Ambient Backscatter Architecture and Working

This page describes ambient backscatter architecture and its working principles, including an ambient backscatter circuit. It explains the operation of ambient backscatter transmitter and receiver (i.e., transceiver) circuits. It also mentions the advantages and disadvantages of using an ambient backscatter communication system.

Introduction

Backscatter communication systems are classified into the following three types:

• Monostatic backscatter communication system
• Bistatic backscatter communication system
• Ambient backscatter communication system

An ambient backscatter communication system is a wireless system that uses ambient RF signals such as TV towers, cellular base stations, and WiFi APs (or routers). It doesn’t use a separate power source or battery for its operation. Instead, it derives its operational power from received ambient RF signals using a simple energy harvester circuit.

It also uses the same ambient RF signals for communication between devices, rather than having its own RF signal generator. The ambient backscatter devices communicate by absorption and reflection of RF signals. A device transmits by modulating the reflection of incident RF signals rather than generating its own RF signals. This communication method is more energy-efficient compared to conventional RF communication that utilizes its own RF source.

Image courtesy: arXiv

Figure 1 depicts ambient backscatter architecture.

• It consists of an RF energy harvester, a transmitter, and a receiver.
• Harvester: This part extracts energy from the received RF signal using a simple diode circuit and stores the converted DC voltage using capacitors. It also houses a power management system that takes care of the effective utilization of the stored power.

Refer to RF energy harvesting system basics for more information.

• Transmitter: It transmits the message by backscattering RF signals that are received on its antenna. It transmits by modulating the antenna to reflect or absorb the RF signals.

Image courtesy: arXiv

• Receiver: At the receiver, it intercepts some of the RF power from the backscattered signal of the transmitter. Later on, the message is decoded using simple decoder logic. This is shown in Figure 3.
• Load Modulation: It is a two-state modulation scheme. It’s used in backscatter communication systems due to its simplicity. It uses a switch between two loads (i.e., Z1 and Z2) to achieve an absorbing state and a reflecting state. Using switching (between Z1 and Z2), the reflection coefficient is allowed to be switched between these two states. In the absorbing state, due to impedance matching, RF signals are absorbed, and it represents bit zero. In the reflecting state, due to impedance mismatching, RF signals are reflected, and it represents bit one.

How Ambient Backscatter Circuit Works

An ambient backscatter transceiver consists of transmitter and receiver modules. The following section describes the working of ambient backscatter transmitter and receiver circuits.

Image courtesy: arXiv

• Let’s assume that backscatter transceiver-A wants to transmit data to backscatter transceiver-B.
• To transmit the data, the harvester first extracts energy from ambient RF signals to be used for transmission.
• Next, the backscatter transceiver uses modulation and reflection of the ambient RF signal for transmission of data. The channel coding and modulation block used before transmission is shown in the figure.
• The input to the transceiver is a stream of ones (‘1’) and zeros (‘0’). A switch or transistor is used to switch between Z1 and Z2.
• When the input is zero (‘0’), the transistor/switch is OFF, and the transceiver will be in a non-reflecting or absorption state.
• When the input is one (‘1’), the transistor/switch is ON, and the transceiver will be in a reflecting state.

Image courtesy: arXiv

• Let’s understand the decoding process at backscatter transceiver-B.
• There are two methods to decode the data at the receiver: one using ADC & Oscillators and the other using an averaging mechanism. The ADC-based method is not used due to its high power consumption. The figure depicts the averaging method.
• As shown, the envelope averager smooths out the received signal. The threshold detector calculates the threshold value. The smoothed-out signal is compared with the threshold value using a comparator to derive ones and zeros.
• The demodulated bits are passed through the decoder to recover the original data transmitted by transceiver-A.

Advantages and Disadvantages of Ambient Backscatter Communication System

Following are the benefits of using an ambient backscatter communication system:

• The system doesn’t require complex and high-power-consuming components such as amplifiers, oscillators, filters, and mixers.
• It offers lower implementation costs.
• The system is easy to implement and deploy.
• The system doesn’t require its own frequency spectrum or any frequency planning, as it relies on existing wireless signals available in the environment, such as cellular signals, TV signals, WiFi, etc.
• It doesn’t require a battery for its operation. The power requirement of the circuit is fulfilled by an energy harvesting circuit from ambient RF signals.

Following are the challenges in using an ambient backscatter communication system:

• The system doesn’t have its own RF signal source, and hence there is no control over the external ambient RF frequency and its power level.
• It offers a low data rate (about 1 kbps) and a low coverage distance (from 1.5 feet to 2.5 feet).
• There is a security risk, as the system relies on an external RF source.
• It generates a very small amount of energy.
• There is no centralized controller, and hence it is difficult to manage all the communication tasks between communicating devices.