A Wireless Full-duplex and Multi-hop Network with
Collision Avoidance using Directional Antennas
Yusuke Sugiyama, Kenta Tamaki, Shunsuke Saruwatari

Takashi Watanabe

Shizuoka University, Japan
Email: saru@inf.shizuoka.ac.jp, {sugiyama, tamaki}@aurum.cs.inf.shizuoka.ac.jp

Osaka University, Japan
Email: watanabe@ist.osaka-u.ac.jp

Abstract—Wireless full-duplexing enables multi-hop wirelessnetworks to increase network performance, such as the endto-end throughput. However, the combination of wireless fullduplexing and multi-hop communication produces a secondarytransmission collision problem due to the increase of transmission
opportunities. This paper proposes directional asynchronous fullduplex medium access control (DAFD-MAC) to avoid interference
on primary and secondary nodes. DAFD-MAC suppresses thesecondary transmission collision problem by sending a data framewith directional antennas and also a Network Allocation Vector(NAV) frame while receiving the data frame. The simulation
results show that the proposed DAFD-MAC outperforms carrier
sense multiple access/collision avoidance (CSMA/CA), full-duplex
MAC (FD-MAC), and multi-hop full-duplex MAC (MFD-MAC).

I.

I NTRODUCTION

Today, wireless communication is indispensable to ourdaily lives. Because it is so widespread, higher performanceof wireless networks is required. Wireless multi-hop communication enables the construction of a wireless networkwith multiple relay nodes needing small transmission power,
even though a single-hop network needs significantly largetransmission power [1].
For many years, wireless communications were achievedin a half-duplex manner, in which a sender node was unable
to receive data in the same band while sending data. However,
the recent progress of hardware and software technologies
provides wireless devices, especially digital signal processors,with more processing power. These devices enable wirelessfull-duplex communication with digital cancellation as well as
analog cancellation of the incoming sending signals throughreceiving antennas [2]–[5]. Since most medium access control (MAC) protocols assume half-duplex communication, theintrinsic difference of the transmission poses a challenge todevelop full-duplex (FD) MAC protocols.
Some MAC protocols have been proposed between anaccess point (AP) and nodes in wireless LANs, i.e., within
a single-hop wireless network [4], [5]. However, this paper
focuses on FD-MAC for a multi-hop wireless network.
In basic MAC protocols for full-duplex wireless communication, a primary sender node, which acquires the channel
first (i.e., initiates the transmission), specifies one node fromits neighboring nodes as a secondary transmission node, andthen the secondary transmission node starts the transmission
according to the primary sender’s transmission [5], [6].

As we discuss in detail in Section II, the secondary transmission interferes with the primary transmission, which causes
a data collision. We define the data collision as a secondarytransmission collision problem. In this paper, to solve thesecondary transmission collision problem, we propose directional asynchronous full-duplex MAC (DAFD-MAC), which
utilizes directional communication. However, it is known thatthe use of directional communication induces the deafness
problem [7]. Because of the deafness problem, neighboring
nodes around the primary and secondary transmissions are
not able to detect these transmissions. In DAFD-MAC, adestination node of the secondary transmission transmits a
Network Allocation Vector (NAV) frame in the opposite direction of the source node of the secondary transmission.
The NAV frame is transmitted by directional antennas withfull-duplexing. The notification by the NAV frame decreases
the secondary transmission collision. We evaluated DAFDMAC by computer simulation. The evaluation results showthat the proposed DAFD-MAC achieves higher throughput
and decreases the collision ratio compared to the conventional
methods of carrier sense multiple access/collision avoidance(CSMA/CA), FD-MAC [5], and multi-hop full-duplex MAC(MFD-MAC) [6].
The rest of the paper is organized as follows. In Section II,
we set the background of the present study by reviewing recent
studies related to multi-hop communication, full-duplex communication, and the secondary transmission collision problem.
In Section III, we propose DAFD-MAC to avoid the secondarytransmission collision problem. In Section IV, we evaluate our
proposed DAFD-MAC by computer simulation. Finally, we
conclude our work in Section V.
II.

