Communication by means of modulated Johnson noise

Edited by William Press, The University of Texas, Austin, TX; received January 24, 2022; accepted November 1, 2022.November 29, 2022119 (49) e2201337119SignificanceThis paper presents experimental realization of an ultralow power wireless communication method that works by selectively connecting or disconnecting an impedance-matched resistor and an antenna. This modulates microwave frequency Johnson noise emitted by the antenna. The data transmission hardware is similar to that of an RFID tag, which communicates by reflecting RF signals; the crucial advantage of the present system is that it requires no preexisting RF signal. An interesting feature of the system is that all components of the system are at the same physical temperature, but it functions because they have different noise temperatures. It is also notable that the elimination of the RF carrier simplifies the system architecture and the reader hardware.AbstractWe present the design of a passive wireless communication method that does not rely on ambient or generated RF sources. Instead, the method modulates the Johnson (thermal) noise of a resistor to transmit information bits wirelessly. By selectively connecting or disconnecting a matched resistor to an antenna, the system can achieve data rates of up to 26 bps and distances of up to 7.3 m. This communication method operates at very low power, similar to that of an RFID tag, with the advantage of not requiring a preexisting RF signal to reflect. Sign up for PNAS alerts. Get alerts for new articles, or get an alert when an article is cited. In passive wireless communication, an energy-constrained data transmitter sends information by modulating a radio frequency (RF) signal generated by an RF source that is not power constrained. Because the data transmitter does not have to generate an RF signal, the power necessary to send data is orders of magnitude less than in conventional RF communication. In modulated backscatter communication, a continuous wave RF carrier is generated by a dedicated device on the high-power side of the link, and the low-power side encodes data by selectively reflecting this RF signal (1). Ambient backscatter is another form of passive communication that makes use of preexisting, ambient RF signals such as those generated by broadcast TV or radio towers (2). While the low power of the data transmission side is attractive, both methods rely on a preexisting RF signal.This paper introduces a form of passive wireless communication, Modulated Johnson Noise, in which the signal to be modulated is the Johnson noise in an unbiased (unpowered) resistor. This scheme retains the benefits of prior passive wireless communication schemes while eliminating the need for an external RF signal. This has the potential to reduce the overall energy consumption of the system, to allow stealthier and low-interference operation, and to allow operation in areas where no ambient RF signals are available.Consider the frequency spectrum measurements shown in Fig. 1A, which shows the measured signal of a 50-Ω terminator and open circuit terminator connected to the input of a receiver. We can see that there is a clear difference between the two measurements, which can be exploited to enable wireless communication. By selectively connecting and disconnecting an impedance-matched 50-Ω resistor to an antenna, information bits can be wirelessly transmitted. For example, in Fig. 1B, we show the received signal of a data packet that was wirelessly transmitted by modulating Johnson noise. While this looks like a noisy signal, after performing demodulation, the data packet can be extracted as shown in Fig. 1C. While thermal noise communication has been proposed from a theoretical perspective (3), we present a system that enables wireless communication by modulating Johnson noise. In this paper, we discuss the design and experimental implementation of the system and evaluate the overall performance of the wireless communication scheme that relies on modulated Johnson noise. The contributions of the paper are summarized below:•We introduce wireless communication by means of modulated Johnson noise, a wireless system that enables devices to communicate without reliance on generated or ambient RF signals.•We present the designs and prototype of hardware that enables Johnson noise communication. Moreover, we demonstrate that the transmitter can be designed to be battery-free by performing solar energy harvesting and charging a supercapacitor rather than using a battery.