## About

156

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September 2004 - present

## Publications

Publications (156)

This paper studies joint spectrum allocation and user association in large heterogeneous cellular networks. The objective is to maximize some network utility function based on given traffic statistics collected over a slow timescale, conceived to be seconds to minutes. A key challenge is scalability: interference across cells creates dependencies a...

This paper studies joint spectrum allocation and user association in large heterogeneous cellular networks. The objective is to maximize some network utility function based on given traffic statistics collected over a slow timescale, conceived to be seconds to minutes. A key challenge is scalability: interference across cells creates dependencies a...

This work studies centralized slow-timescale spectrum management in metropolitan area networks with a very large number of access points (APs) and user equipments (UEs). The joint spectrum allocation and user association problem is first formulated as a convex optimization problem with an exponential number of variables in the network size. A scala...

Facing the challenge of meeting ever-increasing demand for wireless data, the industry is striving to exploit large swaths of unlicensed spectrum which supports open access. Major standards bodies are currently considering a proposal to retool and deploy Long Term Evolution (LTE) technologies in unlicensed bands below 6 GHz. This paper studies the...

Joint allocation of spectrum and user association is considered for a large cellular network. The objective is to optimize a network utility function such as average delay given traffic statistics collected over a slow timescale. A key challenge is scalability: given $n$ Access Points (APs), there are $O(2^n)$ ways in which the APs can share the sp...

In future networks, an operator may employ a wide range of access points using diverse radio access technologies (RATs) over multiple licensed and unlicensed frequency bands. This paper studies centralized user association and spectrum allocation across many access points in such a heterogeneous network (HetNet). Such centralized control is on a re...

A scalable framework is developed to allocate radio resources across a large number of densely deployed small cells with given traffic statistics on a slow timescale. Joint user association and spectrum allocation is first formulated as a convex optimization problem by dividing the spectrum among all possible transmission patterns of active access...

Millimeter wave provides a promising approach for meeting the ever-growing traffic demand in next generation wireless networks. It is crucial to obtain relatively accurate channel state information so that beamforming/combining can be performed to compensate for severe path loss in this band. In contrast to lower frequencies, a typical mobile milli...

In massive multiple-input multiple-output (MIMO) systems, acquisition of the channel state information at the transmitter side (CSIT) is crucial. In this paper, a practical CSIT estimation scheme is proposed for frequency division duplexing (FDD) massive MIMO systems. Specifically, each received pilot symbol is first quantized to one bit per dimens...

Classical multiuser information theory studies the fundamental limits of models with a fixed (often small) number of users as the coding blocklength goes to infinity. This work proposes a new paradigm, referred to as {\em many-user information theory}, where the number of users is allowed to grow with the blocklength. This paradigm is motivated by...

To attain the targeted data rates of next generation (5G) cellular networks requires dense deployment of small cells in addition to macro cells which provide wide coverage. Dynamic radio resource management (RRM) is crucial to the success of such heterogeneous networks due to much more pronounced traffic and interference variations in small cells....

Millimeter wave provides a very promising approach for meeting the
ever-growing traffic demand in next generation wireless networks. To utilize
this band, it is crucial to obtain the channel state information in order to
perform beamforming and combining to compensate for severe path loss. In
contrast to lower frequencies, a typical millimeter wave...

To attain the targeted data rates of next generation (5G) cellular networks requires dense deployment of small cells, in addition to macro cells which provide wide coverage. Dynamic radio resource management (RRM) is crucial to the success of such heterogeneous networks due to much more pronounced traffic and interference variations in small cells....

Next generation (5G) cellular networks are expected to be supported by an
extensive infrastructure with many-fold increase in the number of cells per
unit area compared to today. The total energy consumption of base transceiver
stations (BTSs) is an important issue for both economic and environmental
reasons. In this paper, an optimization-based fr...

As the traffic load in cellular networks grows rapidly, future generation
cellular networks will involve multiple radio access technologies (RATs) over
different frequency bands. This paper studies the spectrum allocation problem
in heterogeneous networks (HetNets) with multiple RATs over different bands
using the average packet sojourn time as per...

A nationwide interoperable public safety wireless broadband network is being planned by the First Responder Network Authority (FirstNet) under the auspices of the United States government. The public safety network shall provide the needed wireless coverage in the wake of an incident or a disaster. This paper proposes a drone-assisted multi-hop dev...

The memoryless additive inverse Gaussian noise channel model describing communication based on the exchange of chemical molecules in a drifting liquid medium is investigated for the situation of simultaneously an average-delay and a peak-delay constraint. Analytical upper and lower bounds on its capacity in bits per molecule use are presented. Thes...

