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Interference dominated wireless communications systems are considered. Joint detection methods are applied to combat the negative effects of the temporal and spatial interference. However, practical joint detectors are not commonly used due to their high complexity. Therefore, there is a constant need to deliver reduced complexity solutions that approach substantial fraction of the channel capacity. In the first part of this thesis it is shown that simple transmission technique employing repetition coding and interleaving combined with interference cancellation is an asymptotically optimal processing strategy when high interference is experienced, for instance due to the relatively high ratio of the number of signals to the number of orthogonal dimensions. Surprisingly, strong, capacity achieving codes exhibit inferior performance and are not well suited for iterative interference cancellation due to their sharp threshold characteristics. Motivated by this observation partitioned modulation is introduced and applied to a multiuser spread spectrum system, which inherently encompasses a repetition coding mechanism. The detection of the resulting signals employs a parallel interference cancellation approach, where the repetition code exchanges information iteratively with the canceller. Precise signal-to-noise ratio evolution of the proposed receiver as a function of demodulation iterations is given. It is shown, that for equal received power system, partitioned demodulator outperforms linear minimum mean squared error processor at a fraction of complexity. This receiver processing for a wide range of parameters delivers estimates that coincide with the optimal processing based on exhaustive search. For unequal received signal powers these advantages are even more visible and for a particular exponential power allocation the proposed system reaches the capacity of the channel. The analytical investigations are verified using computer simulations. In the second part of this dissertation, multi-user MIMO systems compliant with 3GPP LTE standard are considered. Turbo near-far resistant interference cancellation receiver is proposed. It jointly removes multi-user, multi-antenna and inter-symbol interference and outperforms traditional demodulation/decoding method adopted in the LTE standard. Semi-analytical method of predicting the performance of this joint receiver for any system setup is outlined. This approach makes it possible to tune up the performance of the system without running extensive bit-error-rate simulations. / Communications

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In an optimal power control scheme for a Code Division Multiple Access (CDMA) system all mobile stations signals should arrive to the base station at equal power. If not, stronger singals may cause too much interference and block out weaker ones. Commonly used power control schemes utilizes the Signal to Interference Ratio (SIR) to design a Power Control Command (PCC) to adjust the transmit power of the mobile station. A significant problem within the conventional methods is the slow SIR recovery due to deep channel fades. Conventional methods base the PCC on the previous channel state when in fact, the channel state may have significantly changed when transmission occurs. These channel changes may cause the SIR to drop or rise drastically and lead to uncontrollable Multi Access Interference (MAI) resulting in power escalation and making the system unstable. In order to overcome power escalation and improve the recovery from deep fades a novel power control method has been developed. Based on Linear Quadratic Control and Kalman filtering for channel prediction this method designs the PCC based on the coming channel state instead of the current. This optimizes the PCC for the channel state where transmission occurs. Simulations show that this control scheme outperforms previous methods by making the impacts of the deep fades less severe on the SIR and also improves the overall SIR behaviour.

To let many users benefit from the high speed communication in an optical fiber, a way to share the channel is needed. One way to share the channel between the users is the multiple access technique of Code Division Multiple Access, CDMA. This technique is based on coding. By assigning the users different codewords, the receiver is able to distinguish the information sent from each user. One family of codes suitable to use in a fiber optic network is the constant weight Optical Orthogonal Codes, OOC. To influence the users interference tolerability it is possible to assign the users codewords of the same length but with different weights. By letting the users have codewords of different length it is possible to have different data rates in the system. These two versions of optical orthogonal codes, with variable length and weight, are investigated in this thesis for their cardinality. The variable weight and variable length codes were created by combining optical orthogonal codes with different parameters. I have found that for variable length OOC:s of weight 2 it seems to be possible to increase the cardinality of a code, but for codes with higer weights than that, it is better that all users are assigned codewords of the same length. But since an upper bound for the cardinality of these types of codes has not been found, it is not possible to verify if this conclusion is correct. For variable weight OOC:s it is found that it is only possible to increase the cardinality of small, not optimal, codes. For codes including many codewords it is rarely possible to include more codewords without breaking the conditions on cross correlation between the codewords.

In direct-sequence code division multiple access (DS-CDMA) systems, the heuristic optimization algorithms for multiuser detection include genetic algorithms (GA) and simulated annealing (SA) algorithm. In this thesis, we use particle swarm optimization (PSO) algorithms to solve the optimization problem of multiuser detection (MUD). PSO algorithm has several advantages, such as fast convergence, low computational complexity, and good performance in searching optimum solution. In order to enhance the performance and reduce the number of parameters, we propose two modified PSO algorithms, inertia weighting controlled PSO (W-PSO) and reduced-parameter PSO (R-PSO). From simulation results, the performance of our proposed algorithms can achieve that of optimal solution. Furthermore, our proposed algorithms have faster convergence performance and lower complexity when compared with other conventional algorithms.

