An Access Strategy for Downlink and Uplink
Decoupling in Multi-channel Wireless
Networks
Takumi Uekumasu, Makoto Kobayashi, Shunsuke Saruwatari, and Takashi Watanabe
Graduate School of Information Science and Technology, Osaka University, Japan
Email: {uekumasu.takumi, kobayashi.makoto, saru, watanabe}@ist.osaka-u.ac.jp
Abstract—With the increasing popularity of smartphones
and Internet of Things (IoT) devices, there has been an
increase in the amount of mobile traffic on networks. In thispaper, we discuss an access strategy for downlink/uplinkdecoupling (DUDe) with multiple frequency channels toenable the efficient use of frequencies in wireless networks.
DUDe is a wireless communications scheme that effectively
utilizes frequencies when communicating by connecting
the uplink and downlink to different base stations. We
propose two base station selection methods, i.e., signalto-interference-plus-noise ratio (SINR)-based decouplingwith re-association (SBD-RA) and SINR-based decouplingwith first-come first-association (SBD-FCFA). Based on theevaluation results obtained by event driven simulations,
we show that both SBD-FCFA and SBD-RA provide
higher throughput by utilizing frequencies more efficiently
than DUDe when using the same channel, or without
using DUDe. We also show that SBD-RA achieves a highthroughput although SBD-RA has a larger computational
complexity than SBD-FCFA.

I. I NTRODUCTION
In recent years, with the rapid spread in the use ofdevices such as smartphones, tablets, and Internet ofThings (IoT) devices, the demand for wireless communication capacity has continued to increase. Various
efforts have been made to cope with increasing wirelesscommunication capacity, such as the development of 5G
technology [1]. In this paper, of the different approaches
to realizing 5G, we focus on the downlink/uplink decoupling (DUDe). DUDe is a scheme that is used to improvethroughput by independently selecting the source basestation of the downlink and the destination base stationof the uplink in a network having multiple types of basestations [2], [3]. The basic idea of DUDe is shown in
Fig. 1, Fig. 2. In Fig. 1, Fig. 2, we assume that the
distance between a small cell base station and a device
is closer than the distance between the macro cell basestation and the device.
Fig. 1 illustrates a normal connection method. In anormal connection, scheme connects both the uplink andthe downlink to the base station that has the higher
downlink received signal strength indicator (RSSI) at
the device. In the same frequency band, we assume
c 2017 IEEE
978-1-5386-3531-5/17/$31.00 ⃝that the macro cell base station transmits data with hightransmission power, and the small cell base station andthe device transmit data with low transmission power.
In Fig. 1, because the signal from the macro cell basestation has a stronger RSSI at the device, both the uplinkand downlink to the device are connected to the macro
cell base station. On the other hand, in DUDe, scheme
improves the throughput by making a connection to thehigher RSSI on the receiving side. Fig. 2 shows anexample of how to connect with DUDe. As mentioned
above, the macro cell base station transmits data withhigh power, so in Fig. 2, the RSSI on the device is larger
than that of the small cell base station. In this case, forthe downlink, the device connects to the macro cell basestation. On the other hand, in Fig. 2, the small cell basestation is closer to the device than the macro cell basestation, and therefore, the RSSI on the small cell basestation is higher than that on the macro cell base station.
In this case, for the uplink, the device connects to thesmall cell base station.
Previous research on DUDe has assumed that boththe uplink and downlink are in the same frequency
band. However, in this study, we consider the casewhere the uplink and downlink use different frequency
bands. Owing to advances in semiconductor technology
and the miniaturization of wireless modules, current
smartphones are capable of supporting multiple interfaces such as WiMAX, wireless local area network(LAN), Bluetooth, and NFC. In addition, tethering canbe achieved with current technology, enabling the useof multiple wireless interfaces simultaneously. Devices
can connect the uplink and the downlink on different
frequency channels technically, if we show that thefrequency utilization efficiency can be improved, DUDe
can be applied on different frequency channels in future.
To this end, in this paper, we propose two basestation selection methods in DUDe using multiple frequency channels. One is the signal-to-interference-plusnoise ratio (SINR)-based decoupling with re-association(SBD-RA), and the other is SINR-based decoupling with
first-come first-association (SBD-FCFA). The SBD-RA
approach uses the actual SINR as an index for selecting

