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PAPER

Special Section on Implementation, Experiments, and Practice for Ad Hoc and Mesh Networks

A Novel Interference Avoidance Technique on Mobile WirelessRouters Using IEEE802.11n PSMP
Akira KISHIDA† , Takefumi HIRAGURI†† , Masakatsu OGAWA††† , Members,
Kentaro NISHIMORI†††† , Senior Member, Naoki HONMA††††† , and Tetsu SAKATA† , Members

SUMMARY
This paper proposes an interference avoidance techniquethat allows wireless device with similar frequency bands to be operated adjacent to each other for compact mobile wireless routers (MWRs). This
MWR implements two devices of Wireless LAN (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX). The MWR connects WLAN terminals to the backbone network by using WiMAX-WLAN
relay. Generally, different frequency channels are assigned for the wirelesssystems assign in order not to interfere among multiple systems. However,mutual system interference is generated if the space between each device isvery close and if the frequency using each system is adjacent. To suppress
this interference, this paper proposes a novel interference avoidance technique that leverages IEEE802.11n Power Save Multi-Poll (PSMP). First,
we clarify the conditions that raise the issues of mutual interference by experiment. Simulations are conducted to show that the proposed scheme
outperforms the conventional schemes. Finally, the effectiveness of the
proposed scheme is shown by the computer simulation.
key words: IEEE802.11n, IEEE802.16e, mutual system interference,
PSMP, wireless relay

1.

Introduction

Recently, wireless LANs that comply with IEEE802.11
standard [1] are widely implemented in various mobile terminals not only laptop PCs but also small multimedia devices such as music players, gaming machines, and smart
phones. Demand has been growing that these mobile terminals be usable everywhere. However, WLAN service areas
are restricted to WLAN hotspots in public areas. In order tosolve this problem, mobile wireless routers (MWRs), which
can connect WLAN terminals to the backbone network byusing other wireless system with wide communication area,
are a hot research topic [2]–[4]. On the other hand, Worldwide Interoperability for Microwave Access (WiMAX) thatcomplies with IEEE802.16 [5] has received a lot of attention
due to its potential in terms of wide communication area andhigh-speed data transmission. In particular, mobile WiMAX
based on IEEE802.16e [6] is a very attractive system for moManuscript received December 9, 2009.
Manuscript revised March 17, 2010.
†
The authors are with NTT Access Service Systems Laboratories, NTT Corporation, Yokosuka-shi, 239-0847 Japan.
††
The author is with Faculty of Engineering, Nippon Instituteof Technology, Saitama-ken, 345-8501 Japan.
†††
The author is with NTT East Corporation, Tokyo, 141-0022
Japan.
††††
The author is with Niigata University, Niigata-shi, 950-2181
Japan.
†††††
The author is with Iwate University, Morioka-shi, 020-8551
Japan.
DOI: 10.1587/transcom.E93.B.2053

