Development of Versatile Multi-Link MIMO Channel Sounding Sensor
Network Using Software-Defined Radios
Minseok KIM† , Togo IKEGAMI† , and Masamune SATO†† The Graduate School of Science and Technology, Niigata University, Niigata 950-2181, Japan
E-mail: †mskim@eng.niigata-u.ac.jp
Abstract In this study, we develop a versatile multi-link MIMO channel sounding sensor network based on software-defined radios (SDRs). Each radio has a center frequency of 4.85 GHz, a bandwidth of 100 MHz, and a delay
time resolution of 10 ns. It can accommodate a large number of antennas using a switched antenna scheme; further,by associating multiple software radios, it can configure a multi-link MIMO measurement and obtain the synchronizedchannel responses in a short time. It is suitable for various purposes of channel measurements, such as distributed
massive MIMO, cell-free MIMO, localization, and sensing. This paper presents the design and implementation of thedeveloped system and verifies the synchronized operation through laboratory tests. Then, the usability is demonstrated
through an example measurement for indoor device-free localization (DFL).
Key words

1.

multi-link channel sounding, MIMO channels, distributed MIMO, massive MIMO, cell-free MIMO

Introduction

Recently, distributed antenna networks, such as distributed massive MIMO [1], cell-free MIMO [2], localization,and sensing, that leverage a large number of antennas spatially distributed rather than being collocated in a single
array, have emerged. Channel measurement and modelingare essential components of designing and evaluating sucha new wireless system. Thus, channel-sounding techniques
to efficiently obtain the multi-link radio channels among the
distributed massive number of antennas [3] are required.
To cope with the challenge mentioned above, this paper
developed a versatile multi-link MIMO channel sounding sensor network based on a switched antenna channel sounding
scheme based on software-defined radios (SDRs) [4]. This paper presents the design and implementation of the developed
system and verifies the synchronized operation through laboratory tests. Then, the usability is demonstrated throughan example measurement for indoor device-free localization(DFL) [5–7].

2.

Switched Antenna MIMO Channel
Sounder

2.1 Software Defined RadioSDR has high flexibility by implementing the functionsof wireless communication systems in software, allowing forthe realization of various wireless transmission techniques
without the need for hardware changes. In particular, an
open-source development environment, universal software radio peripheral (USRP) (by Ettus Research, USA), is popular for easy and cost-effective prototyping, and it can realizea wide variety of wireless communication techniques using
open-source software platform GNU Radio [4]. The USRPbased prototyping utilizes not only GNU Radio but also software platforms such as LabVIEW and MATLAB/Simulink.
Additionally, a driver and application programming interface (API) called USRP hardware driver (UHD) is provided,

which allows for more advanced control using programming
languages such as C/C++ and Python.
In this study, we adopt the USRP X310, which consists ofa motherboard and two RF daughterboards (UBX 160) witha frequency range from 10 MHz to 6 GHz and a maximum
passband bandwidth of 160 MHz. For the interface between
the USRP and host PC, 10 Gigabit Ethernet is adopted,
allowing for data transfer at up to 200 MS/s (max. passband bandwidth: 200 MHz). However, the maximum bandwidth in this configuration is determined by the low passfilter (LPF) of the daughterboards.
The signal flows for transmitting and receiving are as follows. First, on the transmitting side, the complex baseband
signal sent from the host PC undergoes interpolation processing by the digital upconverter (DUC), converting it into
a complex signal at an intermediate frequency (IF) near DC.
Then, the IF digital signal is converted into an analog signal
via a high-speed DAC and input into the RF upconverter
circuit of the daughterboard. On the receiving side, the received signal is downconverted to a complex signal at an IF
near DC and sampled by a high-speed ADC. After that, it
is converted into a complex baseband signal by the digital
downconverter (DDC) inside the FPGA and finally transferred to the host PC.
2.2 Switched Antenna Multiplexing Scheme
An RF switch connected to the daughterboard’s antenna
port is used to switch the antennas to realize a switchedantenna scheme. The RF switch used is the SR-J030-8S(Universal Microwave Components Corp.), of which detailed
specifications are shown in Table 1. The USRP X310 features
15-pin general-purpose input/output (GPIO) ports which
are directly controlled by the FPGA on the motherboard.
The UHD API provides functions to operate the GPIO ports,
and those are used to control the RF switch in the developed
system. Fig. 1 shows the control sequence diagram for antenna switching. The transmitter (Tx) transmits twice thenumber of sounding signals for the number of receiving antenna elements, then switches the Tx antenna, and the re-©

This article is a technical report without peer review, and its polished and/or extended version may be published elsewhere.
Copyright 2023 by IEICE2 symbol

Measurement Time(�×� MIMO)212

Phase diff. [deg.]