•We evaluate the performance of the wireless system in terms of achievable throughput and communication range. The performance evaluation shows that data rates of up to 26 bps can be achieved at distances of up to 7.3 m.Fig. 1.Johnson noise communication. Wireless communication can be enabled by modulated Johnson noise. (A) shows the measured frequency spectrum at 1.42 GHz, which compares measurements when a 50-Ω load is connected to a receiver and an open circuit load. (B) shows what a received signal looks like when wirelessly transmitting a data packet by modulated Johnson noise, and (C) shows the received data packet after demodulation.Overview of Johnson NoiseJohnson noise is caused by the thermal vibrations of charge carriers inside of an electrical conductor (e.g., resistor) and is characterized by its mean-squared voltage,Vn2=4kTBRe(Z),[1]where k is Boltzmann’s constant, T is temperature, B is bandwidth, and Re(Z) is the real part of the electrical conductor’s impedance (4, 5, 6). A resistor with Johnson noise can be modeled as a Thevenin equivalent circuit that includes a noiseless resistor and a noise voltage generator with voltage given by Eq. 1. With a matched load resistor connected, the maximum noise power provided by the noisy resistor isPn=Vn24R=kTB,[2]which is independent of resistance (7). The thermal noise power is a function of temperature and bandwidth. As an example, a resistor at room temperature (296 K) with a system bandwidth of 500 MHz would result in Pn = −86.9 dBm. We also note that Johnson noise is a white noise source and therefore independent of frequency (within the system’s bandwidth). Moreover, Johnson noise has a Gaussian amplitude distribution.Design and ImplementationWe design and implement a transmitter (TX) and receiver (RX) to enable wireless communication by modulating Johnson noise. First, the transmitter requires an RF switch and a processing unit to control switching between an open or short circuit and resistor load connection shown in Fig. 2A. On the receive side, shown in Fig. 2B, two high gain, low noise amplifiers (LNA) amplify the very low-power signals being transmitted while maintaining a good signal-to-noise ratio. Between the two LNAs is a bandpass filter to prevent feedback oscillation. The entire system is designed to operate at 1.42 GHz with 50-Ω impedance. In Fig. 2 C and D, we show the prototype implementation of the transmitter and receiver. The transmitter uses an RF switch and switches between a 50-Ω RF terminator (with 50-Ω impedance) and an open circuit terminator, both of which are very well shielded (8). On the receive side, there are two LNAs with a combined gain of approximately 84 dB, a 50-MHz bandpass filter, and a software-defined radio (SDR) (9, 10, 11). We constructed pyramidal horn antennas for both the TX and RX sides. The antennas were characterized to have approximately 13.6 dBi of gain.Fig. 2.Design and implementation. (A) shows the transmitter design, which switches between a 50-Ω load and short circuit (or open circuit) to modulated information bits. (B) shows the receiver, which is composed of two low noise amplifiers (LNA) whose output is fed into a software-defined radio. The bandpass filter between the two LNAs is added to prevent feedback oscillation, and the data received by the SDR can be processed by a laptop PC or small form factor computer (e.g., Raspberry Pi). (C) and (D) show the prototype implementation of the transmitter and receiver, respectively.Data Encoding and Modulation.Since the transmitter is switching between two states, data can be modulated by performing ON–OFF keying. A 0 bit is transmitted by simply staying in the OFF state (open circuit), and a 1 bit is transmitted by switching between both ON and OFF states (open circuit and 50 Ω) using a square-wave subcarrier frequency. Mathematically, the transmitted signal is written assrx=m·sgn(sin(2πfsct)),[3]where m represents the bit to be encoded and takes on a 0 or 1 value. The subcarrier frequency is defined as fsc.Packet Detection and Demodulation.Our system assumes that the fsc is known on the RX side, which allows us to perform heterodyne detection. Our demodulation is inspired by techniques used in radio astronomy, in particular, the Dicke radiometer (12). We can view our entire system as a distributed Dicke radiometer, where the switching between two states occurs on the TX side, while the amplification and integration occur on the RX side. The receiver performs heterodyne detection which allows the system to reject noise by filtering out any received signal power that is outside the narrow bandwidth of the subcarrier. In other words, the receiver generates the same square-wave subcarrier signal with amplitude values of +1 and −1, multiplies the received signal with this receive-side subcarrier, and accumulates (integrates) the product values for a duration that is less than or equal to that of the duration of one bit. This process results in a demodulated signal intensity and is given by,⟨srx,ssc⟩=∫0Tsrx(t)ssc(t)dt,[4]where T is the total integration time. The demodulated signal intensity is compared to a threshold value to determine whether a 1 or 0 bit was transmitted. To successfully extract a data payload, data packets are transmitted that are structured to begin with a known preamble which is then followed by the data payload. The preamble is used to perform synchronization and then