bile use, because mobile WiMAX provides the high speedtransmission for mobile users.
The MWR has the functions of WLAN Access Point(AP) and of WiMAX Mobile Station (MS). The WLAN
Stations (STAs) are connected to the AP in the MWR via
an 802.11 link. Hence, the MWR converts media between
IEEE802.11 and 802.16e. This fact enables the WLAN
STAs to connect to the backbone IP network. However,the chassis of the MWR must be downsized because it isassumed to be carried in outdoor. Therefore WLAN and
WiMAX devices are required to be installed next to eachother in these MWRs. Moreover, the assigned frequency
bands of WLAN and WiMAX are 2.4 and 2.5 GHz, respectively. This situation triggers mutual system interference between the two wireless devices and system throughput of thewireless relay is greatly degraded [7].
There are a lot of prior studies concerning MWRs that
use WLANs and WiMAX [8]–[11], however, they concentrated on handover between WLAN and WiMAX. On
the other hand, prior studies [12]–[14] proposed a resource
management scheme for a hybrid network that bridged
WLAN and WiMAX. However, the problem concerning
mutual system interference was not considered. Although
the studies [15], [16] analyzed the effect of mutual systeminterference in a heterogeneous network of WLAN, Bluetooth and/or WiMAX in detail, no interference avoidancetechnique was introduced. There are a lot of schemes thatcan suppress interference in the PHY layer [17]; however,
hardware modification of existing WLAN terminals is required and signal processing is complex when using the interference suppression.
This paper proposes a novel interference avoidancetechnique that achieves a simple implementation using only
MAC protocol. The proposed scheme synchronizes thetransmission and the reception timing of WiMAX andWLAN and avoids the case where interference is generated
in the MWR. Mutual system interference is generated when
WiMAX device of WAN transmits signals while WLAN device of LAN receives signals. In the case when WLAN
transmits signals while WiMAX receives signals, mutual
system interference is generated in the same way. Ordinarily, it is difficult to synchronize transmission and reception timing of WLAN with WiMAX because Carrier-Sense
Multiple Access (CSMA) adapted to IEEE802.11 WLAN isbased on the random access procedure. On the other hand,
Power Save Multi-Poll (PSMP) is regulated in order to savec 2010 The Institute of Electronics, Information and Communication Engineers
Copyright IEICE TRANS. COMMUN., VOL.E93–B, NO.8 AUGUST 2010

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power consumption of WLAN in IEEE802.11n [18]. PSMP
easily allows WLAN AP to control the transmission and reception timing of STAs. Hence, the proposed scheme enables interference avoidance between WLAN and WiMAX
by applying PSMP.
The remainder of this paper is organized as follows.
In Sect. 2, we describe the throughput degradation due toof mutual system interference using experimental equipment. The conventional schemes are introduced in Sect. 3.
Sect. 4 proposes the interference avoidance technique using
IEEE802.11n PSMP and we propose the channel allocation
scheme. The effectiveness of the proposed scheme when
considering actual WLAN and WiMAX scenario is verified
in Sect. 5.
2.

Throughput Degradation due to Mutual System InterferenceThis section describes the condition that the mutual systeminterference is generated and the experiment conducted toverify the issue raised by the mutual system interference between WLAN and WiMAX.
Figures 1 and 2 illustrates the concept of WLANWiMAX relay via the MWR and simplified configuration of
the MWR respectively. MWRs are expected to implement
wireless devices into a size that is smaller than the businesscard. Therefore, the devices of WLAN and WiMAX are
located at adjacent spaces, respectively. In addition to thecondition, the emission of the spectrum mask leaks outside
the band and caused mutual system interference when thefrequency band of WLAN and WiMAX is adjacent. Figure 3 shows this problem conceptually. Here, we assume
that the issue of mutual system interference generated in thecircuit in the MWR is another problem for the future work.
In this paper, we concentrate on the issue of mutual systeminterference caused in the air.
To verify this problem practically, we conducted theexperiment using WLAN AP, STA, WiMAX base station(BS) and MS. Figure 4 illustrates the block diagram of mutual system interference of the experiment. The experimentfocuses on the throughputs of WLAN and WiMAX.
In order to precisely observe the power degradation
caused by mutual system interference, we used wired links
with the same attenuators as wireless links. That is, the attenuators degrade signal power by an amount equal to thefree-space path loss yielded by the distance between wireless devices; fading is not considered in the experimental
system. The parameters of the experiment are shown in
Table 1. Besides, the standards of WLAN and WiMAX
comply with IEEE802.11g and IEEE 802.16e respectively.
The throughput characteristics of WiMAX and of WLAN
are shown in Figs. 5 and 6, respectively. Figure 5 indicatesthat the downlink throughput of WiMAX began to degrade
when the distance between WLAN AP and WiMAX MS
were 10 cm apart, the downlink throughput halved at 5 cm,
and connection lost at approximately 1 cm. Figure 7 illus-

Fig. 3Fig. 4
Fig. 1

The concept of mutual system interference.

