IEICE TRANS. COMMUN., VOL.E94–B, NO.3 MARCH 2011

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PAPER

Special Section on Information and Communication Technologies for Next-generation Mobile Multimedia Life

Development of Multi Channel Ad-Hoc Network System
Kentaro NISHIMORI†a) , Takefumi HIRAGURI†† , Masakatsu OGAWA††† , and Naoki HONMA†††† , Members

SUMMARY
This paper proposes a new Ad-Hoc network system which
comprises the multiple relay access points (APs) with multi channels. AdHoc network systems are recently proposed and incorporated for the communication infrastructure, which relays wireless transmission among access points (APs) in wireless LAN (WLAN) system. System throughput
is decreased due to hidden terminal problem when only a single channel isused for the Ad-Hoc network. In order to solve this problem, a new systemwith multi channels is proposed. However, even if the multi channels are
employed, the co- and/or adjacent-channel interference occurs due to hidden terminal problem and multiple APs in a limited space, when considering a simultaneous transmit and reception at the relay AP. In this paper,
we develop an Ad-Hoc network testbed which can reduce and avoid coand/or adjacent-channel interference by using vertically arranged antennaconfiguration and distributed channel allocation scheme. Moreover, the effectiveness of our testbed is clarified by applying actual WLAN signals.
key words: ad-hoc networks, vertical antenna arrangement, distributed
channel allocation, multi channels

1.

Introduction

Wireless LAN (WLAN) systems have been developed as access connections in order for wireless terminals to connect
networks. WLAN services based on IEEE 802.11 standard[1] are now being used in home and oﬃce situations as wellas many public places. Internet access via WLAN systems
will be realized across extensive areas that cannot be served
by wired network systems due to cost and geographic constraints. The studies [2]–[4] and standardization in IEEE
802.11s WG (Working group) [5] of mesh networks deals
with the communication by the interconnection of WLAN,
which achieves radio relay among two or more WLANs.
Moreover, IETF MANET (Mobile Ad-Hoc Networks)
WG [6] is discussing the routing protocol for the communications among multiple wireless terminals. Ad-Hoc networks, which are temporarily configured by the multiple
terminal stations, are standardized in IETF MANET WG.
Wireless multi-hop using the WLAN is basically used forboth the mesh network in IEEE802.11s and the Ad-Hoc
network in MANET. The wireless multi-hop network is
Manuscript received June 15, 2010.
Manuscript revised October 30, 2010.
†
The author is with the Faculty of Engineering, Niigata University, Niigata-shi, 950-2102 Japan.
††
The author is with the Faculty of Engineering, Nippon Institute of Technology, Saitama-ken, 345-8501 Japan.
†††
The author is with Technical Assistance and Support Center,
NTT East Corporation, Tokyo, 144-0053 Japan.
††††
The author is with the Faculty of Engineering, Iwate University, Morioka-shi, 020-8551 Japan.
a) E-mail: nishimori@m.ieice.org
DOI: 10.1587/transcom.E94.B.667

providing the chance of the communication for the wireless terminals which are placed very far away from eachother, even when they cannot communicate in direct link.
Hence, WLAN has been recently paid much attention. With
using Ad-Hoc systems, the infrastructure built by someAPs will be one of the bearer networks that are available
anytime-anywhere. Therefore, Ad-Hoc network techniques
are expected to provide new multimedia applications, suchas the video delivery to wide area or dead zone, sincethat can create temporary networks for disastrous area, andinter-vehicles communication in intelligent transport system(ITS), etc.
Multi channel systems are proposed on Ad-Hoc networks using the WLAN, in order to avoid interference due
to hidden terminal problem [7]–[11] and increase systemthroughput. Although such studies are based on the theoretical evaluation using computer simulation, there are some issues for achieving the multi-hop system in the actual device
and system. One of the problems is the adjacent channel interference. Even if the multi-hop system operates diﬀerent
channels, the transmitting signal of the other channel aﬀects
the adjacent channels due to its leakage power among thedevices which are installed closely together inside the common aluminum chassis. Hence, the throughput performanceby the relay transmission is degraded by such an interference [12].
There is another issue to realize the multi channel systems. When the number of terminals is increased while theavailable frequency bands are limited, the system throughput is decreased by co-channel interference if the system
employs careless channel number selection. It is reported
that the system throughput is decreased due to hidden terminal problem in wireless multi-hop network even if themulti frequency channels are used [13]–[15]. However,countermeasures relating to these studies are complex to
implement and an extensive modification is required forIEEE802.11 standard. Therefore a simple channel allocation schemes should be implemented in order to develop a
hardware testbed for avoiding the co-channel interference
due to the hidden terminal problem.
In this paper, the Ad-Hoc network system, which comprises the multiple relay access points (APs), is proposed
and implemented by an actual testbed. In the testbed there
are two proposals regarding antenna configuration of the
AP and channel allocation scheme for Ad-Hoc network system using multi channels. Furthermore, two proposed techniques are implemented on the testbed and the eﬀectivenessc 2011 The Institute of Electronics, Information and Communication Engineers
Copyright IEICE TRANS. COMMUN., VOL.E94–B, NO.3 MARCH 2011668

