WIRELESS COMMUNICATIONS AND MOBILE COMPUTING
Wirel. Commun. Mob. Comput. 2011; 11:1551–1563
Published online 3 November 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/wcm.1212

SPECIAL ISSUE PAPER

Recent advances in single-carrier distributed
antenna network
Fumiyuki Adachi* , Kazuki Takeda, Tetsuya Yamamoto, Ryusuke Matsukawa andShinya Kumagai
Department of Electrical and Communication Engineering, Graduate School of Engineering, Tohoku University 6-6-05, Aza-aoba,
Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan

ABSTRACT
For the realization of future wireless networks, gigabit wireless technology, which can achieve higher-than-1 Gbps datatransmission with extremely low transmit power, is indispensable. We have been studying the distributed antenna network(DAN) and the frequency domain wireless signal processing. In DAN, many antennas or clusters of antennas are spatiallydistributed over a service area, and they are connected by means of optical fiber links with DAN signal processing center.
A number of distributed antennas cooperatively serve mobile users using spatial multiplexing, diversity, array, or relaying
technique. In this paper, the recent advances in single-carrier DAN are introduced. Copyright © 2011 John Wiley & Sons,
Ltd.
KEYWORDS
component; distributed antenna network; transmit antenna diversity; space–time coding; spatial multiplexing; frequency domain
equalization*Correspondence
Fumiyuki Adachi, Department of Electrical and Communication Engineering, Graduate School of Engineering, Tohoku University
6-6-05, Aza-aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan.
E-mail: adachi@ecei.tohoku.ac.jp

1. INTRODUCTION
In the early 1980s, cellular mobile communication systems, which made “anytime, anywhere” communications
possible, appeared. Cellular mobile communication systems have evolved from narrowband networks of around
10 kbps (first and second generation systems) to wideband
networks of around 10 Mbps (third generation systems).
Along with the advancement of wireless techniques, wireless services have been shifted from simple voice to data
and then broadband Internet-related data services including video data. Now, the third generation long term evolution systems with 100 Mbps peak data rate are deployedin some countries [1]. A next step is the development ofbroadband wireless technology that will be used in the
fourth generation systems of a peak data rate of around
1 Gbps.
In the future fixed networks, a variety of broadband network services will be made available by the so-called cloudcomputing networks. Along with this evolution of fixed
networks, wireless networks need to further evolve in order
to extend a variety of broadband network services available

Copyright © 2011 John Wiley & Sons, Ltd.

in the fixed networks to wireless users. Wireless access networks need to be enhanced to provide a gigabit wirelesspipe to each user mobile terminal (MT).
However, there are a number of important technicalissues to be addressed, for example, limited bandwidth,
frequency-selective fading, and limited transmit power. In
this paper, some of the aforementioned technical issues are
discussed, and then a distributed antenna network (DAN)
is introduced as a promising solution.
The gigabit wireless channels are extremely frequency
selective. The received signal spectrum is severely distorted. An advanced equalization technique is necessary toachieve high-quality gigabit wireless data transmissions ina strong frequency-selective channel. A promising equalization technique is frequency domain equalization (FDE)[2–4]. FDE is an attractive equalization technique thatcan be combined with transmit diversity, receive diversity,
beamforming, multiplexing, cooperative relaying, hybrid
automatic repeat request combining, and so on.
In addition, the path loss and shadowing loss cause
a severe power loss because the transmit power is limited. The communication range is limited by the uplink

1551

F. Adachi et al.

Single-carrier distributed antenna network(MT-to-base station (BS)). With the same MT transmit
power as in the present wireless systems (i.e., third generation cellular systems), the communication range of thegigabit wireless networks will significantly shrink, andgigabit wireless services may be available near BS only.
Therefore, a fundamental change is necessary in wirelessaccess network architecture.
Distributed antenna system or DAN [5] combined withfrequency domain signal processing has a potential to solve
the aforementioned problems. For uplink signal transmissions, the single-carrier (SC) transmission is promisingbecause it has a lower peak-to-average power ratio thanmulticarrier transmission (using a transmit power amplifier
with the same peak power, SC provides longer communication range than multicarrier). Therefore, we have been
investigating the potential of SC-DAN [5].
In this paper, we first give an overview of DAN in
Section 2 and then present space–time-coded diversity
combined with one-tap FDE for SC-DAN downlink anduplink in Section 3. In Section 4, spatial multiplexing for SC-DAN is presented. Finally, Section 5 offers
conclusions.

2. OVERVIEW OF DISTRIBUTEDANTENNA NETWORKIn DAN, as shown in Figure 1, the conventional BS is
replaced by the signal processing center (SPC), and manyantennas or clusters of antennas are spatially distributed
around the SPC so that some antennas can always be visible from an MT with a high probability. Antennas orantenna clusters are connected to an SPC by means of optical fiber links or wireless links. A number of distributed
antennas cooperate and act as spatial multiplexing, antenna
diversity, relay, or antenna array. Probably the most powerful application is distributed transmit/receive antenna
diversity combined with FDE. The problems that result
from distance-dependent path loss and shadowing loss aswell as from the instantaneous signal power variations due
to fading can simultaneously be mitigated.