R ELATED W ORK

A. Wireless Full-duplex and Multi-hop Communication
Full-duplex communication enables transmission and reception on the same frequency channel at the same time[4], [5]. This particular characteristic of simultaneous communication allows effective use of wireless resources in thetime domain. A conventional radio can only perform halfduplex communicating because the transmitting signal is toostrong to receive a signal from another node. However, thedevelopment of self-interference cancellation has made fullduplex communicating possible [2]–[5], [8], [9]. For example,
Choi et al. [2], Everett et al. [3], Sahai et al. [4] and Jain et al.
[5] succeeded in canceling a received signal by using a fieldprogrammable gate array (FPGA) on a WARP V2 platform[10].

[4], [5] also proposed a wireless full-duplexing MAC protocol referred to as FD-MAC. FD-MAC assumes only singlehop communication between an AP and a client node. We define single-hop full-duplexing as bidirectional full-duplexing.
Figure 1(a) illustrates bidirectional full-duplexing. In this figure, one full-duplex transmission at most is happening between
the AP and node A.
FD-MAC has two candidates for timing adjustment: synchronous [4] and asynchronous [5]. The synchronous method
requires clock synchronization. In this paper, we focus on asynchronous timing adjustment because clock synchronization for
all nodes in a multi-hop network is a daunting task.
When applying wireless full-duplexing to a wireless multihop network, the MAC protocol, which is different from FDMAC, requires a mechanism to select a full-duplex communicating pair in a multi-hop network. The wireless full-duplex
and multi-hop network is able to have not only bidirectional
full-duplexing but also relay full-duplexing. Figure 1(b) shows
the relay full-duplexing. This full-duplex situation is possible
for simultaneous transmissions between node D and E, as an
example of bidirectional full-duplexing. Another possible fullduplex situation is when node B receives a frame from node A
while transmitting a frame to node C, as an example of relay
full-duplexing.
B. Secondary Transmission Collision Problem
The application of wireless full-duplexing to wirelessmulti-hop communication is not simple because of the secondary transmission collision problem. Figure 2 shows threecases of the secondary transmission collision problem. The firstcase, shown in Figure 2(a), occurs when receiver nodes exist
in an overlapping area between a primary transmission and asecondary transmission belonging to the same full-duplexing
process. The second case, shown in Figure 2(b), occurs when
receiver nodes exist in an overlapping area between a secondary transmission and a primary transmission, both of which
belong to another full-duplexing process. Figure 2(c) shows thethird case, which occurs when nodes exist in an overlapping
area between a secondary transmission and another secondarytransmission, both of which belong to another full-duplexing
process.
Generally speaking, we use Request To Send/Clear To Send
(RTS/CTS) frames to avoid collision in a multi-hop communication [11]. However, RTS/CTS frames are not sufficient toavoid the secondary collision in wireless full-duplex and multihop communication. This is assuming that both the primarytransmission node and the secondary transmission node send
RTS/CTS frames to defer the transmission by the neighbornodes. The transmission of RTS/CTS sometimes sets the NAV
to neighbor nodes that will not cause a collision. For example,

APA(a) Bidirectional fullduplexing
Fig. 1.

DE

AB(b) Relay full-duplexing

Bidirectional full-duplexing and relay full-duplexing

C

’
S1st

R
S2nd

S1st

’
S2nd

R
S1st

R

S2nd

’
S1st

R
S1st

S2nd

R

R

(a) First caseFig. 2.

(b) Second case(c) Third caseThree cases of the secondary transmission collision problem

when the destination node of a secondary transmission receives
a CTS frame transmitted by the primary transmission node, thedestination node of the secondary transmission cannot transmit
a CTS frame.
Directional antennas can also be used to reduce interferenceby directing a beam form toward a desired direction [7], [12]–[15]. The use of directional antennas enables a receiver nodeto avoid interference coming from unwanted directions [7].
These antennas also reduce the overlapping area between a primary transmission and a secondary transmission. Directional
MAC (DMAC) is one of the representative MAC protocols
that enable transmission while avoiding interference by using
directional antennas [7], [16]. However, DMAC has a deafness
problem due to the directional antennas [7]. This problem
occurs when a transmission node fails to communicate with its
intended receiver, because the receiver’s directional antenna is
pointed in a different direction [7], [17].
[6] proposed MFD-MAC, which applies wireless fullduplexing to a wireless multi-hop network. MFD-MAC has amechanism in which a primary transmission node specifies asecondary transmission node that likely has a frame to transmit. However, MFD-MAC does not consider the secondarytransmission collision problem.
[18] proposed a synchronous MAC protocol with fullduplexing and directional antennas. However, the MAC protocol must be synchronized across whole networks before thenodes can start transmission. Achieving clock synchronization
for all nodes in a multi-hop network is a daunting task. Additionally, [18] does not consider the secondary transmission
collision problem.
III.

