Wireless Networks
https://doi.org/10.1007/s11276-018-1827-8(0123456789().,-volV)(0123456789().,-volV)

Asym-FDMAC: In-band full-duplex medium access control protocol
for asymmetric traffic in wireless LAN
Md. Abdul Alim1•

Shunsuke Saruwatari1 • Takashi Watanabe1

 Springer Science+Business Media, LLC, part of Springer Nature 2018

Abstract
Recent advancements in antenna design and signal processing have made it feasible to transmit and receive simultaneously
by using the same carrier frequency, which is defined as in-band full-duplex (IBFD) wireless communications. Along withphysical layer advancements, efficient medium access control (MAC) design is required to derive optimal benefit from thislatest technology. In this paper, an IBFD MAC protocol named Asym-FDMAC, is proposed for infrastructure based
WLAN to support asymmetric lengths of traffic for uplink and downlink. This MAC protocol enables multiple users totransmit data to the access point (AP) during the transmission of a single downlink frame from the AP. In Asym-FDMAC,
the AP always initiates the transmission. Therefore, there is no contention period and thus, there is no collision among theuser terminals and the AP for channel access. A mathematical analysis of Asym-FDMAC is presented together with anevaluation of its performance, which involved a comparison with traditional IBFD and half-duplex (HD) communications.
On average, the highest throughput gain achieved by Asym-FDMAC was approximately 54% and 94% compared withtraditional IBFD and HD communications respectively, although the lowest average throughput gain was observed as about
50% and 89%, respectively.
Keywords Full duplex  MAC protocol  FD-MAC  Asymmetric traffic

1 Introduction
In-band full-duplex (IBFD) wireless communication has
emerged as one of the latest techniques to meet the challenges of posed by the upcoming demand in future wirelesscommunications. Wireless communications deployed in
the traditional manner cannot transmit and receive by using
the same carrier frequency at the same time. However, theuse of IBFD wireless communications makes it possible totransmit and receive simultaneously by using the same
carrier frequency. The two most important challenges& Md. Abdul Alim
abdul.alim@ist.osaka-u.ac.jp; alim.ece@gmail.com
Shunsuke Saruwatarisaru@ist.osaka-u.ac.jp
Takashi Watanabewatanabe@ist.osaka-u.ac.jp
1

Department of Information Networking, Graduate School ofInformation Science and Technology, Osaka University,
Osaka, Japan

associated with implementing IBFD are: firstly, the minimization of self-interference and secondly, the mitigationof inter-user interference. To achieve the advantages from
IBFD wireless communication successfully, we need todesign suitable medium access control (MAC) protocol. On
the other hand, asymmetric traffic for uplink and downlink
is very common in wireless communications. As asymmetric traffic is very typical in our practical wireless LAN(WLAN), it should be considered for the MAC design of
IBFD wireless communications.
This paper proposes a novel FD-MAC protocol, which isnamed Asym-FDMAC that supports asymmetric length oftraffic for uplink and downlink. The main feature of AsymFDMAC is that it enables multiple users to transmit their
uplink data to the AP while the AP is transmitting thedownlink data to a single user. This MAC is designed for a
WLAN and its basic structure of the WLAN is shown in
Fig. 1. The WLAN consists of an IBFD-capable accesspoint (FD-AP) and some IBFD clients (FDC). However,this Asym-FDMAC can be utilized for a WLAN thatconsists of both FDC and traditional half-duplex (HD)123

Wireless Networks

Wireless link
Wired link
TNFD
Bidirectional FD••

Fig. 1 Structure of a wireless LAN

clients (HDC). According to this MAC, the AP always
initiates the data transmission and selects user terminals(UTs) to send their data to the AP, and thus there is nobackoff mechanism or contention period for channel
access. The basic concept of this Asym-FDMAC wasproposed in [1]. This paper extends the idea more clearlywith additional performance analysis. This paper provides
new algorithms for the queue management and for the UT
selection. In addition, the research in the current paper
extends and generalizes the mathematical analysis for any
number of uplink frame length to downlink frame length(UFL/DFL) ratio. Apart from the previous performance
analysis in [1], Asym-FDMAC in this paper is more
extensively evaluated by comparing its performance withvarious aspects of traditional IBFD.
FD communications are categorized in this paper in twoways, i.e. bi-directional FD (BFD) and three node FD(TNFD) communications. During BFD communications,
the AP and one user terminal (UT) transmit data to eachother simultaneously; whereas in TNFD, one UT transmits
data to the AP, while the AP transmits data to another UTsimultaneously (Fig. 1). For example, as shown in Fig. 1,
UT-1 is transmitting data to the AP and the AP is transmitting data to UT-6 simultaneously, which is a TNFD
communication. In case of TNFD, the data being transmitted are different, unlike relay FD. The main contributions of Asym-FDMAC are summarized as below:
•
•

•
•

This MAC supports asymmetric as well as symmetrictraffic for uplink and downlink.
If one user terminal (UT) finishes its transmission
earlier, another UT can transmit data to the AP, bywhich multiple UTs can transmit their data in the samedata transmission cycle.
Asym-FDMAC can suppress the inter-user interference
successfully.
The AP always initiates the transmission. Hence, there
neither is any contention period nor is there collision
for channel access.

123

This MAC can be utilized for a WLAN consisting of an
FD-AP and some UTs. The flexibility of this protocol isthat all UTs can either be FD capable or traditional HD
capable devices; even a combination of these two types
is possible.
The performance analysis of the proposed MAC isperformed by mathematical analysis and MATLAB
simulation.

The remainder of this paper is arranged as follows:
Sect. 2 provides the related researches in this area, Sect. 3
provides a brief description of the problem statement
regarding FD-MAC, Sect. 4 describes the proposed MAC
design, Sect. 5 presents the mathematical analysis, andSect. 6 contains the result and discussion.

2 Related works
A substantial amount of research works have been conducted to minimize the self-interference to enable successful IBFD communications. In [13], a balanced/
unbalanced (balun) cancellation technique is used for selfinterference cancellation. Along with balun, a digital cancellation technique is used, which achieves 110-dB cancellation. Two antennas are used to conduct that research.
In contrast, single antenna is used to support IBFD WiFi
radio in [2]. A passive circuit is used here to minimize theself-interference as a first step, after which digital cancellation is used as a second step. A balanced cancellation
technique for front-end RF is proposed in [23]. In addition,other research works have been conducted in the physicallayer for the advancement of IBFD wireless communications [6, 9, 20]. Several research efforts are currently
underway to achieve the highest correctness in IBFD
wireless communications for both analog and digital cancellation techniques [14, 17]. Hence, expectations are thatthe researchers will succeed in suppressing the self-interference almost perfectly in the near future. On the other
hand, to attain the full leverage of IBFD technology inwireless communications, medium access control (MAC)
protocols play a vital role.
On the other hand, a number of MAC protocols havebeen proposed for efficient IBFD communications in recent
years. However, these protocols are not sufficiently capableof addressing all the challenges in this arena. Some MAC
designs are proposed for ad-hoc networks or distribute
networks and some other protocols are proposed for
wireless local area networks (WLANs). A full-duplex (FD)
MAC design is proposed for a WiFi networks, in which
both HD and FD clients are available [8]. However, in that
research, the IBFD communications are limited to a situation in which the AP and an FD client only have packets