3. DIVERSITY
For DAN uplink and downlink transmissions, it is desirable to use as many distributed antennas as possible,
whereas the number of MT antennas is limited to one
or two because there is not enough space to equip too
many antennas at an MT. As a promising DAN downlink transmit diversity technique, we have developed afrequency domain space–time block-coded joint transmit/receive diversity (FD-STBC-JTRD) combined withtransmit FDE [6]. The FD-STBC-JTRD is an extension of
joint transmit diversity/transmit FDE [7] by adding antenna
diversity reception and achieves an Ndan  Nmt th order(full) diversity gain, where Ndan and Nmt represent the
number of distributed transmit antennas and that of MT
receive antennas, respectively.
The FD-STBC-JTRD allows the use of up to Nmt D
4 MT receive antennas while requiring only simple addition, subtraction, and conjugate operations at an MT
receiver and thus alleviates the computational complexity problem of MT receivers. Although the number of MT
receive antennas is limited to Nmt D 4, FD-STBC-JTRD
allows the use of an arbitrary number of distributed transmit antennas, Ndan . There are two types of transmit FDE
weights: minimum mean square error (MMSE) weight andmaximum channel capacity weight.
For the uplink transmissions, frequency domain space–time transmit diversity (FD-STTD) [8] is promising. Thisis because FD-STTD allows the use of an arbitrary number
Ndan of distributed receive antennas although the number
Nmt of MT transmit antennas is limited. A combination of
FD-STBC-JTRD for downlink and FD-STTD for uplink issuitable for DAN.
In Section 3.1, we present the signal representation for
SC-DAN downlink using FD-STBC-JTRD. Section 3.2
introduces the transmit FDE weight for FD-STBC-JTRD.
The SC-DAN uplink using FD-STTD is described in
Section 3.3. Section 3.4 discusses the computer simulation results. Section 3.5 compares the uplink and downlink
performances in terms of bit error rate (BER).

DAN
SPC

Spatialmultiplexing/
diversity

Relay

Distributed antenna Optical fiber link
Figure 1. Distributed antenna network (DAN). SPC, signal processing center.

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Wirel. Commun. Mob. Comput. 2011; 11:1551–1563 © 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/wcm

F. Adachi et al.

Single-carrier distributed antenna network

#1
+ GI

Nc-Point IFFT

STBC- JTRD
encoder

Nc-Point FFT

Data mod.

#Ndan−1#0

SPC

Ndan distributed antennas

Data demod.

Nmt receive antennas

Nc-Point IFFT

#0

STBC- JTRD
decoder− GI#Nmt−1

Nc-Point FFT(a) SPC transmitter(b) MT receiver
Figure 2. Downlink transmitter/receiver structure using frequency domain space–time block-coded joint transmit/receive diversity(FD-STBC-JTRD): (a) signal processing center (SPC) transmitter and (b) mobile terminal (MT) receiver. FFT, fast Fourier transform;GI, guard interval.
Table I. J , Q, and coding rate R .
No. of transmit antennas

No. of receive antennas

J

Q

Coding rate

1
2
3
4

1
2
3
3

1
2
4
4

1
1
3/4
3/4

Arbitrary

The SC signal transmission is a block transmission. In
this paper, a block size of Nc symbols is assumed.
3.1. FD-STBC-JTRD (downlink)
The FD-STBC-JTRD transmitter/receiver structure using
Ndan distributed transmit antennas and Nmt MT receive
antennas is illustrated in Figure 2.
At the SPC, a sequence of J blocks of Nc symbols
each is transformed by an Nc -point fast Fourier transform(FFT) into a sequence of J frequency domain signals andthen encoded into Ndan (the number of distributed transmit antennas) streams of Q-coded frequency domain signal
blocks each. FD-STBC-JTRD decoding at an MT receiverneeds addition, subtraction, and conjugate operations only.
A combination of J , Q, and coding rate R are shown in
Table I for Nmt D 1 to 4.
The achievable diversity gain increases with increasing
Nmt ; however, the coding rate reduces to 3/4 when Nmt D 3
and 4. This suggests that the channel capacity or throughput may be maximized at Nmt D 2. Later, for the sake ofbrevity, only the signal representation for Nmt D 2 (therefore, J D Q D 2) is presented (for Nmt D 3 and 4 [6]).

However, the computer simulation results on the channel
capacity will be shown for Nmt D 1  4 to confirm thatNmt D 2 maximizes the channel capacity.
When Nmt D 2, a pair of Nc symbol blocks (J D 2)
to be transmitted are transformed by Nc -point FFT into
a pair of frequency domain signals fD.k/; D1 .k/I k D
0  Nc  1g .Q D 2/, which is represented by D.k/ D
ŒD0 .k/ D1 .k/T : D.k/ is then encoded into Ndan pairs of
frequency domain-coded signal blocks as2

S0;0 .k/6
S
0;1 .k/6
Sstbc-jtrd .k/ D 6::
4:S0;Ndan 1 .k/

3
S1;0 .k/7
S1;1 .k/7
7::
5:
S1;Ndan 1 .k/

DW.k/Dstbc-jtrd .k/(1)where Sq n .k/, q D 0  1, is the kth frequency component
at the nth transmit antenna. In Equation (1), Dstbcjtrd .k/is the encoding matrix of size 2  2 for Nmt D 2, given by

Wirel. Commun. Mob. Comput. 2011; 11:1551–1563 © 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/wcm


Dstbc-jtrd .k/ D

D0 .k/
D1 .k/

D1 .k/D0 .k/


(2)

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F. Adachi et al.

