Letters

Development of Wireless IP Access
System (STEP 2) in the 26-GHz Band
Katsutoshi Nidaira†, Mitsuhiro Baba, and Toshio SaitoAbstract
To provide fast Internet services to areas where it is difficult to lay optical fiber cables for an accessnetwork, NTT developed a Wireless IP Access System (WIPAS STEP 1), which is a point-to-multipoint
fixed wireless access system using the 26-GHz frequency band that provides a best-effort IP service.
WIPAS combines wireless technology with an optical network and complements FTTH (fiber to thehome) service. This article describes the second-generation WIPAS called STEP 2, which has twice thetransmission rate of STEP 1 while being much more compact. It overviews the concepts and technologies of the STEP 2 system.

1. Wireless IP Access System
High-speed Internet access over optical fiber,ADSL, CATV, etc. has become rapidly available inrecent years. Yet there are still many users who cannot obtain broadband access because of the difficulties of deploying fiber-optic cable or simply because
services have not been made available in their areas
due to cost considerations. With the goal of providing
high-speed Internet access to these previously inaccessible pockets of users, the Access Network Ser† NTT Access Network Service Systems Laboratories
Yokosuka-shi, 239-0847 Japan
E-mail: nidaira@ansl.ntt.co.jp

vice Systems Laboratories developed a Wireless IP
Access System (WIPAS) STEP 1 [1]. WIPAS combines wireless technology with an optical networkand complements the FTTH (fiber to the home) service. STEP 1 services began in September 2002 when
NTT EAST and WEST added them to their
B–FLET’S service menus.
To increase the transmission rate and reduce thesize of the wireless equipment, we developed WIPAS
STEP 2 (Fig. 1). It is a point-to-multipoint Fixed
Wireless Access (FWA)*1 system using quasi-millimeter-waves in the 26-GHz band, one of the fre-

*1 In FWA, both the access point and wireless terminal are fixed.

AP-RFU

AP: Access Point
WT: Wireless Terminal
RFU: Radio frequency unit
IFU: Interface unit

AP-IFUWT
WTNTT

WT-adapter
AP

IP network
E/O
WT
Optical fiber cable

Fig. 1. Wireless IP access system.

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quencies (22, 26, and 38 GHz) allocated to FWA services in Japan. Figure 2 shows the channel plan for
WIPAS. This system comprises wireless terminals(WTs) installed on customer premises (typicallyattached to the balcony or outside wall) and accesspoints (APs) installed on telephone poles and building rooftops. An AP is connected to the core networkby an optical fiber cable. A WT incorporates anantenna, radio frequency (RF) module, baseband processing modules (modulation/demodulation, TDMA,
MAC processing, and so on), and a WT adapter thatfunctions as an interface to a personal computer.
An AP comprises a radio frequency unit (AP-RFU)
that incorporates an antenna and RF module and aninterface unit (AP-IFU) that incorporates baseband
processing modules and a core network interface.
The main specifications of this system are given in
Table 1.
The maximum wireless transmission rate of thissystem is 80 Mbit/s, and the maximum Ethernet
frame transmission rate is 46 Mbit/s. This systemprovides a best-effort type IP service, and the wireless bandwidth is shared fairly among the WTs thatare connected to the same AP. The AP dynamicallyallocates the bandwidth to WTs corresponding to thedemand of each WT and uses a round-robin algor i t h m f o r a l l o c a t i o n p r o c e s s i n g . V L A N *2
(IEEE802.1Q) [2] is used as the protocol for this system, and all IP packets are transmitted transparently
between WTs and the AP. Figure 3 shows the WIPAS
protocol stack.
Some FWA systems in Japan use IEEE802.11b [3](2.4-GHz band). The equipment costs of these FWA
systems are low, but such systems suffer unstable linkquality because they use industrial, scientific, and*2 VLAN is a network technology that organizes and manages virtual networks without physical cable connections.

