IT 17 079

Examensarbete 30 hpOktober 2017

Modulation schemes in ambient backscatter communication

Oliver Harms

Institutionen för informationsteknologiDepartment of Information Technology

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Modulation schemes in ambient backscattercommunication

Oliver Harms

This thesis presents a study of different modulation schemes in the context of backscatter communication. Backscatter communication is a way of wireless communication where no active signal is transmitted. Instead surroundingsignals are modified to transmit data. The goal of this thesis is to explore in how far different modulation schemes in combination with off-the-shelf hardware can be used to tackle the current data rate and distance limitations of backscatter systems.

This thesis compares the modulation schemes on-off keying (OOK) and frequency-shift keying (FSK) using a constant carrier signal as well as a digital television signal. For the use of a constant carrier signal it is shown that high ranges of up to 225 meters in a line-of-sight environment and up to 30 meters in a non line-of sight environment are reachable extending the current distance limitations by far and even the use of high data rates lead to a range of 175 meters. Moreover, this thesis shows the feasibility of replacing the constant carrier with a television signal and achieves ranges of over a meter in surroundings of television signals with a signal strength of not more than -70 dBm.

Tryckt av: Reprocentralen ITCIT 17 079Examinator: Arnold Neville PearsÄmnesgranskare: Christian RohnerHandledare: Ambuj Varshney

Contents

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Problem Statement & Goals . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3.1 Part I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3.2 Part II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.4 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.5 Thesis Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Related Work 5

3 Background 7

3.1 Backscatter Communication . . . . . . . . . . . . . . . . . . . . . . . . . 73.1.1 Signal Strength in Backscatter Communication . . . . . . . . . . 8

3.2 Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.1 Software Defined Radio . . . . . . . . . . . . . . . . . . . . . . . . 93.2.2 Beaglebone Black . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.3 Backscatter Module . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.4 CC2500 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2.5 Arduino Zero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3 Modulation Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.3.1 OOK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.3.2 FSK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.3.3 MSK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4 Design & Implementation 13

4.1 Carrier generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.2 Backscatter unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2.1 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134.3 Receiver unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.3.1 Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.3.2 Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.4 Configuration calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5 Experiments & Results 21

5.1 Experiment Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215.2 Minimal shifting frequency . . . . . . . . . . . . . . . . . . . . . . . . . . 21

V

5.3 Comparison of deviations . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.4 Comparison of OOK and FSK . . . . . . . . . . . . . . . . . . . . . . . . 235.5 Line-of-sight experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.6 Non line-of-sight experiment . . . . . . . . . . . . . . . . . . . . . . . . . 27

6 Television Signals 29

6.1 DVB-T Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.2 Design & Implementation changes . . . . . . . . . . . . . . . . . . . . . . 30

6.2.1 Signal generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.2.2 Backscatter unit . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.2.3 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

7 Comparison of Carrier Signals 33

7.1 Experiment Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337.2 FSK comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337.3 OOK comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

7.3.1 OOK comparison of ranges achievable with a TV carrier . . . . . 34

8 Conclusion & Future Work 37

8.1 Part I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378.2 Part II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378.3 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

VI

List of Figures

3.1 Distances between physical components . . . . . . . . . . . . . . . . . . . 83.2 Signal strength in a backscatter system . . . . . . . . . . . . . . . . . . . 83.3 CC2500 Packet Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.4 Modulation of an OOK signal . . . . . . . . . . . . . . . . . . . . . . . . 113.5 Modulation of an FSK signal . . . . . . . . . . . . . . . . . . . . . . . . . 123.6 Modulation of an MSK signal . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1 C program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.2 Assembly programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.3 Arduino program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

5.1 Noise Floor around carrier frequency . . . . . . . . . . . . . . . . . . . . 225.2 Bit Error Rate for FSK with different deviations . . . . . . . . . . . . . . 235.3 Comparison of different baud rates for OOK and FSK . . . . . . . . . . . 245.4 Line-of-sight experiment for 2.9 kBaud FSK with backscatter tag close to

the carrier generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.5 Line-of-sight experiment for 2.9 kBaud FSK with backscatter tag close to

the receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.6 Line-of-sight experiment for 197 kBaud FSK with backscatter tag close

to the carrier generator . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265.7 Map with locations for the non line-of-sight experiment . . . . . . . . . . 275.8 Non line-of-sight experiment for 2.9 kBaud FSK . . . . . . . . . . . . . . 27

6.1 Recorded TV Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.2 Replayed TV Signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.3 Shifted TV Signal for OOK and FSK . . . . . . . . . . . . . . . . . . . . 31

7.1 Comparison of carrier signals using FSK . . . . . . . . . . . . . . . . . . 337.2 Comparison of carrier signals using OOK . . . . . . . . . . . . . . . . . . 347.3 Comparison of carrier signals using OOK for different distances . . . . . 35

VII

1 Introduction

Wireless communication is more and more important in today’s connected world. It isused to transmit information without the need of a wired connection. However, activelygenerating radio signals as it is done in most wireless communication systems uses a lotof energy which is not compatible with new areas of wireless communication. Especially,internet of things sensors should be able to run on a single battery for a long timeor function even entirely without batteries. To achieve this, a new way of wirelesscommunication called ambient backscatter communication is actively researched.

Ambient backscatter communication does not actively generate radio signals. Insteadit uses ambient signals and modifies those to transmit data. There are different waysof backscatter communication already available, but they need expensive hardware andare limited to a short range. This thesis explores how different modulation schemessupported by off-the-shelf hardware can be used in backscatter communication and howthey perform regarding range and data rate.

This chapter gives a motivation of building such a backscatter system, the goals ofthe project and a summary of the main results.

1.1 MotivationAmbient backscatter is an approach of wireless communication which uses existing radiofrequency (RF) signals, such as radio, television and WiFi, to transmit data without theexpenses of generating RF signals. Data is encoded by reflecting or not reflecting radiosignals, thus modulating data on an existing signal.

The waiver of generating RF signals is a benefit compared to traditional wirelesscommunication systems. The main benefit is the highly reduced complexity of thesystem. Instead of a complex radio, only a microcontroller operating a switch and anantenna is needed. Moreover, a backscatter system has a significantly lower energyconsumption compared to a system generating RF signals and might run on batteriesfor years or even allow harvesting all needed energy. This makes a backscatter systemusable in a wide field of applications such as outdoor sensor nodes or body monitoringequipment where a replacement of a battery is difficult or even impossible.

Different applications in which ambient backscatter communication is a good approachcompared to traditional wireless communication are:

• sensors deployed in hardly reachable environments

• medical implants [11, 23]

1

• Internet of Things (IoT) devices [12, 17]

One major drawback of existing backscatter systems is its limitations in range and datarate. Therefore, the major motivation of this thesis is to explore alternative modulationschemes to those currently used and analyse their potential to address current limitationsin range and data rate.

