Fundamental Survey of Full Duplex
Wireless Communication

Watanabe laboratory
2016/2/28

1

Introduction
In this survey, we have discussed about the basic concept of
in-band full-duplex (IBFD) wireless communication. We
have given emphasis on the Medium Access Control(MAC) design for IBFD wireless communication. Thissurvey has two sections. First section includes thefundamental study and second section includes a example
of IBFD MAC design.
Section-1:
 Concept of In-band Full-duplex (IBFD) wirelesscommunication and MAC design.
Section-2:
 An Example of a IBFD Medium Access Control Design.
2Section-1
Outline of this section









Introduction
Basics of full-duplex
Advantages of in-band full-duplex (IBFD)
Disadvantages of IBFD
Antenna Configuration for IBFD
Transmission modes in IBFD system
Basic concept of MAC design
Challenges of MAC design for IBFD

3

1.1 Introduction:

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 Why Faster WLAN is Needed:
Faster WLAN is required as:o Network traffic is increasing very
rapidly.
o A lot of applications and services
are available now. Many users
want to use more applications at
the same time.
Fig-1. Future trend for Mobile traffic in
exabytes (EB) per month

Section-1

4

1.1 Introduction:

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 Current Technologies for Faster WLAN:
To meet the challenges of increasing traffic in future, we need faster WLAN.
The state-of-the-art techniques for faster WLAN include:• Full Duplex (FD)• Interference Cancellation• Superposition Coding• Multiple Input Multiple Output (MIMO), etc.

5Section-1

1.2 Basics of Full-Duplex:

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In these tutorial, the concept of Full-Duplex (FD) technologies as well as
medium access control design will be described sequentially.
Traditional Full-duplex:
The stations can transmit and receive simultaneously. But, this is done by two
conventional way:i. Time Division Duplexing (TDD)
ii. Frequency Division Duplexing (FDD) So, traditional full-duplex (FD) is performed either in same frequency anddifferent timeslot for transmit and receive, or same time and different
frequency for transmit and receive simultaneously.

Section-1

6

1.2 Basics of Full-Duplex:

2/2 In-band Full-duplex:
 Unlike traditional full-duplex (FD), in-band FD communication is performedby using same time as well as same frequency for transmit and receive
simultaneously.
 But for doing this FD successfully, we need to eliminate the self interference.
 So, self interference cancellation technique (SIC) is very important here.

TX

TX

RX

RX

Node A

Node B
Fig-2. FD wireless transmission

Section-1

1.3 Advantages of In-band FD:

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The main advantages of in-band FD (IBFD) system can be summarized asfollows:
 Can double throughput gain:
In-band FD (IBFD) transmission can utilize the time and frequency
resources properly. As a result, it is theoretically possible to double thelink capacity as compared to HD transmission [1],[2].
 Can reduce feedback delay:
Reception of feedback signaling (such as control information, channel
state information (CSI) feedback, acknowledge (ACK) signals, resource
allocation information, etc.) during data signal transmission enables
shorter air interface latency in feedback information [2].

Section-1

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1.3 Advantages of In-band FD:

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 Can reduce end-to-end delay:
In relay systems, relay nodes with IBFD transmission can reduce end-toend delay because the relay node simultaneously receives data from asource node and transmits data to a destination node.

 Can suppress hidden terminal problem:
Simultaneous listening and sensing can be performed on a frequency
band while the signals are being transmitted. So, each node can decide
whether or not the other nodes are transmitting signals and thus collision
can be avoided.

Section-1

9

1.4 Disadvantages of In-band FD:
The major disadvantages of IBFD can be summarized as below:
 Self Interference:
Simultaneous transmission and reception in a single frequency band cancause the transmitted signals to loop back to their receive antennas. It will
create self interference (SI). Proper self interference cancellation
techniques should be used to minimize this self interference.
 Imperfect interference cancellation:
In practical environments, the SI can not be perfectly canceled for avariety of reasons, such as the non-linearity of hardware components in
the RF chain (the SI power is beyond the feasible range), estimation
errors on the self-channel and the received SI signal (various reflectedinterferences), and incompleteness of various cancellation techniques [1].
 Inter-user interference:
As the FD nodes transmit and receive simultaneously, it will create interuser interference. The number of inter-user interferers increases by almosta factor of two and the aggregate interference at a node increases as well.
Section-1

