Long Term Evolution (LTE) developed by Third Generation Partnership Project (3GPP) is expected to be the standard of the Fourth-Generation (4G) of wireless communication system. LTE supports Frequency Division Duplex (FDD) and Time Division Duplex (TDD), and both of them are based on Orthogonal Frequency Division Multiplexing (OFDM) system in downlink. OFDM systems are sensitive to timing and frequency offset. Therefore, synchronization plays an important role in OFDM systems. In this thesis, we study synchronization problems of a LTE FDD baseband receiver. Particularly, we develop a complete procedure to deal with the synchronization problems. The basic design concept and procedure are as follows: The receiver estimates and compensates the timing and frequency offset by using the repetition property of the cyclic prefix. In the meanwhile, the receiver also detects cyclic prefix mode (or the length of the cyclic prefix). After the frequency offset has been compensated, the receiver then processes cell search. To this end, we multiply each subcarrier by the synchronization sequence provided by LTE specification and transform them into time domain. We then estimate the channel energy in time domain to detect the Cell Identity (Cell ID). Using computer simulations, we verify that the designed receiver performs well.

In many algorithms for Orthogonal Frequency Division Multiplexing (OFDM) systems, the channel estimation is one of the most essential factors. In wireless environment, channel is change very fast, and the channel has multipath effect, the channel length is obtained by channel estimation. In this paper, we estimation the channel length and the SNR by virtual carriers (VC) and Singular value decomposition, when channel estimator known the information for channel length, then calculate complicated can be reduced. Besides, we proposed the estimated method at carriers frequency offset effect. Noise variance (or noise power) can improve performance of channel estimator, e.g. MMSE channel estimator, turbo code or power allocation. In this paper, we were estimate noise variance by using the blind method of property of orthogonality of matrix, which is differed from the traditional method of Pilots.

The efficient use of radio resources is crucial in order for future wireless systems to be able to meet the demand for high speed data communication services. Orthogonal Frequency Division Multiplexing (OFDM) is an important technology for future wireless systems as it offers numerous advantages over other existing technologies, such as robust performance over multipath fading channels and the ability to achieve high spectral efficiency. Dynamic resource allocation can fully exploit the advantages of OFDM, especially in multiple user systems. In this thesis, we investigate a resource allocation problem in a multiple user, multiple cell OFDM cellular network focusing on downlink communications. (For complete abstract open document)

In this thesis, we investigate a wireless communications system with distributed transmit antennas. Under such system scenario, the received signal has multiple carrier frequency offsets (CFOs) since each transmitter has its own oscillator, leading to serious inter-carrier interference (ICI) at the receiver end. Therefore, an ICI self-cancellation scheme is proposed in this thesis, where two different relay nodes use different sub-carriers. When the signals from different relay nodes are combined at the destination node, the ICI self-cancellation can be achieved. In addition, the quality of the received signal can be further improved if the residual CFO can be properly compensated. Traditionally, the medium value of the various CFOs is taken for compensation because of its simplicity. However, a medium value does not result in the optimal performance. In this thesis, a close form expression of optimal CFO is derived to maximize the average signal to interference power ratio. It is shown that the optimal CFO compensation is a function of channel state and individual CFOs. Simulation experiments are conducted to investigate the performance of the proposed scheme. It is shown that the system bit error rate can be substantially improved when the CFO is less than 0.3 subcarrier spacing.

High peak to average power ratio (PAPR) is one of the major drawbacks in orthogonal frequency division multiplexing (OFDM) systems. In recently years, various methods have been proposed to reduce the PAPR performance. The selected mapping (SLM) scheme is perhaps the most popular one because it provides outstanding PAPR reduction performance. In addition, the subcarrier magnitude remains the same in the SLM scheme. However, there are two major shortcomings in the SLM scheme. First of all, it requires a number of inverse fast Fourier transforms (IFFTs) to produce candidate signals, dramatically increasing the computational complexity. In addition, side information has to be transmitted to the receiver to indicate the candidate signal that results in the best PAPR, leading to the decrease in bandwidth utilization. To overcome these two drawbacks, this thesis proposes a novel SLM scheme that does not need side information. The proposed scheme is based on a low complexity SLM scheme [C.-P. Li, S.-H. Wang, and C.-L. Wang, ¡§Novel low-complexity SLM schemes for PAPR reduction in OFDM systems,¡¨ IEEE Trans. Signal Process., vol. 58, no. 5, pp. 2916¡V2921, May 2010] in pilot-aided OFDM system. Simulation experiments are conducted to verify the performance of the proposed scheme. It is shown that the bit error rate (BER) performance of the proposed scheme is very similar to that of the traditional SLM scheme with perfect knowledge of the side information. Therefore, the proposed scheme not only has the advantages of low complexity and high bandwidth utilization, but also has a superior BER performance.

