Stefan Bahrenburg

A hardware demonstrator for TD-CDMA

TD-CDMA is an air interface concept for third-generation mobile radio systems. It utilizes a combination of the three elementary multiple-access schemes: frequency-division multiple access (FDMA), time-division multiple access (TDMA) and code-division multiple access (CDMA). Multiple-access interference (MAI) and intersymbol interference (ISI) are combated by joint detection of all simultaneous signals of the same cell, achieving a high spectrum efficiency. The authors have been involved in the implementation of a first experimental hardware setup of a TD-CDMA mobile radio system. Important objectives of this project are the demonstration of the economic feasibility of CDMA receivers utilizing joint detection and the performance verification of TD-CDMA by measurement campaigns. The authors present a new way of modeling the TD-CDMA receiver in the form of a data-flow model. This model serves as a starting point for the systematic design of an efficient software architecture for TD-CDMA based on a multiprocessor system. The complete digital signal processing (DSP) of a mobile terminal or of a base station can be implemented on a single Texas Instruments digital signal processor TMS320C80. Measurement results obtained by first field trials are presented. The system parameters of the TD-CDMA demonstrator described are similar but not equal to those chosen for the time-division duplex (TDD) mode of the UMTS terrestrial air interface according to the ETSI decision taken in January 1998.

Outdoor measurements with a TD (time division)-CDMA hardware demonstrator for UMTS

In the UMTS air interface standardisation within ETSI, TD (time division)-CDMA has been chosen for the unpaired band operation. TD-CDMA makes use of all three basic multiple access schemes FDMA, TDMA and CDMA. It combines the advantages of these three schemes and seamlessly fits into the world of GSM. In contrast to DS (direct sequence)-CDMA without joint detection (JD), intracell interference is eliminated in the reception by using the a priori knowledge of the CDMA codes of the users of one and the same cell. This elimination is performed by a process termed joint detection and increases spectrum efficiency and capacity. In order to practically verify and to demonstrate the advantages of TD-CDMA, a TD-CDMA hardware demonstrator was implemented, which is suited for a real mobile radio environment. In the paper, first the basic features of the hardware demonstrator are described. Then, measurement results obtained with the hardware demonstrator are presented. Particular emphasis is spent on the critical issues of power control and timing advance control.

JD-CDMA mobile station envelope characteristics and spectra results from a JD-CDMA testbed

The requirements on linearity and the power spectrum of TDMA/CDMA systems are an issue coming up in the standardisation process of the Universal Mobile Telecommunications System (UMTS) standard. The feasibility of transmitting QPSK and 16-QAM signals in multi-code operation with low cost terminals is questioned and the spectral artefacts due to the short code design are said to be unfavourable. A JD-CDMA hardware demonstrator was build to investigate the properties of joint-detection-CDMA in practice. In this paper measurement results and experiences with the demonstrator are presented. It is shown that even with low cost terminals the linearity and spectrum requirements can be fulfilled.