Physical Layer Techniques for Wireless Communication Systems

The increasing diffusion of mobile devices requiring, everywhere and every time, reliable connections able to support the more common applications, induced in the last years the deployment of telecommunication networks based on technologies capable to respond effectively to the ever-increasing market demand, still a long way off from saturation level. Multicarrier transmission techniques employed in standards for local networks (Wi-Fi) and metropolitan networks (WiMAX) and for many years hot research topic, have been definitely adopted beginning from the fourth generation of cellular systems (LTE). The adoption of multicarrier signaling techniques if on one hand has brought significant advantages to counteract the detrimental effects in environments with particularly harsh propagation channel, on the other hand, has imposed very strict requirements on sensitivity to recovery errors of the carrier frequency offset (CFO) due to the resulting impact on correct signal detection. The main focus of the thesis falls in this area, investigating some aspects relating to synchronization procedures for system based on multicarrier signaling. Particular reference will be made to a network entry procedure for LTE networks and to CFO recovery for OFDM, fltered multitone modulation and direct conversion receivers. Other contributions pertaining to physical layer issues for communication systems, both radio and over acoustic carrier, conclude the thesis

Timing and Frequency Synchronization in Practical OFDM Systems

Orthogonal frequency-division multiplexing (OFDM) has been adopted by many broadband wireless communication systems for the simplicity of the receiver technique to support high data rates and user mobility. However, studies also show that the advantage of OFDM over the single-carrier modulation schemes could be substantially compromised by timing or frequency estimation errors at the receiver. In this thesis we investigate the synchronization problem for practical OFDM systems using a system model generalized from the IEEE 802.11 and IEEE 802.16 standards. For preamble based synchronization schemes, which are most common in the downlink of wireless communication systems, we propose a novel timing acquisition algorithm which minimizes false alarm probability and indirectly improves correct detection probability. We then introduce a universal fractional carrier frequency offset (CFO) estimator that outperforms conventional methods at low signal to noise ratio with lower complexity. More accurate timing and frequency estimates can be obtained by our proposed frequency-domain algorithms incorporating channel knowledge. We derive four joint frequency, timing, and channel estimators with different approximations, and then propose a hybrid integer CFO estimation scheme to provide flexible performance and complexity tradeoffs. When the exact channel delay profile is unknown at the receiver, we present a successive timing estimation algorithm to solve the timing ambiguity. Both analytical and simulation results are presented to confirm the performance of the proposed methods in various realistic channel conditions. ..

Analysis and Mitigation of Asynchronous Interference in Coordinated Multipoint Systems

Next generation cellular wireless networks need to achieve both high peak and average data rates. Also, they need to improve the fairness by providing more homogenous quality of service distribution over the entire cell area. Base station (BS) cooperation is one of the techniques which is used to achieve these requirements, especially the fairness requirement. It is able not only to mitigate inter-cell interference, but also to exploit this interference and to use it as a useful signal. Although BS cooperation or what is called coordinated multipoint (CoMP) communications proves that it can achieve high gains in theory, there are some challenges that need to be solved in order for it to be widely deployed. One of the major challenges which prevents the CoMP concept from being widely deployed in new cellular systems is timing synchronization. This problem is particularly challenging when OFDM is employed which is the case in the uplink (UL) and downlink (DL) of WiMAX systems and in the DL of LTE systems. The problem is inherited from the limitations caused by integer time offsets in OFDM systems. In order to achieve the gains promised by CoMP systems, the user equipments' (UEs) signals in UL or the BSs signals in DL should be synchronized such that the time difference of arrivals do not exceed the cyclic prefix length of the transmitted signals. In this thesis, we first provide a detailed mathematical analysis of the impact of integer time offsets on the performance of single-input-single-output (SISO) OFDM systems. In particular, closed-form expressions for the different types of interference caused by the integer time offset are derived. Furthermore, we derive exact closed-form expressions for the bit error rate (BER) and the symbol error rate (SER) of BPSK, QPSK and 16-QAM modulation for transmission over both AWGN and Rayleigh fading channels. The effect of the fractional carrier frequency offset (CFO) is taken into consideration in the derivations. For OFDM systems with a large number of subcarriers, an approximate method for evaluating the BER/SER is given. Next, we generalized our expressions to be suitable for the single-input-multiple-output (SIMO) OFDM systems. The derived closed-form expressions for the interference and probability of error enabled us to investigate the timing synchronization problem of UL CoMP systems, where it is not possible for a UE to be synchronized to more than one BS at the same time. This synchronization problem imposes an upper limit on the percentage of cooperation which could occur in an UL CoMP system. By using geometrical and analytical approaches, we define this upper bound. Moreover, an MMSE-based receiver that mitigates the unavoidable asynchronous interference is proposed. Furthermore, a simple joint channel and delay estimation block is incorporated into the receiver to examine its performance with estimation errors. Finally, an iterative procedure is suggested to reduce the complexity of the proposed mitigation method. Numerical results are provided to show the accuracy of the derived expressions and the robustness of the proposed mitigation method

A Cross Layer Routing Protocol for OFDMA Based Mobile Ad Hoc Networks.

