22-02-2013, 12:56 PM
Pilot-Symbol Assisted Power Delay Profile Estimation for MIMO-OFDM Systems
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Abstract
This letter proposes a power delay profile (PDP)
estimation technique for linear minimum mean square error
(LMMSE) channel estimator of multiple-input multiple-output
orthogonal frequency division multiplexing (MIMO-OFDM) systems.
For practical applications, only the pilot symbols of all
transmit antenna ports are used in estimating the PDP. The
distortions caused by null subcarriers and an insufficient number
of samples for PDP estimation are also considered. The proposed
technique effectively reduces the distortions for accurate PDP
estimation. Simulation results show that the performance of
LMMSE channel estimation using the proposed PDP estimate
approaches that of Wiener filtering due to the mitigation of
distortion effects.
INTRODUCTION
MULTIPLE-INPUT multiple-output orthogonal
frequency division multiplexing (MIMO-OFDM)
is one of the most promising techniques for wireless
communication systems, including the 3rd Generation
Partnership Project Long Term Evolution (3GPP LTE) [1],
[2] and IEEE 802.16 (WiMAX). MIMO-OFDM provides a
considerable performance gain over broadband single-antenna
systems by obtaining the spatial diversity or multiplexing
gain [3], [4]. Most receiver techniques of MIMO-OFDM
systems are designed with the assumption that channel
state information (CSI) is available, in order to achieve
the maximum diversity or multiplexing gain [5]-[7]. The
performance gain depends heavily on accurate channel
estimation, which is crucial for the MIMO-OFDM systems.
SYSTEM MODEL
The system under consideration is a MIMO-OFDM system
with transmit andreceive antennas, and total
subcarriers. Suppose that the MIMO-OFDM system transmits
subcarriers at the central spectrum assigned for data and
pilots with virtual subcarriers, in order to control
interferences with other systems. The CIRs corresponding to
different transmit and receive antennas in MIMO systems
usually have the same PDP [12].
Let be the pilot subcarrier for the th transmit
antenna at the th OFDM symbol, which is a QPSK
modulated signal from known sequences between the transmitter
and receiver. We assume that the pilot subcarriers are
distributed over a time and frequency grid as in Fig. 1, to
preserve the orthogonality of pilots among different transmit
antennas. and represent the index sets for the
pilot subcarriers of theth antenna port in the frequency
and time domains, respectively. At theth OFDM symbol,
the number of pilot subcarriers is defined as The
pilot inserted OFDM symbol is transmitted over the wireless
channel after performing an inverse fast Fourier transform
(IFFT) and adding a CP.
CONCLUSIONS
We proposed a PDP estimation technique for the LMMSE
channel estimator in MIMO-OFDM systems. The CIR estimates
at each path of the MIMO channels were used to obtain
the PDP. For accurate PDP estimation, we considered the
spectral leakage effect from virtual subcarriers, and the residual
noise caused by the insufficient number of estimated CIR
samples. The proposed technique effectively mitigates both the
spectrum leakage and residual noise. Simulation results show
that the performance of LMMSE channel estimation using the
proposed PDP estimate approaches that of Wiener filtering.