1.2 Problem Statement & GoalsAs the development on ambient backscatter continues the data rate and range still haveto improve to be widely used. Most of the backscatter systems already used modulateits information on the carrier wave making it complicated for the receiver to extractthe backscattered information from the received signal and leading to low data ratesand ranges. Some of the systems presented in recent papers already achieve high datarates such as Passive WiFi [12] and Inter-Technology Backscatter [11] but still have apretty low communication range. They achieve the higher data rate by separating thecarrier signal and the backscattered signal making it easier for the receiver to decodethe information and allowing higher data rates; however, complex modulation schemesare used to create WiFi compliant signals.

The main focus of this thesis is to use off-the-shelf radios to build a system notrequiring any expensive hardware and study different modulation schemes like on-offkeying and frequency-shift keying in terms of range and data rate.

1.3 MethodologyTo achieve the goals of this thesis the following methodology is used which can be dividedinto two parts. In the first part a constant carrier is used as ambient signal and the focusof this part is the implementation and the study of the modulation schemes (frequency-shift keying and on-off keying) itself. In the second part the use of a television signalas carrier is introduced and the focus lays in the comparison of the two different carriersignals regarding error rate and range.

1.3.1 Part I

The methodology of the first part is the implementation and testing of an ambientbackscatter system using off-the-shelf hardware and a controlled carrier including thefollowing steps:

• On-off keying (OOK) implementation

• Frequency-shift keying (FSK) implementation

• Experiments regarding comparison of the modulation schemes

• Experiments regarding range and data rate

2

1.3.2 Part II

The methodology of the second part is to compare the performance of the implementedbackscatter system of part one for different carrier signals including the following steps:

• Study of the signal spectrum of a television signal

• Experiments using a television signal as carrier

• Comparison of the carrier signals regarding error rate and range

1.4 ResultsThe results of Part I show a generally better performance for FSK with outdoor rangesof up to 225 meters at a baud rate 2.9 kBaud and up to 175 meters at 195 kBaud andindoor ranges of up to 30 meters at 2.9 kBaud. This shows that the used hardwareand the implementation of this system is capable of extending current limitations inbackscatter systems.

For Part II the results show the general usability of television signals as carrier signalfor the presented backscatter system with ranges of over a meter with a bit error ratewell below 10

�1. This also shows an improvement regarding distance and error ratecompared to previous systems.

1.5 Thesis StructureChapter 1 (Introduction) contains the introduction to this thesis and its motivation.Moreover, the problem, the goals and the main results of this thesis are addressed.

In Chapter 2 (Related Work) different publications with related topics to this projectare summarized including their results.

Chapter 3 (Background) provides useful informations for the reader of this thesis. Itcontains an introduction into backscatter communication and its range constraints aswell as informations on the hardware used for this thesis. Furthermore, the consideredmodulation schemes are explained.

In Chapter 4 (Design & Implementation) the design of the different components of thebackscatter system and its implementations are presented. The presented componentsare the carrier generator, the backscatter unit and the receiver unit. Moreover, a mathe-matical description of the configuration algorithm is given which is used to find a suitableconfiguration for the different modulation schemes working for both the backscatter unitand the receiver unit.

Chapter 5 (Experiments & Results) contains the evaluation of the first part of thethesis. It contains the experiments and their results to find the best working config-uration of the system. Moreover, it presents the experiments performed in differentenvironments and its results regarding range and data rate.

3

Chapter 6 (Television Signals) is the first chapter of the second part of this thesis.This chapter contains a description of digital television signals as well as the requireddesign and implementation changes to be able to use the previously used backscattersystem with television signals as carrier.

Chapter 7 (Comparison of Carrier Signals) contains the evaluation of the second partof this thesis. The experiments as well as its results comparing the two different carriersignals regarding error rate and range are presented.

In Chapter 8 (Conclusion & Future Work) the results of the two parts of this thesisare summarized and future areas of research regarding the work done in this thesis arepresented.

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2 Related Work

The topic of this thesis is related to some recent work in ambient backscatter communi-cation. Recent publications use different ambient signals as well as frequency shifting tobuild backscatter systems for multiple different application areas. A focus of most pub-lications is to achieve higher ranges and data rates. Some of these recent publicationsare summarised in this chapter.

In Ambient Backscatter: Wireless Communication out of Thin Air [14] the authorspresent a battery free backscatter system which uses as its only source of power and asits carrier signal widely available television signals. Their system enables communicationbetween backscatter tags with a bit rate of 1 kbps over a distance of 2.5 meters outdoorsand 1.5 meters indoors.

In Wi-Fi Backscatter: Internet Connectivity for RF-Powered Devices [13] the authorspresent a backscatter system enabled to communicate directly with off-the-shelf WiFiinfrastructure and thus connecting a backscatter system to the internet. It reuses existingWiFi signals as power source and modifies those signals for transmitting data. Thetransmitted data, with a data rate of 1 kbps, can be decoded based on variations insignal strength at a distance of up to 2.1 meters.

In Every Smart Phone is a Backscatter Reader: Modulated Backscatter Compatibil-ity with Bluetooth 4.0 Low Energy (BLE) Devices [9] the authors present a backscattertag which is able to create Bluetooth 4.0 Low Energy (BLE) packets that are indistin-guishable from conventional BLE advertising packets and therefore decodable by everymodern smartphone. For backscattering those signals at a data rate of 1 Mbps a con-stant carrier with a signal strength of 15 dBm is used giving the backscattered signal areceivable range of 9.4 meters with an energy consumption of the backscatter tag over100 times lower than the energy consumption of a conventional BLE transmitter.

In Passive Wi-Fi: Bringing Low Power to Wi-Fi Transmissions [12] the authorspresent a backscatter system creating 802.11b WiFi packets decodable on any WiFidevice. The presented system is able to coexist with other devices in the ISM band dueto a network device doing carrier sense and transmitting the carrier for the backscattertags. The work achieved a range of 30 - 100 feet in non line-of-sight and line-of-sightenvironments with 4 to 5 orders of magnitude lower energy consumption compared toexisting WiFi chipsets.

In Inter-Technology Backscatter: Towards Internet Connectivity for Implanted De-vices [11] the authors present a backscatter system backscattering transmissions of onewireless technology (Bluetooth) to create signals compatible to those of another wire-less technology (WiFi or Zigbee). This technology called interscatter is intended to beused in implanted devices like smart contact lenses communicating with a smartphone.Therefore Bluetooth advertising packets are used as carrier signal and a single sided

5

backscatter signal meeting the 802.11b WiFi standard is created which can be receivedby a smartphone.

In Enabling Practical Backscatter Communication for On-body Sensors [23] the au-thors present ultra-low power on-body sensors using backscatter communication workingtogether with commercial WiFi and Bluetooth radios. They use a system where a smart-phone acts as carrier generator and another wearable device like a wristband receives theinformation of the on-body sensor and extend that to a system with multiple transmit-ters or receivers. They deal with the problems of errors introduced by body movementand the energy consumption used for shifting the signal. Their results show a coverabledistance of up to 4.8 meters with a data rate of 50 kbps consuming not more than 45 µWof energy.

