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Simultaneous Wireless Information and Power Transfer in Modern Communication Systems
Ioannis Krikidis, Stelios Timotheou, Symeon Nikolaou, Gan Zheng, Derrick Wing Kwan Ng, Robert Schober
TL;DR
Wireless communication networks need energy harvesting that can operate from surrounding electromagnetic sources, but SWIPT couples increased power transfer with increased interference. This survey reviews rectenna hardware, practical SWIPT techniques across time, power, antennas, and space, and integration into resource allocation and cooperative cognitive-radio networks. It concludes that SWIPT changes resource allocation and supports information/energy cooperation, while hardware implementations remain largely limited to opportunistic WPT systems.
Problem
Energy harvesting for wireless networks must support terminals using ambient or dedicated electromagnetic sources, while SWIPT creates a coupled power-transfer and interference challenge.
Method
The paper surveys rectenna hardware, SWIPT techniques across time, power, antennas, and space, and applications in resource allocation and cooperative cognitive-radio networks.
Results
SWIPT introduces fundamental changes to resource allocation, including scheduling, power control, and interference management, and enables information/energy cooperation in cognitive-radio networks.
Takeaways & Limitations
Integrating SWIPT can improve overall spectrum efficiency without relying on external energy sources, especially for low-power relay nodes.
Takeaways & Limitations
Hardware implementations have mostly been limited to WPT systems that opportunistically scavenge energy.
Abstract
from arXiv · showhide
Energy harvesting for wireless communication networks is a new paradigm that allows terminals to recharge their batteries from external energy sources in the surrounding environment. A promising energy harvesting technology is wireless power transfer where terminals harvest energy from electromagnetic radiation. Thereby, the energy may be harvested opportunistically from ambient electromagnetic sources or from sources that intentionally transmit electromagnetic energy for energy harvesting purposes. A particularly interesting and challenging scenario arises when sources perform simultaneous wireless information and power transfer (SWIPT), as strong signals not only increase power transfer but also interference. This paper provides an overview of SWIPT systems with a particular focus on the hardware realization of rectenna circuits and practical techniques that achieve SWIPT in the domains of time, power, antennas, and space. The paper also discusses the benefits of a potential integration of SWIPT technologies in modern communication networks in the context of resource allocation and cooperative cognitive radio networks.
I. ABSTRACT
SWIPT unifies wireless information and energy transmission, while requiring practical receiver designs and communication-system adaptations. The paper surveys rectenna hardware, signal-splitting techniques, resource allocation, and cooperative cognitive-radio applications.
- Motivation: Wireless power transfer enables terminals to harvest energy from ambient electromagnetic radiation or dedicated transmitting sources.The paper emphasizes wireless power transfer as an alternative energy-harvesting technology for wireless networks.
- Motivation: SWIPT superposes information and power transfer, potentially improving spectral efficiency, time delay, energy consumption, and interference management.The same signal can charge wireless implants or sensor nodes while carrying control or communication information.
- Scope: The survey explains rectenna circuits, which convert microwave energy into direct-current electricity, and practical SWIPT signal-splitting techniques.Because of practical limitations, SWIPT separates the received signal into two orthogonal parts.
- Scope: The paper presents SWIPT techniques that separate received signals in the domains of time, power, antennas, and space.These techniques address practical ways to realize simultaneous information and energy transfer.
- Network integration: SWIPT fundamentally modifies radio-resource allocation and motivates cognitive-radio scenarios supporting information and energy cooperation between primary and secondary networks.The paper discusses implications for resource allocation and cooperative cognitive-radio networks.
III. WPT MODULE COMPONENTS
Wireless power transfer includes near-field charging, directed far-field power beaming, and ambient RF scavenging. This paper focuses on far-field WPT using antennas communicating in the far field.
- WPT classifications: Wireless power transfer is classified into near-field coupling, far-field directive power beaming, and far-field low-power ambient RF scavenging.The three cases differ in coupling mechanism, distance, directionality, and available power.
