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et al.
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RESEARCH
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Md. Abdul Alim1* , Makoto Kobayashi2 , Shunsuke Saruwatari2 and Takashi Watanabe2
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15*Correspondence:
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Abstract
abdul.alim@ist.osaka-u.ac.jp
117
Department of Information
Full duplex (FD) wireless communication is evolving into a practical technique andNetworking, Graduate School of18
many studies are being conducting in this area, especially regarding the physical
IST,
Osaka
University,
Osaka,
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Japan. Mr. Alim is also a faculty
layer. However, to exploit FD benefit successfully, efficient medium access control
20
member in ECE Discipline, Khulna(MAC) protocols are crucial as well as physical layer advances. Numerous FD21
University, Bangladesh.
MAC protocols have been proposed but these MAC protocols cannot address all
Full list of author information is22
available at the end of the article
the issues encountered in this area. In addition, many half duplex (HD) capable
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devices are present in existing wireless local area networks (WLANs), so there is24
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an urgent need to integrate FD clients and HD clients in the same WLAN. We
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refer to this type of WLAN as a heterogeneous WLAN (Het-WLAN). In this paper,27
we propose an FD-MAC for Het-WLAN, which considers all possible types of FD
28transmissions. Our proposed FD-MAC protocol suppresses inter-user interference.
29
Simulation results demonstrated that a significant throughput gain (about 96%)30
could be achieved by using our proposed FD-MAC compared with traditional HD
31
communications. Moreover, our proposed MAC obtained better performance (av32
erage throughput gain of about 11%) compared with another existing FD-MAC
33
design. In addition, probability analysis suggested that the total probability of FD
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transmissions increased rapidly as the WLAN approached saturation conditions.
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Keywords: Full-duplex; MAC protocol; Het-WLAN; FD-MAC
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1 Introduction
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In general, traditional radio transceivers cannot transmit and receive simultane42
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ously using the same frequency band because of self-interference at the receiver44
end. However, recent technological advances in antenna design and radio frequency
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interference cancellation techniques can reduce self-interference by up to 110 dB [1].
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Similar studies have also been conducted regarding the physical layer by [2, 3, 4].
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The latest technologies for self-interference cancellation allow us to transmit and49
receive signals simultaneously using the same frequency, which is known as in-band
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full duplex (IBFD) communication [5]. However, a suitable medium access control
51(MAC) is crucial to exploit the full advantages of IBFD technology in wireless local
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area networks (WLANs) because the current IEEE standard MAC protocols do not
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support IBFD communications. IBFD is one of the techniques with the greatest
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potential for supporting the huge traffic demands in the near future and researchers
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are attracted to IBFD because it can increase the spectral efficiency without re57
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quiring any additional frequency resources [5]. IBFD can also double the ergodic
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capacity of a multiple-input multiple-output system [1, 3].
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IBFD wireless communication can be categorized as bidirectional FD (BFD), three61
node FD (TNFD), or relay FD (RFD) [5, 6]. BFD and TNFD are illustrated in Fig.
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In-band Full-duplex Medium Access Control
Design for Heterogeneous Wireless LAN

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1. Two nodes comprising the primary transmitter (PT) and primary receiver (PR)transmit to and receive from each other simultaneously (Fig. 1a) in BFD. TNFD
can be described as destination-based TNFD or source-based TNFD. In destinationbased TNFD, PT transmits a signal to PR and PR also transmits a signal to anothersecondary receiver (SR) while receiving data from PT (Fig. 1b). In this case, PRalso acts as a secondary transmitter (ST). In source-based TNFD, PT transmits
data to PR, and ST also transmits data to PT (Fig. 1c). In this case, PT also acts
as a SR. However, in RFD, a user terminal sends data to the destination using
a relaying node [5]. In this study, we consider three modes of transmission, i.e.,
half-duplex (HD), BFD, and TNFD.
All of the user terminals or nodes in existing WLANs are traditional HD capable.
Therefore, it is not possible to replace all of these HD nodes (HDNs) with FD nodes(FDNs) overnight, so it is necessary to incorporate FDNs in existing WLANs in amanner that allows HDNs and FDNs to operate simultaneously. A WLAN thatcomprises HDNs and FDNs is referred to as a heterogeneous-WLAN (Het-WLAN)
because it has different types of clients or nodes.
In this paper, we propose an FD-MAC for Het-WLAN, which we call HFD-MAC.
The basic structure of Het-WLAN is shown in Fig. 2, where the Access Point (AP) is
FD capable (FD-AP) and some FDNs are present as well as HDNs. Moreover, HetWLAN is important because HDNs have some advantages compared with FDNs.
For example, FDNs are more expensive and they consume more power. Therefore,
some people may not like FD devices and they will prefer to choose HD devices tomeet their daily needs. Therefore, a suitable FD-MAC design is required for thistype of Het-WLAN.
The symmetric length of the uplink and downlink data traffic is considered in thedesign of our FD-MAC protocol. This type of traffic is very common in different
areas, such as cellular mobile communications, distant health monitoring, online
video games, peer-to-peer (P2P) TV, P2P file sharing, and video conferencing [7].
Moreover, IBFD wireless communications should be designed in such a way, where
the symmetric traffic length plays a vital role. Otherwise, the link (downlink or
uplink) that finishes its data transmission earlier will be idle for the remainder of
the time of that round.
In this HFD-MAC protocol, all possible types of FD transmission are considered.
To the best of our knowledge, this is a novel research that describes the FD-MAC

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for this kind of Het-WLAN, which describes all possible transmissions. The main
contributions of this research are as follows.
• An FD-MAC is proposed for a Het-WLAN that contains FDNs and HDNs.
• The proposed FD-MAC considers all possible types of FD transmissions inthis Het-WLAN.
• We compared performance of our proposed FD-MAC with traditional HD
communications and another existing FD-MAC protocol.
• A comprehensive probability analysis was performed for the proposed HFDMAC.
The remainder of this paper is organized as follows. In Section 2, we provide a brief
description of related research in this area. In Section 3, we present the problem
statement regarding FD-MAC. In Section 4, we explain the proposed MAC design.
In Section 5, we give a brief description of the method for combating inter-userinterference. In Section 6, we present the mathematical analysis. The results andperformance analysis are given in Section 7. In Section 8, we give our conclusions.

