The theory of groups and group representations have been shown by several researchers to generate promising space-time codes for multiple-antenna links in wireless communication. In this report, potential codes from a root system in a real Euclidean space are examined. In particular, general properties of codes from the infinite irreducible systems A_l, B_l, C_l, and D_l are analytically studied, while codes from the exceptional systems G₂, F₄, E₆, E₇ and E8 are numerically determined. Some of these exceptional codes meet general coding-theoretic expectations, for example in terms of the number of code words. The effort described in this paper is part of a general attempt to assess the use of modular representations of groups, as opposed to classical, i.e. complex-numbered, representations, for antenna diversity. Java technologies are used for the implementation of our numerical experimentation platform.

IEEE 802.11g WLANs operating at 2.4 GHz face interference from Bluetooth which also uses the same frequency. IEEE 802.11g is an orthogonal frequency divison multiplexing (OFDM) based WLAN standard. Adaptive subcarrier selection (ASuS) involves using feedback from the receiver to dynamically allocate subcarriers for OFDM transmission. This paper proposes a method to avoid Bluetooth interference using ASuS. By adaptively choosing subcarriers for OFDM transmission, the frequencies used by Bluetooth can be avoided. Power level deviations in small groups of contiguous subcarriers with respect to other subcarriers in an OFDM symbol can be used as an indication of Bluetooth interference. Simulations show that as compared to the conventional OFDM technique, adaptive subcarrier selection results in significant reduction in the packet error rate.

High Frequency (HF) radio communication channels provide unique challenges to digital communication systems. Typical HF communication systems propagate electromagnetic energy in the 2-30MHz radio spectrum using the earth’s ionosphere and surface as both refractors and reflectors for non line of sight communications. Constantly changing ionosphere conditions result in multipath and severe fading channel characteristics. Through numerical simulation, small block length (520) Low Density Parity Check (LDPC) forward error correction codes used in conjunction with Orthogonal Frequency Division Multiplexing (OFDM) and Space-Time (ST) channel coding are shown to provide excellent communication performance across the HF channel. Average bit error rate performance results will be shown for a 2ms, 2Hz Watterson HF channel model as well as results for the independent Rayleigh fading channel. The space-time simulation parameters were chosen to model an Alamouti dual antenna transmit, single antenna receive communication system utilizing practical data and sampling rates.

Circular Trellis Coded Modulation (CTCM) defines a family of (block) trellis codes which use a unique algebraic constraint, imposed on the start state, to produce a strong tail-biting property without the inefficiency of driving the encoder state to zero by using a sequence of input zeroes. From the beginning of CTCM, elements of the Galois field, GF(p^m), have served dual roles, labeling both s ystem trellis nodes and valid input symbols. This dual use of field elements facilitates exploitation of the algebraic structure of GF(p^m). The system trellis always take a particularly simple and advantageous form (called the pⁿ-fly form) whenever the alphabet of valid input symbols is chosen to be (a coset of) any additive subgroup of the additive group structure of GF(p^m). This paperproposes a family of signal mappings that complete the definition of the CTCM system by providing structurally consistent output labels for the trellis edges. The completion of a structural definition greatly facilitates system analysis, especially the (future) geometrically precise construction of a related signal constellation. At the same time, it preserves the possibility of an advantegeous receiver structure.

Advances in wireless communications and the proliferation of mobile computing devices has led to the rise of a new type of computer network: the ad-hoc wireless network. Ad-hoc networks are characterized by a lack of fixed infrastructure, which give ad-hoc networks a great deal of flexibility, but also increases the risk of security problems. In wired networks, key pieces of network infrastructure are secured to prevent unauthorized physical access and tampering. Network administrators ensure that everything is properly configured and are on-hand to fix problems and deal with intrusions. In contrast, the nodes in an ad-hoc network are responsible for routing and forwarding data in the network, and there are no network administrators to handle potential problems. This makes an ad-hoc network more vulnerable to a misconfigured, faulty, or compromised node. We propose a means for a node in an ad-hoc network to detect and handle these malicious nodes by comparing data available to the routing protocol, such as cached routes in Dynamic Source Routing, ICMP messages, and transport layer information, such as TCP timeouts. This data can then be used along with network probes to isolate the malicious node.

