Performance of MIMO-OFDM systems with adaptive beamforming algorithm
Nam Suk Lee, Yong Seouk Choi, Kyung Soo Kim, Chan Kyu Kim
*
Mobil Telecommunication Lab, Electronics and Telecommunication Institute, Korea
*
Dept. of Electronic Engineering, Hanbat National University of Technology, Korea
Tel : +82-42-860-6602, Fax : +82-42-860-5048 E-mail : namsuk@etri.re.kr
Abstract- In this paper, the new technique combining space-
time decoder with beamforming is proposed for an orthogonal
frequency division multiplexing (OFDM) system with multi-
input multi-output (MIMO). Through the proposed technique
for MIMO-OFDM, the multibeams are formed toward each
multi-transmitter antenna of the desired user. Therefore, the
performance of MIMO-OFDM system is very improved by
the proposed techniques that can reduce cochannel
interference(CCI) and obtain diversity gain. BER
performance improvement of the proposed approach is
investigated through computer simulation in the multi-users
environment.
Key words : OFDM, MIMO, beamforming, space-time code
I. Introduction
MIMO technique can improve greatly the system
capacity as the multi-antennas installed at both transmitter
and receiver construct MIMO channel with rich multipath in
the mobile communication. Also, STC(Space-Time Coding)
technique can transmit higher data rate in the multipath due to
getting more diversity gain by combing the coding algorithm
at transmitter with the signal processing at receiver of the
MIMO system[1][2]. OFDM is multicarrier scheme that can
effectively overcome ISI by inserting longer CP(cyclic prefix)
than delay spread of channel into guard band. Therefore,
MIMO-OFDM technique with STC has been widely studied
for high data rate application such as 4G, WLAN,
WiBro[3][4]. In the multi-users environments, the
performance of MIMO-OFDM is very decreased as the
received signals are much distorted by CCI during the space-
time decoding of multiple antennas receiver. Beamforming
algorithm at mobile communication system with multiple
antennas is known as effective technique to further increase
the system capacity without allocating additional spectra due
to effectively removing CCI[5]. Recently, the technique
combing MIMO-OFDM with beamforming is proposed to
improve the performance of systems through removing
CCI[6][7]. The previous schemes have the limitations that use
only one beam toward the desired user and then, are unable to
provide maximum gain simultaneously to various
DOAs(direction of arrival) of multi-antennas transmitter. In
this paper, to overcome such as the limitation and to improve
the performance of MIMO-OFDM system further in the
multi-users environments, we propose the new technique
combining beamformer with space-time decoder for MIMO-
OFDM. The proposed structure is MIMO-OFDM system with
installing Nt (the number of transmitter antenna) beamfomers
and one S-T decoder at Nr antennas receiver. In the proposed
scheme, as Nt beams with maximum gain are formed toward
each Nt transmitter antennas of the desired user, the
performance of MIMO-OFDM system is greatly improved.
The beamforming algorithm for the proposed structure is
derived by transforming the frequency-domain error signals
into time-domain error signals, due to the nature of the
OFDM system utilizing the IFFT/FFT operation.
+
-
FFT
FFT
S-T
Decoder
DE-
MUX
DE-
MUX
Demodulaor
00
W
01
W
10
W
11
W
11-Nr
W
∑
∑
.
.
.
.
.
.
Weight Control/
Beamforming
Source
Data
)(
0
nR
)(
1
nR
)(
o
ny
~
)(
1
ny
~
)(
po
n
y
~
)(nZ
MUX
Source
Data
Modulaor
Pilot
Symbol
Generator
Alamouti
Encoder
IFFT
IFFT
)(nY
))((
p0
nY
)(
0d
n_YT
))((
p1
nY
)(
0d
n_XT
)(
1d
n_YT
Add
GI
Add
GI
)(
1d
n_XT
)(
p1
ny
~
Re mo ve
GI
SUM
Reference
Pilot
Symbol
)(
p1
nY
)(
p0
nY
Re mo ve
GI
11-Nr
W
Figure 1. A block diagram of MIMO-OFDM system
combing beamfomer with S-T decoder
II. MIMO-OFDM system combing S-T decoder with
beamforming
The figure 1 shows a block diagram of a MIMO-OFDM
system with the transmitter employing Alamouti encoder and
the receiver combing Nt beamformers with S-T decoder. In
this figure, the Alamouti encoding Nt symbols in the
frequency domain are transformed into time domain signals
by IFFT operation. After inserting GI(guard interval), the time
domain signals are transmitted into wireless channel through
Nt transmitters antennas, respectively. The transmitted signals
from M users with Nt antennas arrive at each Nr receiver
antennas with corresponding DOA. The received signals with
spatial phase for each receiver antenna are multiplied by the
weight of Nt beamformers, and then the GI is removed. The
Nt signals are transformed back into frequency domain
signals by the FFT and decoded by space-time decoder.
Diversity gain can be obtained from space-time decoding Nt
signals eliminated CCI, resulting in the increase of signal to
interference and noise ratio. When symbol vector in the
frequency-domain is
[]
T
N
nnnn
yyy
)(...)()()(
110 −
=Y
, the
encoded symbol matrix by Almouti encoder can be written as
follows
1-4244-0264-6/07/$25.00 ©2007 IEEE 656