Block diagram of mutual interference.

Concept of WLAN-WiMAX relay via the MWR.
Table 1

Fig. 2

Simplified MWR configuration.

Parameters of the experiment.

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Fig. 8Fig. 5

Fig. 6

Waveform of WLAN and WiMAX signals in uplink.

Uplink/Downlink throughput of WiMAX.

Uplink/Downlink throughput of WLAN.

5 cm. Figure 8 illustrates the waveform of desired WLAN
signals and interference WiMAX signals that observed at
1 cm distance. In the observation point, WLAN AP is receiving signals from WLAN STA while WiMAX MS istransmitting UL Burst. Point “B” in Fig. 4 is the probe pointof this waveform. As well as Fig. 7, Fig. 8 indicates that desired WLAN signal severely disturbed from the emission ofundesired WiMAX signal. Therefore, WiMAX interference
degrades WLAN throughput in both the uplink and downlink. The reason why not only WLAN UL throughput but
also WLAN DL throughput decreases is as follows;
When WLAN STA receives the signal correctly fromWLAN AP, WLAN STA casts an ACK (Acknowledgement)
frame to WLAN AP. This ACK frame is carried in a signal
sent in the WLAN UL direction. Therefore, this ACK frame
is affected by the mutual system interference from WiMAXUL signals. The throughput of WLAN DL decreases as a result so that WLAN AP, which was not able to receive ACK
frames properly resends, DL signals to WLAN STA. Moreover, WLAN AP transmission was halted due to the carrier
sense function which uses the CSMA algorithm.
3.

Conventional Operation for MWR and Interference
Avoidance Scheme and Issues

This section introduces conventional operation and interference avoidance scheme for MWRs and their issues. MAC
protocol procedures are detailed below.
Fig. 7

Waveform of WLAN and WiMAX signals in downlink.

trates the waveform of interference WLAN signals and desired WiMAX signals that observed at 1 cm distance. In
the observation point, WiMAX MS is receiving DL Burst
while WLAN AP is transmitting signals for WLAN STA.
Point “A” in Fig. 4 is the probe point of this waveform. From
Fig. 7, desired WiMAX signal is almost buried in the emission of undesired WLAN signal.
Therefore, it is clear that WLAN interference degrades
WiMAX throughput. Figure 6 also indicates that WLAN
uplink throughput began to degrade at the separation distance of 32 cm; a 70% throughput reduction occurred at

3.1 Conventional Operation for MWRs
The conventional operation is shown hereafter. WLAN and
WiMAX perform their normal transmission sequences independently. We define this scheme as Autonomous Distributed Control (ADC) as shown in Fig. 9.
In ADC, no synchronization between WLAN and
WiMAX is performed. Although the conventional scheme
using the ADC is simple, mutual system interference occurs if one of the wireless devices in the MWR is receiving signals while the other is transmitting signals. Particularly if mutual system interference occurs while receiving WiMAX control signals such as preamble, FCH (FrameIEICE TRANS. COMMUN., VOL.E93–B, NO.8 AUGUST 2010

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Fig. 9Conventional ADC operation.

Fig. 10

Conventional TDD operation.

Control Header), Downlink (DL)-MAP and Uplink (UL)MAP, throughput is severely degraded by the corruption ofentire WiMAX frames.
3.2 Conventional Interference Avoidance Scheme: TDD
To avoid mutual system interference, a scheme by which
WLAN and WiMAX can coexist, is being discussed in
WiMAX Forum [19]. That scheme leverages the sleep
mode function specified in IEEE802.16e. In the sleep mode,
WiMAX BS divides the transmission period into sleeping
period and listening period. Listening period is the time fornormal transmission, and the sleeping period halts all transmission to cut WiMAX MS power consumption. In thisscheme, WLAN AP executes its normal transmission during WiMAX sleeping period, on the other hand, all WLAN
devices refrain from transmission during the WiMAX listening period as shown in Fig. 10. Time Division Duplex(TDD) scheme prevents mutual system interference. However, it halves the throughput of each wireless system.
4.