DAFD-MAC

From the discussion in Section II, we design the DAFDMAC protocol. The primary goal of our MAC protocol design
is suppression of the secondary transmission collision problem
and improvement of the end-to-end throughput.
A. Concept of DAFD-MAC
DAFD-MAC has two characteristics. The first characteristic is interference avoidance between a primary transmission
and a secondary transmission by using directional antennas.
Figure 3 shows an example of wireless full-duplexing withdirectional antennas. We define S1st as a primary transmission
node, S2nd as a secondary transmission node, D2nd as adestination node of the secondary transmission, and NS1st ,NS2nd , and ND2nd as neighboring nodes of S1st , S2nd , andD2nd , respectively. The solid lines show primary and secondarytransmissions, and the dashed lines show transmission flows.
In Figure 3, relay full-duplexing happens among the primary transmission node S1st , the secondary transmission nodeS2nd , and the destination node of the secondary transmission
D2nd . Node S2nd receives a data frame from node S1st whiletransmitting a frame to node D2nd .
Figure 3(a) shows an example of the secondary transmission collision problem due to the use of omni-directional
antennas. Nodes S1st and S2nd use omni-directional antennas
to transmit data frames, and the transmissions are a primarytransmission and a secondary transmission, respectively. The
receiver node D2nd experiences interference from the primaryand secondary transmissions and cannot receive any frames.

4)

Figure 3(b) shows an example of avoidance of the secondary transmission collision problem by using directional
antennas. Nodes S1st and S2nd use the directional antennas
to send data frames. Therefore, it is possible for receiver
node D2nd to receive the secondary transmission, because
node D2nd does not experience interference from the primarytransmission.
The second characteristic of DAFD-MAC is the 5-way
handshake. The 5-way handshake prevents the deafness problem [17] by sending NAV frames to neighboring nodes ofa primary and a secondary transmission. One example ofneighboring nodes is NS1st , NS2nd , and ND2nd in Figure 3.
The 5-way handshake uses five special frames: Request To
Send (RTS), Request and Clear To Send (RCTS), Set Network
Allocation Vector (SNAV), Directional Set NAV (DSNAV),
and ACK frames, all of which have different roles. We describe
the details of the 5-way handshake in Section III-B.
The operation of DAFD-MAC is as follows:
1)

2)

3)

A node, which has a frame to transmit, uses
CSMA/CA to resolve the contention against neighboring nodes. The node, which wins the contention,transmits an RTS frame. We refer to this node as a
primary transmission node.
The destination node of the RTS frame obtain permission for secondary transmission and transmits an
RCTS frame. We refer to the node as a secondarytransmission node. The RCTS frame includes twodestination addresses: the primary transmission nodeand the secondary transmission node.
The primary transmission node, which receives the
RCTS frame, transmits the SNAV frame. The destination node of a secondary transmission waits until
the end of the transmission of the SNAV frame. Afterthe transmission of the SNAV frame, the destination
ND2nd
ND2nd
D2nd
D2nd

NS 1st

S 1st

S 2nd

NS2nd
NS 1st(a) Secondary transmission collision problem

S 1st

S 2nd

NS2nd

(b) Proposed method

Fig. 3. Collision avoidance with directional antennas: (a) collision problem,(b) avoidance using directional antennas