Tx

1
Rx 2
3

Table 1Isolation
VSWR (on state)
Rise/Fall time∗aOn/Off time∗b∗a∗bSpecification
0.5–12.4 GHz
3.0 dB Max
0.5–6 GHz: 60 dB Min
6–12.4 GHz: 50 dB Min
1.8:1 Max
40 ns Max90 ns Max

Specification
4.85001 GHz
200 MHz
100 MHzMultitone signal [3]256781.25 kHz
1.28 us
10 ns
256
15 dBm (typ.)164 us (8 × 8 MIMO)

ceiver (Rx) switches antennas over a period twice the lengthof the sounding signal. This allows the capture of the sounding signal even with some time delay caused by propagation
and RF switching.
The detailed measurement system specifications areshown in Table 2. Due to the roll-off characteristics of the
DDC, which distorts both ends of the transfer function, themiddle 128 points out of the 256 points of the obtainedtransfer function, excluding 64 points from both ends, areused. As a result, the passband signal bandwidth becomes
100 MHz although the sampling frequency is 200 MHz.
2.3 Evalution of Phase Stability
Accurate measurement of phase information is crucial in
MIMO channel sounding, and the coherence of received signals between elements plays a significant role in array signal processing. A switched antenna MIMO channel sounder
repeatedly transmits the same sounding signal and sequentially switches the Tx and Rx antennas to measure all channels. Therefore, phase drift during the measurement cansignificantly degrade the accuracy of array signal processing.

1.5

2

2.5

0.8
0.7
0.6
0.5
0.4
0.3
0.2

0

0.2

Measurement system specifications.

Item
Carrier frequency
Sampling frequency
Signal bandwidth
Sounding signal
No. subcarriers
Subcarrier interval
Symbol length
Delay resolution
FFT points
Transmit power
Measurement time1(a) Phase stability of the local oscillator.

0.9

10–90%RF, 90–10%RF
50%TTL–90%RF, 50%TTL–10%RF

Table 20.5Time [ms]Specifications of the RF switch.

Item
Frequency range
Insertion loss

-1

0

Sequence diagram of antenna switching process (for2 × 3 MIMO)

Phase diff. [deg.]

Fig. 1

0

-2

time

1 symbol

1

0.4

0.6

0.8

Time offset [ms]11.2(b) Average absolute phase difference at each timeoffset.

Fig. 2Phase stability evaluation.

Hence, we evaluated the phase stability of the local oscillator.
The evaluation was conducted by directly connecting theTx antenna port and Rx antenna port through an attenuator, transmitting a sounding signal, and measuring the phaseof the received signal. The measurement results are shown in
Fig. 2(a). Here, phase variation was calculated by taking the
average phase across all subcarriers and measuring the phasedifference at each time sample with respect to the first sample. The results showed that the maximum phase difference
was approximately 3◦ , with a standard deviation of 0.62◦ .
Furthermore, the average absolute phase difference with respect to time offset is shown in Fig. 2(b). The vertical solidline in the figure indicates the time required for 8 × 8 MIMO
measurement (approximately 164 us). The average absolutephase difference for the measurement duration is found to beapproximately 0.4◦ –0.6◦ . It is considered that the impact of
the switched antenna scheme is minimal, and the results are
sufficiently acceptable for array signal processing.

3.

Multi-Link MIMO Channel Sounding
Sensor Network

In this section, we describe the multi-link MIMO channel sounding system developed to efficiently obtain the radio
channels among the distributed massive number of antennas. Each node has an 8-element switched antenna array for
Tx/Rx (half-duplex) and is designed to measure the impulse
responses of the 8 × 8 MIMO channels between each pair ofnodes. The system is designed to be scalable, and in this

—2—

10Gb
Ethernet

USRP 2

USRP 1GPIO

10Gb
Ethernet

Antenna

GPIO

Slave 1

Slave 2
PPS
Rb
oscillator

10Gb
Ethernet

Rb.
Oscillator

10MHz Ref.

USRP 4

USRP 3GPIO

10Gb
Ethernet

USRP

GPIO

Slave 3

Slave 4Master
TCP・UDP(a) Block diagram.

Fig. 3
Internal time is set
according to PPS signal

Tx/Rx internal time[s]0(b) Photograph.