Proposed Mutual System Interference Avoidance
SchemeThe proposed scheme avoids mutual system interference

without great reducing throughput. The proposed scheme
leverages PSMP function specified in IEEE 802.11n. PSMP
was designed for WLAN STA power saving; however weutilize PSMP for interference avoidance. First, the operation of PSMP is explained, and then the concept of the proposed scheme is introduced. Finally, the whole proposal isdescribed in detail.
Figure 11 depicts PSMP procedure realized byIEEE802.11n. In PSMP, WLAN AP broadcasts a PSMP
frame to all WLAN STAs. This PSMP frame contains information about the timing of transmission and of reception
for STAs. Besides, the PSMP frame indicates the length of
Downlink Phase and Uplink Phase. PSMP procedure enables AP to divide transmission period into Downlink Phaseand Uplink Phase.
In Downlink Phase, only WLAN AP can transmit signals to WLAN STAs and these STAs are prohibited fromsending any signals including ACK frames. On the other
hand, WLAN STAs can transmit during Uplink Phase. In
PSMP sequence, MAC Protocol Data Units (MPDUs) or
Aggregate MAC Protocol Data Units (A-MPDUs) are allowed to use. Moreover, they are delivered in the interval of
Short Interframe Space (SIFS) or Reduced Interframe Space(RIFS). On the other hand, WLAN STAs and AP send an
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Fig. 11

Fig. 12

PSMP Sequence.

Basic concept of the proposed method.

their transmission period (Downlink Phase for AP, Uplink
Phase for STAs) for data received during prior Downlink
Phase or Uplink Phase. A period that consists of Downlink
Phase and Uplink Phase is called a PSMP sequence. As aresult, WLAN STAs only receive or transmit signals duringassigned period which is indicated by PSMP frame. The
concept of the proposed scheme is to make the PSMP sequence as long as a WiMAX frame. That is, the proposed
scheme makes the length of WLAN Uplink Phase equal to
the length of WiMAX DL Subframe. The length of WLAN
Downlink Phase is made equal to the length of WiMAXUL Subframe as well. The basic operation of the proposed
scheme is shown in Fig. 12. Accordingly, the basic operation enables the MWR to transmit signals only in WLAN
Downlink Phase, and to receive signals only in WLAN Uplink Phase. That is, the MWR eliminates mutual systeminterference which realizes highly effective relay transmission.
On the other hand, in the Mobile WiMAX profile [20],the ratio of the number of OFDM symbols of DL to thatof UL has been decided. According to the profile, thelength of DL Subframe is longer than UL Subframe. In
Japan and many other countries, the dominant numbers of
OFDM Symbols in DL and in UL are set at 32 and 15, respectively. In this paper, the length of DL Subframe andUL Subframe complies with above system profile. Therefore, the proposed scheme is more effective than conventional ADC or TDD; however, system throughput might bedegraded by the difference in traffic between WiMAX and
WLAN. In other words, the transmitted WLAN traffic might
be less than WiMAX traffic because the transmission period of WLAN AP (WLAN Downlink Phase) is limited by
the WiMAX UL Subframe period as shown in Fig. 13. In
Fig. 13, the horizontal axis represents time, and the verticalaxis means actual transmission rate of each wireless system
including loss of overhead (for example, Inter Frame Space(IFS) for WLAN, Frame Control Header (FCH), DL-MAP
and UL-MAP for WiMAX).
Our solution is to add a scheduling scheme to the basic proposed scheme. The scheduling scheme modifies thelength of WLAN Downlink Phase in order to make the gapof the amount of traffic between WiMAX downlink andIEICE TRANS. COMMUN., VOL.E93–B, NO.8 AUGUST 2010

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Fig. 13

Key issue with the basic concept.