2 Related Works
Many studies in the area of IBFD wireless communication have aimed to minimize
self-interference, which are related to the physical layer. For example, a single antenna was used to support IBFD WiFi radio by [1], where both analog and digital
self-interference cancellation techniques were used. Various techniques have been
proposed to reduce self-interference, which are related to advances in IBFD wirelesscommunication in the physical layer [2, 4] [8, 9, 10, 11].
Although several MAC designs have been proposed for WLANs using IBFD, they
do not consider all possible types of IBFD transmissions. In addition, some were
proposed for distributed wireless networks or ad-hoc networks, where all the clients/ŶƚĞƌŶĞƚ

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Figure 2: Structure of a Het-WLAN.

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were treated as FDNs [12, 13]. An FD multi-channel MAC (FD-MMAC) was proposed to mitigate multi-channel hidden terminal (HT) problems, which targeted for
eliminating controlling signals [12]. During data transmission between two nodes,
the receiver transmits a beacon packet back-to-back until the data transmission iscomplete. This beacon packet does not contain any user data. Hence, in terms ofuser data communications, the FD capability is not fully utilized in FD-MMAC.
In addition, TNFD communication is not possible because PR always needs totransmit beacon packets while receiving data from PT. Another MAC protocol was
proposed by [13] for IBFD ad-hoc networks, where all the clients were consideredas FDNs. The inter-user interference is not considered during TNFD communications, and thus TNFD communications will be affected greatly by inter-user interference problems. The MAC design proposed by [14] can be used in both ad-hoc
and infrastructure-based WLANs. In this design, all the nodes are considered to be
FD capable. Moreover, in the case of source-based TNFD transmissions, collisions
will occur if more than two clients select the same sub-carrier and AP also selectsthat sub-carrier. Furthermore, this MAC does not include a procedure to allow the
network allocation vector (NAV) to update the timing in different nodes.
An infrastructure-based MAC protocol for IBFD wireless communications was
proposed that includes a shared random back-off mechanism [15], where all thenodes are treated as FDNs and the nodes can switch to the HD or FD mode basedon the traffic availability. According to this MAC, all the nodes first participate in
the normal contention period, before HD transmission occurs. Subsequently, any FDtransmission (BFD or TNFD) can be performed after the shared random back-off
period. Hence, a HD transmission should be performed earlier before initiating an
FD communication. Moreover, the method proposed by [15] restricts TNFD transmission to this case only, where AP wins in the shared random back-off period. A
power-controlled MAC (PoCMAC) was proposed recently for IBFD WiFi networks[16]. This MAC utilizes a contention-based receiver selection scheme to mitigate
inter-user interference during FD transmissions, but only FD-AP and HD clientsare considered in the network. The AP cannot initiate any transmissions in PoCMAC. Another FD-MAC design was proposed for a WiFi network, where both HD
and FD clients are available [17]. However, the authors limit FD communications
when a mobile node and AP have packets for each other.
An FD-MAC was proposed for a WLAN by [18], where AP is FD capable and all
the clients are traditionally HD capable. In this method, AP and all the clients needto calculate and update the signal-to-interference ratio map continuously, which
may increase the computational load as well as the complexity. Transmissions always need to be initiated by clients to establish IBFD communications. If AP winsin the contention period, IBFD communication cannot be established for two reasons: the clients do not support IBFD transmissions or the clients cannot makedecisions about which can start the TNFD transmission. Another AP-based MAC
protocol was proposed by [19], where all of the clients and AP are FD capable. This
protocol has a polling-based MAC design, where AP always needs to initiate thetransmission. However, the control frame formats were not described clearly in thisMAC design.

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Other studies have considered different issues on IBFD communications, such as
relay transmissions or power control-based FD [20, 21, 22], but they did not consider
the MAC design.

3 Problem Statement
The hidden and exposed terminal problems are not so significant when the network
is considered to be an AP-based WLAN and communication is performed using
a request-to-send (RTS) and clear-to-send (CTS) handshake mechanism. This isbecause all the clients in a WLAN can update their NAVs by using the control
frames transmitted by AP.
However, inter-user interference is a major issue in FD-WLAN. The inter-userinterference problem is illustrated in Fig. 3. As shown in the figure, node A sends
an RTS to the AP (FD capable) first and then the AP makes a decision to send data
to node D while also receiving data from node A. If nodes A and D are close to eachother, the packets from node A will interfere with the packets sent by the AP tonode D, thereby producing inter-user interference problems. Node A and the AP are
regarded as PT and PR, respectively. However, node D and the AP are treated as
SR and ST, respectively. Therefore, AP acts as both PR and ST. Without taking
appropriate steps to mitigate this inter-user interference, IBFD communications
cannot be performed perfectly.

4 Proposed MAC Protocol: HFD-MAC
4.1 New Control Frames
A number of control frames are used in HFD-MAC protocol, where some are thesame as those in the IEEE 802.11 standards, such as RTS, CTS, and acknowledgement (ACK). However, some new control frames are introduced in this HFD-MAC,
as shown in Fig. 4, e.g., RTS with duplexing indicator (RTSD) and CTS with duplexing indicator (CTSD). The duplexing indicator (DI) is a two-bit value, which
is appended to the normal RTS and CTS to obtain RTSD and CTSD, respectively,
thereby facilitating FD transmissions. A description of the DI value is given in Table 1. FDNs always use RTSD and CTSD, whereas HDNs use RTS and CTS. In
addition, AP uses RTSD and CTSD to communicate with FDNs. AP also uses RTS

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Figure 3: Inter user interference in FD-WLAN.

