Multicarrier Frequency Hopping Spread Spectrum Techniques With Quasi-Cyclic Low Density Parity Check Codes Channel Coding

This work presents a new proposed Multicarrier Frequency Hopping Spread Spectrum (MCFH-SS) system employing Quasi-Cyclic Low Density Parity Check (QC-LDPC) codes instead of the conventional LDPC codes. A new technique for constructing the QC-LDPC codes based on row division method is proposed. The new codes offer more flexibility in terms of high girth, multiple code rates and block length. Moreover, a new scheme for channel prediction in MCFH-SS system is proposed. The technique adaptively estimates the channel conditions and eliminates the need for the system to transmit a request message prior to transmitting the packet data. The ready-to-use channel will be occupied with a Pseudonoise (PN) code and use for transmission or else, it will be banned

Shuttle Communications and Tracking Systems Modeling and TDRSS Link Simulations Studies

An analytical simulation package (LinCsim) which allows the analytical verification of data transmission performance through TDRSS satellites was modified. The work involved the modeling of the user transponder, TDRS, TDRS ground terminal, and link dynamics for forward and return links based on the TDRSS performance specifications (4) and the critical design reviews. The scope of this effort has recently been expanded to include the effects of radio frequency interference (RFI) on the bit error rate (BER) performance of the S-band return links. The RFI environment and the modified TDRSS satellite and ground station hardware are being modeled in accordance with their description in the applicable documents

Study of spread spectrum multiple access systems for satellite communications with overlay on current services

The feasibility of using spread spectrum techniques to provide a low-cost multiple access system for a very large number of low data terminals was investigated. Two applications of spread spectrum technology to very small aperture terminal (VSAT) satellite communication networks are presented. Two spread spectrum multiple access systems which use a form of noncoherent M-ary FSK (MFSK) as the primary modulation are described and the throughput analyzed. The analysis considers such factors as satellite power constraints and adjacent satellite interference. Also considered is the effect of on-board processing on the multiple access efficiency and the feasibility of overlaying low data rate spread spectrum signals on existing satellite traffic as a form of frequency reuse is investigated. The use of chirp is examined for spread spectrum communications. In a chirp communication system, each data bit is converted into one or more up or down sweeps of frequency, which spread the RF energy across a broad range of frequencies. Several different forms of chirp communication systems are considered, and a multiple-chirp coded system is proposed for overlay service. The mutual interference problem is examined in detail and a performance analysis undertaken for the case of a chirp data channel overlaid on a video channel

Iterative receiver in multiuser relaying systems with fast frequency-hopping modulation

In this thesis, a novel iterative receiver and its improved version are proposed for relay-assisted multiuser communications, in which multiple users transmit to a destination with the help of a relay and using fast frequency-hopping modulation. Each user employs a channel encoder to protect its information and facilitate interference cancellation at the receiver. The signal received at the relay is either amplified, or partially decoded with a simple energy detector, before being forwarded to the destination. Under flat Rayleigh fading channels, the receiver at the destination can be implemented non-coherently, i.e., it does not require the instantaneous channel information to demodulate the users’ transmitted signals. The proposed iterative algorithm at the destination exploits the soft outputs of the channel decoders to successively extract the maximum likelihood symbols of the users and perform interference cancellation. The iterative method is successfully applied for both cases of amplify-and-forward and partial decode-and-forward relaying. The error performance of the proposed iterative receiver is investigated by computer simulation. Under the same spectral efficiency, simulation results demonstrate the excellent performance of the proposed receiver when compared to the performance of decoding without interference cancellation as well as the performance of the maximum likelihood multiuser detection previously developed for uncoded transmission. Simulation results also suggest that a proper selection of channel coding schemes can help to support significant more users without consuming extra system resources. In addition, to further enhance the receiver’s performance in terms of the bit error rate, an improved version of the iterative receiver is presented. Such an improved receiver invokes inner-loop iterations between the channel decoders and the demappers in such a way that the soft outputs of the channel decoders are also used to refine the outputs of the demappers for every outer-loop iteration. Simulation results indicate a performance gain of about 2.5dB by using the two-loop receiver when compared to the performance of the first proposed receiver

Space Station communications and tracking systems modeling and RF link simulation

In this final report, the effort spent on Space Station Communications and Tracking System Modeling and RF Link Simulation is described in detail. The effort is mainly divided into three parts: frequency division multiple access (FDMA) system simulation modeling and software implementation; a study on design and evaluation of a functional computerized RF link simulation/analysis system for Space Station; and a study on design and evaluation of simulation system architecture. This report documents the results of these studies. In addition, a separate User's Manual on Space Communications Simulation System (SCSS) (Version 1) documents the software developed for the Space Station FDMA communications system simulation. The final report, SCSS user's manual, and the software located in the NASA JSC system analysis division's VAX 750 computer together serve as the deliverables from LinCom for this project effort

Capacity and spectrum efficiency analysis of an asymmetric PMR system with DAB downlink

