Chapter 4
Phase 1 of WIA-COMM-009 fixes the vocabulary that every later layer depends on. It is a data-format phase. It defines how a wireless-power-transfer (WPT) system, its resonators and links, its beam hazards and its regulatory standing are written down. The standard accepts that "wireless power" covers very different physical devices. A smartphone pad, a 22 kW vehicle pad, a kilometre-scale rectenna farm and a laser aimed at a drone share almost no hardware. They do share one question: how much power leaves the transmitter, how much arrives as usable DC, and who is allowed to be in the way?
The first decision in the Phase 1 envelope is the transfer mechanism. Every record is grouped under one of six, and the mechanism determines which other descriptors apply.
| Mechanism | Physical basis | Envelope notes from the specification |
|---|---|---|
| IPT (inductive) | Time-varying magnetic field between LF/HF coils | Coupling coefficient k of 0.05–0.5 |
| CPT (capacitive) | Electric-field coupling between conductive plates | Works through a dielectric |
| Resonant | High-Q LC tanks at a fixed frequency | k of 0.05–0.15 typical |
| Microwave | Directed beam with rectenna receivers | 2.45, 5.8, 24, 35 and 94 GHz |
| Laser | Directed near-IR/SWIR beam onto photovoltaic receivers | 808, 976 and 1550 nm |
| Space-solar | Orbital transmitter to an Earth-side rectenna farm | Research-track only |
The system descriptor is the root record. It carries seven canonical items:
Two of these items hold ranges or profiles rather than single numbers. Efficiency in WPT depends on distance, alignment and load, so a single "93%" on a datasheet says little about a deployed system. Recording a profile and a distance envelope lets a consumer of the record judge whether a claimed efficiency applies at its own operating point.
The regulatory authorisation envelope is mandatory in the root descriptor rather than an optional extra. It acknowledges that transmitting power is regulated activity, unlike transmitting data. Spectrum rules, exposure limits and licensing regimes all attach to what the record describes.
The specification builds on one physical fact: how power moves depends on distance measured in wavelengths. In the near field, below roughly λ/2π, the transfer is reactive and non-radiative. Magnetic coupling is modelled through mutual inductance and capacitive coupling through plate capacitance. Beyond about one wavelength the transfer is radiative and follows the Friis relation.
Near-field (d < λ/2π) Φ₂₁ = M × I₁ ; V₂ = -M × dI₁/dt
Far-field (d > λ) P_rx / P_tx = G_tx × G_rx × (λ / 4πd)²
Path loss L_path [dB] = 20 × log₁₀(4πd/λ)The specification defines three regimes, and the distance boundaries are the point of the table below.
| Regime | Distance | Efficiency scaling |
|---|---|---|
| Near-field | d < λ/2π | ∝ (1 + d²/r²)⁻³ |
| Fresnel | λ/2π < d < 2D²/λ | ∝ 1/d |
| Far-field | d > 2D²/λ | ∝ 1/d² |
Simple arithmetic on the standard's own frequencies shows why the mechanisms split as they do. These figures are derived here, not quoted from the specification.
The far-field term 2D²/λ explains why microwave and laser systems are described by aperture size. Their efficiency is bounded by how much of the beam a receiving aperture can capture, not by coupling. The microwave envelope therefore records transmitter aperture and gain, and a beam-steering style (mechanical, electronic phased-array or liquid-crystal).
The specification then turns the physics into a conversion chain that every system, whatever its mechanism, must populate:
DC → AC (Inverter) → Wireless Link → AC → DC (Rectifier)The quoted stage ranges are 92–98% for the inverter, 30–95% for the wireless link (distance dependent) and 85–95% for the rectifier. Overall efficiency is their product. Taking the extremes shows how wide the uncertainty is: roughly 23% for the worst combination and roughly 88% for the best. This spread is why Phase 1 asks for an efficiency profile. The link descriptor expresses the same chain as a link budget: transmit power, then free-space or coupling loss, then receiver-side conversion, then delivered DC power. For space-solar the chain gains two more stages, DC-to-RF conversion in orbit and atmospheric absorption.