Single-carrier distributed antenna networkW(k/ is the transmit FDE weight matrix of size 2  Ndan ,given asW.k/ D A.k/HH .k/

(3)

where

H.k/ D

H0;0 .k/
H1;0 .k/

H0;1 .k/
H1;1 .k/

H0;Ndan 1 .k/
H1;Ndan 1 .k/(4)and . /H denotes the Hermitian transpose. In Equation (3),A.k/ is introduced to keep the transmit power intact after
the encoding and will be discussed in Section 3.2. Hm;n .k/
in Equation (4) is the channel gain between the nth
distributed transmit antenna and the mth MT receive
antenna. An Nc -point inverse FFT (IFFT) is applied to
fS.k/I k D 0  Nc  1g to obtain Ndan pairs of Nc symbol
blocks to be transmitted from Ndan distributed antennas.
At the MT receiver, a superposition of Ndan pairs ofcoded Nc symbol blocks is received by Nmt D 2 antennas.
The received signals are transformed by Nc -point FFT intofrequency domain signals, fRm;0 .k/ and Rm;1 .k/I m D
0  1g, which can be expressed using the matrix form as
R.k/ Ds
D

R0:0 .k/R1:0 .k/R0;1 .k/R1;1 .k/

It can be clearly seen from Equation (7) that the
downlink FD-STBC-JTRD using Ndan distributed transmit antennas and Nmt MT receive antennas can achieve an
Ndan  Nmt .D 2/th order (full) diversity gain.
In the following, we consider an SC-DAN with Ndan
distributed antennas and Nmt MT antennas.
3.2. Transmit FDE weight for
FD-STBC-JTRD (downlink)
Two types of transmit FDE weights are presented: joint
water filling and maximal ratio transmission (WF-MRT)
weight [9] and MMSE weight [6].
3.2.1. Joint WF-MRT weight.
The joint WF-MRT weight is the one that maximizes
the channel capacity. The problem formulation of channel
capacity maximization under the transmit power constraint
can be written from Equation (7) as

max C D

fA.k/g

1
Nc



0

2Es
H.k/Sstbc-jtrd .k/ C N.k/Ts

NX
mt 1 NX
dan 1

@

(5)

mD0

s.t.

where

N.k/ D

N0;0 .k/N1;0 .k/N0;1 .k/N1;1 .k/

kD0

0

ˇ2
Es ˇˇ wf-mrt
ˇB
log2 @1 CˇAstbc-jtrd .k/ˇ
N0

NX
c 1

nD0

121
ˇ
ˇ
ˇHm;n .k/ˇ2A C
A

0
NX
mt 1
c 1
ˇ2 NX
ˇ
@ˇˇAwf-mrt .k/ˇˇ
stbc-jtrd
mD0

kD0



NX
dan 1(6)

1
ˇ2ˇˇHm;n .k/ˇ A D Nc

nD0

is the noise matrix with fNm;q .k/I m D 0  1; q D 0  1g
being independent zero-mean complex Gaussian variables
having variance 2N0 =Ts , where N0 denotes the singlesided power spectrum density of additive white Gaussian
noise and Ts denotes the data symbol duration.
The FD-STBC-JTRD decoding for Nmt D 2 is carriedOout to obtain the frequency domain signal vector D.k/
DiTh
DO 0 .k/ DO 1 .k/ corresponding to the transmitted signal
vector D.k/ D ŒD0 .k/D1 .k/T as
"
#
 .k/
R0;0 .k/ C R1;1
O
D.k/
D
 .k/
R0;1 .k/  R1;0s
1 NX
dan 1Xˇˇ
2EsˇHm;n .k/ˇ2 D.k/
D
A.k/
Ts
mD0 nD0"#
 .k/
N0;0 .k/ C N1;1
C
 .k/
N0;1 .k/  N1;0n
The solution of Equation (8) is denoted by Awf-mrt
stbc-jtrd.k/I
k D 0  Nc  1g and is given by
Awf-mrtstbc-jtrd .k/ D s1ˇˇHm;n .k/ˇ2Nmt
P1 Ndan
P1 ˇ
mD0

nD0

8
ˆ
ˆ
ˆ
<
6
.Es =2N0 /1
6
6max '2D 
;
Nmt
ˆ
4
ˇ2
P1 NdanP1 ˇˇ
ˆ:̂
Hm;n .k/ˇ
2(7)

nFinally, Nc -point inverseo FFT is applied to transformO
D.k/I
k D 0  Nc  1 into a pair of soft decision Nc
symbol blocks (J D 2).
1554

(8)

mD0

93 1
2
>
>
>
=7
7
0 7
>
5
>
>
;

nD0

(9)

where ®2D is set so as to satisfy the transmit power constraint shown in Equation (8). Substituting Equation (9)Wirel. Commun. Mob. Comput. 2011; 11:1551–1563 © 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/wcm

F. Adachi et al.

Single-carrier distributed antenna network

into Equation (3) gives the joint WF-MRT weight. The
channel capacity C (bps/Hz) for the given channel realization is given as
N 1

CD

c
1 X
log2
Nc
kD0
1
0
Nmt 1 Ndan
ˇ2P1 ˇˇ
Es P
ˇ 
1
C
H
.k/
m;n
N0
C
B
B2
8 mD0 nD0
93 C
C
B
ˆ
>
B
<
= C
B6
7C
.Es =2N0 /1; 0 5A@ 4max '2D  Nmt 1 Ndan 1
P
P
>
:̂
;
jHm;n .k/j2
mD0

3.2.2. MMSE weight.
The MMSE weight is the one that minimizes the meanhiTOsquare error between D.k/
D DO 0 .k/ DO 1 .k/
andD.k/ D ŒD0 .k/ D1 .k/T . Similar to Equation (8), finding
the MMSE weight can be formulated asminfA.k/geD

kD0 mD0

s.t.