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and CTS, if it communicates with HDNs. Another two control frames called newCTS (NCTS) and notification with DI value (NDI) are transmitted only by AP.
The transmission (Tx) capabilities of the control frames are summarized in Table
2, where “circles” and “crosses” indicate capability and incapability, respectively.
4.2 Possible FD Transmission in Het-WLAN
HFD-MAC is proposed for a Het-WLAN, which comprises an FD-AP, FDNs, and
HDNs (Fig. 2). All possible cases of FD transmissions are summarized in the following two main cases.
• Case-1: TNFD communicationsi). AP initiates the transmissionii). Any HDN initiates the transmissioniii). Any FDN initiates the transmission• Case-2: BFD communications
In HFD-MAC, the TNFD communication is performed between the AP and twoother nodes, which can be an HDN or FDN. The BFD communication is performed
between the AP and an FDN when they have data for each other. However, HDtransmissions may also occur in the proposed MAC.
4.3 Description of the Proposed HFD-MAC
The working principle of this HFD-MAC is based on the distributed coordination function (DCF) in IEEE 802.11. This protocol uses CSMA/CA and a back-off
mechanism to access the channel and to avoid collisions. For simplicity, the DCF
interframe space (DIFS) time and back-off time are not shown in the figures, but
the short interframe space (SIFS) time is shown by “S” in all of the figures. In
this MAC, a node ignores collisions or the reception of erroneous packets during
the NAV period. Moreover, it is assumed that FDNs can perform self-interference
cancellation perfectly.
According to the IEEE 802.11 standard, a number of management frames areexchanged between the AP and nodes when nodes or devices join the WLAN [23].

Table 1: Description of Duplexing Indicator
Value of DI

Meaning of DI

00

Not Used01

Sender can Rx only

10

Sender can Tx only

11

Sender can Tx and Rx simultaneously

Table 2: Control frames transmission capability
Control Frame

Tx Capability
AP

FDN

HDN
×

RTSD

×

CTSD
RTS

×

CTS

×

NCTS

×

×

NDI

×

×

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For example, after exchanging the prob request and response, the AP grants accessto the device if the device can satisfy the authentication and association procedure.
Some optional bits in these management frames can be used by the nodes to indicate
their FD capabilities. For example, during the joining process, a node moves to theassociation phase after the authentication procedure and an association request
is sent to the AP. The node can indicate its FD capability using this association
request because several optional bits are available. Therefore, the AP can determine
which MAC address has an FD capability and which has an HD capability. Hence,
the AP sends an RTSD to the FDN and an RTS to the HDN. In addition, RTS and
RTSD are transmitted by HDNs and FDNs, respectively.
The description of this MAC protocol is given below for the cases mentioned
above.
4.3.1 Case-1: TNFD Communicationsi) AP initiates the transmission:
The AP initiates the transmission if it wins the contention period. If the AP has
data for a HDN, it sends an RTS to the corresponding HDN. For example, theAP sends an RTS to a HDN (A) and A sends a CTS to AP, as shown in Fig. 5.
If another node (HDN/FDN) then wants to send data to AP, it needs to satisfy
secondary data transmission condition (SDTC)-1: “After receiving RTS from AP,
ST cannot hear the CTS from A.” AP sends the RTS, so all the nodes know that
AP can receive data from another node. Hence, after hearing an RTS from AP, all
the other nodes (HDN and/or FDN) that want to send data to AP must wait fora time (SIFS+CTS). During this time, the nodes that have data to send AP andthat cannot hear A’s CTS stop their NAVs and start their self-timers. The timer
sets a random time within a maximum limit and the timer decreases its value. The
node with the timer that stops first starts to send data to AP. As shown in Fig. 5,Ed^&ƌĂŵĞϮϬǇƚĞƐ

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B’s timer stops first and it starts to send data to AP. Before sending data to AP,
the node (B) senses whether the channel is busy or not. Thus, other nodes (whichstarted their self-timers) will stop their timers sequentially and resume their NAVs.
Immediately after receiving data from ST, AP starts to transmit data to A. The
corresponding receivers transmit ACKs after the data transmission.
In addition, AP sends an RTSD to an FDN if it has data for the FDN. AP always
sends an RTSD to an FDN with a DI value of 11. Hence, all nodes are informedthat AP wants to transmit and receive simultaneously. Suppose that AP has sent
an RTSD to a FDN (E) with a DI value of 11 and E has no data to send AP(Fig. 6). In this case, E will send a CTSD with a DI value of 01, which meansthat E can only receive but cannot transmit because it has no data for AP. AP
can receive data from another node, so it transmits an NDI, where the DI value is
10. This means that AP will only transmit and thus other nodes can send data to
AP. Based on this NDI, other nodes will be informed that AP can only transmit
because it transmitted an RTSD earlier. Therefore, the nodes that want to senddata to AP need to satisfy condition SDTC-2: “After hearing an RTSD from AP,
ST cannot hear a CTSD from E but it can hear an NDI.” The nodes that satisfy
SDTC-2 and that want to send data to AP will stop their NAVs and start theirself-timers. Suppose that B and C have data to send and they satisfy SDTC-2. As
shown in Fig. 6, they start their self-timers after stopping their NAVs. If we supposethat B’s timer expires first, then it will send data after sensing the channel as idle.
Immediately after receiving data from B, AP starts transmission to E. In addition,
C will stop the timer and resume its NAV. All of the other nodes (A, D, and F)
will update their NAVs using the NDI and the ACKs are transmitted accordingly.
ii) Any HDN initiates the transmission:
An HDN initiates the transmission by sending an RTS to AP if the HDN wins
the contention period for the channel access. According to Fig. 7, after receiving an
RTS from a HDN (where the HDN is node A), AP may have data to send to anothernode, which may be another HDN or an FDN. In this case, if we suppose that AP
wants to send data to an SR (B), then AP will mention the address of B in the SR
address (SRA) of its reply in a new-CTS (NCTS), as shown in Fig. 4. The NCTS
will inform both A and B about their data exchange information. Node A updates

Figure 5: AP initiates the transmission to HDN.

