Heterogeneous Control and Data Sub-network for
Millimeter-wave Communications
Xiaoxiao Zhang, Liqiang Zhao, Kai Liang
Key Laboratory of Integrated Service Networks, Xidian University, Xi’an, Shaanxi 710071, China
Abstract:
Millimeter-wave
(mmWave)
communications are highly focused as a powerful
mean enabling to perform very high data transmission.
However it has several inherent shortcomings like
directional transmission and serious attenuation in
atmosphere. So it is difficult to implement random
access in mmWave WLANs. In this paper, a
heterogeneous control and data sub-network
architecture is presented, which decouples the
traditional WLAN into 2.4 or 5GHz control
sub-network and mmWave data sub-network in both
PHY and MAC layers. In control sub-network, DCF is
adopted to transmit control information and in data
sub-network, PCF is adopted to ensure the QoS.
Moreover, an omnidirectional transmission is
employed in the control sub-network to support users’
random access. The data sub-network only covers the
required serving area by using directional antennas for
specific users and can be adjusted dynamically based
on control information. Simulations indicate that
compared
with
the
conventional
WLANs,
heterogeneous mmWave WLANs can provide both
random access and high throughput.
Key words: Millimeter-wave, Heterogeneous network,
WLAN, IEEE802.11
I. INTRODUCTION
The volume of mobile traffic is exploding, driven by
an increment of mobile devices (e.g., laptops, tablets,
and smart TVs) and broadband wireless access
services (e.g., HD video). The research and standard
work for the 5th Generation (5G) Mobile Network are
on the way. One of the preliminary targets for the 5G
is l000-fold capacity increase comparing the 4th
Generation (4G) system [1]. However, the high
bandwidth required by multimedia applications is
seriously stretching the limited wireless spectrum. The
millimeter wave (mmWave) band (especially from 26
to 60 GHz) could break through the limitation of the
traditional bandwidth, and support high data rate by
extending the spectrum resource. Actually, the
abundance of bandwidth in the unlicensed 26-60 GHz
band has attracted more and more interests from both
academia and industry [2-3] in recent years, which is
expected to be used widely in short range indoor
wireless communications including both Wireless
Personal Area Networks (WPANs) and Wireless Local
Area Networks (WLANs). Conventional WLANs at
the unlicensed 2.4 and 5 GHz have got great success
in both academia and business. Currently, they are
pushing forward IEEE 802.11ad mmWave WLAN,
which gets more and more attention due to its ability
to deliver the gigabit-per-second (Gbps) throughput
envisaged for 5G.
Although mmWave technologies promise a bright
future for over 10 Gbps transmission, current
protocols and technologies cannot be applied directly
due to several inherent shortcomings of mmWave
communications.
Atmospheric attenuation: One of the limiting
factors inherent in mmWave transmission is the
considerable atmospheric attenuation caused by
absorption phenomena due to rain drops, water vapor
and oxygen [4].
Sensitive to be blocked: Such short wavelengths in
this band do impose difficulty diffracting around
obstacles. A wall or a desk between a transmitter and
receiver can attenuate the signal by 15 dB or more and
can easily break the link [5].
Directional transmission: Facing such serious
losses in mmWave communications, the directional
antenna with high gain has to be implemented. As
almost all beams concentrate in the range of 4.7
degrees, mmWave communications can be regarded as
a line-of-sight (LOS) transmission.
The
above
characteristics
of
mmWave
communications make it very challenging to provide
the range and robustness similar to what most users
have experienced with conventional WLANs at 2.4
and 5 GHz. For example, IEEE 802.11ad focuses on
60GHz band, and use phased-array antenna to
improve link gain, but it is hard for IEEE 802.11ad to
provide random access for any user, which, on the
other hand, is a basic function in conventional
WLANs. Hence, in this paper, after introducing
aspects of technical challenges, design methodologies,
and
possible
applications
in
mmWave
communications, we provide a heterogeneous network
(HetNet) architecture for mmWave WLANs, and carry
out a detailed discussion and evaluation of the
corresponding performance evaluation.
