Special Feature

Broadband Packet Wireless Access
Mamoru Sawahashi†, Sadayuki Abeta, Hiroyuki Atarashi,
Kenichi Higuchi, Motohiro Tanno, and Taisuke Ihara
Abstract
Packet access in the radio access network (RAN) is essential to reduce network costs and achieve highcapacity multimedia mobile communications capable of supporting services such as high-resolution
video communication. This article presents targets and key techniques for the broadband packet wirelessaccess system that we propose, which enables seamless support of both cellular systems and isolatedcell environments, such as hotspot areas and indoor offices, using the same air interface for efficient
packet access. It also describes efficient packet access techniques for guaranteeing the quality of service(QoS), including handover in the RAN on both the data-link layer and the physical layer.

1. Introduction
A maximum information bit rate of 2 Mbit/s orgreater has been specified as the requirement for
third-generation (3G) mobile communications calledinternational mobile telecommunications-2000(IMT-2000) in the radiocommunication sector of theinternational telecommunication union (ITU-R). We
have experimentally demonstrated high-speed datatransmission above 2 Mbit/s with high quality at an
average bit error rate of less than 10–6 using a 5-MHz
frequency bandwidth in the downlink, applying
three-code multiplexing with a spreading factor (SF)of 4 using two-branch antenna diversity reception.
Meanwhile, the third-generation partnership project(3GPP) has completed specifications focusing on
high-speed packet data transmission in the downlink
called high-speed downlink packet access (HSDPA)
based on an air interface using wideband code division multiple access (W-CDMA) [1]. In HSDPA, amuch higher peak throughput than 2 Mbit/s is possible with low latency and high efficiency by employing adaptive modulation and channel coding (AMC)
including 16 quadrature amplitude modulation† NTT DoCoMo, Inc.
Yokosuka-shi, 239-8536 Japan
E-mail: sawahashi@nttdocomo.co.jp

18(QAM), hybrid automatic repeat request (H-ARQ)
with packet-combining in the medium access control(MAC) layer, and fast packet scheduling (i.e., user
diversity). Moreover, we have shown that a peakthroughput of nearly 10 Mbit/s is possible with a 5MHz bandwidth by using a multipath interference
canceller or chip equalizer even in a multipath fading
channel. However, to meet the current huge increases
in the amount of data traffic and further ones expected in the future, new broadband wireless access methods must provide broadband packet transmission
with a maximum throughput above 100 Mbit/s in thedownlink using a bandwidth of approximately 50 to
100 MHz (note that the target throughput corresponds
to an approximately 10-fold increase over that
achievable in HSDPA with a 5-MHz bandwidth). In
addition, this broadband wireless access must flexibly support both isolated-cell environments (such as
hotspot areas and indoor offices) and cellular systems
from the standpoint of further reducing the cost ofradio access networks (RANs). Furthermore, since it
is presumed that the signal format in the wirelesschannel is a packet format, the service being offeredis basically a best-effort type according to the channel condition of each user and traffic conditions within the cell, where minimum throughput is guaranteedwith the required quality of service (QoS), e.g., delay
and residual packet error rate.
NTT Technical Review

Special Feature

Cellular system

Hotspot area

Supported by the same
air interface

IP-based core network (IP: Internet protocol)

Fig. 1. Concept of the broadband packet wireless access system supportinga cellular system and local areas by the same air interface.

In this article, we present targets for a broadband
packet wireless access system, which is for the systems beyond IMT-2000, and overview key techniques for it. Section 2 describes the targets of broadband packet wireless access. Section 3 then explains
the RAN configuration that we propose, and section
4 presents broadband packet wireless access techniques focusing on wireless access schemes. After
that, section 5 briefly presents efficient packet accesstechniques, and section 6 discusses multiple-antennatransmission methods, which are essential for
enabling wider coverage area and improving theachievable throughput.
2. Targets of broadband wireless accessFigure 1 shows the concept of the proposed broadband packet wireless access system. In the future
RAN, further decrease in the network cost is a very
important requirement for offering rich multimedia
services to customers via wireless communications.
Therefore, the proposed concept can support both acellular system with a multi-cell configuration andlocal areas (such as hotspot areas and indoor officeenvironments) by the same air interface (i.e., thesame carrier frequency, frequency bandwidth, radio
frame structures, etc.). Only the main radio parameters such as spreading factor, data modulation
scheme, and channel coding rate need to be changed.
By changing them according to the cell configuration, channel load, and channel condition of eachuser, we can achieve the maximum system capacity
based on the same air interface, that is, the samebroadband wireless access scheme.
In our broadband packet wireless access system, thetarget of the peak throughput for the downlink in a celVol. 2 No. 9 Sep. 2004

lular environment is 100 Mbit/s, which is approximately ten times the peak throughput forecast for
HSDPA using a 5-MHz frequency bandwidth. Achieving a throughput of 100 Mbit/s will of course help toreduce network cost, but it will also enable largecapacity data downloads and high-resolution videocommunication services among multiple users. However, local areas (generally, short-distance and shorttime-dispersion environments) will probably require
throughput in excess of 100 Mbit/s to support large
amounts of data traffic. We therefore aim for a maximum throughput of 1 Gbit/s (frequency efficiency: 10
bit/s/Hz) for these types of areas and intend to support
throughput flexibly for various environments using thesame air interface.
For the uplink, the target for frequency efficiency
for hotspots and indoor environments is 7 bit/s/Hz.
This comes to a maximum throughput of approximately 300 Mbit/s for a frequency bandwidth of 40
MHz. For a cellular environment, the target is a maximum throughput of 20 Mbit/s per sector*1.
In the broadband wireless access system, all signals
are transmitted by a packet configuration in the RAN,
i.e., to transmit both realtime and non-realtime traffic
data as packet signals. Here, channels can be allocated to multiple users in a cell on the basis of time-division multiplexing using a shared-channel packet format. This will significantly reduce the number ofphysical radio devices making up base-station equipment compared with the circuit-switching transmission system that allocates a dedicated physical channel to each user.

