Monopulse Switching to Cancel Phase Offset inArray Antenna Comprising Multiple COTS SDRs
Masato Miyazawa, Jin Mitsugi

Yuusuke Kawakita and Haruhisa Ichikawa

Auto-ID Laboratory Japan at Keio University, Japan
Email:{right, mitsugi}@sfc.wide.ad.jp

The University of Electro Communications, Japan
Email:{kwkt, h.ichikawa}@inf.uec.ac.jp

Abstract—This paper proposes a phase offset canceler for anarray antenna, which comprises multiple low-cost commercial of
the shelf (COTS) software defined radio (SDR) devices. The mainchallenge is the phase synchronization of SDR devices because
each SDR devices has its own phase offset when it produces
IQ stream. We devise a simple yet effective method to cancel thephase offset of individual SDR device using a double-pole, doublethrow (DPDT) switch which is implemented between antenna
and SDR device. The DPDT switch is controlled according to
the frame timing of each IQ stream to produce two set ofmeasurements, from which we can compute angle of arrival(AoA)
with a simple signal processing. We refer this mechanism as“Monopulse Switching”. We evaluated the working principleexperimentally to reveal Monopulse switching successfully cancel
phase offset to detect AoA without any initial calibration andachieves 2.8 degree measurement accuracy.
Index Terms—Coherent receiver, Software Defined Radio,
Array antenna, Complex monopulse

I. I NTRODUCTION
We have been observing the rapid deployment of shortrange wireless network, such as RFID, WiFi, Zigbee and Bluetooth. Those short range wireless devices are often attached
to physical objects to collect sensor data, its locations anddisposition to information system achieve services such as
supply chain management, warehouse management system andmanufacturing controls. This is the basic notion of Internet ofThings. Almost all of such services, accurate localization ofphysical object is essential and demanded by industries.
Accordingly, we have many commercial products and researches on indoor localization system. Even when we only
confine the survey in RFID, there are a number of commercial
products and researches [1], [2], [3], [4], [5], [6], [7], [8],[9]. Existing technology can be classified into the following
categories[3].
• use markers• transmit power control• antenna beam steering• tag RSSI based• tag phase based
Among the five categories, the phase based localization is,in principle, accurate but the measurement of phase demands
dedicated commercial hardware[1], [2] or a coherent receiver[10].
Existing studies mainly target localization system with aset of geographically fixed reader/writer. But there are many

Fig. 1. Commercial low-cost digital TV tuner can be used as IQ receiver

available handheld RFID reader/writer in the market andextensively used for relatively small scale automatic identification needs particularly in baggage handling in logistics andthe inventory management in apparel [11], [12], [13].
The authors have been working on a mobile localization
system using multiple low-cost commercial off the shelf(COTS) software defined radio (SDR) devices. Our motivation
is to make use of low cost commercial SDR platform such asa low-cost digital TV tuner shown in Fig.1.
Our typical use case is shown in Fig.2 [13]. In the figure, it
is shown that a worker with a handheld reader/writer is looking
for an object, which is applied an RF tag. The handheld
reader/writer is equipped with a receiving only angle of arrival(AoA) detection module. The AoA detection module receives
the signal from the target RF tag and calculates AoA andnavigates the worker by means of, for example, a laser pointer.
As the worker approaches the target object the resolution ofAoA detection increases.
In the course of development of such mobile reader/writer,
we encountered a fundamental problem of the use of multiple
SDRs – synchronizations among SDRs. There are three typesof synchronization, frame synchronization, timing (clock)synchronization and phase synchronization, all of which we
explain in detail later. Among the three synchronizations,the most challenge is the phase synchronization. The phasesynchronization is difficult to be achieved because the sampling and frequency conversion inside a SDR is done byindividual circuitry and thus producing a phase offset. BecauseRF tag attached to each goods