NX
c 1

r 
1
D0 .k/
Dsttd .k/ D
2 D1 .k/

0
mt 1
ˇ2 NX
ˇ
@ˇˇG  Ammse .k/ˇˇ
stbc-jtrd
mD0

NX
dan 11
ˇ2ˇˇHm;n .k/ˇ A D Nc

nD0

(11)
n
.k/I
k
D
With Equation (11) being solved, Ammse
stbc-jtrd
0  Nc  1g is given as [7]3R0;0 .k/R0;1 .k/6
7
R1;1 .k/R1;0 .k/6
7
Rsttd .k/ D 6
7
::
::
4
5
:
:
RNdan 1;0 .k/ RNdan 1;1 .k/
s
2Es T
D
H .k/Dsttd .k/ C N.k/
Ts

nD0

2

N0;0 .k/6N
1;0 .k/6
N.k/ D 6::
4:
NNdan 1;0 .k/

and
G D@

Nc 1 n
mt 1
o2 NX
1 X
Ammse
stbc-jtrd .k/
Nc

(16)

is the noise matrix.
After the FD-STTD decoding, the frequency domainhiTOsignal vector D.k/
D DO 0 .k/ DO 1 .k/ associated withD.k/ D ŒD0 .k/ D1 .k/T is obtained as

mD0

kD0

NX
dan 1

3
N0;1 .k/7
N1;1 .k/7
7::
5:
NNdan 1;1 .k/

N0

(12)

0

(15)

where

 1
ˇ
ˇHm;n .k/ˇ2 C Es

Nmt
P1 Ndan
P1 ˇ
mD0

(14)

Two pairs of FD-STTD codeword are transmitted from Nmt D 2 antennas and are received by
Ndan distributed antennas, followed by Nc -point FFT.
Ndan pairs of the received frequency domain signals,
fRn;0 .k/ and Rn;1 .k/I n D 0  Ndan  1g, are expressedusing the matrix form as

1
1
Nmt



2

ˇ
ˇ2 
ˇ
ˇ
E ˇDO m .k/  Dm .k/ˇ

kD0

Ammse
stbc-jtrd .k/ D

D1 .k/
D0 .k/

nD0

(10)

NX
mt 1
c 1 NX

suitable because FD-STTD allows the use of an arbitrary
number Ndan of distributed receive antennas, whereas thenumber Nmt of MT transmit antennas is limited. A pairof Nc symbol block (subscripts “0” and “1” representing even and odd blocks, respectively) are transformed by
Nc -point FFT into frequency domain signals represented
by D.k/ D ŒD0 .k/ D1 .k/T . The FD-STTD encoding is
expressed using the matrix form as [8]

1 1

ˇ
ˇ
ˇHm;n .k/ˇ2 A

2

(13)

nD0

Substituting Equation (12) into Equation (3) gives the
MMSE weight.

1.0

Distributed antenna
MT of interest

3.3. Receive FDE weight for
FD-STTD (uplink)
The FD-STBC-JTRD is suitable for the downlink transmission. For the uplink transmission, FD-STTD [8] isWirel. Commun. Mob. Comput. 2011; 11:1551–1563 © 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/wcm

MT location area
Figure 3. Antenna distribution. MT, mobile terminal.

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F. Adachi et al.

Single-carrier distributed antenna network

2

Ndan
P1

3

Ndan
P1

 .k/WRn;0n;1 .k/ 7
6
6
7
nD0
nD0
O
D.k/
D 6 N 1
7
Ndan
P
P1 
4 dan
5

Rn;0 .k/Wn;1 .k/ Rn;1 .k/Wn;0 .k/sD .k/ C
Rn;0 .k/Wn;0

nD0

nD0

1 NX
dan 1Xˇˇ
2EsˇHm;n .k/ˇ2 D.k/
A.k/
Ts
mD0 nD0
2
3
Ndan
Ndan
P1
P1 

Nn;0 .k/Wn;0 .k/ C
Nn;0 .k/Wn;1 .k/ 7
6
6
7
nD0
C 6 NnD0
7
Ndan
P1
P1 
4 dan
5

Nn;0 .k/Wn;1 .k/ Nn;1 .k/Wn;0 .k/
nD0

nD0

Similar to the downlink case, the MMSE weight matrix
Othat minimizes the mean square error between D.k/
and
D.k/ is given, for the case of Nmt MT antennas, as [8]
mmse

Wmmse
sttd .k/ D Asttd .k/H .k/

DH .k/
Nmt
ˇ2  E 1P1 NdanP1 ˇˇ
1
s
ˇ C
H
.k/
m;n
Nmt
N
mD0

0

nD0

(18)
where
1

Ammse
sttd .k/ D
1
Nmt

 1
ˇ
ˇHm;n .k/ˇ2 C Es

Nmt
P1 Ndan
P1 ˇ
mD0

nD0

N0

(19)
3.4. Computer simulation results
First, the WF-MRT weight and MMSE weight are compared for SC-DAN downlink using FD-STBC-JTRD.
Then, the channel capacity and BER distributions are evaluated for joint WF-MRT weight and with MMSE weight,
respectively.
Figure 3 illustrates the DAN antenna distribution. N D
16 antennas are uniformly distributed with equal distance
between adjacent antennas. An MT having Nmt antennas
is randomly located in the shaded area. Ndan antennas are
selected for downlink transmissions, on the basis of thelocal average received power (i.e., on the basis of the pathloss plus shadowing loss). The channel is assumed to follow an L D 16-path frequency-selective Rayleigh fading, alog-normally distributed shadowing having standard deviation  D 7:0 dB, and a distance-dependent path loss having
path loss exponent ˛ D 3:5.
3.4.1. Comparison of WF-MRT weight and
MMSE weight for SC-DAN downlink using
FD-STBC-JTRD.
A comparison between the WF-MRT weight and MMSE
weight is made for the case of Ndan D 4 and Nmt D 1.
The normalized transmit Es =N0 is set to 10 dB (i.e.,
the transmit power is the one that provides the received
Es =N0 D 10 dB at the adjacent antenna location).
1556