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its NAV after receiving the NCTS. After receiving the NCTS from AP, B replies
by sending another CTS to AP based on SDTC-3: “SR can only hear the NCTS
but not the RTS from A.” If SDTC-3 is satisfied by B, then it sends a CTS to AP.
The data transmission then occurs as shown in Fig. 7. After the data transmission,
ACKs are sent simultaneously. However, if AP does not receive the CTS from SR,
it receives data from A after finishing the SIFS and CTS time allocated to B. Other
nodes update their NAV times according to the time defined by the control frames.
iii) Any FDN initiates the transmission:
After winning in the contention period, an FDN initiates a transmission by sending an RTSD to AP with a DI value of 11. AP does not have data to send thecorresponding FDN and thus it is a TNFD communication. However, AP has data
for another node (may be another FDN or any HDN). Hence, AP transmits an
NCTS. The subsequent procedure is the same as that described in the previous
subsection for Case-1 (ii). Therefore, sending an NCTS after receiving an RTSD
from AP indicates that AP has data for another node.
4.3.2 Case 2: BFD communications
BFD transmission occurs between AP and one FDN when they have data for eachother. In this case, either AP or an FDN wins in the contention period and sends
an RTSD. For example, AP sends an RTSD with a DI value of 11 to an FDN. TheW^ d^
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ĂƚĂƚŽW^ƚŽƉEsEs;Zd^ͿKƚŚĞƌƐ
;͕͕&ͿdŝŵĞƌZĞƐƚĂƌƚ EsEs;Zd^ͿEs;d^ͿEs;E/Ϳ

Figure 6: AP initiates the transmission to FDN.

Figure 7: A HDN initiates the transmission.

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corresponding FDN also sends a CTSD with a DI value of 11 because the FDN has
data for AP. BFD transmission then occurs.
It should be noted that an RTSD is always sent with a DI value of 11. The
corresponding data transmission may be BFD, TNFD, or HD, which depends on
the reply of the CTSD and other control frames.

5 Combating Inter-user Interference
Our proposed MAC for Het-WLAN suppresses inter-user interference using thefollowing three SDTCs.
• SDTC-1: “An ST can hear an RTS from AP but cannot hear the corresponding
CTS from PR.”• SDTC-2: “An ST can hear an RTSD from AP and cannot hear the corresponding CTSD from PR, but can hear the NDI from AP.”• SDTC-3: “An SR cannot hear an RTS/RTSD from the PT but can hear thecorresponding NCTS from AP.”
SDTC-1 is applied to TNFD communication where AP initiates the transmission
to a HDN. Similarly, SDTC-2 is applied in the case of TNFD where AP initiates
the transmission to an FDN. Thus, after initiating the transmission by the AP to a
FDN, if an ST wants to send data to the AP, the ST needs to satisfy SDTC-2; i.e., if
the ST has data to send to AP and satisfies SDTC-2, it stops its NAV and starts theself-timer, and the corresponding procedures occur for the TNFD communication,
as described in the previous section. In addition, SDTC-3 is used in the case wherean HDN/FDN initiates the TNFD communication. These FD-MAC mechanisms
prevent inter-user interference during TNFD communications.
For example, as shown in Fig. 8, node A (PT) sends an RTS to AP, which is heard
by node C, but it is not heard by B. Next, AP sends an NCTS to A and B because
AP wants to send data to B, which is now an SR. In this case, B will send a CTS
because it has satisfied SDTC-3. If B cannot satisfy the condition, B will not send
the CTS and it becomes an HD transmission. If the conditions are not maintainedby SRs and STs, inter-user interference occurs among the user terminals during
TNFD communications. Thus, TNFD communications will sometimes fail due todata collision.

6 Mathematical Analysis
In this section, we derive the probability equations for different transmissions. The
probability equations for different types of communication (BFD, TNFD, and HD)
are derived by using the packet arrival rate (PAR) at AP for the nodes and thepacket generation rate (PGR) by clients or nodes. It is assumed that a percentage(γ) of total nodes act as HTs. Satisfying SDTC is a key factor that affects theperformance of TNFD communications and it depends on the number of HTs.
Therefore, we considered HTs when deriving the equations. The descriptions ofdifferent symbols and variables in the equations are given in Table 3. All packet
arrivals are assumed to follow a Poisson process and we assume that the system has
the characteristics of the M/M/1 system model.

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,E

&ͲW

Ϯ

ϭ

&E
Ϯ



&
&E

,E
ϯ


,E

&E







^ŽůŝĚůŝŶĞ͗ƵƐĞĨƵůĚĂƚĂ͕ŽƚƚĞĚůŝŶĞ͗ƵŶǁĂŶƚĞĚĚĂƚĂ

Figure 8: SDTC-3: this is satisfied by B, however it is not satisfied by C.

The PAR at AP from the Internet to all clients is:
λAP = mλAPH + nλAPF(1)

The total PAR at AP from the Internet and clients is:
λTotal = λAP + mλH + nλF(2)

If one packet arrives at AP from the Internet, the conditional probability of that
APF
packet arriving at AP for an FDN is nλλTotal and the conditional probability of that
APH
packet arriving for an HDN is mλλTotal . Similarly, if a packet arrives at AP froma node, the conditional probability of that packet arriving from an FDN can beFand the conditional probability of that packet arriving from an
defined as λnλ
Total
mλH
HDN is λTotal . During any data transmission, the packets may still arrive at nodesor AP. However, the nodes differ to start the transmission because the channel isbusy. Therefore, we need to know the average waiting time (Tw ) for a packet in thequeue, which can be derived as follows:Tw =λTotalarrival rate=,service rate ×(service rate - arrival rate)µ(µ − λTotal )(3)

where µ is the average service rate of AP.
The probability analysis is described in the following subsections.
6.1 BFD Communications
To derive the probability of bidirectional FD communications (PBFD ), we present
Lemmas 1 and 2.
Lemma 1: After sending an RTSD to an FDN by the AP, BFD will occur only if
the corresponding FDN also has data to send to the AP.
Proof: To calculate the probability of BFD occurring when AP initiates the transmission to a FDN, we need to calculate the following two probabilities. (a) The conAPF
ditional probability that the packet arriving at AP is for an FDN, which is nλλTotal .
(b) The probability that the corresponding FDN also has a packet to send to the