The rest of this work is organized as follows. After
introducing
current
research
in
mmWave
communications, in section II, we talk of IEEE
802.11ad, the current standard of mmWave WLANs,
and IEEE802.11aj working at 45GHz is also discussed
in section II. In section III, we propose the heuristic
mmWave
heterogeneous
WLANs
(mmWave
HetWLANs), and simulations are carried out to
evaluate its performance in section IV. Finally,
conclusions are drawn in Section V.
II. CURRENT RESEARCH
In this section, we shall introduce current research of
mmWave communications from viewpoints of the
protocol stack. As WLANs only consider PHY layer
and MAC layer, we focus on these two layers in the
following.
2.1 PHY Technologies
A significant challenge that may affect the promising
prospect of mmWave communications is high
propagation attenuation resulting from the high carrier
frequency. To remedy this, antenna arrays can be
adopted at both the source and destination devices,
where a single radio-frequency chain (or a single data
stream) is tied to the antenna arrays, to exploit array
gains via appropriate beam forming schemes [6–8]. In
the meanwhile, the small wave length of mmWave
makes it feasible to equip antenna arrays into even
portable devices [8].
One beam forming scheme has been proposed in [9]
in order to improve beam forming efficiency of
mmWave communication systems, which comprises a
codebook design and a search algorithm. Directional
transmission makes mmWave communication systems
have a strong spatial multiplexing potential. Spatial
multiplexing, by means of the weak correlation of
space channels, could transfer data streams through
different space channels to increase the peak date rate.
In [10], 60 GHz spectrum is divided into three
independent channels, and by using 16 antenna arrays,
it can support 6 links at the same time slot.
Multiple input and multiple outputs (MIMO), which
can increase spectral efficiency without any additional
power consumption, has been widely investigated and
is considered as a viable solution for mmWave
WLANs [11-15].
2.2 MAC Protocols
After deploying directional antenna in mmWave
WLANs, it is prone to the so-called directed hidden
nodes or hidden beam problem. Therefore, equipment
detection and recognition is a primary issue.
Carrier sense multiple access with collision
avoidance (CSMA/CA) is the basic MAC protocol in
IEEE 802.11x, which is effective for omni-directional
antennas. However, [16] shows that existing
directional CSMA/CA protocols do not work well at
60GHz and proposes a novel directional CSMA/CA
protocol for 60GHz WPANs.
So far, some enhanced algorithms can reduce
further the time duration of finding equipment, and
simplify equipment access complexity. For example,
in [17], a complete MAC protocol based on discrete
phase shift codebooks in 60GHz WPANs is proposed
in order to realize Gbps communication.
As suffering from serious attenuation, mmWave is
not fit for long distance transmission. Indoor relay
station is proved to be a feasible solution to extend
transmission distance of mmWave communications.
For example, transmission rate of mmWave network
with 4 relay stations can reach Gbit/s, and its coverage
is as big as WLAN coverage [18].
2.3. mmWave WLAN
IEEE 802.11ad can support multi-gigabit wireless
communications in the 60GHz spectrum. It can
guarantee the best performance, minimized
complexity and lower cost of implementation.
Moreover, the main goal of this standard is to enable
devices to communicate with other devices that
belong to legacy 802.11.
2.3.1 Expected IEEE 802.11ad Architecture
IEEE 802.11ad defines a new topological structure
based on traditional IEEE 802.11x, called personal
basic service set (PBSS). PBSS can handle
transmission loss problem at 60 GHz, and is mainly
used for the high speed video data transmission in the
office or at home. A PBSS is composed of several
types of stations. One of them is a center node, called
PBSS Control Point (PCP), which takes charge
resource allocation, power adjustment and control
functions such as beam forming. Any two nodes in the
network can be two-way data transmission.
The mainly PHY layer enhancement done by
802.11ad is the combination between two types of
modulation and coding schemes Orthogonal
Frequency Division Multiplexing (OFDM) and Single
carrier (SC) respectively [19]. According to different
requirements (e.g., high throughput or stability),