Where the link budget describes the path, the resonator descriptor describes the hardware at each end. For inductive coils the specification gives a coupling coefficient and a loosely-coupled efficiency expression:
k = M / √(L₁ × L₂)
Q = ω × L / R = 2πf × L / R
η = k² × Q₁ × Q₂ / (1 + k² × Q₁ × Q₂) (k² « 1)The practical meaning is that a weak coupling coefficient can be compensated by a high quality factor. The specification's typical k values fall from 0.6–0.9 with excellent alignment (d < r) to below 0.3 when d exceeds the coil radius. The product k²·Q₁·Q₂ is the figure of merit. Design advice follows from it: Litz wire to curb skin effect, ferrite for flux concentration (µᵣ of 1000–3000) and minimal series resistance. The descriptor envelopes quote lower k ranges (0.05–0.5 for IPT, 0.05–0.15 for resonant) because they describe deployed operating points rather than best-case alignment.
Resonant systems operate at f₀ = 1/(2π√(LC)). Coupled-mode theory shows that two coupled resonators split into two eigenmodes, ω₊ = ω₀√(1+k) and ω₋ = ω₀√(1−k). The practical consequence is frequency splitting when coupling gets strong, so a fixed-frequency system that is well tuned at one distance can lose power at a closer one. This is the reason the resonator descriptor records operating-frequency stability across temperature, an impedance-matching envelope and a per-receiver decoupling envelope. Decoupling matters when several receivers share one transmitter, because each one loads the field and detunes the others.
Compensation topology is the other major descriptor field. The specification lists five:
| Topology | TX / RX capacitor | Characteristic |
|---|---|---|
| SS | Series / Series | Constant current, simple |
| SP | Series / Parallel | Constant-current TX, voltage RX |
| PS | Parallel / Series | Constant-voltage TX |
| PP | Parallel / Parallel | Constant voltage |
| LCC | L + C + C / Series or Parallel | Wide ZVS range |
The Phase 1 resonator descriptor names the topology on each side. The Tx side is a planar coil with LCC compensation or a parallel LC. The Rx side is a matched LCC or a series-parallel hybrid. A typical Q for a high-Q resonant system is 100–500. The topology matters downstream because it determines whether the source behaves like a current source or a voltage source, which in turn determines how a load change appears to the transmitter. The link descriptor carries the per-link distance and an alignment envelope giving the lateral and angular tolerance before efficiency falls below policy. For SAE J2954 this is the ±100 mm (X) and ±75 mm (Y) misalignment tolerance.
Capacitive links need only a different pair of formulas. Plate capacitance is C = ε₀·εᵣ·A/d, and capacitive reactance is X_C = 1/(2πfC). The specification's comparison with inductive transfer is qualitative and, for CPT, clear: it tolerates metal well and dielectrics poorly, radiates less EMI, and uses cheaper plates, at a somewhat lower typical efficiency (60–80% against 70–90%). Those trade-offs are why capacitive records appear for rotating machinery, underwater charging and sealed or implanted devices, while inductive records dominate where a user places a device on a surface.
The specification does not give market-size figures, so none are offered here. What it does provide is a map from mechanism to product class. Each class is anchored by an existing industry specification, and each of those specifications occupies a different corner of the frequency, power and range space.
| Industry specification | Frequency | Power | Range | Typical devices |
|---|---|---|---|---|
| Qi (WPC) | 80–300 kHz baseline, 2 MHz extended | 5–15 W baseline, up to 100 W extended | 0–5 mm | Smartphones, wearables |
| AirFuel Resonant | 6.78 MHz | 5–50 W | 0–50 mm | Laptops, tablets |
| SAE J2954 | 85 kHz | 3.7 / 7.7 / 11 / 22 kW (WPT1–WPT4) | 100–250 mm air gap | Electric vehicles |
| WiTricity technology | 6.78 MHz | 1–11 kW | 1–2 m | EV charging, robotics, medical devices |
Reading the table as a market map gives a clear pattern. Mass-market consumer charging sits at the short-range, low-frequency, tightly coupled end. The Qi baseline communicates by ASK modulation at 2 kbps, and AirFuel Resonant uses Bluetooth Low Energy. Spatial freedom costs frequency and regulatory complexity. The vehicle class trades range for kilowatt power and a grid-to-vehicle efficiency target above 85%. Phase 1 does not rank these ecosystems. Its descriptors are written so that a record for any of them can be expressed in the same fields, which lets an integrator compare a pad, a resonant desk and a charging bay on common terms.