In this thesis, we consider the uplink of a cooperative code division multiple access (CDMA) network, where users cooperate by relaying each other¡¦s messages to the base station. The sources adopt CDMA to share the informations offered by relays. In general, spreading waveforms of sources are not orthogonal to each others due to pratical design issues of CDMA network. Therefore, the source signals will suffer from multiple access interference (MAI) at the relays and destination. The MAI results in the increase of bit error rate increased, and diminishes the cooperative network diveristy gains. In order to mitigate MAI, the decorrelating multiuser detection and zero-forcing precoder have been commonly adopted. But, the decorrelating multiuser detection causes noise enhancement, while the zero-forcing precoder causes power expansion. In this thesis, we adopt relay-assisted decorrelating multiuser detector (RAD-MUD) to mitigate MAI as proposed in [1].In this scheme, the relays perform half of decorrelating operation and the destination performs the other half. In this way, neither noise enhancement nor power expansion will occur. However, in the reference [1], the authers assume the transmission is synchronous between sources and relays. The assumption is unrealistic, because it is difficult to achieve synchronization between all sources and relays due to various propagation delays. In this thesis, we extend the research in [1] and relax restriction of synchronization between all sources and relays. Besides, we also adopt cooperative strategies such as transmit beamforming or selective relaying to enhance system performance. Compared with other¡¦s multiuser detection schemes, we show that the proposed schemes can effectively reduce the bit error rate.

In this thesis, a novel pre-coded direct-sequence code division multiple access (DS-CDMA) associated with the multiple-input multiple-output (MIMO) antennas, along with Alamouti¡¦s space-time block code (ST-BC) is considered. In the transmitter, the idea of hybrid pre-coded is exploited; it can be employed to counteract the inter-symbol interference (ISI) introduced by the channel fading duo to multipath propagation, and can be used to exacting the full information of channel impulse response (CIR) in the receiver for semi-blind channel estimation. Under this transceiver framework, we develop a semi-blind adaptive MIMO-CDMA receiver, based on the linearly constrained constant modulus (LCCM) criterion, for symbol recovery. Furthermore, to reduce the complexity of receiver design, the framework of the generalized sidelobe canceller (GSC) associated with the recursive least-squares (RLS) algorithm is adopted, and the gradient approach is applied to track the desired user¡¦s amplitude, simultaneously, and is named as the GSC-CM-IQRD-RLS algorithm. Via computer simulations, the superior performance and the advantages of proposed scheme is verified; it outperforms the conventional LCMV-based algorithm, and slightly better than the convention CM-GSC-RLS receiver, with the paid of spectral efficient.

In this paper, we explore a cooperative code-division-multiple-access(CDMA) network. Users cooperate by forwarding each other¡¦s messages toward the destination. For simplicity, we assume that signal reception at the destination is well-synchronized. Due to practical design issues of CDMA systems, spreading waveforms allocated to users are not perfectly orthogonal in general. This results in multiple-access interference(MAI) at relays and destination. In CDMA uplink networks one common approach is to adopt decorrelating multi-user detection, but it will lead to noise amplification[16,17]. Therefore, we employ relay-assisted decorrelating multiuser detector(RAD-MUD) to mitigate MAI[1] by performing half of decorrelation at the relay and destination respectively. Based on the availability of CSI at relays, we can further adopt cooperative strategies to improve performance, e.g., transmit beamforming and selective relaying. The destination side will use minimum mean-square error(MMSE) detector to demodulate source symbols. In the existing literatures, channel state information(CSI) is assumed to be perfectly known at relay and destination. Actually, CSI is obtained from channel estimation, which usually contains estimation errors. In order to alleviate effects of channel estimation, one goal of this thesis is to design a robust system. Using estimated CSI and statistical property channel estimation errors, we design robust precoder and detector for the relay and destination. It shows that, even with distortion on channel estimations, the system still achieve excellent transmission efficiency. From the simulation results, it shows that the robust design is better than the system without consider channel estimation errors. Finally, we can see that the stable robust design can effectively mitigate effects of imperfect CSI.

In this thesis, the multi-carrier (MC) code division multiple access (CDMA) system in Rayleigh fading channel is considered. The system performance will be degraded due to multiple access interference (MAI) or background noise. It is know that linearly constrained inverse QR-decomposition (LC-IQRD) recursive least-square algorithm can overcome the problems. The main concern of this thesis is to implement the circuit of LC-IQRD algorithm. FPGA components and sets up a high efficient programmable hardware module. In this thesis, we implemented the circuit of LC-IQRD algorithm via a chip of Field Programmable Gate Array (FPGA) with Verilog HDL. The conventional IQRD circuit design employs systolic array architecture. The advantages of systolic array architecture include modularity and hardware simplicity. These properties are extremely desirable for VLSI implementation. In fact, we expect to reduce the execution time of the conventional IQRD algorithm circuit design. Therefore, in this thesis a modified IQRD circuit design is proposed to improve the effect of circuit implementation. It also has advantage of modularity and reduces the execution time. In order to degrade complexity of LC-IQRD algorithm circuit design, the area and speed of circuit are the consideration in this thesis. The data source is produced by Matlab software. We verify the performance of the system in terms of BER (bit error rate) and SINR (signal to interference and noise ratio).Finally, LC-IQRD algorithm circuit is realized in the Altera EP20k1500EFC-33 chip and on the Quartus II of Altera. The algorithm circuit uses 51536 logic elements (LE) for 30 bits fixed point design.