5)

node transmits a DSNAV frame. It should be notedthat the start timing of a primary transmission, asecondary transmission, and the DSNAV transmissionis the same.
At the end of the transmission of the SNAV frame,
the primary and secondary transmissions begin. The
primary and secondary transmission nodes avoid interference by directing a beam forms toward thedestination. The destination node of the secondarytransmission starts the transmission of the DSNAV
frame. We use directional antennas with a beam angleof 4π
3 rad for the transmission of the DSNAV frame.
The directional antennas are focused in the opposite
direction of the secondary transmission node to avoid
interference with the secondary transmission. Detailsof the beam angle are described in Section III-C.
All nodes use location information for the directional
communication. DAFD-MAC assumes that all nodes
know the location information of their surrounding
nodes by using existing techniques such as angleof arrival (AOA) localization and global positioning
system data (GPS) [13], [19].
To prevent collisions, a pair of communication nodes
exchange ACK frames with each other at the sametime by using directional antennas.

B. 5-way Handshake
The 5-way handshake consists of five control frames: RTS,
RCTS, SNAV, DSNAV, and ACK.
The RTS frame is transmitted by a primary transmission
node and includes the destination address and the end time ofthe primary transmission. The destination address also represents the source address of a secondary transmission. The end
time is used for calculating the NAV duration. The neighboring
nodes, which received the RTS frame, set up the NAV, anddefer their transmission. The primary transmission node, whichtransmitted the RTS frame, waits until receiving the RCTS
frame, which is the response of the RTS frame. The primarytransmission node does not start a primary transmission if the
primary transmission node does not receive the RCTS frame
during the assigned time period.
The RCTS frame, which is transmitted by the secondarytransmission node, includes the two destination addresses ofthe primary and secondary transmissions and the end time of
the full-duplexing communication. The secondary transmission
node compares the end times of the primary transmission
and the secondary transmission and selects the later time as
the end time of the full-duplexing. The RCTS frame has
three roles. The first role is the response to the RTS frametransmitted by the primary transmission node. The secondrole is notification to the node selected as the destination of
the secondary transmission. The third role is deferment of
the transmission of neighboring nodes around the secondarytransmission node.
The SNAV frame is transmitted by the primary transmission node and includes the end time of the full-duplexing. The
end time of the full-duplexing uses the same end time in the
RCTS frame. The SNAV frame defers the transmission of theneighboring nodes around the primary transmission node. To

NAV

r
S 1st

r

S 2nd

φ

d

r

r

D2nd

NAV

SIFS SNAV SIFS

RTS
SIFS

DATA

HEADER

SIFS
RCTS

SIFS

SNAV Transmission
Time + 2SIFS

θ

ACK

DATA

HEADER

SIFS
DSNAV

ACK

NAV

NAV

NAV

Fig. 4.

Beam angle when transmitting the DSNAV frametimedefer the transmission of the neighboring nodes, the primarytransmission node needs to be notified of the end time of the
full-duplexing.
The DSNAV frame is transmitted by the node specified
by the destination address of the RCTS frame and includes
the end time of the full-duplexing. The end time is the sameend time in the RCTS frame. The DSNAV frame defers thetransmission of neighboring nodes around the destination nodeof the secondary transmission. To avoid interference with thesecondary transmission, the destination node of the secondarytransmission transmits the DSNAV frame toward the opposite
direction of the source node of the secondary transmission.
The ACK frame is transmitted by the destination nodes ofthe primary and the secondary transmissions. The ACK frame
is for notification of the end of the full-duplexing.
C. Beam Angle of Transmitting the DSNAV Frame
As mentioned in Section III-A, DSNAV is transmittedwith 4π
3 rad as the beam angle of the directional antennas.
If we use 4π
3 rad for the beam angle, the transmission of thenodes that exist in the communication range of the secondarytransmission can be deferred, except for the communication
range of the secondary transmission node.
Figure 4 shows the beam angle while transmitting the
DSNAV frame. We define beam angle θ as
θ = 2π − 2ϕ

㸦1㸧Fig. 5.