The mechanism taxonomy in 4.1 can be traced through the applications the specification names. Inductive transfer covers phones, wearables and vehicles. Resonant transfer covers laptops, tablets, robotics and medical devices. Capacitive transfer covers four niches that share one trait: the environment is hostile to exposed contacts or to coils.
Beamed power addresses a different problem, delivering energy to a receiver that is metres to kilometres away and cannot be wired. Microwave beaming uses rectennas, and laser beaming uses photovoltaic receivers tuned to the laser wavelength. Both introduce a hazard the coil-based mechanisms do not have, a concentrated beam that must never cross a person or an aircraft. That is why 4.4 treats beam safety as a first-class part of the record.
Safety in WPT is organised by frequency, because the biological coupling mechanism changes with frequency. The Phase 1 safety envelope names the limit regime that applies to the system's band:
| Band | Governing limit |
|---|---|
| IPT / CPT, kHz range | ICNIRP guidelines: body-coupled current limit |
| Resonant, MHz range | ICNIRP guidelines: SAR (specific absorption rate) |
| Microwave | Field-strength and SAR limits per IEEE C95.1-2019 |
| Laser | MPE (maximum permissible exposure) per IEC 60825-1 |
The microwave envelope also records the maximum-permissible-exposure envelope per IEEE C95.1 and the regulatory authorisation envelope under the operator's licensing regime. The laser envelope records the laser class per IEC 60825-1. The two classes the specification names are Class 1M and Class 4, and Class 4 requires a documented hazard zone. The envelope also records beam divergence, per-receiver photovoltaic conversion efficiency and an active-safety-scanner entry (LiDAR or camera-based intrusion detection in the beam path). Medical environments add a further requirement: active medical-implant compatibility per ISO 14117.
The central safety mechanism for directed beams is the interlock chain. The transmitter stays gated until the receiver acknowledges its presence and the safety scanner clears the beam path. Beam-blocking interlocks are mandatory for microwave and laser systems above the operator's documented peak-power threshold. The consequence for implementers is that a beaming transmitter is "off unless proven safe". Losing the receiver acknowledgement, or an intrusion in the path, has to remove the beam. The record therefore describes both the interlock and the threshold above which it must exist, so that a reviewer can check the two against each other. The SOTIF envelope in the resonator descriptor extends the same reasoning to any WPT that operates in human-occupied space.
For testing, the reference material points to a public conformance suite that exercises the specification's endpoints and protocol exchanges. Within the 1.x line, existing envelope shapes must remain available. Implementations may add optional fields but may not remove existing ones.
Several practical points follow from the descriptors. First, record the efficiency profile at the operating point rather than the best bench figure. The gap between a bench coupling coefficient of 0.6–0.9 and the 0.05–0.15 typical of deployed resonant systems is where most disappointing field results originate. Second, capture alignment tolerance explicitly. A vehicle pad that meets its efficiency target only when centred is operating outside its own envelope in a normal car park. Third, state compensation topology and detuning behaviour per receiver in any multi-receiver system. Fourth, put the exposure regime and the interlock design into the same record as the power rating, so that a power increase cannot ship without its safety review.
For adoption, the reference container with mock data lets an integrator exercise the contract before connecting real hardware. The sensible order is to swap the mock for real backends one endpoint at a time.
The space-solar record is the standard's most forward-looking envelope, and the specification labels it research-track only. It captures:
The list shows how far the problem has moved from a coil on a desk. Handover between orbits, shared spectrum and clearance for ships and aircraft all become part of what the descriptor must say.
Phase 1 turns WPT physics into a record structure. The mechanism taxonomy separates inductive, capacitive, resonant, microwave, laser and space-solar systems. The system descriptor fixes seven canonical items, from identifier to regulatory authorisation. The three coupling regimes explain why coils are described by coupling coefficient and quality factor while beams are described by aperture and gain. The conversion chain of inverter, link and rectifier shows why efficiency has to be a profile and not a single figure. Resonator and link descriptors carry topology, Q, stability, decoupling, alignment and link-budget detail. The safety envelope ties each frequency band to its exposure regime, and beaming systems additionally require interlocks and a documented hazard zone.
These records describe a system but do not yet move anywhere. The next chapter covers Phase 2, which exposes these envelopes through HTTPS endpoints. There the interest shifts from what a descriptor contains to how a client retrieves it, which status codes it can expect, and how a live link's state is reported against the envelope defined here.