Selected mapping (SLM) schemes are commonly employed to reduce the peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) systems. It has been shown that the computational complexity of the traditional SLM scheme can be substantially reduced by adopting conversion vectors obtained by using the inverse fast Fourier transform (IFFT) of the phase rotation vector in place of the conventional IFFT operations [21]. Unfortunately, however, the elements of these phase rotation vectors of the conversion vectors in [21] do not generally have an equal magnitude, and thus a significant degradation in the bit error rate (BER) performance is incurred. This problem can be remedied by utilizing conversion vectors having the form of a perfect sequence. This paper presents three novel classes of perfect sequence, each of which comprises certain base vectors and their cyclic-shifted versions. Three novel low-complexity SLM schemes are then proposed based upon the unique structures of these perfect sequences. It is shown that while the PAPR performances of the proposed schemes are marginally poorer than that of the traditional SLM scheme, the three schemes achieve an identical BER performance and have a substantially lower computational complexity.

Στις υποθαλάσσιες επικοινωνίες χρησιμοποιούνται ακουστικά κύματα για τη μεταφορά της πληροφορίας. Κατά τη διαδικασία της μετάδοσης εμφανίζονται προβλήματα που σχετίζον- ται με τη καθυστέρηση διάδοσης, την εξασθένηση του σήματος, τις πολλαπλές οδεύσεις λόγω ανάκλασης καθώς και άλλα προβλήματα που παρουσιάζονται στις ασύρματες επι- κοινωνίες. Στην παρούσα εργασία αναλύεται και εξετάζεται η πλειονότητα αυτών. Στο πρώτο κεφάλαιο γίνεται μία συνοπτική περιγραφή του φαινομένου των διαλείψεων καθώς και η ανάλυση βασικών εννοιών, τις οποίες συναντάμε σε επόμενα κεφάλαια. Στο δεύτερο κεφάλαιο περιγράφεται η πολύπλεξη OFDM, η οποία χρησιμοποιείται στις περισσότερες εφαρμογές των υποθαλάσσιων επικοινωνιών. Στο τρίτο κεφάλαιο αναλύεται μαθηματικά η τυφλή εκτίμηση για συστήματα OFDM και αναφέρονται επίσης προσαρμοστικοί αλγόριθ- μοι για την εκτίμηση του καναλιού με βάση την θεωρία η οποία παρουσιάζεται. Τέλος στο τέταρτο κεφάλαιο παρατίθεται κώδικας matlab, στον οποίο γίνεται χρήση των αλγορίθμων των τρίτου κεφαλαίου με σκοπό την εκτίμηση της κρουστικής απόκρισης του καναλιού, ενώ πέραν της γενικής περίπτωσης εξετάζονται επιπλέον λύσεις sparse μορφής. / --

Research in wireless communications has been actively carried out in recent years. In order to enable a high data transmission rate, multiple-input multiple-output (MIMO) communications has been proposed and commonly adopted. Accurate channel identification and reliable data detection are major challenges in the implementation of a communications system operating over a wireless fading channel. These issues become even more challenging in MIMO systems since there are many more parameters involved in the estimation processes. This thesis, consisting of four major parts, focuses on applying convex optimization to solve design problems in both MIMO channel estimation and data detection. The first part proposes a novel orthogonal affine precoding technique for jointly optimal channel estimation and symbol detection in a general MIMO frequency-selective fading channel. Additionally, the optimal power allocation between the data and training signals is also analytically derived. The proposed technique is shown to perform much better than other affine precoding techniques in terms of detection error probability and computational complexity. The second part is concerned with the MIMO orthogonal frequency-division multiplexing (OFDM) systems. The superimposed training technique developed in the first part is applied and extended for MIMO-OFDM systems where all the involved transmitters and receivers are assumed to be uncorrelated. Analytical and numerical results confirm that the proposed design can efficiently identify the unknown wireless channel as well as effectively recover the data symbols, while conserving the transmission bandwidth. The third part considers training and precoding designs for OFDM under colored noise environment. The superiority of the proposed design over the previously-known design under colored noise is thoroughly demonstrated. The last part of the thesis develops the orthogonal affine precoder for spatially correlated MIMO-OFDM systems. The optimal superimposed training sequences are solved by tractable semi-definite programming. To have a better computational efficiency, two approximate design techniques are also presented. Furthermore, the non-redundancy precoder proposed in the third part is employed to combat channel correlation. As a result, the proposed designs are demonstrated to outperform other known designs in terms of channel estimation and data detection.