Fig. 15The scheduling method for uplink.

degradation, the minimum length of WLAN Uplink Phase
is introduced. Accordingly, the beginning of WLAN Downlink Phase is rescheduled to match the end of the minimum
length of WLAN Uplink Phase. Following this reasoning,the minimum length of WLAN Uplink Phase is decided so
as to match the amount of WLAN uplink traffic andWiMAX
uplink traffic as shown in Fig. 15. Hence, system throughput
is improved by this scheduling scheme. Analysis details arepresented in the Appendix.
As a consequence, this scheduling change improves thethroughput via the WLAN-WiMAX relay system.
5.
Fig. 14The scheduling method for downlink.

WLAN downlink minimum. First, the scheduler expands
the WLAN Downlink Phase to match the amount of WLAN
downlink traffic and WiMAX downlink traffic as shown in
Fig. 14. At this time, the MWR requests WiMAX BS not
to allocate WiMAX DL Burst for the MWR after the beginning of WLAN Downlink Phase using the message ofBandwidth Request Header. The reasons for this are that the
WiMAX DL Burst and the transmission in WLAN Downlink Phase after the period might trigger mutual system interference. Second, as well as the downlink scheduling, theoptimal length for WLAN Uplink Phase is decided. The
expansion of the WLAN Downlink Phase equals the reduction of the WLAN Uplink Phase because the total lengthof WiMAX frame is constant. Therefore, excessive expansion of the WLAN Downlink Phase reduces uplink trafficthat needs for WiMAX UL Burst. This causes the degradation of uplink throughput. To prevent this throughput

Performance of Proposed and Conventional Schemes

To clarify the performance of the proposed scheme, systemthroughputs of WiMAX BS and WLAN STA with the conventional and proposed schemes are evaluated by computer
simulations. As the simulation conditions, we assume that if
any packets (in the case of WLAN) or any OFDM symbols(in the case of WiMAX) suffer mutual system interference,
those packets or symbols should be discarded. Moreover,
we assume that there is no interference between the MWR
and WiMAX BS or WLAN STA. These assumptions arereasonable given that the MWRs and WiMAX BS or WLAN
STA will be very far apart compared to the distance between
WLAN AP and WiMAX MS in the MWR. Moreover, downlink and uplink traffic flows occur simultaneously.
WLAN parameters of conventional schemes and of
the proposed scheme comply with IEEE802.11g andIEEE802.11n legacy mode respectively. As well, WiMAX
parameters comply with IEEE802.16e-2005 standard. Under this arrangement, the offered traffic (uplink and downlink) represents the traffic generated from the transmission

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Fig. 16

Table 2The simulation model.

The simulation parameters.

Fig. 18 Uplink throughput characteristics in the proposed andconventional methods.

Fig. 17 Downlink throughput characteristics in the proposed andconventional methods.

edge. Figure 16 shows the simulation model; the simulation
parameters are given in Table 2.
The comparison of proposed and conventional schemesis performedunder the above-mentioned conditions.
Figures 17 and 18 shows the received downlink
throughput characteristics of WLAN STA and the received
uplink throughput characteristics of WiMAX. In Fig. 17,
ADC yields little throughput because most of data traffic arelost by the intermittent mutual system interference. TDD
limits the throughput to less than half the ideal value eventhough it prevents mutual system interference. In particular, uplink and downlink share the communication period
already halved by time division done with WiMAX. Therefore, TDD is able to obtain little throughput after the pointof 11 Mbit/s in the offered traffic. On the other hand, the proposed scheme attained 19.2 times throughput from ADC and2.6 times throughput from TDD at the point of 17 Mbit/s inthe offered traffic. Though the throughput characteristic of
the proposed scheme is saturated after the point of 11 Mbit/sin the offered traffic, the reason is there is no room to expand
WLAN Downlink Phase. In other words, optimal scheduling that WiMAX downlink traffic and WLAN downlink traffic is almost same has been achieved after that point.
In Fig. 18, the reason why little throughput has yieldedin ADC and TDD is the same as the case of downlink.
On the contrary, the proposed scheme obtained nearly
ideal throughput. The proposed scheme obtains 14.4 times
throughput from ADC and twice from TDD at the point of
10 Mbit/s in the offered traffic. Though the throughput characteristic of the proposed scheme and ideal is saturated after
the point of 4 Mbit/s in the offered traffic, the reason is quite
different from the uplink case. That is, because the boundary
is up to 4 Mbit/s of WiMAX uplink throughput, it becomes
impossible to exceed this value.
6.