*1 Sector: Cells are split into sectors for greater efficiency.

19Special Feature

3. Radio access network configuration inbroadband wireless access
3.1 Inter-cell synchronization
A broadband wireless access system must be able
to support a seamless service area covering not only
a cellular environment but also hotspots in underground shopping centers, airports, hotel lobbies, andindoor environments like offices. To achieve flexible
deployment, especially to indoor environments, aninter-cell asynchronous system is advantageous overan inter-cell synchronous system based on time synchronization.
A mobile terminal needs to establish a wireless linkwith the base station that provides the minimum pathloss. In a wireless access system that supports a
multi-cell cellular system, this criterion is nearly thesame as the criterion of finding the reference signal in
the downlink with the highest received power level(in W-CDMA, the common pilot channel is used forthis purpose). Thus, in W-CDMA, the cell or sector
whose common pilot channel in the downlink has thehighest received power is selected as the optimal
cell/sector. The situation is somewhat different, however, in a broadband wireless access system that supports both cellular and isolated cells, as shown in Fig.
2. Specifically, the base-station transmission powerof cellular cells differs from that of hotspot cells,
which means that the cell with the highest received
power level of the common pilot channel in the downlink is not necessarily the cell with the lowest pathloss between the base station and mobile terminal. In
Cellular cell
(outdoors)

Received power of common
pilot channel Pcellular, Photspot

Pcellular > Photspot
Lcellular < Lhotspot

Pathloss (Lcellular)

Hotspot cell(indoors)
Pcellular < Photspot
Lcellular > Lhotspot

Pathloss (Lhotspot)

Pcellular: cellular cell received power
Photspot: hotspot cell received power

Fig. 2. Cell selection when cellular cells and hotspot cells
coexist.

20

the example shown in the figure, the received powerof the pilot channel from an outdoor cellular cell ishighest, but the cell with the lowest path loss is anindoor hotspot. Establishing a wireless link with theindoor cell leads to the maximum reduction in thetransmission power required by the mobile terminal.
In the manner described above, a search must be
performed for the cell/sector having the lowest pathloss between the base station and mobile terminal ina system where isolated environments coexist with acellular environment. This cell search can be divided
into three types: (1) initial cell search, (2) cell searchin active mode, and (3) cell search in idle mode. For
cell searches in the active and idle modes, and standby searches, the currently connected cell (sector)
informs the mobile terminal about the transmission
power and cell type (cellular or isolated) of neighboring cells so that the terminal can find the optimal cellhaving the lowest path loss between itself and thebase station.
For the initial cell search, two methods can be considered. In the first method, which is similar to themethod used in conventional cellular systems, themobile terminal first establishes a wireless link withthe cell whose downlink pilot channel has the highest
received power. The terminal then receives anddecodes a report on neighboring cells from that cell.
If there is a neighboring cell with lower path loss thanthe currently connected cell, the terminal switches its
wireless link to it. In the second method, each cell isallocated beforehand a unique scrambling code indicating a cellular or hotspot cell [2]. As a result, a
mobile terminal in an initial cell-search stage canidentify the cell type and thus search for the optimalcell with the lowest path loss simply using the processing in the physical layer.
Of these three types of cell searches, the initial cellsearch takes the longest time. To reduce the searchtime, three-step cell search algorithms have been proposed using orthogonal frequency and code divisionmultiplexing (OFCDM) or orthogonal frequency
division multiplexing (OFDM) [3], [4]. In the firststep, the system detects the OFCDM or OFDM symbol timing. In the second step, it detects the group towhich a cell-specific scrambling code belongs as wellas the frame/slot timing. To detect this information, amethod that uses a synchronization channel [3] andone that uses a time-multiplexed common pilot channel [4] have been proposed. In the third and final step,
the system detects the scrambling code of the optimalcell among the scrambling-code candidates belonging to the scrambling-code group detected in the secNTT Technical Review

Special Feature

IP-based core network

RouterCell 1
Inter-sector handover→ soft handover(fast sector selection)

Cell 2

Inter-cell handover→ hard handoverFig. 3. Handover control.

ond step.
3.2 HandoverIn addition to handover between cells/sectors in acellular system, a function for achieving handover
between a cellular cell and an isolated cell is essential
to achieving seamless cell deployment. A handover
operation on the data-link or lower layer is calledinter-cell or inter-sector macro-diversity*2, and therequired QoS on these layers is guaranteed by
employing the macro-diversity. On higher application layers, both the data delay via wired transmission
based on the Internet protocol (IP) network in the system and the control signaling delay corresponding to
the mobility of the mobile terminal must be taken into
consideration. In W-CDMA, the mobile terminal sets
up dedicated channels with multiple cells/sectors andobtains a diversity effect by performing a soft handover in which the same information is transmitted tothe mobile terminal via multiple cells/sectors on thedownlink and received by multiple cells/sectors on
the uplink. This macro-diversity effect between
cells/sectors in a soft handover results in high-qualitytransmission in circuit-switching mode.
A broadband wireless access system, however,
employs transmission-slot allocation (packet scheduling) and ARQ, which means that an optimal handover algorithm on the data-link layer or below musttake into account the effects of packet scheduling and
ARQ. In this regard, a study has been performed on*2 Macro-diversity: Improvement obtained by combining the signals
received at multiple cells (or sectors).