dsinθ
AoA=θ

Diversity antennadServo motor driven
RF board

ANT1

ANT2

Laser pointerJust attach

SDR2

SDR1
SDR module

S1: IQ stream 1

S2: IQ stream2

Commercial Reader/WriterFig. 2. A use case of handheld RFID reader/writer equipped with AoA
detection modulePCFig. 3. AoA detection with monopulse of two element antennaof this phase offset, we are forced to perform an extensive
calibration before a measurement. Alternatively we need toadopt a coherent receiver which is usually expensive. This
leads to the scarce use of localization system in industrial
deployments.
In this paper, we propose “Monopulse Switching “ whichmakes use of an RF switch to produce two sets of IQ data from
two antenna. By a simple signal processing, we can cancel thephase offset of SDR device. Although we focus our discussion
to a complex monopulse based AoA detection[13], [14] inthis paper, the core contribution of this study, cancellationof phase offset in SDR device, can be applied to other AoA
detection principles and other applications such as interfering
signal rejection.
We organize the rest of the paper as follows. In Section
II, we explain the basic theory of AoA detection based oncomplex monopulse method and summarize the challenges torealize AoA detection with multiple low-cost SDR devices.
In Section III, the idea of “Monopulse switch” is explained
followed by experimental evaluation of the method in Section
IV. Finally we conclude the paper in Section V.
II. BASIC THEORY OF AOA DETECTION WITH TWO COTS
SDR DEVICES
In this section, we explain a principle of AoA detection
based on complex monopulse and clarify the problem toimplement with two SDR devices.
A. Principle of AoA detection based on Complex Monopulse
methodFigure 3 is the basic technical component of AoA detectionwith two element antenna where θ and d denote the AoA andthe distance between the two element antennas, respectively.
Let λ and ϕ denote the wave length, and the phase differenceof the two antenna, respectively. The phase difference of two
IQ streams, S1 and S2 , can be written as follows.
2πd sin θϕ=(1)λThere are fundamentally three monopulse principles. The firstprinciple is to derive the amplitude of sum signal S = S1 +S2 and the differential signal D = S1 − S2 and is appled
the following formula to derive the approximation of phasedifference.
|D|
ϕ = tan−1(2)|S|This looks simple and robust but suffers from fluctuationof amplitude, fading. The generation of the sum and thedifferential signals would be easy when we use an analog
0/180 degree divider as a front end. The second principle isto derive the phase difference directly from S1 and S2 suchthat
ϕ = ̸ S1 − ̸ S2 .
(3)This is invariant to amplitude fading but we need to synchronize the phase of two signals. The third principle is thecomplex monopulse[14], which takes the complex ratio suchthatD
S1 − S2=.
(4)S
S1 + S2
The complex monopulse, difference of two signal divided bysum of two signals, can be expressed in the following simpleform.
Dej0 − ejϕ=S
ej0 + ejϕ1 − ejϕ=(5)
1 + ejϕ
A graphical representation of Eq.5 is shown in Fig.4. The
denominator of Eq.5 is a linear combination of 1 and ejϕwhose overall phase is denoted as α in Fig.4. The numerator,
on the other hand, constitutes the phase angle β in the complex
plane. Since both triangles ACO and BCO are isosceles
triangles. α + β = π2 , the division in Eq.5 falls on theimaginary axis if there is no multi-path. In other words, wejust discard the measurement where an observed D
S is NOTon the imaginary axis and thus, inaccurate.

Signal Generator

Signal Generator

Divider

Divider

MIMO cableclock
RTL-SDRIm

S1, S2

S2: IQ stream2

S1: IQ stream 1
USB

USRP

USRP

RTL-SDR

GigaEtherUSB
MATLAB

MATLABPCPCAFig. 5. Experimental setup to examine IQ streams from two sets of SDRdevicesSO

φαβCReDNI USRP

B

Connected by MIMO cableFig. 4. Complex monopulse ratio shows up on imaginary axis on complex
plane when there are no multi-path influence

