ERCIM News No.19 - October 1995 - VTT

All Digital Receiver Architectures for a Spread-Spectrum System

by Aarne Mämmelä

In all digital receivers, the A/D conversion is done at the earliest point of the processing chain, either at the intermediate frequency (IF) or at base band, and the feedback loops from the digital part to the analog part are avoided as much as possible.The detection and synchronization algorithms are optimized for digital implementation by using the estimation theory, e.g. maximum likelihood (ML) principles.

The architecture design is based on the knowledge on the receiver algorithms, as well as the DSP code generation and the ASIC design (including programmable logic devices). In the architecture design, the real-time requirements of the receiver will be taken into account. The performance criteria include the speed of execution (time), the amount of circuitry needed (space), the amount of power dissipation, the quality of the processing results, and the cost of the system. Especially, finite-precision effects and different "bottle necks"(caused by memory, feedback loops, etc.) will be studied. Computer-aided design tools will be used in the research.

A prototype DS system developed

As a practical application, a coherent all digital direct sequence(DS) spread-spectrum system was developed and simulated. Similar principles have been applied previously in conventional narrowband systems. A prototype DS system is being implemented with programmable DSP by using C and assembly languages. The coded and interleaved data is transmitted in packets with a preamble for synchronization. There are different packet lengths and bit rates available. The necessary synchronization subsystems, such as the spreading code acquisition and tracking (including symbol synchronization), as well as the carrier and frame synchronization, are included in the receiver.

For mobile applications the synchronization circuits must be modified and a RAKE diversity receiver should be used to exploit multipath propagation. The necessary modifications are small, but the performance calculations will be different due to fading. The optimum and suboptimum structures of the receiver are being studied for fast fading channels, where the Doppler spread is not much smaller than the symbol rate. For example, the linear prediction and smoothing as well as Wiener and Kalman filtering have been studied to improve the performance of RAKE receivers.

Other areas of interest include different modulation techniques, especially combined modulation and coding. An orthogonal convolutional code with a fast Hadamard transformer (FHT) and Viterbi algorithm was simulated and its performance analyzed. This code can be used in a CDMA system as suggested by Viterbi. Besides mobile telecommunications, the DSP research group at VTT Electronics is also studying power line telecommunications.

Please contact:
Aarne Mämmelä - VTT
Tel: +358-81 551 2482