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Fig. 1. Connect both the down- Fig. 2. Connect the downlink to Fig. 3. Connect the uplink to the Fig. 4. Connect both the downlink and uplink to the macro cell the macro cell base station, and macro cell base station, and the link and uplink to the small cell
the uplink to the small cell base downlink to the small cell base base station (case4)
base station (case1)
station (case2)
station (case3)

a base station, and selects the type of base station towhich each device connects, i.e., the macro cell basestation or the small cell base station. We introduced amechanism to reduce the number of calculations requiredfor brute-force computations. The SBD-FCFA is a basestation selection method that further reduces the numberof calculations, as opposed to the SBD-RA, withoutchanging the base station. As a result of the evaluation
using simulations, we find that both the SBD-FCFA and
SBD-RA have a higher throughput compared to existing
methods. In addition, we find that the SBD-RA achieves
a high throughput although it has a larger number ofcalculations compared to the SBD-FCFA.
The rest of the paper is organized as follows. In
Section II, we review the system model of DUDe inheterogeneous wireless networks. Then, in Section III,
we describe the proposed SBD-RA, and in Section IV,
we describe the proposed SBD-FCFA. In Section V, we
present and discuss the performance evaluation results
obtained by simulation, and we confirm the effectivenessof the proposed methods. We conclude the paper in
Section VI.
II. S YSTEM M ODELIn this paper, we assume DUDe using different frequency bands for uplink and downlink. We assume thatthe transmission power of the macro cell base station islarger than that of the small cell base station.
A. Base station selection method
Figs. 1 to 4 show examples of connections in DUDe
where macro cells and small cells use multiple frequency
channels, as assumed in this paper. The green arrow
represents the macro cell frequency band, and the blue
arrow represents the communication in the small cell
frequency band. In DUDe using multiple frequency
channels, the following four types of connection methods
to base stations are conceivable.
case1: Both the downlink and uplink to the macro
cell base station (Fig. 1)• case2: The downlink to the macro cell base station,and the uplink to the small cell base station (Fig. 2)• case3: The uplink to the macro cell base station, andthe downlink to the small cell base station (Fig. 3)• case4: Both the downlink and uplink to the small
cell base station (Fig. 4)•

TABLE II NTERFERENCE IN DUD E USING THE SAME FREQUENCYCHANNELS [4]–[9]Transmitter

Macro
cell
Small
cell

BS
UE
BS
UE

ReceiverMacro cell
Small cell
BS
UE
BS
UE
×
✓
×
✓
✓
×
✓
×
×
✓
×
✓
✓
×
✓
×

TABLE III NTERFERENCE IN DUD E USING MULTIPLE FREQUENCY CHANNELS

Transmitter

Macro
cell
Small
cell

BS
UE
BS
UE

ReceiverMacro cell
Small cell
BS
UE
BS
UE
×
✓
×
×
✓
×
×
×
×
×
×
✓
×
×
✓
×

B. Comparison of interference model with previous research
In DUDe, using the multiple frequency channels handled in this paper, we see that the interference model
is different from the existing research [4]–[9], which isbased on the assumption that the current macro cell basestation and the small cell base station communicate in thesame frequency band. [4]–[9] deals with the base station
selection problem on the premise that the frequency
channels are the same on the uplink and downlink in
DUDe. Table I shows the interference between the basestation and the device assumed in [4]–[9]. In Table I,
the BS is the base station, and the UE is the device. ✓represents the case with interference and × represents thecase with no interference. In [4]–[9], base stations anddevices are assumed to interfere with each other because
they are transmitted in a time-division manner based onscheduling by the base station. For example, [4] defines asystem model on the premise that the frequency channel
is the same on the uplink and the downlink in DUDe,and it shows that the throughput of the whole system
improves by using DUDe.
On the other hand, in DUDe, considering the use ofmultiple frequency channels in this paper, we assume
an interference model where the macro cell base stationand the small cell base station do not interfere with eachother. Table II shows the interference model assumed
in this paper. As can be seen from the table II, in theinterference model, the macro cell base station and thesmall cell base station do not interfere with each other.