B. Problems of AoA detection with COTS SDR devices
Even if we have the same type of SDR devices, theirclock frequency is different because they have their own
oscillator. The difference of clock frequency can be correctedby providing single reference clock. But the absolute phasecannot be synchronized because of the phase offset whichdepends on the hardware and software of MCU and otherelectric circuits inside the SDR.
To quantitatively examine the problem, we set up an experiment shown in Fig.5 where an ASK modulated signal isequally divided and fed into two SDR devices to produce twoIQ streams which is analyzed with MATLAB. We prepared
two sets of SDRs. One set is USRP and another is RTLSDR. The clock of two USRPs are synchronized with MIMO
cable(Fig.6). The master oscillator of one RTL-SDR is fed
into the other one after removing its own oscillator (Fig.7).
Figures 8 and 9 show the results.
From the measurements, we identified the following challenges.
• Although we provide the same radio signal with a divider
to the SDRs, there is a frame time difference, which canbe observed by the difference of ASK timing. In case ofUSRP, the difference is about 0.1 msec. In case of RTLSDR, it is about 0.3 msec. This frame delay stems fromthe timing delay to transfer IQ data to the processing
computer.
• The clock timing can be synchronized both in USRPs andRTL-SDRs, by means of a MINO cable and a dedicatedcoaxial cable, which can be observed by the recovered

Fig. 6. Two USRP are connected with MIMO cable

Sharing one oscillator

Xtal oscillator is removed

Fig. 7. Master oscillator of one is fed into the otherfrequency, the slope of phase, of two SDRs coincides; If
there is a difference of clock, the frequency cannot be thesame.
• Even after we synchronize the frame by properly shifting
the time sequence, we still observe phase offset between
streams S1 and S2 . This phase offset changes at everymeasurement. This problem stems from the timing ofstarting IQ sampling in the SDR device.
Since the starting timing depends on each hardware andsoftware in SDRs, it is difficult to adjust the timing fromMATLAB or other external software. Because of this phaseoffset, a mere subtraction on phase difference does not match

0.06

0-0.5

-1

0.04
-1.5
USRP1
Amplitude

0.03

-2
USRP2
Amplitude

USRP2
Phase

-2.5

0.02

-3

-3.5
00.2

0.4

0.6

0.8

11.2
Time (msec)

dAmplitude

Phase Angle (Radian)θ0.05USRP1Phase

1.4

1.6

1.80.01

DPDT Switch

0.00

Clocksynchronization

2SDR1SDR2

Fig. 8. IQ stream observation with two USRPs
41.4

3

1.2

Signal processing (PC)2

RTL-SDR1
Amplitude

0.800.6

Fig. 10. Introducing a DPDT switch to conventional AoA detection system
to cancel phase offset

Amplitude

Phase Angle (Radian)

1.0
1-1

RTL-SDR2
Amplitude-2

0.2

RTL-SDR2
Phase-3Thus, we can cancel the phase offset ϕo1 − ϕo2 and can derive
the phase difference purely caused from AoA ϕ. In terms ofIQ stream processing, it is simply

0.4

RTL-SDR1
Phase
0.0-4
0

0.2

0.4

0.6

0.8

1
1.2
Time (msec)

1.4

1.6

1.8

ej2ϕ =2Fig. 9. IQ stream observation with two RTL-SDRs

the phase difference caused by AoA. To solve the problem,
it is usual to calibrate before a measurement in which welayout the array antenna and a known source of radio locatedin a designated AoA. This is cumbersome in industrial usages
particularly when we use a mobile reader/writer which is,
in many cases, frequently power controlled to save battery
consumption.
III. C ANCELLATION OF PHASE OFFSET WITH MONOPULSESWITCHING