Vol. 2 No. 9 Sep. 2004

throughput performance versus cell selection interval
during inter-cell macro-diversity reception for
OFCDM on the downlink [5]. For a cell selection
interval of more than 500 ms, cell-selection updating
cannot follow the shadowing variation. This results inthe selection of a cell with a small signal-to-interference power ratio (SIR), i.e., a large path loss, and anincrease in the required ratio of the signal energy per
bit to noise power spectrum density (Eb/N0).
Figure 3 shows the proposed method of handover
control for the data-link layer and below. When taking into account fast packet scheduling and ARQ,
fast hard handover with a cell selection interval of
about 100 ms is considered to be appropriate forinter-cell handover. A hard handover can simplify thecontrol of the core network between different cells
compared with a soft handover. As for inter-sector
handover, processing within the base station makes it
easier to distribute and combine data sequences. A
soft handover (including fast sector selection on thedownlink) is therefore expected to have a traffic-dispersion effect when traffic becomes concentrated in aparticular sector.
4. Broadband wireless access schemes
4.1 Duplexing
Two well-known duplexing schemes are frequency
division duplexing (FDD) and time division duplexing (TDD). TDD has the advantage of not requiring apair of frequency bands, although it does require basestations to be synchronized (frame synchronization)
for application to a cellular system. In an isolated-cell21

Special Feature

f1

fA

Frequency

System band

fB

fC

Frequency

System band(a) One-cell frequency reuse(b) Three-cell frequency reuse

Fig. 4. Comparison of one-cell and three-cell frequency reuse.

4.2 Cell frequency reuse
Figure 4 shows the frequency usage efficiency in acellular system for one-cell and three-cell frequency
reuse. With one-cell frequency reuse (Fig. 4(a)), thetransmitted signals are spread over the whole systemband by spreading in the time or frequency domain
while the despreading process decreases interference
and noise components by the inverse of the spreading
factor on average, which means that a spreading gain
can be expected. In ordinary time division multiple
access and frequency division multiple access, however, three-cell frequency reuse (Fig. 4(b)) is necessary to reduce co-channel interference. As a result, thefrequency bandwidth per cell/sector is 1/3 of the fullsystem band. In short, one-cell frequency reuse is an
essential condition for increasing system capacity in acellular system.
22100
One-cellfrequency reuse
80
Cumulative distribution (%)

environment, moreover, TDD can perform flexible
slot allocation for each link by varying the transmission/reception slot-allocation ratio on the uplink anddownlink using the same frequency band. However,in a cellular system, uplink/downlink slot allocation
ratio needs to be same among a cluster of neighboring cells. FDD, on the other hand, while requiring apair of frequency bands for the uplink and downlink,
does not require base-station synchronization, so it isconsidered to be preferable to TDD from the viewpoint of flexible cell deployment. While both willprobably have to be supported in the end, a system
configuration based on a single wireless interface(frame configuration) is desirable.

Three-cellfrequency reuse60
1 sector
3 sectors

40

20

0
–20

Space attenuation : 3.76 power law
Shadowing : lognormal distribution(standard deviation: 8 dB)
–10

0

10

20

30

40

50

60

Average received SIR

Fig. 5. Cumulative distributions of average received SIR
for one-cell and three-cell frequency reuse.

Figure 5 shows cumulative-distribution of average
received SIR (equal to the ratio of desired-signal
power to interference power from neighboring cells
in this case) for one-cell and three-cell frequency
reuse on the downlink of a cellular system. It is
assumed here that the cell radius is the same for allcells and that the transmission power is the same forall sectors. Distance-dependent path loss follows a
3.76 power law, while shadowing variation follows alognormal distribution with a standard deviation of 8
NTT Technical Review

Special Feature

dB. The 50% values of these cumulative-distribution
plots show that the received SIR for one-cell frequency reuse is about 10 dB less than that for three-cellfrequency reuse, which means that one-cell frequency reuse needs “gain” to suppress about 10 dB ofinterference from neighboring cells. Spreading of thetime or frequency domain can easily achieve one-cellfrequency reuse by effectively reducing interference
from neighboring cells. In addition, to prevent eachsymbol from being affected by same-pattern interference from neighboring cells in one-cell frequency
reuse, it is essential to employ cell-specific (or userspecific) scrambling code.

4.3 Downlink wireless access scheme:
VSF-OFCDM
Table 1 compares broadband wireless access systems on the downlink and Fig. 6 compares DSCDMA and multicarrier transmission systems on abroadband channel. These systems can be roughly
divided into those that perform spreading using cellspecific spreading codes and those that do not.
In a broadband wireless channel, a large number ofdelayed paths are observed. If spreading is applied tothe broadband wireless acess scheme based on thetime-domain-spreading approach taken by DSCDMA, the number of paths that can be resolved
increases. However, at the same time, paths having
different delay times generate interference (multipath
interference), which offsets the Rake diversity effect.