(17)

For both the WF-MRT and MMSE weights, thepower allocation is carried out in antenna and frequency
dimensions. Figure 4 plots the channel transfer function
between transmit antenna 0 and n.D 0  3/ and thesquared value of the corresponding transmit FDE weight.
Both weights allocate the transmit power across the transmit antennas on the basis of maximal ratio strategy. Therefore, irrespective of the weight, less transmit power isallocated to the transmit antennas in a bad channel condition (Figure 4(a1), 4(a2), 4(b1), and 4(b2)), whereas moretransmit power is allocated to the transmit antenna in agood channel condition (Figure 4(a4) and 4(b4)). Power
allocation is carried out across the frequencies. In case of
the WF-MRT weight, more transmit power is allocated to
the frequencies in a good channel condition (Figure 4(b4)).
Figure 5(a) plots the equivalent channel transfer function0NX
mt 1 NX
dan 1

HO .k/ D @

mD0

11
2
ˇ2
ˇ
ˇHm;n .k/ˇ A

nD0

whereas Figure 5(b) plots the squared value of corresponding equivalent transmit FDE weightmt 1 NX
dan 1
ˇ2 NX
ˇ
ˇ
ˇ
ˇ
ˇ O
ˇWm;n .k/ˇ2
ˇW .k/ˇ D

mD0

nD0

It can be seen from Figure 5(b) that joint WF-MRT weight
allocates more transmit power to the frequencies in a betterchannel condition (some frequencies in a bad channel condition are not used at all). On the other hand, the MMSE
weight uses the frequencies even in a poor condition.
3.4.2. FD-STBC-JTRD with joint WF-MRT weight.
Figure 6 plots the 1% outage capacity of SC-DAN downlink using FD-STBC-JTRD with joint WF-MRT weight,
below which the channel capacity falls with 1% probability, for the normalized transmit Es =N0 D 10 dB. The useof Nmt D 2 maximizes the channel capacity. This is a consequence of the trade-off between diversity gain and coding
rate; as Nmt increases, the diversity gain increases, but thecoding rate R reduces to 3/4 when Nmt D 3 (Table I).

Wirel. Commun. Mob. Comput. 2011; 11:1551–1563 © 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/wcm

F. Adachi et al.

Single-carrier distributed antenna network

Figure 7 plots the required transmit Es =N0 for a 1% outage capacity of 5 bps/Hz as a function of Ndan for the caseof Nmt D 2. Increasing Ndan reduces the required transmit
Es =N0 . The required Es =N0 reduction by increasing Ndanfrom 1 to 6 is about 5.6 dB.
3.4.3. FD-STBC-JTRD with joint MMSE weight.
The BER performance of SC-DAN downlink using
FD-STBC-JTRD with MMSE weight is also evaluated.
Figure 8 plots the required transmit Es =N0 for a 1%

outage BER of 104 as a function of Ndan when Nmt D 2.
Increasing Ndan reduces the required transmit Es =N0 . The
required Es =N0 reduction by increasing Ndan from 1 to 6
is about 6.5 dB.
3.5. Uplink/downlink performancecomparison
The BER performances are compared assuming FDSTBC-JTRD using joint MMSE weight for the uplink

122|W0,0(k)|2|H0,0(k)|

10
8
6
4

0

50

100

150

200

0

250

2

|W0,1(k)|2|H0,1(k)|

6
4

200

250

joint WF-MRT
MMSE

1.5
1
0.5

0

50

100

150

200

0

250

0

50

100

150

200

frequency index kfrequency index k(a2) |H0,1(k)|(b2) |W0,1(k)|212

2|W0,2(k)|210|H0,2(k)|150(b1) |W0,0(k)|2

2

8
6
4

250

joint WF-MRT
MMSE

1.5
1
0.5

2
0

50

100

150

200

0

250

0

50

100

150

200

frequency index kfrequency index k(a3) |H0,2(k)|(b3) |W0,2(k)|22

12|W0,3(k)|210|H0,3(k)|100(a1) |H0,0(k)|

8

8
6
4

250

joint WF-MRT
MMSE

1.5
1
0.5

2
0

50

frequency index k

10

0

0

frequency index k

12

0

1
0.5

2
0

joint WF-MRT
MMSE

1.5

0

50

100

150

200

250

0

0

50

100

150

frequency index kfrequency index k(a4) |H0,3(k)|(b4) |W0,3(k)|2

200

250

Figure 4. Channel transfer function and squared value of frequency domain equalization weight when Nmt D 1 and Ndan D 4: (a1)jH0;0 .k /j, (a2) jH0;1 .k /j, (a3) jH0;2 .k /j, (a4) jH0;3 .k /j, (b1) jW0;0 .k /j2 , (b2) jW0;1 .k /j2 , (b3) jW0;2 .k /j2 , and (b4) jW0;3 .k /j2 . WF-MRT,
water filling and maximal ratio transmission; MMSE, minimum mean square error.

