The effects of frequency offsets and aperiodic random spreading sequences on the performance of asynchronous multicarrier code division multiple access (CDMA) systems with correlated fading are investigated in this research. Random parameters including asynchronous delays, correlated Rayleigh fading and spreading sequences are averaged to determine the covariance matrix of the interference-plus-noise vector. An analytic expression for the average signal-to-interference-plus-noise ratio (SINR) is obtained. Furthermore, average bit error probability (BEP) based on the Gaussian approximation is also derived. We show that the system degrades significantly if the frequency offset of the desired user is present, while the system is insensitive to interferers’ frequency offsets. In consequence, it is crucial to estimate and compensate the desired user’s synchronization imperfection. Finally, design tradeoff among the number of sub-carriers, fading correlations, and inter-carrier interference (ICI) are presented in simulation results.

A new way of implementation of MMSE channel estimator for a GMSK signals with Turbo code over multipath fading channels is proposed in the paper. From the recent research results, it is clear that the usage of Turbo Codes with GMSK modulation is highly desirable because some signal processing modules in the receiver can be reused for both turbo decoder and equalization, and thus the receiving complexity can be reduced. Channel estimator is one of the modules that are required by the equalizer and the Turbo code decoder. In this paper we present an estimation technique, used for both GMSK equalization and Turbo decoder (SOVA/Log-MAP) over the multipath fading channel. The proposed MMSE estimator replaces the matched filters that are usually applied at the receiver end. The simulation results show that unified MMSE estimator gives us an edge over the disadvantages of the matched filter and conventional isolated estimators, such that the bit error rate performance is improved and the distorted data could be easily retrieved from the estimation technique.

We provide an iterative and a non-iterative channel impulse response (CIR) estimation algorithm for communication receivers with multiple-antenna. Our algorithm is best suited for communication systems which utilize a periodically transmitted training sequence within a continuous stream of information symbols, and the receivers for this particular system are expected work in a severe frequency selective multipath environment with long delay spreads relative to the length of the training sequence. The iterative procedure calculates the (semi-blind) Best Linear Unbiased Estimate (BLUE) of the CIR. The non-iterative version is an approximation to the BLUE CIR estimate, denoted by a-BLUE, achieving almost similar performance, with much lower complexity. Indeed we show that, with reasonable assumptions, a-BLUE channel estimate can be obtained by using a stored copy of a pre-computed matrix in the receiver which enables the use of the initial CIR estimate by the subsequent equalizer tap weight calculator. Simulation results are provided to demonstrate the performance of the novel algorithms for 8-VSB ATSC Digital TV system. We also provide a simulation study of the robustness of the a-BLUE algorithm to timing and carrier phase offsets.

In this work we first analyze two methods of finding low rank solutions: Steering Independent Conjugate Gradient (SI-CG) and Steering Dependent Conjugate Gradient (SD-CG). These low-rank beamformers have a rank where the output SINR is maximized and a large drop in the output SINR can occur if the beamformer operates at an improper rank. Indirect Dominant Mode Rejection (IDMR) is proposed wherein one first employs a high-resolution spatial spectrum estimation technique to estimate the directions and powers of the dominant interferers. Subsequently, this information is used to construct an estimate of the signal-free (interference plus noise only) autocorrelation matrix (for a given look-direction.) In this process, any residual correlations between the interferers and the signal arriving from the look-direction due to finite sample averaging is effectively removed. Simulations reveal that IDMR yields a dramatic improvement in output SINR relative to CG and PCI/DMR, even when the latter operate at the optimal rank. In the case of correlated signals IDMR shows not to degrade the output SINR, different from CG and PCI/DMR.

An approach to the design of LDPC (low density parity check) error-correction and space-time modulation codes involves starting with known mathematical and combinatorial structures, and deriving code properties from structure properties. This paper reports on an investigation of unital and oval configurations within generic symmetric combinatorial designs, not just classical projective planes, as the underlying structure for classes of space-time LDPC outer codes. Of particular interest are the encoding and iterative (sum-product) decoding gains that these codes may provide. Various small-length cases have been numerically implemented in Java and Matlab for a number of channel models.

