We propose a simulation method for multidimensional Hawkes processes based on superposition theory of point processes. This formulation allows us to design efficient simulations for Hawkes processes with differing exponentially decaying intensities. We demonstrate that inter-arrival times can be decomposed into simpler auxiliary variables that can be sampled directly, giving exact simulation with no approximation. We establish that the auxiliary variables provides information on the parent process for each event time. The algorithm correctness is shown by verifying the simulated intensities with their theoretical moments. A modular inference procedure consisting of a combination between Gibbs through the adaptive rejection sampling and Metropolis Hastings steps is presented. Finally, we compare our proposed simulation method against existing methods, and find significant improvement in terms of algorithm speed. Our inference algorithm is used to discover the strengths of mutually excitations in real dark networks.

In this article, we enable the coexistence of multiple wireless body area networks (BANs) using a finite repeated non-cooperative game for transmit power control. With no coordination amongst these personal sensor networks, the proposed game maximizes each network’s packet delivery ratio (PDR) at low transmit power. In this context, we provide a novel utility function, which gives reduced benefit to players with higher transmission power, and a subsequent reduction in radio interference to other coexisting BANs. Considering the purpose of inter-BAN interference mitigation, PDR is expressed as a compressed exponential function of inverse signal-to-interference-and-noise ratio, so it is essentially a function of transmit powers of all coexisting BANs. It is shown that a unique Nash Equilibrium (NE) exists, and hence there is a subgame-perfect equilibrium, considering best response at each stage independent of history. In addition, the NE is conjectured to be the socially optimal solution according to all possible action profiles. Realistic and extensive on- and inter-body channel models are employed. Results confirm the effectiveness of the proposed scheme in better interference management, greater reliability, and reduced transmit power when compared with other schemes that can be applied in BANs.

OBJECTIVE: We study the use of speech recognition and information extraction to generate drafts of Australian nursing-handover documents. METHODS: Speech recognition correctness and clinicians' preferences were evaluated using 15 recorder-microphone combinations, six documents, three speakers, Dragon Medical 11, and five survey/interview participants. Information extraction correctness evaluation used 260 documents, six-class classification for each word, two annotators, and the CRF++ conditional random field toolkit. RESULTS: A noise-cancelling lapel-microphone with a digital voice recorder gave the best correctness (79%). This microphone was also the most preferred option by all but one participant. Although the participants liked the small size of this recorder, their preference was for tablets that can also be used for document proofing and sign-off, among other tasks. Accented speech was harder to recognize than native language and a male speaker was detected better than a female speaker. Information extraction was excellent in filtering out irrelevant text (85% F1) and identifying text relevant to two classes (87% and 70% F1). Similarly to the annotators' disagreements, there was confusion between the remaining three classes, which explains the modest 62% macro-averaged F1. DISCUSSION: We present evidence for the feasibility of speech recognition and information extraction to support clinicians' in entering text and unlock its content for computerized decision-making and surveillance in healthcare. CONCLUSIONS: The benefits of this automation include storing all information; making the drafts available and accessible almost instantly to everyone with authorized access; and avoiding information loss, delays, and misinterpretations inherent to using a ward clerk or transcription services.

Using narrowband wireless On-Body Area Network (BAN) channel measurements (50 million data points) in diverse environments with multiple subjects, we examine the stationarity of the channel. Wide-Sense Stationarity (WSS) tests and power spectral estimates show that the channel has a 50% probability of stationarity at 500 ms and the probability rapidly diminishes thereafter. We show that non-stationarity is inherent to on-body BANs and not an artifact of experimental setup.

We present a predictor for real Body-Area-Network (BAN) channels that is accurate for up to 2 seconds, even with a nominal channel coherence time of 500 ms. The predictor utilizes the partial-periodicity of measured BAN channels using the previous 4 seconds of channel gain values. We demonstrate use of this predictor for power control with open-access and private channel measurements. When used under a realistic setting for IEEE 802.15.6, with packet loss less than 10%, we show that the accurate channel predictor does not translate into substantial reduction in packet loss or power usage over a simple sample-and-hold method, even though it is a more accurate predictor than sample-and-hold.

