12 research outputs found
Multiuser MIMO Wireless Energy Transfer With Coexisting Opportunistic Communication
This letter considers spectrum sharing between a primary multiuser
multiple-input multiple-output (MIMO) wireless energy transfer (WET) system and
a coexisting secondary point-to-point MIMO wireless information transmission
(WIT) system, where WET generates interference to WIT and degrades its
throughput performance. We show that due to the interference, the WIT system
suffers from a loss of the degrees of freedom (DoF) proportional to the number
of energy beams sent by the energy transmitter (ET), which, in general, needs
to be larger than one in order to optimize the multiuser WET with user fairness
consideration. To minimize the DoF loss in WIT, we further propose a new
single-beam energy transmission scheme based on the principle of time sharing,
where the ET transmits one of the optimal energy beams at each time. This new
scheme achieves the same optimal performance for the WET system, and minimizes
the impact of its interference to the WIT system.Comment: submitted for possible publicatio
Multi-antenna Wireless Powered Communication with Co-channel Energy and Information Transfer
This letter studies a multi-antenna wireless powered communication (WPC)
system with co-channel energy and information transfer, where a wireless device
(WD), powered up by wireless energy transfer (WET) from an energy transmitter
(ET), communicates to an information receiver (IR) over the same frequency
band. We maximize the achievable data rate from the WD to the IR by jointly
optimizing the energy beamforming at the ET and the information beamforming at
the WD, subject to their individual transmit power constraints. We obtain the
optimal solution to this problem in closed-form, where the optimal energy
beamforming at the ET achieves a best energy/interference tradeoff between
maximizing the energy transfer efficiency to the WD and minimizing the
co-channel interference to the IR. Numerical results show that our proposed
optimal co-channel design is superior to other reference schemes.Comment: IEEE Communications Letters. Accepted. 9 pages, 4 figure
Optimal Energy Beamforming under Per-Antenna Power Constraint
Energy beamforming (EB) is a key technique to enhance the efficiency of
wireless power transfer (WPT). In this paper, we study the optimal EB under
per-antenna power constraint (PAC) which is more practical than the
conventional sum-power constraint (SPC). We consider a multi antenna energy
transmitter (ET) with PAC that broadcasts wireless energy to multiple randomly
placed energy receivers (ER)s within its cell area. We consider sum energy
maximization problem with PAC and provide the optimal solution structure for
the general case. This optimal structure implies that sending one energy beam
is optimal under PAC which means that the rank of transmit covariance matrix is
one similar to SPC. We also derive closed-form solutions for two special cases
and propose two sub-optimal solutions for general case, which performs very
close to optimal beamforming
Wireless Energy Transfer to a Pair of Energy Receivers using Signal Strength Feedback
This paper focuses on wireless energy transfer (WET) to a pair of low complex
energy receivers (ER), by only utilizing received signal strength indicator
(RSSI) values that are fed back from the ERs to the energy transmitter (ET).
Selecting the beamformer that maximizes the total average energy transfer
between the ET and the ERs, while satisfying a minimum harvested energy
criterion at each ER, is studied. This is a nonconvex constrained optimization
problem which is difficult to solve analytically. Also, any analytical solution
to the problem should only consists of parameters that the ET knows, or the ET
can estimate, as utilizing only RSSI feedback values for channel estimation
prohibits estimating some channel parameters. Thus, the paper focuses on
obtaining a suboptimal solution analytically. It is proven that if the channels
between the ET and the ERs satisfy a certain sufficient condition, this
solution is in fact optimal. Simulations show that the optimality gap is
negligibly small as well. Insights into a system with more than two ERs are
also presented. To this end, it is highlighted that if the number of ERs is
large enough, it is possible to always find a pair of ERs satisfying the
sufficient condition, and hence, a pairwise scheduling policy that does not
violate optimality can be used for the WET.Comment: 7 pages, 2 figures, To appear in International Symposium on Modeling
and Optimization in Mobile, Ad Hoc and Wireless Networks (WiOpt), 201
UAV-Enabled Wireless Power Transfer: Trajectory Design and Energy Region Characterization
This paper studies a new unmanned aerial vehicle (UAV)-enabled wireless power
transfer (WPT) system, where a UAV-mounted energy transmitter (ET) broadcasts
