymmetry. For this purpose, a
centralised DCA is developed. It is found that this algorithm indeed enables neighbouring cells
to adopt different rates of asymmetry without a significant capacity loss.
Moreover, a decentralised DCA algorithm based on the TS–opposing principle is developed.
In this context, a novel TS assignment concept is proposed which reduces the complexity associated
with the TS–opposing technique. In addition, the TS assignment plan allows for full
spatial coverage. It is shown that the capacity of a TD–CDMA/TDD interface can be greater
than the capacity of an equivalent FDD interface. The performance of the decentralised DCA
algorithm is limited by the interference in the uplink. Therefore, additional methods which assist
in reducing the interference in the uplink are envisaged to further improve the performance
of the decentralised DCA algorithm.
The exploitation of the TS–opposing technique in two different ways demonstrates that this
method can be used to improve the performance of a TD–CDMA/TDD system significantly.|
|Appears in Collections:||Engineering thesis and dissertation collection|
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