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

658

PAPER

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

Power Saving Control Method for Battery-Powered Portable
Wireless LAN Access Points in an Overlapping BSS Environment
Masakatsu OGAWA†a) , Takefumi HIRAGURI†† , Members, Kentaro NISHIMORI††† , Senior Member,
Naoki HONMA†††† , Kazuhiro TAKAYA† , and Kazuo MURAKAWA† , Members

SUMMARY
This paper proposes a power saving control method forbattery-powered portable wireless LAN (WLAN) access points (APs) in an
overlapping basic service set (OBSS) environment. The IEEE802.11 standard does not support power saving control for APs. Some conventional
power saving control methods for APs have been proposed that use the
network allocation vector (NAV) to inhibit transmission at stations (STAs)
while the AP is sleeping. However, since with these approaches the actual beacon interval in the OBSS environment may be extended due to the
NAV as compared to the beacon interval which is set at the AP, the power
consumption and delay may be increased as compared to a single BSS unaﬀected by interference from neighboring APs. To overcome this problem,
this paper introduces a new action frame named power saving access point(PSAP) action frame which the AP uses to inform STAs within its BSS
about the AP’s sleep length. In addition, a function of the PSAP action
frame is that STAs enter the sleep state after receiving the PSAP action
frame. The proposed control method avoids the postponement of beacon
transmission and reduces the power consumption in an OBSS environment,
as compared to the conventional control method. Numerical analysis andcomputer simulation reveal that the newly proposed control method conserves power as compared to the conventional control method. The proposed control method achieves the minimum consumed power ratio at theAP, which is 44% as compared to the standard, when the beacon interval
is 100 ms and the sleep length is 60 ms, even if the number of neighboringAPs in an OBSS environment is increased.
key words: IEEE802.11, power saving, portable access point, overlapping
BSS

1.

Introduction

Over the past decade, wireless LAN (WLAN) devices havebeen installed in many types of electronic devices suchas personal computers, portable gaming machines, andportable music players. Although there are major WLAN
areas such as in the homes, oﬃces, and public hotspots, environments in which electronic devices can access the Internet are still limited. In other words, electronic devices that
only incorporate WLAN devices cannot access the Internet
in a mobile environment. To address this, some vendors
have developed WLAN access points (APs) that incorporate
Manuscript received June 19, 2010.
Manuscript revised October 28, 2010.
†
The authors are with Technical Assistance and Support Center, NTT East Corporation, Tokyo, 144-0053 Japan.
††
The author is with the Faculty of Engineering, Nippon Institute of Technology, Saitama-ken, 345-8501 Japan.
†††
The author is with the Faculty of Engineering, Niigata University, Niigata-shi, 950-2102 Japan.
††††
The author is with the Faculty of Engineering, Iwate University, Morioka-shi, 020-8551 Japan.
a) E-mail: ogawa@east.ntt.co.jp
DOI: 10.1587/transcom.E94.B.658

Fig. 1

Battery-powered portable APs in OBSS environment.

a cellular network function, which is used as a wide area network (WAN) interface, for systems such as 3G, WiMAX, orPHS, as shown in Fig. 1 [1], [2]. In battery-powered APs,
power consumption is a very important problem. Although
the IEEE 802.11 standard [3] supports power saving control
for stations (STAs), it does not support this for APs because
it is assumed that APs are always supplied by mains electric
power. This means that the specification does not consider
battery-powered portable APs. To address this problem, it isnecessary to introduce a method of power saving control for
APs.
Recently, several power saving control methods for
APs have been proposed [4]–[9]. In one of the conventional
control methods is that the network allocation vector (NAV)
specified in the IEEE 802.11 standard is extended to allow
an AP to go into the sleep state during one or more future
non-overlapping time periods [4], [5]. Since STAs set theNAV for these periods and the AP also wakes up in these periods, it is required that the AP and STAs have a scheduler tomanage one or more future time period. Hence, this type ofcontrol methods requires a modification to the IEEE802.11
standard. In other type of conventional control methods, theAP uses the duration field of frames that it transmits to set
the NAV in the STA, so allowing it to enter the sleep state[6]–[9]. However, these conventional control methods do
not take account of the overlapping BSS∗ (OBSS) environment, as shown in Fig. 1, because, except for [8], [9], these
do not focus on battery-powered portable APs that may be
used anywhere. In addition, even for the control methods
that do focus on battery-powered portable APs [8], [9], thespecial problems of an OBSS environment are not clearly∗
The overlapping BSS means that the two or more APs areoperating in the same channel and can hear each other.

