29-10-2012, 02:36 PM
SPACE-TIME/FREQUENCY CODING FOR MIMO-OFDM IN NEXT GENERATION BROADBAND WIRELESS SYSTEMS
SPACE-TIMEFREQUENCY.pdf (Size: 231 KB / Downloads: 64)
ABSTRACT
With the advent of next generation (4G)
broadband wireless communications, the combination
of multiple-input multiple-output
(MIMO) wireless technology with orthogonal
frequency division multiplexing (OFDM) has
been recognized as one of the most promising
techniques to support high data rate and high
performance. In particular, coding over the
space, time, and frequency domains provided by
MIMO-OFDM will enable a much more reliable
and robust transmission over the harsh wireless
environment. In this article we provide an
overview of space-time (ST) coding, space-frequency
(SF) coding, and space-time-frequency
(STF) coding for MIMO-OFDM systems. Performance
results show that STF coding can
achieve the maximum diversity gain in an endto-
end MIMO-OFDM system over broadband
wireless channels. Furthermore, for orthogonal
frequency division multiple access (OFDMA),
we propose a multiuser SF coding scheme that
can achieve the maximum diversity for each user
while minimizing the interference introduced
from all the other users.
INTRODUCTION
Swifter, higher, stronger — the Olympic motto is
also being pursued for the upcoming 4G broadband
wireless communication systems. Motivated
by the huge demands for fast and reliable communications
over wireless channels, future broadband communication systems should provide
swifter data processing (low-complexity),
higher data rate, and stronger (robust) performance.
In practice, however, the broadband
channel is a typically non-line-of-sight channel
and includes many impairments such as timeselective
and frequency-selective fading. To
address these challenges, one promising solution
is to combine two powerful technologies, namely,
multiple-input multiple-output (MIMO)
antennas and orthogonal frequency division multiplexing
(OFDM) modulation [1].
MIMO-OFDM
MIMO
MIMO wireless communication refers to the
transmissions over wireless links formed by multiple
antennas equipped at both the transmitter
and receiver. The key advantages of employing
multiple antennas lie in the more reliable performance
obtained through diversity and the achievable
higher data rate through spatial multiplexing
[2]. These concepts are briefly discussed below.
Diversity — The signal transmission over broadband
wireless channels always suffers from attenuation
due to the detrimental effect of multipath
fading, and this can severely degrade the reception
performance. In MIMO systems, the same
information can be transmitted from multiple
transmit antennas and received at multiple
receive antennas simultaneously. Since the fading
for each link between a pair of transmit and
receive antennas can usually be considered to be
independent, the probability that the information
is detected accurately is increased. Apart
from the spatial diversity, other forms of diversity
are commonly available, namely, temporal
diversity and frequency diversity.
SPACE-FREQUENCY CODED OFDM
This strategy, which consists of coding across
antennas and OFDM subchannels, is called SF
coding [22]. A straightforward way of realizing
SF coding for two transmit antennas is to directly
spread the Alamouti code over two subchannels
in one OFDM block. Figure 4a shows the
example of SF coding for two transmit antennas.
The two symbols s1 and –s2 * are sent from subchannels
k and l of the same OFDM block n at
antenna 1, respectively, where k and l denote the
indices of two separated subchannels. Meanwhile,
s2 and s1 * are sent from subchannels k and
l of the same OFDM block n at antenna 2,
respectively. However, this simple SF coding
approach can only achieve space diversity gain,
whereas the maximum diversity gain in frequency-
selective MIMO channels is MtMrL, shown
earlier. In [12] the full diversity SF code design
criteria were derived. To exploit the full diversity
in MIMO multipath fading channels, an SF code
design approach was proposed by multiplying
the input information stream with a part of the
DFT matrix [23]. The resulting SF codes can
achieve full diversity at the expense of a large
bandwidth efficiency loss. The symbol rate is not
more than 1/(MtL). In [24] a systematic design of
full diversity SF block codes (SFBC) was proposed.
By repeating each row of the ST codes
matrix on L different subchannels of the same
OFDM block, the SF codes provide higher data
rates than the approach described in [23]. However,
they cannot achieve a rate larger than 1/L.
The detailed SF coding structure via mapping
for two transmit antennas is shown in Fig.