In this paper, we describe new lower bounds on error variance of phase offset estimation schemes used in IEEE 1588 based synchronization. The motivation for this study is to determine the feasibility of providing microsecond-level time synchronization over mobile backhaul networks for the backend in 4G cellular systems. Many packet filtering/selection techniques for phase offset estimation have been proposed in synchronization literature, however, lower limits on the performance of such estimators have not yet been described. In this paper, we re-derive two Bayesian estimation bounds, namely the Ziv-Zakai and Weiss-Weinstien bounds, for use under a non-Bayesian formulation. This enables us to apply these bounds to the problem of phase offset estimation. Simulation results compare the performance of existing estimation schemes against these lower bounds under a variety of different network scenarios.

The IEEE 1588 protocol has received recent interest as a means of delivering sub-microsecond level clock phase synchronization over packet-switched mobile backhaul networks. Due to the randomness of the end-to-end delays in packet networks, the recovery of clock phase from packet timestamps in IEEE 1588 must be treated as a statistical estimation problem. A number of estimators for this problem have been suggested in the literature, but little is known about the best achievable performance. In this paper, we describe new minimax estimators for this problem, that are optimum in terms of minimizing the maximum mean squared error over all possible values of the unknown parameters. Minimax estimators that utilize information from past timestamps to improve accuracy are also introduced. Simulation results indicate that significant performance gains over conventional estimators can be obtained via such optimum processing techniques. These minimax estimators also provide fundamental limits on the performance of phase offset estimation schemes.

In packet-based time synchronization protocols such as IEEE 1588, clock phase offsets are determined via two-way message exchanges between a master and a slave. Since the end-to-end delays in packet networks are inherently stochastic in nature, the recovery of phase offsets from message exchanges must be treated as a statistical estimation problem. Recently, minimax estimators for this problem were proposed by the authors, which are optimum in terms of minimizing the mean-squared estimation error over all values of the unknown parameters. In this paper, we consider a restricted class of estimators referred to as L-estimators, which are linear functions of order statistics. The problem of designing optimum L-estimators is studied under several hitherto unconsidered criteria of optimality. Our results address the case where the queuing delay distributions are fully known, as well as the case where network model uncertainty exists. Optimum L-estimators that utilize information from past observation windows to improve performance are also described. The derived L-estimators have a much lower computational complexity than minimax estimators, and also require lesser statistical knowledge of the queuing delays. Simulation results indicate that L-estimators exhibit a mean-squared estimation error very close to minimax estimators under many network scenarios.

This paper proposes a Real-Time Pricing (RTP)-based power scheduling scheme as demand response for residential power usage. In this scheme, the Energy Management Controller (EMC) in each home and the service provider form a Stackelberg game, in which the EMC who schedules appliances' operation plays the follower level game, and the provider who sets the real-time prices according to current power usage profile plays the leader level game. The sequential equilibrium is obtained through the information exchange between them. Simulation results indicate that our scheme can not only save money for consumers, but also reduce peak load and the variance between demand and supply, while avoiding the "rebound" peak problem.

This paper presents the joint transmitter and receiver de sign with adaptive beamforming for multiple-input multiple output (MIMO) single carrier-frequency division multiple access (SC-FDMA) systems. Overall signal to interference plus noise ratio (SINR) of estimated symbols is maximized by selecting appropriate transmit power, transmit and receive beamforming weights and frequency domain-linear equalization (FD-LE) weight. Closed-form solutions unveil a symmetric design of the transmitter and receiver. In addition, two suboptimal system designs with closed form solutions are investigated.

We consider three-user multiple access cognitive radio channels and obtain achievable rates for reliable communication over these channels. A synopsis of the channel model is as follows: before establishing communication with the receiver, one of the transmitters (named primary) splits its message-set into two parts and shares them among the remaining two transmitters (named secondary) via a noiseless channel of infinite capacity. We thus have a cooperative scenario with each secondary transmitter having noncausal knowledge of a part of the primary''s message. First, we precisely define the channel model and then derive a set of achievable rates for this channel by employing a combination of two well-known techniques: superposition coding introduced for broadcast channels and Gel'f and-Pinkser's binning principle used for coding channels with random parameters. Lastly, we consider the Gaussian model of this channel to numerically evaluate and plot the rate region. Results demonstrate that, by sharing its message in parts, the primary user achieves higher rates, while the two secondary users continue to transmit at the same rate as that of the classical three-user multiple access channel without message-sharing between the transmitters.