different MCSs are used.
SC has a low complexity and strong stability, but it
only provides 4.6 Gbit/s. OFDM allows a higher data
rate up to 7Gbps and supports communications over
longer distances with greater delay.
The SC and OFDM according to data rates provide
flexibility to several intensive applications requiring
high transmission data rate.
2.3.2 MAC Layer Enhancements
The 802.11ad MAC protocol divides transmission
time into small beacon interval (BI), and using BI as
basic unit for slot time allocation. Fig. 1 is a BI time
slot allocation chart, according to the different access
rules are divided into four different types of child
interval [20].
BI
DTI
BTI
AATI
BFT
CP1
CFP1
CFP2
CP2
BTI:
BTI: Beacon
Beacon Transmission
Transmission Interval
Interval
Time
A-BFT:
A-BFT: Association
Association Beamforming
Beamforming Training
Training
ATI:
ATI: Announcement
Announcement Time
Time Interval
Interval
DTI:
DTI: Data
Data Transfer
Transfer Interval
Interval
Fig.1 BI time slot allocation
BTI is begin of BI, AP will broadcast beacon frame
carried with control information to coverage of signal
in this interval, so as to guarantee the STA is
synchronization with AP in subsequent three intervals.
In A-BFT, AP and STA accepted beacon frame will
initialize the beam forming training. Then AP and all
the STA will transmit frame about network
management in ATI, such as association,
communication, relay frames. DTI is a core time of BI,
used for transmitting information between of nodes.
Two channel access time: CFP (contention free period)
and CP (contention period) is in DTI. This is same as
legacy IEEE 802.11x, guarantying 802.11ad
compatible with legacy 802.11x.
2.3.3 IEEE 802.11aj
However, the coverage of 802.11ad is very short
because of much higher path loss at 60 GHz band than
at 2.4/5 GHz bands. That is because 802.11ad focus
on 60GHz, however 60GHz is just in absorption peak,
attenuation loss is about 14 dB/km, it conduces
serious attenuation. But signals below 50GHz do not
have such problem, their attenuation losses are under
1db/km [21]. Besides, as for path loss, it is direct
proportion to the square of frequency, that is to say
path loss of 60GHz mmWave is born to more 20db
than 5.8GHz. So a new study group [22] proposed
IEEE 802.11aj, which works in 45GHz. 802.11aj
avoids problem of excessive loss in transmission
process and enable multi-Gbps throughput and lower
power.
III. HETERGENEOUS MMWAVE WLANS
3.1 Description of HetNet
In traditional WLANs, control frames and data frames
are transmitted at the same physical channel, which
indicates that the Control plane (C-plane) and the User
plane (U-plane) are totally overlapped or closely
coupled. However, in mmWave WLANs, the C-plane
has to provide random access for any user, and the
U-plane has to support directional transmissions.
Since the C-plane and the U-plane are closely coupled,
it is hard to fulfill such contradictory requirements in
the conventional network architecture. Hence, we
present a heterogeneous mmWave WLAN in this
paper, which includes a control sub-network at the
2.4GHz or 5GHz band, and a data sub-network at the
mmWave band by decoupling C-plane and U-plane at
both the PHY and the MAC layers, as shown in Fig. 2.
In IEEE 802.11x, there are two access modes, one
is a fundamental Distributed Coordination Function
(DCF), and the other is the optional Point
Coordination Function (PCF). The DCF is a
contention-based protocol, while PCF is based on a
centralized polling protocol where a point coordinator
controls the access to the radio resource.
In mmWave HetWLANs, the conventional IEEE
802.11x is employed for the control sub-network, and
IEEE 802.11ad is used for the data sub-network to
keep compatible with legacy 802.11 standards as more
as possible. In this way, the control sub-network can
provide omnidirectional coverage for any user to
access the network, and the data sub-network only
covers the required serving area in the directional
mode to provide directional coverage and Gbps
transmission for given users at the mmWave band.
Upon considering the relevant context of mmWave
WLANs, DCF is more suitable for the control
sub-network to support random access, and PCF is
more suitable for the data sub-network to ensure QoS
of users. In fact, the control sub-network covers all the
deployment areas, supporting access requests of users,
synchronization and other signaling processes.
