15-05-2012, 11:03 AM
Time Domain Signal Detection for MIMO OFDM
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Abstract
The MIMO techniques with OFDM is regarded as a
promising solution for increasing data rates, for wireless access
qualities of future wireless local area networks, fourth generation
wireless communication systems, and for high capacity, as well as better
performance.
Hence as part of continued research, in this paper an attempt
is made to carry out modelling, analysis, channel matrix estimation,
synchronization and simulation of MIMO-OFDM system. A time
domain signal detection algorithm can be based on Second Order
Statistics (SOS) proposed for MIMO-OFDM system over frequency
selective fading channels. In this algorithm, an equalizer is first inserted
to reduce the MIMO channels to ones with channel length shorter than
or equal to the Cyclic Prefix (CP) length. A system model in which the
ith received OFDM block left shifted by j samples introduced.
MIMO OFDM system model which uses the equalizer can
be designed using SOS of the received signal vector to cancel the most
of the Inter Symbol Interference (ISI). The transmitted signals are then
detected from the equalizer output. In the proposed algorithm, only 2P
(P transmitted antennas / users in the MIMO-OFDM system) columns
of the channel matrix need to be estimated and channel length
estimation is unnecessary, which is an advantage over an existing
algorithms.
In addition, the proposed algorithm is applicable for irrespective
of whether the channel length is shorter than, equal to or longer than
the CP length. Simulation results verify the effectiveness of the
proposed algorithm and shows that it out performs the existing one in
all cases.
Keywords: Multiple Input Multiple Output (MIMO), Orthogonal
Frequency Division Multiplexing (OFDM), Cyclic Prefix (CP), second –
order statistics (SOS), Inter symbol Interference (ISI).
I. Introduction
Multiple input multiple outputs (MIMO)
using multiple antennas at both transmit and receive
ends becomes an important system for future wireless
Communications [1]–[5] as it has the potential to
greatly increase the system capacity without extra
bandwidth. Generally, MIMO is applied in two
situations. One is in Space-Time Coding (STC)
systems where the transmission quality [Bit-Error Rate
(BER)] is improved due to spatial diversity. The other
is in spatial multiplexing or spatial multiple access
systems where independent data streams are
transmitted over different antennas, thus increasing the
transmission rate or improving the system capacity.
The MIMO technique can be designed with multiple
antennas at both the transmitter and the receiver (i.e., a
MIMO system). MIMO can be sub-divided into three
main categories. So the functions of the MIMO
techniques are Pre-Coding, Spatial Multiplexing and
Diversity.
Orthogonal Frequency Division Multiplexing
(OFDM) technique has been widely utilized in Digital
Audio Broadcasting (DAB), Digital Video
Broadcasting (DVB), and broadband wireless local
area networks (IEEE 802.11a) [5]–[8], due to its
ability to resist frequency selective fading. It is,
therefore, desirable to combine OFDM with MIMO
for high system capacity, as well as better
performance. A lot of research interest has thus been
attracted to MIMO OFDM systems in recent years [9]
- [15].
In MIMO OFDM systems over frequency
selective fading channels, signal detection can easily
implemented by a set of parallel per sub carrier signal
detectors applicable to flat fading channels when the
channel length is shorter than or equal to the cyclic
prefix (CP) length. However, when the channel length
is longer than the CP length, Inter Block Interference
(IBI) occurs and the orthogonal property of sub
carriers will be destroyed, resulting in substantial
performance degradation of the signal detection
algorithm. In this paper, a Time-domain signal
detection algorithm based on second-order statistics
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Special Issue of IJCCT Vol.1 Issue 2, 3, 4; 2010 for International Conference [ACCTA-2010], 3-5 August 2010
Time Domain Signal Detection for MIMO OFDM
(SOS) is proposed for general MIMO-OFDM systems
over frequency selective channels.
A new system model is first introduced in
which the ith received OFDM block is left shifted by
samples. The new system model has certain structural
properties that enable an equalizer to be designed to
cancel most of the Inter Symbol Interference (ISI)
using SOS of the received signals. At the output of the
equalizer, only two paths of the transmitted signals are
retained and the signals can readily be detected. Due to
the special structure of the new system model, it turns
out that only columns of the channel matrix need to be
estimated. It follows that the minimum number of pilot
symbols required to estimate the columns of the
channel matrix is (compared with and is independent of
the channel length. It also means that channel length
estimation is unnecessary, which implies that the
proposed algorithm has substantial advantage,
computationally as well as avoiding performance
degradation due to channel length estimation error,
over existing algorithms [17], [18]. Furthermore, the
proposed algorithm is applicable irrespective of
whether the channel length is shorter than, equal to or
longer than the CP length. Simulation results confirm
the effectiveness of the proposed algorithm, and show
that it outperforms the existing ones in all cases.
The rest of the paper is organized as follows.
In section II the new MIMO-OFDM system model is
introduced. The time domain signal detection algorithm
is present in section III and in section IV, the
performance of the proposed algorithm is demonstrated
by simulation. Finally, section V draws the conclusion.