Wireless Networks

for each other. This MAC design considers the same lengthof frame size for downlink and uplink. A cross-layer protocol design for both single-channel and multi-channel
networks is proposed for distributed FD networks in [15].
That paper also considers the same frame size for downlink
and uplink. In PocMAC, a power-controlled FD-MAC is
proposed, where symmetric traffic lengths are used for
uplink and downlink [7]. A MAC protocol is proposed for
wireless ad-hoc networks, which supports multi-hop
communications [21].
Asymmetric length of traffic is considered in this proposed MAC, as this type of traffic is very common in theexisting WLANs. The uplink and downlink traffic statistics
are discussed in [18]. It is mentioned in that paper that theratio of the uplink frame length (UFL) to the downlink
frame length (DFL) is 1:5, although the ratio depends on
the applications. The packet arrival ratio for uplink anddownlink is observed as about 1:1. However, the DFL ismuch higher than the UFL and the downlink transmission
is not always paired with the corresponding UT for the
uplink transmission. Therefore, more uplink packet
requests from different users are received in the uplinkqueue of the AP, while the AP transmits a single downlink
packet. In another paper, the authors observe that the UFL/DFL ratio is about 1:3 [24]. This ratio is observed to be
1:3.47 in another similar study [5]. Some technical reports
also confirm this traffic asymmetry in wireless communications. For example, NOKIA reports that traffic asymmetry is downlink dominated during normal day time hours
by 1/10 [19]. Moreover, the UFL/DFL ratio is reported tohave increased from 1/2 to 1/10 in Sweden during thecourse of 2.5 years only (from 2012 to 2014) [12]. Thisratio is observed as being approximately 1/6 in China andJapan. Based on these traffic statistics and trends, the UFL/DFL ratio is expected to be in the range of 1/7–8 for theperiod up to 2024 [12].
A few research studies have been conducted to address
the asymmetric traffic in designing MAC protocols for
IBFD wireless communications. A cross-layer model is
proposed to accommodate asymmetric traffic for IBFD
wireless networks in [16]. However, the paper does notdescribe any MAC protocol in detail. Another MAC protocol named as A-Duplex was proposed for WLANs andthe authors considered both symmetric and asymmetrictraffic for uplink and downlink [22]. However, the FD
communication is performed only, when the clients sendrequest to send (RTS) first. If the AP sends the RTS first, a
half-duplex (HD) communication takes place. In A-Duplex, during asymmetric traffic in IBFD, the transmitter
sends a busy tone, if it finishes its transmission earlier thanits counterpart. Another paper also describes an FD-MAC
that uses a busy tone to support the asymmetric traffic [10].
Other researchers also consider the traffic asymmetry in

FD-MAC, but without using the busy tone [4]. In that case,the transmitter only waits for the transmission of its
counterpart to finish, if it finishes its transmission earlier.
The MAC protocol in this paper is proposed to support
this traffic asymmetry. Although some MAC protocols areproposed to support traffic asymmetry, which is discussed
earlier; but the supports are insufficient. Those protocols
use a busy tone or wait technique to support traffic asymmetry. However, for the duration of the busy tone or
waiting period, only one-way transmission (i.e., HD communication) is allowed to take place. Depending on theapplications, the period of this busy tone or waiting period
may be shorter or longer. If we are able to use this time (thebusy tone or wait period) for any other data transmission, it
may be possible to increase the throughput by using IBFD
wireless communications. This is the main motivation ofthis research work in this paper.

3 Problem statement of IBFD
communications in WLAN
3.1 Inter-user interference
The hidden terminal and exposed terminal problem is an
important matter in WLANs. However, the severity of thisproblem is reduced, when the network is designed as an AP
based WLAN and the communication is maintained byusing the RTS and clear to send (CTS) mechanism. The
reason is that all UTs in a WLAN update their network
allocation vectors (NAV) by using the control frames thatare transmitted by the AP and UTs.
On the other hand, in case of IBFD in wireless LANs,
inter-user interference is a crucial problem. This problem is
depicted in Fig. 2. Here, suppose user terminal-1 (UT-1)
sends data to the AP, after which the AP takes the decision
to send data to UT-2 while receiving data from UT-1.
However, if UT-1 and UT-2 are close to each other, thedata frame from UT-1 is likely to interfere at UT-2 with the

Useful data
Collision (interference)
Fig. 2 Inter-user interference in IBFD wirelss LAN

123

Wireless Networks

data frame being sent by the AP; this phenomenon isregarded as the inter-user interference problem. Here, UT-1
and the AP are considered as the primary transmitter (PT)and primary receiver (PR) respectively. However, UT-2
and the AP are considered as the secondary receiver (SR)and secondary transmitter (ST), respectively. Thus, the AP
has the role of both PR as well as ST. Clearly, the performance of IBFD cannot be optimized without taking
appropriate steps to minimize the inter-user interference.

3.2 Busy-tone during FD transmission
A busy tone is not required in symmetric FD transmissions,where the uplink and downlink frame sizes are the same.
However, in most of the cases, the traffic volume isasymmetric for uplink and downlink, as mentioned in
Sect. 2. Hence, in case of asymmetric traffic, someresearchers proposed the busy tone in FD-MAC as inshown Fig. 3(a), where the UT sends a busy tone if it
finishes its transmission earlier than its counterpart [10, 22]or waits for the transmission of its counterpart to finish [4].
However, the problem is that no uplink data transmission
can take place during the time of the busy tone or while
waiting. Therefore, it becomes HD communication interms of data communication in that case. Hence, if other
UTs could transmit their uplink data for the duration of thisbusy tone or while waiting, the overall throughput would
be higher as in Fig. 3(b). This concept is utilized in thisresearch for the proposed Asym-FDMAC.

AP

R S

UT-1

S

C

Data to UT-1

S A

Data to AP Busy Tone / Idle timeA

4 Proposed FD-MAC protocol
4.1 Description of Asym-FDMAC
The AP always initiates the transmission in AsymFDMAC. Hence, there is no contention period or backoff
time for channel access. According to the IEEE 802.11
standard, a number of management frames are exchanged
between the AP and UTs at the time when the UTs or
devices join the WLAN [11]. For example, after
exchanging the probe request and response, the AP grants
access to the device, if the device can satisfy the authentication and association procedure. The AP assigns an
association ID for each UT to which it grants the channel toaccess. This association ID is a numerical number and isregarded as the slot number (SN), which is used in thisMAC design. The maximum SN is broadcasted by the AP
by using the beacon frame.
The AP transmits RTS, if it has data to send to a UT.
The AP sends RTS to the UT, based on first in first out(FIFO) queuing system, if it has packets for multiple UTs.
Suppose the AP has data to send to UT-1 as shown in
Fig. 4. Therefore, the AP sends an RTS to UT-1 and UT-1
sends a CTS to the AP. After that, packet-length declaration and interference prevention (PDIP) is transmitted byeach UT according to the assigned SN. The PDIP is a newcontrol frame of two bytes. Each UT transmits PDIP to the
AP to inform its UFL according to the time sequences(Figs. 4, 6). This control frame is used in Asym-FDMAC tocombat the inter-user interference, which is described in
Sect. 4.2. The inter-user interference mainly occurs whenthe AP has downlink data to a UT and another one or more
UTs have uplink data to the AP in the same transmission
cycle (Fig. 4). In this case, after exchanging the RTS andCTS, all UTs send their UFL to the AP by using PDIP,
which is also used to minimize the inter-user interference.

