Cooperative relays have been used in many wireless applications to reduce transmit power as well as adding receiver diversity. However, the energy efficiency trade-offs of relay networks have not been thoroughly investigated using a more complete energy model of the system. In this paper we present an energy model for a cooperative dual-relay system using decode-and-forward (DF) protocol with receiver diversity. Energy consumption is based on models of both analog and digital components for transmission and reception. A quartic transmit power amplifier (PA) energy equation is derived that is a function of the following parameters: relay location, relative transmit power allocation amongst the relays, spectral efficiency, transmission distance, pathloss, and target bit-error rate (BER). The energies of the remaining circuit components are added to form a multi-relay DF system energy equation, which is then numerically evaluated to minimize total energy by finding the optimal relay locations, relay transmit PA power allocations, and spectral efficiency. Results are compared to our previous work that considered the energy of direct transmission and a single-relay system, and the minimum-energy system as a function of source-destination distance is considered.

High data rates and multiple channel profiles of a MIMO system are naturally well-suited to carry video content. However, a video communication scheme to exploit these desirable properties of MIMO systems is widely unexplored. Even the most sophisticated MIMO rate adaptation methods rely on channel BER which shows a monotonic decrease as the channel quality improves. However, video quality, for which PSNR is a better measure than BER, does not show a proportional increase with improved channel quality because of error concealment. In this paper, we present a novel application-aware rate adaptation method which can detect variations in a MIMO channel at a video receiver, quantify the impact of these variations on the received video quality, and adaptively select a transmission profile (consisting of modulation, coding and MIMO mode) to provide unprecedented improvements in video quality. The proposed application-aware MIMO Video Rate Adaptation (MVRA) method relies on a comprehensive model of source- and channel-induced distortions in video quality. Using this model, a MIMO receiver can select an appropriate transmission profile on a per-GOP basis. Trace-driven evaluations over an 802.11n channel show that the proposed MVRA method's PSNR is very close to the PSNR of an optimal RA scheme. Comparison with state-of-the-art MIMO RA methods shows that the proposed MVRA method provides consistently better PSNR, with an average improvement of 19.29% (5.5486 dBs) over the best existing MIMO RA scheme.

Previous studies on clock synchronization problem have resulted in diverse techniques ranging from computationally expensive to simple statistical methods. The common conclusion from this investigation has been the classic complexity-performance tradeoff. This research presents a synchronization scheme that is computationally simpler and robust to the underlying network delay density function.

A sender-receiver paradigm, in which a master and slave node exchange timing packets to estimate the clock offsets of the slave node and other nodes located in the common broadcast region of master and slave nodes, is adopted herein for synchronizing the clocks of individual nodes in a wireless sensor network (WSN). The maximum likelihood estimate of the clock offset of the listening node hearing the broadcasts from both the master and slave nodes was derived, assuming symmetric exponential link delays. This paper advances those results in two directions. First, some improved estimators, each being optimal in its own class, are derived for the clock offset of the listening node and mean link delays. Second, the results are generalized by addressing the more realistic problem of clock offset estimation under asymmetric exponential delays. The results presented in this paper are important for time synchronization of WSNs, where these techniques can be utilized to achieve accurate clock estimates with reduced power consumption.

For a meaningful processing of the information sensed by a wireless sensor network (WSN), the clocks of the individual nodes need to be matched through some well-defined procedures. Extending the idea of having silent nodes in a WSN overhear the two-way timing message communication between two active (master and slave) nodes, this paper derives the maximum-likelihood estimator (MLE) for the clock offsets of the listening nodes located within the communication range of the active nodes by assuming an exponential link delay modeling, hence synchronizing with the reference node at a very low cost. A vital advantage for adopting such an approach is that the performance of sender-receiver protocols can be compared with receiver-receiver protocols on equal footings, because their main critical aspect was associated with the high-communication overhead induced by the point-to-point nature of communication links relative to broadcast communications. The MLE is also shown to be the minimum variance unbiased estimator (MVUE) of the clock offset when the mean of exponential link delays is known. Since it is attractive to know in advance the extent to which an estimator can perform through its lower bound, the Chapman-Robbins bound and the Barankin bound for the clock offset estimator are also derived. It is shown that for an exponential link delay model, the mean square error of the clock offset estimator is inversely proportional to the square of the number of observations, and hence its performance is on a similar scale, albeit slightly lesser, as compared to the usual sender-receiver clock offset estimator. In addition, a novel method referred to as the Gaussian mixture Kalman particle filter (GMKPF) is proposed herein to estimate the clock offsets of the listening nodes in a WSN. GMKPF represents a better and flexible alternative to the MLE for the clock offset estimation problem due to its improved performance and applicability in arbitrary and generalized non-Gaussian random delay models.

For many applications, distributed networks require the local clocks of the constituent nodes to run close to an agreed upon notion of time. Most of the widely used clock synchronization algorithms in such systems employ the sender-receiver protocol based on a two-way timing message exchange paradigm. Maximum likelihood estimator (MLE) of the clock offset based on the timing message exchanges between two clocks was derived in D. R. Jeske, On maximum likelihood estimation of clock offset[IEEE Trans. Commun., vol. 53, pp. 53-54, Jan. 2005], when the fixed delays are symmetric and the variable delays in each direction assume an exponential distribution with an unknown mean. Herein, the best linear unbiased estimate using order statistics (BLUE-OS) of the clock offset between two nodes is derived assuming both symmetric and asymmetric exponential network delays, respectively. The Rao-Blackwell-Lehmann-Scheffe¿ theorem is then exploited to obtain the minimum variance unbiased estimate (MVUE) for the clock offset which it is shown to coincide with the BLUE-OS. In addition, it is found that the MVUE of the clock offset in the presence of symmetric network delays also coincides with the MLE. Finally, in the presence of asymmetric network delays, although the MLE is biased, it is shown to achieve lesser mean-square error (MSE) than the MVUE in the region around the point where the bidirectional network link delays are symmetric and hence its merit as the most versatile estimator is fairly justified.