c 2011 The Institute of Electronics, Information and Communication Engineers
Copyright OGAWA et al.: POWER SAVING CONTROL METHOD FOR BATTERY-POWERED PORTABLE WIRELESS LAN AP IN OBSS ENVIRONMENT

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evaluated. If the conventional control methods [8], [9] areapplied to an OBSS environment, the actual beacon intervalin the OBSS environment may be extended, as compared tothe beacon interval which is set at the AP, on account of theNAV. As a result, the power consumption and delay may be
increased as compared to a single BSS unaﬀected by interference from neighboring APs. The problem with conventional control methods is that they use the NAV to allow an
AP to enter the sleep state. Since the STAs or APs, which
receive a frame, are set their NAV according to the duration
field of its received frame, they cannot transmit in an OBSS
environment.
This paper presents a method of power saving control
for battery-powered portable WLAN APs which is applicable to an OBSS environment. To overcome the problem
mentioned above, this paper uses a new action frame named
power saving access point (PSAP) action frame which the
AP uses to inform STAs within its BSS that the AP enters asleep state. In addition, a function of PSAP action frame isthat STAs enter the sleep state after receiving a PSAP action
frame. The proposed control method avoids the postponement of beacon transmission and reduces the power consumption in an OBSS environment, as compared to conventional control methods.
The rest of the paper is organized as follows. Section 2 reviews the related studies and the remaining problems. Section 3 describes the proposed power saving control
method. Section 4 presents the numerical analysis and simulation results, and compares the performance of the standard
and conventional control method with the proposed control
method. Finally, Sect. 5 concludes the paper.
2.

Fig. 2 Beacon frame format and operation procedure in standard,
PS STD.

Fig. 3CTS-to-self frame format and Broadcast frame format.

Related Studies and Remaining Problems, Concerning Power Control Method

2.1 Related Studies Concerning Power Control Method
The IEEE 802.11 standard specifies power control method
for STAs [3]. This paper refers to this control method as
PS STD. The power control has two states, an awake state
and a sleep state. An STA wakes up from the sleep state
when the STA has frames to transmit to the AP or when it
receives a delivery traﬃc indication message (DTIM). The
STA enters the sleep state when the AP has no frames tosend to the STA and the STA has no frames to send to the
AP. The power control method for STAs is described briefly
as follows.
An AP periodically transmits a beacon that includes a
traﬃc indication map (TIM) element, as shown in Fig. 2.
The TIM element consists of the DTIM Count, DTIM Period, Bitmap Control, Partial Virtual Bitmap, etc. When
the DTIM Count is zero, this is a special case called theDTIM. The DTIM Count represents the number of beacons
before the next DTIM is transmitted. The DTIM Period represents the number of beacon intervals between successive
DTIMs. Whenever an AP broadcasts a beacon, the valueof the DTIM Count is reduced by one until it reaches zero.

Fig. 4

Operation procedure in conventional control method, PS NAV.

