The novel use of a Maximum Likelihood Decision-Directed (MLDD) synchronization scheme for a Wavelet Packet Modulation (WPM) System is discussed. MLDD synchronization allows symbol synchronization without edge detection. Multi-Carrier Modulation (MCM) techniques are being increasingly employed in military communication networks to combat time-dispersive and time-variant channel effects. One MCM method, Coded Orthogonal Frequency Division Multiplexing, has been particularly effective against multipath fading environments. Wavelet packet modulation has recently been introduced to address the need for improved transmission performance over channels with joint time and frequency interference components. Wavelet Packet Modulation's strength rests in a flexible, custom mapping of the desired signal on the communications channel at the transmitter to avoid a variety of a priori noise/interference patterns. The use of wavelet packet modulation on wireless channels has been hindered due to symbol synchronization not being achievable with conventional edge detection techniques. A MLDD receiver symbol timing recovery algorithm is presented here that should help expedite the adoption of WPM on wireless channels.
This paper demonstrates through numerical simulation the performance of moderate (8E+3) block length Low Density Parity Check (LDPC) codes on a High Frequency (HF) multipath fading channel utilizing Orthogonal Frequency Division Multiplexing (OFDM). Some results are also shown for the Additive White Gaussian Noise (AWGN) and Rayleigh channels. The simulation parameters were chosen to compare results with the standard OFDM 39-tone HF waveform. LDPC codes are linear block codes that have outstanding error correction and detection capabilities.
A packet-based power control scheme is proposed in this paper. The proposed power control scheme aims at minimizing the total number of transmission that a packet needs before it is received successfully over a Rayleigh fading channel subject to two constraints. One is that the transmission power should be greater than zero. The other is imposed by the constraint on the total transmission power at the base station. We use the augmented Lagrangian multiplier method to solve this problem and provide a theoretical solution. The simulation results show that, with the proposed power control scheme, the number of re-transmission can be reduced and the delay of a packet wasted in the channel can be decreased as well.
A microcell is a cell with 1-km or less radius which is suitable for heavily urbanized area such as a metropolitan city. This paper deals with the microcell prediction model of propagation loss which uses probabilistic techniques. The RSL (Receive Signal Level) is the factor which can evaluate the performance of a microcell and the LOS (Line-Of-Sight) component and the blockage loss directly effect on the RSL. We are combining the probabilistic method to get these performance factors. The mathematical methods include the CLT (Central Limit Theorem) and the SPC (Statistical Process Control) to get the parameters of the distribution. This probabilistic solution gives us better measuring of performance factors. In addition, it gives the probabilistic optimization of strategies such as the number of cells, cell location, capacity of cells, range of cells and so on. Specially, the probabilistic optimization techniques by itself can be applied to real-world problems such as computer-networking, human resources and manufacturing process.
The performances of direct sequence-code division multiple access (DS-CDMA), multicarrier-CDMA (MC-CDMA) and multicarrier-direct sequence-CDMA (MC-DS-CDMA) systems under different channel conditions are compared in this work. In a frequency-selective slowly fading channel, MC-CDMA and MC-DS-CDMA outperform DS-CDMA, since the former two systems partition the frequency band into sub-channels, each of which has a nearly constant frequency response. Thus, MC-CDMA and MC-DS-CDMA do not suffer much from the multipath effect. The performance of MC-CDMA and MC-DS-CDMA can be further differentiated in severe fading conditions. In a frequency-selective fast fading channel, the larger spreading ratio of MC-DS-CDMA in the time domain prevents the chip duration of a sub-carrier from being longer than the channel coherence time. Hence, the sub-carrier orthogonality is maintained in MC-DS-CDMA, leading to its better performance in this case.
Current high quality audio coding techniques mainly focus on coding efficiency, which makes them extremely sensitive to channel noise, especially in high error rate wireless channels. In our previous work, we developed a progressive high quality audio codec, which was shown to outperform MPEG-4 version 2's scalable audio codec. In this work, we extend the error-free progressive audio codec to an error-resilient scalable audio codec by re-organizing the bitstream and modifying the noiseless coding module. A dynamic segmentation scheme is used to divide an audio bitstream into several segments. Each segment contains independently decodable data so that errors will not propagate across segment boundaries. An unequal error protection scheme is then adopted to improve error resilience of the final bitstream. The performance of the proposed algorithm is tested under different error patterns of WCDMA channels with several test audio materials. Our experimental results show that the proposed approach achieves excellent error resilience at a regular user bit rate of 64 kb/s.
