Zugl.: Ulm, Univ., Diss.

Zugl.: Darmstadt, Techn. Univ., Diss., 2008

Universiẗat, Diss., 2001--Kaiserslautern.

University, Diss., 2002--Essen.

Techn. University, Diss., 2003--Berlin.

A multiple-antenna-multiple-equalizer (MAME) system is proposed for overcoming cochannel interference (CCI) in code-division multiple access (CDMA) indoor wireless systems. The main advantage of the MAME system is the enhanced interference suppression capability as compared with many existing approaches. Thus, the use of the MAME system can lead to an increase in the capacity of the CDMA system. In the MAME system, a fractionally-spaced equalizer (FSE) is used to process the signal at each antenna. The number of antennas or the tap spacing of the FSEs is not fixed and inherent flexibility is available to the designer. In particular, CDMA indoor wireless systems are best suited to use the interference suppression capabilities of the MAME system. It is shown that spectral correlation present in user signals is the reason for the interference suppression capability of the MAME system. Moreover, the MAME system is interpreted as a dual-domain diversity combiner. Spatial and bandwidth-domain diversity are used and the relative importance of the diversity domains is discussed. These discussions offer new insights into the interference suppression capabilities of the MAME system and give a clear picture of its workings. Extensive simulation results are presented to illustrate the performance of the MAME system under various conditions. Optimal or minimum mean-squared error (MMSE) results are first presented to illustrate the superior interference suppression performance. The effects of the number of antennas, tap spacing, receive filtering, spectral correlation, diversity domains, and near-far conditions on performance are examined and results obtained support the arguments presented earlier in the thesis. The FSEs in the MAME system are implemented as adaptive filters and the mean-squared- error (MSE) performance is investigated. A quasi-Newton (QN) algorithm is recommended over other adaptive filtering algorithms because of ill-conditioning of the autocorrelation matrix in the MAME system. Simulation results confirm the superior convergence performance of the QN algorithm. Decision-directed equalization is also investigated and bit-error rate (BER) results presented illustrate that the gains in the MMSE performance will most likely translate into gains in BER performance. The BER performance in near-far and birth of interferers conditions illustrate that the MAME system is a promising solution to counter these problems. The thesis concludes with an indoor wireless strategy based on the MAME system which offers the following advantages: 1. More users than the processing gain of the CDMA system can share the same bandwidth. 2. No information about code sequences is needed at the receiver. 3. Simple code sequence allocation schemes can be used at the transmitter. 4. Variable numbers of users can be accommodated. 5. Simple power control and error-correction coding schemes can be used. / Graduate

The rapid demand for broadband wireless access with fast multimedia services initiated a vast research on the development of new wireless systems that will provide high spectral efficiencies and data rates. A potential candidate for future generation wireless systems is multi-carrier code division multiple access (MC-CDMA). To achieve higher user capacities and increase the system data rate, various multiple-input multiple-output (MIMO) technologies such as spatial multiplexing and spatial diversity techniques have been proposed recently and combined with MC-CDMA.This research proposes a chip level coded ordered successive spatial and multiuser interference cancellation (OSSMIC) receiver for downlink MIMO MC-CDMA systems. As the conventional chip level OSIC receiver [1] is unable to overcome multiple access interference (MAI) and performs poorly in multiuser scenarios, the proposed receiver cancels both spatial and multiuser interference by requiring only the knowledge of the desired user's spreading sequence. Simulation results show that the proposed receiver not only performs better than the existing linear detectors [2] but also outperforms both the chip and symbol level OSIC receivers. In this work we also compare the error rate performance between our proposed system and MIMO orthogonal frequency division multiple access (MIMO OFDMA) system and we justify the comparisons with a pairwise error probability (PEP) analysis. MIMO MC-CDMA demonstrates a better performance over MIMO OFDMA under low system loads whereas in high system loads, MIMO OFDMA outperforms MIMO MC-CDMA. However if all users' spreading sequences are used at the desired user receiver, MIMO MC-CDMA performs better than MIMO OFDMA at all system loads.In the second part of this work, user grouping algorithms are proposed to provide power minimisation in grouped MC-CDMA and space-time block code (STBC) MC-CDMA systems. When the allocation is performed without a fair data rate requirement, the optimal solution to the minimisation problem is provided. However when some fairness is considered, the optimal solution requires high computational complexity and hence we solve this problem by proposing two suboptimal algorithms. Simulation results illustrate a significantly reduced power consumption in comparison with other techniques.

Orthogonal Frequency-Division Multiplexing (OFDM) has been proved to be a promising technology that enables the transmission of higher data rate. Multicarrier Code-Division Multiple Access (MC-CDMA) is a transmission technique which combines the advantages of both OFDM and Code-Division Multiplexing Access (CDMA), so as to allow high transmission rates over severe time-dispersive multi-path channels without the need of a complex receiver implementation. Also MC-CDMA exploits frequency diversity via the different subcarriers, and therefore allows the high code rates systems to achieve good Bit Error Rate (BER) performances. Furthermore, the spreading in the frequency domain makes the time synchronization requirement much lower than traditional direct sequence CDMA schemes. There are still some problems when we use MC-CDMA. One is the high Peak-to-Average Power Ratio (PAPR) of the transmit signal. High PAPR leads to nonlinear distortion of the amplifier and results in inter-carrier self-interference plus out-of-band radiation. On the other hand, suppressing the Multiple Access Interference (MAI) is another crucial problem in the MC-CDMA system. Imperfect cross-correlation characteristics of the spreading codes and the multipath fading destroy the orthogonality among the users, and then cause MAI, which produces serious BER degradation in the system. Moreover, in uplink system the received signals at a base station are always asynchronous. This also destroys the orthogonality among the users, and hence, generates MAI which degrades the system performance. Besides those two problems, the interference should always be considered seriously for any communication system. In this dissertation, we design a novel MC-CDMA system, which has low PAPR and mitigated MAI. The new Semi-blind channel estimation and multi-user data detection based on Parallel Interference Cancellation (PIC) have been applied in the system. The Low Density Parity Codes (LDPC) has also been introduced into the system to improve the performance. Different interference models are analyzed in multi-carrier communication systems and then the effective interference suppression for MC-CDMA systems is employed in this dissertation. The experimental results indicate that our system not only significantly reduces the PAPR and MAI but also effectively suppresses the outside interference with low complexity. Finally, we present a practical cognitive application of the proposed system over the software defined radio platform.

Medium access mechanisms are one of the most important aspects of buses, which are shared mediums. Almost all standard buses use Time Division Multiple Access (TDMA) as their medium access scheme. Such buses usually are multi-wire, very sensitive to time synchronization, and often managed by a master node. In this thesis, we develop new non-TDMA schemes for bus communications which are based on Code Division Multiple Access (CDMA), or Frequency Division Multiple Access (FDMA) that do not have the intrinsic limitations of traditional buses. The proposed schemes are based on a single wire bus setting. Since, in theory, any node can have a dedicated communication link to any other node on the bus, the nodes virtually form a fully-connected mesh, hence the name Mesh-Bus. In such schemes, no master node is required on the bus; therefore, we can have a distributed bus in which all the nodes have the same functionality. Also, no time synchronization is required. Every node, using its unique code/frequency, creates a virtual private link to the other nodes, and using such interference-resistant virtual private links, the nodes communicate data to each other. This dissertation explores the underlying principals of such non-TDMA schemes and through extensive software simulations investigates various scenarios in for CDMA scheme, and studies the performance for the system. Finally, a hardware implementation of the CDMA scheme is presented, and some experimental results are provided to validate the simulation results.

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