Design and performance of CDMA codes for multiuser communications

Walsh and Gold sequences are fixed power codes and are widely used in multiuser CDMA communications. Their popularity is due to the ease of implementation. Availability of these code sets is limited because of their generating kernels. Emerging radio applications like sensor networks or multiple service types in mobile and peer-to-peer communications networks might benefit from flexibilities in code lengths and possible allocation methodologies provided by large set of code libraries. Walsh codes are linear phase and zero mean with unique number of zero crossings for each sequence within the set. DC sequence is part of the Walsh code set. Although these features are quite beneficial for source coding applications, they are not essential for spread spectrum communications. By relaxing these unnecessary constraints, new sets of orthogonal binary user codes (Walsh-like) for different lengths are obtained with comparable BER performance to standard code sets in all channel conditions. Although fixed power codes are easier to implement, mathematically speaking, varying power codes offer lower inter- and intra-code correlations. With recent advances in RF power amplifier design, it might be possible to implement multiple level orthogonal spread spectrum codes for an efficient direct sequence CDMA system. A number of multiple level integer codes have been generated by brute force search method for different lengths to highlight possible BER performance improvement over binary codes. An analytical design method has been developed for multiple level (variable power) spread spectrum codes using Karhunen-Loeve Transform (KLT) technique. Eigen decomposition technique is used to generate spread spectrum basis functions that are jointly spread in time and frequency domains for a given covariance matrix or power spectral density function. Since this is a closed form solution for orthogonal code set design, many options are possible for different code lengths. Design examples and performance simulations showed that spread spectrum KLT codes outperform or closely match with the standard codes employed in present CDMA systems. Hybrid (Kronecker) codes are generated by taking Kronecker product of two spreading code families in a two-stage orthogonal transmultiplexer structure and are judiciously allocated to users such that their inter-code correlations are minimized. It is shown that, BER performance of hybrid codes with a code selection and allocation algorithm is better than the performance of standard Walsh or Gold code sets for asynchronous CDMA communications. A redundant spreading code technique is proposed utilizing multiple stage orthogonal transmultiplexer structure where each user has its own pre-multiplexer. Each data bit is redundantly spread in the pre-multiplexer stage of a user with odd number of redundancy, and at the receiver, majority logic decision is employed on the detected redundant bits to obtain overall performance improvement. Simulation results showed that redundant spreading method improves BER performance significantly at low SNR channel conditions

Digital data transmission over mobile radio channels

The aim of this work is to study data transmission over a microwave digital mobile radio channel at 900 MHz, where the channel is subjected to multipath fading. Besides the fading, the other impairments assumed here are additive noise, co-channel interference and adjacent channel interference. Two modulation techniques are investigated in this work, namely Quadrature-Amplitude-Modulation (QAM) and Quadrature-Phase-Shift- Keying (QPSK). The channel is characterised digitally, assuming multipath Rayleigh fading in the presence of noise. The detection process studied here are near-maximum likelihood schemes: non-linear equalisation methods are also considered in detail. The thesis is also concerned with carrier synchronisation and channel estimation under conditions of Rayleigh fading. Since the carrier syncn,honisation is a most important requirement in mobile radio, a Digital Phase Locked Loop (DPLL) technique has been designed and investigated in the form of a feedback digital synchronisation system. Two types of channel estimation technique, namely feedforward and feedback estimators, are also investigated in this work. The feedback estimator is modified by the addition of a digital control system, in order to reduce its delay, and to cope with rapidly fading signals. Successful carrier synchronisation is demonstrated by the use of space diversity. The study was completed using models of the component parts of the system, and by the use of extensive computer simulations to analyse the system under various operating conditions