In Augmenting IoT Networks with Backscatter-Enabled Passive Sensor Tags [17] theauthors present a concept of adding battery free passive sensing capabilities to existingIoT deployments without the need to modify those deployments. The sensor tags aredesigned to collect sensor readings and transmit those to nearby active 802.15.4 IoTdevices using backscatter communication. The carrier for those tags is generated byone of the existing devices which removes the need for an external carrier generator.The strength of the produced carrier signal is maximally 0 dBm creating a range ofthe backscattered signal of 20 cm which is sufficient for the use case of close distancecommunication.

In HitchHike: Practical Backscatter Using Commodity WiFi [24] the authors presenta low-power backscatter system using codeword translation to transmit valid 802.11bWiFi signals using existing WiFi infrastructure. The system introduces a one-sidedsignal shift to remove the normally found copy of a backscatter system with frequencyshift. The HitchHike system transmits with a data rate of 1 Mbps and covers distancesof up to 54 meters in a line-of-sight environment using as little energy as 33µW .

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3 Background

This chapter starts with an introduction how backscatter communication works. After-wards, the hardware used for the project is introduced, followed by a description of threedifferent modulation schemes possible with the presented hardware.

3.1 Backscatter CommunicationBackscatter communication is a form of wireless communication not requiring to ac-tively generate RF signals. Instead the radar cross-section (RCS) of the antenna of thebackscatter tag is modulated to either absorb or reflect a carrier signal. This modulationof the RCS is performed by changing the impedance of the antenna circuit between twostates.

To communicate on a frequency different to the frequency of the carrier signal, ashifting operation in addition to the RCS modulation is performed. To achieve this, thecarrier signal is multiplied with a square wave generated by the backscatter tag. Thefrequency of this square wave is the required offset �f , between the carrier frequency fcand the desired frequency. The square wave can be written as a Fourier series as shownin Equation 3.1.

Stag(�ft) =4

⇡

1X

n=1,3,5,..

1

nsin(2⇡n�ft) (3.1)

Multiplying the carrier signal Sc = sin(2⇡fct) with this square wave leads to theresulting signal r(t) in Equation 3.2 [12].

r(t) = Sc ⇥ Stag(�ft)

= sin(2⇡fct)⇥ Stag(�ft)

=

4

⇡

1X

n=1,3,5,..

1

nsin(2⇡n�ft)⇥ sin(2⇡fct)

=

2

⇡

1X

n=1,3,5,..

1

n{cos(2⇡(fc � n�f)t)⇥ cos(2⇡(fc + n�f)t)} (3.2)

Using the first harmonic (n = 1) of Equation 3.1 results in two signals created, by theperformed multiplication in Equation 3.2, at an offset of �f on both sides of the carriersignal’s frequency fc. Thus, the backscattered signal is shifted to the desired frequencyfc +�t and a copy of it to the frequency fc ��f . [12, 24]

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3.1.1 Signal Strength in Backscatter Communication

In a backscatter system consisting of three components, a carrier generator, the backscat-ter tag a receiver, the communication range depends on two parameters. These twoparameters are the distance between the carrier generator and the backscatter tag (d1)as well as the distance between the backscatter tag and the receiver (d2).

Figure 3.1: Distances between physical components

The signal strength at the receiver in free space (Pr) can be modelled using Friis pathloss as given in Equation 3.3.

Pr =

✓PtGt

4⇡d21

◆K

✓�2Gr

4⇡d224⇡

◆(3.3)

The equation consists of three parts, where the first term in the first parenthesiscontains the signal propagation from the carrier generator to the backscatter tag withPt being the transmit power of the carrier generator and Gt its antenna gain. Similarto that, the third term in the second parenthesis describes the signal propagation fromthe backscatter tag to the receiver with the antenna gain at the receiver Gr and thewavelength of the transmitted RF signal �. The second term of the equation, the factorK, is a constant accounting for the return loss and antenna gain of the used backscattertag.

Figure 3.2: Signal strength in a backscatter system

Figure 3.2 illustrates how the general curve of the Friis model looks like for fixedcarrier generator and receiver positions and a variable placement of the backscatter tag.This shows that the highest achievable ranges in a backscatter system are those wherethe backscatter tag is located close to the carrier generator or to the receiver. [12, 16]

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3.2 HardwareThis section describes the hardware components needed to build the backscatter systempresented in this thesis. These are a Software Defined Radio as carrier generator, aBeaglebone Black in combination with a backscatter module and a receiver (CC2500)in combination with an Arduino.

3.2.1 Software Defined Radio

A Software Defined Radio or SDR is a "radio in which some or all of the physical layerfunctions are Software Defined" [5]. That means that any device which can transmitor receive radio frequency signals with at least one parameter changeable by software isa Software Defined Radio. Examples of those parameters important for this thesis arefrequency, transmit power and signal amplitude.

The SDR used in this project is the USRP B200 by Ettus Research [20]. It is used togenerate a simple carrier and to record and replay a television signal.

3.2.2 Beaglebone Black

The Beaglebone Black is a development platform like the Raspberry Pi with GPIOconnectors to interface with external hardware. The speciality of the Beaglebone andthe reason for its usage in this project are the on-board Programmable Real-time Units[3].

The Programmable Real-time Units (PRU) are 32-bit microcontrollers which are partof the Texas Instruments AM3358 processor. The PRU is a microcontroller running at200 MHz with single-cycle access to some of the pins and access to the memory of theprocessor. Because of the real-time behaviour of this chip and its high frequency, theBeaglebone Black is an optimal development platform for this project [4].

3.2.3 Backscatter Module

The backscatter module is a module developed by the Uppsala Networked Objects group(UNO) at Uppsala University. It has an external antenna which can be switched on andoff by external hardware through the Analog Devices HMC190BMS8 RF switch whichwas used in [11] as well. The power consumption of this module is 0.3µW .

3.2.4 CC2500

The CC2500 is a transceiver by Texas Instruments operating in the 2.4GHz-ISM band(2400MHz-2483.5MHz) [21]. It is a widely used transceiver and especially highly config-urable regarding frequency, baud rate1 and modulation scheme which is of importancefor this project.

1rate of symbols transmitted per second

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The CC2500 used in this project is part of a ccRF click module with an PCB traceantenna and running at a voltage of 3.3 volts. [15]

In this project the CC2500 is used as a receiver for the data transmitted by thebackscatter tag. To be able to decode the received data, the transmitter has to usea certain packet format given by the CC2500. This packet format is explained in thefollowing paragraph.

Packet Format

A CC2500 packet consists always of a preamble, sync word and data field. If the chipis configured to receive packets of variable lengths, a length byte is inserted betweenthe sync word and the data. Moreover, it is possible to insert an address byte forcommunication between multiple CC2500 modules and add a checksum to the end ofthe packet to check whether the received data is valid. The general structure of a packetis given in image 3.3.