This paper evaluates the performance of a high frequency (HF) wireless network for transporting packet multimedia services. Beyond of allowing civil/amateur communications, HF bands are also used for long distance wireless military communications. Therefore, our work is based on NATO Link and Physical layer standards, STANAG 5066 and STANAG 4539 respectively. At each HF channel, a typical transmission bandwidth is about 3 kHz with the resulting throughput bit rate up to 12800 bps. This very low bit rate by itself imposes serious challenges for reliable and low delay real time multimedia communications. Thus, this paper discusses the performance of a real time communication system designed to allow an end-to-end communication through “best effort” networks. With HF channel diversity, the packet loss percentage, on average considering three channel conditions, is decreased by 16% in the channel SNR range from 0 to 45 dB.

In this paper, multi-channel estimation schemes for a GMSK-based system with transmit diversity (space-time coding) are presented. For such a system, the channel information (impulse response) is critical for both space-time decoding and equalization at the receiver. Three non-blind estimation schemes, which decompose the channel in the process, are proposed for the GMSK receiver to obtain the impulse response of each of the multipath channels (i.e. transmit antennas): oversampling deconvolution, minimum mean-square error, and joint adaptive and correlation estimation. Since the received signal is the sum of emitted GMSK signals, interference cancellation is employed to facilitate the estimation process. Three cancellation algorithms, including direct cancellation, mean-square cancellation, and iterative cancellation, combined with each channel estimation method are investigated and compared. The estimated channel information will feed to the receiver consisting of space-time decoder and equalizer to decode the symbols of interest. Two receiver architectures are investigated in this paper, where the first design is the space-time decoder followed by the equalizer, the other is in the reverse way (equalizer followed by space-time decoder). In each of the two receiver architectures, the channel estimation needs additional modification and so does the equalizer. The equalizer in the design is a maximum likelihood sequence estimation (MLSE) based on Viterbi algorithm. To prove the concept and algorithms, both simulation and hardware implementation are performed. From the experimental results, it is shown that all the channel estimation algorithms can produce acceptable impulse response for space-time decoding and equalizer, in which the joint adaptive estimation with iterative cancellation is superior to the others. It is also shown that the diversity gain of this transmit diversity system is as good as a system with the same degree of receive diversity.

A reduced-rank space-time channel estimation scheme is presented for uplink multiuser DS-CDMA (direct-sequence code-division multiple access) systems in dispersive MIMO (multiple input and multiple output) channels. The proposed method was based on the AP method with training sequences. It is shown that both the mean squared error of channel estimation and the bit error probability can be improved via rank order selection. The optimal performance can be achieved using the generalized likelihood ratio test (GLRT).

In this paper, a brief overview of the power control problem will be presented along with different directions in power control design. In addition, a new approach to power control design inspired from control theory will be introduced. In this approach, linear quadratic control will be utilized to determine the transmission power of each node in the network. The system states will represent the signal quality and the error between the signal-to-interference level and the desired target. The necessary modifications to the IEEE 802.11 MAC layer to incorporate the proposed power control algorithm will be described.

Successful power management in a wireless sensor network requires optimization of the protocols which affect energy-consumption on each node and the aggregate effects across the larger network. System optimization for a given deployment scenario requires an analysis and trade off of desired node and network features with their associated costs. The sleep protocol for an energy-efficient wireless sensor network for event detection, target classification, and target tracking developed at Sandia National Laboratories is presented. The dynamic source routing (DSR) algorithm is chosen to reduce network maintenance overhead, while providing a self-configuring and self-healing network architecture. A method for determining the optimal sleep time is developed and presented, providing reference data which spans several orders of magnitude. Message timing diagrams show, that a node in a five-node cluster, employing an optimal cyclic single-radio sleep protocol, consumes 3% more energy and incurs a 16-s increase latency than nodes employing the more complex dual-radio STEM protocol.