We investigate the impact of wireless channel temporal variations on the design of medium access control (MAC) protocols for body area networks (BANs). Our measurements-based channel model captures large and small time-scale signal correlations, giving an accurate picture of the signal variation, specifically, the deep fades which are the features that mostly affect the behavior of the MAC. We test the effect of the channel model on the performance of the 802.15.4 MAC both in contention access mode and TDMA access mode. We show that there are considerable differences in the performance of the MAC compared to simulations that do not model channel temporal variation. Furthermore, explaining the behavior of the MAC under a temporal varying channel, we can suggest specific design choices for the emerging BAN MAC standard.

This paper gives expressions for the capacity of ergodic multiple-input multiple-output channels with finite dimensions, in which the channel gains have a correlated complex normal distribution and receivers experience independent Gaussian noise. The particular correlated normal distribution considered corresponds to flat Rayleigh fading with arbitrary transmit and receive correlation. Knowledge of the correlation matrices is assumed at both the transmitter and receiver, while the receiver, but not the transmitter, has complete knowledge of the channel realization. The optimal input density is characterized via a necessary and sufficient condition for optimality, along with an iterative algorithm for its numerical computation. The resulting capacity is expressed in terms of hypergeometric functions of matrix argument, which depend on the channel correlation matrices only through their eigenvalues. Some closed-form expressions are also given in the case of single-sided correlation. Some consideration is given to high- and low-power asymptotics. Easily computable asymptotic expressions are also given for receive-side only correlation in the case that the number of transmitters is large. In that case, the capacity can be divided into two components: one arising from the dominant eigenvalues of the receiver-end correlation matrix, and the other from the remaining spherically distributed eigenvalues. Some numerical results are also presented.

Signal variation in Body Area Networks (BANs) is dominated by slow, macroscopic fading. The quantized (state) channel has long, and variable state-holding times. We motivate the use of a semi-Markov approach to model the quantized channel gain and show that for everyday activities and on-body (real-world) measurements this approach leads to tractable limits for state holding times and transition probability. We show that while activity does influence the parameters of the semi-Markov model, the impact is not strong.

Methods for transmit power control based on simple long-term channel prediction for the general body-area communications channel are presented. The power control methods are based on large sets of empirical every-day activity data. Numerous transmit-receive pair (Tx-Rx) locations on the human body, i.e. on-body, for a typical body-area-network (BAN) are considered. With the use of a simple prediction method based on held samples, and an enhanced held simple prediction method that uses short term mean path loss with the held sample, optimal power allocation for long-term transmit power control is described. When tested, according to the draft IEEE 802.15.6 BAN radio standard, on empirical data, both power allocation methods are shown to be more reliable, and also more energy efficient in terms of transmit circuit power consumption, than systems that use typical set Tx power levels for BAN.

Coexistence of multiple wireless body area networks (WBAN) is a very challenging problem because piconets move frequently and each piconet can have hundreds of sensors. The IEEE 802.15.6 task group is developing a standard based on superframe-level frequency-hopping scheme, which is a good solution to the piconet coexistence problem without requiring coordination between piconets. However, its spectrum efficiency is very low when only a single piconet is operating. In this paper we propose a superframe-level time-hopping scheme with variable contention access period. Compared to the frequency-hopping scheme, the proposed scheme has similar interference mitigation capability, but can achieve significant improvement in energy consumption, latency and spectrum efficiency.

A dynamic narrowband on-body area communications scenario is characterized with respect to link margin as a difference between system operating point, in terms of receive power, and receiver sensitivity. The characterization is based on an extensive measurement campaign near the 900 MHz ISM bands, with a number of different human subjects moving at a range of speeds in an indoor-office scenario. Key implications for operating reliability in terms of outages, meeting latency requirements, infeasibility of interleaving and limits upon packet duration are drawn from this link margin analysis pertinent to body-area-communications system design. The need for receive hardware with good receiver sensitivity is highlighted. Further to this context important second-order statistics are given, such as fade duration, as well as a novel measure: average fade magnitude. Distributions are given to accurately characterize these second order statistics. Along with link margin analysis, this modeling can be used to verify possible implementations, and help in system design.

