Vishakan Ponnampalam

On cell parameter ID assignment in UTRA-TDD

The use of short spreading sequences in UTRA-TDD may lead to large variability in uplink performance due to the level of cross correlation between spreading codes. The effective spreading code for a particular user is derived from a cell-specific scrambling code and the channelization code allocated to that user. We show that the interference between intercell users is dependent on their cell-specific scrambling code. This leads to a proposed cell-id allocation scheme for UTRA-TDD that reduces intercell interference. We also outline how the uplink air interface for future TD-CDMA systems may be designed to reduce this effect.

Diversity reception in short-code CDMA

Short spreading sequences facilitate efficient multiuser detectors and have hence been incorporated into practical code division multiple access systems such as 3rd Generation Partnership Project UTRA-TDD. However, as the number of users approaches the spreading code length, the asymptotic performance and the numerical stability of linear multiuser detection algorithms degrade. Given some simplifications such as synchronous users, it is shown that receive diversity effectively mitigates these degradations and allows the system to be operated with a high load. A lower bound on the mean asymptotic multiuser efficiency is presented and the numerical stability of linear multiuser detection algorithms for systems with receive diversity is analysed. It is further shown that the performance variability is significantly reduced by receive diversity.

MIMO processing for HSDPA in UTRA TDD

Multiple input multiple output (MIMO) processing is expected to provide significant enhancements to the capacity of UMTS systems by improving the spectral efficiency. A number of candidate MIMO techniques have been proposed and are currently being studied by 3GPP. While MIMO techniques for UTRA FDD have been thoroughly studied, little effort has been spent on investigating MIMO for UTRA TDD. We analyse the suitability of various MIMO techniques for UTRA TDD and present initial simulation results.

Multiuser detection of short-code CDMA with antenna diversity

Many current and proposed wireless protocols that employ a code division multiple access (CDMA) element, incorporate short spreading sequences in order to facilitate efficient multiuser detection architectures. One such standard is the time division duplex (TDD) mode of the 3rd Generation Partnership Project (3GPP) universal terrestrial radio access (UTRA) system. One potential drawback with using short spreading codes is that as the number of users approaches the spreading code length the near-far resistance and the numerical stability of linear multiuser detection algorithms degrades. One method to mitigate these effects, and hence operate the system with a higher load, is to employ diversity antennas at the receiver. In this paper we study the mean asymptotic multiuser efficiency and analyse the numerical stability of linear multiuser detection algorithms employing receiver diversity.