Therefore, it is the basis of mmWave HetWLANs.
I.
Omnidirectional
antenna transmit
control
information and
build control
network
II. Directional
antenna
transmit high
speed data
based on
control
network
TIME
Directional
beam
Control
network
Fig.2 Heterogeneous mmWave WALNs
In our proposed mmWave HetWLAN, all the
control functions are implemented in the control
sub-network, so we can simplify the data sub-network
by removing the three optional access types
mentioned in Section 2.3.3 ( such as i.e., BTI, A-BFT,
and ATI in Fig. 1). Only DTI is used for the data
sub-network to transmit data. In this way, the control
sub-network can provide random access for any user,
and the data sub-network only focuses on Gbps
transmission.
In order to implement the proposed heterogeneous
network architecture, the legacy IEEE 802.11x
protocol stack has to be modified, as shown in Fig.3.
An additional management sub-layer is introduced for
C/U plane. When packets arrive from the upper layer,
the management sub-layer differentiates them into
C-frames and U-frames, and then sends these frames
to the appropriate sub-network.
In IEEE802.11x, there are mainly three kinds of
frame named data frame, control frame and
management frame. Management frame is responsible
for joining or quitting WLANs, such as association
request frame, association response frame, beacon
frame and so on. Obviously, data frames should be
transmitted in data sub-network, and management
frames be transmitted in control sub-network. Control
frames, such as request-to-send (RTS), clear-to-send
(CTS), acknowledgement (ACK), and power
saving-polling frame (PS-Poll), are usually used with
data frames to deal with channel acquisition, carrier
sensing
maintenance,
and
giving
positive
acknowledgement of received data, to improve the
reliability of data transmission between stations. Thus,
control frames may be transmitted in both
sub-networks.
When C/U plane sends its packets to the upper layer,
management sub-layer can read the information of
packets and save the network information, such as
which nodes need to transmit data and network
topology obtained through control sub-network. We
can focus on user data transmitting in business layer
with aid of management sub-layer，which offers a
uniform interface for high layer protocols. The control
sub-network and the data sub-network do not
exchange information directly as there is not such an
interface. However, both of these two sub-networks
could get the related management and control
information of each other by means of the
management sub-layer.
Network layer
The management sublayer
MAC layer
Control
Omnidirectional
network
transmission
MAC layer
Data
network
PHY layer
Control
network
PHY layer
Data
network
transmission
Directional
transmission
Fig.3 Protocol stack of mmWave HetWLAN
As mmWave HetWLAN keeps compatible with
legacy 802.11x and 802.11ad, most enhanced
protocols, algorithms, and technologies for 802.11x
and 802.11ad could also be implemented for mmWave
HetWLAN.
3.2 Analysis of control sub-network
As DCF is employed in control sub-network,
two-dimensional discrete-time Markov chains model
in [23] is introduced.
DCF is based on CSMA/CA, which uses a basic
acknowledgment mechanism for verifying successful
transmissions and an optional RTS/CTS handshaking
mechanism to decrease overhead from collisions.
There is a binary exponential backoff mechanism in
DCF. If a node has a new packet to be transmitted, this
node will generate a random backoff interval before
transmitting. The backoff time is slotted and the
number of backoff slots is uniformly chosen in the
range [0, CW]. At the first transmission attempt, the
Contention Window, CW, is set equal to a value CWmin
called the minimum contention window. After each
unsuccessful transmission, CW is doubled up to the
maximum value CWmax = 2m∙CWmin. The value m is
called the maximum backoff stage, and CWmax is
called the maximum contention window. Once CW
reaches CWmax, it will remain at the value until the
packet is transmitted successfully or the
retransmission time reaches the retry limit (m). When
the limit is being reached, retransmission attempts will
cease and the packet will be discarded. Let W0 be the
initial backoff window, W in retransmission attempt i
is:
Wi 
 2i W , i  m
 m
2 W , i  m
(1)
Then the average delay slot time for every packet
is:
 p i  p m1 Wi  1 
 2 