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Single-carrier distributed antenna network

12

15(Es/N0)req. (dB)and FD-STTD using MMSE weight for the downlink. Ndan
arbitrary distributed antennas and Nmt D 2 MT antennas
are assumed. The same antenna distribution pattern as in
Section 3.4 is assumed.

FD-STBC-JTRD(downlink)
α=3.5, σ=7.0
L=16
Nmt =2

10

5

10

^

H(k)

8

0

6

1

2

3

4

5

6

Ndan

4
2
0

0

50

100

150

200

250

Figure 7. Required transmit Es /N0 for a 1% outage capacityof 5 bps/Hz. FD-STBC-JTRD, frequency domain space-time
block-coded joint transmit/receive diversity.

frequency index k(a) Hˆ (k)15

FD-STBC-JTRD(downlink)
α= 3.5, σ = 7.0
L = 16
Nmt =2

joint WF-MRT
MMSE

1.5

|W(k)|2(Es/N0)req. (dB)

2

^

1

10

5

0.5
0

0
0

50

100

150

200

250

frequency index kˆ (k)|2(b) |W
Figure 5. Equivalent channel transfer function and squared
value of equivalent transmit frequency domain equalization
O .k /| and (b) jW
O .k /j2 .
weight when Nmt D 1 and Ndan D 4: (a) |H
WF-MRT, water filling and maximal ratio transmission; MMSE,
minimum mean square error.

1% outage capacity (bps/Hz)

7

Normalized transmit Es/N0 = 10dB
α=3.5, σ=7.0, L=16
Nc=256

6

2
Nmt=1

5

4

3

1

2

3

4

5

6

7

2
3

3

4

5

6

Ndan
Figure 8. Required transmit Es =N0 for a 1% outage bit error
rate of 104 . FD-STBC-JTRD, frequency domain space–timeblock-coded joint transmit/receive diversity.

The complementary cumulative distribution function of
the measured downlink and uplink BERs are plotted in
Figure 9 for the normalized transmit Es =N0 D 5 dB. It can
be clearly seen from the figure that well-balanced downlink
and uplink transmissions can be achieved. By increasing
Ndan (the number of distributed transmit/receive antennas),
the probability that the received signal power drops due
to path loss, shadowing loss, and frequency-selective fading can be more reduced and hence the BER can besignificantly reduced.

4. SPATIAL MULTIPLEXING

4
8

9

10

Ndan
Figure 6. 1% outage channel capacity of single-carrier distributed antenna network downlink using frequency domain
space–time block-coded joint transmit/receive diversity withjoint water filling and maximal ratio transmission weight.

1558

2

N mt = 1

⎧
⎪
⎪
⎨
⎪
⎪⎩

3,4

1

The spatial multiplexing is an attractive technique toachieve highly spectrum-efficient transmission [10]. Multiple distributed antennas in DAN can be also used not
only for space diversity but also for the spatial multiplexing to improve the throughput of the systems [11]. Here, we
consider the uplink case using Nmt transmit antennas andNdan receive distributed antennas (Ndan receive antennas
are selected among N distributed antennas).

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F. Adachi et al.

Prob [BER>abscissa]

1

Single-carrier distributed antenna network

Assuming the Nmt  Ndan spatial multiplexing, the frequency domain received signal vector at the nth receive
distributed antenna Yn D ŒYn .0/; : : : ; Yn .k/; : : : ; Yn .Nc 
1/T is expressed as

Normalized transmit Es/N0 = 5dB
α=3.5, σ=7.0, L=16
Nc=256, Ng=32
FD-STTD(MMSE)
Nmt =2(uplink)

Yn D

Ndan =1
2

10-2

4

10-3
1.00E-05

1.00E-04

1.00E-03

1.00E-02

1.00E-01

BER
Figure 9. Uplink/downlink performance comparison. BER, biterror rate; FD-STTD, frequency domain space–time transmit
diversity; FD-STBC-JTRD, frequency domain space–time blockcoded joint transmit/receive diversity; MMSE, minimum meansquare error.

4.1. Signal detection for spatial
multiplexingŶ

MLD usingM-algorithm…
…

Multiplication
of QH

Recently, we proposed a maximum likelihood detection(MLD) employing QR decomposition and M-algorithm(QRM-MLD) [12] for the SC spatial multiplexing ina frequency-selective fading channel [13]. The SC frequency domain maximum likelihood block signal detection
employing QR decomposition and M-algorithm (QRMMLBD) is a powerful signal detection scheme combinedwith FDE. The receiver structure of SC frequency domain
QRM-MLBD for a cyclic prefix (CP)-inserted SC (CP-SC)
spatial multiplexing is illustrated in Figure 10.

Y1

N

mt
2Es X
Hn;m Fdm C Nn
Ts

(20)

mD1

where F is the discrete Fourier transform matrix of size
Nc Nc , Hn;m D diagŒHn;m .0/; : : : ; Hn;m .k/; : : : ; Hn;m.Nc  1/ is the frequency domain channel matrix between
the mth transmit antenna and nth receive antenna, and
Nn D ŒNn .0/; : : : ; Nn .k/; : : : ; Nn .Nc  1/T is the frequency domain noise vector. From Equation (20), theNdan Nc  1 overall frequency domain received signal Y
is given by

3

YNr

s

FD-STBC-JTRD(MMSE)(downlink)

10-1



T

fY1 gT    fYNdan gT2
H1;1 F
H1;2 Fs
6
F
H2;2 F
H
2;1
2Es 6
D
6
::
::
Ts 4
:
:

YD

HNdan ;2 F
3d1
N1
6 :: 7 6
7::
4 : 5C4
5
:
dNmt
NNdan
3
2
s
d1
2Es 6 : 7
H 4 :: 5 C N
D
Ts
dNmt
2

HNdan ;1 F
3 2Figure 10. Single-carrier frequency domain maximum likelihood block signal detection employing QR decomposition andM-algorithm. MLD, maximum likelihood detection.

