Letters

Improvements to WIPAS (Wireless IP AccessSystem) for Application-area Expansion
Yukihide Mizumoto†, Tomotaka Yoshie, Yasuhiro
Hatakeyama, Hideyuki Maruyama, and Mitsuhiro Baba
Abstract
Over the last year, we have developed new functions and improved construction technologies to further expand the application area of NTT’s wireless IP access system (WIPAS). This article describes anadaptive modulation scheme, a minimum bandwidth guarantee function, a point-to-point communication system, improved radio zone design system, and better connectors, metal fittings, and waterproofing.

1. WIPAS
The wireless IP (Internet protocol) access system(WIPAS) was developed to provide broadband IP service primarily to SOHO (small office/home office)and mass users at an early stage. It is a point-to-multipoint 26-GHz-band fixed wireless access (FWA)

system. Figure 1 illustrates the services and Table 1
gives their main specifications [1]. To further expand
the application area of WIPAS, we developed thefunctions and technologies described in sections 2
and 3.
2. Improved functions
2.1 Adaptive modulation scheme
The adaptive modulation scheme (Fig. 2) is automatically switched according to the transmission† NTT Access Network Service Systems Laboratories
Yokosuka-shi, 239-0847 Japan
E-mail: mizumoto@ansl.ntt.co.jp

APWTAPWTAPIP network
WT

E/O

Optical fiber
AP: access point

WT: wireless terminal

Fig. 1. Service concept.

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Table 1. Main specifications.
Frequency band

26-GHz band

Modulation

QPSK,16QAM

Transmission

TDM/TDMA/TDD

Transmission capacity (Ethernet layer)

QPSK: 23 Mbit/s,16QAM: 46 Mbit/s

Number of WTs

Max. of 239 WTs per AP

QoS

Fairness assignment among WTs,Minimum bandwidth guarantee*

OthersAdaptive modulation function*
Point-to-point communication function**: Recently developed

Under poor quality conditions,the modulation scheme
automatically switches tomaintain communications.

Rainfall, etc.

WTAP

WT

Modulation
scheme isswitched basedon transmission
conditions.

Under good quality conditions,the high-speed communication area isexpanded.

Fig. 2. Adaptive modulation scheme.

conditions of the wireless segment, so high-speed
communications can be achieved when the transmission conditions are good. When rain causes deterioration in the transmission conditions, the modulation
scheme is changed to the appropriate scheme, so stable communication is achieved. The current version
of WIPAS can use two types of fixed setup: 16QAM(quadrature amplitude modulation) and QPSK (quadrature phase shift keying). The transmission distancefor 16QAM is approximately 400 m. At this distancethe yearly non-operational period due to rain attenuation is stated as being less than two minutes. However, in good weather, the transmission distance is 800
to 900 m, which is equivalent to that of QPSK underrainy conditions. Thus, the new adaptive modulation
scheme that automatically switches between 16QAMand QPSK in the wireless segment according to the56transmission conditions lets us expand thetransmission distance to 800 to 900 m ingood weather, while maintaining a peak
speed of 46 Mbit/s.
Figure 3 shows the wireless frame format of WIPAS. The payload area of thewireless frame is partitioned based on themodulation scheme, and the wireless terminal (WT), which can communicate
using a different modulation scheme, has aconstruction that allows mixing. The WT
periodically measures the quality of theuser data in the downward payload(16QAM), and reports the measurement
results to the access point (AP). Based on
the quality report received from the WT,
the AP chooses the modulation scheme. At
that time, if the quality degrades, the AP
immediately switches to QPSK. If thequality level recovers, the AP switches
back to 16QAM after verifying the stability a few times and verifying that there is no
instability due to throughput degradation. The chosen
modulation scheme reflects the allocation to theQPSK or 16QAM payload area. Since switching isaccomplished by changing the area, the modulation
scheme is switched without interruption [2].
2.2 Minimum bandwidth guarantee function
The minimum bandwidth guarantee function
enables guaranteed-type services by adding a preferred bandwidth assignment function to the current
version of WIPAS. It also enables services to be provided to a mixture of best-effort-type general users
and guaranteed-type special users.
Table 2 shows the upward and downward payloads,
which are independent of each other, for the guaranteed bandwidth set for each WT. Figure 4 shows anexample of bandwidth assignment. The guaranteed
NTT Technical Review

LettersTime

1.024 ms
Wireless PHY layer

Wireless PHY layer

Wireless PHY layer

Downward section
Downward
header

MACframe

Wireless PHY layer

Upward section

Downward payload

•••

DMF

Slot demand•••

Upward payload•••

Downward payload area Downward payload area(QPSK)(16QAM)•••

Upward payload area
(QPSK)

Upward payload area
(16QAM)

DMF: delay measurement frame
MAC: media access control

Fig. 3. Wireless frame format.