However, the interference model considered in thispaper uses an ideal model that does not interfere withthe uplink and downlink in a small cell base station. For
example, assuming a wireless LAN as a small cell, theuplink and downlink interfere with each other. However,
if we use the frequency band of the wireless LAN,
controlling wireless LAN is technically impossible. if
the interference between the uplink and downlink does
not occur by using a technology such as licensedassisted access (LAA), controlling wireless LAN is possible. We also consider the simplicity of comparisonswith previous studies assuming that macro cell basestations and small cell base stations perform DUDe on
the same frequency channel.
C. Interference model
In this paper, we define the SINR model of DUDe
using multiple frequency channels related to the SINR
model defined in [4]. ΦM ，ΦF ， and Φd are the set ofpoints representing the positions of macro cell base stations, small cell base stations, and devices, respectively,
which are randomly positioned.
xMj (∈ ΦM ), xFl (∈ ΦF ), xdm (∈ Φd ) are the positionsof the jth macro cell base station (Mj ), the locations of
the lth small cell base station (Wl ), and the position of
the mth device (dm ), respectively. j, l, and m are natural
numbers that are assigned to base stations or devices thatexist in countless numbers for convenience. We define
PM , PF , and Pd as the transmit powers of the macro cellbase station, the small cell base station, and the device.
We consider the downlink received power and SINR
from the base station, focusing on the mth device (dm ).
D
The downlink receive power (SM) from the jthj dmDmacro cell base station (Mj ) at the device is SM=
j dm
PM hMj dm ∥xMj − xdm ∥−α ，The downlink receive power(SFDl dm ) from the lth small cell base station (Wl ) is
SFDl dm = PF hFl dm ∥xFl − xdm ∥−α . hMj dm and hFl dm
represents Rayleigh fading, which is caused by radio
wave propagation from the macro cell base station (Mj )and the small cell base station (Fl ) at the device (dm ).
hMj dm ，hFl dm are independent exponential distributions
with an average of 1. ∥xMj − xdm ∥ and ∥xFl − xdm ∥ are
the distances between the macro cell base station (Mj )and the device (dm ), and the distance between the small
cell base station (Fl ) and the device, respectively.
In DUDe, by using multiple frequency channels, the
SINR at the device when the mth device (dm ) connects
to the ĵth macro cell base station (Mĵ ) on the downlink
D
is
SM
ĵ dm
D
SINRMĵ dm = ∑D
2
xM ∈ΦM \{xM } SMj dm + σjĵ

the SINR at the device when the mth device (dm )connects to the ˆlth small cell base station (Wl̂ ) on thedownlink is
SFD dml̂
SINRDFl̂ dm = ∑D
2
xF ∈ΦF \{xF } SFl dm + σll̂

σ 2 represents the noise.
The interference in the uplink in DUDe using multiple
frequency channels depends on whether each device isconnected to a macro cell base station or a small cellbase station. Specifically, the device connected to themacro cell base station in the uplink is not affected by
the interference caused by the device connected to thesmall cell base station. It is necessary to treat devices thatare connected to the macro cell base station and the small
cell base station separately. Therefore, we define a set ofdevice locations connecting the uplink to the macro cellbase station as Φ̃Md (∈ Φd ), and a set of device locations
connecting the uplink to the small cell base station asΦ̃Fd (∈ Φd ). They satisfy Φ̃Md ∩ Φ̃Fd = ϕ.
D
Like the downlink, the uplink received power (SM)
j dmfrom the mth device (dm ) in the macro cell base stationD(Mj ) is SM= PM hMj dm ∥xMj − xdm ∥−α . hMj dm ，j dm
hFl dm represents the Rayleigh fading caused by radio
wave propagation from the macro cell base station (Mj )and the small cell base station (Fl ) at the device (dm ),respectively. hMj dm ，hFl dm are independent exponential
distributions with the average of 1. ∥xMj − xdm ∥ and∥xFl − xdm ∥ are the distances between the macro cellbase station (Mj ) and the device (dm ), and the distance
between the small cell base station (Fl ) and the device.
The SINR at the macro cell base station when the m̂thdevice (dm̂ ) connects to the jth macro cell base station(Mj ) on the uplink is
SdUm̂MjU
SINRdm̂ Mj = ∑U2xdm ∈Φ̃Md \{xdm̂ } SdmMj + σ
The SINR at the small cell base station when the m̂thdevice (dm̂ ) connects to the lth small cell base station(Fl ) on the uplink is
SdUm̂FUl∑
SINRdm̂ Fl =U
2
xd ∈Φ̃Fd \{xd } SdmF + σ
m