Mobile ad hoc networking (MANET) supports self-organizing, mobile infrastructures and enables an autonomous network of mobile nodes that can operate without a wired backbone. Ad hoc networks are characterized by multihop, wireless connectivity via packet radios and by the need for efficient dynamic protocols. All routers are mobile and can establish connectivity with other nodes only when they are within transmission range. Importantly, ad hoc wireless nodes are resource-constrained, having limited processing, memory, and battery capacity. Delivery of high quality-ofservice (QoS), real-time multimedia services from Internet-based applications over a MANET is a challenge not yet achieved by proposed Internet Engineering Task Force (IETF) ad hoc network protocols in terms of standard performance metrics such as end-to-end throughput, packet error rate, and delay. In the distributed operations of route discovery and maintenance, strong interaction occurs across MANET protocol layers, in particular, the physical, media access control (MAC), network, and application layers. The QoS requirements are specified for the service classes by the application layer. The cross-layer design must also satisfy the battery-limited energy constraints, by minimizing the distributed power consumption at the nodes and of selected routes. Interactions across the layers are modeled in terms of the set of concatenated design parameters including associated energy costs. Functional dependencies of the QoS metrics are described in terms of the concatenated control parameters. New cross-layer designs are sought that optimize layer interdependencies to achieve the “best” QoS available in an energy-constrained, time-varying network. The protocol design, based on a reactive MANET protocol, adapts the provisioned QoS to dynamic network conditions and residual energy capacities. The cross-layer optimization is based on stochastic dynamic programming conditions derived from time-dependent models of MANET packet flows. Regulation of network behavior is modeled by the optimal control of the conditional rates of multivariate point processes (MVPPs); these rates depend on the concatenated control parameters through a change of probability measure. The MVPP models capture behavior of many service applications, e.g., voice, video and the self-similar behavior of Internet data sessions. Performance verification of the cross-layer protocols, derived from the dynamic programming conditions, can be achieved by embedding the conditions in a reactive routing protocol for MANETs, in a simulation environment, such as the wireless extension of ns-2. A canonical MANET scenario consists of a distributed collection of battery-powered laptops or hand-held terminals, capable of hosting multimedia applications. Simulation details and performance tradeoffs, not presented, remain for a sequel to the paper.

Clustering is necessary for data aggregation, hierarchical routing, optimizing sleep patterns, election of extremal sensors, optimizing coverage and resource allocation, reuse of frequency bands and codes, and conserving energy. Optimal clustering is typically an NP-hard problem. Solutions to NP-hard problems involve searches through vast spaces of possible solutions. Evolutionary algorithms have been applied successfully to a variety of NP-hard problems. We explore one such approach, Particle Swarm Optimization (PSO), an evolutionary programming technique where a 'swarm' of test solutions, analogous to a natural swarm of bees, ants or termites, is allowed to interact and cooperate to find the best solution to the given problem. We use the PSO approach to cluster sensors in a sensor network. The energy efficiency of our clustering in a data-aggregation type sensor network deployment is tested using a modified LEACH-C code. The PSO technique with a recursive bisection algorithm is tested against random search and simulated annealing; the PSO technique is shown to be robust. We further investigate developing a distributed version of the PSO algorithm for clustering optimally a wireless sensor network.

Traditionally, the measuring or monitoring system of manufacturing industries uses sensors, computers and screens for their quality control (Q.C.). The acquired information is fed back to the control room by wires, which - for obvious reason - are not suitable in many environments. This paper describes a method to solve this problem by employing the new Bluetooth technology to set up a complete new system, where a total wireless solution is made feasible. This new Q.C. system allows several line scan cameras to be connected at once to a graphical user interface (GUI) that can monitor the production process. There are many Bluetooth devices available on the market such as cell-phones, headsets, printers, PDA etc. However, the detailed application is a novel implementation in the industrial Q.C. area. This paper will contain more details about the Bluetooth standard and why it is used (nework topologies, host controller interface, data rates, etc.), the Bluetooth implemetation in the microcontroller of the line scan camera, and the GUI and its features.

An enhanced CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) protocol to be used in the Medium Access Control (MAC) layer of the IEEE 802.11 standard for wireless local area networks (wireless LANs) is proposed in this work. In wireless LANs, the CSMA/CA protocol supports asynchronous data transfer, and adopts an acknowledgement mechanism to confirm successful transmissions and a handshaking mechanism to reduce collisions. In both cases, a binary exponential backoff mechanism is used. The enhanced protocol improves the exponential backoff scheme by dynamically adjusting the contention window (CW) around the optimal value. Moreover, an analytical model based on the Markov chain is developed to analyze the system performance in terms of throughput and delay. Numerical results are presented to show the effect of the proposed backoff mechanism.