Coexistence of multiple wireless body area networks (WBAN) is a very challenging problem because each piconet can have a large number of sensors and their movement is unpredictable. Moreover, suitable global coordination schemes do not exist as there is no natural choice of coordinator between piconets. Adaptive schemes that work well with low-occupancy channels, such as listen before transmit, are not a wise global solution because of the potential for high levels of traffic in any one area. In this paper we investigate the performance of three classic multiple-access schemes - namely TDMA, FDMA and CDMA - for (inter-network) piconet coexistence. We first consider a theoretical analysis of these schemes and then simulate each scheme using real-world interference measurements. It is found that co-channel interference could significantly degrade system performance if left unchecked, and that TDMA and FDMA are better choices than CDMA in terms of co-channel interference mitigation.

A word-valued source Y = Y1,Y2,...is discrete random process that is formed by sequentially encoding the symbols of a random process X = X1,X2,...with codewords from a codebookC. These processes appear frequently in information theory (in particular, in the analysis of source-coding algorithms), so it is of interest to give conditions onXandCfor whichYwill satisfy an ergodic theorem and possess an asymptotic equipartition property (AEP). In this paper, we prove the following: 1) ifXis asymptotically mean stationary (AMS), thenYwill satisfy a pointwise ergodic theorem and possess an AEP; and 2) if the codebookCis prefix-free, then the entropy rate ofYis equal to the entropy rate ofXnormalized by the average codeword length.

A characterization of the dynamic narrowband on- body to off-body area channel is presented based on realtime measurements of the time domain channel response at carrier frequencies near the 900 MHz and 2400 MHz Industrial, Scientific and Medical (ISM) bands. A statistical characterization is presented of received signal amplitude when the subject's body is standing and walking, transmitted from the body to a receiver, Rx, off the body, with various orientations of the subject's body with respect to the receiver, and various distances from the receiver. Two locations of the transmitter, Tx, on the human body are considered. The Lognormal distribution provides a good fitting model with and without movement. Further, the stability is characterized based on a measure of channel response variance, which is called here the channel variation factor and can characterize the channel coherence time. The on-body to off-body area channel is determined to be generally stable over a period of 25 ms, but the amount of stability is found to be dependent both on movement, Tx location on-body, and carrier frequency.

We propose a low complexity noncoherent receiver operating at twice the symbol-rate for systems where each data symbol consists of multiple frames/chips. The receiver does not require explicit timing and channel estimation. It implements simple delayed-autocorrelation, followed by sampling at twice the symbol-rate. Simulation results show the receiver achieves performance close to a conventional one with perfect timing.

Inter-network interference is a significant source of difficulty for wireless body area networks. Movement, proximity and the lack of central coordination all contribute to this problem. We compare the interference power of multiple Body Area Network (BAN) devices when a group of people move randomly within an office area. We find that the path loss trend is dominated by local variations in the signal, and not free-space path loss exponent.

Network-to-network interference is a challenging problem in wireless body area networks which move frequently and operate in a non-coordinated way. In this paper, we characterize interference based on field measurements. We examine the correlation and independence between the desired (signal) and interference channels. The results show that over long periods, the two channels are statistically un-correlated. Over a short periods, the channels may become correlated, particularly when the channel variation is small.

A new differential unitary space-time code is presented that is a product of group diagonal matrices of different orders and a unitary Vandermonde matrix. Design is simplified through properties of the union bound on block error probability for the presented code. Detection is simplified via properties of the diagonal matrices which form a cyclic group. Superior performance to best known differential space-time codes is shown, particularly for 3, 5 and 6 transmit antennas.