Conclusion

This paper proposed a novel interference avoidance technique on MAC protocol. The proposed scheme synchronizes the transmission and the reception timing of WiMAX
and WLAN using IEEE802.11n PSMP and avoids mutual
system interference. In addition to this proposition, we proposed the scheduling scheme to eliminate the gap of traffic that improves throughput characteristic via the WiMAXWLAN relay system. F irst, we described that the impact ofmutual system interference between WLAN and WiMAX
units was clarified by the experiment. We showed that theconventional scheme of ADC allows mutual system interference to significantly degrade throughput, and the addi-

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tion of TDD, while preventing mutual system interference,
halves the throughput since it shares the communication period. On the other hand, the proposed scheme achieves
good throughput; it eliminates mutual system interferenceby careful scheduling without wasting the communication
period. The computer simulation confirms its good performance. The proposed scheme succeeded in obtaining about
20 times throughput compared with ADC and about 3 times
throughput compared with TDD.
This paper discussed the scenario of one MWR connected to WiMAX BS as an initial investigation. As future
work, we must extend the scheduling algorithm to improvethroughput performance when multiple MWRs are connected to WiMAX BS; of the goal is to ensure that the optimal WiMAX DL Burst allocation is achieved. We note thatthe time position of DL Burst for each MWR in WiMAX
frame may limit the length of WLAN Downlink phase, andthis limitation might degrade system performance. Even
without this extended algorithm, the proposed scheme ensures that mutual interference problem is suppressed. Thatis, the basic concept of the proposed scheme, which is tomake the PSMP sequence as long as the WiMAX frame, iseffective.
Acknowledgement
We are deeply grateful to Mr. Kengo Nagata and Mr. Makoto
Umeuchi whose enormous support and insightful comments
were invaluable during the course of our study.
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Appendix:

Calculation for the Optimal WLAN Downlink Phase

Details of the theoretical calculation used in Sect. 4 of the
proposed scheme are explained here. Figure A· 1 illustrates
how to calculate for the optimal WLAN Downlink Phase.
First, on the basic proposed scheme, we define theamount of traffic that received at WiMAX MS as C x in a

Fig. A· 1

Calculation for the optimal WLAN Downlink Phase.

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WiMAX frame. As well, we also define the amount of traffic that transmitted at WLAN AP as Cw in a WiMAX frame.
Then traffic difference Cex shown as (A· 1) will not be transferred to WLAN STAs.
(A· 1)

Cex = C x − Cw

In addition, the number of all subcarriers that WiMAXDL Burst uses is s, the number of OFDM symbols that DL
Burst allocated to the MWR is L, and modulation bit number
is m. C x is represented as (A· 2).
(A· 2)

Cx = s × L × m × e

Variable e is a coefficient of proportion of data payload
in a DL Burst. We assume that the allocation of DL Burst
is to use all subchannels, and to arrange in the time direction for simplification. We define that the number of WLAN
packets that can be transmitted in WLAN Downlink Phaseis nw . We assume that the Ethernet frame in 1500 bytes ispayload of WLAN packet hereafter. Payload of the Ethernet
frame that doesn’t come up to 1500 bytes is assumed to be aas well. Therefore, Cw is represented as (A· 3)
Cw = 1500 × 8 × nw + a × 8(A· 3)

Therefore, nw is given as follows.
nw = floor [tDP − tO ] /tPtO(a) = tPS MP + tACK + tPDU(a) + tIFS
tP = tPDU(1500) + tIFS