Quasi-random nanostructures have attracted significant interests for photon management purposes. To optimize such patterns, typically very expensive fabrication processes are needed to create the pre-designed, subwavelength nanostructures. While quasi-random photonic nanostructures are abundant in nature (for example, in structural coloration), int...

Joint optimization of nonlinear precoders and receive filters is studied for
both the uplink and downlink in a cellular system. For the uplink, the base
transceiver station (BTS) receiver implements successive interference
cancellation, and for the downlink, the BTS station pre-compensates for the
interference with Tomlinson-Harashima precoding (TH...

Deploying relays and/or mobile base stations is a major means of extending
the coverage of a wireless network. This paper presents models, analytical
results, and algorithms to answer two related questions: The first is where to
deploy relays in order to extend the reach from a base station to the maximum;
the second is where to deploy a mobile bas...

This paper studies the optimization of resource allocation in wireless
heterogenous networks (HetNets) on a relatively slow timescale. Unlike most
previous work on physical layer resource allocation, this work considers
traffic dynamics and uses the average packet sojourn time rather than the sum
rate and the outage probability as the performance m...

Facing the challenge of providing sufficient network capacity for wireless
data, the industry is currently debating how to take advantage of hundreds of
megahertz of unlicensed spectrum. One specific proposal being considered by the
3GPP is to retool and deploy Long Term Evolution (LTE) technologies in
unlicensed bands. This paper studies the funda...

In recent years, a number of mathematical relationships have been established between information measures and estimation measures for various models, including Gaussian, Poisson and binomial models. In this paper, it is shown that the second derivative of the input-output mutual information with respect to the input scaling can be expressed as the...

Most work on wireless network resource allocation use physical layer
performance such as sum rate and outage probability as the figure of merit.
These metrics may not reflect the true user QoS in future heterogenous networks
(HetNets) with many small cells, due to large traffic variations in overlapping
cells with complicated interference condition...

Classical multiuser information theory studies the fundamental limits of
models with a fixed (often small) number of users as the coding blocklength
goes to infinity. This work proposes a new paradigm, referred to as many-user
information theory, where the number of users is allowed to grow with the
blocklength. This paradigm is motivated by emergi...

Classical multiuser information theory studies the fundamental limits of
models with a fixed (often small) number of users as the coding blocklength
goes to infinity. Motivated by emerging systems with a massive number of users,
this paper studies the new {\em many-user paradigm}, where the number of users
is allowed to grow with the blocklength. T...

In recent years, a number of new connections between information measures and estimation have been found under various models, including, predominantly, Gaussian and Poisson models. This paper develops similar results for the binomial and negative binomial models. In particular, it is shown that the derivative of the relative entropy and the deriva...

In a peer-to-peer wireless network, efficient distributed scheduling algorithms generally require all active nodes to acquire the state information of their respective neighbors, e.g., their queue lengths. However, information exchange between nodes is challenging in the absence of a collision-free schedule, mainly due to the half duplex design of...

Collisions with hidden terminals is a major cause of performance degradation
in 802.11 and likewise wireless networks. Carrier sense multiple access with
collision avoidance (CSMA/CA) is utilized to avoid collisions at the cost of
spatial reuse. This report studies receiver design to mitigate interference
from hidden terminals. A wireless channel m...

In-band full-duplex (IBFD) operation has emerged as an attractive solution
for increasing the throughput of wireless communication systems and networks.
With IBFD, a wireless terminal is allowed to transmit and receive
simultaneously in the same frequency band. This tutorial paper reviews the main
concepts about in-band full-duplex wireless .A key...

This paper studies communication networks with a very large number of users simultaneously communicating with an access point. A new notion of many-access channel (MnAC) is introduced, which is the same as a multiaccess channel except that the number of users increases unboundedly with the coding block length. Unlike the conventional multiaccess ch...

Location-based services in a wireless network require nodes to know their
locations accurately. Conventional solutions rely on contention-based medium
access, where only one node can successfully transmit at any time in any
neighborhood. In this paper, a novel, complete, distributed ranging and
localization solution is proposed, which let all nodes...

The throughput and stability properties of wireless network coding are evaluated for an arbitrary number of terminals exchanging broadcast traffic with the aid of a relay. First, coding and scheduling schemes are derived that minimize the number of transmissions needed for each node to broadcast one packet. For stochastically varying traffic, the s...

In recent years, a number of results have been developed which connect information measures and estimation measures under various models, including, predominantly, Gaussian and Poisson models. More recent results due to Gil Taborda and Pérez-Cruz relate the relative entropy to certain mismatched estimation errors in the context of binomial and nega...

