Near-Field Magnetic Induction for Wireless Audio and Data Streaming

By: Jean-Daniel Wu, Antenna Specialist AE, Future Electronics

Today, the magnetic induction technique is being widely used for near-field communication (NFC) and wireless charging applications. Compared to Bluetooth communication, which works at 2.4GHz, near-field magnetic induction (NFMI) technology can provide a more reliable, more secure, and much lower power radio link within, on, and in the immediate proximity of a human body.

First, this paper will give an overview of the NFMI technology compared to NFC and Bluetooth. Then, NXP’s NFMI products are introduced as solutions for wireless body-area network (WBAN) communication. Next, NFMI antenna design and near-field link budget calculation are shown. Finally, some typical application examples using NXP’s NFMI chipsets for wireless audio and data streaming are discussed.

Introduction to Wireless Body-Area Network (WBAN)

The rapid growth of miniaturized body sensor units, wireless connectivity technologies, and single body central gateways have enabled wireless communication near body areas and real-time transmission of sensor data through the internet. This WBAN^[1] is expected to have initial applications in healthcare, allowing inexpensive and continuous monitoring of health conditions of patients. The vital parameters of chronic diseases, like diabetes mellitus, cardiovascular diseases and respiratory diseases, can be collected and exchanged between the patients and the hospital, which is the key concept of mobile health (mHealth) or Telehealth.

Figure 1. Illustration of (a) Bluetooth communication (b) NFMI communication

Comparing NFMI, NFC and Bluetooth for WBAN

When we think of a wireless connectivity solution for a WBAN, that’s Bluetooth. When a Bluetooth signal is transmitted by an antenna, it then travels as far as it possibly can till it runs out of energy. This is called far-field transmission. It is good if the signal needs to be transmitted over a long distance. However, Bluetooth can have problems if the wireless communication needs to be very low power and confines to a fairly short distance near body areas. First of all, human body is an integral part of the Bluetooth communication channel. Improper placement of Bluetooth devices close to a human body may result in a detuned antenna input impedance, reduced antenna efficiency, and distorted antenna radiation pattern. Penetration of a Bluetooth signal into the human body is another problem. Bluetooth can’t be used to communicate with deeply implanted medical devices. This is because the Bluetooth signal can be quickly absorbed and greatly attenuated due to the very conductive body tissues. Second, interference with the Bluetooth band can be very high due to coexistence of Bluetooth, Wi-Fi, and ZigBee at the ISM 2.4GHz band. Third, Bluetooth earpieces only last for a few hours. Reducing Bluetooth power consumption and increasing continuous working time are very important for continuous operation of sensor units. Last but not least, Bluetooth has potential security problems. Its signal can be intercepted and decrypted after propagating into free space. Soldiers using Bluetooth on the battlefield could set themselves up as obvious targets. The National Security Agency (NSA) has issued warnings against Bluetooth vulnerabilities that strictly restrict its use in the Armed Forces.

NFMI is a short-range wireless technology that communicates by a tightly coupled magnetic field among devices^[2]. NFMI enables human body friendly, reliable, secure, and power efficient wireless communication. In NFMI communication systems, the modulated signal sent out from a transmitter coil is in the form of a magnetic field. This magnetic field induces voltage on the receiving coil, which in turn will be measured by an NFMI receiver. The power density of NFMI signals attenuate at a rate inversely proportional to the distance to the sixth power compared to the second power for Bluetooth signals. This means for the same distance, the power density of NFMI signals is 10000 times weaker than Bluetooth signals provided that both transmitting power are equal. This type of wireless transmission is referred to as “near-field.” NFC is based on the same principles and uses the same high-frequency (HF) band. However, NFMI is a remarkable evolution of NFC that extends the reading distance from 1-4 inches for NFC, to up to 9 feet for NFMI. At around 13MHz, NFMI provides a data rate of over 400Kbps per frequency channel, up to 10 separate frequency channels and 10 sub-channels per frequency channel using time division. That’s a hundred separate wireless links per smartphone inside a single wireless Personal Area Network! Bluetooth can only do a few – very poorly. The key parameters and performances of the NFMI, NFC, and Bluetooth wireless protocols are summarized in Figure 2.