Traditional TCP performance degrades over lossy links, as the TCP sender assumes that packet loss is caused by congestion in the network path and thus reduces the sending rate by cutting the congestion window multiplicatively, and a mechanism to overcome this limitation is investigated in this research. Our scheme identifies the network path condition to differentiate whether congestion happens or not, and responds differently. The basic idea of separating congestion and non-congestion caused losses is to compare the estimated current available bandwidth and the average available bandwidth. To minimize the effect of temporary fluctuation of measurements, we estimate the available bandwidth with a higher weight on stable measurements and a lower weight on unstable fluctuations. In our scheme, packet loss due to congestion invokes the TCP Newreno procedure. In cases of random loss that is not related to congestion, the multiplicative decrease of the sending rate is avoided to achieve higher throughput. In addition, each duplicate acknowledgement after a fast retransmission will increase the congestion window to fully recover its sending rate. Extensive simulation results show that our differentiation algorithm achieves high accuracy. Accordingly, the TCP connection over lossy link with the proposed scheme provides higher throughput than TCP Newreno.

Quality of Service (QoS) is an important issue in the next generation wireless networks providing multimedia services. To address this complex problem, we consider QoS in wireless multimedia networks at two levels: connection-level QoS, measured by the connection blocking and dropping probabilities, and packet-level QoS, measured by bandwidth, delay/jitter and loss sensitivity. First, different multimedia applications are classified into three service classes, namely CBR, VBR and UBR, according to their bandwidth, delay/jitter and reliability requirements. Then, network can choose appropriate resource management schemes for different service classes accordingly in order to meet their QoS requirements. CBR traffic requires a constant bit rate available during the entire lifetime of the connection. Both connection-level and packet-level QoS is guaranteed by connection-oriented service. Thus the joint optimization problem can be reduced to optimization of connection-level QoS (handoff dropping probability and new call blocking probability) for CBR traffic. VBR traffic could generally tolerate a certain amount of delay or packet loss, thus improve resource utilization by connection-less service. We formulate the trade-off of the QoS parameters on both levels based on a VBR traffic model, and find the near-optimal solutions for the joint optimization problem. UBR traffic does not have specified requirement on QoS, thus being served by the best-effort service. Network uses whatever the resource not being used to improve the overall system utilizations.

The paper defines self-similarity for vector processes by employing the discrete-time continuous-dilation operation which has successfully been used previously by the authors to define 1-D discrete-time stochastic self-similar processes. To define self-similarity of vector processes, it is required to consider the cross-correlation functions between different 1-D processes as well as the autocorrelation function of each constituent 1-D process in it. System models to synthesize self-similar vector processes are constructed based on the definition. With these systems, it is possible to generate self-similar vector processes from white noise inputs. An important aspect of the proposed models is that they can be used to synthesize various types of self-similar vector processes by choosing proper parameters. Additionally, the paper presents evidence of vector self-similarity in two-channel wireless LAN data and applies the aforementioned systems to simulate the corresponding network traffic traces.

A new scheme to control the transmission power, allocate subcarriers, and choose modulation schemes for each mobile terminal in a multiuser OFDM system is proposed in this paper. This scheme will make OFDM systems more flexible and more robust to the channel variation along time. We divide the problem into two stages. In the first stage, we maximize the minimum signal-to-noise ratio (SNR) of subchannels subject two constraints. The total system performance can be maintained to an acceptable level in this stage. In the second stage, based on the result of the first stage, we choose the modulation schemes of subchannels of each mobile terminal in the hope that the total transmission rate can be maximized and the performance requirement of each mobile terminal can be satisfied. Simulations are conducted under a different total number of mobile terminals and a different SER (symbol error rate) requirements. Simulation results show that the proposed scheme works well under the simulated environments.

