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Ambient backscatter architecture | Ambient backscatter working

This page describes ambient backscatter architecture and ambient backscatter working with ambient backscatter circuit. The working operation of ambient backscatter transmitter and receiver (i.e. transceiver) circuit is described. It mentions advantages and disadvantages of ambient backscatter communication system.

Introduction:
Backscatter communication systems are classified into following three types.
• Monostatic backscatter communication system
• Bistatic backscatter communication system
• Ambient backscatter communication system

Ambient backscatter communication system is a wireless system which uses ambient RF signals such as TV towers, cellular base stations, WiFi APs (or routers) and so on. It does not use separate power source or battery for its operation. It derives its operational power from received ambient RF signals using simple energy harvester circuit. It also uses 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 reflection of incident RF signals rather than generating its own RF signals. This communication method is more energy efficient in comparison to conventional RF communication utilizing its own RF source.

Ambient backscatter architecture | Ambient backscatter working

Ambient backscatter architecture
Image courtesy: arXiv

• The figure-1 depicts ambient backscatter architecture. It consists of RF energy harvester, transmitter and receiver.
• Harvester: It is the part which does extraction of energy from received RF signal using simple diode circuit and stores the converted DC voltage using capacitors. It also houses power management system which takes care of effective utilization of stored power. The figure-2 below, depicts RF energy harvesting circuit diagram. Refer RF energy harvesting system basics >> for more information.
RF Energy Harvesting Circuit Diagram
• Transmitter: It transmits the message by backscattering RF signals which are received on its antenna. It does transmission by modulating the antenna to reflect or absorb the RF signals.
ambient backscatter transceiver
Image courtesy: arXiv

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

How ambient backscatter circuit works

Ambient backscatter transceiver consists of transmitter and receiver modules. Following section describe working of ambient backscatter transmitter circuit and receiver circuit.

Ambient backscatter transmitter circuit

ambient backscatter transmitter circuit
Image courtesy: arXiv

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

Ambient backscatter receiver circuit

ambient backscatter receiver circuit
Image courtesy: arXiv

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

Advantages and disadvantages of ambient backscatter communication system

Following are the benefits of ambient backscatter communication system.
- The system does not require complex and high power consuming components such as amplifier, oscillator, filter and mixer.
- It offers lower implementation cost.
- The system is easy to implement and deploy.
- The system does not require its own frequency spectrum or any frequency planning as it relies on existing wireless signals avaiable in the environment such as cellular signal, TV signal, wifi etc.
- It does not require battery for its operation. The power requirement of the circuit is fulfilled by energy harvesting circuit from ambient RF signals.
Following are the challenges in ambient backscatter communication system.
- The system do not have its own RF signal source and hence there is no control on external ambient RF frequency and its power level.
- It offers low data rate (about 1 kbps) and low coverage distance (from 1.5 feet to 2.5 feet).
- Security risk as the system relies on external RF source.
- It generates very small amount of energy.
- There is no centralised controller and hence it is difficult to manage all the communication tasks between communicating devices.
Refer advantages and disadvantages of ambient backscatter >> for more information.



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