When wireless sensors are capable of variable transmit power and are battery powered, it is important to select the appropriate transmit power level for the node. Lowering the transmit power of the sensor nodes imposes a natural clustering on the network and has been shown to improve throughput of the network. However, a common transmit power level is not appropriate for inhomogeneous networks. A possible fitness-based approach, motivated by an evolutionary optimization technique, Particle Swarm Optimization (PSO) is proposed and extended in a novel way to determine the appropriate transmit power of each sensor node. A distributed version of PSO is developed and explored using experimental fitness to achieve an approximation of least-cost connectivity.

Genetic Algorithms are powerful tools, which when set upon a solution space will search for the optimal answer. These algorithms though have some associated problems, which are inherent to the method such as pre-mature convergence and lack of population diversity. These problems can be controlled with changes to certain parameters such as crossover, selection, and mutation. This paper attempts to tackle these problems in GA by having another GA controlling these parameters. The values for crossover parameter are: one point, two point, and uniform. The values for selection parameters are: best, worst, roulette wheel, inside 50%, outside 50%. The values for the mutation parameter are: random and swap. The system will include a control GA whose population will consist of different parameters settings. While this GA is attempting to find the best parameters it will be advancing into the search space of the problem and refining the population. As the population changes due to the search so will the optimal parameters. For every control GA generation each of the individuals in the population will be tested for fitness by being run through the problem GA with the assigned parameters. During these runs the population used in the next control generation is compiled. Thus, both the issue of finding the best parameters and the solution to the problem are attacked at the same time. The goal is to optimize the sensor coverage in a square field. The test case used was a 30 by 30 unit field with 100 sensor nodes. Each sensor node had a coverage area of 3 by 3 units. The algorithm attempts to optimize the sensor coverage in the field by moving the nodes. The results show that the control GA will provide better results when compared to a system with no parameter changes.

The impact of the non-uniform individual sensor node lifetime on the connectivity of a data gathering tree over time is studied in this research. The lifetime of sensor devices depends on the device failure rate and/or battery energy depletion, and surviving nodes may not preserve the uniform node density across the network as nodes age. We first examine the general node aging problem by considering the energy consumption rate and the node failure rate. The energy consumption rate in a data gathering tree is presented with or without data aggregation. The nodes in each hop level show a different energy depletion rates even with data aggregation, which is studied by mathematical analysis as well as simulation results. Then, the resulting non-uniform connectivity over time in a data gathering tree is examined with a node's survivor function. It is shown by mathematical analysis and simulation results that the node aging process has a significant impact on the connectivity as the hop distance increases.

The need for a robust predictive sensor communication network inspired this research. There are many critical issues in a communication network with different data rate requirements, limited power and bandwidth. Energy consumption is one of the key issues in a sensor network as energy dissipation occurs during routing, communication and monitoring of the environment. This paper covers the routing of a sensor communication network by applying an evolutionary algorithm -- the ant system. The issues considered include optimal energy, data fusion from different sensor types and predicting changes in environment with respect to time.

In the distributed operations of route discovery and maintenance, strong interaction occurs across mobile ad hoc network (MANET) protocol layers in the transport of service packets between source and destination nodes. Quality of service (QoS) requirements are specified for multimedia service classes by the application layer. Due to node mobility, knowledge of their locations is inherently incomplete. Network topology is thus only partially known in time and in space. Cross-layer interactions in the protocol are modeled in terms of a set of concatenated design parameters, updates on node positions and associated energy costs. Functional dependencies of the QoS metrics on the concatenated parameters are described. An analytical framework is established for development of new cross-layer designs that optimize layer interdependencies to achieve the “best” QoS available in the MANET given a partially known, time-varying topology. The designs, based on a reactive MANET protocol, adapt provisioned QoS to dynamic network conditions and residual energy capacities. Cross-layer optimization, given partially known topology, is based on stochastic dynamic programming and predictive estimation conditions derived from time-dependent models of MANET behavior. Models of real-time behavior are based on the control of conditional rates of multivariate point processes (MVPPs). These rates depend on the concatenated protocol and resource parameters. The analytical framework supports predictive models to estimate node mobility in addition to measurement-based estimates of probability distributions for voice, video, data, and other Internet traffic. Estimates of QoS metrics are given in terms of recursive stochastic filters of the network state, based on complete or partial observations of topology and events.