Table 2. Bandwidth guarantee assignment summary.
Downlink

UplinkSetting

Bandwidth guaranteeSetting

Bandwidth guarantee

0Best effort

0Best effort

11 Mbit/s

11 Mbit/s

22 Mbit/s

22 Mbit/s

33 Mbit/s

33 Mbit/s

44 Mbit/s

44 Mbit/s

66 Mbit/s

66 Mbit/s

88 Mbit/s

88 Mbit/s

10

10 Mbit/s

10

10 Mbit/s

10

If the requested bandwidthis less than the guaranteedbandwidth, only therequested bandwidth isassigned.

Guaranteed

9Requested

8Bandwidth (Mbit/s)

bandwidth (red) is set so that it does not to exceed theallowed bandwidth; once it has been set, this bandwidth is completely guaranteed. The bandwidth toassign (blue) is calculated when the guaranteed bandwidth (red) is set. The bandwidth guarantee function
does not assign the bandwidth in a fixed manner. If therequested bandwidth (yellow) is less than the guaranteed bandwidth (WT3, 4, and 8), then only the requested bandwidth is assigned, leaving some surplus bandwidth that can be reassigned to other WTs. Then, eachof the remaining WTs is assigned its guaranteed bandwidth plus a fair share of the surplus. Thus, the bandwidth assignment is carriedout in a flexible manner,which avoids wasting thewireless bandwidth.

Assigned

765

Each of the remaining
WTs is assigned its
guaranteed bandwidth
plus a fair share of thesurplus.

4321012345
WT number

678Fig. 4. Bandwidth assignment (example).

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2.3 P-P communication
systemFigure 5 illustrates the PP (point-to-point) communications system. Two WTs
are paired to achieve P-P
communications simply
and cost-effectively. Thissystem can be applied toenterprise users and used to
expand application areas
such as entrance circuits to
hotspot services. Furthermore, a maintenance function is implemented in the57

Letters

Slave station

Master station
IP network

Network management system

Application for enterprise users

Slave station

Master station
IP network

Hotspot service area
Entrance for hotspot service

Fig. 5. P-P communication system.

Table 3. Main supervision and control items for P-P communication system.

Setting

DisplayControl

MasterSlave

Ethernet interface setting○○

Time setting○—*1

Equipment information (serial number, etc.)○○

Equipment status○○

Equipment reset○○Transmission on/off○—*2*1: Slave station is synchronized to the time of master station.
*2: Slave station interrupts transmission when master station has a transmission interruption.

master station and remote operation of both stations
is possible. This system uses the QPSK and 16QAM
modulation schemes. The transmission capacities are
16 and 32 Mbit/s, respectively, and the upward anddownward sections are allocated in the same fixedbandwidth.
Table 3 lists the main supervision and control
items. Remote operation uses SNMP (simple network management protocol) and can perform functions such as setting up the two P-P stations, dealing
with transmission interruptions, and performing
resets.
2.4 Improved radio zone design system
Since WIPAS uses the 26-GHz frequency band, aguaranteed line of sight (LOS) is necessary between
the AP and WT. The design system for establishingan AP station has an application to judge the degreeof LOS considering the buildings surrounding the58

intended installation point. It can calculate the degreeof LOS inside the simulation area and generate theAP-WT profile. This system is used to deploy WIPAS
to cover a wide area. For this system, we verified thatLOS calculation results agreed closely with actualLOS measurements. To achieve greater accuracy, thebuilding and vegetation information used by the system should be improved. Therefore, we developedand implemented a function that easily obtains 3D
data such as the shape and height of buildings andvegetation from a photograph taken from a comparatively high location. The photograph and map are displayed on the system’s screen, and by plotting several reference points on the map and photograph, thisfunction mutually links data such as positions. Next,by drawing ground features such as buildings andvegetation in the photograph, 3D data can be recognized on the map (Fig. 6). The addition of this function improved the reliability of the radio zone design
NTT Technical Review

LettersMap

PhotographBased on the drawing on the picture, information such as the shapeof the building and height is reflected in the map.

Fig. 6. Linkage of map/picture.

Approximately 14 mm
Signal lines
Outdoor Ethernet cable
Approximately
1.3 mmConnector
Flat cable

Double-sided tape(shock absorbing material)

Fig. 7. Unit configuration (prototype).

Power lines(protective lines)

Fig. 8. Construction of flat cable.

results [3].
Furthermore, we improved the usefulness of thesystem by adding functions that readily search abuilding, register service conditions, and easily
switch among the LOS figures.
3. Improved construction technology
3.1 Simpler connectors
To enable access through apartment windows without air-conditioning conduits, we developed a window-frame-gap traversing unit that has a flat cablesection for the window frame. This unit comprises an
outdoor Ethernet cable, a flat cable, and a connector
to connect to an indoor cable (Fig. 7). As shown in
Fig. 8, the flat cable is as thin as possible to allow it
to pass through the gap between the window and theframe and it incorporates protective lines and shock
absorbing material to enable it to withstand repeatedimpacts when the window is opened and closed. Even
if the cable core is shorted, the user terminal is proVol. 2 No. 9 Sep. 2004

Fig. 9. Example of the fixture of the developed flat cableon the window frame.

tected by a pulse transformer. The flat cable is fixed
to the window frame using double-sided tape andreinforcement tape, as shown in Fig. 9.
3.2 Smaller and lighter metal fittings forinstalling WTs
In response to requests from construction workers,
59

LettersFewer constraints Use of thinner materials
based on ensuring
a good appearance

Volume: decreased by 22%
Mass: decreased by 29%

For side reception
Downsizing

Before improvement
Providing more variety, using thinnermaterials, and reducing the size to allow
greater selection to better fit usage

For front reception

Fig. 10. Downsizing of the metal fittings.