Multi-user detection (MUD) techniques are applied to the GPS interference suppression/rejection problem, such that recovery of GPS satellite signals out of an environment contaminated with one or more correlated interference sources is afforded. In modeling the narrowband digital interference as a set of multiple users it is shown that recovery is possible via any of several MUD algorithms, including the conventional detector, the decorrelating detector, the optimum linear detector, and the minimum mean square error (MMSE) detector. The relationship between the underlying structure of the GPS symbol (C/A code) and the performance of these detectors is studied such that bit error rate (BER) and signal-to-interference ratio (SIR) are developed analytically.
This paper discusses nonstationary interference excision techniques in spread spectrum communications systems. Excision techniques to remove, or at least suppress, sinusoidal, auto-regressive, and digital communication signals have matured relative to those dealing with nonstationary interferers. Time-frequency domain methods for analysis and estimation of the frequency modulated (FM) interference are summarized. Domains other than time and frequency, such as the Gabor-domain, the Wavelet-domain, and quadratic time-frequency signal representations, are appropriate for non-traditional smart jamming in which the interference parameters are highly dependent on time. Excision methods can be linear, bilinear, or nonlinear with performance dependent on the interference power relative to the desired signal and noise. The receiver SINR expressions and curves are presented in this paper for some of the key interference excision techniques.
This paper further examines a computationally simple scheme for estimating the symbol period in the presence of multipath that was recently proposed by Breinholt and Zoltowski. The algorithm is not premised on the symbol pulse shape making it robust to multipath effects. This paper relates simplifications that facilitate real-time implementation of the algorithm in a per sample update mode. The algorithm is presented in the context of the 8-VSB digital TV modulation standard but is equally applicable to other modulation formats. Simulations show that the algorithm performs well in the presence of multipath. Comparisons with a Gardner timing recovery loop show a reduction of jitter and faster frequency acquisition than that of the Gardner loop. The algorithm can be used alone or to speed the acquisition of other timing recovery methods. As an example, the algorithm is combined in a simple manner with a Gardner loop to provide significantly faster timing acquisition.
In this paper, we show how the convergence time of equalizers for 8-VSB based on the conjugate gradient (CG) algorithm can be considerably improved through initialization based on a channel estimate. We derive real and complex minimum mean-square error (MMSE) equalizers and implement them adaptively using the conjugate gradient, recursive least squares (RLS), and least mean squares (LMS) algorithms. We show that both CG and RLS have similar convergence times --- both are much faster than LMS. Since the CG algorithm is easily initialized, we compare several methods of initialization to determine how each affects convergence and then apply the best methods to initialize equalizers using channel estimates. We find that initializing the correlation matrices and filling the feedback taps with training symbols greatly speeds convergence of the CG adaptive equalizer, potentially approaching the rate of convergence when running the algorithm on the matrix equations using the actual channel.
A cost function, utilizing the signal amplitude as well as the phase information, yields fast convergence and small mean square errors, when used for equalizing dynamic wireless channels. Recently, it has been shown that the minimization of cost functions that include both amplitude-dependent and constellation-dependent terms leads to improved receiver performance for high-order signal constellations, compared to that achieved by only using either term. The amplitude-dependent term corresponds to the constant modulus algorithm (CMA), which ensures proper global convergence properties. On the other hand, the constellation-matched error (CME) term provides desirable local convergence properties. Two possible schemes can be used for presenting a combined cost function. One scheme is based on a weighted sum of the CMA and the CME terms, where the weights are fixed, whereas the cost function in the other scheme has time-varying weights that are recursively update using gradient techniques. In this paper, performance comparison is conducted between techniques implementing combined cost functions with fixed and variable weighting parameters. The comparison also includes the CMA and DD techniques as well as adaptive equalizations based on dual modes. In the latter, one term of the cost function is considered at one time. The common dual-mode approach calls for initialization with CMA and thereafter switching to a proper constellation-matched algorithm.
One of the emerging trends in advanced human-computer interaction is the increasing portability of computing devices. Some experts predict that wearable computers are the next generation of portable machines. Worn on the body they provide constant access to computing and communications resources. The objective of wearable computer design is to merge the user's information space with his or her workspace. The wearable computer should offer seamless integration of information processing tools with the existing work environment. To accomplish this, the wearable system must offer functionality in a natural and unobtrusive manner, allowing the user to dedicate all of his or her attention to the task at hand with no distraction provided by the system itself. In this survey paper, definition, main components and characteristics of wearable PCs are introduced. A survey of the academic and research institutes leading research projects in wearable PCs is presented. These on-going projects are generally falling into four research areas, namely, local communications, eyeglass displays, speech interaction and mobile applications. In this overview paper, the wireless link methods and wireless communication features in the area of local communication are emphasized, progress to meet the technical challenges in eyeglass display is reported; and on-going research in the area of speech interaction is briefly described.