of the proposed techniques is confirmed by throughput experiments using actual WLAN signals.
The remainder of this paper is organized as follows. In
Sect. 2, we describe the target and issue on multi channel
systems. Section 3 proposes the interference suppression
using a vertical array antenna. In Sect. 4, we propose thechannel allocation scheme. The eﬀectiveness of the testbed,
in which the proposed schemes are implemented, is shownusing actual WLAN signals in Sect. 4.
2.

Target Scenario and Problem

The network configuration of the Ad-Hoc network systemcomprises the multiple relay access points (APs) with multichannels as shown in Fig. 1. LN denotes a wireless relay
link and N is the number of links in Fig. 1. There is a possibility that each AP links to neighboring APs dynamically inmulti-hop relay access system with multi channel access. To
realize this network configuration, a relay AP must be able
to transmit and receive the signals at the same time using
the diﬀerent frequency channels. Figure 2 shows a configuration of the developed AP which can realize the multi-hop
network system with multi channel access. The specification of developed AP is shown in Table 1. This AP has four
WLAN devices, and these devices control communication
link and data transfer, independently.
The main function of MAC protocols is Carrier Sense
Multiple Access with Collision Avoidance (CSMA/CA),

which is implemented in WLAN system. CSMA/CA attempts to prevent a terminal from transmitting at the sametime as other terminals within the same transmitting range
by listening to the signals within the same channel before
transmitting on each terminal. Therefore, CSMA/CA is useful function for a single cell situation, where same channel is shared by the terminals. However, since the distance between APs will be far rather than a single cellin the Ad-Hoc network, the hidden terminal problem occurs as shown in Fig. 3, since the signal of the AP1.cannot
reach AP2, and vice versa. The most famous scheme uses
frame exchange with Request-To-Send (RTS) and Clear-ToSend (CTS) frames (Two-way handshake) in order to eliminate the interference between hidden terminals [1]. However, since RTS/CTS have large overhead, throughput performance is degraded. Moreover, the interference occurs,
if RTS frames are sent from multiple terminals at the sametime. Therefore, in order to avoid this issue, multi channels
are utilized in the developed system.
Figure 4 shows an example for two kinds of interference in multi-hop network system with multi channels. For
Table 1Item
WLAN
Ethernet
CPU
OS
RAM
ChassisWeightFig. 1

Fig. 2

Specification of developed AP.

Specifications
Number of WLAN device: 4
WLAN IEEE802.11b/g (2.4 GHz band)
Number of interfaces: 1
10/100Base-T
SH4Linux Kernel 2.6
DDR-SDRAM: 64 MByte
Aluminum 230×210×60mm
500g

Fig. 3

Hidden terminal problem.

Fig. 4

Two kinds of interferences.

Configuration of Ad-Hoc network system.

Configuration of multiple APs with multi channels.

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simplification, we assume that the developed AP uses thechannel 1 (CH-1) and channel 2 (CH-2) for receiving andtransmitting signals, respectively, in Fig. 4. As a first interference, the interference due to hidden terminal problem isconsidered. As shown in Fig. 4, when the AP3 tries to transmit the signals to the AP1 with CH-2, the interference fromthe AP3, which uses the same frequency channel with thedeveloped AP, also arrives at the AP2. The hidden terminal
problem occurs as shown in Fig. 3, because the AP2 receives
the signal from the AP3 while the developed AP cannot recognize the AP3 due to the obstacle such as buildings and outof range on the carrier sense as shown in Figs. 3 and 4.
The second problem is that the interference due to aleakage power among devices, which are implemented inside same aluminum chassis. As shown in Fig. 4, the developed AP tries to communicate with AP1 and AP2 at thesame time using CH-1 and CH-2, respectively. Even if theadjacent channels are used among WLAN devices, the interference from CH-2 arrives at the spurious on the CH-1,
because the received power between the transmitter and receiver at the developed AP is much higher than the receiverat the developed AP from the AP1. It is reported that the mutual interference among adjacent channels is a big problem[12], when WLAN devices are mounted in adjacent spaceand each device use neighbor channels.
In this paper, we propose two techniques in order tosolve two kinds of interference problems. One of the proposed techniques is the interference suppression using thenull direction of the radiation pattern of the dipole antenna.
The proposed antenna can reduce the mutual interference
inside the developed AP by arranging dipole antenna vertically since this direction corresponds to the null directionof the radiation pattern. Second proposal is relating to distributed channel allocation scheme. The second proposal
recognizes simple network topology and allocates channels