When d = r，we obtain
ϕ=

π
3

(4)

㸦4㸧

㸦5㸧

DAFD-MAC time chart

D. Operation Example of DAFD-MAC
Figure 5 shows an example of DAFD-MAC operation.
S1st is a primary transmission node, S2nd is a secondarytransmission node, D2nd is a destination of a secondary transmission, and the nodes that possibly can communicate with
S1st , S2nd and D2nd are NS1st , NS2nd and ND2nd , respectively.
The operation is presented in the following.
1)

2)

where ϕ is the angle of the isosceles triangle in Figure 4. We
assume0≤d≤r(2)where the length of the base is d and the length of two sides
is r in the isosceles triangle in Figure 4. From equations (2)and (3), we obtain
π
π
≤ϕ≤ .
3
2

㸦3㸧

as the minimum angle of ϕ. We substitute equation (4) for
equation (1) and obtain4π
θ=3
as the maximum beam angle.

(1)

where the communication range of the secondary transmission
node is r, and the distance between the secondary transmission
node S2nd and the destination of the secondary transmission
D2nd is d. In addition, ϕ is derived as( )πdϕ = − arcsin
(3)
2
2r

㸦2㸧

3)

4)

RTS frame transmission
The primary transmission node S1st transmits an RTS
frame to defer the transmission of its neighbor nodes
with omni-directional antennas. The nodes receiving
an RTS frame, such as NS1st and D2nd , set up the
NAV. As shown in Figure 5, the end of the NAV
duration corresponds to the end time of the SNAV
frame transmission.
RCTS frame transmission
The secondary transmission node S2nd , which is
specified by the RTS frame, obtains permission for asecondary transmission. Node S2nd receives the RTS
frame and waits for the Short Interframe Space (SIFS)
time, and then it starts the RCTS frame transmission
with the omni-directional antennas. The neighbornode NS2nd , which received the RCTS frame, is set
to the NAV duration. As shown in Figure 5, the end
time of the secondary transmission is later than theend time of the primary transmission. Node NS2nd
sets up the NAV duration, which is calculated withthe end time of the secondary transmission.
SNAV frame transmission
Node S1st , which received the RCTS frame, transmits
the SNAV frame with omni-directional antennas after
waiting for the SIFS time. The neighboring nodeNS1st , which received the SNAV frame, sets up theNAV duration, which is calculated with the end timeof the secondary transmission.
Primary transmission, secondary transmission, and
DSNAV frame transmission
Nodes S1st , S2nd , and D2nd start transmissions at
the same time. Node S1st , which transmitted the

5)

0.35
0.3
Throughput [Mbps]

SNAV frame, waits for the SIFS time, and starts
the primary transmission to Node S2nd with thedirectional antennas. Node S2nd , which received the
SNAV frame, waits for the SIFS time and starts
the secondary transmission to node D2nd with thedirectional antennas. Node D2nd , which received the
RCTS frame, waits for the duration of transmitting
the SNAV frame + 2SIFS time. After the wait, nodeD2nd starts the transmission of the DSNAV frameto the neighboring nodes ND2nd with the directional
antennas of beam angle 4π
3 rad.
ACK frame transmission
After the secondary transmission, node S2nd andnode D2nd wait for the SIFS time. After passing the
SIFS time, the pair of communication nodes exchange
ACK frames with each other.

0.25
0.2
0.15
0.1

0
0

Fig. 6.

IV.

We conduct a computer simulation to evaluate DAFDMAC. We compare the end-to-end throughput and collision
ratio with the following four MAC protocols:

2)

3)

4)

0.4

0.8
1.2
1.6
Generated Traffic [Mbps]

2

End-to-end throughput vs. generated traffic

E VALUATION

A. Simulation Environment

1)

CSMA/CA
FD-MACMFD-MACDAFD-MAC (Proposed)

0.05

CSMA/CA (4-way handshake)
In CSMA/CA, nodes exchange RTS/CTS frames before a data transmission to prevent collisions. Dueto the benefit of exchanging RTS/CTS frames, thecollision ratio is the lowest compared to the others.
Therefore, CSMA/CA is the baseline for performance
without full-duplex communication.
FD-MAC
FD-MAC [5] uses full-duplex communication. However, FD-MAC is restricted to bidirectional fullduplexing, that is, primary and secondary transmission nodes having frames whose destination is eachother. We use FD-MAC as the baseline for comparing
relay full-duplexing to bidirectional full-duplexing.
MFD-MAC
MFD-MAC [6] has a mechanism in which a primarynode specifies a secondary transmission node thatlikely has a frame to transmit. The mechanism isaimed toward the maximum use of opportunities forfull-duplexing; however, it suffers from the secondarytransmission collision problem mentioned in Section
II-B. Because of the secondary transmission problem,the collision ratio of the MFD-MAC is the largest
among all the others. We use MFD-MAC as the baseline for evaluating performance without suppressionof the secondary transmission collision problem.
DAFD-MAC
DAFD-MAC is the proposed approach shown in
Section III. DAFD-MAC avoids the secondary transmission collision problem and the deafness problem
by using the directional antennas and the 5-way handshake. We use the beam angle when transmissions
and receptions of data and ACK frames are at π6 rad.
Note the beam angle is a reference angle and may not
be the optimal angle. This angle should be determinedby taking into account the influence of interference[15].