Figure 3.3: CC2500 Packet Format [21, p. 29]

The data sheet provides additional informations for the different parts of this packetformat. Beginning with the preamble, which has a length of 2 to 24 bytes of alternating1 and 0 bits. Furthermore, the sync word consists of two predefined bytes which aretransmitted 0 to 2 times. [21]

In the following chapters, a packet consists always of 4 preamble bytes and 4 syncbytes which are the standard settings of configurations created with SmartRF Studio 72

for the CC2500. A length byte is not used because only packets with a fixed data lengthare transmitted and therefore the data length is configured using a separate register.The address field and checksum are not used either.

3.2.5 Arduino Zero

The Arduino Zero is an Arduino board using a 32-bit ARM Cortex M0+ microcontroller.It is used to communicate with the CC2500 using SPI and is chosen due the matchingvoltage level of its pins and the CC2500 board. Moreover, it has a USB connector to beprogrammed easily and to exchange data with a connected computer. [1]

2Software by Texas Instruments to find register configurations for their RF chips, like the CC2500.

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3.3 Modulation SchemesThe CC2500 supports different modulation schemes like the binary modulation schemesOOK, FSK and MSK. Binary modulation scheme means that every bit is transmittedas its own symbol resulting in an equality of bit rate and baud rate for those modula-tions schemes. In the following sections, the three mentioned modulation schemes aredescribed.

3.3.1 OOK

On-off keying (OOK) is the simplest form of Amplitude-shift keying3 (ASK). A one isencoded as the presence of a signal and a zero is encoded as the absence of a signal[19]. OOK uses only one frequency for transmitting data and requires therefore lessbandwidth than other modulation schemes. Moreover, it needs less energy than theother presented modulation schemes because it only needs to emit a signal when a oneis transmitted [7]. Figure 3.4 shows an example of an OOK signal.

Figure 3.4: Modulation of an OOK signal [10, p. 6]

3.3.2 FSK

Frequency-shift keying (FSK) is a modulation scheme encoding different symbols asdifferent frequencies. The simplest form of FSK is binary frequency-shift keying (BFSKor 2-FSK) which encodes a one as one frequency and a zero as another frequency. Itis less susceptible to errors than OOK because a receiver looks for specific frequenciesfor the different symbols and therefore noise spikes cannot cause as many errors as forOOK. [22, 10]

The version of FSK used in this project is binary FSK with a continuous phase(CPFSK). Continuous phase means that there are no phase jumps at the frequencytransitions. An example of such a signal can be seen in Figure 3.5. vc1(t) and vc2(t)

3One and zero are represented as different amplitudes of a carrier wave.

11

Figure 3.5: Modulation of an FSK signal [10, p. 9]

are the two signals of frequencies used to generate the respective FSK signal vFSK(t)according to the bit pattern. In this project, contrary to Figure 3.5 the higher frequencyis used for encoding a one instead of a zero.

3.3.3 MSK

MSK is a special form of continuous-phase frequency-shift keying (CPFSK). The spe-ciality is that the frequency difference between transmitting a zero and a one is equal tohalf the data rate resulting in a difference of the waveform for transmitting a one or azero of half a period. This difference of half a period can be seen in Figure 3.6. [18]

Due to the fixed relationship between the frequency deviation and the data rate forMSK and the accompanying incomparability with the other modulation schemes forfixed deviations and data rates, MSK was not further used in this project.

Figure 3.6: Modulation of an MSK signal [18]

12

4 Design & Implementation

This chapter addresses the design used in this project. It presents the physical connectionof the components presented in Chapter 3 and the software implementation used withinthe subsystems, in which the design can be divided. Those subsystems are the carriergenerator, the backscatter unit and the receiver unit. Figure 3.1 gives an overview onthe physical placement of the subsystems. Moreover, a mathematical description tocalculate a configuration usable for the communication between the backscatter unitand the receiver unit is presented.

4.1 Carrier generatorThe SDR described in the previous chapter is connected to a computer and is used togenerate a carrier wave with a constant frequency. It transmits a carrier signal at aspecific frequency within the 2.4 GHz ISM band.

4.2 Backscatter unitThe backscatter unit consists of the Beaglebone Black and the backscatter module de-scribed in the previous chapter. The backscatter module is connected to the BeagleboneBlack using two connections. It is connected to pin P8.11 of the Beaglebone Black,which is accessible as an output by the first PRU of the Beaglebone Black [6], and toground to have a common ground.

4.2.1 Implementation

The implementation of the backscatter logic consists of two parts. A C program forwriting the data to send into the shared memory of the Beaglebone’s processor andstarting the second program which is a program written in assembly running on thePRU-core. There are each a C program and a PRU-assembly program for OOK andFSK. The structure of the C program can be seen in Figure 4.1.

Depending on its purpose (FSK or OOK) the data written to the shared memory isslightly different. For OOK the first bytes contain the number of toggles for transmittinga one, the corresponding delay time and the delay time for transmitting a zero, whereasFSK starts directly with the actual packet data. For FSK, each bit in preamble, syncword and payload is represented by the number of toggles and the delay time betweentwo toggles. For OOK the data consists of the actual bits which are saved as one byteeach for being processable by the PRU program.

13

Figure 4.1: C program

14

(a) FSK program

(b) OOK program

Figure 4.2: Assembly programs

15

The assembly programs, of which the flow charts for FSK and OOK are given inFigure 4.2a and Figure 4.2b respectively, perform the main task of the backscatter unitby toggling the antenna according to the data in the shared memory.

The assembly program for FSK executes for each toggle-delay pair its program routine.This means, the antenna is toggled, followed by the predefined delay, for the number ofgiven toggles. For OOK, the number of toggles for transmitting a one, the correspondingdelay time and the delay time for transmitting a zero are read first. Afterwards, thesame routine as for FSK is executed when a 1 is read from memory and a second routineis executed when a 0 is read. This second routine delays the further execution for thedelay time read in the beginning of the program execution.

4.3 Receiver unitThe receiver unit consists of a CC2500 module and an Arduino Zero. In the following,a short overview about the physical design is given followed by an explanation of theprogram implementation.

4.3.1 Design

The CC2500 module is connected to the Arduino according to the following table:

CC2500 3V3 GND MISO MOSI CLK CS GDO0Arduino Zero 3V3 GND ICSP-1 ICSP-4 ICSP-3 A2 D5

To communicate between the Arduino Zero and the CC2500, SPI is used, therefore theCC2500 connectors for MOSI, MISO, clock (CLK) and chip select (CS) are connectedto those configured in the SPI library for the Arduino Zero. [2] The GDO0 pin, used fornotifications of incoming data from the CC2500 can be connected to any digital inputpin of the Arduino, here pin 5 was chosen.

4.3.2 Implementation

The CC2500 is configured using different registers. The values for those are easilyconfigurable using SmartRF Studio 7. Some of those values have to be configured inaccordance with the results of the configuration algorithm presented in section 4.4. Themost important registers are those setting the centre frequency, the deviation between thetwo frequencies used for FSK transmission, the baud rate, the receive filter bandwidthand the modulation scheme.