No uplink data
UT-2
AP

(a)
AP

R S

S

Data to UT-1

S A

UT-1

R S

S

S NFC S

Data
(to AP)C

Data(to UT-1)

S MA

S

S A

UT-2

UT-1

C

Data to AP

A
UT-3

UT-2

(b)
R: RTS, S: SIFS time, C: CTS, A: ACK
Fig. 3 a Busy-tone in traditional IBFD communication and b efficientuse of busy tone time

123

S

Data
(to AP)

Data to AP
UT-4
PDIP

R: RTS, S: SIFS time, C: CTS, PDIP: Packet-length declaration and interference
prevention, NFC: Notification for Clients, A: ACK, MA: Multiple ACK.

Fig. 4 Time sequence for Asym-FDMAC (AP has data to send)Wireless Networks

Therefore, this control frame is named as packet-length
declaration and interference prevention, i.e. PDIP. However, this interference does not occur when the AP has
downlink data only or the UTs have uplink data to the AP
only (Fig. 6). In this case, the PDIP is sent to inform theuplink packet length only, whereupon the AP selects the
UTs and inform the UTS by sending NFC (notification forclients). All the control frames that are used in AsymFDMAC are shown in Fig. 5. The duration of each timeslot (TS) is 2.6 ls in which the PDIP are sent. As the CRC
and frame control are not included in PDIP, the PDIP isvery short and some PDIP may have error during thetransmission from the UT to the AP in real radio environment. However, after exchanging the RTS and CTS,
for a successful IBFD communication, at least one successful PDIP transmission is required. Therefore, alltransmissions of PDIP do not need to be error freetransmissions; only a few successful transmissions of
PDIP are enough for the IBFD transmission. Therefore,
the PDIP is considered as a control frame of short time
duration to reduce the overhead. Each UT sends the PDIP
during its allocated SN after satisfying the secondary datatransmission condition (SDTC). The SDTC is defined asfollows:
the UT can hear the RTS, but cannot hear the corresponding CTS.
In this MAC protocol, all UTs can hear the RTS.
However, this condition is important for another reason. In
the real wireless environment, there are different kind ofnoises. Therefore, sometimes the RTS cannot be received
correctly by all the UTs. Hence, this SDTC is required forreal environment implementation. This SDTC is notapplicable to the UT, to which the AP transmits the RTS.
The UT sends its uplink frame duration by using PDIP to
the AP, if it satisfies the SDTC. However, if it has data but
cannot satisfy the SDTC, the UT sends ‘00000001’ in
PDIP. This is transmitted to the AP to inform that the UT
has data, but it cannot send data to prevent the inter-user

Fig. 5 Format of control frames (in octet)

interference. On the other hand, if the UT has no data tosend, it sends only zeros in its PDIP. After receiving thePDIPs from all of the UTs, the AP selects the UTs that willtransmit uplink data in this transmission cycle. After theslection of UTs, the AP broadcasts NFC to inform theselected UTs as shown in Fig. 4.
The AP maintains two queues for this proposed MAC.
One queue (Q1 ) is for the UTs that have uplink packets to
the AP and another queue (Q2 ) is for the UTs, to which the
AP has downlink packets to send. The AP maintains thequeues to store the MAC address and corresponding frame
length of the UT. The AP maintains the Q2 according to the
FIFO queueing system, based on downlink frame requests,
but the Q1 is maintained by the AP according to the
Algorithm-1. Q1 is the queue that store the UFL of the UTthat has uplink data or packet to the AP. For this purpose,Q1 stores the UFL and corresponding address of the UT.
The Q1 is maintained according to the FIFO queueing
system.
In each transmission cycle, the AP rearranges the orderof the queue after getting all PDIPs from all UTs. The UT
to which the AP sends the RTS has the first priority in the
Q1 , as it becomes bi-directional IBFD communication
without inter-user interference. All other uplink requests
are served as FIFO system. This is performed by the first‘‘FOR loop’’ in Algorithm 1. After that, if any UT sends
UFL request in both current cycle and earlier cycle, the AP
updates the UFL duration of the corresponding UT (if the
UFL duration is changed in current cycle), while keeping
the same position in the queue. However, if any UT sends
the UFL in the current cycle only, its information isappended in the queue, which is the last priority. This
procedure is performed by using the last ‘‘FOR loop’’ in
Algorithm 1.
The AP selects the UTs for NFC according to the
Algorithm 2. The AP use this algorithm for each datatransmission cycle, after receiving PDIPs from all the UTs.
The AP collects the UFL of the first UT of Q1 and compares the UFL with the DFL. If the UFL of the UT is lessthan the DFL, the AP selects the UT for NFC. Therefore,
the AP assigns the duration of the UFL and MAC addressof the corresponding UT in NFC. After that, the AP selects
another UT from the queue to perform the same procedure
until Total UFL [ DFL. If Total UFL [ DFL, the AP
permits lower UFL for the last UT to transmit, so that thetransmission time of the total uplink frame becomes thesame as the downlink frame in each cycle. However, if the
UFL is larger than the DFL for the first UT during theselection procedure, the AP selects only one UT for thatcycle, but permits the UT to transmit the data of the same
duration of DFL. After selecting the UTs for the NFC, Q1
is updated for the next cycle. This procedure of UT
selection is depicted in Algorithm 2.

123Wireless Networks

Algorithm 1 : Q1 management by the AP
Inputs:DF L : Downlink frame length for the current
cycle from (Q2 ).
U F L : Uplink frame length (Q1 ).
Request P DIP : The array of all PDIP information sent by all UTs.
Output:Q1 : Queue of the UTs that have uplink packets
to send the AP.

AP

WBT DR S

S

NFC

S

S MA

UT-1

UT-2

S

Data
(to AP)

UT-3

Data(to AP)

S

UT-4
PDIP

1: Start Q1 Management
2: n = length of Request P DIP
3: for i = 1 to n do
4:
if MAC address of Request P DIP [i] == MAC

address of the UT that sends CTS && U F L of
Request P DIP [i] > 1 then5:
Q1 [1] ← insert the MAC address and UFLof Request P DIP [i]6:
Q1 ← rearrange the order of the queue7:
end if
8: end for
9: for i = 1 to n do
10:
if U F L of Request P DIP [i] > 1 && MAC address of Request P DIP [i] ⇒ Match with any MAC
address of Q1 [i] then11:
Q1 [i] ← Update the corresponding UFL
12:
else
13:
At the end of Q1 ← add the MAC address
and UFL of Request P DIP [i]14:
end if
15: end for
16: End Q1 Management

After broadcasting the NFC by the AP, the data transmission takes place as shown in Fig. 4. The UTs are
selected to transmit their uplink data while the AP transmits the downlink data. The acknowledgements (ACKs)
are transmitted after finishing the data transmission. The
AP transmits multiple ACK (M-ACK) to the UTs thattransmit data to the AP. As shown in Fig. 5, M-ACK
contains all the MAC addresses of the UTs that transmit the123

WBT: Wait Before Transmit, DR: Dummy RTS, S: SIFS time, C: CTS, PDIP:
Packet-length declaration and interference prevention, NFC: Notification for Clients,
MA: Multiple ACK.