Packet dropping in Mobile Ad-hoc Networks could be a result of wireless link errors, congestion, or malicious packet drop attack. Current techniques for detecting malicious behavior either do not consider congestion in the network or are not able to detect in real time. Further more, they usually work at network layer. In this paper, we propose a TCP-Manet protocol, which reacts to congestion like TCP Reno protocol, and has additional capability to distinguish among congestion, wireless link error, and malicious packet drop attack. It is an end-to-end mechanism that does not require additional modifications to the nodes in the network. Since it is an extension of existing TCP protocol, it is compatible with existing protocols. It works in conjunction with the network layer and an unobtrusive monitor to assist the network in the detection and characterization of the nature of the behavior. Experimental results show that TCP-Manet has the same performance as that of TCP-Reno in wired network, and performs better in wireless ad-hoc networks in terms of throughput while having good detection effectiveness.

Rapid technological advances in wireless communication have made it possible for networking sensor devices. Given the low computation and battery power capacities of these sensor nodes, the key design factors of network protocols are self-configuring, energy-efficient, adaptive, and scalable. We presented the multi-hop infrastructure network architecture (MINA) for a wireless sensor network consisting of a few hundred sensors that communicate data to a base station (BS). We designed a Unified Network Protocol Framework for MINA that encompasses network organization, medium access control (MAC) and routing protocols. In this paper, we improve it by adaptively varying transmission range to maintain network connectivity. It is a derivative-free optimization algorithm. The BS periodically evaluates the objective function, chooses the appropriate transmission range and broadcasts it to the sensor nodes that then update the transmission range. The advantages are: (i) Avoids the disconnectivity; (ii) Maximizes the number of nodes that can be connected to the BS, (iii) Minimizes the energyxdelay metric and (iv) Avoids the "hot-spot" nodes in the network. The performance in terms of delay, throughput, energy consumption and network lifetimes, is studied in detail using discrete-event simulation compared with other protocol. The results show that it is energy efficient in a large scale network.

A maximum likelihood estimator (MLE) that jointly estimates the carrier frequency offset (CFO) and the channel response of each user in uplink OFDMA systems is investigated in this research. The proposed MLE distinguishes itself from existing methods by its applicability to more flexible carrier assignment schemes. It achieves high computational efficiency by transforming a multidimensional optimization problem into a one-dimensional optimization problem. A suboptimal method is developed to further reduce the computational complexity. It is demonstrated by simulation results that the proposed MLE can provide accurate CFO and channel estimation in both SISO and SIMO environments.

A symbol-by-symbol maximum likelihood (ML) detection scheme for multicarrier (MC) systems is proposed in this work. When the number of subchannels is sufficiently large, the received symbols across all subchannels are approximately uncorrelated. Then, the proposed symbol-by-symbol ML detection, which is obtained by minimizing the symbol error probability, is nearly optimal. Furthermore, we will show how to reduce the complexity of the symbol-by-symbol ML detection when the constellation size is large. Simulation results show that the proposed symbol-by-symbol ML detection scheme outperforms the symbol-by-symbol minimum distance (MD) detection scheme by up to 2 dB in a noisy environment with crosstalk such as the DMT-ADSL system.

A new scheme to control the transmission power, allocate subcarriers, and choose modulation schemes for each mobile terminal in a multiuser OFDM system is investigated in this work. This proposed scheme attempts to make OFDM systems more flexible and robust to channel variations along time. The problem is decomposed to two stages. In the first stage, we maximize the minimum signal-to-noise ratio (SNR) of subchannels subject to constraints. The total system performance should be maintained at an acceptable level in this stage. Based on the result of the first stage, we choose modulation schemes for subchannels of each mobile terminal with an objective to maximize the total transmission rate while satisfying the performance requirement of each mobile terminal in the second stage. Simulations are conducted under a varying number of mobile terminals with a range of symbol error rates (SERs). Simulation results show that the proposed scheme works well under certain operating environments.