6 link MIMO channel measurement configuration with 4 nodes.
Repeat transmission
at regular intervals

0.1

Node 130.2

Node 21
4

5

2
Tx Signal

Node 3Rx Signal
Receive for � sample
at any given time

Fig. 4

Timing synchronization for transmission and reception.

study, it is configured as a 6 link system with 4 nodes.
3.1 Multi-Link Channel Measurement Scheme
Each node has an 8-element switched antenna array for
Tx/Rx, and the propagation path between each pair of nodes
is defined as a link. When N denotes the number of nodes,
there exist N2 links. In this study, we designed a sixlink channel measurement system with four nodes. Fig. 3
shows an overview of the measurement system. Each nodeis equipped with one USRP, and a slave PC controls each
USRP. Additionally, there is a master PC that sends transmission and reception commands to the slave PCs. The
communication is carried out through TCP, and the data
received by each slave PC is sent to the master PC using
UDP.
3.2 Timing Synchronization
To achieve propagation channel measurements with physically separated multiple radio transceivers, carrier frequency,
and transmission/reception timing must be synchronizedamong them. USRPs have 10 MHz REF and pulse-persecond (PPS) ports, which function as interfaces for synchronizing multiple devices. In this study, the REF and PPS signals generated by a rubidium oscillator are commonly input
to each node’s USRP, achieving frequency and timing synchronization between the nodes. The UHD API provides afunction to set the USRP device time synchronized with the
PPS. By using this function to set the device time to 0 whencommunication between the USRP device and the host PC
is established, the USRP device time is synchronized with
the PPS signal. This synchronizes the internal time (elapsed
time from PPS) between the nodes. Fig. 4 shows an imageof the synchronized transmission and reception timing. According to this figure, the Tx transmits the sounding signal
every 100 ms. The receiver receives the signal of an arbitrary

Fig. 5

6

Node 4Phase 1
Phase 2Phase 36 link measurement procedure with 4 nodes.

transmission cycle. The timing of transmission and reception
can be specified by setting the device time to start transmission and reception using the function provided by the UHD
API.
3.3 Multilink Measurement Procedure
By the method described above, it is possible to measure
the impulse responses of the MIMO channels for each Tx-Rx
node pair. This is extended to realize multi-link measurement by appropriately switching the transmission or reception mode of each node. Fig. 5 shows the Tx-Rx pairs foreach link measurement. In this scheme, the measurement of
all links is divided into three phases. In the first phase, Node1 is set as the Tx node, and measurements for Links 1 to
3 are conducted. In the second phase, Node 2 is set as the
Tx node, and measurements for Links 4 to 5 are conducted.
Finally, in the third phase, Node 3 is set as the Tx node, andthe measurement for Link 6 is conducted.
In each phase, communication is carried out between themaster PC and slave PCs as shown in Fig. 6. To ensure thattransmission has started, the command to start reception
on the Rx slave PC is given after receiving a transmission
start notification from the Tx slave PC. The Tx node, uponreceiving the transmission start command, transmits continuously at intervals of 100 ms. On the other hand, the Rxnode, upon receiving the reception start command, receives
the necessary samples in accordance with the transmission
timing. If there are multiple Rx nodes, a broadcast is madeto each slave PC on the Rx side to start reception simultaneously. The received data is transferred to the master PC via
UDP by each slave PC. On the master PC side, the received
data is checked, and the transmission for the next phase isstarted. Furthermore, to speed up the process, it is also possible to save the received data on each slave PC and skip thedata transfer to the master PC, moving directly to the next—3—

(a) Measurement scene.

3

3
4

2

2

Fig. 6
Table 3

4

Communication flow diagram.

1

Node coordinates, (x, y) [m], in two deployment
configuration.
Anchor Node12
34

Config. 1(1.6, 0.0)
(0.2, 4.75)(4.0, 5.68)(7.04, 2.0)

Config. 2(4.5, 0.0)
(0.0, 2.0)
(1.5, 5.68)(7.04, 4.75)

1(b) Antenna deployment configurations(Blue: Config. 1, Orange: Config. 2).

Fig. 7Channel measurement environment.

phase.

4.