Table 1. Comparison of wireless access systems on the downlink.
Single/multicarrier
DS-CDMA

Access system

OFCDM

OFDM

Single/multicarrier
TDMA

Spreading

Yes

Yes

No

No

Number of carriers

1 or a few

Many

Many

1 or a few

Effects of multipath
interference

Degrades the Rake
diversity effect

Robust against multipath interference

Robust against multipath interference

Degrades signal due tointer-symbol interference

Frequency reuse

One-cell frequency
reuse

One-cell frequency
reuse

Cell frequency reusebasically needed

Cell frequency reusebasically needed

DS-CDMA system

Multicarrier transmission system(OFCDM, OFDM)
Path #1

Path #1

Path #2

Path #2

Path #3

Rake combining
Multipath interference component

Path #3

Guard interval(>maximum delay time of multipath)

Frequency

Frequency

Fig. 6. Comparison of wireless access systems in a broadband channel.

Vol. 2 No. 9 Sep. 200423

Special Feature

If the SIR of each despread path is very small, the signal after Rake combining cannot achieve the required
SIR. Therefore, a wireless access system robust
against multipath interference is therefore essential toachieving high-quality signal transmission in a
broadband channel. In this regard, a multicarriertransmission scheme first subjects a high-speed data
stream to a serial-parallel conversion generating
many subcarrier signals whose symbol length is sufficiently longer than the multipath propagation delay
time. It then proceeds to transmit these low-symbolrate data streams in parallel. In other words, in multicarrier transmission, the frequency band is divided
into a large number of narrow-band signals resulting
in a small bandwidth per subcarrier. This means that
amplitude and phase variations within a subcarrier
can be treated as flat fading and that the effects ofwaveform distortion caused by frequency-selective
fading can be reduced. Even if there is a subcarrier
whose received power has dropped due to fading, its
decoded error can be compensated for by applying
error-correction channel coding across multiple subcarriers whose received power has not dropped,
resulting in high-quality reception.
On the downlink of a cellular system, OFCDM,
which is based on multi-carrier CDMA (MC-CDMA)[6], [7], can reduce the effects of multipath interference while obtaining a frequency diversity effect by
spreading and mapping channel-coded symbols
across multiple subcarriers. For this reason, OFCDM
is more suitable for increasing capacity than otherCodewireless access systems [8], [9]. Thus, in addition to
being robust against multipath fading, which is acharacteristic of multicarriers, OFCDM can achieve
one-cell frequency reuse in a flexible manner andincrease system capacity by applying spreading ineither the time or frequency domain. In an isolatedcell environment, however, interference from neighboring cells is small and system capacity can generally be increased by not using spreading. The reason
for this is that when spreading in the frequency
domain is used, code orthogonality among multiplexed code channels collapses due to frequencyselective fading caused by multipath interference.
Similarly, when spreading in the time domain is used,
amplitude fluctuation occurs in the time domain in ahigh-speed Doppler environment, giving rise to intercode interference that in turn destroys the orthogonality in the time domain. In either case, it is difficult
to multiplex as many code channels as corresponds tothe spreading factor.
To deal with the above situation, OFCDM with avariable spreading factor (VSF-OFCDM) has been
proposed [10], [11]. This system adapts the variablespreading factor in the OFCDM time and frequency
domains according to the cell configuration, propagation conditions (delay spread, maximum Doppler frequency, and interference from other cells), channelload, radio parameters (data modulation scheme andchannel coding rate), and other conditions. Figure 7
shows the concept of VSF-OFCDM with two-dimensional spreading. Here, one data modulation symbol

Time domain
spreading (SFTime)Time

Frequency domain
spreading (SFFreq)

User 1
User 2User 3Frequency

Total spreading factor SF > 1

Time-domain-spreading only
(SFFreq = 1)

Hotspot indoor cell

Cellular system

Control of variable spreading factors(SFTime, SFFreq)

Fig. 7. Concept of VSF-OFCDM using two-dimensional spreading.