In this paper, a subspace-based semi-blind channel identification scheme is developed for a zero padded-orthogonal frequency division multiplexing (ZP-OFDM) system with asynchronous interferers. In our model of multiuser OFDM signals, an asynchronous interferer has a guard interval within the transmitted data block when the observation window is synchronized with respect to the desired user. As a result, the interfering user's channel vector lies in a different null space relative to the desired user's subspace, allowing the desired user's channel vector to be separated and determined uniquely up to a multiplicative scalar. However, in practice, a noisy sample correlation matrix produces a number of small eigenvalues, which can cause poor performance when conventional subspace techniques are employed. This problem is overcome by using several eigenvectors corresponding to the smallest eigenvalues; the channel estimates are obtained by linear combinations of these vectors. In order to find the linear combining coefficients, known pilot symbols are required. However, the pilot-only estimation method is not feasible when the channel order and the number of active users are large. Therefore, we use the preceding method to develop a subspace-based semi-blind algorithm that guarantees estimation of every user's channel vector. Simulation results demonstrate that the semi-blind channel estimator offers more than a 2dB gain over pilot-only channel estimation in the bit error rate (BER) performance.

This paper will investigate differential space frequency coding and its applicability to multipath fading High Frequency (HF) radio channels. Orthogonal Frequency Division Multiplexing (OFDM) will be combined with differential Alamouti space frequency codes to measure performance on the Watterson HF channel model. Differential coding facilitates non-coherent reception and can thus also reduce receiver complexity. Numerical results will be shown for the CCIR poor (2ms, 1Hz) and extra poor (2ms, 2Hz) channel conditions for a system comprised of 2 transmit antennas and a single receive antenna. For comparison, performance will also be shown vs. a new single transmit and receive antenna HF OFDM Code Division Multiple Access (CDMA) scheme.

One of the key limitations of Orthogonal Frequency Division Multiplexing (OFDM) waveforms when used for data transmission on multipath fading channels is frequency selective fading. This type of fading can cancel out or severely degrade the signal strength of many of the OFDM tones, producing an irreducible error rate. In the early 1990’s, researchers combined some of the characteristics of Code Division Multiple Access (CDMA) and Spread Spectrum (SS) with OFDM in order to create a more robust modulation scheme which could survive frequency selective fading and thus OFDM-CDMA was born. This paper will investigate the performance of OFDM-CDMA waveforms using different modulation schemes on HF multipath fading channels. In addition, several Multi-User Detection (MUD) schemes will be applied in the demodulation process to determine their benefits.

Cascaded Integrator Comb (CIC) filters are one of the most economical multirate filters widely used as decimators in digital receivers. Size/power improvements in this crucial unit of a digital receiver may essentially improve system performance including battery lifetime of the wireless portable system. Since, the only block in this filter structure that can be improved for better size/power performance is the adder, we have designed several CIC decimators using different means of arithmetic and the designs are synthesized for Altera's Field Programmable Gate Arrays (FPGAs). The arithmetic schemes used are two's complement addition, Carry Save Adder (CSA) using parallel counter logic and the Modified Carry Save Adder (MCSA) that incorporates Wallace tree structure. Each of these CIC decimators is a 16 bit I/O bit-width, 5-stage design with a rate change factor of 1000. Due to the presence of the integrators, the internal required bit-width is 66 bits. In order to maintain the same number of input and output bit-widths, two different Pruning schemes are used, pruning done at each stage of the design and pruning at the final stage alone. Results of synthesis are tabulated for the individual adder designs and the CIC designs with both the pruning schemes for all possible synthesis options. Depending on the requirements of the application, the results of the synthesis can be used to choose a CIC decimator that consumes less silicon or a design that provides better speed or a design which is most cost effective in terms of area/speed product.

The Hybrid Emergency Radiation Detection (HERD) system is a rapidly deployable ad-hoc wireless sensor network for monitoring the radiation hazard associated with a radiation release. The system is designed for low power, small size, low cost, and rapid deployment in order to provide early notification and minimize exposure. The many design tradeoffs, decisions, and challenges in the implementation of this wireless sensor network design will be presented and compared to the commercial systems available. Our research in a scaleable modular architectural highlights the need and implementation of a system level approach that provides flexibility and adaptability for a variety of applications. This approach seeks to minimize power, provide mission specific specialization, and provide the capability to upgrade the system with the most recent technology advancements by encapsulation and modularity. The implementation of a low power, widely available Real Time Operating System (RTOS) for multitasking with an improvement in code maintenance, portability, and reuse will be presented. Finally future design enhancements technology trends affecting wireless sensor networks will be presented.