Fig. 6 Time sequence for Asym-FDMAC (AP has no data to send)

uplink data to the AP. However, only one ACK is transmitted to the AP by the UT that receives the downlink data.
However, if none of the UTs have any data to send theAP, the UTs send PDIPs that only contain zeros. In thatcase, the AP simply mentions the receiver’s address in the
NFC to which the AP sends data, whereupon it becomes
HD communication, as there is no uplink data.
On the other hand, if the AP has no downlink data to any
UT, it waits for a predefined time known as wait before
transmit (WBT). As shown in Fig. 6, if the AP has no data
to send, it sends a dummy RTS (DR) after spending thetime of WBT. The PDIPs are transmitted by the UTs after
the SIFS time. The remaining part of the procedure issimilar to the previous description. However, in this case it
is HD communication, as the AP has no downlink data tosend (Fig. 6).
Now, we provide an example of this Asym-FDMAC fora clear understanding. First, suppose the AP has data tosend to UT-1 as shown in Fig. 4. Then, after exchanging
RTS/CTS by the AP and UT-1, all UTs transmit their
PDIPs as shown in the figure. Suppose, all UTs have data tosend and all of them send their corresponding UFL to the
AP by sending PDIPs. The AP manages the Q1 after
receiving all PDIPs according to the Algorithm 1, whereupon the AP selects two UTs as uplink transmittersaccording to the Algorithm 2.

Wireless Networks

Algorithm 2 : UT selection for NFC
Inputs:DF L : Downlink frame length for the current
cycle.
Q1 : Queue of the UTs that have uplink packets
to send the AP.
Output:U T N F C : Selected UTs for NFC.
1: Start UT selection for the current cycle
2: T otal U F L = 0
3: U T N F C = Φ
4: n ← Length of Q1
5: for i = 1 to n do
6:x ← assign the UFL of the UT from Q1 [i]7:
if T otal U F L + x ≤ DF L then8:
U T N F C ← insert MAC address and UFLof the UT from Q1 [i]9:T otal U F L = T otal U F L + x
10:
else
11:
y ← (DF L − T otal U F L)
12:
if y > 0 then13:
U T N F C ← insert MAC address of the
UT from Q1 [i]14:
U T N F C ← insert “y” as the durationof U F L of the corresponding UT
15:
end if16:
break for17:
end if
18: end for
19: Q1 ← Update the queue for the next cycle
20: End UT Selection for the current cycle

Therefore, the AP writes the MAC addresses of UT-1
and UT-3 as RA-1 and RA-2 respectively in the NFC asshown in Fig. 5. The AP also include the corresponding
data length in this NFC. After broadcasting the NFC, thedata transmission starts as in Fig. 4. M-ACK and ACK aretransmitted after the data transmission finishes. As two UTs
send data to the AP, the M-ACK contains the addresses of
those two UTs. However, UT-1 only sends one ACK, as it
has only received data from the AP.
The control frames like RTS, CTS and ACK are compatible to the IEEE 802.11 standard. However, some newcontrol frames (such as DR, NFC, M-ACK and PDIP) areintroduced in this proposed Asym-FDMAC, which aredescribed in Fig. 5. Therefore, necessary software upgrade
is required for these new control frames as well as for thisMAC protocol.

4.2 Minimization of inter-user interference
The inter-user interference is one of the major problems for
IBFD wireless communications. Asym-FDMAC proposesa technique to combat this interference during IBFD. When
the AP sends an RTS, it is received by every UT in the
WLAN. However, when the corresponding UT sends a
CTS, it is not received by every UT, since some UTs are
hidden to the UT that sends the CTS. This combating
technique is depicted in Fig. 7. As shown in that figure,suppose the AP sends RTS to UT-1 and this RTS is
received by all UTs in the network. Then, UT-1 sends CTS
to the AP and this CTS is received by the AP and the UTs
that are within the transmission range of UT-1. Therefore,
only UT-2 and the AP can receive the CTS in this case as
shown in Fig. 7. However, UT-3 and UT-4 cannot receive
the CTS, as they are the hidden terminals to UT-1. However, they can receive the RTS from the AP. Therefore,
these two UTs can satisfy the SDTC that is described in
Sect. 4.1 and thus they can send their PDIPs (with actualUFL) during their time slot. As a result of this, UT-3 andUT-4 will be allowed to transmit their uplink data, while
the AP transmits the downlink data to UT-1. As these two
UTs are the hidden terminals to UT-1, no inter-userinterference will occur for this IBFD communication andthus Asym-FDMAC combats inter-user interference.

5 Mathematical analysis
5.1 Probability calculation
It is assumed that the packet arrival rate (PAR) follows the
Poisson arrival process and the service time is almost fixed
or deterministic. Therefore, the system follows the

HT

RTS
RTS
CTS
CTS
RTS

RTS
HT

Fig. 7 Combating inter-user interference123Wireless Networks

characteristics of an M/D/1 queueing system. The downlink traffic length is higher than that of the uplink traffic
and it is assumed in this MAC protocol that the average
downlink traffic length is fixed. As total uplink traffic
length is matched with the downlink traffic length in thisAsym-FDMAC, the service time is almost the same.
Therefore, the system is assumed to be approximately
M/D/1 system. The total packet arrival rate at the AP fromthe Internet in the form of downlink packets is defined as:
kDT ¼ ðkD1 þ kD2 þ       kDn ÞnX
kDi) kD T ¼

ð1Þ

i¼1

where n is the total number of UTs, kD1 is the downlink
packet arrival rate at the AP for UT-1, kD2 is the downlink
packet arrival rate at the AP for UT-2, and so on. If thepacket arrival rate at the AP for each UT is the same, (1)can be written as
kDT ¼ nkD

ð2Þ

where kD is the downlink packet arrival rate at the AP foreach UT. Similarly, the total packet generating rate (PGR)
by the UTs as uplink packets to the AP is:kUT ¼ ðkU1 þ kU2 þ       kUn ÞnX
kUi) kU T ¼

ð3Þ

i¼1

where kU1 is the uplink packet generated by UT-1 to theAP, kU2 is the uplink packet generated by UT-2, and so on.
If the PGR by each UT is the same, (3) can be written as,
kUT ¼ nkU

ð4Þ

where kU is the PGR by each UT. Therefore, the total
packet arrival rate at the AP for both the uplink anddownlink becomes:
kTotal ¼ kDT þ kUT

ð5Þ

Suppose, one packet arrives at the AP as a downlink packet
for a UT. Therefore, the RTS and other control frames aretransmitted before data transmission takes place. If there is
no uplink packet request before the data transmission, it
becomes half-duplex (HD) communication. The conditional probability of the HD communication (PHD ) is:PHD ¼ ekUT Twhd