Construction time:decreased by 75%
After improvement

Before improvement
Waterproofing using self-amalgamating
tape and PVC* tape
Construction time: approx. 2 min.

Waterproofing using normal-temperature
constricting sleeve
Construction time: approx. 0.5 min.

* PVC: polyvinyl chloride

Fig. 11. Improvements in waterproofing.

users, etc., we reduced the size and weight of metal
fittings for installing the WT. As shown in Fig. 10, thevolume and mass of the installation metal fittings
were decreased by 22% and 29%, respectively, while
making efforts to improve the appearance and working efficiency.
3.3 Better waterproofing for WTs
To improve the waterproofing, we developed a normal-temperature constricting sleeve for the Ethernet
cable used in conjunction with the WT. This enabled
us to devise a simple waterproofing method. Compared with the current method of wrapping using selfamalgamating tape and PVC (polyvinyl chloride)tape, the new method reduces the work time to 25%
while easily ensuring adequate waterproofing andachieving good work reproducibility (Fig. 11).

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4. Concluding remarks
WIPAS was implemented in “IP data communication network service for specified areas” in July 2003.
In December 2003, it was added to NTT’s B-FLET’sservice menu. In the future, we plan to expand the service area using these newly developed technologies
and investigate additional application areas.
References
[1]

[2]

[3]

K. Nidaira, M. Baba, and T. Saito, “Development of Wireless IP
Access System (STEP 2) in the 26-GHz Band,” NTT Technical
Review, Vol. 1, No. 5, pp. 68-73, 2003.
Y. Yasui, Y. Mizumoto, H. Kuwabara, M. Baba, S. Yoneda, S. Katsumata, and F. Murakami, “An Adaptive Modulation-Control For
Wireless IP Access System,” IEICE General Conference B-5-282,Mar. 2004.
T. Yoshie and H. Maruyama, “A Study on Accuracy Improvement ofRadio Design System for Wireless IP Access System,” IEICE General Conference B-5-279, Mar. 2004.

NTT Technical Review

Letters

Yukihide Mizumoto

Hideyuki Maruyama

FWA Development Project, First Promotion
Project, NTT Access Network Service Systems
Laboratories.
He received the B.E. degree in electronics
engineering from Tokai University, Hiratsuka,
Kanagawa in 1994. He joined NTT in 1988. He
has mainly been engaged in R&D and commercial introduction of wireless access systems
using the 1.9-GHz band. He is currently engaged
in WIPAS R&D.

Senior Research Engineer, FWA Development
Project, First Promotion Project, NTT AccessNetwork Service Systems Laboratories.
He received the B.E. degree in electrical engineering from Niigata University, Niigata in 1985
and joined NTT the same year. He has mainly
been engaged in link design of fixed microwave
systems; research, service development, andcommercial introduction of satellite communications systems; and development of portable wireless systems for digital video transmission. He iscurrently involved with a radio zone design system and operation support system for WIPAS.

Tomotaka YoshieMitsuhiro Baba

FWA Development Project, First Promotion
Project, NTT Access Network Service Systems
Laboratories.
He received the B.E. degree in electronic engineering and M.S. degree in information sciencefrom Tohoku University, Sendai, Miyagi in 1993
and 1995, respectively. Since joining NTT in
1995, he has mainly been engaged in R&D andcommercial introduction of wireless access systems. He is currently engaged in the developmentof portable wireless systems for digital video
transmission and a radio zone design system for
WIPAS. He is a member of the Institute of Electronics, Information and Communication Engineers (IEICE) of Japan.

Senior Research Engineer, FWA Development
Project, First Promotion Project, NTT AccessNetwork Service Systems Laboratories.
He received the B.S. degree in electronics fromWaseda University, Tokyo in 1986. Since joining NTT in 1986, he has mainly been engaged in
the development of relay digital microwave systems and their commercial introduction andR&D of broadcasting TV relay systems andband-occupation-type wireless access systems
using quasi-millimeter waves. He is currentlyengaged in WIPAS R&D. He is a member ofIEICE of Japan.

Yasuhiro Hatakeyama
Chief Examiner, Access Business Department,Engineering Business Headquarters.
He joined NTT in 1977. He has mainly been
engaged in R&D and commercial introduction of
wireless access systems using the 1.9-GHz bandand 26-GHz band.

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