3
H1;Nmt F
H2;Nmt F 7
7
7
::
5
:
HNdan ;Nmt F(21)
where H is an equivalent channel matrix of size Ndan Nc 
Nmt Nc . QRM-MLBD can be applied to the SC spatial
multiplexing by treating a concatenation of the space andfrequency domain channel and discrete Fourier transformas this equivalent channel.
The QRM-MLBD consists of two steps: QR decomposition and M-algorithm. First, the QR decomposition
is applied to the equivalent channel matrix H to obtain
H D QR, where Q is an Ndan Nc  Nmt Nc unitary matrix
and R is an Nmt Nc  Nmt Nc upper triangular matrix.
O D
The transformed frequency domain received signal Y
iT
h
O
O
O
Y .0/; : : : ; Y .k/; : : : ; Y .Nmt Nc  1/ is obtained as

Decision variable

QR decomposition



::
:


s
O D QH Y D
Y

2
2Es 6
R4
Ts

3
d1
:: 7 C QH N
: 5
dNmt

(22)

It can be understood from Equation (22) that the MLD
can be converted to the successive tree search problem andthat the computational complexity can be reduced by introducing the M-algorithm into the successive tree search.
SC frequency domain QRM-MLBD can achieve the BER
performance close to the MLD with significantly reduced
computational complexity compared with the MLD.

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Single-carrier distributed antenna network

…

CP(0)

CP(1)

Data symbols(0)Ng symbols

Data symbols(1)…

Data symbols(1)…

Nc symbols
DFT block(a) CP-SC…

TS

Data symbols(0)Nc symbols

TS
Ng symbols

DFT block(b) TS-SC
Figure 11. (a) Cyclic prefix inserted single carrier (CP-SC) and (b) training sequence-aided single carrier (TS-SC). DFT, discrete
Fourier transform.

However, the CP-SC block transmission requires a fairly
large number of surviving paths in the M-algorithm; therefore, its computational complexity is still very high. To
overcome this problem, we suggested using a known training sequence (TS)-aided SC (TS-SC) transmissions, inwhich the known TS in the previous block acts as the CP
in the present block, as shown in Figure 11 [14],[15]. The
known TS is exploited in the M-algorithm to reduce thenumber of surviving paths.

10−1
2×2 spatial multiplexing
QRM-MLBD
TS-SC MIMO

Average BER

10−2

CP-SC MIMO
M=4
M=16
M=256
MMSED

10−3

4.2. Computer simulation results
First, we compare CP-SC and TS-SC for point-to-pointtransmission using 2  2 spatial multiplexing. We assume
a block transmission of Nc D 64 symbols. The channel isassumed to be a symbol-spaced L D 16-path frequencyselective block Rayleigh fading channel having uniform
power delay profile. Ideal channel estimation is assumed.
The average BER performance of SC frequency domain
QRM-MLBD is plotted in Figure 12. Also plotted is the
BER performance achievable by MMSE detection. As thenumber M of surviving paths in the M-algorithm increases,
the BER performance improves. When TS-SC is used, therequired number of surviving paths in the M-algorithm
is greatly reduced while achieving almost the same BER
performance as CP-SC.
In DAN, seven antennas are distributed over an entire
cell to compare with the conventional cellular network as
illustrated in Figure 13. Figure 14 plots the complementary
cumulative distribution function of the computer-simulated
BER when Nmt D Ndan D 2. The single-user and singlecell uplink SC-DAN is considered. The normalized transmit Es =N0 is set to 10 dB (i.e., the transmit power is the

10−4

10−5

QPSK
N =64
N =16

0

2/3

5

10

15

20

Average received Eb/N0 per antenna (dB)r2
1.0

Figure 12. Bit error rate (BER) performance comparison
between cyclic prefix-inserted single carrier (CP-SC) and training sequence-aided single carrier (TS-SC). QPSK, quadraturephase shift keying; QRM-MLBD, maximum likelihood block signal detection employing QR decomposition and M-algorithm;
MIMO, multiple input multiple output; MMSED, minimum meansquare error detection.

1560

r

1.0

r1

(a) DAN(b) Conventional network

Figure 13. System model: (a) distributed antenna network(DAN) and (b) conventional network. Nmt D Ndan D 2.

Wirel. Commun. Mob. Comput. 2011; 11:1551–1563 © 2011 John Wiley & Sons, Ltd.
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F. Adachi et al.

Single-carrier distributed antenna network

100

Prob [BER>abscissa]

Nmt =Ndan =2(Spatial multiplexing)

10−1

the transmission performance compared with the MMSE
spatial filtering.