24

NTT Technical Review

Special Feature

Table 2. Comparison of wireless access systems on the uplink.
Access system

Single/multicarrier
DS-CDMA

OFCDM

OFDM

Single/multicarrier
TDMA

Spreading

Yes

Yes

No

No

Number of carriers

1 or a few

Many

Many

1 or a few

Low power-consuming
terminal

Low peak-power transmission
possible by spreading

Large peak power

Large peak power

Large peak power

Large capacity

Performance improvedby the Rake diversity

Accuracy of channel
estimation degrades

Accuracy of channel
estimation degrades

Signal degrades due tointer-symbol interference

Frequency reuse

One-cell frequency
reuse

One-cell frequency
reuse

Cell frequency reusebasically needed

Cell frequency reusebasically needed

will be spread across several consecutive OFCDM
symbols and several consecutive subcarriers (thesenumbers are given by SF Time and SF Freq , which
denote the spreading factor in the time and frequency
domains, respectively) with the total spreading factor
represented as the product SF = SFTime × SFFreq. As
indicated in Fig. 7, two-dimensional VSF-OFCDM
can (i) control the total spreading factor in accordance with the cell configuration (the mobile terminal
sets the spreading factor based on control informationfrom the base station), (ii) adaptively control SFTimeand SFFreq in accordance with propagation conditions, channel load, and radio parameters, and (iii)maximize channel capacity for both a cellular system
and an isolated-cell environment.
4.4 Uplink wireless access scheme:
VSCRF-CDMA
Table 2 compares broadband wireless access systems on the uplink. From the viewpoint of low
mobile-terminal power consumption, the mostimportant requirement on the uplink, the DS-CDMAapproach is more advantageous than the multicarrier
approach as in OFDM and OFCDM, which use many
subcarriers having a large peak-to-average power
ratio. The uplink, moreover, requires a dedicated pilot
channel for each mobile-terminal physical channel toperform coherent detection and demodulation. Consequently, for OFDM and OFCDM, channel estimation must be performed for each subcarrier, and forthe same pilot power condition, the pilot channel signal power per subcarrier is small compared with thatof DS-CDMA. As a result, the DS-CDMA system
has been reported to maintain more accurate channelestimation, to reduce the received Eb/N0 that satisfies
the required packet error rate compared with multicarrier systems, and to be capable of increasing thelink capacity [12]. Moreover, in DS-CDMA, there isVol. 2 No. 9 Sep. 2004

a bandwidth that can minimize the required transmission power (i.e., received Eb/N0). This optimal subcarrier bandwidth is determined based on the tradeoff
between the Rake diversity and increasing multipath
interference. When propagation conditions (such as
delay profile shape and number of paths) and the
spreading factor are given as parameters, the received
Eb/N0 required can be reduced the most by a subcarrier bandwidth from 20 to 40 MHz [12]. Accordingly, a multicarrier/DS-CDMA system configured on
the basis of this optimal subcarrier bandwidth inaccordance with the system band is a promising wireless access system from the viewpoint of link capacity.
The DS-CDMA system can achieve one-cell frequency reuse in a flexible manner through spreadingin the time domain. The advantage of one-cell frequency reuse diminishes, however, in an isolated-cell
environment for which interference from neighboring
cells is small. Here, link capacity turns out to be 2030% of the spreading factor when the voice activity
factor is not used. To support a single air interface forboth multi- and isolated-cell-environment DSCDMA, the link capacity needs to be increased for
isolated cells. Since interference from other users andmultipath interference make it difficult to achieveorthogonalization in the code domain when the bandwidth is broadened, orthogonalization between
simultaneous users in the frequency or time domain
must be established.
Figure 8 shows the concept called variable spreading and chip repetition factors CDMA (VSCRFCDMA) [13]. In a multi-cell environment, it is usual
to perform spreading only in the time domain, whichmakes one-cell frequency reuse easy to achieve. In an
isolated-cell environment, on the other hand, theprinciple of symbol repetition is applied to the spreadchip sequence and chip repetition is performed. Here,25

Special Feature

1 symbol =SFcellular chip

1 symbol =SFhotspot chip × repetition CRF times

Time

User
1
2
3
4

Time

Time/frequency conversion

Time/frequency conversion

Frequency

Frequency

Hotspot indoor cell
Variable spreading factor,
chip repetition factor(SF, CRF)

Cellular system

Fig. 8. Concept of VSCRF-CDMA.

SF denotes the spreading factor for total band broadening. Its value is determined by the symbol rate of
the physical channel. In an isolated-cell environment,
CRF is set to 1 or greater, and the time-domain
spreading factor SFhotspot is made small based on therelationship SF = SFhotspot × CRF. This kind of control
makes it possible to allocate received signals from anumber of simultaneous users equal to CRF to a setof subcarriers that are mutually orthogonal in the frequency domain. In the method proposed here, CRF
and SFhotspot are varied adaptively according to thecell configuration (multiple cells or an isolated cell),number of simultaneously accessed channels, andpropagation condition (number of multipaths).
4.5 Physical channel configuration
Figure 9 shows the configuration of physical channels. First, the common control channel transmits
broadcast and paging information at a fixed level oftransmission power. This channel uses a fixed modulation scheme, quadrature phase shift keying(QPSK), and a low coding rate so that reception canbe performed at the required quality and coverage
within the cell. Next, the shared packet channeltransmits high-speed packet data at a fixed level oftransmission power. It applies AMC using a modulation scheme and channel coding rate appropriate forthe received SIR and provides a maximum throughput that guarantees the required received-packet
26

error rate. Finally, the associated control channel
sends control information for the physical and datalink layers to facilitate high-quality transmission on
the shared packet channel. It features a fixed modulation scheme (QPSK) and coding rate and applies
transmission-power control to compensate for fluctuations in the received level caused by instantaneous
fading.
5. Efficient packet access techniques
Table 3 lists realtime (RT) and non-realtime (NRT)
traffic requirements [14], [15]. The transmission of
RT traffic data such as audio and video broadcasts
must provide guaranteed reception quality at or
below the required residual packet error rate while
meeting delay requirements. In contrast, NRT traffic
data such as file transfer and WWW browsing mustensure the delay not more than a few seconds, which
is a less stringent requirement than that of RT traffic
data, but the data transmission must be error free
within this delay. Figure 10 shows packet control on
the data-link and physical layers [16]. On these layers, scheduling is performed to satisfy the required
delay and IP packet loss rate on the RAN in accordance with the traffic data in question. On the downlink, efficient transmission-slot allocation (scheduling) is performed according to the received SIR andtype of traffic data (RT or NRT) of each mobile terNTT Technical Review

Special Feature

Good
Received SIR
BadCommon control channel:
Fixed transmission power andfixed modulation scheme

Fixed transmission power

Shared/common packet channel:
Fixed transmission power with AMC

Fixed transmission power

Associated control channel:
Transmission-power control andfixed modulation scheme

Transmission-power control
Time

Information bit rate
High speed

Low speed

Fig. 9. Physical channel configuration.