An integrated Mach-Zehnder interferometer made of electro-optic polymer, which has excellent broadband (>100 GHz) response, was fabricated as a mm-wave receive antenna. When an electric field is applied to the interferometer arm(s) made of EO material, a phase delay is generated which results in a net imbalance in the interferometer and thus a change in the output intensity. This output intensity change, which contains electric field strength and temporal profile information, is then read by a photodetector and processed. To test this antenna in free space, a micro-strip travelling electromagnetic cell, which has uniform electric field distribution in the 1 GHz range, was constructed. The test results show the antenna had good linear response over a 40 dB power range, at 1 GHz center frequency. The measured minimum detectable E-field strength was about 0.22 V/m (or 6.7 nW/cm²) at 1 kHz bandwidth with a laser power of 7.9 μWatt (-21dBm) measured after the sensor, which agrees with our theoretical calculations. The measured E-field signal increases with increasing laser power, which indicates that significant sensitivity improvement, can be easily obtained by lowering passive losses. The antenna was found to be thermally stable over a temperature range from -30 to 50 C. The antenna sensitivity can be further improved by lowering the device insertion loss, optimizing the photodetector and detection circuitry, and using EO polymers with higher electro-optic coefficients.

The effects of inter-carrier interference (ICI) and aperiodic random spreading sequences on the performance of asynchronous multicarrier code division multiple access (CDMA) systems with correlated fading among sub-carriers are investigated in this research. To obtain the maximal ratio combining (MRC) filter, random parameters including asynchronous delays, correlated Rayleigh fading and spreading sequences are averaged to find the unconditional covariance matrix of the interference-plus-noise vector. We demonstrate that the ICI in the system proposed by Kondo and Milstein can be mitigated by assigning a common random spreading sequence over all sub-carriers for each user, rather than using a set of distinct spreading sequences. Moreover, the analytic expression for the bit error probability (BEP) can be obtained with the Gaussian approximation. Simulation results are used to demonstrate the accuracy of our analysis. Finally, various design tradeoffs including the number of sub-carriers, fading correlations, ICI and multipath effect are also presented in simulation.

In this paper we introduce a new structured channel impulse response (CIR) estimation method for sparse multipath channels. We call this novel CIR estimation method Time-Of-Arrival based Blended Least Squares (TOA-BLS) which uses symbol rate sampled signals, based on blending the least squares based channel estimation and the correlation and cleaning followed by TOA estimation. TOA estimation is accomplished in the frequency domain and is based on AR model parameter estimation via unconstrained least squares. Simulation examples are drawn from the ATSC digital TV 8-VSB system. The delay spread for digital TV systemscan be as long as several hundred times the symbol duration; however digital TV channels are, in general, sparse where there are only a few dominant multipaths.

The recent rapid growth of multimedia communications has efficiently allowed delivering different services, formats and contents over an enormous variety of digital networks with IP acting as an integration protocol. The main objective of this research work is to evaluate the performance of an high frequency (HF) wireless network for transporting multimedia services according to UDP/IP protocol stack. Besides, 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. A typical transmission bandwidth is about 3 kHz resulting in a varying bit rate in the range between 75 and 12800 bps. This very low bit rate by itself imposes serious challenges for reliable real time multimedia communications. This paper discusses optimal combinations of channel coders, modulators and packet sizes in order to achieve the greatest throughput in function of the signal-to-noise ratio and HF channel conditions.

Error propagation is a serious concern when a decision feedback equalizer (DFE) is used in a communication system. This paper describes a method of mitigating the effects of error propagation by constraining the feedback tap coefficients. It is shown that the most natural method of constraining the feedback taps is to constrain the 1-norm of the tap vector. The paper also considers a constraint on the 2-norm of the feedback tap vector. The proposed method is demonstrated using the trellis coded 8-VSB system used by the ATSC terrestrial broadcast standard for digital television. Results show that the constraints do reduce error propagation in the DFE, but the performance is considerably better when a zero delay trellis decoder is used to determine the decisions in the feedback filter.

Infrared (IR) is a new medium for wideband wireless applications especially at indoors. Infrared has abundant bandwidth and does not require FCC approval enabling rapid deployment. Intensity modulated systems do not experience multipath fading. However, health regulations limit the laser output power [IEC 825]. Photodetectors with large photosensitive areas have high sensitivity but they also have high junction capacitance that limits the bandwidth. High ambient noise is another concern in infrared systems. Spreading the spectrum, which enables low power transmission, can be used to overcome these limitations. In this paper, we evaluate the performance of a direct spread CDMA type infrared wireless system in a multiuser environment considering: multi-user interference and inter-symbol-interference assuming different channel models. According to the simulation a BER of 10-7 can be achieved with a moderate bit rate when, the channel impulse response exponentially decays. The maximum bit rate, however, depends on the mean square value of the channel dispersion. Lab prototype agrees with the theory and shows ambient light is the biggest concern in a pico cellular environment with few users.