In a molecular communication system, molecules convey the information by traversing from the transmitter to the receiver through the medium, which is often liquid. The time for a molecule to travel a fixed distance according to Brownian motion with a constant drift has the inverse Gaussian distribution. Hence the molecular communication channel is...

This paper addresses relay design for a wireless multiple-input-multiple-output (MIMO) switching scheme that enables data exchange among multiple users. Here, a multi-antenna relay linearly precodes the received (uplink) signals from multiple users before forwarding the signal in the downlink, where the purpose of precoding is to let each user rece...

A nationwide interoperable public safety broadband network is being
planned by the United States government. The network will be based on
long term evolution (LTE) standards and use recently designated spectrum
in the 700 MHz band. The public safety network has different objectives
and traffic patterns than commercial wireless networks. In particul...

This paper studies the optimal use of limited-rate feedback of channel state information (CSI) in the case of a wideband multicarrier channel with multiple transmit antennas and a single receive antenna. With full knowledge of the CSI, the receiver should compute the optimal beamforming vectors, jointly quantize them, and feed them back to the tran...

Device-to-device (D2D) communications can be underlaid with a cellular infrastructure to increase resource utilization, improve user throughput and extend battery lifetime of user equipment. This paper studies a hybrid D2D and cellular system where D2D transmissions are controlled by the cellular base transceiver stations (BTSs) and use the cellula...

Future cellular networks are expected to be heterogeneous with dense deployments of base transceiver stations and access points (wireless nodes). Energy consumption and interference management are two major issues in such networks. The goal of this work is to develop a framework for deciding if particular nodes should be powered off to reduce inter...

We consider the diffusion of innovation in two-sided markets where both sides choose between an incumbent technology and an innovation. Each player chooses whether to adopt the innovation or not, and repeatedly learns how many players on the other side of the market adopted the innovation and revises her decision accordingly. Using large-system ana...

In many wireless communication systems, radios are subject to a duty cycle
constraint, that is, a radio only actively transmits signals over a fraction of
the time. For example, it is desirable to have a small duty cycle in some low
power systems; a half-duplex radio cannot keep transmitting if it wishes to
receive useful signals; and a cognitive r...

Multicell joint processing can mitigate inter-cell interference and thereby increase the spectral efficiency of cellular systems. Most previous work has assumed phase-aligned (coherent) transmissions from different base transceiver stations (BTSs), which is difficult to achieve in practice. In this work, a noncoherent cooperative transmission schem...

A wireless relay with multiple antennas is called a multiple-input-multiple-output (MIMO) switch if it maps its input links to its output links using "precode-and-forward." Namely, the MIMO switch precodes the received signal vector in the uplink using some matrix for transmission in the downlink. This paper studies the scenario of K stations and a...

In recent years, a number of results have been developed which connect
information measures and estimation measures under various models, including,
predominently, Gaussian and Poisson models. More recent results due to Taborda
and Perez-Cruz relate the relative entropy to certain mismatched estimation
errors in the context of binomial and negative...

This paper addresses joint transceiver and relay design for a wireless
multiple-input-multiple-output (MIMO) switching scheme that enables data
exchange among multiple users. Here, a multi-antenna relay linearly precodes
the received (uplink) signals from multiple users before forwarding the signal
in the downlink, where the purpose of precoding is...

Two users at the two ends of a bidirectional channel wish to estimate the common state of the channel. The problem is usually treated as two separate one-way channel estimation problems: User 1 sends deterministic pilots to assist user 2 in estimating the channel, and vice versa. This paper questions whether such separation is optimal. In other wor...

A wireless relay which forms a one-to-one mapping from the inputs (uplinks) to the outputs (downlinks) is called a multiple-input-multiple-output (MIMO) switch. The MIMO switch carries out precode-and-forward, where all users send their signals in the uplink and then the MIMO switch precodes the received vector signal for broadcasting in the downli...

Many wireless communication systems are subject to duty cycle constraint, that is, a radio only actively transmits signals over a fraction of the time. For example, it is desirable to have a small duty cycle in some low-power systems; a half-duplex radio cannot keep transmitting if it wishes to receive useful signals; and a cognitive radio needs to...

Interference between nodes is a critical impairment in mobile ad hoc networks. This paper studies the role of multiple antennas in mitigating such interference. Specifically, a network is studied in which receivers apply zero-forcing beamforming to cancel the strongest interferers. Assuming a network with Poisson-distributed transmitters and indepe...