The frequency is configured using the registers FREQ2, FREQ1 and FREQ0. The twomost significant bits (of FREQ2) are always 01. The 24 bit register value is calculatedusing Formula 4.1 containing the centre frequency fc and the oscillator frequency fXOSC

of the CC2500 which is 26 MHz.

FREQ[23 : 0] = fc ⇤2

16

fXOSC

(4.1)

16

Figure 4.3: Arduino program

17

The deviation is configured in register DEVIATN and calculated according to Formula 4.5.The value of i is configured using the three least significant bits (2, 1 and 0) of theregister and the value of j using the bits 6, 5 and 4. The baud rate and the receivefilter bandwidth are configured in the registers MDMCFG4 and MDMCFG3. The baud rate iscalculated using Formula 4.3 with i representing the register MDMCFG3 and j representingthe four least significant bits of the register MDMCFG4. The four most significant bits areused to configure the filter bandwidth. Formula 4.2 is used to calculate this bandwidth,with i being bit 5 and 4 of the register and j being bit 7 and 6.

BW =

fXOSC

8 ⇤ (4 + i) ⇤ 2j , for i, j 2 [0, 7] (4.2)

The last important register is MDMCFG2. The bits 6, 5 and 4 of this register are used toconfigure the modulation scheme used and OOK is represented by the value 011 whereasFSK is represented by the value 000 [21].

The general algorithm for the receiver unit is shown in Figure 4.3. The main part of thealgorithm consists of the following procedure. Whenever the CC2500 starts receivinga packet, the GDO0 pin is pulled up and when the complete packet is received, thepin is pulled down. This is used to notify the Arduino that a new packet is in thebuffer. Afterwards the Arduino requests the data from the CC2500 and receives it. Thecorresponding RSSI value (signal strength) to this transmission is calculated as well asthe RSSI value of the background noise (noise floor). In the end the data and the twoRSSI values are transmitted to the connected computer for further processing.

4.4 Configuration calculationThe formulas and equations used to calculate the settings for the backscatter unit arebased on the following formulas given in the data sheet of the CC2500.

The possible baud rates can be calculated with Formula 4.3: [21, p.26]

RDATA =

(256 + i) ⇤ 2j

2

28⇤ fXOSC , for i 2 [0, 255], j 2 [0, 15] (4.3)

And the possible deviations from the centre frequency with Formula 4.4: [21, p. 33]

fdev =fXOSC

2

17⇤ (8 + i) ⇤ 2j, for i, j 2 [0, 7] (4.4)

The oscillator frequency fXOSC for the CC2500 module used in this thesis is 26 MHz.Therefore fXOSC will be replaced with 26000000 in all further occurrences of the formulasabove.

Based on these formulas and the constraint that the delays of the Beaglebone can onlybe multiples of 10 ns with a minimum of 80 ns, Equation 4.5 can be used to calculatethe possible delays for the two FSK frequencies for a deviation with an error of less thanone percent.

18

������

���10000000002⇤t1 � 10000000002⇤t2

���2

� fdev

������< 0.01 (4.5)

All times (t, t1 and t2) in this and the following equations and formulas are timesin nanoseconds. The first term in the equation above is the formula for the achievabledeviations using the Beaglebone whereas fdev represents the deviations possible for theCC2500 as described in Formula 4.4.

Using those delays for which Equation 4.5 is true Formula 4.6 can be used to calculatethe centre frequency by which the signal is shifted.

fshift =1000000000

2⇤t1 +

10000000002⇤t2

2

(4.6)

The possible baud rates for the delay times calculated with Equation 4.3 with min-imum deviation from the baud rates provided by the CC2500 can be calculated withEquation 4.7. The modulo operator used in this formula and the next one is the modulooperator of the programming language Python, with support for floating point numbers.

����1000000000

RDATA ⇤ t mod 1� 0.5

���� < " (4.7)

The first term of this equation denotes the number of toggles, corresponding with adelay time t, needed to transmit one baud. This should not deviate much from a naturalnumber. Otherwise, errors would be introduced because it would lead to an inaccuratebaud rate. The equation has to be true for both delay times t1 and t2 using the samebaud rate from Formula 4.3. The comparison value " might have different values basedon the needed accuracy.

Similar to Equation 4.7, Equation 4.8 is used to find reasonable baud rate configu-rations for OOK. OOK has one more constraint in comparison to FSK. For OOK thenumber of toggles, corresponding with a delay time t, needed to transmit a one has tobe an even number to have the antenna again in low state after that number of toggles.

����1000000000

RDATA ⇤ t mod 2� 1

���� < " (4.8)

19

5 Experiments & Results

In this chapter, the experiment setup as well as the different experiments and its resultsare presented. First of all, the setup used for the different experiments including thedata sent to achieve comparable results is explained. Afterwards the minimal shiftingfrequency for sufficiently rejecting the carrier is determined followed by the determinationof the best deviation needed for FSK. After comparing the two modulation schemesextended evaluations in line-of-sight and non line-of-sight environments are presentedfor the use of FSK.

5.1 Experiment SetupThe physical setup for all experiments out of the first one (Chapter 5.2) is as shown inFigure 3.1. The position of the SDR is fixed within an experiment whereas the placementof the backscatter tag and the receiver is changed modifying the distances d1 and d2.

Each performed experiment is divided into multiple experimental runs. Each of theseexperimental runs consists of sending 100 predefined random packets consisting of a onebyte packet number followed by 63 bytes of random data.

All experiments from section 5.3 on consist of three experimental runs with differentorientations of the receiver to find the average error. For the experiments from section 5.5onwards, several experimental runs were performed for each location and orientation toreach a higher accuracy.

The carrier frequency for all of the following experiments was set to 2480 MHz for notinterfering with surrounding signals.

The transmit power of the SDR and the backscatter module differ between the ex-periments due to a change of the used antenna. For the first experiments a simpleomni-directional 2.4 GHz antenna was used resulting in a transmit power of 12 dBm.From section 5.5 on instead, the SDR signal was amplified and omni-directional antennaswith a higher range were used to achieve higher ranges. The amplified SDR signal hasa signal strength of 26 dBm. The overall trends of the first experiments are not effectedby this choice.

5.2 Minimal shifting frequencyThe first experiment was done to find out which frequency shift is minimally neededthat the backscattered signal is minimally affected by noise created by the SDR.

For this experiment only the SDR and the receiver unit were used and placed about30 cm away from each other. The receiver was set to listen on a fixed frequency while the

21

Figure 5.1: Noise Floor around carrier frequency

frequency of the carrier generator was changed in steps of 250 kHz from 10 MHz belowthe receiver frequency to 10 MHz above the receiver frequency. For each frequency,the carrier generator was set to, the RSSI value at the receiver was recorded. TheseRSSI values represent the noise floor introduced by the carrier signal for different offsetsbetween carrier frequency and receiver frequency. To receive a backscatter signal at thereceiver, the strength of this signal has to be stronger than the noise floor. Therefore alow noise floor value is appreciated.