When the DTIM Count reaches zero, it is reset to the valueof the DTIM Period. In addition, Bit 0 of the Bitmap Control represents the buﬀer status for broadcast or multicast
frames, and the Partial Virtual Bitmap represents the buﬀer
status per destination for the STAs. Hence, the AP and STAs
manage broadcast or multicast frames and unicast frames
separately, and the STAs know the buﬀer status at the APfrom the Bitmap Control and Partial Virtual Bitmap. To
simplify, it is assumed in Fig. 2 that no data frames are being generated. Hence Bit 0 of the Bitmap Control is set tozero and all the bits of the Partial Virtual Bitmap are set tozero. In this situation, after receiving the DTIM, each STA

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Fig. 5

Problems of conventional control method, PS NAV, in OBSS environment.

enters the sleep state. As shown in Fig. 2, the STAs remainin the sleep state except when receiving a DTIM, but the AP
is always in the awake state. Hence, the power consumed by
the AP is greater than that consumed by the STA.
To reduce the power consumption at the AP, power saving control method for battery-powered portable APs has
been proposed [9]. This control method uses the NAV toensure conditions in which the AP can enter the sleep state.
This paper refers to this control method as PS NAV. To makepossible the sleep state for the AP using the frames that are
specified in the IEEE802.11 standard as shown in Fig. 3,
the AP causes the STAs to refrain from transmission using
the NAV while the AP is sleeping. The operation procedure when no data frames are being generated is shown in
Fig. 4. The Overhead shown in this figure represents the average waiting time for transmissions in a CTS-to-self or abroadcast frame, which is given later. At first, an AP checks
whether or not its own buﬀer is empty during the awake state
before transmitting a DTIM. To force the STAs to maintainthe awake state after the AP broadcasts a DTIM if the buﬀer
is empty, according to a sleep length for the AP the AP creates one or more CTS-to-self and one broadcast frames orcreates one broadcast frame. This broadcast frame contains
null data in the Frame Body and is broadcasted only whenan AP allows the STAs to enter the sleep state. In addition,
this frame has the value of zero in the More Data field of the
Frame Control, which is a part of the MAC header. After
that, the AP broadcasts the DTIM with the value of one in
Bit 0 of the Bitmap Control. After the DTIM has been transmitted, the created frames are also transmitted. Using the
NAV, NAVcts created by the CTS-to-self frame and NAVbro
created by the broadcast frame, the AP enters the sleep state.
Since these frames are transmitted after the DTIM to allow
the AP to enter the sleep state, the DTIM Period is set to
one in the TIM elements so as to increase the opportunity
for the AP to enter the sleep state. If the buﬀer is not empty

during the awake state before transmitting the DTIM, theAP executes PS STD. That is, the AP continues to remainin the awake state. Therefore, this method of control is effective when the downlink traﬃc (AP to STAs) is low. In
addition, this method of control is fully compatible with theIEEE802.11 standard. The detail is described in [9].
2.2 Problems of Conventional Control Method in an
OBSS Environment
Since battery-powered portable APs may be used anywhere,
an OBSS environment is formed in places where many people gather, such as stations, buses, and trains. Hence, it isnecessary to address the problems of an OBSS environment
because the frequency of occurrence of this situation is highin a urban area. The problem of which the PS NAV is applied to an OSBB environment is described as follows.
If two or more APs in OBSS environment use the NAV
to allow each AP to enter the sleep state, the actual beacon
interval may be extended as compared to the normal beacon interval which is set at the AP. Assuming that two APs,
AP1 and AP2 in Fig. 1, are present, the NAVs set by thetwo APs aﬀect each other’s BSS as shown in Fig. 5. This
figure shows the behaviors in an OBSS environment whenno data frames are being generated. For example, if AP1
receives a CTS-to-self or a broadcast frame from a neighboring AP (AP2), AP1 sets the NAV by the duration fieldin the received frame. Since AP1 cannot transmit frames
while it sets NAV, the actual beacon interval of AP1 may
be extended. Moreover, since it may be considered that thesleep length should be set to a large value so as to reduce
power consumption more eﬀectively, the actual beacon interval is greater than the normal beacon interval which is set
at AP.
The problems resulting from an extended beacon interval are that the delay is increased and the consumed power is

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661

also increased as compared to a single BSS. The evaluations
related to these problems are discussed in Sect. 5.
3.