In this paper, we show that the characterisation of all determinant
inequalities for $n \times n$ positive definite matrices is equivalent to
determining the smallest closed and convex cone containing all entropy
functions induced by $n$ scalar Gaussian random variables. We have obtained
inner and outer bounds on the cone by using representable fun...

This paper fully determines the degree-of-freedom (DoF) region of two-user interference channels with arbitrary number of transmit and receive antennas in the case of isotropic and independent (or block-wise independent) fading, where the channel state information is available to the receivers but not to the transmitters. Thus, the DoF gap between...

This monograph surveys the interactions between information measures and estimation measures as well as their applications. The emphasis is on formulas that express the major information measures, such as entropy, mutual information and relative entropy in terms of the minimum mean square error achievable when estimating random variables contaminat...

Assuming additive Gaussian noise, a general sufficient condition on the input distribution is established to guarantee that the ratio of mutual information to signal-to-noise ratio (SNR) goes to one half nat as SNR vanishes. The result allows SNR-dependent input distribution and side information.

This paper studies a problem frequently seen in wireless networks: Every node wishes to broadcast information to nodes within a single hop, which are referred to as its peers. We call this problem mutual broadcast. A novel solution is proposed, which exploits the multiaccess nature of the wireless medium and addresses the half-duplex constraint at...

Distributed medium access control (MAC) protocols are proposed for wireless networks assuming that one-hop peers can exchange a small amount of state information periodically. Each station maintains a state and makes state transitions and transmission decisions based on its state and recent state information collected from its one-hop peers. A stat...

Joint optimization of linear and nonlinear precoders and receive filters is considered for both the uplink/downlink in a single cell. The users and base station have multiple antennas and channel state information is initially unknown and not explicitly estimated. In the nonlinear case, for the uplink we assume successive interference cancellation...

A wireless relay with multiple antennas is called a multiple-input-multiple-output (MIMO) switch if it forms a one-to-one mapping from the inputs (uplinks) to the outputs (downlinks). This paper studies the case with N source stations and N destination stations (which may be the same set), so that the mapping is any permutation of the N inputs. Mor...

Distributed medium access control (MAC) protocols are proposed for wireless
networks assuming that one-hop peers can periodically exchange a small amount
of state information. Each station maintains a state and makes state
transitions and transmission decisions based on its state and recent state
information collected from its one-hop peers. A stat...

A novel scheme for full-duplex neighbor discovery in wireless networks is proposed. The scheme allows all nodes to simultaneously discover one-hop neighbors and identify their network interface addresses (NIAs) within a single frame of transmission, which typically consists of no more than a few thousand symbols. The key technique is to assign each...

This paper studies the capacity region of a two-user ergodic interference channel with fading, where only one user is subject to interference from the other user, and the channel state information (CSI) is only available at the receivers. A layered erasure model with arbitrary fading statistics is studied first, whose capacity region is completely...

Multicell joint processing can mitigate inter-cell interference and thereby increase the spectral efficiency of cellular systems. Most previous work has assumed phase-aligned (coherent) transmissions from different base stations (BSTs), which is difficult to achieve in practice. In this work, a noncoherent cooperative transmission scheme for the do...

The design of scheduling schemes for wireless communication systems has been driven by a compromise between the objectives of system throughput and fairness among users. In case the quality of all user channels is known to the controller, proportional fair scheduling has been well understood. However, to acquire the channel quality information may...

The capacity of a fading channel can be substantially increased by feeding back channel state information from the receiver to the transmitter. If the feedback rate is limited, what state information to feed back and how to encode it are important questions. This paper studies power loading in a multicarrier system using no more than one bit of fee...

This paper studies the problem of neighbor discovery in wireless networks,
namely, each node wishes to discover and identify the network interface
addresses (NIAs) of those nodes within a single hop. A novel paradigm, called
compressed neighbor discovery is proposed, which enables all nodes to
simultaneously discover their respective neighborhoods...

A novel solution is proposed to undertake a frequent task in wireless
networks, which is to let all nodes broadcast information to and receive
information from their respective one-hop neighboring nodes. The
contribution is two-fold. First, as each neighbor selects one
message-bearing codeword from its unique codebook for transmission, it
is shown...

This paper introduces a novel paradigm for design- ing the physical and medium access control (MAC) layers of mobile ad hoc or peer-to-peer networks formed by half-duplex radios. A node equipped with such a radio cannot simultaneously transmit and receive useful signals at the same frequency. Unlike in conventional designs, where a node's transmiss...