Randomized network coding (RNC) has greatly reduced complexity of implementing network coding in large-scale, heterogeneous networks. Two tradeoffs are studied here that further reduce the overhead in applying RNC. The first examines how RNC performance varies with a node's randomizing capabilities. Specifically, a limited randomized network coding (L-RNC) scheme - in which intermediate nodes perform randomized encoding based on only limited number of random coefficients - is proposed and its performance bounds are analyzed. L-RNC is applicable to networks in which nodes have either limited computation/storage capacity or have ambiguity about downstream edge connectivity (e.g., as in ad hoc sensor networks). A second tradeoff studied here examines the relationship between reliability and capacity gains of RNC, i.e., how the outage probability of RNC relates to the transmission rate at source node. This tradeoff reveals that significant reductions in outage probability are possible when the source deliberately transmit at (only slightly) below network capacity. It therefore provides an effective alternative to improve RNC feasibility when the size of finite field is fixed.

This paper presents theoretical results on the convergence of the distributed consensus timing synchronization (DCTS) algorithm for wireless sensor networks assuming general Gaussian delay between nodes. The asymptotic expectation and mean square of the global synchronization error are computed. The results lead to the definition of a time delay balanced network in which average timing consensus between nodes can be achieved despite random delays. Several structured network architectures are studied as examples and their associated simulation results are used to validate analytical findings.

This paper proposes a novel discrete time second- order distributed consensus time synchronization (SO-DCTS) algorithm for wireless sensor networks. The consensus properties and convergence rates of the SO-DCTS algorithm are analyzed for both directed and undirected networks. Additionally, the convergence region and optimal convergence rate of the SO- DCTS algorithm are determined for undirected networks and this convergence rate is shown to be superior to that of the first-order DCTS (FO-DCTS) algorithm under careful algorithm design. Finally, simulation results are provided to verify these analytical results.

We develop a game theoretic framework for effective and adaptive inter-cell interference management in the OFDMA-based WiMAX/3GPP-LTE systems. Such approach requires no fixed spectrum planning beforehand and does not affect the spectrum usage for users that are not subject to inter-cell interference. Based on the handoff mechanisms in WiMAX systems, we also examine means of categorizing users who are, or are not, subject to potential interference. Different optimization opportunities for BSs are identified and analyzed. The interactions between BSs are modeled by a Stackelberg game in which BSs can make intelligent and rational decisions to reduce interference with minimum optimization cost. The algorithm to attain stable Nash Equilibrium is investigated and the behaviors of BSs with different preferences in the game are examined.

A game theoretic framework is developed in this paper to facilitate inter-cell interference management through cognitive sensing distributively performed by mobile stations (MSs). Using stochastic geometry, we reveal the relationship between the effectiveness of interference management and MS's "willingness" to perform cognitive sensing. Such cognitive sensing performed by MS is motivated by the associated beneficial results as well as by the rewards from base station(BS) that encourage sensing. Different tradeoffs for BS and MSs exist in their interactions, which are modeled as a Stackelberg game in this paper. While both BS and MS seek to manage interference at its own minimum cost, we design algorithm to achieve Nash equilibrium in such a game and investigate the optimal strategies taken by the players (BS and MSs).

Two approximations to the Shannon capacity of a maximal-ratio combining (MRC) diversity system are proposed in terms of the diversity factor (DF). Asymptotic analysis shows that the approximations are identical to the true capacity in the low-SNR regime and have the same slope but different power offsets in the high-SNR regime. For Rayleigh-fading environments, the power offset differences are derived and shown to be considerably small quantities. Thus, the DF can be considered as an effective measure of the asymptotic capacity improvement offered by MRC diversity reception in Rayleigh fading channels.