m̂

l

III. SBD-RA
In the system model shown in Section 2, if thenumber of devices is m, the number of macro cellbase stations is j, and the number of small cell basestations is l. The combination of the devices and thebase station is O((j + l)m ). If (j + 1)m combinations
are tried, we determine all of the connections of thedevices with the maximum total throughput. However,
because the number of combinations of devices and basestations exponentially increases as the number of devices
increases, it is not realistic to try (j + 1)m combinationsof devices and base stations in a real environment having
a large number of devices.
To reduce the computational complexity at the timeof connection, we propose a base station selection
method called the SBD-RA. The SBD-RA first determines whether to connect the uplink of each device tothe macro cell base station or the small cell base station.
If the number of devices is m, the number of macro
cell base stations is j, and the number of small cell

Algorithm 1 SBD-RA base station selection method.
1: for all do ∈ Φ̃Md + Φ̃Fd do
2:for all B ∈ ΦM + ΦF do
D
3:
ThBd
← calcDLThroughput(B, do , ΦM , ΦF )o
4:
end forD
5:
BdDo ← arg max ThBdo
B∈ΦM +ΦF

6:
do connect to the station (BdDo ) in the downlink
7: end for
8: for all Ro do
9:decideUEway(Ro )10:for all do ∈ Φ̃Md + Φ̃Fd do

if do ∈ Φ̃Md thenfor all B ∈ ΦM do
ThdUo B
calcULThroughput(B, do , Φ̃Md , Φ̃Fd )14:
end for15:
end if16:
if do ∈ Φ̃Fdo then17:
for all B ∈ ΦF do
18:
ThdUo B
caluULThroughput(B, do , Φ̃Md , Φ̃Fd )19:
end for20:
end if
21:
end∑for22:
if ( B∈ΦM +ΦF ThdUo B > ThM AX ) then∑23:
T hM AX ← B∈ΦM +ΦF ThdUo B
24:
Rb ← Ro25:
end if
26: end for27: decideUEway(Rb )28: for all do ∈ Φ̃Md + Φ̃Fd do
29:
BdUo ← arg max ThdUo B
11:
12:
13:

←

←

B∈ΦF

do connect to the station (BdUo ) in the uplink
31: end for30:base stations is l. The number of combinations of eachdevice connecting to each base station in the uplink isO(m(j + l)2m ).
Algorithm 1 shows the selection method of a basestation to which the SBD-RA is connected. B is a basestation, do is a device, and BdDo and BdUo are base stations
selected by the device do in the downlink and uplink,respectively. Ro is an arrangement that stores the typeof macro cell base station or small cell base station
to which the device connects in the uplink, and Rb isthe array with the highest throughput of the sequencesthat store which type of macro cell base station or
small cell base station to which the device connects
in the uplink. ΦM andΦF are the set of positions of
the macro cell base station and small cell base station, as defined in Section II-C. Φ̃Md and Φ̃Fd are theset of devices that are connected to the macro cellbase station and the small cell base station defined in

Algorithm 2 SBD-FCFA base station selection method.
1: for all B ∈ ΦM + ΦF do
D
2:
SINRBd← calcDLSINR(B, do , ΦM , ΦF )o
3:
SINRdUo B ← calcULSINR(B, do , Φ̃Md , Φ̃Fd )4: end forD
5: B D ← arg max SINRBdo
B∈ΦM +ΦF

6: connect to the station (B D ) in the downlink
U
7: B U ← arg max SINRdo B
B∈ΦM +ΦF