QPSK
Nc =64, Ng =16
16-path uniform

Normalized transmit Es/N0 =10dB
CN
MMSED

10

10

−2

−3

10

REFERENCES

QRM-MLBD
M=1
M=4
M=16

−5

10

−4

DAN

−3

10

−2

10

−1

10

BER
Figure 14. Complementary cumulative distribution function ofbit error rate (BER). Nmt D Ndan D 2. QPSK, quadraturephase shift keying; MMSED, minimum mean square error detection; QRM-MLBD, maximum likelihood block signal detection
employing QR decomposition and M-algorithm; CN, conventional network; DAN, distributed antenna network.

one that provides the received Es =N0 D 10 dB at the celledge). MT transmits Nmt parallel data streams by using
spatial multiplexing. It is assumed that Ndan distributed
antennas nearest from the MT are selected as receive antennas. It can be seen that DAN can significantly reducethe BER compared with the conventional network. When
QRM-MLBD with M D 16 is used, DAN can reduce theoutage probability by 20% compared with the conventional
network (the outage probability is defined as the probability that the BER exceeds the required BER D 103in this paper). Furthermore, QRM-MLBD can reduce the
BER compared with the MMSE signal detection. The
performance improvement is significant in DAN.

5. CONCLUSION
Distributed antenna network is a promising wireless technology to realize gigabit wireless data transmissions. In
this paper, we have introduced gigabit DAN combinedwith SC frequency domain signal processing. Distributed
transmit/received diversity can solve the problems arising from severe channel selectivity and limited transmit
power. It is desirable to use as many distributed antennas as possible to achieve higher diversity gain while
limiting the number of MT antennas to one or two so
as to alleviate the complexity problem of MT. It was
shown that the balanced downlink/uplink performance canbe achieved by using FD-STBC-JTRD for the downlink
and FD-STTD for the uplink. We also presented a spatial
multiplexing using TS-SC and QRM-MLBD to improve

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1984 to September 1985, he was a United Kingdom SERC
Visiting Research Fellow in the Department of ElectricalEngineering and Electronics at Liverpool University. He
is an IEICE Fellow and was a co-recipient of the IEICE
Transactions Best Paper of the Year Award 1996, 1998,
and 2009 and also a recipient of the Achievement Award
2003. He is an IEEE Fellow and was a co-recipient of
the IEEE Vehicular Technology Transactions Best Paperof the Year Award 1980 and again 1990 and also a recipient of the Avant Garde Award 2000. He was a recipientof Thomson Scientific Research Front Award 2004, Ericsson Telecommunications Award 2008, Telecom System
Technology Award 2009, and Prime Minister Invention
Prize 2010.
Kazuki Takeda received his B.S.,
M.S., and Dr. Eng. degrees in
Communications Engineering fromTohoku University, Sendai, Japan,
in 2006, 2008, and 2010, respectively. From April 2008 to March
2011, he was a Japan Society for
the Promotion of Science (JSPS)
Research Fellow. Since April 2011,
he has been with Panasonic Corporation. He was a recipient of the 2009 IEICE RCS (Radio Communication Systems) Active Research Award.

AUTHORS’ BIOGRAPHIESFumiyuki Adachi received B.S.
and Dr. Eng. degrees in Electrical Engineering from Tohoku University, Sendai, Japan, in 1973 and1984, respectively. In April 1973,
he joined the Electrical Communications Laboratories of Nippon
Telegraph & Telephone Corporation (now NTT) and conducted various types of research related to digital cellular mobile
communications. From July 1992 to December 1999, he
was with NTT Mobile Communications Network, Inc.
(now NTT DoCoMo, Inc.), where he led a research groupon wideband/broadband CDMA wireless access for IMT2000 and beyond. Since January 2000, he has been with
Tohoku University, Sendai, Japan, where he is a Professor of Electrical and Communication Engineering at theGraduate School of Engineering. He was appointed as aDistinguished Professor in 2011.
His research interests are in CDMA wireless accesstechniques, equalization, transmit/receive antenna diversity, MIMO, adaptive transmission, and channel coding,with particular application to broadband wireless communications systems. He is a program leader of the
5-year Global COE Program "Center of Education andResearch for Information Electronics Systems" (20072011), awarded by the Ministry of Education, Culture,
Sports, Science and Technology of Japan. From October
1562

Tetsuya Yamamoto received his
B.S. degree in Electrical, Information and Physics Engineering in2008 and M.S. degree in Communications Engineering in 2010 fromTohoku University, Sendai, Japan.
Currently, he is a Japan Society for
the Promotion of Science (JSPS)
Research Fellow, studying toward
his Ph.D. degree at the Department of Electrical andCommunications Engineering, Graduate School of Engineering, Tohoku University. His research interests include
frequency-domain equalization and signal detection techniques for mobile communication systems. He was arecipient of the 2008 IEICE RCS (Radio Communication
Systems) Active Research Award.
Ryusuke Matsukawa received his
B.S. degree in Electrical, Information and Physics Engineering fromTohoku University, Sendai, Japan,
in 2010. Currently, he is a graduate student at the Department
of Electrical and Communications
Engineering, Graduate School ofEngineering, Tohoku University.
His research interests include distributed antenna networks
and multiple antenna techniques.

Wirel. Commun. Mob. Comput. 2011; 11:1551–1563 © 2011 John Wiley & Sons, Ltd.
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F. Adachi et al.

Single-carrier distributed antenna network

Shinya Kumagai received his B.S.
degree in Information and Intelligent Systems from Tohoku University, Sendai, Japan, in 2011.
Currently, he is a graduate student
at the Department of Electricaland Communication Engineering,Tohoku University. His researchinterests include distributed MIMO
diversity and multiplexing techniques for mobile communication systems.

Wirel. Commun. Mob. Comput. 2011; 11:1551–1563 © 2011 John Wiley & Sons, Ltd.
DOI: 10.1002/wcm

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