Fig. 5most suitable for Ad-Hoc network, in order to avoid the interference due to the hidden terminal problem. Moreover,the proposed two techniques are implemented in the developed AP. We introduce the testbed and clarify the eﬀectiveness of the testbed.
In the conventional technology implemented on testbed[20], the destination AP was specified by using the directional antennas, and the routing paths were constructed. In
addition, the channel allocation scheme uses an algorithm
that considers the directional antenna. For the feature of proposed techniques in this paper, multi-antennas are also used
to relay communication, but the omni directional antennas
are adopted instead of directional antennas. These omni directional antennas are utilized to decrease the interference
between antennas for two combined APs. Furthermore, acomplex algorithm is not required even when the relay APs
are are simply arranged without taking care of the interference, and the wireless relay systems/Ad-Hoc systems can beachieved simply. Therefore, the proposed channel allocation
is suitable for APs with omni directional antennas.
3.

Proposed Antenna Configuration

3.1 Principle
In this section, we describe the requirement of AP antenna
and propose a novel antenna configuration which can eﬃciently suppress the mutual interference inside the developed AP. The objective of this study is developing the AP
system suitable for the Ad-Hoc relay system. For thissystem, the antenna pattern in the horizontal plane shouldbe omni-directional because the relay station must transmit/receive signal to/from all of directions. Another requirement is that the antenna must suppress the interference
among jointly placed APs while allowing APs to receive sig-

Photo of developed AP.

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Fig. 7

Fig. 6 Proposed antenna configuration using vertically deployed dipole
antennas.

nals from separated APs.
Figure 5 is the photograph of the developed AP. Fourantennas are configured inside of a cylinder. The verticallyarranged dipole array is introduced to the AP system while
the horizontally spaced dipole array is equipped in Fig. 3.
The other functions are designed based on the concept
shown in Fig. 3. Four frequency channels on IEEE802.11b/g(1, 6, 11, 14 ch) are assigned on the testbed. Figure 6 reveals
the antenna geometry of the proposed configuration in detail. The distance between neighboring antennas is 20 cm,
and the total length of the array antenna is 80 cm. Four vertically polarized dipole antennas are arranged along the null
direction of their radiation pattern. This arrangement provides high isolation among the antennas even when they areplaced closely each other since no radiation power is delivered to neighboring antennas.
Other important feature of this antenna is that all antennas have omni-directional patterns in the horizontal plane.
This property satisfies the requirement that AP must be able
to communicate with other separated APs. All antennas are
printed dipoles and configured on the common substrate.
Each printed dipole has an independent feedline, which isconnected to sole AP module inside the chassis. Conventional array antenna using dipole antenna in the verticalplane is called as co-linear array. However, those ports
are connected with one another in order to receive/transmit
signals on the conventional co-linear array, because the colinear array is used in order to obtain the higher antenna gain
compared to the single dipole antenna. On the other hand,each dipole antenna in the proposed system is used for themutual interference suppression among antennas in the vertical plane. This configuration achieves high interference
suppression between adjacent channels and enables arbitrary AP in the system to transmit the signals while the otherone is receiving the signal from another STA. As describedabove, this antenna configuration satisfies both two requirements: 1) to be able to suppress the interference among thejointly placed APs, and 2) to be able to communicate withother separated APs.

Radiation pattern of dipole antennas on vertical plane.