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Table 3: Declaration of Variables
Variables

Explanation

m
n
γ
λAP
λAPH
λAPF
λH
λF
PBFD
PTNFD
PHD
TRTS
TCTS
TRTSD
TCTSD
TSIFS
TNDI

Number of HDNs
Number of FDNs
Percentage of total nodes as hidden terminals
Packet arrival rate at AP for all clients (packets/s)
Packet arrival rate at AP for each HDN (packets/s)
Packet arrival rate at AP for each FDN (packets/s)
Packet generating rate by each HDN for AP (packets/s)
Packet generating rate by each FDN for AP (packets/s)
Probability of bidirectional FD communicaiton
Probability of TNFD communication
Probability of HD communication
Time for RTS
Time for CTS
Time for RTSD
Time for CTSD
Time for SIFS
Time for NDI

AP within the time T1 , where T1 = TRTSD + TSIFS + Tw . The probability
that
 the

FDN has at least one packet to send to AP within time T1 is 1 − e−λF T1 . By
multiplying (a) and (b), we can calculate the probability of BFD occurring in thiscase.
Lemma 2: After sending an RTSD to the AP by an FDN, BFD will occur only if
the AP also has data to send to the corresponding FDN.
Proof: To calculate the probability of BFD occurring when an FDN initiates thetransmission, we need to calculate the following two probabilities. (a) The condiF
tional probability that the packet arriving at AP is sent by an FDN, which is λnλ.
Total(b) The probability that AP also has a packet to send to FDN within the time T1 .
The probability thatone packet to send to the corresponding FDN
 AP has at least
within time T1 is 1 − e−λAPF T1 . By mltiplying (a) and (b), we can calculate theprobability of BFD occurring in this case.
By using Lemmas 1 and 2, we can derive an equation to calculate the probabilityof BFD occurring (PBFD ), as follows.

P BFD =

nλAPF 1 − e−λF T1
λTotalnλF +
1 − e−λAPF T1
λTotal(4)

6.2 TNFD Communications
To derive the probability of the TNFD communication (PTNFD ), we present Lemmas
3–6 as follows.
Lemma 3: After initiating the transmission by a HDN to the AP, TNFD communications will occur only if AP also has data to send to any other node that cannot
hear the RTS from the HDN.

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Proof: To calculate the probability of TNFD occurring in the case when an HDN
initiates the communication, we need to calculate the following two probabilities.
(a) The conditional probability that the packet arriving at AP is sent by an HDN,
H
which is λmλ. (b) The probability that AP also has a packet to send to any
Totalother node that cannot hear the RTS from the HDN within time T2 , where T2 =TRTS + TSIFS + Tw . The probability
that AP has at least one
 packet for one of thehidden nodes within time T2 is 1 − e−{(m−1)λAPH +nλAPF }T2 γ. By multiplying (a)and (b), we can calculate the probability of TNFD occurring in this case.
Lemma 4: After initiating the transmission by a FDN to the AP, TNFD communications will occur only if: (i) the AP has no data for the corresponding FDN and(ii) the AP has data for another node that cannot hear the RTSD from the FDN.
Proof: To calculate the probability of TNFD occurring in this case, we need to
calculate the following three probabilities. (a) The conditional probability that anF
FDN has a packet to send to AP, which is λnλ. (b) The probability that AP has
Total


no data to send to the corresponding FDN within time T1 , which is e−λAPF T1 .

(c) The probability that the AP has a packet for another node that cannot hear

the RTSD from the FDN within time T1 , which is 1 − e−{mλAPH +(n−1)λAPF }T1 γ.
We can calculate the probability of TNFD communications occurring in this caseby multiplying (a), (b), and (c).
Lemma 5: After initiating the transmission by the AP to a HDN, TNFD communications will occur only if another node that cannot hear the RTS has data tosend to AP.
Proof: To calculate this probability, we need to calculate the following two probabilities. (a) The conditional probability that the AP has a packet to send a HDN,
APH
which is mλλTotal . (b) The probability that any other node that cannot hear the RTS

has packets for the AP within time T
3 , where, T3 = TRTS + TSIFS
 + TCTS + Tw . This
probability can be written as T3 is 1 − e−{(m−1)λH +nλF }γT3 . By multiplying (a)and (b), we can calculate the probability of TNFD occurring in this case.
Lemma 6: After initiating the transmission by the AP to a FDN, TNFD will occur
only if: (i) the corresponding FDN has no data to send to the AP and (ii) any othernode that cannot hear the RTSD has data for the AP.
Proof: To calculate the probability of TNFD communications occurring in thiscase, we need to calculate the following three probabilities. (a) The conditional
APF
probability that the AP has a packet to send to an FDN, which is nλλTotal . (b) The

probability that
corresponding FDN has no data to send to the AP within time the T1 , which is e−λF T1 . (c) The probability that any other node that cannot hear the
RTSD has data for the AP within time T4 , where T4 = TRTSD + 2TSIFS + TCTSD +TNDI + Tw . This probability is calculated as

1 − e−{mλH +(n−1)λF }γT4 . We cancalculate the probability of TNFD occurring in this case by multiplying (a), (b),and (c).
By using Lemmas 3–6, we can derive an equation for calculating the probabilityof TNFD occurring (PTNFD ):

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Page 14 of 22mλH 
1 − e−{(m−1)λAPH +nλAPF }T2 γ
λTotalnλF  −λAPF T1 +
1 − e−{mλAPH +(n−1)λAPF }T1 γeλTotalmλAPH +1 − e−{(m−1)λH +nλF }γT3
λTotal
nλAPF  −λF T1 
1 − e−{mλH +(n−1)λF }γT4e+λTotalPTNFD =