As the war on terrorism continues abroad, the need to maintain security at home remains a major concern. Every aspect of the security effort can benefit from rapid information transfer. Advances in Commercial-Off-the-Shelf (COTS) radios make them valuable in applications ranging from deployed sensors on unsecured borders, to organizing an emergency response team. Additionally, the reduced cost and availability of these transceivers make them a viable alternative to custom developments. As an example, COTS radios can serve as the communication element in Unattended Ground Sensor (UGS) and munitions systems. Other applications include networked sonobuoys and networked RFID tracking. An additional advantage of COTS solutions is that they are available in a small form factor. They have extremely small mechanical outlines and are easily installed in systems requiring miniature designs and light payloads such as man-pack emplaced radios for the Army’s Future Combat System (FCS), as well as missions carried out by Unmanned Aerial Vehicles (UAV). The availability of the COTS radios result in a low-cost alternative for communication links that are applicable to military and Homeland Defense projects. They provide an easily obtainable, low-cost radio alternative enabling quick design cycles to meet mission timeliness. COTS solutions eliminate months from a typical design cycle and have many features already implemented such as ad-hoc routing and encryption. This enables robust hardware to be fielded quickly when a new need arises. The focus of this paper is to identify different COTS modules that can efficiently and cost effectively be applied to these and other various applications.

Wireless sensor network has attracted considerable research attention as the world becomes more information oriented. This technology provides an opportunity of innovations in traditional industries. Management and control of streetlight system is a labor-intensive high-cost task for public facility operations. This paper applies wireless sensor network technology in streetlight monitoring and control. Wireless sensor networks are employed to replace traditional physical patrol maintenance and manual switching on every lamp in the street or along the highway at the aim of reducing the maintenance and management expense. Active control is used to preserve energy cost while ensuring public safety. A proof-of-concept network architecture operated at 900 MHz industrial, scientific, and medical (ISM) band is designed for a two-way wireless telemetry system in streetlight remote control and monitoring. The radio architecture, multi-hop protocol and system interface are discussed in detail. MOTES sensor nodes are used in simulation and experimental tests. Simulation results show that the sensor network approach provides an efficient solution to monitor and control lighting infrastructures through wireless links. The unique application in this paper addresses an immediate need in streetlight control and monitoring, the architecture developed in this research could also serve as a platform for many other applications and researches in wireless sensor network.

Lockheed Martin and the University of Central Florida (UCF) are jointly investigating the use of a network of COTS video cameras and computers for a variety of security and surveillance operations. The detection and tracking of humans as well as vehicles is of interest. The three main novel aspects of the work presented in this paper are (1) the integration of automatic target detection and recognition techniques with tracking (2) the handover and seamless tracking of objects across a network, and (3) the development of real-time communication and messaging protocols using COTS networking components. The approach leverages the previously developed KNIGHT human detection and tracking system developed at UCF, and Lockheed Martin’s automatic target detection and recognition (ATD/R) algorithms. The work presented in this paper builds on these capabilities for surveillance using stationary sensors, with the goal of subsequently addressing the problem of moving platforms.

A scheme that uses the hidden Markov model (HMM) is proposed in this work to detect unauthorized nuisance packets in IP networks, which waste network resources and may result in the denial of service (DoS) attack. The proposed HMM is designed to differentiate the attack traffic from the normal traffic systematically. The design of the basic HMM model is first introduced, and the operations of the detector are then described in detail. Finally, we show that the detector using HMM is not sensitive to various attack types and able to detect the attack at an earlier stage.

Next generation communication networks will be required to support many multimedia services including voice, video and data. Since, bandwidth is an extremely valuable and scarce resource in multimedia networks, efficient bandwidth management is necessary in order to provide high quality service to users. Some applications need bandwidth reservations at an earlier time so that desired QoS requirement could ben ensured in the future. In this paper, we propose a new bandwidth management algorithm for advance reservation. The main design principle underlying our algorithm is adaptive online control based on real time estimation. This online approach is dynamic and flexible that responds to current network conditions. Therefore, our scheme is able to resolve conflicting QoS requirements while ensuring efficient network peformance. With a simulation study, we demonstrate the excellent performance of our scheme under widely diverse traffic load intensities.