Table 3. Traffic requirements.
Servise class

ExampleRealtime (RT)
Non-realtime(NRT)

Pacekt loss rate

Audio and video
broadcasts<150 ms

0.5% or lessFile transfer, WWW
browsing<2-3 s0%minal. On the uplink, each terminal sends a reservation packet beforehand to report the QoS requirements of subsequent data packets, data size, channel
conditions, etc., to the base station and the base station performs transmission-slot allocation on the data
packet channel for each mobile terminal based on thisinformation. The system also applies MAC-layer
ARQ to the packet data channel according to therequired delay time. The above approach can reduce
the required transmission power through a time diversity effect, especially in the case of NRT traffic data
[17].
6. Multi-antenna transmission/reception
techniques
6.1 Configurations
A broadband wireless access system is expected touse a high carrier frequency to support high-speedtransmission bit rates, and if such a service is to beprovided to a wide area, small zones (micro-cells)
Vol. 2 No. 9 Sep. 2004

QoS requirements (examples)
End-to-end delay

and low required received Eb/N0 will be indispensable. Therefore, it would be useful to have an adaptive antenna array that can generate directional beams
in the angular direction of each user. At the sametime, a multiple-input-multiple-output (MIMO), spatially multiplexed multi-antenna transmitting/receiving scheme is effective for increasing the throughput(frequency efficiency). This scheme uses multiple
transmitting/receiving antennas and radio devices
and transmits different data streams from each transmitter. On the downlink, VSF-OFCDM wirelessaccess with a frequency bandwidth of 100 MHz
enables a peak throughput of 200/300 Mbit/s to beachieved by combining 16QAM/64QAM data modulation and a channel coding rate of 3/4. A four-antenna MIMO, for example, might achieve 1-Gbit/s transmission (frequency efficiency: 10 bit/s/Hz) for a frequency bandwidth of 100 MHz in an isolated-cell
environment. Below we discuss three multi-antennatransmitter/receiver configurations from the viewpoint of improving frequency efficiency. Table 427

Special Feature

Traffic data
(User #1)

Traffic data
(User #K)

Detect QoS class(delay, IP packet loss rate)

Detect QoS class(delay, IP packet loss rate)

Determine priority

Determine priority

Feed back channel
quality information
Perform high-speed packet
scheduling
Allocate transmission slots to each user

Fig. 10. Packet scheduling on the data-link and physical layers.

Table 4. Comparison of multi-antenna transmitter/receiver configurations.

EffectMIMO multiplexing

MIMO diversity

Adaptive-antenna-arraytransmission

Increases information bit rate

Increases diversity gain

Increases average received SIR

Required transmission-antennaintervalInter-antenna fading
correlation

Large

Small

Large

Small signal-separation ability

Small transmission-diversity gain

Large antenna gain

Small

Large signal-separation ability

Large transmission-diversity gain

Small antenna gain

Own-cell/other-cellinterference-suppression effect

No

Yes

RF circuit calibration

Unnecessary

Necessarysummarizes the comparison.
• MIMO multiplexing method: transmits different
data streams on independent radio propagation
paths using the same frequency band and time slot
[18]
• MIMO diversity method: performs space-time coding and transmits data streams amongst multiple
antennas [19]• Adaptive-antenna-array transmission method: performs directional transmission using multiple
transmitting antennas(1) The MIMO transmitter/receiver configuration(MIMO multiplexing method) is shown in Fig.
11(a). This method performs serial/parallel conversion on a modulated data stream to produce N
separate streams and transmits them by spatial
parallelism. Since different data streams aretransmitted on the same frequency band and time28

slot, the received signals carrying these N data
streams must be separated at the receiver. For thispurpose, the following methods have been proposed .
• Separate signals by maximum likelihood detection (MLD)• Combine received signals by weighting so as tominimize the mean square error• Successively extract regenerated data replicas
from the received signal (Vertical Bell Laboratories layered space time (V-BLAST)) [20](2) In the MIMO diversity method, the transmitter
performs space time block coding (STBC) [19]after information bits have been channel encodedand data modulated and generates and transmits
N streams of coded data. The receiver performs
STBC decoding at each antenna and then performs antenna-diversity reception through maxiNTT Technical Review

Special Feature

Data
modulation
section 2
Datamodulation
section N

Transmits different data bydifferent antennas

Data
demodulation
section 1
Data
demodulation
section 2
Data
demodulation
section N

Receiver

Combines N data
streams

Data
modulation
section 1

Received-signal
separation section

Serial-to-parallel conversionof among N antennas

Transmission
data

Arrangement of M
separated antennas

Arrangement of N
separated antennas

Transmitter

Received
data

Increases information bit rate by N times(a) MIMO transmission/reception

Close arrangementof N antennas

Transmission
data

Data
modulation
sectionDirectional transmission

Make N-antennas
worth of copies

Transmitter

ReceiverIncreases received-signal
power by N times

Data
demodulation
sectionReceived
data

Antenna weightgeneration section(b) Adaptive antenna array transmission

Fig. 11. Multi-antenna transmission/reception methods.