Power Saving Control Method for Battery-Powered
Portable WLAN APs in an Overlapping BSS Environment

To overcome the above problem, this paper proposes a
power saving control method for battery-powered portable
WLAN APs which is applicable to an OBSS environment.
The key of this proposal is the introduction of a new action frame named power saving access point (PSAP) action
frame. The function of this frame is that the AP informs the
STAs within its BSS about a sleep length† which is going
to be initiated at the AP. Using the PSAP action frame, theSTAs within its AP which received the frame set the sleep
length of the AP. That is, since the STAs decide whether theframe is received or not by the source address in the frame,
the sleep length of the AP is set up for each BSS. Anotheraim of this proposal is to maintain the compatibility with theIEEE802.11 standard. This is achieved by using the venderspecific action frame shown in Fig. 6. This proposed control
method is called PS ACT in this paper. In addition, the function of power saving control is called PSAP. As well as theconventional control method, PS NAV, since these frames
are transmitted after the DTIM to allow the AP to enter the
sleep state, the DTIM Period is set to one in the TIM elements so as to increase the opportunity for the AP to enter
the sleep state. The proposed control method is eﬀective
in low downlink traﬃc, as well as the conventional control
method.
First, after an STA has been associated with an AP, the
STA informs the AP of its PSAP capability using a PSAP
action frame whose action value shown in Table 1 is zero.
After the AP receives this frame, the AP replies to the STA
using a PSAP action frame whose action value is one. By
this negotiation, the AP can manage information about thecapability information of STAs within its BSS. Only if all
associated STAs have the PSAP capability, the AP can at-

Fig. 6

PSAP action frame format.

Table 1

PSAP action frame contents.

tempt to enter the sleep state.
The procedure of operation of the proposed control
method is described as follows. Figure 7 shows the operation procedure when no data frames are being generated.
In this figure, the STA1 and STA2 shown in Fig. 1 are associated with the AP1 and AP2, respectively. The Overhead
shown in this figure represents the average waiting time fortransmissions in a PSAP action frame or a broadcast frame
which is given later. An AP checks whether or not its own
buﬀer is empty while in the awake state before transmitting a DTIM. If the buﬀer is empty, the AP creates a PSAP
action frame whose action value shown in Table 1 is two
and also a broadcast frame. The content of the PSAP action
frame is the sleep length of the AP. In addition, the broadcast frame contains null data in the Frame Body, and thisframe has the value of zero in the More Data field of the
Frame Control, which is a part of the MAC header. The APthen broadcasts the DTIM with the value of one in Bit 0 of
the Bitmap Control, and after that also transmits the PSAP
action frame. After the AP has broadcast the PSAP action
frame, the AP enters the sleep state. On the other hand,
the STA receiving the DTIM, maintains the awake state toreceive broadcast or multicast frames. Although STAs normally maintain the awake state to receive broadcast or multicast frames, STAs within the BSS can enter the sleep state
to avoid the unnecessary power consumption, but they only
do this after receiving the PSAP action frame. Here, thesleep length for the AP is the value in the information fieldof the PSAP action frame. However, the sleep length for
STAs is extended as compared to the value in the information field of the PSAP action frame. The amount of extension is DIFS. The reason why the extended length is usedis that the AP broadcasts the created broadcast frame on a

Fig. 7†

Operation procedure in proposed control method, PS ACT.

The sleep length is a system parameter for products.

IEICE TRANS. COMMUN., VOL.E94–B, NO.3 MARCH 2011662

high priority basis. When the sleep length has expired, theAP or STA returns to the awake state. Finally, the AP broadcasts the broadcast frame with the value of zero in the More
Data field. The procedure after that is the same as for the
standard, PS STD.
4.