ð6Þ

where Twhd is the average waiting time of a uplink packet
before transmission, when it becomes HD communication
after sending the RTS by the AP. The average waiting timeis derived in the following Sect. 5.2.
As described before in Sect. 4.1, the nonzero UFL istransmitted by the UTs that cannot hear the CTS. However,123

a nonzero UFL can be transmitted by the UT that transmits
a CTS, if it has data to send. Suppose, the AP sends an RTS
to UT-1 and it sends a CTS to the AP. Hence, the UTs thatcannot hear the CTS and have data to send the AP transmit
nonzero UFL and this number of UTs depends on thenumber of hidden terminals (HT) in the network, as
described in Sect. 4.2. If b is the average percentage of HT
in the network, the number of UTs that cannot hear thecorresponding CTS is m ¼ bb  nc. Therefore, the conditional probability of the FD communication (PFD1 ), whereexactly one UT sends a nonzero UFL to the AP is:
PFD1 ¼ eðmþ1ÞkU Twfd ðm þ 1ÞkU Twfd
ð7Þ
where 1  m\n and Twfd is the average waiting time of a
uplink packet before transmission, when it becomes FDcommunication after sending an RTS by the AP. The termm þ 1 includes the UT that sends the CTS and the UTs thatcannot hear the CTS.
Similarly, the probability of exactly two UTs sending
nonzero UFL to the AP is:2
kTwfd kTwfdPFD2 ¼ ðeÞð8Þ
2!
where k ¼ ðm þ 1ÞkU .
Therefore, we can generalize the probability equation
for the FD communication, where exactly k UTs sendnonzero UPL to the AP, after sending an RTS. This
probability equation can be generalized as:k
kTwfd kTwfdPFDk ¼ ðeÞð9Þk!
where 1  k  m þ 1.
Now, suppose the UFL/DFL ratio is 1:x. Therefore, atleast dxe UTs are required to send nonzero UFL to the AP
to enable the AP to select UTs in such a way that the total
UFL becomes almost the same as the DFL. Hence, tomaximize the throughput by using Asym-FDMAC for a
UFL/DFL ratio of 1:x, at least k ¼ dxe number of nonzero
UFL requests need to be transmitted to the AP. However, if
the number of UTs that send the PDIP with nonzero UFL ismore than x, the AP selects only dxe number of UTs totransmit the uplink data. Therefore, the probability of FD
communications, in which at least dxe number of UTs sendnonzero UFL is written as:

i
mþ1
X
kTwfd
kTwfdPFDk  dxe ¼
ð10Þ
ðe
Þ
i!
i¼dxe
where 1  dxe  m þ 1.

Wireless Networks

5.2 Average waiting time calculation
It is possible to calculate the utilization factor by using thecorresponding arrival rate and service time. The utilization
factor (q) for this MAC protocol can be defined as:
q¼

Service time
Average packet interarrival time

ð11Þ

Therefore, the uplink utilization factor is defined as:
qu ¼ 1

1
Ts
=l
kU
¼1
¼ T
=kUT
=kUT
l

ð12Þ

where Ts is the service time and l is defined as the average
service rate that is the inverse of Ts . Here, both the service
time for downlink and uplink are the same for this MAC
design, which is referred to as the time required to complete one cycle. The value of Ts depends on both HD and
FD transmission. After sending RTS by the AP, if HD
communication takes place, Ts for HD is given by (Tshd ):Tshd ¼ TRTS þ TCTS þ nTPDIPþ TNFCHD þ TDPL þ TACK þ 5TSIFS

ð13Þ

The symbols in the equations are explained in Table 1.
However, if the FD communication takes place, Ts for FD
is given by (Tsfd ):Tsfd ¼ TRTS þ TCTS þ nTPDIPþ TNFCFD þ TDPL þ TMACK þ 5TSIFS

ð14Þ

The main differences between (13) and (14) are the timerequired for NFC and ACK. The M-ACK is transmitted by

the AP, when more than one UTs transmit uplink data
during FD communications. If there is no nonzero UFL
request, the AP only includes the address of the UT designated to receive the downlink data in the NFC. Therefore,
for HD communications the NFC contains only the addressof one UT, but for FD communications the NFC contains
additional addresses and the time duration of the UFL.
Replacing the value of Ts in (12) by Tshd and Tsfd from(13) and (14) respectively, the uplink utilization factor for
HD (quhd ) and FD (quFd ) are achieved respectively. Therefore, we obtain,
1
Tshd
=l
kU
¼ 1 hd ¼ T=kUT=kUT lhd

ð15Þ

1
Tsfd
=l
kU
¼ 1 fd ¼ T
=kUT
=kUT
lfd

ð16Þ

quhd ¼ 1
qufd ¼ 1

According to the well-known Pollaczek–Khinchine(P–K) formula for an M/D/1 system, the average waitingtime of an uplink packet before transmission is derived as:
quhd
Twhd ¼
ð17Þ
2lhd ð1  quhd ÞTwfd ¼

qufd
2lfd ð1  qufd Þ

ð18Þ

5.3 Throughput calculation
It is assumed that the average values of DFL and UFL are
ld and lu respectively. If HD transmission occurs after
sending the RTS, the throughput is defined as:

Table 1 Declaration of variables
Variables

Explanation

Variables

Explanationk

Number of UTs that send nonzero UPL to the APb

Average percentage of hidden terminal in the networknTotal number of UTs

mAverage number of HTs in the WLAN

kD

Average downlink packet arrival rate at the AP for each
UT (packets/s)kU

Average PGR by each UT (packets/s)kðm þ 1ÞkU

PHD

Conditional probability of HD communications after
sending RTS

PFDkConditional probability of FD communications after
sending RTS, where k UTs send nonzero UPL

TACKTime for ACK

TCTSTime for CTS

TDPLTime for downlink packet length

TMACKTime for M-ACK

TNFCHDTime for NFC in HD communication

TNFCFDTime for NFC in IBFD communication

Tshd

Service time for HD communication

Tsfd

Service time for IBFD communication

lhd

Average service rate for HD communication

lfd

Average service rate for IBFD communication

TPDIP

Time for PDIP

TRTS

Time for RTS

TSIFSTime for SIFS

Twfd

Average waiting time of a uplink packet before
transmission, when it becomes FD communication after
sending RTS by the AP

Twhd

Average waiting time of a uplink packet before
transmission, when it becomes HD communication after
sending RTS by the AP123

Wireless Networks

Thhd ¼

ld
Tdatahd

ð19Þ

The probability of obtaining this throughput is PHD , which
is given in (6). Thus, multiplying PHD with (19), we obtain
the expected value of Thhd . We categorize the FD communication according to the number of transmitters thatwant to send data to the AP. If there is only one UT thatwants to send data to the AP, the throughput is defined as:
Thfd1 ¼ld þ lu
Tdatafd

ð20Þ

The probability of this Thfd1 occurring is defined in (7).
Similarly, we can derive Thfd2 , where two UTs aim to senddata to the AP and for which the corresponding probability
can be found in (8). If the UFL/DFL ratio is 1:x, at least dxe
number of UTs are required to send data to the AP toenable it to select the UTs in such a way that the total UFL
and DFL become almost the same. Therefore, thethroughput in this case is achieved as follows:
Thfdk  dxe ﬃ

ld þ dxelu
Tdatafd

ð21Þ

Here, Thfdk  dxe is the throughput, when the number of UTs
that want to send uplink data is greater than or equal to dxe.
However, if this number is less than dxe, we obtain thethroughput by using the following equation:
Thfdk\dxe ﬃ

ld þ klu
Tdata

ð22Þ

From the above analysis, it is obvious that thethroughput (Th) has different values for different probabilities. Thus, the expected value of Th is given by:
X
E½Th ¼xpðxÞð23Þx:pðxÞ [ 0Here, p(x) is the probability mass function (PMF) of Th. By
using the values of PMF from (6) to (10), we obtain themean value of Th.
E½Th ¼ ðThhd  PHD Þ þ ðThfd1  PFD1 Þþ ðThfd2  PFD2 Þ       þ ðThfdk  PFDk Þð24Þ