Cataloged from PDF version of article.Different trunked Private Mobile Radio (PMR) systems have been designed over the last several decades, all of which have symmetric downlink and uplink channel capacities. Due to this symmetry, those systems may not be spectrally efficient in case of different types of services which are only supported by PMR systems, such as group (acknowledged or unacknowledged) and broadcast calls, either voice or data. In this thesis, a new asymmetric trunked PMR system comprising an OFDM based broadband, wide-area downlink and a narrowband cellular uplink, is proposed to achieve a higher capacity and higher spectral efficiency than current digital trunked PMR systems have. This thesis concentrates on the system capacity analysis of the proposed system associated only with the downlink part for voice communications, as well as the spectrum efficiency comparison of the proposed system with the Terrestrial Trunked Radio (TETRA) system, which is accepted as the spectrally most efficient PMR system. In this study, we study the performance and capacity of the proposed system using Digital Audio Broadcasting (DAB) downlink. In particular, we study the capacity of such a system for voice calls using voice activity detection and statistical multiplexing. Moreover, we show that, the capacity of the system can significantly increase, if the incoming calls, which cannot find an available channel, are allowed to wait a certain amount of time before occupying a channel. The system is shown to have high trunking efficiency since all users are assumed to use the pool of channels available in the wideband downlink. Spectral efficiency of the proposed system and a standard TETRA system are compared using numerical case studies against different traffic loads, cell sizes and number of clusters. The optimum point, with respect to number of clusters, up to which the proposed system is more efficient, is determined. It is shown that for a realistic PMR scenario the proposed system is more efficient up to 5 clusters, i.e. 35 cells, and therefore it can be concluded that the proposed system can be used efficiently in realistic situations.Şengül, ErsinM.S

Reducing Communication Delay Variability for a Group of Robots

A novel architecture is presented for reducing communication delay variability for a group of robots. This architecture relies on using three components: a microprocessor architecture that allows deterministic real-time tasks; an event-based communication protocol in which nodes transmit in a TDMA fashion, without the need of global clock synchronization techniques; and a novel communication scheme that enables deterministic communications by allowing senders to transmit without regard for the state of the medium or coordination with other senders, and receivers can tease apart messages sent simultaneously with a high probability of success. This approach compared to others, allows simultaneous communications without regard for the state of the transmission medium, it allows deterministic communications, and it enables ordered communications that can be a applied in a team of robots. Simulations and experimental results are also included

Collaboration Enforcement In Mobile Ad Hoc Networks

Mobile Ad hoc NETworks (MANETs) have attracted great research interest in recent years. Among many issues, lack of motivation for participating nodes to collaborate forms a major obstacle to the adoption of MANETs. Many contemporary collaboration enforcement techniques employ reputation mechanisms for nodes to avoid and penalize malicious participants. Reputation information is propagated among participants and updated based on complicated trust relationships to thwart false accusation of benign nodes. The aforementioned strategy suffers from low scalability and is likely to be exploited by adversaries. To address these problems, we first propose a finite state model. With this technique, no reputation information is propagated in the network and malicious nodes cannot cause false penalty to benign hosts. Misbehaving node detection is performed on-demand; and malicious node punishment and avoidance are accomplished by only maintaining reputation information within neighboring nodes. This scheme, however, requires that each node equip with a tamper-proof hardware. In the second technique, no such restriction applies. Participating nodes classify their one-hop neighbors through direct observation and misbehaving nodes are penalized within their localities. Data packets are dynamically rerouted to circumvent selfish nodes. In both schemes, overall network performance is greatly enhanced. Our approach significantly simplifies the collaboration enforcement process, incurs low overhead, and is robust against various malicious behaviors. Simulation results based on different system configurations indicate that the proposed technique can significantly improve network performance with very low communication cost

HetNets with Random DTX Scheme: Local Delay and Energy Efficiency

Heterogeneous cellular networks (HetNets) are to be deployed for future wireless communication to meet the ever-increasing mobile traffic demand. However, the dense and random deployment of small cells and their uncoordinated operation raise important concerns about energy efficiency. On the other hand, discontinuous transmission (DTX) mode at the base station (BS) serves as an effective technology to improve energy efficiency of overall system. In this paper, we investigate the energy efficiency under finite local delay constraint in the downlink HetNets with the random DTX scheme. Using a stochastic geometry based model, we derive the local delay and energy efficiency in a general case and obtain closed-form expressions in some special cases. These results give insights into the effect of key system parameters, such as path loss exponents, BS densities, SIR threshold and mute probability on the system performance. We also provide the low-rate and high-rate asymptotic behavior of the maximum energy efficiency. It is analytically shown that it is less energy-efficient to apply random DTX scheme in the low-rate regime. In the high-rate regime, however, random DTX scheme is essential to achieve the finite local delay and higher energy efficiency. Finally, we extend the analysis to the loadaware DTX scheme where the mute probability depends on the user activity level