mal ratio combining (MRC).
(3) A transmitter/receiver configuration for performing adaptive-antenna-array transmission is shownin Fig. 11(b). In this configuration, the system
subjects a data stream to channel coding andmodulation (including data modulation andspread modulation), copies the result N times corresponding to the number of antennas, and multiplies each stream by an antenna weight unique toeach antenna branch. The antenna weights are
generated adaptively by first estimating the direction of a signal received from the target user on
the uplink and then computing them so that themain beam of the transmitting antenna pattern isdirected toward the user [21]. On the receivingside, the system performs antenna diversity
reception by MRC. This type of adaptive-antenna-array transmission can ideally increase antenna gain by N times, where N is the number oftransmitting antennas.
6.2 Comparison of configurations for ultrahighspeed signal transmission
As shown in Table 4, the MIMO multiplexing
Vol. 2 No. 9 Sep. 2004

method, which transmits information in parallel via
multiple transmitting antennas, can increase theinformation bit rate by simply increasing the numberof transmitting antennas, unlike the other two methods. In MIMO diversity and adaptive-antenna-arraytransmission, the throughput can be increased byincreasing either the modulation level or channelcoding rate. For VSF-OFCDM wireless access with afrequency bandwidth of 100 MHz, assuming fourantennas for transmission and four for reception, theMIMO multiplexing method can achieve a highspeed, high-efficiency throughput of 1 Gbit/s (10bit/s/Hz) using 16QAM or 64QAM data modulation.
The MIMO diversity or adaptive-antenna-array transmission methods, on the other hand, would have toemploy high-efficient multilevel modulation, so theachievable throughput performance is degraded dueto the decrease in the minimum Euclidean distance.
Overall, the MIMO multiplexing method, whichtransmits multiple data streams by spatial multiplexing, is the most appropriate method for achieving
high-efficiency transmission such as 10 bit/s/Hz [22].

29Special Feature

7. Conclusion
This article gave an overview of broadband wireless access technology featuring seamless support ofvarious communication environments from cellular
systems to isolated cells like hotspots and offices all
by a single wireless interface. In future research, weplan to perform indoor and outdoor experiments tomeasure propagation characteristics and evaluate
packet-signal transmission on a broadband channel.
References
[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

30

3GPP, TR25.848, “Physical Layer Aspects of UTRA High Speed
Downlink Packet Access.” (UTRA: UMTS (universal mobile
telecommunications system) terrestrial radio access)M. Tanno, H. Atarashi, K. Higuchi, and M. Sawahashi, “Fast Cell
Search Algorithm for System with Coexisting Cellular and Hot-spot
Cells for OFCDM Forward Link Broadband Wireless Access,” IEEE
VTC 2003-Spring, pp. 133-138, Apr. 2003.
Y. Ishii, K. Higuchi, and M. Sawahashi, “Three-step Cell Search
Algorithm Employing Synchronization and Common Pilot Channels
for OFCDM Broadband Wireless Access,” IEICE Trans. Commun.,Vol. E85-B, No. 12, pp. 2672-2683, Dec. 2002.
M. Tanno, H. Atarashi, K. Higuchi, and M. Sawahashi, “Three-step
Cell Search Algorithm Employing Common Pilot Channel for
OFCDM Broadband Wireless Access,” IEICE Trans. Commun., Vol.
E86-B, No. 1, pp. 325-334, Jan. 2003.
A. Morimoto, S. Abeta, and M. Sawahashi, “Cell Selection Based on
Shadowing Variation for Forward Link Broadband OFCDM Packet
Wireless Access,” IEEE VTC 2002-Fall, pp. 2071-2075, Sep. 2002.
N. Yee, J. P. Linnartz, and G. Fettweis, “Multi-Carrier CDMA in
Indoor Wireless Radio Networks,” PIMRC’93, pp. 109-113, Sep.
1993.
K. Fazel and L. Papke, “On the Performance of Convolutional-coded
CDMA/OFDM for Mobile Communication Systems,” PIMRC’93,
pp. 468-472, Sep. 1993.
S. Abeta, H. Atarashi, M. Sawahashi, and F. Adachi, “Performance ofCoherent Multi-Carrier/DS-CDMA and MC-CDMA for Broadband
Packet Wireless Access,” IEICE Trans. Commun., Vol. E84-B, No. 3,
pp. 406-414, Mar. 2001.
H. Atarashi, S. Abeta, and M. Sawahashi, “Broadband Packet Wireless Access Appropriate for High-speed and High-capacity Throughput,” IEEE VTC2001-Spring, pp. 566-570, May 2001.