D1: 25.270 - 25.330 GHz
Low frequency group

D1’: 26.125 - 26.185 GHz
High frequency group795
MHz

Upper Ch

Lower Ch
VH

Upper Ch

Lower Ch
VH

15 MHz30 MHz15 MHz15 MHz30 MHz15 MHzFig. 2. Channel plan for WIPAS.
Table 1. Main specifications of WIPAS STEP 2.

Vol. 1 No. 5 Aug. 2003

Frequency

26-GHz band

Modulation

QPSK/16QAM

Transmission

TDMA/TDD

TX capacity

For wireless: 40 Mbit/s (QPSK), 80 Mbit/s (16QAM)
For Ethernet: 23 Mbit/s (QPSK), 46 Mbit/s (16QAM)

TX power

AP: 14 dBm (QPSK), 11 dBm (16QAM)
WT: –6 to 14 dBm (QPSK), –9 to 11 dBm (16QAM)(auto power control: 0.5-dB step)

Antenna

AP: Omni, Sector (90 degrees), Horn, Cassegrain
WT: Flat

User interface

10 Base-T/100 Base-TX (full/half duplex)

Network interface

100 Base-FX or100 Base-TX (full/half duplex)

MAC processing

VLAN (IEEE802.1Q)

TX distanceApprox. 700 m (QPSK), 400 m (16QAM)

Number of WTs

Max. 239 WTs per AP

Downtime

Less than 2 minutes per year69

Letters

IP
PPP

PPP

PPPoE
PPP

PPPoE

EtherPHY

802.1Q802.1Q

Ether

Ether

Wireless MAC

Wireless MAC

Wireless PHY

Wireless PHY

EtherPHYPC

WT

802.1Q

802.1Q

Ether

Ether

PHY

PHY

AP

10/100Base-TX

Air

100Base-FX

NAS(Network Access Server)

Fig. 3. Protocol stack.

medical (ISM) bands. Because WIPAS uses the 26GHz band, which FWA services are permitted to use,
there is no interference from other wireless services.
FWA systems that use the 5-GHz band will begin services soon. These systems are based on
IEEE802.11a, and the transmission rate is up to 36
Mbit/s. WIPAS STEP 2 surpasses the 5-GHz FWA
systems in maximum transmission rate with the sameequipment cost.
2. New features of STEP 2 system
WIPAS STEP 2 has four major improvements over
the STEP 1 system.
(1) In addition to QPSK (quadrature phase shift keying), it also uses 16QAM (quadrature amplitude
modulation) for modulation .
(2) WT-RFU and WT-IFU are unified.
(3) It can select either the upper or lower channel in
the AP-RFU and WT equipment.
(4) It also supports point-to-point communication.
The use of 16QAM raises the maximum wirelesstransmission speed to 80 Mbit/s, twice that of theSTEP 1 system. The WTs are made significantly
smaller and easier to produce by integrating WT-RFU

and WT-IFU. Because the AP-RFU and WT equipment have a polarization option (vertical or horizontal polarization), users can select one of four channels
in the STEP 2 system. The STEP 2 system can beused to connect an AP to a core network instead ofoptical fiber cable using point-to-point mode.
3. Increased transmission rate
Figure 4 shows the frame format of the STEP 2 system. If QPSK is used for modulation, the wirelesstransmission rate is 40 Mbit/s, because one symbol*3(the symbol rate is 20 Msymbol/s) can carry two bitsof information in QPSK. If 16QAM is used, the airtransmission rate is 80 Mbit/s, because one symbol
can carry four bits of information.
The 16QAM modulation is only used in payload
fields for safety concerns pertaining to equipment
control (QPSK is more robust against C/N+I*4 degradation than 16QAM). The maximum Ethernet frame*3 A symbol is an information unit that is transmitted over the baseclock of a wireless signal and can multiplex a few bits of data bymeans of modulation.
*4 C/N+I: The ratio of carrier power to noise plus interference.

