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Application of Smart Antenna Technologies in Simultaneous Wireless Information and Power Transfer
Zhiguo Ding, Caijun Zhong, Derrick Wing Kwan Ng, Mugen Peng, Himal A. Suraweera, Robert Schober, H. Vincent Poor
TL;DR
Energy-constrained wireless networks need alternatives to battery replacement and intermittent conventional harvesting, motivating SWIPT, which uses RF signals for simultaneous information and energy transfer. This article studies MIMO and relaying applications across network topologies, finding improved reliability and outlining design challenges and performance trade-offs.
Problem
Energy-constrained wireless networks face limited battery lifetimes, while SWIPT requires changes to wireless-network design and balances information rates against harvested energy.
Method
The article investigates MIMO and relaying technologies for SWIPT across different network topologies, including their separate and combined use.
Results
An energy-harvesting relay decreases outage probability from 7 × 10^-1 to 5 × 10^-2 compared with direct transmission.
Takeaways & Limitations
Smart antenna technologies have the potential to improve SWIPT energy and spectral efficiency while exposing performance trade-offs and future design challenges.
Abstract
from arXiv · showhide
Simultaneous wireless information and power transfer (SWIPT) is a promising solution to increase the lifetime of wireless nodes and hence alleviate the energy bottleneck of energy constrained wireless networks. As an alternative to conventional energy harvesting techniques, SWIPT relies on the use of radio frequency signals, and is expected to bring some fundamental changes to the design of wireless communication networks. This article focuses on the application of advanced smart antenna technologies, including multiple-input multiple-output and relaying techniques, to SWIPT. These smart antenna technologies have the potential to significantly improve the energy efficiency and also the spectral efficiency of SWIPT. Different network topologies with single and multiple users are investigated, along with some promising solutions to achieve a favorable trade-off between system performance and complexity. A detailed discussion of future research challenges for the design of SWIPT systems is also provided.
I. INTRODUCTION
SWIPT combines wireless information transmission with RF energy extraction, addressing energy constraints in wireless sensor networks and requiring new network-design criteria. The article examines MIMO and relaying as smart-antenna technologies for improving SWIPT efficiency across network topologies.
- Wireless power transfer can extend wireless sensor network lifetimes, avoiding costly battery replacement and limitations from intermittent natural energy harvesting.
- SWIPT uses information-carrying signals for energy extraction, making the information-rate–harvested-energy trade-off an important performance criterion.
- MIMO and relaying are studied as smart-antenna technologies with potential to improve SWIPT energy and spectral efficiency.
- MIMO can focus RF energy from a multi-antenna data fusion center on sensors needing wireless charging, improving energy efficiency compared with a single-antenna transmitter.
- Relays can harvest RF energy from a source and use it to forward information, supporting efficient RF-signal use and information-and-energy cooperation.
- The article introduces SWIPT concepts, discusses separate and joint MIMO and relaying applications, and outlines future design challenges.
II. SWIPT: BASIC RECEIVER STRUCTURES
SWIPT receiver design must accommodate the separation of information decoding and energy harvesting, while antenna arrays or relaying can help provide sufficient operating power. The section surveys several receiver structures.
- Information decoding and energy harvesting generally cannot be performed on the same received signal in SWIPT systems.
- A single-antenna receiver may not collect enough energy to ensure reliable power supply.
- MIMO and relaying are presented as antenna-array deployments for generating sufficient power for reliable device operation.
- The receiver structures reviewed are power splitting, separated, time-switching, and antenna-switching architectures.
A. Separated Receiver
The separated receiver architecture assigns energy harvesting and information decoding to distinct receivers with separate antennas served by a common multi-antenna transmitter. Its information–energy trade-off can be optimized using channel information and feedback.
- The separated architecture implements separate EH and ID circuits in two receivers with separated antennas served by a common multiple-antenna transmitter.
- The architecture can be implemented with off-the-shelf components for the two individual receivers.