10

1.5 Antenna Configuration for IBFD:As IBFD requires in-band operation for transmitting and receiving radio
frequency (RF) chains, the conventional duplexers cannot separate these
two RF transmissions. So, IBFD can be performed by using the following
antenna configurations: Shared Antenna [3]: In shared antenna configuration, a single antenna isused for simultaneous in-band transmission and reception by using athree-port circulator. Ideally, a circulator prevents the leakage signals
from the transmit RF chain to the receive RF chain. But, in real case,the transmit signal causes also causes interference to the received
signals.
 Separated Antenna [3]: In this configuration, there are separate
antennas for transmission and reception. This division of spatial
resources, however, introduces a trade-off. As such, a fair comparison
between HD and FD transmission should consider the exact number of
RF antennas required to establish FD transmission.
Section-1

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1.6 Transmission Modes in IBFD System:

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 Half-Duplex: In half-duplex (HD) mode the transmission is performed
in a single direction (Fig. 3(a)).
 Bi-directional FD (BFD): In this case, two node transmit signal toeach other and thus receive signal simultaneously. That means two nodetransmit and receive simultaneously (Fig. 3(b)).

A

B

(a)

A

B

(b)

Fig-3. FD transmission mode: (a) HD mode and (b) BFD mode
Section-1

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1.6 Transmission Modes in IBFD System:

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 Relay FD (RFD): In RFD system, a FD node transmits signal to one
node while receiving from another one. As in Fig. 4, node C
transmits to node A and A transmits to node B simultaneously. Here,node A is in FD mode, however node B and C are in HD mode.

C

A

B

Fig-4. FD transmission mode: Relay mode

Section-1

1.7 Basic Concept of MAC design:

13

1/8

 In OSI (Open Systems Interconnection)
terms, higher-layer protocols (layer 3
and above) are independent of networkarchitecture and are applicable to LANs,
MANs, and WANs. Thus, a discussion of
LAN protocols is concerned principallywith lower layers of the OSI model.
 This figure relates the IEEE LAN
protocols to the OSI architecture.

Fig-5. IEEE 802 Protocol Layers Compared to
OSI Model

Section-1

14

1.7 Basic Concept of MAC design:

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 So the 2nd layer (Data link layer) has two sub-layer:(i) Logical Link Control (LLC) and (ii) Medium Access Control(MAC).
 The Logical Link Control (LLC) is responsible for error and flow
control.
 On the other hand, MAC layer is responsible for framing and MAC
address and Multiple Access Control.
 MAC layer sets the rules how the medium will be accessed by theusers.

Section-1

1.7 Basic Concept of MAC design:

15

3/8

 Taxonomy of Multiple-Access Protocols
 To understand the MAC design, one should have the basic concept ofmultiple access control. The taxonomy of multiple access protocols can beshown by the following chart.

Source:Behrouz A. Forouzan, “Data Communications and Networking,” 4th edition.

Section-1

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1.7 Basic Concept of MAC design:

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 Taxonomy of Multiple-Access Protocols
 Random Access (or contention) Protocols: In this type of protocols, no station is superior to another station and none isassigned the control over another.
 A station with a frame to be transmitted can use the link directly based on aprocedure defined by the protocol to make a decision on whether or not tosend data.
 There are a number of protocols in this category. However, as Carrier
Sense Multiple Access with Collision Avoidance (CSMA/CA) is widelyused in wireless communication, we will discuss briefly about CSMA/CA.

Source:Behrouz A. Forouzan, “Data Communications and Networking,” 4th edition.
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Section-1

1.7 Basic Concept of MAC design:

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 CSMA Protocol
 By listening before transmitting, stations try to reduce the vulnerability
period to one propagation delay. This is the basis of CSMA (Kleinrock andTobagi, UCLA, 1975).
 If a frame was sent by a station, All stations knows immediately so they can
wait before start sending A station with frames to be sent, should sense the medium to check thepresence of another transmission (carrier) before it starts its own
transmission.
 This can reduce the possibility of collision but it cannot eliminate it.

Source:Behrouz A. Forouzan, “Data Communications and Networking,” 4th edition.

Section-1

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1.7 Basic Concept of MAC design:

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 CSMA Protocol
 However, collision can still be occurred when more than one station begin
transmitting within a short time (the propagation time period) as shown in
the following figure.