Time1.024 ms
Wireless PHY

Wireless PHY

Wireless PHY

Downward section
Downward
Header

Upward section

Downward payloads
QPSKMACframe

Wireless PHY

•••

DMF

Slot demand

Upward payloads

16QAM

QPSK

16QAM

•••••••••

Pilot signal

GT

DMF: Delay measurement frame GT: Guard time
PHY: Physical layer

Fig. 4. MAC frame format.

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transmission rate is 46 Mbit/s, if 16QAM is only used
for payload modulation and all payloads are allocated to the downlink. If all payloads are allocated to theuplink, the transmission rate is 31 Mbit/s.
4. Wireless terminal technologies
To unify the WT-RFU and WT-IFU, we developedi) a one-chip application-specific integrated circuit(ASIC) that handles modulation/demodulation, MAC
processing, and TDMA control, ii) a surface-mountmicrowave monolithic IC (MMIC) for the RF modules, and iii) a slotted-waveguide-array flat antenna[4].
Unifying the baseband modules into a one-chip
ASIC reduced the WT equipment size and cost andimproved device reliability. The MMICs consist of ahigh-power amplifier, low-noise amplifier, and up
and down converters. Their surface-mounting design
reduces the equipment manufacturing cost because

they can be installed without special manufacturing
processes.
We developed a slotted-waveguide-array flat antenna with the RF and baseband modules and powersupply circuit on its rear side to reduce the size andcost of the WT equipment. The flat antenna has a narrow beamwidth of 4° to obtain a high antenna gain.
Table 2 gives the antenna specifications. Figure 5
shows the appearance of the WT and compares theSTEP 1 and 2 systems.
We developed a WT-adapter having a PC interface,power supply, and alarm indication. It connects to theWT and user PC via Ethernet cables. Vacant coreswithin the Ethernet cable are used to supply power tothe WT.
5. Access point technologies
We reduced the size of the AP-IFU (size: 27 × 32 ×16 cm, weight: 6.5 kg) by using two ASICs for theTable 2. Antenna specifications of WIPAS.
APTypeWT

Omni

SectorDirectivity

H plane:Omni directional
V plane:Fitting cosec2 curve

H plane:Beam width:90°V plane:
Fitting cosec2 curve

Gain

6.0 dBi

12.0 dBi

5.5 dBi

31.5 dBi

Polarization

V or HV or H

V and H

V and H

V.S.W.R.

HornFlat

Beam width:
90° × 90°

Beam width:
4°

Up to 1.5

XPD

Greater than 20 dB

WT equipment

Size: 190 × 190 × 55 mm
Weight: 2.0 kg
Ethernet cableconnection

Greater than 25 dB

WT-RFU

Size: Dia. 254 × 125 mm
Weight: 2.8 kg
RF cableconnection
WT-IFU

WT-adapter

Size: 94 × 40 × 36 mm
Weight: 0.1 kg
STEP 2Size: 200 × 200 × 50 mm
Weight: 1.3 kg
STEP 1

Fig. 5. Appearance of wireless terminal.

Vol. 1 No. 5 Aug. 2003

71Letters

Size: 275 × 325 × 160 mm
Weight: 6.5 kg

Size: 330 × 380 × 380 mm
Weight: 14 kg

STEP 2

STEP 1

Fig. 6. Appearance of AP-IFU.