A game-theoretic analysis of decode and forward cooperative communications is presented for additive white Gaussian noise (AWGN) and Rayleigh fading channels. Cooperative communications is modeled as a repeated game in which the two participating terminals are selfish and seek to maximize their own payoff, a general utility function that monotonically increases with signal-to-noise ratio. Results show a Nash Equilibrium in which users mutually cooperate can be obtained for AWGN channels when strict power control is enforced and users care about future payoff. However, such power control may not be necessary to achieve cooperative Nash Equilibrium when the game is played in Rayleigh fading channels. We study the Rayleigh fading channel as a two state Markov model in this paper. In this case, a mutually cooperative Nash Equilibrium 1) always exists when the utility function is convex and users care somewhat about future payoff; and 2) may not always exist when the utility function is concave, especially in adverse channel conditions. Examinations of several widely-used concave functions, however, demonstrate that mutual cooperation is more likely when users increase their value on future payoff. Additionally, it is shown that improving the effective uplink channel conditions of users, e.g., by using multiple transmit antennas, further encourages cooperation.

In wideband code division multiple access (WCDMA) system capacity analysis and simulations, downlink interference due to loss of orthogonality between spreading codes is modeled by means of the orthogonality factor (OF). This study derives closed-form expressions for the time-averaged OF over independent Rayleigh fading multipath channels, in which the average power gains of each path can be distinct or identical. Two approximation approaches proposed in related literature are tested against the analytical results presented here

Amplify-and-forward (AF) relaying strategies are studied for Nakagami-fading channels in an (N + 2)-terminal wireless cooperative system where a source terminal transmits information to a destination terminal with the assistance of N relay terminals. The asymptotic outage behavior is investigated in terms of coding and diversity gains for channel-state information (CSI)-assisted AF relaying and semi-blind AF relaying in various channel profiles. It is shown that semi-blind AF relaying system achieves strictly higher average SNRs. Further, when there exists fading severity difference between the source-to-relay and relay-to-destination links, both AF strategies achieve the same diversity gain whereas semi-blind AF achieves strictly lower coding.

This paper investigates the diversity gain offered by implementing network coding (R. Ahlswede et al., 2000) over wireless communication links. The network coding algorithm is applied to both a wireless network containing a distributed antenna system (DAS) as well as one that supports user cooperation between users. The results show that network-coded DAS leads to better diversity performance as compared to conventional DAS, at a lower hardware cost and higher spectral efficiency. In the case of user cooperation, network coding yields additional diversity, especially when there are multiple users

This paper examines the uplink user capacity in a two-tier code division multiple access (CDMA) system with hotspot microcells when user terminal power is limited and the wireless channel is finitely-dispersive. A finitely-dispersive channel causes variable fading of the signal power at the output of the RAKE receiver. First, a two-cell system composed. of one macrocell and one embedded microcell is studied and analytical methods are developed to estimate the user capacity as a function of a dimensionless parameter that depends on the transmit power constraint and cell radius. Next, novel analytical methods are developed to study the effect of variable fading, both with and without transmit power constraints. Finally, the analytical methods are extended to estimate uplink user capacity for multicell CDMA systems, composed of multiple macrocells and multiple embedded microcells. In all cases, the analysis-based estimates are compared with and confirmed by simulation results.

The number of simultaneous users (or user capacity) supportable on the uplink of a multiple-macrocell code division multiple-access (CDMA) system with multiple "hotspot" microcells embedded within is studied. These microcells operate on the same frequency as the macrocells and are installed in regions of high user demand. It is shown that the user capacity depends on how the users are distributed among cells, and that the maximum (called the attainable capacity) occurs when all cells serve roughly the same number of users. The approach builds on a two-cell analysis published previously, for a single microcell embedded in a single macrocell. First, this analysis is expanded upon to estimate the attainable capacity for M macrocells, where the center one contains L microcells. Then the case in which L microcells are distributed randomly among the M macrocells is analyzed. In each case, the formula for attainable capacity is very simple and highly accurate (as demonstrated via simulations) up to reasonably high values of L. For example, with L microcells distributed among M macrocells, the analysis is accurate at least up to eight microcells per macrocell. The analysis and results are general with respect to cell geometries, propagation parameters, and other variables of the two-tier CDMA system