(A· 4)(A· 5)(A· 6)The variables tDP and tPDU represent the length of the
WLAN Downlink Phase before the scheduling and PDU
length, respectively. tPS MP , tACK , and tIFS represents eachthe length of PSMP frame, ACK, and IFS length.
Second, for our expanded scheduling scheme, we define the expanded length of WLAN Downlink Phase as t1 .
The period t1 is led from the following calculations. We assume that the number of all OFDM symbols in DL Burst
is Lmax , and the length of an OFDM symbol is l. Therefore, the amount of traffic that WiMAX MS receives after
the scheduling in a WiMAX DL Subframe is given by as C x
shown below.
 t 
1
C x =C x − s × (Lmax −L)+ceil
×m×e(A· 7)l
The amount of traffic that WLAN AP transmits after thescheduling is given by Cw .
(A· 8)
Cw = 1500 × nw + a × 8


As well as before the scheduling, nw is the number ofpackets that have 1500 byte payload of Ethernet frames in
WLAN Downlink Phase after the scheduling, and a represents the payload of Ethernet frame whose size is less than1500 bytes. Therefore, nw is given as follows.
nw = floor [(t1 + tDP ) − tO ] /tPtO(a ) = tPS MP + tACK + tPDU(a ) + tIFS

(A· 9)(A· 10)(A· 11)tP = tPDU(1500) + tIFSThe scheduling scheme determines t1 for which C x = Cw .
Therefore, t1 is given by (A· 12).

s·m·e·(2L−Lmax )−1500·8· tDP −tO(a ) /t p
(A· 12)t1 =s · m · e/l + 1500 · 8/t p
Finally, the final expanded length of WLAN DownlinkPhase t1 is decided. To calculate t1 , the minimum lengthof WLAN Uplink Phase t2 is decided as follows. We define
the amount of traffic that transmitted from WiMAX MS as
G x in a WiMAX frame. The number of all subcarriers that
WiMAX UL Burst uses is u, the number of OFDM symbols
that UL Burst allocated to the MWR is J, and modulation
bit number is i. G x is represented as (A· 13).
Gx = u × J × i × e(A· 13)Here, the quotient to divide G x by 1500 bytes is assumed
to nu and the surplus is assumed to be c. Therefore, t2 isrepresented as follows.
t2 = tP × nu + tR(c)tR(c) = tACK + tPDU(C) + tIFS

(A· 14)(A· 15)

We assume that tUD represent the length of the WLAN Uplink Phase before the scheduling. Accordingly, the final ex
panded length of WLAN Downlink Phase t1 is decided as(A· 16).
t1 =tUD − t2
t1

(t2 < tUD − t1 )(t2 ≥ tUD − t1 )(A· 16)

Akira Kishidareceived his B.S. and M.S.
degrees from Tokyo University of Agriculture
and Technology, Tokyo, Japan, in 2005 and2007, respectively. He joined NTT Access Service Systems Laboratories in 2007. He is engaged in the research of interference avoidancetechnique on MAC protocol for multi-wirelesssystems such as between Wireless LAN and
WiMAX. His current interests are high efficient interference avoidance technique on unified MAC and PHY.

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 Corporation (NTT) in
Japan. He has been engaged in research and development of high speed and high communication quality wireless LANs systems. He is now
associate professor in Nippon Institute of Technology (NIT). He is a member of IEEE.

IEICE TRANS. COMMUN., VOL.E93–B, NO.8 AUGUST 2010

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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.

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.

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.

Tetsu Sakata
received the B.E. and M.E.
degrees from Kyoto Institute of Technology,
Kyoto, Japan, in 1986 and 1988, respectively.
He joined NTT Radio Communication Systems
Laboratories in 1988. Since then he has been
engaged in research and development of wireless LAN systems, digital modems, transmission power control system, and digital LSIs forsatellite communications and personal communications. He is currently a Senior Research Engineer of Wireless Access Systems Project, NTT
Access Network Service Systems Laboratories. He is a member of IEEE.