Evaluation for Proposed Control Method

4.1 Performance Criteria and Parameter Setting
To evaluate the performance of the proposed control
method, the following criteria are defined.
- Consumed power ratio:
This represents the ratio of the consumed power in
PS ACT to that in PS STD, or the ratio of the consumed
power in PS NAV to that in PS STD.
- Diﬀerence in delay† :This represents the diﬀerence in delay between
PS ACT and PS STD, or the diﬀerence in delay between
PS NAV and PS STD.
- Throughput:
This represents the amount of traﬃc successfully transmitted with PS ACT or with PS NAV.
The main parameters for analysis and simulation are
given in Table 2†† . The power consumption refers to Cisco
Aironet 350 series [10], which was also used in [9]. In thesimulation, the data frames are generated following a Poisson Process. In addition, the number of stations within a
BSS is one, and the volume of oﬀered traﬃc††† at an AP is
the same value at a STA.
4.2 Numerical Analysis
First of all, to clarify the performance when no data frames
are being generated, the consumed power is evaluated forthe standard, PS STD, the conventional control method,
PS NAV, and the proposed control method, PS ACT.
To analyze the performance, some terms need to be defined. Let the time taken by the AP to broadcast a beacon
frame, the time taken by the AP to broadcast a PSAP action
frame, and the time taken by the AP to broadcast a broadcast
frame be T beacon , T act , and T bro , respectively. In addition, let
the time taken by the AP to transmit a CTS-to-self frame,
used in PS NAV, be T cts .
The awake state may be sub-divided into the transmission, receiving, and listening states. In the related studies
Table 2

Parameters for evaluations.

[4], [6], [9] the consumed power in the receiving state was
set to the same value as that in the listening state, and, forsimplicity, this paper makes the same assumption. Let thevalue of the consumed power in the transmission state, thevalue in the listening/receiving state, and the value in thesleep state be Et , El , and E s , respectively.
Using these terms, the consumed power is evaluated
without considering the contention among neighboring APs,
for simplicity. First, the values of the consumed power for
the AP and STA during DTIMs in the standard, PS STD,
Eap ps−std and E sta ps−std , are expressed below.
Eap ps−std = Et ·T beacon+El (Beacon Interval−T beacon ),E sta ps−std = El ·T beacon+E s (Beacon Interval−T beacon )

(1)
(2)

where, the DTIM Period is set to one, to evaluate the proposed method, standard, and conventional control method
fairly. Even if neighboring APs are present, the above equations are satisfied. Since the STAs are in the sleep state except when the DTIM is received, the STAs do not receive
frames from neighboring APs. Hence, the consumed power
for the STA is not changed by the number of neighboring APs. In addition, since the AP is always in the listening/receiving state except when the DTIM is received, theconsumed power for the AP is not changed by the numberof neighboring APs.
The values of the consumed power for the AP and STA
during DTIMs in the conventional control method, PS NAV,
Eap ps−nav and E sta ps−nav , are evaluated. As shown in Fig. 5,
the actual beacon interval may be extended as compared tothe beacon interval which is set at the AP. The actual beacon
interval is changed by the number of neighboring APs in the
OBSS environment, Nap , and the sleep length. The sleep
length including Overhead related to transmission is shownas τ in Fig. 5. If the beacon interval which is set at the AP
is smaller than τ(Nap + 1), the actual beacon interval is extended to τ(Nap + 1). Hence, two conditions for evaluation
are considered.
If Beacon Interval ≥ τ(Nap + 1);
Eap ps−nav = Et (T beacon +T cts ·Ncts +T bro )+E s ·S LEEP ps−nav + El ·(Beacon Interval−(T beacon + T cts ·Ncts +T bro + S LEEP ps−nav )),(3)
E sta ps−nav = El (T beacon + (T cts +NAVcts )Ncts+T bro +Overhead) + E s (Beacon Interval−(T beacon +(T cts +NAVcts )Ncts +T bro +Overhead)) (4)If Beacon Interval < τ(Nap + 1);
Eap ps−nav = (Et (T beacon +T cts ·Ncts +T bro )†The delay represents the time from frame generation until theframe is completely sent out, i.e., the ACK is successfully received.
††
The transmission rate means the speed of wireless links.
†††
The oﬀered traﬃc means the traﬃc generation rate at each AP
or STA.