A doubly optimal binary signature set is a set of binary spreading sequences that can be used for code division multiplexing purposes and exhibits minimum total-squared-correlation (TSC)and minimum maximum-squared-correlation (MSC) at the same time. In this article, we focus on such sets with signatures of odd length and we derive closed-form expressions for the signature cross-correlation matrix, its eigenvalues, and its inverse. Then, we derive analytic expressions for (i) the bit-error-rate (BER) upon decorrelating processing,(ii) the maximum achievable signal-to-interference-plus-noise (SINR) ratio upon minimum-mean-square-error (MMSE) filtering, and (iii) the total asymptotic efficiency of the system. We find that doubly optimal sets with signature length of the form 4m+1, m=1, 2,..., are in all respects superior to doubly optimal sets with signature length of the form 4m-1 (the latter class includes the familiar Gold sets as a small proper subset). "4m+1" sets perform practically at the single-user-bound (SUB) after decorrelating or MMSE processing (not true for "4m-1" sets). The total asymptotic efficiency of "4m+1" sets is lower bounded by 2/e for any system user load. The corresponding lower bound for "4m-1" sets is zero.

In this paper we quantify theoretically the effect of the desired-signal power level on the mean square filter estimation error and the normalized output signal-to-interference-plus-noise-ratio (SINR) of sample matrix inversion (SMI)-type estimates of the minimum mean-square-error (MMSE) and the linearly constrained minimum variance (LCMV) filters. We prove that in finite data support situations filters that utilize a sample average estimate of the desired-signal-absent input correlation matrix exhibit superior normalized filter output SINR and mean square filter estimation error when compared to filters that utilize a sample average estimate of the desired-signal-present input correlation matrix. Finally, we investigate pilot-assisted and decision-directed adaptive filter implementations that exhibit near desired-signal-absent SMI-filtering performance while they are trained using desired-signal-present data/observations.

In this work we consider the problem of detecting the information bit of a direct-sequence code-division-multiple-access (DS-CDMA) user in the presence of spread spectrum interference and AWGN using a multi-layer perceptron neural network receiver. We develop a fast converging adaptive training algorithm that minimizes the mean square error (MSE) at the output of the receiver. The proposed adaptive algorithm has two key features: (i) it utilizes constraints that are derived from properties of the optimum single-user decision boundary for AWGN multiple-access channels, and (ii) it embeds importance sampling principles directly into the receiver optimization process. Simulation studies illustrate the BER performance of the proposed scheme.

An array processing approach to joint PN (pseudo-noise) code acquisition and DOA (direction-of-arrival) estimation in asynchronous direct-sequence code-division multiple access (DS-CDMA) systems is proposed. This problem is traditionally treated as a 2-D search problem in the space-time domain. Our approach transforms the 2-D space-time matrix data from the antenna output to several time domain vectors using a set of beams steering to different directions. Then, the acquisition process searches from these 1-D temporal vectors to provide soft information for the next stage, i.e. DOA estimation. Our algorithm is a blind approach, where the training sequence is not needed. Besides, the number of antennas required can be much less than that of the incoming signals, and the DOA search range can be largely reduced based on the soft information from the acquisition process. Numerical simulations are presented to demonstrate that the proposed solution is resistant to the near-far effect and robust to the change of the fading environment.

The multi-code CDMA problem, and the corresponding data and noise model is discussed. The problem of "crosstalk", which occurs when using an MMSE receiver in colored noise, is addressed. A method to avoid "cross-talk", by choosing the eigenvectors of the interference plus noise matrix as codes, is introduced. This approach is shown to produce codes that are orthogonal not only in the conventional sense, but also with respect to the interference plus noise matrix. The gain on the codes is shown to be inversely proportional to the corresponding eigenvalue. Simulation results demonstrate that the codes corresponding to the smallest eigenvalues dramatically out-perform the MMSE receiver for Walsh-Hadamard codes in terms of mean-square error (MSE).