This paper targets the problem of clock synchronization for a set of receivers lying within the broadcast range of two nodes implementing a general sender-receiver protocol using a wireless channel. The maximum likelihood estimate of the clock offset of the inactive node (and mean link delay) hearing the broadcasts from both the master and slave nodes was derived in assuming symmetric exponential link delays. In this paper, the minimum variance unbiased estimate for the clock offset of such nodes is derived by applying the Rao-Blackwell-Lehmann-Scheffe theorem. The result is important in the realm of wireless sensor networks, where a tight network synchronization along with a conservative energy utilization plays a major role in network performance.

Clock synchronization is an important issue for the design of a network composed of small sensor nodes. Based on the two-way timing message exchange mechanism and assuming an exponential network delay distribution, many analytical results have been presented in the literature by applying the techniques from statistical signal processing. This paper derives the minimum variance unbiased estimator for the clock offset for both symmetric and asymmetric exponential delay cases. For the asymmetric delays, it is shown to be a function of both the minimum and the mean link delays. This result is a very significant contribution since only the minimum link delay observations have been used to estimate the clock offset in the past. For the symmetric case, it is shown to coincide with the maximum likelihood estimator. In addition, the result is also applicable to clock synchronization problem in general computer networks.

Wireless Sensor Networks (WSN) are a key vehicle for driving many future applications, most of which require the logical clocks of the constituent nodes to by closely synchronized in time with each other. Applying the concepts from the field of statistical signal processing in this area can yield many fruitful results and advance the state of the art in this technology. This paper illustrates a summary of the main results achieved so far by working in this particular direction, which includes the derivation of Maximum Likelihood Estimators (MLE) of the clock parameters, namely the clock offset, skew and drift as well as the link delays, the lower bounds on their performance and two simplified schemes for power conservation in the context of WSNs.

Clock synchronization plays a fundamental role in the operation of wireless sensor networks. This paper derives the Maximum Likelihood Estimator (MLE) of clock phase offset and skew between two nodes that assume a two-way timing message exchange mechanism to achieve synchronization. The MLE is derived under quite general conditions assuming an exponential distribution for the variable network delays and the presence of an unknown deterministic component due to the fixed network delays.

Motivated by the necessity of having a good clock synchronization amongst the nodes of wireless ad-hoc sensor networks, the joint maximum likelihood (JML) estimator for clock phase offset and skew under exponential noise model for reference broadcast synchronization (RBS) protocol is formulated and found via a direct algorithm. The Gibbs sampler is also proposed for joint clock phase offset and skew estimation and shown to provide superior performance relative to JML- estimator. Lower and upper bounds for the mean-square errors (MSE) of JML-estimator and Gibbs Sampler are introduced in terms of the MSE of the uniform minimum variance unbiased (UMVU) estimator and the conventional best linear unbiased estimator (BLUE), respectively.

Clock synchronization represents a crucial element in the operation of Wireless Sensor Networks (WSN). For any general time synchronization protocol involving a two way message exchange mechanism (e.g., Timing Synch Protocol for Sensor Networks (TPSN) [1]), the Maximum Likelihood Estimate (MLE) for clock offset under the exponential delay model was derived in [2] assuming no clock skew between the nodes. Since all practical clocks are running at different rates with respect to each other; the skew correction becomes important for achieving long term synchronization which in turn results in reduction of message exchanges and hence minimal power utilization. The MLE for clock offset as well as skew under the exponential delay model for TPSN were derived in [6]. In this paper; due to complexity reasons for finding the MLE, a simple and efficient algorithm is presented as an alternative to the ML estimation which particularly suits the low power demanding regime of wireless sensor networks.

Recently, a few efficient timing synchronization protocols for wireless sensor networks (WSNs) have been proposed with the goal of maximizing the accuracy and minimizing the power utilization. This paper proposes novel clock skew estimators assuming different delay environments to achieve energy-efficient network-wide synchronization for WSNs. The proposed clock skew correction mechanism significantly increases the re-synchronization period, which is a critical factor in reducing the overall power consumption. The proposed synchronization scheme can be applied to the conventional protocols without additional overheads. Moreover, this paper derives the Cramer-Rao lower bounds and the maximum likelihood estimators under different delay models and assumptions. These analytical metrics serves as good benchmarks for the thus far reported experimental results

Recently, a few protocols for synchronizing the nodes of wireless sensor networks (WSNs) to a common time frame have been proposed with the goal of maximizing the accuracy and minimizing the power utilization. Thus far, the performance of the existing protocols for time synchronization is assessed only through computer simulations and experiments without using any rigorous analytical metrics. The goal of this paper is to fill up this gap by deriving the Cramer-Rao lower bound (CRLB) for the clock offset in one of the most popular synchronization algorithms, namely timing-sync protocol for sensor networks (TPSN), assuming commonly used exponential and Gaussian delay models, respectively. Furthermore, this paper proposes novel and practical clock skew estimators requiring no prior information of the fixed portion of delays, which makes the TPSN algorithm very suitable to synchronization in light of its power efficiency constraint.