Interference analysis of and dynamic channel assignment algorithms in TD–CDMA/TDD systems

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 assignment techniques. 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.