This paper examines the effect of soft handoff on the uplink user capacity of a code division multiple access system consisting of a single macrocell in which a single hotspot microcell is embedded. The users of these two base stations operate over the same frequency band. In the soft-handoff scenario studied here, both macrocell and microcell base stations serve each system user, and the two received copies of a desired user's signal are summed using maximal ratio combining. Exact and approximate analytical methods are developed to compute uplink user capacity. Simulation results demonstrate a 20% increase in user capacity compared to hard handoff. In addition, simple approximate methods are presented for estimating soft-handoff capacity and are shown to be quite accurate.

This paper studies a two-tier code-division multiple-access (CDMA) system in which the microcell base is converted into a data access point (DAP), i.e., is a limited-range base station that provides high-speed access to one user at a time. The microcell (or DAP) user operates on the same frequency as the macrocell users and has the same chip rate. However, it adapts its spreading factor, and thus its data rate, in accordance with interference conditions. By contrast, the macrocell serves multiple simultaneous data users, each with the same fixed rate. The achievable throughput for individual microcell users is examined and a simple accurate approximation for its probability distribution is presented. Computations for average throughputs, both per-user and total, are also presented. The numerical results highlight the impact of a desensitivity parameter used in the base-selection process.

This paper examines the effect of soft handoff on the uplink user capacity of a CDMA system consisting of a single macrocell in which a single hotspot microcell is embedded. The users of these two base stations operate over the same frequency band. In the soft handoff scenario studied here, both macrocell and microcell base stations serve each system user and the two received copies of a desired user's signal are summed using maximal ratio combining. Exact and approximate analytical methods are developed to compute uplink user capacity. Simulation results demonstrate a 20% increase in user capacity compared to hard handoff. In addition, simple, approximate methods are presented for estimating soft handoff capacity and are shown to be quite accurate.

This paper examines the downlink user capacity in a CDMA system composed of a macrocell and a microcell, where both cells use the same frequency channel. The effect of channel dispersion on the number of simultaneous users supported is studied for three different downlink power control methods. Channel dispersion produces in-cell interference and variable fading of the received signal power. Novel analytical methods are developed to account for both effects; these techniques derive user capacity for any delay profile using the results for the uniform delay profile. The downlink user capacities for the three power control methods are compared to the uplink user capacity under the same channel conditions. The results point to the need for fast power control to obtain uplink-limited performance.

This paper examines the uplink user capacity in a CDMA system composed of one macrocell and one microcell when user terminal power is limited and the wireless channel is finitely-dispersive. A finitely-dispersive channel causes variable fading of the signal power at the output of the RAKE receiver. Analytical methods are developed to compute the user capacity as a function of a dimensionless parameter that depends on the transmit power constraint and cell radius. Additionally, a novel analytical method is developed to study the effect of variable fading due to a finite number of multipaths. This technique derives the user capacity for any delay profile using the results for the uniform multipath channel. Simulation results confirm the accuracy of these simple analytical solutions.

This paper studies the number of voice users (user capacity) supported on the uplink of a single-macrocell/single-microcell code-division multiple-access system. A "hotspot" microcell is embedded within a larger macrocell and operates over the same bandwidth as the larger cell. Analytic methods are presented for computing user capacity which account for propagation loss, multiple-access interference, power control, and random locations of user terminals, as well as two distinct methods by which users select base stations (tiers). Along with the exact user capacity, a technique for making accurate approximations is also presented. Simulation results verify both the exact and approximate analytical methods. This simulation is also employed to study the capacity gains of a third, more optimal, tier-selection scheme. These results point to differences in capacity performance based on the tier-selection method, as well as on the traffic density within the hotspot region.