OGAWA et al.: POWER SAVING CONTROL METHOD FOR BATTERY-POWERED PORTABLE WIRELESS LAN AP IN OBSS ENVIRONMENT

663+E s ·S LEEP ps−nav + El (τ(Nap +1)−(T beacon +T cts ·Ncts +T bro +S LEEP ps−nav )))·Beacon Interval/τ(Nap +1),
(5)
E sta ps−nav = (El (T beacon +(T cts +NAVcts )Ncts+T bro +Overhead+τ(Nap +1)−Beacon Interval)+E s (Beacon Interval−(T beacon +(T cts +NAVcts )Ncts +T bro +Overhead)))·Beacon Interval/τ(Nap +1)(6)
where Overhead, S LEEP ps−nav , and τ are expressed below.
Overhead = DIFS +CWmin /2 · S lotT ime,
S LEEP ps−nav = (NAVcts −Overhead)Ncts +NAVbro ,τ = T beacon +(T cts +NAVcts )Ncts +T bro +NAVbro+2 · Overhead.

Fig. 8 Consumed power ratio at AP as compared to PS STD if no data
frames are generated.

Moreover, Ncts is the number of transmitted CTS-toself frames. This value is determined by the sleep length.
The detail is described in [9]. In addition, since the beacon interval is extended, the consumed power is normalized
by the beacon interval which is set at the AP if the beacon
interval which is set at the AP is smaller than τ(Nap +1).
Next, the values of the consumed power for the AP
and STAs during DTIMs in the proposed control method,
PS ACT, Eap ps−act and E sta ps−act , are expressed below.
Eap ps−act = Et (T beacon +T act +T bro )+E s ·S LEEP ps−act + El (Beacon Interval−(T beacon +T act +T bro +S LEEP ps−act )),E sta ps−act = El (T beacon +T act +T bro+2·Overhead−DIFS ) + E s (Beacon Interval−(T beacon +T act +T bro +2·Overhead−DIFS ))Fig. 9 Consumed power ratio at STA as compared to PS STD if no data
frames are generated.

(7)

(8)

where Overhead is expressed by the equation given above.
Moreover, as mentioned above, the values of the consumed
power for the AP and STA are not changed by the numberof neighboring APs.
4.3 Analysis and Simulation Results
First, the consumed power ratio is evaluated when no data
frames are being generated, using the above equations.
The consumed power ratio of PS ACT and PS NAV as
compared to PS STD is expressed as Eap ps−act /Eap ps−std
and Eap ps−nav /Eap ps−std for the AP and is expressed as
E sta ps−act /E sta ps−std and E sta ps−nav /E sta ps−std for the STA.
The analysis results for the AP and STA are shown in Figs. 8
and 9, respectively. As shown in Fig. 8, the consumed power
ratio of PS ACT is reduced. For example, the proposed control method achieves the minimum consumed power ratio at
the AP, which is 44% as compared to the standard, when
the sleep length is 60 ms, even if the number of neighboring APs in an OBSS environment is increased. However
the consumed power ratio of PS NAV at the AP in an OBSS
environment, i.e., Nap is one or two, is not reduced even

if the sleep length is increased. The reason for this is thatthe actual beacon interval is extended as compared to the
beacon interval which is set at the AP. Moreover, when thesleep length is short or the number of neighboring APs iszero, the consumed power ratio of PS ACT at the AP is thesame as that of PS NAV, because the actual beacon interval
is not extended as compared to the beacon interval which is
set at the AP. As shown in Fig. 9, the consumed power ratio of PS NAV at the STA increases as Nap increases. The
reason why the consumed power ratio with Nap =0 is morethan 100% is that the STA is in the awake state while the
AP is sleeping. Moreover, the consumed power of PS ACTat the STA is small as compared to PS NAV because the
STA enters the sleep state while the AP is sleeping, but isslightly increased as compared to PS STD. Hence, the AP
of PS ACT achieves a lower power as compared to both
PS STD and PS NAV. In addition, although the consumed
power at the STA of PS ACT is slightly increased as compared to PS STD, this is lower than PS NAV.
Next, the performance considering data traﬃc† when
the number of neighboring APs is two, that is in an OBSS
environment, is evaluated by computer simulation. Figures 10 and 11 show the consumed power ratio of PS ACT†
It is assumed that data frames are unicast frames. The AP
informs the STAs about the buﬀer status per destination for the
STAs using the Partial Virtual Bitmap of the TIM element.