Throughput estimation of users in a wireless packet data communication system utilizing the General Packet radio Service (GPRS) is investigated in this work. GPRS allows dynamic allocation of the bandwidth to a mobile terminal according to its traffic demand, which results in better resource utilization and a lower communication cost. The proposed user throughput estimation scheme relies on packet data traffic modeling. Since future wireless data services are mainly in web browsing, we consider a traffic model for today's WWW traffic that consists of ON/OFF two states with the ON period containing a sequence of document transmissions. A realistic traffic model is derived based on actual traffic data measurements. The throughput and transmission delay for fixed coding schemes can be obtained accordingly. This research effort contributes to the capacity planning of GPRS for its data services.
The authors have previously studied simulation of network traffic traces using a fractional pole-zero system derived from a discrete-time dilation or scaling operation based on warping transforms that convert discrete-time frequency to a continuous-time frequency. However, such a model can be used to analyze and simulate only stationary discrete-time self- similar signals. This paper propounds a more general model for discrete-time linear scale-invariant (DLSI) systems built on linear kernel approaches. It is shown that this model can synthesize self-similar processes for simulating non-stationary network traffic.
In this paper we propose an adaptive routing using a fuzzy system. The traffic in the network is re-routed to nodes, which are less congested, or have spare capacity. Based on a set of fuzzy rules, link cost is dynamically assigned depending upon the present condition of the network. Distance vector algorithm, which is one of the shortest path routing algorithms is used to build the routing tables at each node in the network. The proposed fuzzy system determines the link cost given the present congestion situation measured via the delays experienced in the network and the offered load on the network. Delay in the links, was estimated by the time taken for the test packets to travel from the node to its neighbors. The delay information collected by the test packets and the number of packets waiting in the queue, are the two inputs to the fuzzy system. The output of the fuzzy system is the link cost. This algorithm was applied on a simulated NSFNET, the USA backbone, as well as to another test network with a different topology. Robustness and optimality of the proposed fuzzy system was tested by simulating various types of load patterns on these networks. Simulation studies showed that the performance of the fuzzy system was very close to or better than the best performance of the composite metric under different load conditions and topologies.
The performance of the fixed-user-per-slot allocation method and the dynamic slot allocation method for time slot allocation in SDMA (Space-Division Multiple Access) is evaluated. The unsorted/sorted first come first serve (USFCFS/SFCFS), first fit (FF) and modified first fit (MFF) methods are studied. Each slot allocation scheme is evaluated with a number of beamforming algorithms. Two different capacity efficiencies are defined as the thresholds in the slot assignment, with one threshold potentially achieving higher efficiency but sacrificing some of the benefits an adaptive antenna system can provide. The computational complexity of the algorithms is considered when evaluating their performance. We define a new performance indicator, the effective relative angle (ERA), and show that it can be used in evaluating not only the performance of the power gain achieved by the antenna array, but also the capacity efficiency. The system performance benefit of the sorting process for the fixed-user-per-slot allocation method is also studied. Theoretical and simulation results are examined to try to find a rule of thumb for the capacity improvement over a conventional (single antenna) system of an adaptive antenna system using each slot allocation method.
High data rate networks are now being designed to rapidly exchange high value sensor and video information among various users located on the surface and airborne. Phased array antennas can provide rapid switching amongst the members in addition to the gain necessary to connect these members. For many applications, these antennas would be configured to either transmit or receive. Designing a phased array antenna that is full duplex significantly complicates the design because much of the emphasis in the design is placed on keeping the transmit energy out of the receive channel. A phased array antenna could be designed to receive only when the antenna is not transmitting; thus the design would only need to protect the receiver components during the transmission. The antenna would be then full duplex. This paper describes a small pseudo full duplex wideband multi-frequency band (X, Ku and K) phased array antenna with +/-27 degrees of the beam scanning, using a novel multi-line time delay phase shifter controlled by a piezoelectric transducer (PET). This antenna design is simple, and would use no more space than an antenna used solely for either transmit or receive. Also, some of the isolation between the transmit and receive channels is achieved using novel low-cost fabrication techniques.