- Near-field WPT: Near-field transfer uses inductive, capacitive, or resonant coupling to deliver tenths of Watts over short distances up to one meter.These distances are sub-wavelength.
- Far-field WPT: Far-field directive power beaming uses directive antennas to transfer several mWatts over distances up to several meters.The stated environments include indoor and outdoor settings.
- Ambient RF scavenging: Ambient RF scavenging opportunistically collects power from public transmitters, including cellular base stations and TV broadcasting stations.The collected power is in the range of several µWatts and supports communication with peer nodes.
- Scope: The paper’s main focus is far-field WPT, which uses antennas communicating in the far field.Near-field applications such as electric-car and cellphone charging are noted but are not the main focus.
A. Wireless Power Receiver Module
A wireless power receiver combines an antenna, matching network, RF-to-DC converter, power management unit, and energy storage. Practical performance depends strongly on antenna conditions and rectifier input power, which can make ambient harvesting inefficient.
- Receiver architecture: The receiver module comprises an antenna or array, matching network, RF-to-DC converter, power management unit, and energy storage unit.The storage unit supplies the CPU, sensors, and low-duty-cycle communication transceiver.
- Receiver architecture: A field implementation scavenged ambient power from carrier wireless digital TV signals broadcast 6.3 km from Tokyo TV Tower.The cited prototype field measurement was conducted in downtown Tokyo.
- Antenna conditions: Maximum received power requires transmitter-receiver alignment, high antenna gain, and polarization alignment, but multipath can randomize polarization.Dual orthogonal linear polarization or omnidirectional antennas can address polarization or direction uncertainty, respectively.
- Antenna conditions: Broadband or multiband antennas are preferred because received power is integrated over frequency and broadband reception can collect more power than narrowband reception.The effective area is Ae = (λ^2G_R)/(4π).
- Rectifier design: RF-to-DC conversion is a critical design challenge because efficiency depends on incident RF power, delivered load, and DC voltage.Rectifiers may use single-diode circuits, Dickson charge pumps, Schottky diodes, or transistor-based integrated implementations.
- Rectifier design: At low power levels, diode forward voltage can reduce rectifier efficiency to zero, so reported high efficiencies may not occur in practical RF scavenging.A Tokyo measurement found approximately 50 µW received power, while high-efficiency rectifiers required 0.5−5 mW input power; measured efficiency was consequently small.
- Power management: The power management unit maintains an optimum rectifier load while charging the energy storage unit without additional loss.This function must accommodate changing received RF power levels.
IV. TECHNIQUES FOR SWIPT
Practical SWIPT must separate the received signal into information-decoding and energy-harvesting paths because RF-domain harvesting destroys information content. The paper organizes these techniques across time, power, antenna, and space domains.
- Motivation: Early information-theoretic studies treated one signal as carrying both energy and information without losses, but practical RF harvesting destroys the information content.This creates a fundamental information-power transfer trade-off in practical systems.
- Signal splitting: Practical SWIPT splits the received signal into distinct parts for energy harvesting and information decoding.The paper discusses splitting in the time, power, antenna, and space domains.
- Signal splitting: The four practical signal-splitting domains are time, power, antenna, and space.Figure 3 labels these as time, power, antenna, and space transmission techniques.
A. Time Switching (TS)
Time switching separates SWIPT operations by scheduling receiver slots for information decoding or energy harvesting. It simplifies hardware but depends on accurate synchronization and scheduling.
- A. Time Switching (TS): Time switching makes the receiver alternate between information decoding and energy harvesting in separate time slots.The entire signal in a slot serves either information decoding or power transfer.
- A. Time Switching (TS): TS offers simple receiver hardware implementation but requires accurate time synchronization and information/energy scheduling.Its main trade-off is implementation simplicity versus coordination requirements.
B. Power Splitting (PS)
Power splitting performs SWIPT simultaneously by dividing the received signal into streams with different power levels. It increases receiver complexity but supports instantaneous information and power transfer.