Asym-FDMAC needs to satisfy two conditions for IBFD
communications. Firstly, the AP should have packets tosend any UT and secondly, at least one UT should havepackets to send to the AP at the same time. However, if
more than one UT has packets to send, the throughput
becomes higher.
It is possible to derive the equation to determine thethroughput under the saturation condition. The saturation
condition is defined as the situation in which all the UTs
and the AP always have packets for transmission, i.e., the123transmission queues of each of the UTs and the AP arealways nonempty [3, 22]. Under this condition, the AP and
all UTs have data to send. Therefore, the communication isalways IBFD communication in saturation condition. For
this reason, the value of PHD becomes zero in (24). However, the values of the other probabilities depends on the
total number of UTs. Thus, under the saturation condition,
the probabilities have the following values:
–
–
–
–
–

PHD ¼ 0; for n  1, andPFD1 ¼ 1; for n ¼ 1.
PFD2 ¼ 1; for n ¼ 2.
PFDk\dxe ¼ 1; for n ¼ k.
PFDk  dxe ¼ 1; for n  dxe.

By using these values in (24), we can calculate the mean
throughput in the saturation condition.

6 Results and discussion
The performance of Asym-FDMAC is described in thefollowing two subsections.

6.1 Performance result based on mathematicalanalysis
This analyses is performed based on the mathematicalequations which is described in Sect. 5. Here, both theaverage uplink and downlink packet arrival rates at the AP
are set to 42 packets/s for each UT. The parameters formathematical analyses as well as for the simulation(Sect. 6.2) are provided in Table 2. The result of thisevaluation is shown in Fig. 8, which shows the performance for an average HT ratio of 45%, 30% and 15% in
Fig. 8(a)–(c), respectively. The HT is considered in termsof one UT in the netwrok, i.e. if a UT is considered totransmit data, there are some UTs that cannot receive thedata as they are hidden to that corresponding UT. For each
UT, a number of hidden UTs are found in a wireless network. Therefore, an average number of HT can be obtained
and thus we can calculate a percentage of the average HT,
which is regarded as the average HT ratio in this paper. In
the simulation, the average HT ratio is assumed to be therespective number, such as 45%, 30% and 15% in Figs. 8
and 9. On the other hand, the AP can receive data from all
UTs in the network and vice versa. The UFL/DFL ratio isassumed to be 1/3 for this analysis, where the downlink
frame length is 2000 bytes.
In this simulation, four different cases are considered, if
the AP has a downlink packet:••

There is no uplink packet.
Exactly one UT sends uplink data.

Wireless Networks
Table 2 Simulation parameter

Parameter

Value

DFL (payload for downlink)

2000 bytes

TS duration for PDIP

2.6 lsData rate

54 Mbps

Control frame (RTS, CTS, etc.) rate

12 Mbps

WBT time

25 ls

DIFS time

28 ls

SIFS time

10 ls

Minimum backoff window size (CWmin ) for traditional IBFD

32 ls

PLCP preamble duration

16 ls

PLCP header duration

4 ls

Mean throughput
HD (Only DL)
FD (DL + Exactly one UT sends UL data)
FD (DL + Exactly 2 UTs send UL data)
FD (DL + At least 3 UTs send UL data)

60
50
40
30
20
10
0

70

Mean throughput (Mbps)

70

Mean throughput (Mbps)

Mean throughput (Mbps)

70

Mean throughput
HD (Only DL)
FD (DL + Exactly one UT sends UL data)
FD (DL + Exactly 2 UTs send UL data)
FD (DL + At least 3 UTs send UL data)

60
50
40
30
20
10
0

0

5

10

15

20

25

30

35

40

45

Mean throughput
HD (Only DL)
FD (DL + Exactly one UT sends UL data)
FD (DL + Exactly 2 UTs send UL data)
FD (DL + At least 3 UTs send UL data)

60
50
40
30
20
10
0

0

5

10

15

20

25

30

35

40

45

0

5

10

15

20

25

30

Number of UTs

Number of UTs

Number of UTs

(a)

(b)

(c)

35

40

45

Fig. 8 Mean throughput under different situations, after sending RTS by the AP. a Average HT ratio is 45%. b Average HT ratio is 30%.
c Average HT ratio is 15%••

Exactly two UTs send uplink data.
At least three UTs send uplink data.

As shown in Fig. 8(a), the mean throughput decreases
gradually as the number of UT increases from 5 to 35,
where the average HT ratio is 45%. The main reason lies in
the fact that the decreasing rate of throughput for HD ishigher than the total increasing rate of other cases of FD
transmissions. However, the mean throughput increases
rapidly from 33 to 54 Mbps as the number of UTs
increases from 35 to 45. As the average PAR is fixed, theprobability of FD communication (where at least 3 UTs
send uplink data) is very low up to 30 UTs. As the UFL/DFL ratio is 1/3, at least three UTs are required to sendnonzero UFL to achieve the highest throughput. However
this probability increases very rapidly as the number of
UTs increases from 30 to 45 and thus the mean throughput
increases sharply. The figure shows that the throughput for
HD communication follows a decreasing trend towards
zero as the number of UTs increases. Because, the probability of HD communication decreases as the number of
UTs increases while the PGR remains fixed. Contrary tothis, the throughput for FD communications with exactly
one or two uplink frame increases gradually up to a certain

level and then decreases sharply towards zero as the network size increases. Because, since the network size
increases, the probability that at least three or more UTs
will send uplink data to the AP.
Almost similar trends are found in Fig. 8(b, c). As
shown in Fig. 8, the lowest value of the mean throughput
decreases as the average HT ratio decreases. For example,
the lowest mean throughput is 33 Mbps, 28 Mbps and22 Mbps for an average HT ratio of 45%, 30% and 15%
respectively. Similarly, the highest peak of the mean
throughput decreases as the HT ratio is reduced. The reason
is that if the HT ratio decreases, the number of UTs that
send uplink data decreases and the probability of FD
communications decreases, as the PAR and PGR are fixed,
and thus the mean throughput also decreases. As theaverage PGR is fixed, the maximum uplink utilization ratio
is about 98% for all the cases, i.e. the utilization factor is98%, when the total number of UTs is 45. Since thethroughput of HD only in the figure is decreases to zero as
the number of UTs increases to 45, this performance
analysis suggests that the proposed MAC can achieve
higher throughput in high traffic load; because the utilization factor is 98% in this case.