E Z    

1  p m1
i 0




m
(2)
where p represents the collision probability. The delay
is given by
(3)
E  D   E  Z  E  slot 
where E[slot] is the average time length of a slot.
Since DCF is contention based, collision probability
will increase and delay will become larger with the
increasing number of users. Therefore, for the
relatively less and small control packets transmitted in
the control sub-network, DCF mode is more
appropriate to ensure the establishing of network
connection.
3.3 Analysis of data sub-network
In the data sub-network, we adopt PCF. In the PCF
protocol, time is divided into super-frames [24]. A
super-frame consists of a Contention Free Period
(CFP) in which a Point Coordinator (PC) controls
channel access, and a Contention Period (CP) in
which the DCF rules apply.
The average amount of payload successfully
transmitted in CFP is given by
ECFP   1  PU EU  1  PD ED 
(4)
where EU and ED are the average payloads of the
upload frame and download frame， respectively.
After considering the limit of CFP, the average
amount of payload successfully transmitted is
obtained as follows
guaranteed in directional transmission in PCF mode.
The data sub-network transmits high-speed data via
mmWave and the data packets are normally massive
and long, leading to high real-time requirements. PCF
mode can improve the throughput and reliability of
mmWave transmission, while mmWave can
compensate the delay defects of PCF mode. Therefore,
DCF and PCF are used for the control sub-network
and the data sub-network, respectively.
IV SIMULATION RESULTS
In order to evaluate our proposed MmWave
HetWLANs, the following simulations are carried out
in OPNET. We consider an indoor area of 15 × 15m2
where an AP is located in center and 10 STA are
around randomly. High-speed video data information
is transmitted between them. For comparison, we
consider two network architectures, which are our
proposed HetWLAN, and the legacy 802.11ad
network. The values of the most parameters of the
control sub-network and the data sub-network used to
obtain numerical results for simulations are specified
in IEEE 802.11a and 802.11ad, respectively.
In the first 10 seconds, by exchanging control
frames through the AP in the omnidirectional mode at
2.4GHz band, all the users access the HetWLAN, as
shown in Fig. 4. The accessed users begin to exchange
data frames through the AP in the directional mode at
26GHz band. Besides, for comparison, a legacy
802.11ad WLAN is evaluated within the same
scenario.
ECFP
2.5

9
Heterogeneous HetW LAN
Legacy 802.11ad


CFPmax  B  E
 min  ECFP ,
1  PU  EU  1  PD  ED  

1

P
U

SIFS

1

P
D

SIFS








U
D


2
Throughtput (bps)
(5)
The average amount of payload successfully
transmitted in CP is given by
ECP  nPtr Ps Enrt
(6)
where Enrt is the average payload of non-real-time
traffic-frames. Hence, the system throughput η can be
expressed as follows:
x 10
1.5
1
0.5
Payload successful transmission time in a superframe
0
Average length of a superframe