(5)

6.3 HD Communications
To derive the probability of HD communication (PHD ), Lemmas 7–10 are presented
as follows.
Lemma 7: After initiating the transmission by a HDN, HD communications will
occur if: (i) AP has no data to send any other node or (ii) AP has data to send toanother node that is exposed to the corresponding HDN. Because although the AP
has data for the nodes that are exposed to the corresponding HDN, the SDTC-3
cannot be satisfied, and thus a HD communication will occur.
Proof: To calculate the probability of HD communications occurring in thiscase, we need to calculate the following probabilities. (a) The conditional probability that the HDN initiates a transmission and the probability that the AP
has no nodes within time T2 , which is calculated as data to send to othermλH−{(m−1)λAPH +nλAPF }T2. (b) The probability that the HDN initiates thee
λTotaltransmission and the AP has at least one
are exposedto packet for the nodes that−{(m−1)λAPH +nλAPF }T2
H
the HDN, which is calculated as λmλ
1
−
γ
.
1
−
e
Total
By adding (a) and (b), we can calculate the probability of HD communications
occurring in this case.
Lemma 8: After initiating the transmission by an FDN, HD communications
will occur if: (i) The AP has no data to send to any other node including thecorresponding FDN, or (ii) The AP has data to send to another node that is exposedto the corresponding FDN. SDTC-3 cannot be satisfied so HD communications will
occur in this case.
Proof: To calculate the probability of HD communications occurring in this case,
we need to calculate the following probabilities. (a) The conditional probability that
the FDN initiates the transmission and the probability that the AP has no data
to send to other 
nodes, including the FDN, within time T1 , which is calculatednλF−λAP T1
as λTotal e. (b) The probability that after initiating the transmission by
a FDN, the AP has no data for the corresponding FDN and the AP has at least
one packet for the nodes thattimeare exposed to the FDN within T1 , which is−λAPF T1−{mλAPH +(n−1)λAPF }T1
F
e
1
−
e
1
−
γ
. By adding (a)calculated as λnλ
Total
and (b), we can calculate the probability of HD communications occurring when an
FDN initiates the transmission.
Lemma 9: After initiating the transmission by the AP to an HDN, HD communications will occur if other nodes that cannot hear the RTS have no data forAP.
Proof: To calculate the probability of HD communications occurring in this case,
we need to calculate the following two probabilities. (a) The conditional probability

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APH
that the AP has a packet to send to an HDN, which is mλλTotal . (b) The probability
that other nodesthat satisfy SDTC-1 haveno data for the AP within time T3 , which−{(m−1)λH +nλF }γT3
. By multiplying (a) and (b), we obtain theis calculated as eprobability of HD communications occurring in this case.
Lemma 10: After initiating the transmission by the AP to an FDN, HD communications will occur if: (i) the corresponding FDN has no data for the AP and (ii)other nodes that cannot hear the RTSD have no data for the AP.
Proof: To calculate the probability of occurring HD in this case, we need to calculate the following three probabilities. (a) The conditional probability that the AP
APF
has a packet to send to an FDN, which is nλthe correλTotal . (b) The probability that
sponding FDN has no data to send to the AP within time T1 , which is e−λF T1 . (c)The probability that other nodes that cannot
 hear the RTSD have no data for the
AP within time T4 , which can be written as e−{mλH +(n−1)λF }γT4 . By multiplying(a), (b), and (c), we can calculate the probability of HD communications occurringin this case.
By using Lemmas 7–10, the equation for calculating the probability of HD communication occurring (PHD ) can be derived as follows.

mλH  −{(m−1)λAPH +nλAPF }T2 eλTotalmλH +
1 − e−{(m−1)λAPH +nλAPF }T2 1 − γ
λTotalnλF  −λAP T1 e+λTotalnλF  −λAPF T1 +e
1 − e−{mλAPH +(n−1)λAPF }T1 1 − γ
λTotalmλAPH  −{(m−1)λH +nλF }γT3 +eλTotalnλAPF  −λF T1  −{mλH +(n−1)λF }γT4 ee+
λTotalPHD =(6)Furthermore, the probability equations should satisfy the following condition:
PBFD + PTNFD + PHD = 1(7)

7 Result and Performance Analysis
Simulations were performed in MATLAB to analyze the performance of the proposed HFD-MAC. The performance analysis for HFD-MAC is presented in thefollowing two subsections in terms of the probability analysis and throughput analysis.
7.1 Probability Analysis
The probability analysis was performed in unsaturated conditions, where every node
did not have data to send at all times. By contrast, in saturation conditions, allclients and the AP always have packets to transmit [24]. The saturation condition
represents the maximum load in a stable condition, i.e., the queue for arriving
packets is assumed to always be nonempty for each node in the network.