Figure 5.1 shows that the noise floor for shifts bigger than 2 MHz changes only slightly.Therefore, the difference between a 2 MHz shift or a larger shift is only minimal whichleads to a minimal frequency shift of 2 MHz used for further experiments.

5.3 Comparison of deviations

Calculating all possible deviations for which Equation 4.5 is true leads to different shift-ing frequencies all above 2 MHz. Therefore all of them could be used. To be ableto compare the different deviations, all deviations with delay times corresponding to afrequency shift of 2 to 2.5 MHz were taken into account.

Due to the fact that the number of toggles for the two different frequencies for FSKhas to differ by at least one, the maximum baud rate for a deviation of 41 kHz is164 kBaud. Therefore three different baud rates below 164 kBaud were chosen to havecomparable results for the different deviations. These baud rates are around 2.4 kBaud,75 kBaud and 150 kBaud. The used baud rates for each configuration were calculatedin accordance with equation 4.7 with a value of 0.3 for ". The distances between theSDR and the backscatter tag and between the backscatter tag and the receiver wereboth chosen to be 1 meter for this experiment.

The results of this experiment in Figure 5.2 shows that the best deviation is 95 kHzDeviations above 95 kHz show overall worse results than small deviations. Moreover, thesmallest tested deviation shows worse results than a deviation of 95 kHz at high baud

22

Figure 5.2: Bit Error Rate for FSK with different deviations

rates. This is due to the difference in the number of toggles between the two frequencies.This difference is only one for the smallest baud rate which means that the differencebetween the frequencies of the two symbols is only half a period. For this configurationof half a period difference it is hard to find the best fitting baud rate because the exactone is not available on the CC2500. Therefore it is better to not use a difference betweenthe toggle numbers of one. That means that the highest tested baud rate will providebetter results with deviations larger than 41 kHz.

The following experiments are all done with the baud rate configuration of 95 kHzand a frequency shift of 2.178 MHz.

5.4 Comparison of OOK and FSKTo compare the general performance of OOK and FSK a couple of experiments wereperformed inside a lab. For those experiments, the backscatter tag was placed one, twoor three meters away from the carrier generator and the receiver was placed in differentdistances from the carrier generator, building a straight line with the carrier generatorand the backscatter tag. The maximum distance coverable between the carrier generatorand the receiver was 8 meters. The results of these experiments are shown in Figure 5.3.

The evaluation of the results clearly shows that lower baud rates perform generallybetter than higher baud rates. One of the reasons for that is the sensitivity of the CC2500which is higher for signals of low baud rates, resulting in a higher range for those signals.Furthermore, FSK performs always better than OOK which was expected due to FSKbeing less susceptible to noise than OOK but is also due to a higher sensitivity of theCC2500 for FSK than for OOK which can be experimentally seen. The comparisonof the three figures also shows that the performance is better for a placement of thebackscatter tag close to the signal generator which is in accordance with Chapter 3.1.1.

23

(a) 1 meter distance between SDR and backscatter tag

(b) 2 meters distance between SDR and backscatter tag

(c) 3 meters distance between SDR and backscatter tag

Figure 5.3: Comparison of different baud rates for OOK and FSK with different distancesbetween SDR and backscatter tag

24

The peak with a high bit error rate for all baud rates and modulation schemes, es-pecially at 5 meters distance in Figure 5.3a, is due to the lab environment with a largemetal tube at the ceiling. At this distance the backscatter tag was placed approximatelybelow that tube. The reflections of this tube are likely to create interferences leading toway more errors than in the neighbouring configurations.

Because of the much better performance of FSK compared with OOK, the followingexperiments were done for FSK only.

5.5 Line-of-sight experimentThe next experiments were done to ascertain the performance of FSK in line-of-sightenvironments. To reach the best possible performance and avoid interferences with othersignals, the line-of-sight experiments were performed outdoors between the universityand a forest.

Figure 5.4: Line-of-sight experiment for 2.9 kBaud FSK with backscatter tag close tothe carrier generator

The first experiment analyses the possible range of FSK with a baud rate of 2.9 kBaudfor different distances between the carrier generator and the backscatter tag. As visiblefrom Figure 5.4 ranges of up to 225 meters are reachable. Moreover, it can be seenthat even at these high distances the bit error rate is below 10

�4 for a placement ofthe backscatter tag close to the carrier generator. Higher distances between the carriergenerator and the backscatter tag lead to higher bit error rates but for the most coverabledistances the bit error rate is well below 10

�2.Figure 5.5 shows the results of the second experiments studying the performance of

FSK with the same baud rate as above for small distances between the backscatter tagand the receiver. With small distances between the backscatter tag and the receiver dis-tances of up to 200 meters from the carrier generator are coverable. For higher distancesbetween the backscatter tag and the receiver smaller distances between the receiver and

25

Figure 5.5: Line-of-sight experiment for 2.9 kBaud FSK with backscatter tag close tothe receiver

the carrier generator are coverable which is in accordance with the model described inChapter 3.1.1. The coverable ranges for the same tag distances as in Figure 5.4 arealmost the same. The bit error rate is especially for the 200 meter range higher than inthe previous experiment but still for the most configurations below 10

�2.

Figure 5.6: Line-of-sight experiment for 197 kBaud FSK with backscatter tag close tothe carrier generator

The last figure (Figure 5.6) for line-of-sight experiments shows the experiments donefor the use of a high baud rate (197 kBaud). Using this baud rate, a shorter maximumrange of 175 meters is coverable. Moreover, the bit error rate is significantly higherfor the same distance configurations compared with the usage of a low baud rate of2.9 kBaud. Generally it is possible to reach high distances with a high baud rate but

26

this should only be considered if a high amount of data has to be transmitted and incombination with a strong error correction algorithm.

5.6 Non line-of-sight experimentThe previous experiment showed the distances possible in an outdoor environment. Herethe focus lays on the performance of 2.9 kBaud FSK in a non line-of-sight indoor envi-ronment. Therefore the carrier generator and the backscatter tag are placed in the sameroom with different distances between each other and the receiver is placed in a differentroom as seen in Figure 5.7.

Figure 5.7: Map with locations for the non line-of-sight experiment

Figure 5.8 shows the results of this experiment. The vertical lines show the positionsof the walls in the test environment. This experiment shows as well as the previous onesbetter performance for short distances between the carrier generator and the backscat-ter tag. Especially, for a distance of 6 meters between the carrier generator and thebackscatter tag the signal-to-noise ratio is close to 0 resulting in a maximal range of20 meters communicating through 4 walls whereas a maximal range of 30 meters, com-municating through 8 walls is possible for a carrier generator-tag distance of 1 meter.The bit error rate for this experiment is for almost all cases below 10

�3.

Figure 5.8: Non line-of-sight experiment for 2.9 kBaud FSK

27

6 Television Signals

The second part of this thesis deals with a study whether television signals can be usedas carrier signal for the previously described communication system.