Figure 2. Comparison of different wireless communication protocols

NXP NxH22xx NFMI Product Portfolio

NXP NxH2280/81/61 products are ultra-low-power single-chip solutions. They are optimized for high quality wireless audio and data streaming using NFMI to provide a reliable and tightly- coupled body-area network around the user.

Figure 3. Key parameters of NxH22xx chipset at 48kHz sampling rate

Figure 4. (a) NFMI chip with coil antenna (b) NxH22xx SDK (c) NxH22xx ADK with CSR Bluetooth [Source: NXP Semiconductors]

The NxH22xx series contains 3 products which are listed in Figure 3^[3]. NxH2280 is the initial NFMI offering for hearing and hearables. NxH2281 increased the audio quality to the level of existing Bluetooth A2DP headsets. NxH2261 chipsets are the standard package for CE products.

The key features of NxH2280/81/61 are^[3]:

• Human Body Compatible: NFMI signal can penetrate through human body tissue with low absorption rate. The specific absorption rate (SAR) is 100 times lower than Bluetooth.
• More Secure in the Near-Field Tens of devices can communicate inside the magnetic bubble around the human body while outside of bubble the magnetic signal is greatly attenuated.
• Customer Programmable Controller: These products integrate a customer programmable Cortex M0 processor with flexible interfaces, making it possible to create ultra-low-power audio streaming applications without a host MCU.
• Embedded Network Protocol: Very flexible embedded networks can be implemented. It can support up to 15 devices and 2 transmit audio streams, 2 receive audio streams, and multiple data streams in parallel. It is an optimized protocol with low latency ear-to-ear communication.
• Customer Programmable CoolFlux DSP: CoolFlux DSP with I2S interface to host or codecs. Audio sample rates between 16KHz and 48KHz are supported.
• Highly Integrated and Ultra-Low Power: The NxH22XX chipsets are packaged as a bumped die (<11mm²) with on-chip supply regulators. They support crystal-free operation and need only a few small external decoupling capacitors. The peak current draw is as low as 1.35mA at 1.2V with one TX/RX stream. It’s just a fraction of Bluetooth.

To simplify the development using the NxH2280/81/61 chipset and reduce time-to- market, NXP offers a Software Development Kit (SDK) and Application Development Kit (ADK) board as shown in Figure 4. The SDK comes with NxH22xx IC, LPC1115 host MCU, audio codec, connectors for SWD interface, and peripherals such as displays and switches. The SDK can demonstrate uni-directional, bi-directional, stereo audio streaming, and Bit-Error Rate (BER) application to evaluate transmit power versus link distance.^[3] The ADK board uses NxH2281 with CSR8670. There are two different types of ADK boards. One is with both NxH2281 and CSR9670. The other is with only the NxH2281 NFMI chip. The CSR8670 code is simplified and minimally changed from the default Sink firmware. Battery powered ADK boards are very useful to simulate earbud or headphone applications for hearables.

NFMI Antenna Design and Layout Guideline

Antenna selection and placement have a substantial influence on the range and performance that can be maximally achieved out of the NFMI radio. Unlike commonly used mono-pole antennas (ceramic chip/PIFA/Inverted-L) or dipole antennas (whip antennas for example) for far-field long distance communication, NFMI antennas are nothing but loosely coupled coil inductors. In this section, the equivalent circuit model of NFMI antenna coil is introduced first. Then the NFMI link model is discussed, followed by the discussion of antenna coil sensitivity versus Tx-Rx range. Finally, the guidelines for antenna design and placement are discussed.

This section is very useful for understanding and designing all types of near-field antennas for NFC and wireless charging as well.

