Marcus Benthin

Influence of channel estimation on the performance of a coherent DS-CDMA system

Direct-sequence code division multiple access (DS-CDMA) has become one of the favourite candidates for future mobile radio communication systems. The uplink of a coherent DS-CDMA system is investigated. The channel estimation is carried out via a control channel, which is transmitted simultaneously to the data channel. This concept is very closely related to the CODIT-CDMA concept. In the channel estimation, an averaging process is involved, which for long averaging periods, suppresses the noise influence efficiently. On the other hand, long averaging intervals lead to systematic errors in the estimated channel coefficients due to the time-variant behavior of the channel. Taking into account these effects, a closed-form analysis of the data-channel symbol-error rate (SER) is presented. This analysis allows the prediction of the systems performance and helps to optimize the parameter settings for channel estimation.

Viterbi decoding of convolutional codes with reliability information for a noncoherent RAKE-receiver in a CDMA-environment

The generation and the use of reliability information for the decoding process of convolutionally encoded data in the context of a CDMA-environment is addressed. Special emphasis lies on the noncoherent detection of M-ary orthogonal symbols with a RAKE-receiver. Although symbol by symbol decisions are carried out, it is possible to characterize the reliability of the individual bits with specific soft information. This can be exploited successfully by a Viterbi decoder. A significant improvement of the bit error performance is achieved, even on low signal to noise conditions. The paper starts with the derivation of the maximum likelihood (ML) criteria for the detection of binary data on the AWGN channel and the binary symmetric channel (BSC).

On the optimisation of a noncoherent CDMA system based on M‐ary orthogonal modulation

This paper concerns the optimisation of a DS-CDMA based mobile communication system with respect to the transmission quality in the case of data and video services respectively. For combatting transmission errors, forward error protection based on convolutional codes and interleaving in connection with soft decision maximum-likelihood (ML) decoding through the Viterbi algorithm is applied. In this context, a technique to generate post-decision reliability information in the case of a noncoherent DS-CDMA receiver for a given ML metric is presented. The proposed error control strategy involves the application of equal error protection for raw data transmission with the average post-decoding BER as an optimisation criterion. In the case of a video transmission, a-priori knowledge regarding the error sensitivity of individual parameter sets suggests an unequal error protection strategy based on punctured convolutional codes, where the segmental SNR at the source codec output represents the optimisation criterion. Due to the high complexity of the system under investigation, corresponding results are obtained through simulation of the point-to-point transmission link. A significant gain regarding the achievable transmission quality with respect to conventional approaches can be achieved in connection with a compound design of source coding, channel coding and spectral spreading.

Digital video and speech transmission in existing land mobile radio networks

The development of very low bit rate image compression algorithms renders the transmission of moving and still video with associated speech in a variety of existing and future narrowband telecommunication networks. Existing analog, narrowband (mobile) networks represent a potential field for the introduction of digital bearer services under utilization of the underlying system resources in a compatible manner. A broader variety of services in coexistence with existing analog services may be provided without new network planning. This paper presents corresponding techniques for video and speech transmission in the existing Nonpublic Land Mobile Network under application of commercially available radio telephones and an additional, external digital signal processing unit. Special attention is given to the choice of a digital modulation method under consideration of network constraints such as adjacent channel interference and linear distortions due to nonideal analog IF filters. In this context a modified maximum likelihood detector based on an equivalent linear model of the baseband transmission link is presented. For robust low bit rate video transmission a combined source-channel coding scheme with unequal error protection matched to the source and channel characteristics in case of the target mobile narrowband network is proposed.