The design of a scheduling scheme is crucial for the efficiency and user-fairness of wireless networks. Assuming that the quality of all user channels is available to a central controller, a simple scheme which maximizes the utility function defined as the sum logarithm throughput of all users has been shown to guarantee proportional fairness. Howe...

This paper fully determines the degree-of-freedom (DoF) region of two-user
interference channels with arbitrary number of transmit and receive antennas
and isotropic fading, where the channel state information is available to the
receivers but not to the transmitters. The result characterizes the capacity
region to the first order of the logarithm...

This work studies the capacity region of ergodic fading Gaussian interference channels, where only one of the users is subject to interference from the other user, and that channel state information is not available at transmitters (no CSIT). In particular, inner and outer bounds of the capacity region are obtained. The inner bound is achieved by a...

This paper studies medium access control (MAC) protocols for regular wireless networks, where only nearest-neighbor interactions are involved. Each station chooses a state in the current time slot, which determines whether it transmits or not, based on its own state and the states of all its nearest neighbors in the previous time slot. The dynamics...

Consider the minimum mean-square error (MMSE) of estimating an arbitrary random variable from its observation contaminated by Gaussian noise. The MMSE can be regarded as a function of the signal-to-noise ratio (SNR) as well as a functional of the input distribution (of the random variable to be estimated). It is shown that the MMSE is concave in th...

We consider the problem of signal estimation (denoising) from a statistical mechanical perspective, using a relationship between the minimum mean square error (MMSE), of estimating a signal, and the mutual information between this signal and its noisy version. The paper consists of essentially two parts. In the first, we derive several statistical-...

We consider broadcasting from a single source to multiple destinations in a linear wireless erasure network with feedback. The problem is to find the maximum stable throughput under different transmission policies with opportunistic network coding and forwarding. Given stochastically varying traffic, we assume that network coding decisions are base...

This paper studies a layered erasure model for interference channels, which is a simplification of the Gaussian fading interference channel using the deterministic model approach. It is assumed that interference is one-sided, so that one of the receivers is not subject to interference. Moreover, the channel state information (CSI) is known to the r...

Transmitter preceding is a crucial technique for harnessing the potential of multiple-input multiple-output (MIMO) fading channels. In many practical wireless systems, a limited amount of feedback from the receiver is available at the transmitter, which can be used to direct the choice of the precoder from a codebook to match the channel state. Ass...

Channel uncertainty and co-channel interference are two major challenges in the design of wireless systems such as future generation cellular networks. This paper studies receiver design for a wireless channel model with both time-varying Rayleigh fading and strong co-channel interference of similar form as the desired signal. It is assumed that th...

We consider multiple-input multiple-output (MIMO) systems exploiting the full diversity order of a MIMO fading channel via optimal beamforming and combining. Specifically, an analytical characterization of the transient regime of a training-based MIMO system over arbitrarily correlated channels is presented. No channel state information is assumed...

This work studies the capacity region of a two-user ergodic interference channel with fading, where only one of the users is subject to interference from the other user, and the channel state information (CSI) is only available at the receivers. A layered erasure model with one-sided interference and with arbitrary fading statistics is studied firs...

This paper studies two-user MIMO interference channel with isotropic fading. We assume that users are equipped with arbitrary number of antennas and the channel state information (CSI) is available at receivers only. An outer bound is obtained for the degree of freedom region, which suggests the loss of degrees of freedom due to the lack of CSI at...

In this paper we consider the application of the Ising model from statistical mechanics to model interactions among interfering stations in a wireless network with a line topology. Each station employs a medium access control (MAC) protocol such that it decides to transmit or not depending on the states of its nearest neighbors and itself in the pr...

Fast and efficient neighbor discovery is crucial to the deployment of wireless ad hoc networks. Conventional random access neighbor discovery schemes assume a collision model for the medium access control layer and require each neighbor to transmit its identity repeatedly with random delay to resolve collision. This paper proposes a compressed neig...

Compressed sensing deals with the reconstruction of a high-dimensional signal from far fewer linear measurements, where the signal is known to admit a sparse representation in a certain linear space. The asymptotic scaling of the number of measurements needed for reconstruction as the dimension of the signal increases has been studied extensively....

This paper establishes new information-estimation relationships pertaining to models with additive noise of arbitrary distribution. In particular, we study the change in the relative entropy between two probability measures when both of them are perturbed by a small amount of the same additive noise. It is shown that the rate of the change with res...

The achievable rate of a wideband multi-input single-output channel with multi-carrier transmission is studied with limited feedback of channel state information (CSI). The set of sub-channel vectors are assumed to be jointly quantized and relayed back to the transmitter. Given a fixed feedback rate, the performance of an optimal joint quantization...