The development of wireless cellular communication systems has evolved from the first-generation (1G) of analogue stage to second-generation (2G) and third-generation (3G) of digital stage. Now, due to market oriented demands, it is has stepped into fourth-generation (4G) of broadband stage. As per recommendations of International Mobile Telecommunications-2000 (IMT-2000), the future wireless communication is bound to occupy the features including high-speed data and broadband transmission, high capacity to support a huge number of simultaneous users, global mobility, high security, and scalable quality of service (QoS) along with low cost for both operators and subscribers. The above features are imposing technical challenges on system design and stimulating various research topics on capacity, complexity and performance. In order to increase the capacity of wireless networks, various multiple access schemes have been reported in the literature. The credit of most competent multiple access scheme in 2G systems goes to CDMA scheme which offers an even better bandwidth-efficiency than TDMA and FDMA schemes, however, its implementation is quite difficult due to involvement of rather complex technologies including complex power-control, and multiuser detection techniques etc. The performance of CDMA scheme is mainly limited by multiple access interference (MAI) and inter-symbol interference (ISI). In conventional CDMA systems, the spreading sequences were employed for the purpose of user separation, however due to poor aperiodic correlations amongst spreading codes including Gold, Walsh, Kasami sequences, the direct-sequence (DS) spreading mechanism demonstrates low spreading efficiency (SE) in case of high user count.
The requirement of alternate mechanism for user separation has been solved by Interleave-Division Multiple-Access (IDMA) scheme, in which, most of above stated problems do not exist due to application of user-specific interleavers having low cross-correlation amongst them. The interleaved data resulted from user-specific interleavers, demonstrates better orthogonality amongst each other in the channel. The condition of orthogonality is maintained for reducing the risk of collision amongst the interleavers during communication process.
In IDMA scheme, orthogonal interleavers are employed as the only means for user separation and, hence, are referred as the heart of the scheme. The selection of interleaver along with optimum design methodology for IDMA system leads to satisfactory spectral efficiency. During initial run of IDMA scheme, random interleavers (RI) were employed for user separation. Later, random interleavers were replaced by master random interleavers (MRI) in order to reduce the memory requirement raised due to storage of random interleavers at transmitter and receiver ends. Many more interleavers are reported in literature but most of them are based on methodology of selection of user-specific interleavers amongst available random interleavers. Still, the problem of computational complexity involved in interleaving and de-interleaving mechanism is unresolved for user specific interleavers. These problems in user-specific interleavers have provided the motivation for development of an optimum interleaving mechanism for IDMA scheme.
In this thesis, an optimum interleaving mechanism has been proposed named as tree based interleaving (TBI) mechanism, for the solutions related to computational complexity, bandwidth requirement and optimization of memory size, at transmitter and receiver ends. The orientation of the work has been maintained towards the analysis and design ofproposed tree based interleaving (TBI) mechanism for IDMA scheme fulfilling the requirement of orthogonality and easy implementation.
In the beginning of thesis, the mechanism of interleaving with necessary conditions is presented. Later, the performance and analysis of proposed TBI mechanism with IDMA scheme has been presented. Apart from the bit error rate (BER) performance analysis, the interleavers have also been analyzed on the basis of memory requirement and computational complexity at transmitter and receiver ends. The performance evaluation of IDMA scheme with proposed tree based interleaving (TBI) mechanism, in uncoded and coded environments, has been duly investigated. After investigation, it is noticed that the computational complexity of tree based interleaver is extremely less in comparison to that for master random interleaver while it is marginally higher to that occurring in case of random interleaver. However, the bandwidth and memory requirement of proposed tree based interleaver is found to be considerably less than that of random interleaver and slightly higher to that of master random interleaver. It has been observed that BER performance of this interleaver is similar to that of random interleaver and master random interleaver.
The second goal of the thesis is to evaluate BER performance of IDMA scheme with maximal ratio combining (MRC) diversity, for proposed TBI mechanism along with RI and MRI mechanisms with various architectures. After simulations, the BER performance of proposed TBI mechanism is observed to be very near to that of random interleaver.
Further, the correlation analysis of all the concerned interleavers have been carried out in this thesis which reveals the behavior of interleavers with increment in user count. It is observed that the cross-correlation of TBI mechanism is almost similar to that of MRI and RI mechanisms.
It again confirms that BER performance of TBI mechanism should be similar to that of other mechanisms. The increment in multiple access interference (MAI) is also observed with increment in user count for all the interleaving mechanisms.
Finally, the proposed tree based interleaving mechanism has been implemented on field-programmable gate-array (FPGA) system for observing the performance related to hardware requirements and timing constraints in comparison to that required with RI and MRI mechanisms, for IDMA systems. During the implementations, it is observed that the hardware requirement of tree based interleaving (TBI) mechanism is at the minimum level in comparison to other considered mechanisms. The timing constraints of TBI mechanism are also found to be at its minimum level. In addition to it, the tree based interleaving mechanism inherits comparably lesser hardware complexity to that required for master random interleaving mechanism due to requirement of lesser looping operations. After the analysis, it has been observed that proposed tree based interleaver requires least hardware along with least timing constraints for its operation. The proposed tree based interleaving (TBI) mechanism has demonstrated optimum overall BER and other performances in comparison to RI, and MRI mechanisms. Thus, the proposed interleaving mechanism along with IDMA scheme provides an alternative to conventional CDMA scheme for future wireless communication systems.
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