A use of TV signals instead of a constant carrier signal would make the backscattersystem being usable in a much wider variety of situations and save a lot of energy andcost by not needing extra hardware for generating a specific carrier wave.

This chapter deals with properties of DVB-T television signals and the design andimplementation changes needed to use those signals as carrier signals.

6.1 DVB-T SignalTerrestrial television signals are available in the most places and transmitted continu-ously. Therefore they are theoretically usable as a carrier signal. The terrestrial TVsignals available in Europe and many other places worldwide are DVB-T signals usingthe signal format OFDM1 with a maximum channel width of 8 MHz. [14, 8]

Figure 6.1: Recorded TV Signal

A 15 second sample of such a signal with a centre frequency of 594 MHz was recordedwith the SDR used in this thesis to replay it as carrier signal. The 20 MHz wide signalwas recorded using the SDR set to a sample rate of 20 million, in combination with anantenna suitable for Sub-GHz signals. An averaged FFT-plot of the recorded signal isshown in Figure 6.1.

1Orthogonal Frequency Division Multiplex: large number of close spaced carriers, orthogonal to eachother to avoid interferences. [8]

29

6.2 Design & Implementation changesIn this section the relevant changes performed to use a TV signal as a carrier with thepreviously used hardware are described.

6.2.1 Signal generator

For replaying the signal using the SDR, a program using GNURadio was developed. Theprogram consists of a file source connected to a USRP sink for outputting the signal. Theparameters needed are the sample rate which is the same as for recording the signal andthe centre frequency around which the signal should be replayed. The centre frequencywas chosen to be inside the 2.4 GHz ISM band to be able to use the same receiver asin the previous part of this thesis and to be able to compare the performance of thedifferent modulation schemes for different signals used as carrier. The frequency chosenis 2476 MHz to achieve that the right edge of the replayed TV signal is at approximately2480 MHz which is the same frequency used for the carrier signal in the previous partof this thesis. The FFT-plot in Figure 6.2 shows the average replayed signal.

Figure 6.2: Replayed TV Signal

6.2.2 Backscatter unit

For the OOK program running on the Beagelobone Black no changes were needed. Onlyfor FSK some changes were necessary. The approach of shifting the carrier signal bysmall different amounts like in the previous chapters was not possible because the higherfrequency shift for transmitting a one would lead to a signal not only at the higher ofthe two frequencies the receiver is listening on but also at the lower frequency due to thewidth of the TV signal. Therefore the program was changed to allow smaller delay timesthan 80 ns and make a larger frequency shift possible. The chosen frequency shift fortransmitting a one was chosen to be 10 MHz. The shifting frequency for transmitting azero was set to 2.5 MHz to be still above the minimum shifting frequency and have the

30

(a) OOK: TV Signal (blue) and shifted Signal(red)

(b) FSK: TV Signal (blue), shifted Signaltransmitting 0 (red), shifted Signal trans-mitting 1 (green)

Figure 6.3: Shifted TV Signal for OOK and FSK

edges of the shifted signals at the right positions to reuse the previously used deviationof 95 kHz. An FFT-plot with the carrier signal and the shifted signals for FSK andOOK can be seen in Figure 6.3.

6.2.3 Receiver

For the receiver program only minor changes had to be made. First of all the centrefrequencies for using a TV Signal as carrier had to be adjusted. The other adjustmentis the filter bandwidth of the CC2500 which was set to its maximum value of 812.5 kHzinstead of 325 kHz for using FSK with a TV Signal as carrier. This adjustment wasmade to better receive the FSK signal after testing with the old value first.

31

7 Comparison of Carrier Signals

This chapter contains the study of usability of TV signals used as a carrier signal forbackscattering instead of a constant carrier signal.

7.1 Experiment SetupThe experiments described in the following sections were all performed inside a lab ina line-of-sight environment. Due to the maximum signal strength of the replayed signalof around �70 dBm (see Figure 6.2), the amplifier used in Chapter 5 could not be usedfor these experiments. Because of the small signal strength only short distances werecoverable with a TV signal as carrier. To be still able to compare the carrier signals,a different measure was needed. This measure is the signal-to-noise ratio (SNR) whichis used in the following sections to compare the bit error rate of the different carriersignals.

7.2 FSK comparisonDue to the low signal strength of the carrier signal, only measurements for an SNRof up to 16 were possible. The results of the comparison using FSK can be found inFigure 7.1. The result for a constant carrier shows that a higher SNR leads to a lower

Figure 7.1: Comparison of carrier signals using FSK

33

bit error rate which is expected. For the TV signal as carrier, the results indicate onlya small downwards trend with a bit error rate always well below 10

�1 and for some ofthe highest measured SNRs even below 10

�2.

7.3 OOK comparisonFor the comparison of carrier signals for OOK an SNR of up to 17 was possible. Figure 7.2shows the results of the comparison of the TV signal as carrier and the constant carrier.It indicates decreasing bit error rates for increasing SNRs especially using a televisionsignal as carrier. Furthermore, the results indicate for a signal strength close to the noisefloor that the usage of a constant carrier leads to lower bit error rates than a TV signalused as carrier. For a stronger signal the results imply that the use of TV signals as acarrier or the use of a constant carrier is more or less equally good.

Figure 7.2: Comparison of carrier signals using OOK

7.3.1 OOK comparison of ranges achievable with a TV carrier

For OOK even higher ranges than for FSK were coverable. Therefore, using Figure 7.3,the bit error rate for different distances from the signal generator with a distance of 30 cmbetween signal generator and backscatter tag can be compared. For this comparison anexperiment using a constant carrier with a signal strength of �70 dBm and the samedistances as with the TV signal as carrier was performed to be able to compare rangesbased on signals of similar strength. This comparison shows that OOK is generallyperforming better using a constant carrier but also a range of up to 1.3 meters is coverablewith a bit error rate well below 10

�1, using a TV signal as carrier.

34

Figure 7.3: Comparison of carrier signals using OOK for different distances

35

8 Conclusion & Future Work

This chapter contains the discussion of the results of Part I and Part II of this thesis aswell as relevant future work to improve the backscatter system. The results show thatthe presented backscatter system with the studied modulation schemes, which allow forsimpler receivers, achieves new ranges for a backscatter system with a controlled carrieras well as with an uncontrolled carrier. It especially exceeds the ranges of backscattersystems modulating the data on top of the ambient signal instead of using a frequencyshift.

8.1 Part I

The results of the first part of this thesis, using a constant carrier signal, showed thatfrequency-shift keying (FSK) has a much better performance regarding error rate andrange compared to on-off keying (OOK). With FSK ranges of up to 225 meters areachievable using a baud rate of 2.9 kBaud and even a high baud rate of 197 kBaud isusable to reach distances of 175 meters. Furthermore, the presented backscatter systemis capable of transmitting data through multiple walls in an indoor environment andachieving a range of up to 30 meters using a baud rate of 2.9 kBaud.