Adaptive array antenna is proposed to reduce the cochannel interference [16]. The adaptive array supresses theinterference by using antenna arrays and signal processing
techniques. Moreover, the interference cancellation techniques are proposed using the array antenna which is arranged in the vertical plane [17], [18]. However, as mentioned above, the proposed antenna configuration utilizes
the feature on the antenna pattern which is arranged in thevertical plane for each antenna: There is no signal processing unlike the adaptive array. Figure 7 shows the comparison between the calculated and measured antenna patterns
in the vertical plane. The radiation pattern F of the halfwavelength dipole antenna in the vertical plane is shown as
follows: cos π cos(θ) 2F(θ) = (1)sin(θ)
where θ denotes the angle in Fig. 7. As can be seen in Fig. 7,
the measured radiation pattern agrees with the calculated
pattern well. Moreover, we found that, the null over −30 dB
can be created for the directions on the adjacent antennas.
3.2 Eﬀectiveness of Proposed Antenna Configuration
We confirm the eﬀectiveness of proposed antenna configuration by comparing the conventional array antenna whose
antenna is arranged in the horizontal plane. Figure 8 shows
evaluation scheme. APB is the AP on conventional or proposed system and APA and APC are APs which are used
for this evaluation. The measurements are carried out inan anechoic chamber, because the eﬀect on sole antenna isevaluated. The signal (1 ch) from APA is received at APB
and the signal (14 ch) is transmitted from APB to APC . For
the proposed configuration, the element spacing, d is fixed
to be 20 cm and vertically arranged dipole array as shownin Fig. 6 is used. The element spacing, d is changed from
20 to 200 cm in the conventional configuration (horizontallyarranged dipole array).
Figure 9 shows the measurement results of the averagethroughput versus the element spacing, d. As can be seen in
Fig. 9, the throughput without the interference (6.7 Mbps) isachieved when the proposed antenna configuration is used.

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On the other hand, d must be separated 120 cm in the conventional antenna arrangement.
We focus on the mutual coupling level between two
antennas in Fig. 10. As can be seen in Fig. 10, the array antenna with the vertical arrangement realizes the mutual coupling reduction of over 20 dB compared to the array antenna

Fig. 8Configurations for evaluation.

Fig. 9Fig. 10

Throughput of UDP flow.

Mutual coupling between antennas.

with the horizontal arrangement. Hence, we found that theantenna configuration with vertical antenna arrangement iseﬀective in reducing the mutual interference power with fair
array antenna size.
4.

Channel Allocation Scheme

It is essential to mitigate the interference from hidden APs
in a multi-channel configuration as it is in a single-channel
configuration. Channel-allocation schemes play an important role in mitigating hidden APs. Although there havebeen numerous studies on channel allocation [19], actual
performance has not been evaluated with an actual testbed
in any of these.
The hidden terminal problem was not evaluated suﬃciently in [19], because they did not properly describe how
the network topology was configured. However, because of
the eﬀect of the antennas we propose, our new ad-hoc network system can disregard interference between antennas
and it simply allocates the best channel. Moreover, since
the purpose of the proposed channel-allocation scheme is to
avoid the hidden-terminal problem, the purpose of this paper is completely diﬀerent from the work in [19]. Finally,
while we understand the importance of a channel-allocation
scheme for routing that suits all applications and maintains
the quality of links (transmission rate and delay), studies onthese issues will remain to be done in future work because
their purpose regarding these issues was diﬀerent from that
dealt in this paper.
It is important to implement a channel-allocation
scheme on a testbed and clarify how eﬀective the implemented scheme is to confirm its behavior. Since the ad-hoc
network is built in autonomously in a distributed manner, we
focused on an autonomous-distributed scheme for allocating
channels that used localized information.
There is a brief overview of the channel-allocation
scheme we developed in Fig. 11. As can be seen from thisfigure, we assumed that the network consisted of nine APsthat were deployed in a grid pattern. In addition, the rangeof communications for each AP reaches adjacent APs. The
point of this scheme was to allocate channels by taking into

Fig. 11

Concept underlying method of channel allocation.