We bound the number of electromagnetic signals which may be observed over a frequency range [F-W, F+W] a time interval [0, T] within a sphere of radius R. We show that the such constrained signals may be represented by a series expansion whose terms are bounded exponentially to zero beyond a threshold. Our result implies there is a finite amount of information which may be extracted from a region of space via electromagnetic radiation.

This paper presents a novel design method for a spatially interpolated beamformer. By separating the design of the physical antenna array and the digital signal processors, we remove the restriction on the half-wavelength spacing requirement of a uniform linear array to achieve higher efficiency. To be more realistic, however, we fix the element positions regardless of the steering angles. The maximum energy concentration properties of discrete prolate spheroidal sequences are exploited to optimize the weighting coefficients and filtering window. Two examples are given to demonstrate the high flexibility, low computational complexity, and non compromised performance of the proposed beamformer.

We consider t-input r-output Rayleigh fading channels with transmit-sided correlation, where the receiver knows the channel realizations, and the transmitter only knows the channel statistics. Using Lagrange duality, we develop an easily computable, tight upper bound on the loss in information rate due to the use of any given input covariance for this channel. This bound is applied to two simple transmission strategies. The first strategy is a reduced-rank uniform allocation, in which independent, equal power Gaussian symbols are transmitted on the at strongest eigenvectors of the transmit covariance matrix, where 0 les alpha les 1 is chosen to optimize the resulting information rate. The second strategy is water-filling on the eigenvalues of the transmit covariance matrix. The upper bound on loss shows these strategies are nearly optimal for a wide range of signal to noise ratios and correlation scenarios

This paper presents a novel unitary space-time signal design for two to six transmit antennas which is applicable to both coherent and differential space-time coding. In the context of differential modulation, this new space-time signal design demonstrates good bit-error performance when compared with previously designed codes both theoretically and by simulation. The design for two antennas is shown to subsume previous differential unitary space-time signal constellation with the property that the number of optimum codes scales with the size of the constellation. As part of analysis of this novel design an exact expression for the union bound on error probability is given for differential unitary space-time modulation. The calculation of the union bound on bit-error probability can also shown to be considerably simplified for these codes allowing for the simplified design of larger constellations.

This paper presents an efficiently parametrised model for second-order-statistics dominated MIMO channels. Recently, new MIMO models have been developed to emulate the statistics of real measurements: (1) analytic models which parametrise the statistics of the channel gains, and (2) geometric models which interpret the channel as separate multi-paths. Unfortunately analytic models are tied to the measurement array geometry, while geometric models significantly increase model complexity. We present a new stochastic framework, based on a modal decomposition of the MIMO channel, which allows channel models for arbitrary array geometries from a single set of measured data. Such a framework yields simple MIMO models that efficiently parametrise the channel, with adjustable accuracy. Results show that the new models match the capacity of real and simulated data as well as similar models

Node mobility and physical channel effects cause the quality of links in wireless networks to fluctuate randomly. At the network layer, these changes are accommodated by the control information (overhead) of routing protocols. We provide lower bounds on the minimum average control overhead of deterministic routing protocols. When routing devices use noisy, out-of-date, or guess topology information, routing errors will occur. The number of "routing errors" experienced by well known routing schemes grows non-trivially with increased estimation noise. Route error growth is a novel characterization of protocol robustness for switched packet networks. This work motivates an information theoretic view of routing protocols

This paper presents a novel physical-modeling approach to wireless systems with multiple antennas. The fundamental problem of modeling the communication channel is studied, where the channel consists of a finite spatial volume for transmitting, a finite spatial volume for reception, and an arbitrary set of reflective-scattering bodies. The number of communication modes (or degrees of freedom) for such a system is calculated, using the procedure developed. We present a simple model for multipath channels, which allows insight into the development of a correlated multiple-input multiple-output (MIMO) channel model. In particular, the model is independent of transmitter and receiver elements and relies on the physical parameters of the channel involved. Our work explains which physical parameters determine the channel model and its channel capacity.