This section introduces a proposal to cancel phase offset.
We insert Double-Pole Double-Throw(DPDT) switch between
antennas and SDR devices, and acquire two set of phasedifferences with a DPDT switch. Figure 10 shows the proposed“Monopulse switch” system overview. The DPDT switch
provides SW 1 and SW 2 states such that
• SW 1 : SDR1 = ϕ01 + ϕ and SDR2 = ϕ02• SW 2 : SDR1 = ϕ01 and SDR2 = ϕ02 + ϕwhere ϕ0i denotes the phase offset of SDRi
The phase difference at SW1 state is ∆SW 1 = ϕ01 +ϕ−ϕ02
and the phase difference at SW2 state is ∆SW 2 = ϕ01 −(ϕ02 + ϕ). The subtraction of the two states yields thefollowing.
∆SW 1 − ∆SW 2= 2ϕ(6)S11 S22S21 S12(7)where we denote the stream i at SW j state as Sij . Since theswitching response of commercial DPDT switch is usually lessthan 20 nsec, we can neglect the influence of even fast fadingin “Monopulse switch” .
IV. E VALUATIONThis section introduces a prototype phase difference estimation system with complex monopulse switching with COTS
SDR devices, experimental environment and the results ofexperiment evaluation. An schematic of the prototype system
is shown in Fig.11. Since this is a functional experiment,
the experiment was conducted by wired configuration rather
than wireless AoA detection. We use Skyworks SKY13411374LF which is less than 50 cents as a DPDT switch. A signal
generator generates an RF signal at 918MHz with 1kHz ASK
modulation. The signal is divided into two signals with a 3dBdivider. We insert a phase shifter (Pastermack PE8244) to oneof the two routes to apply a predefined phase shift. We shifted
the phase of one route by adjusting PE8244. Since the phaseshifter is dial type and there are no gauge, we used a network
analyzer (HP 8753D) to accurately adjust the phase. Thistime, we applied four phase shifts, 0 deg, 30 deg, 60 deg and90deg with PE8244 and measure the phase difference with astraightforward complex monopulse method and the proposedcomplex monopulse switching method. The two SDR devices
which connected by MIMO cable are connected to a PC
through Gigabit Ethernet cables, and IQ signals generated
by SDR devices are processed by a custom made virtualSignalGenerator

Divider

PhaseShifter

DPDTSwitch

USRP
USRP

PC
(Signalprocessing)90°

Fig. 11. Experimental setup schematics

USRP

USRP

60°

Phase shifter

30°0°MIMO cable

DPDT switch
DividerTo PC

To signal
generator

Fig. 14. Result of phase difference measurement with complex monopulse
switching

Monopulse switchig

Fig. 12. Experiment set up of monopulse switching evaluation

Source1Source2

Triggerdetection

Triggerdetection

Frame
synchronization

Frame
synchronization

Straightforward Monopulse

Fig. 15. Occurrence probability of measured phase difference

Signal processing basedon complex monopulsewith and withoutswitching

the occurrence probability of the corresponding error range.
In Fig.15, it is clearly seen that the Monopulse Switching canprovide stable and accurate measurements.

Fig. 13. The function flow of the developed virtual instrument of LabVIEW

V. C ONCLUSION

instrument(VI) in LabVIEW. Figure 13 shows the schema ofVI program.
A. Result of experimentFigure 14 shows the result. It is shown Monopulse Switching can provide exact phase difference measurement by eliminating the phase offset. On the other hands, the straightforward
complex monopulse produces erroneous phase difference. Thisis because every time we restart the SDR devices the phaseoffset would be different. The average phase measurement
error of complex monopulse switching is 2.8 degree. Thisremaining error seems to be resulted from the imperfect frame
synchronization. Figure 15 shows the occurrence probabilityof the measurements. Horizontal axis of the figure denotes
the phase difference measurement error. The vertical axis isAoA detection with multiple COTS SDR devices is attractive particularly when we use a mobile reader/writer. But
it suffers from the phase offset caused by the software andhardware in each SDR device. Therefore, calibrations before
measurement are demanded in conventional AoA detection
with non-coherent array antenna. We can cancel the phaseoffset with proposed Monopulse Switching, which can beimplemented by using a low-cost DPDT switch and simple signal processing. An experimental evaluation reveals Monopulse
Switching can achieve stable AoA detection with 2.8 degree
error demanding no calibration before experiments.
ACKNOWLEDGEMENT
This work is supported by Ministry of Internal Affair andCommunications Japan under SCOPE grant 15003007 and
JSPS Kakenhi Grant 25240010.

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