8: connect to the station (B U ) in the uplink

II-C, respectively. calcDLThroughput(B, do , ΦM , ΦF ) isa function that is employed to calculate the throughputof the downlink from the base station (B) to the device(do ). calcULThroughput(B, do , Φ̃Md , Φ̃Fd ) is a function
to calculate the throughput of the uplink from the device(do ) to the base station (B), and decideUEway(Ro ) is afunction that is used to calculate the throughput of themacro cell base station or the small cell base station, andto decide whether to connect.
Algorithm 1 first calculates the throughput on thedownlink between each device and all base stations. In
the first to fourth lines of Algorithm 1, each device
calculates the throughput on the downlink between allof the base stations. In lines 5 and 6 of Algorithm
1, the device is connected downlink to the base station
with highest downlink SINR. Next, each device verifies
whether to connect the uplink to the macro cell basestation or the small cell base station on a brute-force
basis. The idea of SBD-RA is to decide in advance towhich of the macro cell base station or small cell basestation the device should be connected. In the 9th lineof Algorithm 1, method determines whether each device
connects to macro cell base station or a small cell basestation. After that, each device obtains the maximum
throughput from either the macro cell base station or
the small cell base station. the device connects to themacro cell base station obtains the maximum uplinkthroughput in the 11th to 15th lines of Algorithm 1,
and the device connects to the small cell base station
obtains the maximum uplink throughput in the 16th to20th lines of Algorithm 1. Finally, devices connect by
a combination in which maximizes the total of uplinkthroughput in all devices in uplink.
IV. SBD-FCFA
In the SBD-RA shown in Section III, although SBDRA reduces the number of calculations when comparedwith the case of using the brute-force approach for allof them, the number of calculations remains large as
O(m(j +l)2m ). In real networks, devices repeat additionand exit operations, so the number of calculations may
be problematic depending on the scale of the network.
In the DUDe for heterogeneous wireless networks,
the SINR of the device uplink depends on the type of

connected base station of the other device. When we
use the RSSI base in a heterogeneous wireless network,
the interference increases and the throughput of the uplink decreases. Specifically, in a heterogeneous wirelessnetwork, when using the RSSI base when the numberof small cell base stations is larger than the number ofmacro cell base stations, the device that connects its
uplink to the small cell with a large number of basestations is connected to the device. When many devices
connect their uplink to a small cell, the interference atthe small cell base station increases, and the throughputof the device connected to the small cell base station
decreases. To improve the throughput, we should selectthe base station for all of the devices based on the SINR
in the uplink.
However, it is not realistic to determine the uplink
destination base station for all devices on a brute-force
basis. In the case of the brute-force approach, assumingthat the number of devices is m, the number of macro
cell base stations is j, and the number of small cellbase stations is l, there are (j + 1)m combinations ofstreets. As the number of devices increases, the numberof computation increases exponentially. In a wirelessnetwork in which a large number of wireless devices
exists and fluctuate dynamically, it is necessary to selectbase stations based on the SINR using a small numberof calculations.
In this paper, we propose a base station selection
method, SBD-FCFA. SBD-FCFA reduces the number ofcalculations by not changing the destination base stationof the device that is connected to the base station oncein the wireless network. Specifically, when each device
connects, method selects the base station based on the
SINR considering only the interference of the devices
already connected to the base station. Further, by notchanging the base stations once connected, SBD-FCFA
reduces the computational complexity.
Algorithm 2 shows the base station selection methodof SBD-FCFA. B is an arbitrary base station, do is
the newly connected device, and B D and B U are thebase stations selected for the downlink and the uplink,respectively. ΦM , ΦF is the set of macro cell base stations
and small cell base station positions defined in Section
II-C, Φ̃Md , Φ̃Fd represents the set of devices connected tothe macro cell base station, and small cell base station,
respectively. calcDLSINR(B, do , ΦM , ΦF ) is a function
to calculate the downlink SINR from the base station
B to the device do . calcULSINR(B, do , Φ̃Md , Φ̃Fd ) is afunction to calculate the uplink SINR from the device
do to the base station B.
First, the method calculates the downlink and uplink
SINR with all base stations. Specifically, the method
estimates the SINR between all of the base stations onlines 14 of Algorithm 2. When method calculates the
SINR of the uplink, method considers to which basestation the other device connects macro cell base stationor small cell base station. Subsequently, in lines 5 and 6of Algorithm 2, the device makes a downlink connection
to the base station that achieves the highest downlink
SINR. Finally, in lines 7 and 8 of Algorithm 2, thedevice makes an uplink connection to the base stationthat achieves the highest uplink SINR.
If the number of devices is m and the number of basestations is j +l, there are m(j +l) combinations in SBDFCFA. This is less than m(j +l)2m in SBD-RA, and thenumber of calculations decreases. In addition, even if thenumber of devices increases, the amount of computations
does not increase exponentially, so we consider that it
can be applied to a large-scale network.
V. E VALUATIONIn this section, we show the effectiveness of the
proposed method using computer simulations to describe
the performance evaluation. Specifically, we verify theeffectiveness of SBD-RA and SBD-FCFA by evaluating
the average throughput for each device in a small-scale
network and a large-scale network.
A. Evaluation environment
We evaluated the throughput of the uplink and downlink by performing computer simulations of the proposed
method. The transmission power of the macro cell basestation was 46 [dBm], the transmission power of thesmall cell base station was 20 [dBm], and the transmission power of the device was 20 [dBm]. We set the noiseσ 2 to -90 [dBm], and the path-loss coefficient α to 4. The
traffic model was the full-buffer model. The throughput
was the average of the Shannon capacities between thedevices and base stations. We performed the simulation
1000 times, and we used the average as the throughput.
We used downlink reference signal received power(DRP) and RSSI base as comparison targets. DRP isa method that is used to connect the uplink and the
downlink to the same base station based on the downlink
received power, as described in Section II-C. RSSI Baseis a method used in [4], and independently connects
the uplink and downlink to base stations based on thereceived power. We perform comparisons using the four
methods, which include the proposed methods SBDFCFA and SBD-RA.
B. Evaluation in small networks
We randomly placed base stations and devices within
the range 1 × 1 [km2 ]. Then, we set two macro cellbase stations and five devices. We evaluated the averagethroughput by varying the number of small cell basestations from 2 to 10. In Fig. 5, we show the downlink
throughput. From the figure, we observe two things.
First, SBD-FCFA and SBD-RA improved the downlink
throughput by up to 1.2 [bps/Hz] compared with the DRP
and RSSI Base. This is because the device selected theFig. 5.
Average downlink Fig. 6. Average uplink through- Fig. 7.
Average downlink Fig. 8. Average uplink throughthroughput in small networks.
put in small networks.
throughput in large networks.
put in large networks.