PhDMobile ad hoc networks are of growing interest because of their unique characteristics and advantages in many practical applications. QoS provision acts as a major challenge in the routing protocol design in the real-world mobile ad hoc networks, especially for the real-time services. OFDM is a new technology which has many advantages over the other modulation schemes. Because of its prominent features, many popular wireless standards have adopted it as physical layer modulation, such as IEEE 802.11 series, WiMAX, 3GPP LTE etc, and it is extended to multiuser environment known as OFDMA. So far none of the existing ad hoc routing protocols fully account for the OFDMA based mobile ad hoc networks. In this thesis, a QoS routing protocol is proposed for OFDMA based mobile ad hoc networks. A signal strength-based sub-channel allocation scheme is proposed in the routing protocol aiming to reduce the signalling overhead and cochannel interference. The performance of the proposed routing protocol is compared with other alternative proposals through simulations using OPNET simulator. Moreover, a partial time synchronization and a null subcarrier based frequency synchronization algorithms are also proposed for OFDMA based ad hoc network to further support and facilitate the proposed sub-channel allocation scheme and routing protocol

On future aeronautical communications: implementation of a real-time AeroMACS waveform for software-defined radios (SDR)

This master thesis deals with the implementation of a real-time waveform compliant with WiMAX-based standard suitable for aeronautical communications (AeroMACS). All the transmitter/receiver functions are software-implemented, focusing on the acquisition techniques, software optimization (real-time goal) and performance evaluation

Identification of Technologies for Provision of Future Aeronautical Communications

This report describes the process, findings, and recommendations of the second of three phases of the Future Communications Study (FCS) technology investigation conducted by NASA Glenn Research Center and ITT Advanced Engineering & Sciences Division for the Federal Aviation Administration (FAA). The FCS is a collaborative research effort between the FAA and Eurocontrol to address frequency congestion and spectrum depletion for safety critical airground communications. The goal of the technology investigation is to identify technologies that can support the longterm aeronautical mobile communication operating concept. A derived set of evaluation criteria traceable to the operating concept document is presented. An adaptation of the analytical hierarchy process is described and recommended for selecting candidates for detailed evaluation. Evaluations of a subset of technologies brought forward from the prescreening process are provided. Five of those are identified as candidates with the highest potential for continental airspace solutions in L-band (P-34, W-CDMA, LDL, B-VHF, and E-TDMA). Additional technologies are identified as best performers in the unique environments of remote/oceanic airspace in the satellite bands (Inmarsat SBB and a custom satellite solution) and the airport flight domain in C-band (802.16e). Details of the evaluation criteria, channel models, and the technology evaluations are provided in appendixes

On future aeronautical communications: implementation of a real-time AeroMACS waveform for software-defined radios (SDR).

This master thesis deals with the implementation of a real-time waveform compliant with WiMAX-based standard suitable for aeronautical communications (AeroMACS). All the trasnmitter/receiver functions are software-implemented, focusing on the acquisition techniques, software optimization (real-time goal) and performance evaluation

Performance analysis of 4G wireless networks using system level simulator

Doutoramento em Engenharia ElectrotécnicaIn the last decade, mobile wireless communications have witnessed an explosive growth in the user’s penetration rate and their widespread deployment around the globe. In particular, a research topic of particular relevance in telecommunications nowadays is related to the design and implementation of mobile communication systems of 4th generation (4G). 4G networks will be characterized by the support of multiple radio access technologies in a core network fully compliant with the Internet Protocol (all IP paradigms). Such networks will sustain the stringent quality of service (QoS) requirements and the expected high data rates from the type of multimedia applications (i.e. YouTube and Skype) to be available in the near future. Therefore, 4G wireless communications system will be of paramount importance on the development of the information society in the near future. As 4G wireless services will continue to increase, this will put more and more pressure on the spectrum availability. There is a worldwide recognition that methods of spectrum managements have reached their limit and are no longer optimal, therefore new paradigms must be sought. Studies show that most of the assigned spectrum is under-utilized, thus the problem in most cases is inefficient spectrum management rather spectrum shortage. There are currently trends towards a more liberalized approach of spectrum management, which are tightly linked to what is commonly termed as Cognitive Radio (CR). Furthermore, conventional deployment of 4G wireless systems (one BS in cell and mobile deploy around it) are known to have problems in providing fairness (users closer to the BS are more benefited relatively to the cell edge users) and in covering some zones affected by shadowing, therefore the use of relays has been proposed as a solution. To evaluate and analyse the performances of 4G wireless systems software tools are normally used. Software tools have become more and more mature in recent years and their need to provide a high level evaluation of proposed algorithms and protocols is now more important. The system level simulation (SLS) tools provide a fundamental and flexible way to test all the envisioned algorithms and protocols under realistic conditions, without the need to deal with the problems of live networks or reduced scope prototypes. Furthermore, the tools allow network designers a rapid collection of a wide range of performance metrics that are useful for the analysis and optimization of different algorithms. This dissertation proposes the design and implementation of conventional system level simulator (SLS), which afterwards enhances for the 4G wireless technologies namely cognitive Radios (IEEE802.22) and Relays (IEEE802.16j). SLS is then used for the analysis of proposed algorithms and protocols.FC