Source:Behrouz A. Forouzan, “Data Communications and Networking,” 4th edition.
19

Section-1

1.7 Basic Concept of MAC design:

7/8

 CSMA / CA Protocol
 CSMA / CA protocol is proposed to avoid collision as well as to avoid
hidden and exposed terminal problem. In general, CSMA / CA works
according to the following way:
− It uses four-frame exchange for better reliability.
− Source transmit a Request to Send (RTS) frame to destination.
− Destination responds with Clear to Send (CTS).
− After receiving CTS, source transmits data
− Destination responds with ACK, after receiving data successfully.

Section-1

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1.7 Basic Concept of MAC design:

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 CSMA / CA Protocol
 So CSMA / CA protocol has following advantages:− RTS alerts all stations within the range of the source. As a result thosestations do not start their transmission.
− CTS alerts all stations within the range of destination. Similarly, thosestations also do not start their transmission.
− As a result of RTS and CTS, the collision is reduced significantly.
− RTS/CTS exchange is required function of MAC but may be disabled,
if required.

Section-1

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1.8 Challenges of MAC Design for IBFD:

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i. Inter-user Interference:o Here, suppose Alice sends RTS to AP (FDAP) first and then AP takes decision to senddata to Bob while receiving data from Alice.
o If Alice and Bob are close to each other, thepackets from Alice will interfere with thepackets from AP at Bob, which creates interuser interference problem.
o The first one is primary transmission andsecond one is secondary transmission. So,
Alice is primary transmitter (PT) and Bob issecondary receiver (SR). However, theaccess point is working as both primary
receiver and secondary transmitter (PR/ST).
Section-1

AP

PR/ST

PT

SR

Alice

Bob

Useful data
Collision (interference)Fig-6. Inter user interferencein FD-WLAN

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1.8 Challenges of MAC Design for IBFD:
ii.

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Selecting FD Transmission Modes and Nodes:

o In wireless networks, clients can operate in any of the FDtransmission modes that are illustrated in section 6. Hence, selecting
a set of clients and an FD transmission mode is a vital issue tomaximize the overall utility of FD transmission.
o The basic approaches that are currently used in FD-MAC protocols
to select proper clients and modes for FD transmission are sharedrandom backoff, header snooping, and request-to-send (RTS)/ clearto-send (CTS) mechanisms [3].

Section-1

1.8 Challenges of MAC Design for IBFD:

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iii. Hidden Node Problem:o In practical case, the primary and secondary transmitted packets may
have different packet lengths. Therefore, the transmission of allnodes will not finish at the same time. As a result, relying only on
FD data transmission (even in case if BFD mode) does notcompletely solve the hidden node problem.
o The hidden node problem in FD transmissions due to asymmetricdata traffic at the transmitter and the receiver can be referred to as
the residual hidden node problem [3].
o However, the node that finishes data transmission earlier can solve
this problem by transmitting busy tone signals until the other node
completes its transmission [5].
Section-1

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1.8 Challenges of MAC Design for IBFD:

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iv. Deafness in Directional Antennas:
o If directional antennas are used in FD systems, neighboring nodes
around the primary transmitter and secondary transmitter unable todetect these transmissions. This scenario is known as the deafness
problem in directional transmission [3, 5].
o Due to the deafness problem, the neighboring nodes try to access thechannel assuming that the channel is available for their transmission.
This will end up with a collision in the ongoing transmission.
o To minimize this problem, a centralized MAC protocol can be used,as the central coordinator knows the locations of its registered nodes
and their corresponding transmission directions.
Section-1

25

Section-2
An Example of a IBFD Medium Access Control Design26

2.1 Introduction:
In this section, we will describe briefly a MAC design for IBFD, which is
published in IEEE transaction of Wireless Communication. This is a
CSMA/CA (carrier sensed multiple access with collision avoidance) based
MAC protocol. From this discussion, we will get an idea how to design a
MAC protocol for in-band FD wireless communication. The information of
the paper that will be discussed is given below:
 Title:“Power-Controlled Medium Access Control Protocol for Full-Duplex WiFi
Networks”
 Authors:
W. Choi, H. Lim and A. Sabharwal
 Journal:
IEEE Transactions on Wireless Communications
Volume: 14, Issue: 7, pp. 3601 - 3613, 2015.
Section-2

27

2.2 Main contribution of the paper: This paper proposed a MAC protocol to manage inter-client interference
in a wireless network.
 Here the network consists of an FD-AP and some half-duplex clients.
 They uses power control mechanism and distributed selection to
manage inter-clients interference.
 Firstly, a RSSB (received signal strength based) back-off mechanism is
proposed to provide a higher reception opportunity to the client with a
low inter- client interference.
 Secondly, the transmit powers of the AP and transmitter are adjusted tominimize inter-user interference, which is called power controlled MAC(PocMAC).