This paper studies the problem of quantization of a source that lives on the complex Grassmann manifold. The special structure of the Grassmann manifold and the distortion measures that are defined on it differentiates this problem from the traditional problem of vector quantization in Euclidean spaces. Assuming a uniform source distribution along with a distortion based on chordal distance, codebook design algorithms are mentioned and rate distortion tradeoffs are studied. The expected distortion for such a quantizer is approximately characterized. These results are then applied to the performance analysis of a multiple antenna wireless communication system.

Wireless communication occurs through continuous fields, over regions of space. Although the communication channel may be modelled in terms of infinite dimensional vector spaces, it is paramount to develop finite dimensional approximations for this channel. We extend previous work which examined the fundamental finite dimensionality of fields over a finite region of space to incorporate restricted angles of signal arrival. By considering a subspace of fields where direction of arrival of the field components, or source spatial distribution, is restricted we show the received field dimensionality is linearly related to the product of the radius of the region of interest and the angular restriction. This result provides a rigourous foundation for analysing the performance and capacity of MIMO systems in non isotropic environments. The proof is presented for the two dimensional case. It is apparent that this work can be extended to the case of three dimensions for a similar result

We give an optimality condition for the input covariance for arbitrary ergodic Gaussian vector channels under the condition that the channel gains are independent of the transmit signal, the transmitter has knowledge of the channel gain probability law and the receiver has knowledge of each channel realization. Using this optimality condition, we find an iterative algorithm for numerical computation of optimal input covariance matrices

We consider the standard (point-wise) linear channel model for MIMO wireless systems in terms of a continuous operator channel. We show analytically that the point-wise representation overestimates the (true) modal connection strengths and produces artificially distorted channel singular values. Analytic results are compared with simulations for simple channel models and the convergence of the point-wise model toward the continuous model is shown.

We present information theoretic capacity results for information transfer in space, under the general assumption that the receiver applies a spatial filter to receive signals. Our work presents novel applications of linear, bounded, invariant operators to communication channels. We outline the procedure for calculating the information theoretic capacity for this restricted class of operator channels and apply this result to wireless information transfer.

This paper presents a new physical modeling approach for wireless systems with multiple antennas. The fundamental problem of modelling the communications channel if we are given an arbitrary spatial volume for transmitting, an arbitrary spatial volume for receiving, and a set of scattering bodies is studied. We show how to calculate the number of communication modes, both for direct (point-to-point) and for scattering environments. Our work explains the physical parameters, which determine the channel model and its channel capacity.

This paper deals with the problem of computing channel capacity for wireless communications systems with multi-antennas. The system is modeled using a transmit volume, a receive volume and a set of reflective scatterers. Based on the author's previous work on channel modeling of such a system, we give a multi-input-multi-output model for communications between arbitrary antenna arrays in two volumes. Together with a novel noise model for describing interferences, we derive formulas for channel capacity and capacity bounds. In particular, our results give conclusions which agree with intuition: (i) in the case where the transmitter knows the channel, the channel capacity saturates to a constant when the number of transmit/receive antennas increases beyond a certain threshold; (ii) in the case where the transmitter des not know the channel, the channel capacity still saturates when the receive antennas increases but decays when the number of transmit antennas increases beyond a certain threshold.

We present a general multiple path scattering model for multiple transmit, multiple receive wireless systems. The model is generated using physical modelling of the scatterers surrounding transmit and the receive arrays. The condition of the resulting multiple-input multiple-output (MIMO) transfer matrix is then examined. A key parameter /spl eta/ which determines the channel condition is identified. This parameter depends on the local scatter geometries, the separation of arrays, and the wavelength of the transmitted signal. We show that there exists a critical value for this parameter at which the channel condition changes sharply. The implication is that the promised linear growth in channel capacity may not eventuate if the separation of the transmit and receive arrays is large (/spl ap/10/spl times/ or 20/spl times/ at 2 GHz) compared with the distance from array elements to local scatterers. Monte-Carlo simulations are used to demonstrate these claims.