123Wireless Networks

6.2 Performance result in saturation condition

sends CTS (a total of three UTs) can send uplink data to the
AP. For this reason, in case of 15% HT ratio, it is notpossible to get the sufficient number of UTs that can satisfy
the SDTC and thus the throughput is lower in case of fiveUTs (Fig. 9c). The lowest average throughput gain isobserved as about 50% and 89% for the average HT ratioof 15% compared with traditional IBFD and HD respectively (Fig. 9c).
The performance analysis also shows that there is little
difference between the simulation and analytic result of
Asym-FDMAC as shown in the Fig. 9. The main reason
lies in the fact that the SIFS time between the transmissionsof different UTs is not considered in (21) and (22).
Therefore, the analytic throughput is slightly higher thansimulation throughput for the cases in which the minimum
number of hidden UTs (this number is two in this simulation) are available to achieve the maximal throughput. On
the other hand, the average number of UTs (m) in (7)–(10)is calculated by rounding down the product of the average
HT ratio and total number of UTs. Therefore, the analytic
result requires more UTs than the simulation environment
to achieve the maximal throughput and thus the analytic
value falls behind the simulation throughput for lower
average HT ratios and for lower number of total UTs asshown in Fig. 9(b) (for 5–10 UTs) and Fig. 9(c) (for 5–15
UTs). However, this is not observed in Fig. 9(a), as theaverage HT ratio is 45%. Hence, the minimum number ofhidden UTs is obtained for five UTs as well as for a higher
number of UTs.
The result of another evaluation of the throughput performance is depicted in Fig. 10 in terms of the UFL/DFL
ratio for 20 UTs under saturation condition, where thedownlink frame size is fixed at 2000 bytes. The figure shows that Asym-FDMAC outperforms traditional
IBFD by about 52% throughput gain for the UFL/DFL ratioof 1/3, which is the maximal gain in this analysis. The
throughput gain decreases as the UFL/DFL ratio decreases.
This happens for two reasons. Firstly, the number of UTs
available for transmitting uplink data increases to match

80

80

70

70

70

60

60

60

50
40
30
Asym-FDMAC (analytic)
Asym-FDMAC (simulation)
Traditional IBFD
Traditional HD with RTS/CTS

20
10

50
40
30
Asym-FDMAC (analytic)
Asym-FDMAC (simulation)
Traditional IBFD
Traditional HD with RTS/CTS

20
10

0

Throughput (Mbps)

80

Throughput (Mbps)

Throughput (Mbps)

The performance of Asym-FDMAC under the saturation
condition is evaluated by conducting an extensive
MATLAB simulation. Asym-FDMAC is compared withtraditional IBFD and HD communications performed with
RTS/CTS. Traditional IBFD is explained in Fig. 3(a).
The simulation results are shown for different HT ratioswith a fixed UFL/DFL ratio of 1/3 in Fig. 9. As shown in
Fig. 9(a), the maximum simulation throughput for AsymFDMAC is about 73 Mbps for 5 UTs under saturation
conditions, where the average HT ratio is 45%. The
throughput decreases gradually to 58 Mbps, as the numberof UTs increases to 45. Because, the number of TS
increases for transmitting PDIP, as the number of UTs
increases and thus the throughput decreases. On the other
hand, as traditional IBFD and HD use the distributed
coordination function (DCF) to access the channel, thenumber of collisions increases as the number of UTs
increases and thus the throughput decreases. AsymFDMAC outperforms traditional IBFD and HD by 59%
and 100% throughput gain respectively for 5 UTs in termsof the maximal throughput (Fig. 9a). However, in case ofminimal throughput for 45 UTs, the gains are achieved as
55% and 98% in comparison with traditional IBFD and HD
respectively, and the average throughput gain achieved by
Asym-FDMAC is 54% and 94% respectively.
It is noticeable from Fig. 9 that the maximal throughput
decreases for Asym-FDMAC as the average HT ratio
decreases. For example, the maximal simulation throughputs are 73 Mbps for 5 UTs (Fig. 9a), 71 Mbps for 5 UTs(Fig. 9b) and 69 Mbps for 10 UTs (Fig. 9c) for different
average HT ratios, i.e., 45%, 30% and 15% respectively.
This can be understood by considering that, if the average
number of HT decreases, the number of UTs available tosatisfy the SDTC is insufficient. In this simulation, as the
UFL/DFL ratio is 1/3, at least two hidden UTs are required
after the CTS transmission to achieve the maximal
throughput. Therefore, two hidden UTs and the UT that

5

10

15

20

25

30

35

40

45

40
30
Asym-FDMAC (analytic)
Asym-FDMAC (simulation)
Traditional IBFD
Traditional HD with RTS/CTS

20
10
0

0
0

50

0

5

10

15

20

25

30

35

40

45

0

5

10

15

20

25

30

Number of clients

Number of clients

Number of clients

(a)

(b)

(c)

35

40

45

Fig. 9 Throughput comparison under saturation conditions (the UFL/DFL ratio is 1/3). a Average HT ratio is 45%. b Average HT ratio is 30%.
c Average HT ratio is 15%123Wireless Networks

first reason and the total transmission time increases for thesecond reason, which leads to a lower throughput as the
UFL/DFL ratio decreases. However, in case of traditional
IBFD, the total transmission time remains the same for all
the UFL/DFL ratios. Moreover, comparing to the totaltransmission time only a little amount of uplink data
decreases as the UFL/DFL ratio decreases. Since thedownlink frame size is fixed at 2000 bytes, the throughputof traditional IBFD remains almost stable as shown in
Fig. 10.
A comparison in terms of overhead generated undersaturation condition is shown in Fig. 11. This comparison
is depicted in two different ways. For example, Fig. 11(a,b) compare Asym-FDMAC with traditional IBFD in termsof total overhead. However, Fig. 11(c, d) compare them byconsidering overhead/byte, where the unit is nanosecond/byte (ns/B). In this comparison, the overhead is calculated
in terms of time from the beginning to the end of thetransmission cycle except for the time required for user
data (payload) transmission. For example, in the time
sequence of the AP as shown in Fig. 4, the overhead time
includes the time starting at the point of transmitting the
RTS until the ending point of M-ACK transmission, except
the time of the data transmission. It is observed that the
total overhead generated by of Asym-FDMAC is almost
the same as that of traditional IBFD for the UFL/DFL ratioof 1/3 (Fig. 11a). However, for a UFL/DFL ratio of 1/9,

70
Asym-FDMAC
Traditional IBFD

50
40
30
20
10
0
1/6

1/9

1/12

1/15

UFL/DFL ratio
Fig. 10 Throughput comparison with respect to UFL/DFL ratio (for20 UTs)300

Asym-FDMAC
Traditional IBFD

200
100

20 UTsAsym-FDMAC
Traditional IBFD

200
100

20 UTs40 UTs40 UTsNumber of UTs

Number of UTs

(a)

(b)

120
100

300

0

0

Overhead/byte (ns/B)

Fig. 11 Overhead comparison:a, b show total overhead
comparison and c, d show
overhead/byte comparison.
a UFL/DFL ratio is 1/3, b UFL/DFL ratio is 1/9, c UFL/DFL
ratio is 1/3, d UFL/DFL ratio is
1/9

Total overhead ( s)the downlink data transmission, as the UFL/DFL ratio
decreases and thus the number of SIFS time increases inbetween the transmission of uplink data by different UTs.
Therefore, the total uplink data decreases. Secondly, as thenumber of uplink data transmitting UTs increases, the totaltransmission time increases, because the AP needs toinclude their addresses in both NFC and M-ACK. Therefore, the total amount of transmitted data decreases for the

Total overhead ( s)1/3Overhead/byte (ns/B)

Throughput (Mbps)

60

Asym-FDMAC
Traditional IBFD

80
60
40
20

120
100

Asym-FDMAC
Traditional IBFD

80
60
40
20
0

0
20 UTs40 UTs20 UTs40 UTsNumber of UTs

Number of UTs

(c)