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Fig. 10 Consumed power ratio at AP as compared to PS STD if data
frames are generated.

Fig. 11 Consumed power ratio at STA as compared to PS STD if data
frames are generated.

and PS NAV as compared to PS STD, at the AP and STA,
respectively. As well as the previous results, it is verified
that PS ACT outperforms PS NAV. Moreover, if the oﬀered
traﬃc is greater than about 0.5 Mbps, the consumed power
ratio is approximately 100%. This means that the AP executes PS STD because the AP has frames while in theawake state before transmitting a DTIM. This is analyzed
as follows. Let λ, P(λ), and L be defined as the generation
rate of data frames at the AP, the probability that the AP
has data frames before transmitting a DTIM, and the data
frame length, respectively. This probability is expressed as
P(λ) = 1 − exp(−λ · Beacon Interval). From this equation,
it is confirmed that the oﬀered traﬃc, λL, is greater than
0.55 Mbps when the P(λ) is greater than 0.99.
Figures 12 and 13 show the diﬀerence in delay as compared to PS STD, at the AP and STA, respectively. As
shown in Fig. 12 for AP, the diﬀerence in delay of PS NAV
increases with the sleep length. The reason for this is thatthe actual beacon interval is larger than the beacon interval
which is set at the AP. The delay of PS ACT is the same asthat of the PS STD because the actual beacon interval is notextended in an OBSS environment. On the other hand, as
shown in Fig. 13 for the STA, the delay of both PS ACT and
PS NAV are increased as compared to PS STD. The reason
for this is that the STA cannot transmit data frames to the AP
while the AP is sleeping. In addition, the delay of PS NAV

Fig. 12

Diﬀerence in delay at AP as compared to PS STD.

Fig. 13

Diﬀerence in delay at STA as compared to PS STD.

Fig. 14

Throughput characteristics of PS ACT and PS NAV at AP.

Fig. 15

Throughput characteristics of PS ACT and PS NAV at STA.

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is increased as compared to PS ACT because the actual beacon interval is larger than the beacon interval which is set atthe AP.
Finally, Figs. 14 and 15 show the throughput characteristics at the AP and STA, respectively. Both PS ACT and
PS NAV show the same throughput, with no degradation.
The reason for this is that the AP enters the sleep state only
when the oﬀered traﬃc is low. Therefore, even if the offered traﬃc is greater than 1.0 Mbps, the throughput characteristics of PS ACT and PS NAV are the same as those of
PS STD.
As mentioned above, numerical analysis and computer
simulation results show that the proposed control method,
PS ACT, is eﬀective for a battery-powered AP in an OBSS
environment. Specifically, the proposed control method iseﬀective for low oﬀered traﬃc at the AP. That is, this situation means that the frequency of transmitting frames at
the AP is low after STAs are associated with the AP. Usually, the AP does not always transmit frames to STAs because STAs intermittently request to transmit. For example,
the AP may not transmit frames to STAs while the user isreading e-mails. Therefore, low power consumption in low
oﬀered traﬃc condition is important.
5.