8.2 Part II

The results of the second part indicate that the presented backscatter system performsgenerally better using a constant carrier instead of a television signal. For FSK, a con-stant carrier is leading to a much better performance whereas for OOK the performanceusing a constant carrier is on average only slightly better. This shows that FSK is thepreferred modulation scheme in a scenario with a constant carrier whereas OOK is thepreferred modulation scheme in a scenario with a highly fluctuating carrier. The usageof OOK as modulation scheme also shows that ranges of over a meter are coverable witha weak carrier signal of around �70 dBm in the tested environment.

8.3 Future Work

Future research based on the presented results is especially needed for uncontrolledcarrier signals.

37

First of all, a different receiver supporting sub-GHz frequencies should be used to beable to work with actual television signals and to evaluate how capable the system is indifferent environments.

Moreover, it would be interesting to analyse the performance of the presented backscat-ter system in the context of other available ambient signals used as carrier. Analogue anddigital radio signals which are available in many more environments would be interestingto remove the limitations of television signals not being available in most indoor environ-ments. Other signals like WiFi or Bluetooth would be especially interesting for indoorenvironments or for wearable Internet of things applications due to its large availability.

Another interesting area for this backscatter system would be a bidirectional com-munication where the backscatter tag is capable of receiving and decoding signals fromanother backscatter tag. Furthermore, it would be interesting to see how good sucha backscatter system could be used in the field of communications between movableobjects.

38

Bibliography

[1] arduino.cc. Arduino ZERO & Genuino ZERO. [Online; accessed 30-September-2016]. 2016. url: https://www.arduino.cc/en/Main/ArduinoBoardZero.

[2] arduino.cc. SPI library. [Online; accessed 30-September-2016]. 2016. url: https://www.arduino.cc/en/Reference/SPI.

[3] beagleboard.org. BeagleBone Black. [Online; accessed 19-September-2017]. 2016.url: http://beagleboard.org/black.

[4] beagleboard.org. PRU-ICSS Resources. [Online; accessed 30-September-2016]. 2015.url: http://beagleboard.org/pru.

[5] Cognitive Radio Working Group and others. SDRF cognitive radio definitions(SDRF-06-R-0011-V1.0.0). [Online; accessed 30-September-2016]. 2007. url: http://www.sdrforum.org/pages/documentLibrary/documents/SDRF-06-R-0011-

V1_0_0.pdf.[6] credentiality. Beaglebone PRU GPIO example. [Online; accessed 19-September-

2017]. 2015. url: http://credentiality2.blogspot.se/2015/09/beaglebone-pru-gpio-example.html.

[7] Darrell L. Ash, RF Monolithics Inc. A COMPARISON BETWEEN OOK/ASKAND FSK MODULATION TECHNIQUES FOR RADIO LINKS. [Online; ac-cessed 03-October-2017]. url: http://www.digikey.com.au/Web%20Export/Supplier%20Content/RFM_583/PDF/rfm-an-ook-ask-fsk-comparison.pdf.

[8] electronicsnotes. What is DVB-T: Digital Video Broadcasting. [Online; accessed18-July-2017]. url: https://www.electronics-notes.com/articles/audio-video/broadcast-tv-television/what-is-dvb-t-basics-tutorial.php.

[9] Joshua F Ensworth and Matthew S Reynolds. “Every Smart Phone is a BackscatterReader: Modulated Backscatter Compatibility with Bluetooth 4.0 Low Energy(BLE) Devices”. In: RFID (RFID), 2015 IEEE International Conference on. IEEE.2015, pp. 78–85.

[10] Andrea Goldsmith. Analog Transmission of Digital Data: ASK, FSK, PSK, QAM.[Online; accessed 30-September-2016]. 2016. url: http://web.stanford.edu/class/ee102b/contents/DigitalModulation.pdf.

[11] Vikram Iyer et al. “Inter-Technology Backscatter: Towards Internet Connectivityfor Implanted Devices”. In: Proceedings of the 2016 conference on ACM SIGCOMM2016 Conference. ACM. 2016, pp. 356–369.

V

[12] Bryce Kellogg et al. “Passive Wi-Fi: Bringing Low Power to Wi-Fi Transmissions”.In: 13th USENIX Symposium on Networked Systems Design and Implementation(NSDI 16). 2016, pp. 151–164.

[13] Bryce Kellogg et al. “Wi-Fi Backscatter: Internet Connectivity for RF-Powered De-vices”. In: ACM SIGCOMM Computer Communication Review. Vol. 44. 4. ACM.2014, pp. 607–618.

[14] Vincent Liu et al. “Ambient Backscatter: Wireless Communication out of ThinAir”. In: Proceedings of the ACM SIGCOMM 2013 Conference on SIGCOMM.SIGCOMM ’13. Hong Kong, China, 2013, pp. 39–50. isbn: 978-1-4503-2056-6.doi: 10.1145/2486001.2486015. url: http://doi.acm.org/10.1145/2486001.2486015.

[15] MikroElektronika d.o.o. ccRF click. [Online; accessed 30-September-2016]. url:http://www.mikroe.com/click/ccrf/.

[16] Pavel V Nikitin and KV Seshagiri Rao. “Antennas and propagation in UHF RFIDsystems”. In: RFID, 2008 IEEE International Conference On. IEEE. 2008, pp. 277–288.

[17] Carlos Pérez-Penichet et al. “Augmenting IoT Networks with Backscatter-EnabledPassive Sensor Tags”. In: 3rd Workshop on Hot Topics in Wireless, HotWireless.2016, pp. 23–27.

[18] Ian Poole. What is MSK, Minimum Shift Keying Modulation. [Online; accessed 21-July-2017]. url: http://www.radio-electronics.com/info/rf-technology-design / pm - phase - modulation / what - is - msk - minimum - shift - keying -

tutorial.php.[19] Prashanth Holenarsipur, Maxim Integrated Products Inc. FSK: Signals and De-

modulation. [Online; accessed 15-September-2017]. 2008. url: http://www.eetimes.com/document.asp?doc_id=1276362.

[20] Ettus Research. USRP B200 (Board Only). [Online; accessed 15-September-2017].2017. url: https://www.ettus.com/product/details/UB200-KIT.

[21] Texas Instruments. “CC2500 Low-Cost Low-Power 2.4 GHz RF Transceiver”. In:CC2500 datasheet (2010).

[22] Bob Watson. Analog Transmission of Digital Data: ASK, FSK, PSK, QAM. [On-line; accessed 15-September-2017]. 2001. url: http://s.eeweb.com/members/mark_harrington/answers/1335729484-FSK_signals_demod.pdf.

[23] Pengyu Zhang et al. “Enabling Practical Backscatter Communication for On-bodySensors”. In: Proceedings of the 2016 conference on ACM SIGCOMM 2016 Con-ference. ACM. 2016, pp. 370–383.

[24] Pengyu Zhang et al. “HitchHike: Practical Backscatter Using Commodity WiFi”.In: Proceedings of the 14th ACM Conference on Embedded Network Sensor Sys-tems. SenSys ’16. New York, NY, USA: ACM, 2016, pp. 259–271.

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