(d)

123

Wireless Networks
Table 3 Effect of missing frames

Name ofmissing
controlframes
RTS

Nodes that missthe control
frame
Corresponding
receiver client
Any other client

CTS

AP

Any other client

PDIP

NFC

Impact on that node
(In terms of data
communication)
The client cannot
transmit to and receivefrom the AP
The client cannot send
PDIP during its slot
number (SN)
Still the AP can receive
data from other clients
The corresponding client
can send PDIP during its
SN

Any other client

The corresponding client
cannot send data to AP
No eﬀect

Corresponding
client

The corresponding client
cannot send data to AP

Any other client

No eﬀect
The corresponding client
cannot send PDIP
It seems that the client
does not receive datafrom APIt seems that the APdoes not receive datafrom the clientAPDR

Any other client

ACK froma client

AP

ACK from
the AP

Corresponding
clientData froma clientAP

AP will not send ACK tothe corresponding client

Data fromthe AP

Corresponding
client

The client will not send
ACK to AP

Asym-FDMAC generates 33% and 23% more overhead for
20 UTs and and 40 UTs respectively (Fig. 11b). On theother hand, Asym-FDMAC outperforms traditional IBFD
with lower overhead/byte. Although Asym-FDMAC has
higher total-overhead in most of the cases, it has a lower
overhead/byte compared to traditional IBFD in all cases(Fig. 11c, d).
Overhead/byte is an important parameter for performance analysis, as it describes the overhead as well as theamount of transmitted data. Therefore, the MAC protocol
with lower overhead/byte performs more effectively.

123

Overall impact on datatransmission
HD transmission is possible
from other clients to AP.
FD communication is possible.
HD transmission is possible.
Inter-user interference canoccur. HD communication ispossible. Sometimes FDcommunication is also possible.
FD communication is possible.
FD communication is possible.
Corresponding client cannot
send data to the AP, FD
communication is possible withlower throughput.
No eﬀect on FD communication.
HD communication is possible.
Transmission from AP to client
is discarded and re-transmission
is required.
The transmission is discarded
and re-transmission is required.
FD is possible with other clientswith lower throughput.
Re-transmission is required fromthe client.
HD transmission is possible.
Re-transmission is required fromthe AP.

Asym-FDMAC outperforms traditional IBFD with lower
overhead/byte in all cases as shown in Fig. 11(c, d).

6.3 Effect of missing frames
Any control frames or any transmitted data can be lost andthus the corresponding frame cannot be received by a nodeor by the AP in real environment due to noise or interference. Table 3 describes the impact of any missing control
frames as well as data on the proposed Asym-FDMAC. In
case of traditional HD communication, if any RTS or CTS

Wireless Networks

is missed out, the communication is discarded and the retransmission of RTS as well as CTS is required. This retransmission is also required, if the data or the ACK are
lost. However, in case of Asym-FDMAC (as shown in the
Table 3), most of the cases the data transmission can be
possible; only the exception is for missing of data and
ACK. Even in case of missing RTS and CTS, some times
communication is possible with lower throughput. Therefore, this proposed Asym-FDMAC is theoretically morerobust than the existing traditional IBFD and HD communication. For example, after sending the RTS by the AP,
if the RTS is not received by the corresponding UT, HD
transmission is still possible. In this case, the corresponding
UT cannot send the CTS. Therefore, all the UTs that haveuplink data can send the PDIP, as all UTs satisfy the SDTCin this case. Now, as the AP does not receive the CTS, it
will not transmit the downlink data. However, the AP canselect the UTs for the uplink data transmission and thus HDtransmission is possible. On the other hand, if the RTS is
not received by an individual UT, it cannot send the PDIP
in this transmission cycle only, but FD communication ispossible as other UTs receive RTS. However, if the RTS is
totally lost (i.e. no UTs can receive the RTS correctly), there-transmission is required.

7 Conclusion
This paper proposes a MAC protocol for IBFD wirelesscommunications. This MAC protocol supports asymmetrictraffic for uplink and downlink communication, which canbe utilized in an infrastructure based WLAN. As asymmetric traffic is very common in WLANs, we need toinclude this feature in future FD wireless communications.
The proposed Asym-FDMAC supports the transmission of
uplink data by multiple user terminals during a single
downlink transmission. Hence, the busy time or idle time(used in other FD-MAC protocols) can be used for datatransmission. The simulation results showed that the proposed FD-MAC outperforms traditional IBFD and HD
communications significantly.
This MAC protocol is not compatible with the IEEE
standard. As IBFD is a new technology and the existingIEEE standard does not support IBFD communication, we
need new MAC protocols to achieve the full advantage ofthis latest technology. In addition, to implement this AsymFDMAC with the existing devices, the software up-gradation and proper scheduling are required. For example,
the AP can give chance to the existing WiFi devices byswitching the channel access mechanism to the contention
based mechanism. This issues can be considered moredeeply in the upcoming future research. The concept ofhidden terminal is utilized here for combating inter-userinterference. However, other techniques can be utilized infuture research for this purpose, such as the capture effect.
Acknowledgements This research and development work is supported by the MIC/SCOPE No. 155007006. This work is also supported by JSPS KAKENHI Grant No. JP16H01718.

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Md. Abdul Alim received the
B.Sc. in Electronics and Communication Engineering (ECE)
degree from Khulna University,
Khulna,Bangladesh.
He
received the M.Eng. degreefrom Chongqing University,
Chongqing, China. He hascomplete the Ph.D. degree from
Graduate School of IST, Osaka
University, Japan on March 22,
2018. He is now serving as afaculty member in Electronics
and Communication Engineering Discipline, Khulna University in Bangladesh. Mr. Alim has professional experiences in cellular123

wireless communications as an executive engineer from SIEMENS
Bangladesh. His research interests include MAC design for in-bandfull-duplex wireless communications, wireless power transmissions
and estimations.
Shunsuke Saruwatari received
the Dr. Sci. degree from the
University of Tokyo in 2007.
From 2007 to 2008, he was avisiting researcher at the Illinois
Genetic Algorithm Laboratory,
University of Illinois at UrbanaChampaign. From 2008 to 2012,he was a research associate at
the RCAST, the University ofTokyo. From 2012 to 2016, he
was an assistant professor at
Shizuoka University. He has
been an associate professor at
Osaka University Since 2016.
His research interests are in the areas of wireless networks, sensor
networks, and system software.
Takashi Watanabe is a Professorof Graduate School of Information Science and Technology,
Osaka University, Japan. He
received his B.E., M.E. and
Ph.D. degrees from Osaka
University, Japan, in 1982, 1984
and 1987 respectively. He
joined Faculty of Engineering,
Tokushima University as an
assistant professor in 1987 andmoved to Faculty of Engineering, Shizuoka University in1990. He was a visiting
researcher at University of California, Irvine from 1995 through 1996. He has served on many
program committees for networking conferences, IEEE, ACM, IPSJ
(Information Processing Society of Japan), IEICE (The Institute ofElectronics, Information and Communication Engineers, Japan). His
research interests include mobile networking, ad hoc networks, sensor
networks, ubiquitous networks, specially MAC and routing. He is amember of IEEE, IEEE Communications Society, IEEE Computer
Society, as well as IPSJ and IEICE.