ECFP  ECP
F
(7)
PCF is polling based, the AP can allocate resources
like channel access to every STA in turn, and thereby
the packet dropped probability is small. From (7) we
can know the throughput and reliability can be
0
1000
2000
3000
Load (pkt/s)
4000
5000
Fig.4 Network throughput
Fig. 4 shows that compared with legacy 802.11ad,
HetWLAN can improve network throughput slightly
when the traffic load is light. However, along with
load increasing, 802.11ad get saturated soon, and the
gap between HetWLAN and 802.11ad gets larger and
larger. Obviously, as most resources are used for
transmit control frames, 802.11ad cannot take full
advantages of the mmWave band. This is mainly
because in legacy 802.11ad network control and data
information are transmitted in the same channel, and
the complexity of the directional transmission makes
the control load larger than the omnidirectional
control sub-network, which causes data information
throughput decreases. Besides, in legacy 802.11ad
network, there are directional hidden nodes problems
with directional transmission. Thus this problem bring
down the network throughput. However, HetWLAN
achieves the omni-directional coverage control subnet
and the data subnet using PCF mechanism. It can
reduce the collision of the data and allocate resources
reasonably, improving the throughput.
0.2
Legacy 802.11ad
Control sub-network
Data sub-network
0.18
0.16
Delay (s)
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0
0.8
0.9
1
1.1
1.2
1.3
Load (pkt/s)
1.4
1.5
1.6
1.7
x 10
4
Fig.5 Delay
Fig. 5 shows that delay in HetWLAN is much
superior to 802.11ad as it is very hard for 802.11ad to
support both random access and Gbps transmission.
This is because legacy 802.11ad which only has
directional antenna that cannot discover STA in time.
When data quantities get larger, it need much more
time than HetWLAN to access network. Besides, it is
easier for HetWLAN to avoid collision because of
control sub-network. These reasons make HetWLAN
have smaller delay than legacy 802.11ad.
V. CONCLUSIONS
In this paper, a deep survey of mmWave WLANs
including network topology, protocol, technology, and
spectrum is provided. In order to provide random
access and Gbps transmission simultaneously,
heterogeneous network architecture is proposed for
mmWave WLAN, which includes a control
sub-network in the omnidirectional mode at 2.4/5GHz
and a data sub-network in the directional mode at the
mmWave band. After C/U-plane splitting, different
MAC and PHY protocols are deployed for two
sub-networks, e.g., DCF for the control sub-network
and PCF for the data sub-network, respectively. Our
simulation results show that compared with legacy
802.11ad, mmWave HetNet can increase network
throughput and reduce delay.
ACKNOWLEDGEMENT
This work was supported in part by National Natural
Science Foundation of China (No. 61372070), Natural
Science Basic Research Plan in Shaanxi Province of
China (2015JM6324), Hong Kong, Macao and Taiwan
Science & Technology Cooperation Program of China
(2014DFT10320), EU FP7 Project MONICA
(PIRSES-GA-2011-295222), and the 111 Project
(B08038).
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Biographies
Xiaoxiao Zhang is now studying for his master’s
degree of electronics and communication
engineering in Xidian University. He received his
bachelor degree of Communication Engineering
from
Xi`an
University
of
Posts
&
Telecommunications. His main research interests
lies in mmWave networks and distributed
spacecrafts of precise formation flying. Email:
anypoint2009@163.com.
Liqiang
Zhao
obtained
his M.Sc. in
Communications and Information Systems and
Ph.D. in Information and Communications
Engineering from Xidian University, China in 2000
and 2003, respectively. His current research
focuses on broadband wireless access, green
communications,
and
near
space
communications. Due to his excellent works in
education and research, in 2008, Prof. Liqiang
was s awarded by the Program for New Century
Excellent Talents in University, Ministry of
Education. In addition, he is the corresponding
author, Email: lqzhao@mail.xidian.edu.cn.
Kai Liang is now a Ph.D. student in Xidian
University and major in communication and
information systems. He received his bachelor’s
degree in communication engineering from Xi’an
University of Architecture And Technology, China
in 2007. His current research interests include
broadband wireless access, wireless network
design and coordinated multipoint-to-multiuser
communication systems.