- Its achievable information rate and harvested energy can be traded off using channel state information and receiver feedback.
- The transmit covariance matrix can be optimized to maximize ID capacity subject to a minimum energy requirement at the EH receiver.
B. Time Switching Receiver
The time-switching receiver periodically assigns each receive antenna to energy harvesting or information decoding according to a switching sequence. The sequence and transmit signal can be jointly optimized using channel statistics and energy-transfer service requirements.
- Each receive antenna periodically switches between the EH and ID circuits according to a time-switching sequence.
- The time-switching sequence and transmit signal can be jointly optimized using channel statistics and energy-transfer quality-of-service requirements.
C. Power Splitting Receiver
The power splitting receiver divides received power into energy-harvesting and information-decoding streams, allowing their trade-off to be adjusted through per-antenna splitting ratios. Jointly optimizing the signal and splitting ratios can further improve performance.
- The receiver splits each antenna’s received power into two streams sent respectively to an energy harvester and an information decoder.This enables simultaneous energy harvesting and information decoding before active analog/digital processing.
- Varying the power splitting ratios balances system achievable information rate against harvested energy.The ratio can be optimized separately for each receive antenna.
- Joint optimization of the transmit signal and power splitting ratios provides further performance improvement.
- The evaluation considers average total harvested energy in mJ/s and average system achievable rate in bit/s/Hz.The setup uses a 915 MHz carrier, a 10-meter transmitter–receiver distance, and 10 Watt total transmit power.
- The channel model uses independent and identically distributed Rician fading coefficients with a Rician K-factor of 6 dB.
D. Antenna Switching Receiver
Antenna switching assigns subsets of receive antennas to information decoding or energy harvesting, offering a practical alternative to time switching and power splitting. In the evaluated comparison, optimized power splitting yields the largest trade-off region but incurs the greatest complexity and computational burden.
- Antenna switching selects L of N_R antennas for information decoding and uses the remaining N_R − L antennas for energy harvesting.
- The antenna switching protocol is easy to implement and attractive for practical SWIPT designs.It avoids the stringent synchronization required by time switching and performance degradation associated with hardware imperfections in power splitting.
- Antenna switching is theoretically a special case of power splitting with binary splitting ratios at each receive antenna.
- For the 2-transmit-antenna, 2-receive-antenna point-to-point scenario, the comparison evaluates optimal resource allocation across receiver structures.The separated receiver uses one antenna for energy harvesting and one for information decoding.
- The optimized power splitting receiver achieves the largest trade-off region among the considered receivers.This comes at the expense of the highest hardware complexity and computational burden for resource allocation.
III. MIMO SWIPT NETWORKS
MIMO provides additional spatial degrees of freedom for jointly supporting information transfer and wireless power transfer in single- and multiuser SWIPT networks. Interference management, user scheduling, antenna selection, and interference alignment expand design opportunities while introducing coordination and beamforming constraints.
- Additional transmit antennas can support beamforming that improves the efficiency of information and energy transfer.
- Multiuser MIMO: In multiuser MIMO, signals intended for information receivers can also wirelessly charge energy-harvesting receivers.
- Interference management: Co-channel interference must be considered in multiuser systems, motivating block diagonalization precoding and user scheduling.Scheduling can switch receivers between energy-harvesting and information-decoding roles according to channel quality.
- Multi-source networks: Multi-source multiuser MIMO introduces shared-spectrum interference, making interference control and coordination challenging.
- Interference alignment: Antenna selection and interference alignment can partition received signals into an information-transfer subspace and an energy-transfer subspace.
- Interference alignment: The aligned-interference design protects information transfer from interference while using formerly discarded interference as an energy source.The approach reframes interference control by exploiting undesired signals for SWIPT performance.
- Beamforming constraints: RF energy harvesting imposes additional constraints on transmit beamforming, requiring conventional solutions such as zero forcing and maximum ratio transmission to be modified.