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consideration the eﬀect from interfering links† . For example, when the channel on link 1 (L1) is allocated as shownin Fig. 11, the channels on its links (L5, L7, L8, and L9) interfering with L1 have to be taken into account. That is, aninterfering link means a link of hidden APs (AP6, AP7 andAP8) interfering with APs (AP1 and AP2) on the target link.
Basically, the target link (L1) is allocated a channel that is
diﬀerent from the channel of its interfering links (L5, L7,
L8, and L9).
The channel-allocation scheme we developed involves
four operrations:(1) Detection of neighboring APs.
(2) Recognition of network topology.
(3) Selection of AP for establishing links.
(4) Algorithm for allocating channels.
Each AP has a unique ID (APID) to autonomously allocate
channels and they exchange information concerning channels that are allocated. When an AP is initially deployed, it
detects its neighboring APs. After the AP has collected theinformation for allocating channels from neighboring APs,
it recognizes the network topology around itself. Then, theAP selects an AP from APs to establish a link. Finally,
the AP allocates a channel for the link with the channelallocation algorithm, which uses the information from interfering links. As stated above, the proposed scheme is distributed autonomously and is scalable because the scheme
only uses local information around a newly deployed AP.
We developed a channel allocation module operated in the
MAC layer to accomplish a channel-allocation scheme inactual devices. We used the MAC layer functions provided
by the software design kit (SDK) to establish the number ofchannels for all WLAN devices.
4.1 Neighboring AP Detection
A newly deployed AP broadcasts an AP information request
on a common channel to detect its neighboring APs. APsthat receive the AP information request reply with their AP
information response. The newly deployed AP recognizes
its neighboring APs after it receives their AP information
responses. An AP information request consists of a source
APID. An AP information response consists of a destination
APID, a source APID, AP information (neighboring APIDs,
channel number, and number of hops from the gateway), andneighboring AP information (APID and the channel numbers for all WLAN devices). In this way, the newly deployed
AP collects the local information around itself.
4.2 Network Topology Recognition
After the newly deployed AP has collected the AP information responses from its neighboring APs, it recognizes the
network topology so that it can allocate channels. For example, a newly deployed AP (AP1) detects the AP2 in Fig. 11.
To allocate a channel on L1, AP1 has to recognize channels with interfering links (L5, L7, L8, and L9). As seenin Fig. 12, AP1 recognizes the network topology from thechannel numbers and links in that information by receiving
AP information responses from AP2 and AP4.
More specifically, after AP1 has received an AP information response from AP2, it obtains information thatis from AP2’s neighboring APs (AP3 and AP5), channel
numbers connected to neighboring APs, and channel numbers that are used in neighboring APs. Then, after AP1
has received an AP information response from AP4, it obtains information that is from AP4’s neighboring APs (AP5and AP7), channel numbers connected to neighboring APs,and channel numbers that are used in neighboring APs. Finally, AP1 recognizes the network topology around itself, as
shown in Fig. 13. This channel-allocation scheme only uses
a localized network topology and does not use the entire network topology.
4.3 Selection of AP for Establishing Link
The newly deployed AP recognizes the number of hops fromthe gateway to all neighboring APs by receiving an AP information response from its neighboring APs. It then selects an
AP from the APs to establish a link with two rules. 1) The
received signal strength indicator (RSSI) is greater than apre-defined value and 2) the smallest number of hops fromthe gateway. A neighboring AP satisfying these two conditions is selected. In other words, these rules mean the link isstable and it has the shortest path to the gateway. RSSI in the
MAC frame is aggregated (desired and undesired) signals.
4.4 Channel-Allocation Algorithm
The newly deployed AP recognizes the network topology
around itself and it selects an AP from neighboring APs toestablish the link according to the procedure described in theprevious subsections. Finally, a channel on the link is allocated. The policy in this algorithm is to mitigate interference
from hidden APs. For the sake of simplicity, this subsection
explains how a newly deployed AP establishes links for allof its neighboring APs without taking into consideration thenumber of hops from the gateway. By recognizing the network topology described in Sect. 4.2, the newly deployed
AP knows the channels on interfering links. If more than
one available channel diﬀers from the one that is present, thenewly deployed AP selects one of the available channels.
However, if no available channel is present, the newly
deployed AP selects the channel with the smallest RSSI.
Let us more specifically explain the channel-allocation algorithm using the target link (L1) in Fig. 13. The available
channels are initially CH1, CH2, CH3, and CH4. After the
newly deployed AP (AP1) receives an AP information response, it knows CH2, CH3, and CH4 are used in interfering links. Here, since the available channel is CH1, which is
diﬀerent from the channels on interfering links (CH2, CH3,†Interfering links mean those between hidden APs and theneighboring APs of APs on a target link.

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Fig. 12

Example of AP information response.