- B. Power Splitting (PS): Power splitting divides the received signal into two streams with different power levels, directing one to the rectenna and the other to baseband decoding.A power-splitting component performs the signal division.
- B. Power Splitting (PS): PS has higher receiver complexity than TS and requires optimization of the power-splitting factor α.The trade-off arises from adding simultaneous signal paths and a tunable split.
- B. Power Splitting (PS): PS enables instantaneous SWIPT because the received signal in one time slot supports both information decoding and power transfer.This contrasts with TS, which assigns a slot to one function at a time.
- B. Power Splitting (PS): PS is more suitable for applications with critical information or energy requirements or delay constraints and is closer to the information-theoretical optimum.These benefits are stated alongside its simultaneous operation.
C. Antenna Switching (AS)
Antenna switching dynamically assigns receiving antennas between information decoding and energy harvesting, while GSC-based variants select antennas according to channel strength. These approaches involve per-frame optimization and can have high complexity.
- C. Antenna Switching (AS): Antenna switching dynamically assigns each receiving antenna to information decoding or energy harvesting.The antennas are divided into two groups, with assignments optimized for each communication frame.
- C. Antenna Switching (AS): Optimal antenna switching requires solving an optimization problem in every communication frame.For MIMO decode-and-forward relay channels, the relay uses harvested energy to retransmit the received signal.
- C. Antenna Switching (AS): Optimal antenna switching can have high complexity, motivating lower-complexity mechanisms based on generalized selection combining.The antenna-assignment problem has also been formulated as a knapsack problem and solved using dynamic programming.
- C. Antenna Switching (AS): GSC-AS uses the L strongest channel paths for either energy or information and assigns the remaining antennas to the other function.The GSCE and GSCI variants prioritize energy harvesting and information decoding, respectively.
D. Spatial Switching (SS)
Spatial switching realizes SWIPT by assigning MIMO eigenchannels to information or energy, with switches directing each output to decoding or rectification. In the relay example, GSCI generally outperforms GSCE, while PS beats AS and TS performs poorly.
- D. Spatial Switching (SS): SVD transforms the MIMO link into parallel eigenchannels that independently convey either information or energy.A switch at each eigenchannel output directs the signal to a decoding or rectification circuit.
- D. Spatial Switching (SS): Eigenchannel assignment and power allocation form a nonlinear combinatorial optimization problem.An optimal polynomial-complexity algorithm is available for the special case of unlimited maximum power per eigenchannel.
- D. Spatial Switching (SS): In the relay example, GSCI outperforms GSCE for L = 1 and L = 2.The setup uses a battery-free MIMO relay that harvests energy to power relaying transmission, with outage probability as the metric.
- D. Spatial Switching (SS): GSCI with L = 1 achieves a diversity gain of two, indicating that diversity gain is more important than energy harvesting in this setting.This conclusion is attributed to the high RF-to-DC efficiency η.
- D. Spatial Switching (SS): PS outperforms AS by 2.5 dB at high P, whereas TS performs poorly because of its required time division.The comparison is reported for outage probability versus transmit power P with r0 = 2 BPCU and NT = 3.
V. RESOURCE ALLOCATION FOR SYSTEMS WITH SWIPT
SWIPT resource allocation must jointly manage information, energy, time, and interference because RF signals serve dual purposes. The discussed examples show trade-offs between harvesting and throughput, while power control and receiver scaling improve the achievable capacity–energy region.
- V. RESOURCE ALLOCATION FOR SYSTEMS WITH SWIPT: SWIPT makes efficient energy transfer an additional QoS requirement alongside throughput, reliability, energy efficiency, fairness, and delay.Resource-allocation studies therefore address energy and information scheduling, interference management, and joint power control.
- V. RESOURCE ALLOCATION FOR SYSTEMS WITH SWIPT: The large sensitivity gap between energy harvesting at −10 dBm and information decoding at −60 dBm obstructs practical SWIPT realization.Joint power control and user scheduling are identified as key responses.