base station with little interference by other devices by
the SBD-FCFA. Second, SBD-FCFA achieved the same
downlink throughput as SBD-RA. This is because theinterference does not change in the downlink even if thebase station to which the device is connected changes.
Fig. 6 shows the uplink throughput. From the figure,
we observe two things. First, the SBD-FCFA improved
the uplink throughput by up to 2.2 [bps/Hz] comparedwith DRP, and by up to 1.1 [bps/Hz] higher than the
RSSI Base. Second, the SBD-RA improved the uplinkthroughput by up to 2.3 [bps/Hz] compared with SBDFCFA. This is because devices do not change base stationin the SBD-FCFA when a new device is connected, so
method does not consider the interference of the newly
added device. Therefore, the device that was connected
earlier could not connect to the base station with the
maximum SINR.
C. Evaluation in large networks
We randomly placed base stations and devices in therange 2 × 2 [km2 ]. We set 10 macro cell base stations
and 25 devices. We evaluated the average throughput by
varying the number of small cell base stations from 5 to
50. Because the SBD-RA calculations are complicated,
we compared DRP, RSSI Base, and SBD-FCFA.
In Fig. 7, we show the downlink throughput. From
Fig. 7, SBD-FCFA improved the downlink throughput
by up to 1 [bps/Hz] compared to DRP and RSSI Base.
This is because the device selected the base station withlittle interference by the SBD-FCFA.
Fig. 8 shows the uplink throughput. From Fig. 8, SBDFCFA improved the uplink throughput up to 1.6 [bps/Hz]
compared to DRP, and up to 0.7 [bps / Hz] compared to
RSSI Base. This is because the device selected the basestation with little interference by the SBD-FCFA.
VI. C ONCLUSION
In this paper, we proposed DUDe using different
frequency bands for the uplink and downlink, and we
proposed two base station selection methods, SBDRA and SBD-FCFA. SBD-RA reduces the number ofcalculations required to obtain the maximum throughput
by routing all kinds of base stations in the uplinkin each device. SBD-FCFA is an extension of RSSI
Base used in DUDe in cellular. By using SINR as thebase station selection index, SBD-FCFA selects the basestation considering interference using a small numberof calculations. We evaluated the performance of theeffectiveness of SBD-RA and SBD-FCFA by performing
computer simulations.
ACKNOWLEDGMENTS
We acknowledge the support received by JSPS Science Research Grant JP16H01718 and NTT AccessNetwork Service Systems Laboratories.
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