IV. RELAY ASSISTED SWIPT SYSTEMS
Relay-assisted SWIPT uses energy-harvesting relays to improve reliability, but harvested power couples relay transmission to channel conditions and changes relay-selection design. User cooperation and power allocation can mitigate these effects.
- EH relays can extend relay battery lifetime by harvesting energy from received RF signals for forwarding.
- 7 × 10^-1 to 5 × 10^-2: an EH relay decreases outage probability compared with direct transmission.This corresponds to a more than ten-fold improvement in reception reliability.
- Without a direct source-destination link, EH-relay outage decays with SNR at log SNR, slower than the 1/SNR rate of conventional systems.The slower decay results from relay transmission power fluctuating with source-relay channel conditions.
- User cooperation and advanced power allocation can restore a 1/SNR outage-decay rate in networks with multiple user pairs and an EH relay.Water-filling-based and auction-based approaches are identified as examples of advanced allocation strategies.
- EH relays require different selection strategies because source-relay channels determine both transmission reliability and harvested energy.Moving the relay toward the conventional midpoint can worsen outage probability, and max-min selection may achieve only a small fraction of full diversity gain.
V. THE COMBINATION OF MIMO AND COOPERATIVE RELAYING IN SWIPT
Combining MIMO with cooperative relaying gives SWIPT systems additional spatial-processing and cooperation capabilities. The paper describes improved information–energy trade-offs and shows that more antennas and strong interference can increase achievable rates.
- Inactive MIMO-capable devices can act as relays for active users, including low-cost single-antenna sensors in heterogeneous networks.SWIPT encourages such cooperation because helping other users does not reduce relay-battery lifetime.
- MIMO relays can serve multiple source-destination pairs simultaneously while providing an additional dimension for SWIPT performance improvement.
- The combination of MIMO and cooperative relaying can improve the trade-off between information rate and harvested energy.
- Increasing relay antennas from three to six nearly triples the achievable ergodic rate.The rate is evaluated as a function of average CCI strength with an optimized power-splitting ratio.
- When CCI is strong, exploiting it produces substantial rate improvement, with higher gain as CCI becomes stronger.For weak CCI, specifically ρI ≤ −10 dB, the rate difference from the no-CCI case is negligible.
VI. RESEARCH CHALLENGES
The section identifies research challenges for energy-efficient MIMO and relay-assisted SWIPT, including optimization, relaying, and communication security. It also highlights practical constraints involving interference, circuit costs, energy storage, and channel-state information.
- Energy efficient MIMO SWIPT: Energy-efficient MIMO SWIPT requires jointly optimizing spatial, power, frequency, and time degrees of freedom while accounting for circuit consumption and finite energy storage.
- Energy efficient MIMO SWIPT: The non-convex energy-efficiency objective makes optimal algorithm design difficult, motivating low-complexity but efficient algorithms.
- Energy efficient SWIPT relaying: Relay-assisted SWIPT faces half-duplex spectral-efficiency loss, while successive and full-duplex relaying offer alternatives that introduce exploitable inter-relay or loopback interference.
- Energy efficient SWIPT relaying: Relay-assisted SWIPT can harvest energy at the source or destination, but protocols must add a phase for transmitting energy to those nodes.
- Energy efficient SWIPT relaying: Practical relaying designs must include circuit power, amplifier inefficiency, storage losses, coordination energy, and the signalling overhead required for channel-state information.
- Communication security management: Higher information-signal power can increase energy transfer but also increases information-leakage susceptibility, making communication security a critical SWIPT issue.
VII. CONCLUSIONS
The article discusses SWIPT concepts and receiver trade-offs, then investigates MIMO and relaying across different network topologies. It concludes by outlining future challenges for energy-efficient MIMO- and relay-assisted SWIPT systems.
- The article discusses basic SWIPT concepts and corresponding receiver architectures alongside system performance trade-offs.
- MIMO and relaying are investigated as smart antenna technologies for SWIPT across different network topologies.
- Future research challenges are outlined for designing energy-efficient MIMO- and relay-assisted SWIPT systems.