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The probability analysis results for BFD, TNFD, and HD transmissions in the
proposed HFD-MAC relative to the downlink PAR (from AP to nodes) are shownin Fig. 9. The PGRs by each HDN and FDN were 20 and 25 packets/s, respectively,
and the numbers of HDNs and FDNs were both 10 (fixed). The PGR was higher for
FDNs than HDNs because FDNs can handle more data than HDNs. The downlink
PAR at AP for each node was varied from 5 to 65 packets/s, as shown in Fig. 9. The
packet arrival rate cannot be increased more than 65 packets/s for this simulation
parameters because the utilization factor becomes higher than 100%, which is aunstable condition. The maximum utilization factor in this case was about 94%,
i.e., the utilization factor was about 94% when the average PAR at AP for eachnode was 65 packets/s.
The simulation results in Fig. 9 show that the probability of HD communications
decreased significantly as PAR increased at AP for each node, whereas PT N F D
increased significantly as PAR increased for each node. Moreover, PBF D also increased. Thus, the total probability of FD communications (PT N F D + PBF D ) increased significantly. Moreover, the figures present the effects of HTs on the probability of different communications. As shown in the figures, it is observed that withthe higher percentage of HTs in the network had higher probability of FD communications and lower probability of HD communications as compared between two
sub-figures in Fig. 9. However, even with a small percentage of HTs (10% of total
nodes), the HFD-MAC provided about 70% FD communications out of all possible
communications in saturation conditions (Fig. 9(a)).
The probability analysis results according to the total number of nodes are shownin Fig. 10, where the PGRs and PARs were the same as those used in the previous
analysis, i.e., 20 and 25 packets/s for HDNs and FDNs, respectively. In this simulation, 50% of total nodes were FDNs and 50% were HDNs always. The results in thisfigure exhibit a similar trend to those in the previous figure. The total probabilityof FD communications (PT N F D + PBF D ) increased greatly as the number of nodes
increased whereas the probability of HD communications decreased. The maximum
utilization factor in this case was about 97%, i.e., the utilization factor was about
97% if the total number of nodes was 40. The results in Fig. 10 also depicts the effects of HTs on the probability of different types of communications in HFD-MAC.

1

1

0.9

0.9

0.8

0.8

0.7

0.7

0.6

0.6

Probability

Probability

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0.5
0.4

0.5

P TNFD + P BFD
P TNFD

0.4

P TNFD + P BFD

0.3

P HD

0.2

P HD

0.3

P TNFD

P BFD

0.2

P BFD

0.1

0.1

0

0
0

10

20

30

40

50

60

0

10

20

30

40

50

Downlink packet arrival rate at the AP (packets/s)

Downlink packet arrival rate at the AP (packets/s)(a) Average HT ratio is 10%(b) Average HT ratio is 30%

Figure 9: Probability vs. downlink packet arrival rate at the AP.

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Page 17 of 22

From this figure, it is observed that the probability of TNFD decreased with theincrease of nodes’ number, when the nodes’ number was higher than 36 (Fig. 10(b)). Because, PHD continued to decrease and PBFD continued to increase.
Based on this probability analysis, it is observed that the probability of FD communications (PTNFD and PBFD ) increased significantly and the probability of HD
communications decreased substantially as the system moved toward the saturation
condition from an unsaturated condition. FD communications could not always be
performed under saturation conditions because the SDTC was not always satisfied
when HDNs initiated the transmission. In addition, the figures show that the sumof all the probabilities (PTNFD , PBFD and PHD ) always equals 1, thereby validating
the derivation of the equations.
7.2 Throughput Analysis
An extensive simulation was performed in MATLAB to analyze the throughput ofour proposed HFD-MAC under saturation conditions. The simulation parameters
are given in Table 4. The simulation results are shown in Fig. 12. HFD-MAC was

Table 4: Simulation Parameter
Packet payload (data)
RTS
CTS
RTSD
CTSD
ACK
NDI
Data rate
Control frame (RTS, CTS, etc.) rate
DIFS time
SIFS time
Slot time
Minimum backoff window size (CWmin )PLCP preamble duration
PLCP header duration
Maximum time of self-timer

1

1

0.9

0.9

0.8

0.8

0.7

0.7

0.6

0.6

Probability

Probability

1
2
3
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9
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0.5
0.4

2000 bytes
20 bytes
14 bytes
20.25 bytes
14.25 bytes
14 bytes
14.25 bytes
54 Mbps
6 Mbps
34 µs
16 µs
9 µs
15
16 µs
4 µs
50 µs

0.5
0.4

P TNFD + P BFD

P TNFD + P BFD

0.3

P HD

0.2

P TNFD0.3

P TNFD

P HD

0.2

P BFD

0.1

P BFD

0.1

0

0
0

5

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30

35

Total number of nodes(a) Average HT ratio is 10%

40

0

5

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15

20

25

30

35

Total number of nodes(b) Average HT ratio is 30%

Figure 10: Probability vs. total number of nodes.

40

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compared with an existing FD-MAC [17] and the traditional HD with RTS/CTS.
We compared HFD-MAC with the existing FD-MAC because it also works in HetWLANs. However, FD communications were limited only to cases where the AP
and other FDNs had packets for each other [17].
We assumed that the reception of any control frame and data is successful if
the receiving node lies within the range of the transmitter. A typical topology for
an AP-based WLAN was considered for this simulation, as shown in Fig. 11. As
mentioned earlier, satisfying the SDTCs is crucial for performing TNFD communications in HFD-MAC. Moreover, satisfying SDTC depends on the topology, wheresome hidden nodes exist. A WLAN was considered in this simulation with 30% hidden nodes on an average. For example, as shown in Fig. 11, if node B is a PT, thennode E and D cannot hear the signal transmitted from B, i.e., E and D are HTs withrespect to node B. Similarly, node F is the HT with respect to node C. Therefore,
if B initiates the transmissions, E or D can participate in TNFD communications
because they satisfy the SDTC. Similarly, if C initiates the transmissions, only F
can participate in the TNFD communications.
As shown the Fig. 12, there were two FDNs and HDNs initially, and each number
was then increased by two, whereas only one AP was considered. In this simulation,
we used 10 samples as the throughput with each different number of nodes andthen calculated the average throughput, as shown in Fig. 12. The results show thatthe mean average throughput with HFD-MAC was 55.17 Mbps, whereas the values
were 28.12 Mbps and 49.69 Mbps for the traditional HD and existing FD-MAC [17],respectively. Thus, the mean average throughput increased by 96.19% and 11.03%
using our proposed MAC compared with the traditional HD and existing FD-MAC,
respectively.
In this simulation, we considered that about 30% of total nodes were hidden whena particular node was considered. Satisfying the SDTC depends on the number ofhidden nodes in the network. The throughput with HFD-MAC was lower than thatusing the existing FD-MAC when the number of nodes was less than 10 because
the SDTC cannot always be satisfied if the number of nodes is lower. However, thisSDTC condition is not required by the FD-MAC proposed by [17]. Hence, either

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Figure 11: Typical topology of a WLAN that consists of hidden terminals.