wireless energy to charge distributed energy receivers (ERs) on the ground. In
particular, we consider a basic two-user scenario, and investigate how the UAV
can optimally exploit its mobility to maximize the amount of energy transferred
to the two ERs during a given charging period. We characterize the achievable
energy region of the two ERs, by optimizing the UAV's trajectory subject to a
maximum speed constraint. We show that when the distance between the two ERs is
smaller than a certain threshold, the boundary of the energy region is achieved
when the UAV hovers above a fixed location between them for all time; while
when their distance is larger than the threshold, to achieve the boundary of
the energy region, the UAV in general needs to hover and fly between two
different locations above the line connecting them. Numerical results show that
the optimized UAV trajectory can significantly improve the WPT efficiency and
fairness of the two ERs, especially when the UAV's maximum speed is large
and/or the charging duration is long.Comment: Submitted for possible conference publicatio
Wireless Information and Power Transfer Design for Energy Cooperation Distributed Antenna Systems
Distributed antenna systems (DAS) have been widely implemented in
state-of-the-art cellular communication systems to cover dead spots. Recent
studies have also indicated that DAS have advantages in wireless energy
transfer (WET). In this paper, we study simultaneous wireless information and
power transfer (SWIPT) for a multiple-input single-output (MISO) DAS in the
downlink which consists of arbitrarily distributed remote antenna units (RAUs).
In order to save the energy cost, we adopt energy cooperation of energy
harvesting (EH) and two-way energy flows to let the RAUs trade their harvested
energy through the smart grid network. Under individual EH constraints, per-RAU
power constraints and various smart grid considerations, we investigate a power
management strategy that determines how to utilize the stochastically spatially
distributed harvested energy at the RAUs and how to trade the energy with the
smart grid simultaneously to supply maximum wireless information transfer (WIT)
with a minimum WET constraint for a receiver adopting power splitting (PS). Our
analysis shows that the optimal design can be achieved in two steps. The first
step is to maximize a new objective that can simultaneously maximize both WET
and WIT, considering both the smart grid profitable and smart grid neutral
cases. For the grid-profitable case, we derive the optimal full power strategy
and provide a closed-form result to see under what condition this strategy is
used. On the other hand, for the grid-neutral case, we illustrate that the
optimal power policy has a double-threshold structure and present an optimal
allocation strategy. The second step is then to solve the whole problem by
obtaining the splitting power ratio based on the minimum WET constraint.
Simulation results are provided to evaluate the performance under various
settings and characterize the double-threshold structure.Comment: 11 pages, 7 figure
Cognitive Wireless Power Transfer in the Presence of Reactive Primary Communication User
This paper studies a cognitive or secondary multi-antenna wireless power
transfer (WPT) system over a multi-carrier channel, which shares the same
spectrum with a primary wireless information transfer (WIT) system that employs
adaptive water-filling power allocation. By controlling the transmit energy
beamforming over sub-carriers (SCs), the secondary energy transmitter (S-ET)
can directly charge the secondary energy receiver (S-ER), even purposely
interfere with the primary WIT system, such that the primary information
transmitter (P-IT) can reactively adjust its power allocation (based on
water-filling) to facilitate the S-ER's energy harvesting. We investigate how
the secondary WPT system can exploit the primary WIT system's reactive power
allocation, for improving the wireless energy harvesting performance. In
particular, our objective is to maximize the total energy received at the S-ER
from both the S-ET and the P-IT, by optimizing the S-ET's energy beamforming
over SCs, subject to its maximum transmit power constraint, and the maximum
interference power constraint imposed at the primary information receiver
(P-IR) to protect the primary WIT. Although the formulated problem is
non-convex and difficult to be optimally solved in general, we propose an
efficient algorithm to obtain a high-quality solution by employing the Lagrange
dual method together with a one-dimensional search. We also present two
benchmark energy beamforming designs based on the zero-forcing (ZF) and
maximum-ratio-transmission (MRT) principles, respectively, as well as the
conventional design without considering the primary WIT system's reaction.