This paper studies the number of simultaneous voice users (or user capacity) supportable on the uplink of a multiple macrocell CDMA system with multiple "hotspot" microcells embedded within. These microcells operate over the same bandwidth as the macrocells and are assumed to be installed in regions of high user demand. First, conditions for optimal performance of such a system are established. This analysis shows that the total user capacity is maximum when there are equal numbers of active users in each cell. Along with these conditions, analytical approximations are presented for computing maximum user capacity as a function of the number of microcell bases in the system. In particular, the approximations show the total user capacity as growing linearly with the number of hotspot microcells. Simulation results verify this trend when the number of microcells in the system is not large. The numerical constants for this linear trend are related to basic system and propagation parameters. When the number of microcells gets large, it is shown that the same linear growth in capacity can be sustained by desensitizing the macrocells, i.e., raising the received power requirements at the macrocell bases.

This paper studies the number of voice users supported on the uplink (user capacity) of a single-macrocell/single-microcell CDMA system. A "hotspot" microcell is embedded within a larger macrocell and operates over the same bandwidth as the larger cell. Analytic methods are presented for computing user capacity which account for propagation loss, multiple-access interference, power-control, and random locations of user terminals, as well as two distinct methods by which users select base stations (tiers). Along with the exact user capacity, a technique for approximations is also presented. Simulation results verify both the exact and approximate analytical methods. This simulation is also employed to study capacity gains of a third, more optimal, tier-selection scheme. The results point to differences in capacity performance based on the tier-selection method as well as on the traffic density within the hotspot region.

User mobility management is one of the important components of mobile multimedia systems. In a cell-based network, a mobile should be able to seamlessly obtain transmission resources after handoff to a new basestation. This is essential for both service continuity and quality of service assurance. We present strategies for accommodating continuous service to mobile users through estimating resource requirements of potential handoff connections. A diverse mix of heterogeneous traffic with diverse resource requirements is considered. We investigate static and dynamic resource allocation schemes. The dynamic scheme probabilistically estimates the potential number of connections that will be handed off from neighboring cells, for each class of traffic. The performance of these strategies in terms of connection blocking probabilities for handoff and local new connection requests are evaluated. The performance is also compared to a scheme previously proposed by Yu and Leung (see IEEE Journal on Selected Areas in Communications, vol.15, p.1208-25, 1997). The results indicate that using dynamic estimation and allocation, we can significantly reduce the dropping probability for handoff connections.

User mobility management is one of the important components of mobile multimedia systems. In a cell-based network, a mobile should be able to seamlessly obtain transmission resources after handoff to a new base station. This is essential for both service continuity and quality of service assurance. In this paper, we present strategies for accommodating continuous service to mobile users through estimating resource requirements of potential handoff connections. A diverse mix of heterogeneous traffic with diverse resource requirements is considered. The investigate static and dynamic resource allocation schemes. The dynamic scheme probabilistically estimates the potential number of connections that will be handed off from neighboring cells, for each class of traffic. The performance of these strategies in terms of connection blocking probabilities for handoff and local new connection requests are evaluated. The performance is also compared to a scheme previously proposed by Yu and Leung (see IEEE J. Select. Areas Commun., vol.15, p.1208-25, 1997). The results indicate that using dynamic estimation and allocation, we can significantly reduce the dropping probability for handoff connections.

Energy efficiency is an important issue in mobile wireless networks since the battery life of mobile terminals is limited. Conservation of battery power has been addressed using many techniques. This paper addresses energy efficiency in medium access control (MAC) protocols for wireless networks. The paper develops a framework to study the energy consumption of a MAC protocol from the transceiver usage perspective. This framework is then applied to compare the performance of a set of protocols that includes IEEE 802.11, energy-conserving MAC (EC-MAC), PRMA, multiservices dynamic reservation-TDMA (MDR-TDMA), and distributed-queueing request update multiple access (DQRUMA). The performance metrics considered are transmitter and receiver usage times for packet transmission and reception. The analysis here shows that protocols that aim to reduce the number of contentions perform better from a energy consumption perspective. The receiver usage time, however; tends to be higher for protocols that require the mobile to sense the medium before attempting transmission.

Mobile computers typically have limited energy for computing and communications due to short battery lifetimes. Encoding, decoding, and transmission of video information require significant computing and communication resources. Low power encoding and decoding schemes have been researched extensively. In this paper, we focus on processing encoded video for transmission under low battery power conditions. Such processing, while conserving battery power, attempts to reduce deterioration of video quality.