Section-228

2.3 Description of PoCMAC:

1/81) Frame structures:
 PoCMAC uses five types ofcontrol frames and two types of
DATA frame headers, as shown in
Fig. 10 and 11.
 The five control frames are:
RTS, CTS-Uplink (CTS-U),
CTS-Downlink (CTS-D),
ACK-Downlink (ACK-D), andACK-Uplink (ACK-U),

Fig-10. Control frame structure.

 The two types of DATA frame
headers are:
The header of the AP (HA) and
The header of the client (HC)Fig-11. Header of data frame for AP and client.

Section-2

2.3 Description of PoCMAC:

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1) Frame structures: The CTS-U is transmitted by the AP after it receives an RTS
from a client. In addition, using the CTS-U, the AP informs thecandidate clients that it wants to transmit the DATA frame.
 The number of RX candidates that can be listed in the CTS-U
frame is set to M. But here, it is only 2.
 The CTS-D frame is transmitted by the candidate client that wins
the RSSB contention after the AP broadcasts a CTS-U frame.
 CTS-D also includes inter-client interference information, whichis the received power of the RTS from the TX. If the RX cannot
overhear the RTS from TX and cannot measure the signal
strength, that field is filled with zeroes.

Section-230

2.3 Description of PoCMAC:

3/82) Proposed PoCMAC:

Fig-12. Operation example of the full-duplex AP and clients.
(1st transmission period: C1 - TX, C4 - RX / 2nd transmission period: C5 - TX, C2 - RX).

Section-2

2.3 Description of PoCMAC:

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2) Proposed PoCMAC:
 TX side(1) All clients that want to transmit a DATA frame perform a back-off
mechanism.
(2) The client that wins in back-off mechanism transmits an RTS frame to
the AP and waits for a CTS-U from the AP.
(3) After receiving CTS-U from AP, TX waits for the HA of the DATA
frame and others set network allocation vector (NAV).
(4) As soon as the TX receives the HA of the DATA frame from the AP, it
starts to send uplink DATA with the transmit power specified in thereceived HA frame.
(5) After completing data transmission, the TX waits for an ACK-U
frame from the AP.
(6) If the acknowledgement bit of the ACK-U frame is ‘1’, the TX canverify that the transmission was successful, and then return to theinitial state.
(7) Otherwise, the TX returns to the initial state for retransmission.
Section-2

32

2.3 Description of PoCMAC:

5/82) Proposed PoCMAC:
 RX side(1) All clients that do not want to transmit a DATA frame to the AP, orthat lose the contention, continue to overhear the RTS frametransmitted from other clients or wait for a CTS-U frame from the AP.
(2) After the clients overhear the CTS-U frame from the AP, they canidentify the clients that are nominated as the RX candidates.
(3) If the client is one of the candidates for the RX, it performs the RSSB
contention mechanism. Otherwise, it sets the NAV.
(4) The client that wins the contention among the candidates transmits a
CTS-D frame, including the information on the inter-clientinterference from the TX, and waits for the HA of the DATA frame.

Section-2

2.3 Description of PoCMAC:

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2) Proposed PoCMAC:
 RX side(5) If the client that transmitted the CTS-D frame receives the HA frameof the DATA frame, the client is considered to be the RX and starts
the downlink DATA reception.
(6) If the downlink DATA reception is successful, the RX transmits an
ACK-D frame to the AP. Otherwise not.
(7) After hearing an ACK-U from the AP, the RX returns to the initialstate.