With the rapid growth of wireless data demands and saturation of radio frequency (RF) capacity, visible light communication (VLC) has become a promising candidate to complement conventional RF communication, especially for indoor short range applications. However the performance of the system depends on the propagation and type of system used. An optical Orthogonal Frequency Division Multiplexing (O-OFDM) together with multiple input multiple output (MIMO) in different scenario and modulation techniques are studied in the thesis. A novel optical wireless communication (OWC) multi-cell system with narrow field of view (FOV) is studied. In this system the intensity modulated beam from four light sources are used for communication. The system allows beams to be concentrated in specific areas of the room to serve multiple mobile devices with low interference and hence increase system capacity. The performance of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM), direct current biased optical OFDM (DCO-OFDM) and single carrier (SC) modulation are then compared in this system considering single user and multiusers scenarios. The performance of the multi-cell is compared with single cell with wide FOV. It is shown that the capacity for multi-cell system increases with the number of users to 4 times the single user capacity. Also the findings show that multi-cell system with narrow beams can outperform a single wide beam system in terms of coverage area and hence average throughput of about 2.7 times the single wide beam system capacity. One of the impairments in line of sight (LOS) OWC systems is coverage which degrades the performance. A mobile receiver with angular diversity detectors in MIMO channels is studied. The objective is to improve the rank of the channel matrix and hence system throughput. Repetition coding (RC), spatial multiplexing (SMP) and spatial modulation (SM) concepts are used to evaluate throughput across multiple locations in a small room scenario. A novel adaptive spatial modulation (ASM) which is capable of combating channel rank deficiency is devised. Since the receiver is mobile, the channel gains are low in some locations of the room due to the lack of LOS paths between transmitters and receivers. To combat the situation adaptive modulation and per antenna rate control (PARC) is employed to maximise spectral efficiency. The throughputs for fixed transmitters and receivers are compared with the oriented/inclined detectors for different cases. Angular diversity detectors offer a better throughput improvement than the state of the art vertical detectors, for example in ASM angular diversity receiver gives throughput of about 1.6 times that of vertical detectors. Also in SMP the angular detectors offer throughput about 1.4 times that of vertical detectors. SMP gives the best performance compared to RC, SM and ASM, for example SMP gives throughput about 2.5 times that of RC in both vertical detectors and angular diversity receivers. Again SMP gives throughput about 6 times that of SM in both vertical detectors and angular diversity receivers. Also SMP provides throughput about 2 times that of ASM in both vertical detectors and angular diversity receivers. ASM exhibit improvement in throughput about average factor of 3.5 times SM performance in both vertical detectors and angular diversity detectors. As the performance of the system may be jeopardized by obstructions, specular and diffuse reflection models for indoor OWC systems using a mobile receiver with angular diversity detectors in MIMO channels are considered. The target is to improve the MIMO throughput compared to vertically oriented detectors by exploiting reflections from different reflecting surfaces in the room. The throughput across multiple locations in the small room by using RC, SMP and SM approaches is again evaluated. The results for LOS only channels against LOS with specular or diffuse reflection conditions, for both vertical and angular oriented receivers are then compared. The results show that exploiting specular and diffuse reflections provide significant improvements in link performance. For example the reflection coefficient (α) of 0.9 and the antenna separation of 0.6 m, RC diffuse model shows throughput improvement of about 1.8 times that of LOS for both vertical detectors and angular diversity receivers. SM diffuse model shows throughput improvement of about 3 times that of LOS for both vertical detectors and angular diversity receivers. ASM diffuse model shows throughput improvement of about 2 times that of LOS for both vertical detectors and angular diversity receivers. SMP diffuse model shows throughput improvement of about 1.5 times that of LOS for both vertical detectors and angular diversity receiver.