Evaluation of Multi-Link MIMO Measurement System

In this section, we evaluate the operation of the multi-link
MIMO channel sounding network developed in the previous
section.
4.1 Measurement Environment and Specifications
In this system, a 6 link 8 × 8 MIMO channel measurement was conducted with 4 nodes. Each node was equippedwith an equally spaced linear array antenna with an element
spacing of half a wavelength. Fig. 7 shows the measurement
environment. Propagation channel measurements were conducted with two types of antenna arrangements, as shownin Fig. 7(b). The positions of the anchor nodes for each antenna deployment are shown in Table 3. Since the anchor
nodes’ positions differ, two datasets were combined to extend to 12 link measurements with 8 nodes. Fig. 8 shows the
propagation paths obtained by RT simulation for 12 links.
4.2 MIMO CTFs and CIRs
Fig. 9 shows an excerpt of the channel measurement results for the link between Node 1 for Tx and Node 3 for Rx,
under Config 1. The straight-line distance of this link is
6.17 m, hence the theoretical value of the delay time is 20.6
ns. Furthermore, the free space propagation loss is 62.0 dB,
and since the gain of the Tx and Rx antennas is 4 dBi, thetheoretical value of the propagation loss is 54.0 dB. As seenin Fig. 9(b), there is a peak around the delay time of 20

Fig. 8Propagation paths obtained by RT simulationns, and the received power is confirmed to be approximately−50 ∼ −60 dB. Given that the delay time resolution of thedeveloped system is 10 ns, it can be seen that the results
obtained are reasonable in comparison to the theoretical values.
4.3 DFL Evaluation
4.3.1 Multipath-RTI TechniqueIn the radio tomographic imaging (RTI) method [5], thereceived signal strength (RSS) values y ∈ RN for each propagation path are represented as the linear sum of voxel values
along the propagation path as
y = Wx + n,(1)M

where x ∈ R represents the voxel values in the target localization area, and n ∈ RN is zero mean white Gaussian
noise process. The change in RSS due to power attenuation
caused by obstruction at time t is,∆y(t) = W∆x(t) + ∆n(t)

(2)
—4—

Table 4-30

Tx� - Rx�Tx� - Rx�Tx� - Rx�-40

Power [dB]

-50
-60
-70

Pos.
A
B
C
D
E

Target’s coordinates (x, y) [m](3.52, 3.15)(3.00, 1.40)(1.40, 4.50)(5.50, 3.15)(5.50, 5.00)
Table 5

-80
-90
-100
4.8

4.82

4.84

4.86

Frequency [GHz]4.88(a) Transfer function.
-30

Tx� - Rx�Tx� - Rx�Tx� - Rx�-40

Power [dB]

-50

-70
-80

Parameter
RTI voxel size
RTI ellipse param., γ
Regularization param., λRegularization param., α
DBSCAN 
DBSCAN NminPts

Value
0.05 m0.05 mby 5-fold cross-validation
0.25
0.5 m
3

∆y (φTx , φRx , τ ) = aH (φTx , φRx , τ ) h

-90

-600

-400

-200

0

Delay [ns]

200

400

− aH (φTx , φRx , τ ) h0

600(b) Impulse response.

Fig. 9

RTI parameters

are pre-estimated using ray-tracing simulations based on thesimple 3D model of the measurement environment. Then,based on the simulation results, the vector of RSS changes
for each link can be obtained through MIMO beamforming
for the measured MIMO channel matrix h as

-60

-100

Coordinates of positioning target

Example of MIMO channel measurement results(Legend: Element number of Tx and Rx antennas)

where ∆y(t) = y(t) − y0 , ∆x(t) = x(t) − x0 , and ∆n(t) =n(t) − n0 . Moreover, y0 , x0 , and n0 are the values of thebaseline measurement. W ∈ RN ×M is a matrix for weighting
the voxels along the line-of-sight (LoS) path and is definedby( √m
1/ dl , dm
l,s + dl,d < dl + γ(3)[W]ij = wij =0,elsewhere
Here, dl is the distance between nodes in the lth link, dm
l,s
and dml,d are the distances from the mth voxel to the Tx andRx nodes, respectively. Furthermore, γ is a parameter that
affects the width of weighting.
In the multipath RTI method [6, 7], in addition to thedirect wave, reflected waves up to two reflections are utilized. Specifically, it is assumed that the reflected waves are
generated by virtual sensor nodes outside the localization
target area, and conditions considering this are added to theweighting matrix as
 √
m
m

1/√dl , dl,s + dl,r < dl,1 + γ
m[W]ij = wij = 1/ dl , dl,r + dm
(4)l,d < dl,2 + γ
0,elsewheremHere, dl is the path length of the lth path, dm
l,s , dl,d , andmdl,d are the distances from the mth voxel to the transmitting
node, reflection point, and receiving node, respectively.
To realize multipath RTI, it is necessary to know thechanges in RSS for each propagation path. In this study, we
use array antennas at each anchor node to extract the RSS
of individual propagation paths through MIMO beamforming. Specifically, the propagation paths between each link