Section-2

34

2.3 Description of PoCMAC:

7/82) Proposed PoCMAC:
 AP side(1) The AP waits for an RTS frame from clients.
(2) After receiving the RTS, the AP transmits a CTS-U including the addressof the TX and the addresses of the RX candidates to which the AP wants
to transmit the DATA and waits for a CTS-D.
(3) After receiving the CTS-D, the AP can calculate the optimal transmit
powers for the AP and TX; then, it starts the transmission of HA, which
includes the transmit power for the TX.
(4) After transmitting the HA and stabilizing the interference nulling, the APcontinues the downlink DATA transmission to the RX and starts theuplink DATA reception from the TX.

Section-2

2.3 Description of PoCMAC:

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2) Proposed PoCMAC:
 AP side(5) After transmitting and receiving the DATA frames simultaneously, the AP
waits for an ACK-D from the RX.
(6) If the ACK-D frame is received, the AP can determine that the downlink
DATA transmission was successful, and then, it transmits an ACK-U withacknowledgement bit ‘1’.
(7) Otherwise, the AP determines that the downlink DATA transmission has
failed, and then, it transmits the ACK-U frame with acknowledgement bit‘0’ .
(8) After transmitting the ACK-U frame, the AP returns to the initial state.

Section-2

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2.4 Performance Evaluation:

1/3

The performance of the proposed PocMAC is performed by followingway: The simulation is done by MATLAB.
 SDR (software defined radio) based evaluation is performed.
 The comparison is performed between PoCMAC and CSMA/CA
based half-duplex scheme.

For the simplicity, only the simulation result by MATLAB has beenpresented here.

Section-2

2.4 Performance Evaluation:

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2/3

Simulation for Average Throughput
 They consider a single-cell systemwith an AP having full- duplex
capability and its associatedclients with backlogged user
datagram protocol (UDP) packets.
 The transmission rates are set to6Mbps for the control frames and54 Mbps for the DATA frames.
 The simulation parameters areshown in Table-1.

Fig-15. SINRUplink and SINRDownlink with respect to the
suppression level of self-interference cancellation when theangle between the TX and the RX is 90◦.

Section-2

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2.4 Performance Evaluation:

3/3

Simulation for Average Throughput
 From the Fig. 15, there is nochangeinthethroughput
performance in case of CSMA/CA
half-duplex, as it is not affected by
the self-interference cancellation.
 On the other hand, PoCMAC
achieves a higher throughput
performance.
 When α is greater than 65 dB, thethroughput performance levels off
because the AP already uses its
maximum transmit power and theself-interference is sufficiently
suppressed.
Section-2

Fig-15. Average throughput with respect to the suppression
level of self- interference cancellation when the number ofclients is 10.

39

Conclusion:
In this study, the fundamental concept of in-band full-duplex wirelesscommunication has been described. This tutorial has mainly two sections: 1st
section includes basic study for the IBFD and medium access control designand 2nd section includes an example of this kind of MAC protocol. So, after
studying the first section, the second section will provide a deeper
knowledge on the MAC protocol for IBFD wireless communication.

Section-2

40

References:[1] D. Kim, H. Lee, and D. Hong, “A Survey of In-band Full-duplex Transmission: From the
Perspective of PHY and MAC Layers” IEEE Communications Surveys & Tutorials, vol. pp,
issue. 99, 2015.
[2] D. Kim, S. Park, H. Ju, and D. Hong, “Transmission capacity of full- duplex based two-way
ad-hoc networks with ARQ protocol,” IEEE Trans. Veh. Technol., vol. 63, no. 7, pp. 3167–
3183, 2014.
[3] K.M. Thilina, H. Tabassum, E. Hossain and D. I. Kim, “Medium access control design for full
duplex wireless systems: challenges and approaches,” IEEE Communications Magazine, vol.
53, Issue. 5, pp. 112 – 120, 2015.
[4] A. Tang and X. Wang, “Medium Access Control for a Wireless LAN with a Full Duplex AP
and Half Duplex Stations,” in Proc. of IEEE GlobeCom Conference, 2014.
[5] K. Miura and M. Bandai, “Node Architecture and MAC Protocol for Full Duplex Wirelessand Directional Antennas,” Proc. IEEE PIMRC, 2012.
[6] W. Choi, H. Lim and A. Sabharwal, “Power-Controlled Medium Access Control Protocol for
Full-Duplex WiFi Networks,” IEEE Transactions on Wireless Communications, vol. 14, issue.
7, pp. 3601 - 3613, 2015.

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