Underlay signaling is a robust physical layer technique, allowing for transmitting a very low power signal in conjunction with the primary signals across the entire frequency band of the primary signals. The secondary users of the secondary network (i.e., a wireless network consisting of primary and secondary networks) primarily utilize the underlay, which increases spectral efficiency and improves the network capacity. This thesis focuses on underlay signaling in the context of the cellular (primary) network, where the underlay is an auxiliary channel made available to the primary users and the network, that is, the base stations and users of the cellular network. The current fifth-generation (5G) cellular networks are constructed using Orthogonal Frequency Division Multiplexing (OFDM) modulation. Hence, this thesis delves into the study of underlay coexistence with OFDM, specifically 5G, by performing extensive simulations and analytical analysis and investigating the impact of underlay signaling on the throughput performance of 5G networks. We develop the underlay signaling based on the frequency-domain spread spectrum and add the underlay signal prior to the Inverse Fast Fourier Transform (IFFT) operation of OFDM. Furthermore, we present a real 5G setup built on the srsRAN project, where we showcase a proof-of-concept demonstration of underlay coexistence with the 5G over the air, where the 5G base station transmits both 5G NR and underlay signal simultaneously. Through our research, we conclusively demonstrate that a low-data rate underlay signal can be successfully transmitted alongside the existing 5G signal. Our study concludes by carefully selecting the appropriate design parameters, such as the signal-to-interference power level (5G power in relation to underlay), spreading factor, and coding gain at which we can reliably detect and decode underlay signals having no impact on the 5G performance. The integration of underlay in 5G brings forth a multitude of benefits using underlay for military and tactical applications, massive Machine Type Communications (mMTC) alongside Ultra-Reliable Low Latency Communications (URLLC), and the offloading of crucial control information of 5G to the underlay channel. Thus, this underlay operates as a low-data rate error-free conduit, with the potential to provide Low Probability of Interception (LPI) and Low Probability of Detection (LPD) attributes and heightened reliability while concurrently transmitting with the 5G NR, bolstering the overall effectiveness of the communication. / Master of Science / Underlay signaling is a technique that allows for transmitting a low-power signal alongside the primary signals, occupying the entire frequency band of the primary signals. The secondary users of the secondary network (i.e., a wireless network consisting of primary and secondary networks) primarily utilize the underlay, which increases spectral efficiency and improves the network capacity. This thesis focuses on underlay signaling in the context of cellular (primary) networks where the underlay is an auxiliary channel made available to the primary users and network, that is, the base stations and users of the cellular network. The current fifth-generation (5G) cellular networks are constructed using Orthogonal Frequency Division Multiplexing (OFDM) modulation. OFDM is a multicarrier modulation scheme that divides the available frequency band into multiple narrow subcarriers, each carrying a portion of the data. The key advantage of OFDM is its ability to combat frequency-selective fading, where different frequencies experience different levels of fading and interference. By using many closely spaced sub-carriers, OFDM can effectively mitigate the impact of fading, allowing for robust and reliable communication even in challenging channel conditions. Thus, this thesis investigates the co-existence of underlay signaling and OFDM in 5G. We develop the underlay signaling based on the frequency-domain spread spectrum. Extensive simulations and analytical analysis are performed to understand the impact of underlay signaling on OFDM performance in terms of bit error rates and throughput. Additionally, a real 5G setup is presented, demonstrating a proof-of-concept of underlay co-existence with 5G NR, where the 5G base station transmits both 5G NR and underlay signal simultaneously. Through the research, it is conclusively demonstrated that a low-data rate error-free underlay signal can be successfully transmitted alongside the existing 5G signal. The integration of underlay in 5G brings forth a multitude of benefits using underlay for military and tactical applications, massive Machine Type Communications (mMTC) alongside Ultra-Reliable Low Latency Communications (URLLC), and the offloading of crucial control information of 5G to the underlay channel. Thus, this underlay operates as a low-data rate error-free conduit, characterized by its low interception and low detection attributes, enhancing reliability while concurrently transmitting with 5G NR, bolstering the overall effectiveness of the communication.