2

(5)

2

,

Here, h0 is the MIMO channel matrix measured when
there is no target in the positioning area. Furthermore,a (φTx , φRx , τ ) is the steering vector for the angle of departure (AoD), angle of arrival (AoA), and delay time (DToA).
Considering a MIMO configuration using linear array antennas with K elements at half-wavelength intervals for boththe transmitter and receiver, when the AoD, AoA, and delay
time τ arrive in the broadside direction as φTx , φRx , respectively, the amplitude of the received signal is denoted as Γ,and the MIMO channel matrix for each link is representedas2

h = A(Ω)Γ ∈ CK N ×1 ,(6)

2

A(Ω) = [a (Ω1 ) , . . . , a (ΩL )] ∈ CK N ×L ,
L×1

(7)

Γ = [Γ1 , . . . , ΓL ] ∈ C,(8)Ω = [Ω1 , . . . , ΩL ] ∈ C

L×1

(9)

,

Ωl = [φTx,l , φRx,l , τl ] ,a (Ωl ) = aT (φTx,l ) ⊗ aR (φRx,l ) ⊗ aτ (τl ) ,

(10)
(11)

where N and L represent the number of delay time bins andthe number of propagation paths, respectively. Moreover,⊗ denotes the Kronecker product operator. The steeringvectors for transmission, reception, and delay time are represented asaR (φRx ) = [1, exp (−jπ sin φRx ) ,· · · , exp (−jπ(K − 1) sin φRx )]T ,(12)aT (φTx ) = [1, exp (−jπ sin φTx ) ,· · · , exp (−jπ(K − 1) sin φTx )]T ,aτ (τ ) = [aτ (τ0 − τ ) , . . . , aτ (τN −1 − τ )]T ,aτ (τ ) =sin πN ∆f τ
1
exp (−jπ∆f τ ),Nsin π∆f τ

(13)
(14)
(15)

—5—

Pos.B

Pos.D

Pos.C

Pos.E

Measurement Simulation

Pos.A

Fig. 10

Generated RTI images and position estimation results

where ∆f is the frequency sampling interval.
4.3.2 Evaluation Results
We present the results of verifying the effectiveness ofthe proposed positioning method based on the measurement
results obtained by using the developed system. Table 4shows the positioning target’s locations. A radio wave absorber with a radius of 0.3 m was used as the positioning
target. Channel measurements were conducted both in theabsence of the positioning target (baseline measurement) andwith the positioning target in each configuration, and postprocessing was performed to generate RTI images and estimate positions.
Fig. 10 shows the RTI images created for the target placed
as shown in Table 4 in the measurement environment, where
the anchor node deployment was described in section 4. For
comparison, RTI images generated by simulation are also
shown. Here, the various parameters used for generating the
RTI images and position estimation are listed in Table 5. In
the figure, the yellow circles represent the actual positionsof the positioning targets, and the red crosses represent theresults of the position estimation.
From the measurement results, it can be seen that theestimated positions are generally in the vicinity of the actual target positions. For Pos E, there are artifact images in
the results generated by simulation as well, making theoretical estimation difficult; therefore, the estimation accuracy
is lower compared to other positions in the measurement
results. On the other hand, for Pos B, the simulation wasable to estimate the position within the radius of the positioning target, but the measurement results estimated it
at a distance position. It is seen that, for Pos B, the direct wave path is not obstructed, and the influence of the
RSS estimation of the reflected waves is larger compared toother positions. Additionally, the difference between the actual propagation path and that generated by RT simulation
could lead to a degradation in estimation accuracy. That is,there might be paths that exist in the RT simulation but not
in reality, or vice versa. To solve this, it is necessary to compare the measurement results with the RT simulation results
and remove the paths that do not actually exist. Furthermore, by extracting MPCs from the measurement values andassociating them with the results of the RT simulation, it isexpected that more accurate RTI images can be generated.

5.

Conclusion

In this study, we developed a system capable of multilink MIMO channel measurement in a short period, aimed
at various scenarios of a distributed massive number of antennas, and verified its effectiveness. The system utilizes the
USRP X310 software radio to implement antenna switching
MIMO techniques and enables simultaneous measurement ofmultiple links by associating multiple USRPs. We conducted
multi-link MIMO channel measurements in a typical room toverify the system.

Acknowledgement
This work was supported by JAEA Nuclear Energy S&T
and Human Resource Development Project Grant Number
JPJA22P22683407.
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