Figure 5. NFMI antenna coil model

Antenna Design: Figure 5 shows the equivalent circuit model of the antenna coil. L_m is the inductance of the antenna coil. Rm is the equivalent resistance and C_m is the parasitic capacitance between turns of coils.

Where β is a inductance modifier constant, l, d and A are the length, the diameter, and the cross section of the antenna coil respectively. N is the number of turns of antenna coils. μ₀ is the permeability of free-space and μ_rod is the equivalent permeability of ferrite rod.

The NFMI antenna can be connected to the NFMI chip directly without matching network. This is because tunable Resistors and Capacitors (RC) tanks are implemented inside the chip. Both of the tunable capacitance and resistance can vary within a certain range to compensate automatically the detuning of NFMI antennas.

Figure 6: System requirements of NxH2280/81/61 chipsets

Assuming C_tune is 60pF, as carrier frequency f_c is 10.6MHz, we have

As the bandwidth is B=400kHz, the system quality factor Q_sys can be calculated as

To have a system Q_sys =26.5. Taking all the system losses into consideration, the total system resistance R_sys

so the antenna resistance R_m shall be chosen as

And then the quality factor Q_m

The last key parameter of NFMI antennas is the self-resonant frequency f_res, which is preferably chosen to be 3 to 4 times higher than the carrier frequency f_c. The calculated inductance L_m, resistance R_m, quality factor Q_m, and self-resonant frequency f_res can be verified with the NFMI antenna datasheet in Figure 7.

Measuring Conditions:
Temperature 23°C (73.4°F) ±5°C (64.4°F-82.4°F)

Figure 7. Electrical parameters of commercial NFMI antenna^[6].

Link Budget Calculation: The link budget calculation for loosely coupled coil antennas is very different from the Friis transmission equation commonly used to estimate the range of far-field long-distance wireless communication^[7].

Figure 8. NFMI radio link for sensitivity calculation

As is shown is Figure 8, the transmitting power of NFMI transmitter is P_TX, then the voltage on the antenna coil can be calculated as

Assuming the coupling coefficient between the receiver and transmitter is K, so we have

Where k ∝ l² d ⁄ r³ if we keep L_m constant. The voltage at the input of receiver circuit can be calculated as

From equation (7) – (9), we can calculate the receiver sensitivity as

Equation (10) is very important in guiding one to design or select the best antenna for NFMI, NFC, and wireless charging applications. The process of defining the antenna parameters can be summarized as:

• First select a carrier frequency f_c and determine the required nominal inductance value L_m based on the capacitance C_tune of the tuned R_C tank.
• Then determine the system quality factor Q_sys based on the system bandwidth B and carrier frequency f_c.
• And then calculate the total resistance R_sys from the Q_sys, L_m and carrier frequency f_c. So the maximum resistance of the antenna R_m can be estimated using equation (5).
• Finally, the self-resonant frequency f_res can be chosen 3-4 times higher than f_c.

After making sure the antenna inductance L_m is unchanged, in order to extend the range of NFMI communication, from equation (10):

• Increasing the diameter d of the antenna coil by 2 can increase the sensitivity by 6dB.
• Increasing the length l of the antenna coil by 2 can increase the sensitivity by 12dB.
• Increasing the system quality factor Q_sys (decreasing system resistance) by 2 can increase the sensitivity by 6dB.
• Increasing the distance r between the two coils by 2 will decrease the sensitivity by 18dB.
• The sensitivity is not dependent on the number of turns N of the antenna coil as we keep the inductance L_m constant.

Figure 9. Relationship between Rx Sensitivity, Tx range and Tx power required

Figure 9 shows how a change in Rx sensitivity (column 1) is translated to a change in range (column 2) when the Tx power is kept constant, or to a change in transmit power required (column 3) when the distance between antenna coils is kept constant.

Integration considerations: Proper NFMI antenna placement and layout are very important in achieving the theoretical maximum range. There are some general guidelines we can follow to minimize the noise that can be picked up by NFMI antennas.