Fig. 14

Fig. 15

Fig. 13

Recognized network topology at AP1.

and CH4), AP1 allocates CH1 to L1, which connects toAP2.
4.5 Evaluation of Performance with Channel Allocation
How eﬀective the testbed was with the proposed schemes
is clarified in this subsection by comparing throughput between single and multi-channels using WLAN signals. Figure 14 shows the configuration for the measurement system.
We employed the setup for allocating channels and measuring throughput using the five APs in Fig. 14. The standard
for the WLAN was IEEE802.11g and the common channel

Experimental configuration.

Experimental results for channel allocation.

was 13 ch. The available channels were set to be 1, 6, and11 ch. The gateway was the AP5 in Fig. 14.
Figure 15 denotes the channels for each link that wereassigned by the proposed channel-allocation scheme using
the configuration in Fig. 14. As can be seen in Figs. 14 and
15, Links 3 and 4 use diﬀerent channels from Links 1 and2. Here, the interference in Links 1 and 2 is Links 3 and
4. A simple network configuration of only five APs wasestablished without STAs in this experiment to clarify thebasic characteristics of the proposed antenna configurationand channel-allocation scheme. This experimental configuration is diﬀerent from that in Fig. 13 and is a very simple 1-dimensional model. However, the main point in thechannel-allocation scheme is that APs select diﬀerent channels in neighboring links to establish a link. Therefore, theIEICE TRANS. COMMUN., VOL.E94–B, NO.3 MARCH 2011

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5.

Fig. 16

Experimental results for channel allocation.

proposed scheme will still be eﬀective even if it is carriedout with the configuration in Fig. 13.
Next, we measured the UDP throughput for configurations of single and multi-channels using Netperf [21]. The
physical distance between APs was set to 10 m and thesending pattern for UDP packets was set to CBR. The offered traﬃc was set to 40 Mb/s to fill the channel capacityof IEEE 802.11g (physical data rate of 54 Mb/s). The UDP
packets were 1472 bytes. We measured the UDP throughput for the links of AP1-AP2 (1 hop), AP1-AP3 (2 hops),
AP1-AP4 (3 hops), and AP1-AP5 (4 hops). Figure 16 denotes the normalized UDP throughput versus the number ofhops. We set 1 as the value of throughput for the link from
AP1 to AP2. As can be seen in this figure, the normalized
throughput is 1/N when the number of hops is N for the
single-channel configuration.
The normalized throughput, on the other hand, is muchgreater than 1/N when the number of hops is N for the multichannel configuration. Hence, we found that the multichannel configuration achieved higher throughput than thatfor the single channel. The isolation between antennas, theprocessing performance of the test bed, and the transfer delay in multi-hops were assumed to cause the slightly degraded throughput for 2 and 3 hops. For example, the influence of interference cannot completely be removed fromthe proposed antenna configuration, because slight mutual
coupling (from about −40 to −50 dB) between antennas occurred, as shown in Fig. 10. Although the throughput for
the 4th hop is low for the multi-channel configuration due
to interference between Links 1 and 4 in Fig. 16, we consider such problems can be solved by combining the use of
5 GHz band in IEEE802.11a. A basic experimental evaluation was employed by using a simple network to clarify the
key characteristics of both the proposed configuration forantennas and allocation of channels that is implemented inan actual device. The experimental results from our ad-hoc
system suggested that reliable operation could be obtained.
However, more detailed evaluations are required when the
proposed system in this paper is replaced in practice forlarge-scale network services. Hence, we intend to simulatea large-scale model in future work.

Conclusion

We proposed an ad-hoc relay transmission system withmultiple-relay APs and channels to avoid the eﬀects of interference due to the hidden-terminal problem and multiple APs inside a single piece of equipment. Moreover, wedeveloped a testbed to evaluate the schemes we propose.
The new system has two main features: 1) the suppressionof interference from adjacent channels by using antennaradiation patterns with a vertical arrangement of antennas
and 2) a distributed scheme for channel allocation which
recognizes simple network topologies and allocates channels that are the most suited to the ad-hoc network.
We clarified how eﬀective the proposed scheme was byusing a testbed we developed. We confirmed that 1) an antenna configuration with a vertical antenna array could suﬃciently reduce interference among APs inside a single pieceof equipment, and 2) the proposed distributed allocation ofchannels could automatically avoid channels with interfering links due to the hidden-terminal problem. Moreover,
it is confirmed that transmission was eﬀective using multichannels by measuring UDP throughput to compare it to thatusing a single channel.
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Appendix:

Time Spent in Allocating Channels

The appendix explains the time spent by the system we developed in allocating channels. To evaluate the analyticaltime in the environment in Fig. 14, let the transmission time
for an AP information request be Trequest and an AP information response be Tresponse, which are including overhead. For simplicity, we will evaluate the analytical time for
1 hop, 2 hops, 3 hops, and 4 hops. For example, AP4 andAP5 in Fig. 14 are only present for 1 hop, and AP3, AP4,and AP5 are only present for 2 hops. For 1 hop, since AP4wants to allocate a channel to communicate with the gateway (AP5), AP4 broadcasts an AP information request andthen AP5 transmits an AP information response. AP4 canallocate the channel through this frame exchange. Hence,
the analytical time is Trequest + Tresponse for 1 hop. Since
AP selects the links by the number of hops from the gateway
and allocates a channel on the links as stated in Sects. 4.3
and 4.4, AP3 allocates the channel after AP4 has finishedallocating the channel for 2 hops. After AP4 has broadcast an AP information request, AP3 and AP5 transmit an
AP information response. Considering the analytical time
for AP3, Trequest + Tresponse, the analytical time for 2
hops is Trequest + 2×Tresponse + Trequest + Tresponse =2×Trequest + 3×Tresponse. The analytical time in the otherhops is calculated similarly. The analytical times are 1.6 ms
for 1 hop, 4.3 ms for 2 hops, 6.9 ms for 3 hops, and 9.6 ms
for 4 hops. For 4 hops, the AP1 farthest from the gateway
takes the longest to finish operation, because information
concerning the gateway is recognized last.

Kentaro Nishimori
received the B.E., M.E.
and Dr.Eng. degrees in electrical and computerengineering form Nagoya Institute of Technology, Nagoya, Japan in 1994, 1996 and 2002, respectively. In 1996, he joined the NTT Wireless Systems Laboratories, Nippon Telegraphand Telephone Corporation (NTT), in Japan. He
was senior research engineer on NTT Network
Innovation Laboratories. He is now associate
professor in Niigata University. He was a visiting researcher at the Center for Teleinfrastructure (CTIF), Aalborg University, Aalborg, Denmark in 2006. He was an
editor for the Transactions on Communications for the IEICE Communications Society and Assistant Secretary of Technical Committee on Antennas
and Propagation of IEICE. He received the Young Engineers Award fromthe IEICE of Japan in 2001, Young Engineer Award from IEEE AP-S Japan
Chapter in 2001, Best Paper Award of Software Radio Society in 2007 andDistinguished Service Award from the IEICE Communications Society in2005 and 2008. His current research interest is Multi-user MIMO systems
and cognitive radio systems. He is a member of IEEE.

Takefumi Hiraguri
received the M.E. and
Ph.D. degrees from the University of Tsukuba,
Ibaraki, Japan, in 1999 and 2008, respectively.
In 1999, he joined the NTT Access Network
Service Systems Laboratories, Nippon Telegraph and Telephone (NTT) Corporation. He is
now associate professor in Nippon Institute ofTechnology. He has been engaged in researchand development of high speed and high communication quality wireless LANs systems. He
is a member of IEEE.

Masakatsu Ogawa
received the B.E.,
M.E. and Ph.D. degrees from Sophia University, Tokyo, Japan, in 1998, 2000 and 2003,
respectively. He was an adjunct member at
National Institute of Informatics from 2003 to
2004, and a visiting researcher at Sophia University from 2003 to 2005. In 2004, he joined NTT
Access Network Service Systems Laboratories,
NTT Corporation. From 2004 to 2009, he wasengaged in research and development of highspeed wireless LANs. In 2009, he moved to theTechnical Assistance and Support Center, NTT East Corporation. He received the IEICE Young Researcher’s Award in 2007, and the IEICE Communications Society Distinguished Contributions Award in 2005, 2008 and2009. He is a member of IEEE.

Naoki Honma
received the B.E., M.E., andPh.D. degrees in electrical engineering from Tohoku University, Sendai, Japan in 1996, 1998,
and 2005, respectively. In 1998, he joined the
NTT Radio Communication Systems Laboratories, Nippon Telegraph and Telephone Corporation (NTT), in Japan. He is now associate professor in Iwate University. He received
the Young Engineers Award from the IEICE ofJapan in 2003, the APMC Best Paper Award in
2003, and the Best Paper Award of IEICE Communication Society in 2006, respectively. His current research interest isplanar antennas for high-speed wireless communication systems. He is amember of IEEE.