This paper proposes an improved form of authentication security for Mobile IPv6 called buddy enhanced return routability. Enhanced return routability builds on the foundation of return routability and extends that technique through the use of stochastic route selection for authentication messages. Theoretical work and simulation results show that enhanced return routability strengthens authentication security without adding restrictive overhead requirements or opening up new security holes.

Multiple channel access interference is a major cause of throughput degradation in wireless networks because of the shared channel. IEEE 802.11 MAC protocol is a standard for medium access in wireless LANs, but suffers from contention and co-channel interference and thus performs poorly. We propose to use receiver-initiated MAC protocol, instead of the sender-oriented 802.11, to address these issues. The proposed protocol is based on carrier sensing and resolves collisions among senders based on a deterministic tree splitting algorithm to minimize the retransmissions of data packets due to hidden terminals. Further, subchannel assignment is used to exploit the parallel transmissions that are possible in multi-channel networks; thus, reducing the channel contention and improving the throughput. We also present simulation results, using ns-2 simulator, to evaluate our approach and compare it with 802.11. The simulation results indicate that collision resolution with multiple subchannel access is more effective in throughput and provides better packet delays than 802.11 MAC. We also observe that the maximum throughput is achieved when the channel is divided into three or four subchannels irrespective of the size and shape of the network and traffic load.

Next generation high-capacity (> 40 Gb/s) optical communication systems, whether for fiber or free-space transmission, will likely require high spectral efficiency techniques that enable optimization of performance, SWAP (size, weight and power), as well as the support of critical features such as transmission security. Modulation techniques with high number of bits per symbol coding such as differential quaternary phase-shift-keying (DQPSK) at lower symbol rates are more advantageous than conventional binary on-off keyed format with one bit per symbol coding. DQPSK is more robust to impairments of the transmission medium such as fiber dispersion than 40 Gb/s on-off keying. DQPSK is also less vulnerable to eavesdropping than direct-detection of on-off keyed signals at the physical layer. We report results of a 2-bit/s/Hz spectral efficiency multi-channel transmission system with four bits per symbol coding using 12.5 Gsymbol/s DQPSK and polarization multiplexing by bit-interleaved orthogonal polarizations. Transmission of 14×40 Gb/s 20-GHz spaced optical channels over 4×102 km of standard SMF-28 optical fiber with bit-error-ratio below the forward-error-correction code threshold (10^-3) required for error-free operation was achieved. Key enabling components used in the transmission experiment are: DQPSK optical modulator and demodulator, channel demultiplexing tunable band-pass optical filter, and balanced photoreceiver.

Signal space diversity is a power and bandwidth efficient diversity technique. To exploit the signal space diversity, joint maximum-likelihood (ML) detection at the receiver is usually needed, where the complexity grows expontentially with the dimension of a lattice. In this paper, we propose a serial concatenated scheme and two simple iterative methods to exploit the signal space diversity. The simple iterative methods are based on the idea of soft interference cancellation. The first iterative method is based on a scalar Gaussian approximation while the second one is a vector Gaussian approximation. The complexity of the first iterative method grows linearly with the dimension of the lattice, and the simulations show that when the dimension of the lattice N = 32, at BER = 10^-5, the performance gap between the Rayleigh fading channel and the Gaussian channel is only 0.3 dB. The complexity of the second iterative method grows cubically with the dimension of the lattice and the simulations show that its performance approaches that of the optimal MAP detection method.

This paper presents an algorithm for deriving uniformly spaced samples from nonuniformly spaced samples of a signal (and visa versa). The nonuniformity must be a function of linear sampling i.e. to derive uniformly spaced samples from an exponentially sampled signal. One place this situation occurs is in the calculation of some integral transforms like the Hankel transform. The algorithm runs quickly, in linear time and can thus be used with the FFT to create fast transform algorithms and provide a superior alternative to linear interpolation.