[10] M. Sawahashi and H. Atarashi, “Broadband TD-OFCDM Packet
Transmission using Variable Diffusion Rate,” 2001 IEICE General
Conference, B-5-97, Mar. 2001 (in Japanese).
[11] H. Atarashi, S. Abeta, and M. Sawahashi, “Variable Spreading Factor
Orthogonal Frequency and Code Division Multiplexing (VSFOFCDM) for Broadband Packet Wireless Access,” IEICE Trans.
Commun., Vol. E86-B, No. 1, pp. 291-299, Jan. 2003.
[12] S. Suwa, H. Atarashi, S. Aheta, and M. Sawahashi, “Optimum Bandwidth per Sub-carrier of Multicarrier/DS-CDMA for Broadband
Packet Wireless Access in Reverse Link,” IEICE Trans. Fundamentals, Vol. E85-A, No. 7, pp. 1624-1634, July 2002.
[13] H. Atarashi, T. Kawamura, and M. Sawahashi, “Variable-diffusionrate Multi-carrrier/DS-CDMA Uplink Broadband Wireless Accessusing Symbol Repetition,” 2003 IEICE General Conference, B-5-53,
Mar. 2003 (in Japanese).
[14] Recommendation G.114, Telecommunication Standardization Sectorof ITU, “One-way Transmission Time”, Geneva, Switzerland, Mar.
1993.
[15] Recommendation G.1010, Telecommunication Standardization Sector of ITU, “End-user Multimedia QoS Categories,” Nov. 2001.
[16] A. Harada, S. Abeta, and M. Sawahashi, “Adaptive Radio Parameter
Control Considering QoS for Forward Link OFCDM WirelessAccess,” IEICE Trans. Commun., Vol. E86-B, No. 1, pp. 314-324,
Jan. 2003.
[17] Y. Iizuka, M. Tanno, and M. Sawahashi, “Efficient random accesschannel transmission method utilizing soft-combining of retransmitted message packets according to QoS,” IEEE 1CCS2002, pp. 441445, Nov. 2002.
[18] G. J. Foschini, Jr., “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” Bell Labs Tech. J., pp. 41-59, Autumn 1996.
[19] V. Tarokh, H. Jafarkhani, and R. Calderbank, “Space-Time Block
Coding for Wireless Communications: Performance Results,” IEEE J.
Select Areas Commun., Vol. 17, No. 3, pp. 451-460, Mar. 1999.
[20] P. W. Wolniansky, G. J. Foschini, G. D. Golden, and R. A. Valenzuela, “V-BLAST: an architecture for realizing very high data rates
over the rich-scattering wireless channel,” 1998 URSI International
Symposium on Signals, Systems and Computers, pp. 295-300, Sep.
1998.
[21] H. Taoka, S. Tanaka, T. Ihara, and M. Sawahashi, “Adaptive Antenna Array Transmit Diversity in FDD Forward Link for W-CDMA andBroadband Packet Wireless Access,” IEEE Wireless Communications, Vol. 9, pp. 34-41, Apr. 2002.
[22] J. Kawamoto, T. Asai, K. Higuchi, M. Sawahashi, “Comparison of
Signal Transmission Methods using MIMO Channel of VSFOFCDM toward 1-Gbit/s Data Transfer,” IEICE Technical Report,
RCS2003-23, Apr. 2003 (in Japanese).

NTT Technical Review

Special Feature

Mamoru Sawahashi

Kenichi Higuchi

Director, IP Radio Network Development
Department, NTT DoCoMo, Inc.
He received the B.S. and M.S. degrees from the
University of Tokyo, Tokyo in 1983 and 1985,
respectively, and the Dr. Eng. degree from the
Nara Institute of Technology in 1998. In 1985, hejoined NTT Electrical Communications Laboratories. In 1992, he transferred to NTT Mobile
Communications Network, Inc. (now NTT
DoCoMo, Inc.). He is a member of IEEE and theInstitute of Electronics, Information and Communication Engineers of Japan (IEICE). He iscurrently serving as an editor for IEEE Transactions on Wireless Communications.

Assistant Manager, IP Radio Network Development Department, NTT DoCoMo, Inc.
He received the B.S. degree from Waseda University, Tokyo in 1994 and the Ph.D. degree inelectrical and communication engineering fromTohoku University, Sendai, Miyagi in 2002. In
1994, he joined NTT Mobile Communications
Network, Inc. (now, NTT DoCoMo, Inc.). He isa member of IEICE.

Sadayuki Abeta

Motohiro Tanno

Manager, IP Radio Network Development
Department, NTT DoCoMo, Inc.
He received the B.E., M.E., and Ph.D. degrees
in electrical communication engineering from
Osaka University, Suita, Osaka in 1993, 1995,
and 1997, respectively. In 1997, he joined NTT
Mobile Communications Network, Inc. (nowNTT DoCoMo, Inc.). He is a member of IEEEand IEICE.

Assistant Manager, IP Radio Network Development Department, NTT DoCoMo, Inc.
He received the B.E. and M.E. degrees in electrical and electronic engineering from Kyoto
University, Kyoto in 1993 and 1995, respectively. In 1995, he joined NTT Mobile Communications Network, Inc. (now NTT DoCoMo, Inc.).
He is a member of IEICE.

Hiroyuki Atarashi

Taisuke Ihara

Assistant Manager, IP Radio Network Development Department, NTT DoCoMo, Inc.
He received the B.E., M.E., and Ph.D. degrees
in electrical engineering from Keio University,
Tokyo in 1994, 1996, and 1999, respectively. In
1999, he joined NTT Mobile Communications
Network, Inc. (now NTT DoCoMo, Inc.). He is amember of IEEE and IEICE.

Manager, IP Radio Network Development
Department, NTT DoCoMo, Inc.
He received the B.S. degree from the Science
University of Tokyo, Tokyo in 1992 and the M.S.
and Ph.D. degrees from Tohoku University,
Sendai, Miyagi in 1994 and 2003, respectively.
In 1994, he joined NTT Mobile Communications
Network, Inc. (now NTT DoCoMo, Inc.). He is amember of IEICE.

Vol. 2 No. 9 Sep. 200431