919 research outputs found
Low Complexity Blind Equalization for OFDM Systems with General Constellations
This paper proposes a low-complexity algorithm for blind equalization of data
in OFDM-based wireless systems with general constellations. The proposed
algorithm is able to recover data even when the channel changes on a
symbol-by-symbol basis, making it suitable for fast fading channels. The
proposed algorithm does not require any statistical information of the channel
and thus does not suffer from latency normally associated with blind methods.
We also demonstrate how to reduce the complexity of the algorithm, which
becomes especially low at high SNR. Specifically, we show that in the high SNR
regime, the number of operations is of the order O(LN), where L is the cyclic
prefix length and N is the total number of subcarriers. Simulation results
confirm the favorable performance of our algorithm
Subspace-Based Blind Channel Identification for Cyclic Prefix Systems Using Few Received Blocks
In this paper, a novel generalization of subspace-based blind channel identification methods in cyclic prefix (CP) systems is proposed. For the generalization, a new system parameter called repetition index is introduced whose value is unity for previously reported special cases. By choosing a repetition index larger than unity, the number of received blocks needed for blind identification is significantly reduced compared to all previously reported methods. This feature makes the method more realistic especially in wireless environments where the channel state is usually fast-varying. Given the number of received blocks available, the minimum value of repetition index is derived. Theoretical limit allows the proposed method to perform blind identification using only three received blocks in absence of noise. In practice, the number of received blocks needed to yield a satisfactory bit-error-rate (BER) performance is usually on the order of half the block size. Simulation results not only demonstrate the capability of the algorithm to perform blind identification using fewer received blocks, but also show that in some cases system performance can be improved by choosing a repetition index larger than needed. Simulation of the proposed method over time-varying channels clearly demonstrates the improvement over previously reported methods
A Semi-Blind Pilot-Assisted Channel Estimation Algorithm in OFDM Systems
In this paper we study a new semi-blind channel estimation algorithm in orthogonal frequency division multiplexing (OFDM) systems. The proposed scheme is an extension of a recently reported subspace-based blind channel estimation algorithm in cyclic prefix systems which requires very few received blocks. The semi-blind estimation algorithm is devised by using the information obtained both from the blind channel estimation method and a pure pilot-assisted method. The proposed algorithm uses a small amount of received data and can be applied to any types of communication constellations. Simulation results show that, with the same number of pilot samples, the semi-blind algorithm has a clear improvement in system performance over the pure pilot-assisted method. To achieve the same bit- error-rate performance, the proposed semi-blind algorithm uses fewer pilot samples
On the Persistency of Excitation for Blind Channel Estimation in Cyclic Prefix Systems
Recently, a new subspace-based blind channel estimation
algorithm in cyclic prefix (CP) system was reported.
A persistency of excitation (PE) property of the input signal is
required for the algorithm to work. In this paper, the probability
of fulfilling the PE property under different situations is studied.
Four factors in the algorithm affect the PE property of the input
signal: 1) signal constellation used; 2) precoder coefficients; 3)
number of consecutive blocks; 4) a number called the repetition
index. Theoretical derivations as well as numerical simulations
are given to demonstrate the main points of this paper. Important
conclusions are 1) that the probability of fulfilling the PE
property increases and converges to unity when the number
of received blocks increases but is always upper-bounded by
a value less than unity when the repetition index increases;
2) that the probability of fulfilling the PE property is smaller
when the algorithm is applied in orthogonal frequency division
multiplexing (OFDM) systems than in single-carrier-cyclic-prefix
(SC-CP) systems
New Blind Block Synchronization for Transceivers Using Redundant Precoders
This paper studies the blind block synchronization problem in block transmission systems using linear redundant precoders (LRP). Two commonly used LRP systems, namely, zero padding (ZP) and cyclic prefix (CP) systems, are considered in this paper. In particular, the block synchronization problem in CP systems is a broader version of timing synchronization problem in the popular orthogonal frequency division multiplexing (OFDM) systems. The proposed algorithms exploit the rank deficiency property of the matrix composed of received blocks when the block synchronization is perfect and use a parameter called repetition index which can be chosen as any positive integer. Theoretical results suggest advantages in blind block synchronization performances when using a large repetition index. Furthermore, unlike previously reported algorithms, which require a large amount of received data, the proposed methods, with properly chosen repetition indices, guarantee correct block synchronization in absence of noise using only two received blocks in ZP systems and three in CP systems. Computer simulations are conducted to evaluate the performances of the proposed algorithms and compare them with previously reported algorithms. Simulation results not only verify the capability of the proposed algorithms to work with limited received data but also show significant improvements in the block synchronization error rate performance of the proposed algorithms over previously reported algorithms
Joint data detection and channel estimation for OFDM systems
We develop new blind and semi-blind data detectors and channel estimators for orthogonal frequency-division multiplexing (OFDM) systems. Our data detectors require minimizing a complex, integer quadratic form in the data vector. The semi-blind detector uses both channel correlation and noise variance. The quadratic for the blind detector suffers from rank deficiency; for this, we give a low-complexity solution. Avoiding a computationally prohibitive exhaustive search, we solve our data detectors using sphere decoding (SD) and V-BLAST and provide simple adaptations of the SD algorithm. We consider how the blind detector performs under mismatch, generalize the basic data detectors to nonunitary constellations, and extend them to systems with pilots and virtual carriers. Simulations show that our data detectors perform well
Blind block synchronization algorithms in cyclic prefix systems
In orthogonal frequency division multiplexing
(OFDM) systems, symbol synchronization is a critical step for
successful data transmission. While this task is done in most
current systems by using training symbols, a few studies have
been dedicated to solving the problem blindly, that is, where
training symbols are not available. Blind symbol synchronization
problem is especially important in many blind channel estimation
algorithms in the literature which assume that OFDM symbol
synchronization is perfect. In this paper, a broader version of the
blind symbol synchronization problem is studied, namely, blind
block synchronization in cyclic-prefix (CP) systems. The proposed
algorithm for this broader problem covers the blind symbol
synchronization problem in OFDM systems. Unlike previously
reported algorithms which are based on obtaining sufficient
statistics of received samples, the proposed algorithm is capable
of identifying the correct block boundaries using much less
received data in absence of noise. Simulation results of the
proposed algorithm not only verify the declared property but also
demonstrate improvement in accuracy of symbol synchronization
over previously reported algorithms in presence of noise
Diversity techniques for blind channel equalization in mobile communications
A blind algorithm for channel distortion compensation is presented which can be employed in spatial or temporal diversity receivers. The proposed technique can be used in frequency selective and frequency flat fading mobile channels, using burst transmission schemes in the first case and OFDM modulation in the second one. The algorithm is base on a deterministic criteria and is suited for estimation when short sets of data are available.Peer ReviewedPostprint (published version
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