• Make sure the antennas are coaxially aligned and avoid placing them orthogonal to each other.
• Connect the NFMI antenna to the PCB using twisted lines if possible.
• Route the antenna traces away from other signals like I²S, I²C, and other high speed signals.
• Make the area under the antenna connections free from signal traces and vias.
• Place the antenna away from copper PCB planes and noisy components like DC/DC converters.
• Avoid using current loops; they are radiative. Use ground layers.
• Reserve a space or shielding cans to reduce potential radiation signals.

Applications of NxH2280/81/61 Products

Wireless Earbuds: Bluetooth-only wireless earbuds have potential “Pausing” problems due to missing radio links signal which is caused by human body absorption. NFMI is the only solution for high-quality truly wireless music streaming near body area thus far. Figure 10 shows the music streaming from a Bluetooth enabled device while at the same time collecting sensor information measured in the ear canal. The left earbud has both a Bluetooth device and an NFMI radio. The right one has no need for a Bluetooth device and needs only one NFMI radio. In order to compensate for the latency introduced by the audio transmission between the NFMI radios, it is important to implement a delay line in the Bluetooth device in the left earbud before the audio codec. Typically, an audio sampling rate of 48kHz is recommended. It is also possible to establish a bidirectional transmission channel for sensor data, which allows for a data rate of 50kbps throughput on top of the audio stream.

Figure 10. Music streaming + vital parameters monitoring using Bluetooth and NFMI

Music streaming and vital parameters monitoring using NFMI devices without Bluetooth are also possible. As shown in Figure 11, in this case, the music is not streamed from any Bluetooth devices. Instead, the NFMI radio can play the music file which is already stored in an embedded flash memory located in the left earbud. The sensor data can be stored in the embedded flash as well. The music streaming and data transmission share the same bandwidth.

The earbuds can also be used as a Bluetooth headset (Figure 12). In this application, no sensor data is collected and exchanged. Comparing to Bluetooth only earbuds, the headset application uses NFMI to replay the music streaming from one earbud to the other. By doing this, the music streaming radio link is never disturbed by the human head and the reliability of the radio link is greatly improved.

Figure 11. Standalone Music streaming + vital parameters monitoring using NFMI

Figure 12. Headset application using Bluetooth and NFMI

Wireless Implantable Sensor: Microfabrication technologies have advanced to the level that wireless sensors can be incorporated into implants. These sensors can be used to measure key parameters such as blood pressure, temperature, and forces inside the human body. Those passive implantable sensors must be compatible with human tissue, small in size, and robust enough to withstand any physical forces applied by human body. They must also be able to transmit data wirelessly. Since NFMI radio communications are very sensitive to antenna alignment, once the data is collected and sent out via NFMI radio inside the implantable sensors, it is required to have multiple receiving NFMI antennas to detect the transmitted signal. At least one of those receiving antennas has a good connection with the sensor.

Figure 13. Transmission of data for wireless implantable sensors

Conclusion: NFMI is a short-range wireless technology that is perfectly suited for body-area networks because NFMI is human body friendly, naturally private, highly reliable and power efficient. It is the perfect technology for applications for wireless earbuds, hearable instruments and wireless implantable sensors. Combining NFMI technology with wireless charging by sharing the same antenna, NFMI holds great potential in providing fully wireless, small form factor, and high-quality hearable devices and implantable sensors. Without any doubt, NFMI will significantly impact the hearable/wearable industry and our daily medical practice.

References:
[1] https://en.wikipedia.org/wiki/Body_area_network
[2] https://en.wikipedia.org/wiki/Near-field_magnetic_induction_communication
[3] http://www.nxp.com/documents/leaflet/MIGLOTECHFS.pdf
[4] https://www.ap.com/technical-library/more-about-thdn-and-thd/
[5] https://community.nxp.com/docs/DOC-331010
[6] http://www.sonion.com/wp/wp-content/uploads/RF02AA10-3110605.pdf
[7] https://en.wikipedia.org/wiki/Friis_transmission_equation
[8] AN11699 – NXH2280 magnetic induction radio