The common simulation parameters are as follows. The
physical rate is 2 [Mbps], the radio range is 250 [m], the frame
size is 1,500 [bytes], the area size is 1,500 [m] × 1,500 [m],the number of nodes is 100, the number of flows is 5, and thesimulation time is 300 [sec]. We use Dynamic Source Routing(DSR), which is an on-demand routing protocol that makes
use of source routing, as the routing protocol [20].
B. Throughput vs. Generated Traffic
We evaluate end-to-end throughput by changing the generated traffic. The aggregate end-to-end throughput versus thegenerated traffic is shown in Figure 6. Figure 6 shows thefollowing:
1)

2)

The throughput of the proposed DAFD-MAC
achieves the highest throughput among the others.
The reason it has the highest throughput is as follows: DAFD-MAC has full-duplex communicationcompared to that of CSMA/CA. While FD-MAC
only supports bidirectional full-duplexing, DAFDMAC supports not only bidirectional full-duplexing
but also relay full-duplexing. DAFD-MAC suppresses
the secondary transmission collision problem eventhough MFD-MAC does not.
The throughput of CSMA/CA is the lowest among
all the others. This is because CSMA/CA does notsupport full-duplexing.

C. Collision Rate vs. Generated Traffic
To verify the suppression of the secondary collision problem in DAFD-MAC, we also evaluate the collision ratio bychanging the generated traffic. The aggregate collision ratio
versus the generated traffic is shown in Figure 7. Figure 7
shows the following:
1)

2)

The collision ratio of the proposed DAFD-MAC islower than that of FD-MAC and MFD-MAC. Thisis because DAFD-MAC succeeds in suppressing thesecondary transmission collision problem by using
the directional antennas and the 5-way handshake.
The collision ratio of CSMA/CA is the lowest among
all the others. This is because it has the lowest collision ratio for the following reason:. in CSMA/CA,

[5]

0.5

Collision Ratio

0.4
[6]

0.3
0.2

[7]

CSMA/CA
FD-MACMFD-MACDAFD-MAC (Proposed)

0.1
0
0

Fig. 7.

3)

4)

0.4

0.8
1.2
1.6
Generated Traffic [Mbps]

[8]

2

[9]

Collision ratio vs. generated traffic

every node belonging to a wireless network initiates
its transmission after exchanging RTS/CTS frames,
and the exchange of RTS/CTS frames prevents collisions in half-duplex communication.
The collision ratio of MFD-MAC is the highest
among all the others. This is because MFD-MAC
does not have any mechanisms to suppress the secondary transmission collision problem, as shown in
Section II-B.
The collision ratio of FD-MAC is lower than thatof MFD-MAC. This is because FD-MAC has fewer
full-duplex opportunities, and so the collision ratio of
FD-MAC is lower than that of MFD-MAC. FD-MAC
only supports bidirectional full-duplexing, and MFDMAC supports not only bidirectional full-duplexing
but also relay full-duplexing.

[10]
[11]

[12]

[13]

[14]

[15]

[16]

V.

C ONCLUSION

In this paper, we proposed the DAFD-MAC protocolthat avoids the secondary transmission collision problem byusing directional antennas. The secondary collision problem
occurs when combining wireless full-duplexing with multi-hop
communication. We evaluated the proposed DAFD-MAC bycomputer simulation. The evaluation results show that DAFDMAC achieves higher throughput than do CSMA/CA, FDMAC, and MFD-MAC.
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