Conclusion

This paper has proposed a power saving control method forbattery-powered portable WLAN APs which can accommodate an OBSS environment. The proposed control method
introduces the PSAP action frame, by which the AP informs STAs within its BSS about the AP’s sleep length. The
main contribution is that the proposed power saving control
method for APs is compatible with the IEEE 802.11 standard, and the proposed control method can coexist with theexisting WLAN systems with no adverse eﬀect because thispower saving control is performed for each BSS. The analysis and simulation results reveal the following.
1) As compared to the standard, the consumed power at
the AP is decreased by the proposed control method, while
the consumed power at the STA is slightly increased. The
degree of this increase is small as compared to the conventional control method in an OBSS environment. For example, the proposed control method achieves the minimum
consumed power ratio at the AP, which is 44% as comparedto the standard, when the beacon interval is 100 ms and thesleep length is 60 ms, even if the number of neighboring APs
in an OBSS environment is increased.
2) As compared to the standard, the delay of the proposed control method at the AP is the same, and the delay ofthe proposed control method at the STA is increased. However, the degree of this increase is also small as compared tothe conventional control method in an OBSS environment.
3) The throughput at both AP and STA is not degradedwhen the proposed control method is introduced.
The proposed control method does not take account ofunscheduled automatic power saving delivery (U-APSD) for
QoS specified in the IEEE802.11e because the STA requires

the transmission to the AP at an arbitrary timing. Therefore,
the cooperation of control between the power control for an
AP and U-APSD is a challenging issue for future study.
References[1] Buﬀallo, Portable WiFi, http://buﬀalo.jp/products/catalog/network/
dwr-pg/[2] Huawei, Pocket WiFi, http://www.huawei.com/jp/catalog.do?id=1021[3] “IEEE Std 802.11, Part 11: Wireless LAN medium access. control(MAC) and physical layer (PHY) specifications,” ISO/IEC 8802-11,
2008.
[4] Y. Li, T.D. Todd, and D. Zhao, “Access point power saving insolar/battery powered IEEE 802.11 ESS mesh networks,” Proc.
QShine 2005, Florida, OL USA, Aug. 2005.
[5] A.M. Kholaif, T.D. Todd, P. Koutsakis, and M.N. Smadi, “QoSenabled power saving access points for IEEE 802.11e networks,”
Proc. WCNC 2008, pp.2331–2336, Las Vegas, NV USA, MarchApril 2008.
[6] F. Zhang, T.D. Todd, D. Zhao, and V. Kezys, “Power saving access points for IEEE 802.11 wireless network infrastructure,” IEEE
Trans. Mobile Comput., vol.5, no.2, pp.144–156, Feb. 2006.
[7] M. Choi, S. Jin, and S. Choi, “Power saving for multi-radio relay
nodes in IEEE 802.11 infrastructure networks,” Proc. IEEE APWCS
2007, Hsinchu, Taiwan, Aug. 2007.
[8] K. Igarashi, A. Yamada, and T. Ohya, “Power saving method of
wireless LAN access point,” IEICE Technical Report, RCS2008123, Oct. 2008.
[9] M. Ogawa and T. Hiraguri, “Power saving control for batterypowered portable WLAN APs,” IEICE Trans. Fundamentals,
vol.E92-A, no.9, pp.2253–2256, Sept. 2009.
[10] Cisco, Aironet 350 series: http://www.cisco.com/en/US/products/
hw/wireless/

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.

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666

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.

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.

Kazuhiro Takaya
received the B.E. and
M.E. degrees in Electrical and Electronic Engineering from Okayama University in 1993 and1995, respectively. He joined NTT Telecommunication Network Laboratory, NTT Corporation, in 1995. He has studied electromagnetic
interference in wireless and wired communication systems, disaster prevention countermeasures using communication systems, and human
centered design in telework support systems. He
is now a senior manager at NTT East Corporation.

Kazuo Murakawa
received the B.E. and
M.E., and Ph.D. (engineering), degrees from thedepartment of electronics at Kumamoto University, Japan in 1984, 1986 and 2000, respectively.
He joined NTT Electrical Telecommunication
Laboratory, NTT Corporation, in 1986. His research field has been in measurement and analysis of time series signals in EMC fields, design of broadband antenna using optical fibers,
and evaluation of electromagnetic field fromtelecommunication systems. Currently, he is asenior manager at NTT East Corporation. He received the 1990 IEICE
Shinohara Scholarship Award, the 1995 Japan Institute of Printed Circuits
Research Scholarship Award, the 1997 and 2003 Japan Electric Association
Shibusawa Award (43rd and 48th award), and the 2001 ITU International
Scholarship Award.