Alim et al.

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BFD or HD communication occurred when using FD-MAC. Therefore, the averagethroughput was higher using FD-MAC than the proposed HFD-MAC when thenumber of nodes was less than 10. In addition, the average throughput decreased
with FD-MAC [17] when the number of nodes increased. When the number of nodes
increased, BFD communications did not increase as much compared with TNFD
communications. Therefore, the average throughput decreased with FD-MAC ([17])because TNFD communications were not considered here. However, the probabilityof HD communications decreased greatly whereas that of TNFD increased significantly as the number of nodes increased, and thus the average throughput increasedin our proposed HFD-MAC.
Some differences are observed between the simulation throughput and thethroughput that is calculated analytically as in Fig. 12. The reason is that thepercentage of hidden terminals are fixed in the mathematical derivations. However, there are some variations of this number in different network topology in thesimulation. Moreover, the contention period for channel access may vary in simulations and thus the exact time of the transmission also changes. On the other
hand, a fixed average transmission time is assumed in the mathematical analysis.
Therefore, some variations are observed between the simulation throughput and theanalytical throughput. The mean average throughput is achieved by the simulation
and by analytically are 55.17 Mbps and 54.29 Mbps respectively, which are very
close to each other.
The average HT ratio is considered as 30% for the simulation in Fig. 12. Thisconsideration is clarified by the following discussion. The MAC design is proposed
for an infrastructure based WLAN by using the game theory [25]. The authors
evaluated the performance in the presence of HT and considers the percentage ofHT from 10% to 60% [25]. The performance of IEEE 802.11 DCF is analyzed in
the presence of HT, where the HT ratio is varied from about 5% to 60% [26]. The
effect of HT on the performance of MAC protocol based on IEEE 802.11 standard
are analyzed in [27]. The authors considered the HT ratio from 10% to 30% in that

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Figure 12: Average throughput with respect to number of nodes.

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paper. The average HT ratio in practical radio environment can vary depending on
various parameters, such as network topology, line of sight, transmitting power or
range, etc. Therefore, we chose the average HT ratio as 30% for this simulation in
Fig. 12.
The performance of HFD-MAC in terms of average HT ratio for 25 nodes is shownin Fig. 13, which is performed in saturation conditions. As shown in the figure, thethroughput of HFD-MAC increases as the average HT ratio increases. Because, theSDTC conditions cannot be satisfied in most of the cases for the lower average HT
ratio. However, the SDTC conditions are satisfied in most of the cases for higher
average HT ratio and thus throughput increases. On the other hand, the throughput
of existing FD-MAC [17] remains the same with the increase of average HT ratio,as there is no effect of HT on the throughput for that MAC protocol.
Further results are shown in Fig. 14 with 20 nodes, which compares the achievable maximum, average, and minimum throughput under saturation conditions. It
is observed that the maximum achievable throughputs using our proposed HFDMAC and the existing FD-MAC were almost the same at about 63 Mbps, whereas
the value was 32.61 Mbps with traditional HD. Hence, the maximum achievablethroughput gain using our HFD-MAC was about 93.19% compared with HD communication. However, the gains in the average throughput were 96% and 11% comparing to the traditional HD and FD-MAC [17], respectively. On the other hand,
HFD-MAC achieved lowest throughput in case of the minimum throughput comparison. This happened in some rare cases in saturation conditions, when the TNFD
was initiated only by a HDN and the SDTC was not satisfied, and thus HD communications occurred. However in all other cases of TNFD, BFD communications
occurred in saturation conditions. Moreover, as FD communications (either BFD
or TNFD) occurred in most of times in saturation conditions, higher throughput
was achieved by using the proposed HFD-MAC.

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Figure 13: Average throughput with respect to average HT ratio.

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Page 21 of 22ϳϬϲϯ͘Ϯϭ ϲϯ͘Ϯϵ

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The MAC design for this kind of Het-WLAN is very important, as the coexistenceof HDNs and FDNs in the same WLAN will be inevitable in near future. Some FDMAC were proposed, where FDNs can switch between FD and HD mode [14, 15].
However, this is not sufficient because there are some differences in terms of MAC
protocols between the WLAN that consists of only FDNs and the WLAN that
contains both HDNs and FDNs. Although the FDN can switch between FD and
HD mode, it depends on the packet availability and also on the response of therespective FDN (type-1). However, it is fixed that the HDN always performs HDtransmission, which does not depend on packets arrival rate (type-2). The decisionin type-1 is taken by the node and the AP acts accordingly. However, the decisionin type-2 is always fixed and the AP takes decision without getting any feedback
from the HDN. Therefore, different MAC protocols are required for these two kindsof networks.

8 Conclusion
In this paper, we proposed an FD-MAC protocol for the Het-WLAN, which comprises FDNs, HDNs, and an FD-AP. In our proposed HFD-MAC, all possible FDtransmissions were considered in Het-WLANs. This protocol minimizes the interuser interference during FD transmissions by using SDTCs. The simulation results
showed that HFD-MAC increased the overall throughput significantly comparedwith that using traditional HD transmissions. In addition, the performance of HFDMAC protocol was better than that of another existing FD-MAC. The probability
analysis results suggested that the total probability of IBFD transmission increased
significantly as the WLAN approached saturation conditions. This type of FD-MAC
may be very important for supporting high speed FD-WLANs in the near future.
Competing interests
The authors declare that they have no competing interests.
Acknowledgements
This research and development work was supported by the MIC/SCOPE # 155007006. This work was also
supported by JSPS KAKENHI Grant Number JP16H01718.
Author details
1
Department of Information Networking, Graduate School of IST, Osaka University, Osaka, Japan. Mr. Alim is alsoa faculty member in ECE Discipline, Khulna University, Bangladesh.. 2 Department of Information Networking,Graduate School of IST, Osaka University, Osaka, Japan.

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