Omni/90 degrees sector type
Size: Dia. 150 × 600 mm
Weight: 7.0 kg

Horn/Cassegrain type
Size: Dia. 254 × 125 mm
Weight: 2.9 kg

Fig. 7. Appearance of AP-RFU.

baseband processing. One, which is commonly usedin WTs, functions as a modem, and the other performs TDMA control and MAC processing. Figure 6shows the appearance of AP-IFU and compares theSTEP 1 and 2 systems.
In the AP, we separated the AP-RFU from the APIFU in the same way as in the STEP 1 system,
because different AP-RFU antennas are used to suit
the service type. For example, an omni-directional
antenna with a 360° horizontal directional pattern isused for services in residence areas without high-risebuildings (Table 2). A horn antenna with a 90° × 90°
directional pattern is used for services targeted at
occupants of apartment or office buildings because
these users are widely spread both horizontally andvertically. On the other hand, a Cassegrain antenna
with a 4° pattern is used for a wireless entrance system. The appearance of the AP-RFU is shown in Fig.
7.
6. Equipment installationof the equipment and installation.
7. Conclusion
NTT has improved Wireless IP Access System(WIPAS). The STEP 2 system is designed for homeand SOHO (Samll Office Home Office) users in areas
where it is difficult to lay optical fiber cables. It uses
the 26-GHz frequency band and has a transmission
capacity of 80 Mbit/s. Compared with the previousversion (STEP 1), we reduced the wireless terminal
cost by employing MMICs and ASICs in the radio
frequency and baseband modules, respectively.
Services based on this system will commence thisyear. In the future, we will add quality-of-service andadaptive modulation functions and develop a wirelessrepeater.
References[1][2]

The equipment is now small enough to be mounted
on telephone poles or customers homes (e.g., balconies or outside walls). Figure 8 shows a photograph
72

T. Saitoh, M. Baba, and Y. Yasui, “Development of 26-GHz Band
Wireless IP Access System,” NTT Review, Vol. 15, No. 1, 2003.
LAN MAN Standards Committee of the IEEE Computer Society,“IEEE Standards for Local and Metropolitan Area Networks: Virtual
Bridged Local Area Networks,” Draft Standard P802.1Q/D11, Jul.
1998.

NTT Technical Review

Letters(a) Wireless terminal
AP-RFU
Omni antenna

AP-RFU
Horn antenna

AP-IFU

AP-IFU(b) Access pointFig. 8. Equipment installation.
[3][4]

ANSI/IEEE Std 802.11, MEDIUM ACCESS CONTROL (MAC)
AND PHYSICAL (PHY) SPECIFICATIONS, 1999 Edition.
Y. Kimura, T. Hirano, J. Hirokawa, and M. Ando, “Alternating phasefed single layer slotted waveguide arrays with chokes dispensing withnarrow wall contacts,” IEE Proc. Microw. Antennas and Propagation
Vol. 148, No. 5, pp. 295-301, 2001.

Katsutoshi Nidaira
Research Engineer, FWA Development Project, First Promotion Project, NTT Access Network Service Systems Laboratories.
He received the B.S. and M.S. degrees inphysics from Ibaraki University, Mito, Ibaraki in
1989 and 1991, respectively. He joined NTT in
1991. He has mainly been engaged in researchand development of satellite communications
and wireless multi-hop networks. He is currently
engaged in research and development of WIPAS.
He is a member of the Institute of Electronics,Information, and Communication Engineers(IEICE) of Japan.

Vol. 1 No. 5 Aug. 2003

Mitsuhiro Baba
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 andresearch and development of broadcasting TV
relay systems and band occupation type wirelessaccess systems using quasi-millimeter waves. He
is currently engaged in WIPAS research anddevelopment. He is a member of IEICE of Japan.

Toshio Saito
Senior Research Engineer, Supervisor, FWA
Development Project, First Promotion Project,NTT Access Network Service Systems Laboratories.
He graduated from the department of information engineering, faculty of engineering, Ibaraki
University in 1982. He completed his Masters
Course at Ibaraki University, Hitachi, Ibaraki in
1984 and joined NTT in 1984. He was mainlyengaged in the development of relay digital
microwave systems and their commercial introduction and business planning of long-distancecommunications. He is currently engaged in theresearch and development of a wireless IP accesssystem (WIPAS). He is a member of IEICE ofJapan.

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