- V. RESOURCE ALLOCATION FOR SYSTEMS WITH SWIPT: With optimal power control, the capacity–harvested-energy trade-off region increases significantly with NT, and average harvested energy improves with more energy-harvesting receivers.The example considers one information receiver and K energy-harvesting receivers served by an NT-antenna transmitter.
- V. RESOURCE ALLOCATION FOR SYSTEMS WITH SWIPT: Harvest-then-transmit allocates downlink time to harvesting before uplink transmission, trading greater harvested energy for less uplink transmission time.Varying the allocation can change the resulting system throughput.
- V. RESOURCE ALLOCATION FOR SYSTEMS WITH SWIPT: Strong interference can serve as an energy source, and artificial interference may benefit systems whose receivers lack sufficient operating energy.Interference alignment or coordination can concentrate multicell interference into wireless charging zones.
VI. JOINT INFORMATION AND ENERGY COOPERATION IN CR NETWORKS
SWIPT extends cooperative cognitive radio networks by allowing primary transmitters to provide both information and energy to low-power secondary relays. The proposed cooperation enlarges achievable rate regions and can substantially improve secondary and primary rates over information-only cooperation.
- VI. JOINT INFORMATION AND ENERGY COOPERATION IN CR NETWORKS: Conventional cognitive-radio cooperation requires the secondary transmitter to have sufficient transmit power, which low-power relays may lack.SWIPT addresses this challenge by linking information cooperation with energy transfer.
- VI. JOINT INFORMATION AND ENERGY COOPERATION IN CR NETWORKS: The proposed scheme has the primary transmitter send information and energy to the secondary transmitter, which relays primary information in return.This creates cooperation at both information and energy levels.
- VI. JOINT INFORMATION AND ENERGY COOPERATION IN CR NETWORKS: The additional energy cooperation provides a substantial gain over information-only cooperation and is presented as a promising solution for future CCRNs.The conclusion is supported by the reported achievable-rate improvements.
- VI. JOINT INFORMATION AND ENERGY COOPERATION IN CR NETWORKS: The amplify-and-forward scheme uses power splitting, with the secondary transmitter allocating α and 1−α of the received signal to processing and harvesting.It then jointly optimizes α and precoding vectors to maximize primary and secondary rates.
- VI. JOINT INFORMATION AND ENERGY COOPERATION IN CR NETWORKS: Extra energy cooperation enlarges achievable rate regions even at RF-to-DC efficiency η = 0.1.The comparison is against conventional information-cooperation-only cognitive radio schemes.
- VI. JOINT INFORMATION AND ENERGY COOPERATION IN CR NETWORKS: When the required PU rate is 2 bps/Hz, the SU rate doubles or triples as η increases from 0.1 to 1.When the SU rate is 1.5 bps/Hz, the PU achieves a 75% higher data rate.
VII. CONCLUSION AND FUTURE WORK
The survey concludes that SWIPT changes fundamental communication-network operations and creates new research challenges spanning resource allocation, hardware, security, path loss, applications, and cooperative cognitive radio networks.
- The survey overviews SWIPT techniques and discusses information/energy cooperation between primary and secondary systems in cognitive radio networks.
- SWIPT introduces fundamental changes in resource allocation, influencing scheduling, power control, and interference management.
- Long-distance SWIPT transmission is expected to have unsatisfactory efficiency unless advanced resource allocation and antenna technologies are combined.Massive MIMO can steer directive energy/information beams, while coordinated multipoint can reduce transmitter–receiver distance and provide spatial diversity.
- SWIPT creates communication and energy security concerns because stronger information signals can increase eavesdropping susceptibility and receivers can falsify channel-state information.
- SWIPT hardware development remains important because implementations have mostly been limited to opportunistic ambient-energy harvesting, while circuit inefficiencies create technique tradeoffs.TS is theoretically less efficient than PS, but PS suffers power-splitting losses not accounted for in the theoretical comparison.
- Potential SWIPT applications include structural, healthcare, and building monitoring, but realization requires challenges to be overcome across hardware, protocols, and architecture.