Numerical results show that our proposed design leads to significantly improved
energy harvesting performance at the S-ER, as compared to these benchmark
schemes
Joint Transmit and Reflective Beamforming Design for IRS-Assisted Multiuser MISO SWIPT Systems
This paper studies an intelligent reflecting surface (IRS)-assisted multiuser
multiple-input single-output (MISO) simultaneous wireless information and power
transfer (SWIPT) system. In this system, a multi-antenna access point (AP) uses
transmit beamforming to send both information and energy signals to a set of
receivers each for information decoding (ID) or energy harvesting (EH), and a
dedicatedly deployed IRS properly controls its reflecting phase shifts to form
passive reflection beams for facilitating both ID and EH at receivers. Under
this setup, we jointly optimize the (active) information and energy transmit
beamforming at the AP together with the (passive) reflective beamforming at the
IRS, to maximize the minimum power received at all EH receivers, subject to
individual signal-to-interference-plus-noise ratio (SINR) constraints at ID
receivers, and the maximum transmit power constraint at the AP. Although the
formulated SINR-constrained min-energy maximization problem is highly
non-convex, we present an efficient algorithm to obtain a high-quality solution
by using the techniques of alternating optimization and semi-definite
relaxation (SDR). Numerical results show that the proposed IRS-assisted SWIPT
system with both information and energy signals achieves significant
performance gains over benchmark schemes without IRS deployed and/or without
dedicated energy signals used
Cognitive Wireless Powered Network: Spectrum Sharing Models and Throughput Maximization
The recent advance in radio-frequency (RF) wireless energy transfer (WET) has
motivated the study of wireless powered communication network (WPCN), in which
distributed wireless devices are powered via dedicated WET by the hybrid
access-point (H-AP) in the downlink (DL) for uplink (UL) wireless information
transmission (WIT). In this paper, by exploiting the cognitive radio (CR)
technique, we study a new type of CR enabled secondary WPCN, called cognitive
WPCN, under spectrum sharing with the primary wireless communication system. In
particular, we consider a cognitive WPCN, consisting of one single H-AP with
constant power supply and distributed users, shares the same spectrum for its
DL WET and UL WIT with an existing primary communication link, where the WPCN's
WET/WIT and the primary link's WIT may interfere with each other. Under this
new setup, we propose two coexisting models for spectrum sharing of the two
systems, namely underlay and overlay based cognitive WPCNs, depending on
different types of knowledge on the primary user transmission available at the
cognitive WPCN. For each model, we maximize the sum-throughput of the cognitive
WPCN by optimizing its transmission under different constraints applied to
protect the primary user transmission. Analysis and simulation results are
provided to compare the sum-throughput of the cognitive WPCN versus the
achievable rate of the primary user in two coexisting models. It is shown that
the overlay based cognitive WPCN outperforms the underlay based counterpart,
thanks to its fully cooperative WET/WIT design with the primary WIT, while it
also requires higher complexity for implementation.Comment: This is the longer version of a paper to appear in IEEE Transactions
on Cognitive Communications and Networkin
A General Design Framework for MIMO Wireless Energy Transfer with Limited Feedback
Multi-antenna or multiple-input multiple-output (MIMO) technique can
significantly improve the efficiency of radio frequency (RF) signal enabled
wireless energy transfer (WET). To fully exploit the energy beamforming gain at
the energy transmitter (ET), the knowledge of channel state information (CSI)
is essential, which, however, is difficult to be obtained in practice due to
the hardware limitation of the energy receiver (ER). To overcome this
difficulty, under a point-to-point MIMO WET setup, this paper proposes a
general design framework for a new type of channel learning method based on the
ER's energy measurement and feedback. Specifically, the ER measures and encodes
the harvested energy levels over different training intervals into bits, and
sends them to the ET via a feedback link of limited rate. Based on the
energy-level feedback, the ET adjusts transmit beamforming in subsequent
training intervals and obtains refined estimates of the MIMO channel by
leveraging the technique of analytic center cutting plane method (ACCPM) in
convex optimization. Under this general design framework, we further propose
two specific feedback schemes termed energy quantization and energy comparison,
where the feedback bits at each interval are generated at the ER by quantizing
the measured energy level at the current interval and comparing it with those
in the previous intervals, respectively. Numerical results are provided to
compare the performance of the two feedback schemes. It is shown that energy
quantization performs better when the number of feedback bits per interval is
large, while energy comparison is more effective with small number of feedback
bits.Comment: This is a longer version of a paper to appear in IEEE Transactions on
Signal Processin