Ultra-wide band (UWB) is a relatively new term used to describe a technology that has been known since the 1960's as carrier free, baseband or impulse technology. The basic concept is to develop, transmit and receive an extremely short duration burst of radio frequency energy-typically a few tens of pico seconds to a few nanoseconds in duration. The resultant waveforms are extremely broadband, so much so that it is often difficult to determine an actual RF center frequency- thus the term carrier free. Since UWB waveforms are of such short time duration, they have some rather unique properties. In communications, for example, UWB pulses can be used to provide extremely high data rate performance in multi-user network applications . For radar applications, these same pulses can provide very fine range resolution and precision distance and/or positioning measurement capabilities. These short duration waveforms are relatively immune to multi-path cancellation effects as observed in mobile and in-building environments. As a consequence, UWB systems are particularly well suited for high-speed, mobile wireless applications. As bandwidth is inversely related to pulse duration, the spectral extent of these waveforms can be made quite large. With proper engineering design, the resultant energy densities (i.e., transmitted watts of power per unit hertz of bandwidth) can be quite low. This low energy density translates into a low probability of detection (LPD) RF signature. An LPD signature is of particular interest for military applications (e.g., for covert communications and radar); however, an LPD signature also produces minimal interference to proximity systems and minimal RF health hazards, significant for both military and commercial applications. In this paper we consider the development of a simulation model to calculate ultra-wide band signal propagation characteristics in urban indoor and outdoor environments. The simulation is accomplished using a hybrid model that combines ray tracing and FDTD. The model takes into account the material characteristics of the surrounding walls and buildings, and other obstructions, and accounts for effects due to multiple reflections. The application operates on a 3D terrain database representation of an urban area. The ultimate goal of the simulation is to determine is to maximize coverage in and urban environment given a fixed number of base stations, or, conversely, to optimize the number and location of base stations given a predetermined coverage pattern.
The paper presents two adaptive techniques, namely, the phase-only adjustment and the amplitude-phase optimization. In each case it creates multiple nulls to suppress multiple jammers. The adaptive technique is based on forward and backward smoothed ESPRIT (estimation of signal parameters via rotational invariance technique), which is capable of handling both coherent and non-coherent narrow-band multiple signals. For the amplitude-phase optimization both current amplitudes and phases are simultaneously adjusted to maximize the SIR (signal-to-interference ratio), which does not require iteration. The phase-only adaptive technique assumed fixed amplitudes while phases are adjusted, which required iterative algorithm. In both cases it is demonstrated to be effective and efficient. Computer simulations for a linear array of 32 elements and eight incoming signals have shown that the optimization technique can create wide as well as deep nulls, which correspond well to jammer widths and strengths. In each case the SIR improves substantially and the phase-amplitude optimization has much better SIR improvement than that of the phase-only adjustment.
Rapid advances in information technology and telecommunications, and more specifically wireless and mobile communications, and their convergence (telematics) are leading to the emergence of a new type of information infrastructure that has the potential of supporting an array of advanced services for healthcare. The objective of this paper is to provide a snapshot of the applications of mobile technology in healthcare. A brief review of the spectrum of these applications and the potential benefits of these efforts will be presented, followed by success case studies in electronic patient record, emergency telemedicine, teleradiology, and home monitoring. It is anticipated that the progress carried out in these efforts, and the potential benefits of emerging mobile technologies will trigger the development of more applications, thus enabling the offering of a better service to the citizen.
This paper proposes the integration of an antenna with a photodetector for high capacity wireless communications. The side-illuminated waveguide photodetector (WGPD) is used to convert the RF-modulated optical power into a microwave signal, which in turn is fed to an antenna. The WGPD is a standard p-i-n device grown on a semi-insulating InP substrate and fabricated using conventional techniques. The performance of this RF/photonic antenna in the frequency range 17-20 GHz is studied theoretically and experimentally. An equivalent circuit model for the WGPD is developed to estimate the photodetector impedance as a function of frequency to assist in the impedance matching between the photodetector and the antenna. The agreement between measurement and circuit model results for the WGPD impedance is very good. It is envisioned that a large number of such RF/Photonic antenna elements could be networked together into a star configuration, feeding in and out of a radio hub.
In this paper, we briefly cover the basic idea of hierarchical cellular networks with mobile routers and its channel planning issues. Then we introduce fault recovery protocols using backup routers and adaptive energy control. Simulation results are presented for evaluation purposes.
This paper presents a comprehensive review of the published algorithms on power control for cellular systems. The majority of the research is focused on Code Division Multiple Access (CDMA) systems, although a small fraction of the reviewed literature pertains to Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).
This paper presents a summary of polarization diversity schemes used in wireless communications. Results of various polarization schemes using dipoles, evaluated with respect to parameters such as SINR, mean effective gain, correlation coefficient etc. are presented. Analytical results show a noticeable improvement of the performance of the polarization diversity scheme when combined with the space diversity scheme.