1,534 research outputs found
A Two-Phase Maximum-Likelihood Sequence Estimation for Receivers with Partial CSI
The optimality of the conventional maximum likelihood sequence estimation
(MLSE), also known as the Viterbi Algorithm (VA), relies on the assumption that
the receiver has perfect knowledge of the channel coefficients or channel state
information (CSI). However, in practical situations that fail the assumption,
the MLSE method becomes suboptimal and then exhaustive checking is the only way
to obtain the ML sequence. At this background, considering directly the ML
criterion for partial CSI, we propose a two-phase low-complexity MLSE
algorithm, in which the first phase performs the conventional MLSE algorithm in
order to retain necessary information for the backward VA performed in the
second phase. Simulations show that when the training sequence is moderately
long in comparison with the entire data block such as 1/3 of the block, the
proposed two-phase MLSE can approach the performance of the optimal exhaustive
checking. In a normal case, where the training sequence consumes only 0.14 of
the bandwidth, our proposed method still outperforms evidently the conventional
MLSE.Comment: 5 pages and 4 figure
Coded DS-CDMA Systems with Iterative Channel Estimation and no Pilot Symbols
In this paper, we describe direct-sequence code-division multiple-access
(DS-CDMA) systems with quadriphase-shift keying in which channel estimation,
coherent demodulation, and decoding are iteratively performed without the use
of any training or pilot symbols. An expectation-maximization
channel-estimation algorithm for the fading amplitude, phase, and the
interference power spectral density (PSD) due to the combined interference and
thermal noise is proposed for DS-CDMA systems with irregular repeat-accumulate
codes. After initial estimates of the fading amplitude, phase, and interference
PSD are obtained from the received symbols, subsequent values of these
parameters are iteratively updated by using the soft feedback from the channel
decoder. The updated estimates are combined with the received symbols and
iteratively passed to the decoder. The elimination of pilot symbols simplifies
the system design and allows either an enhanced information throughput, an
improved bit error rate, or greater spectral efficiency. The interference-PSD
estimation enables DS-CDMA systems to significantly suppress interference.Comment: To appear, IEEE Transactions on Wireless Communication
Interference Coordination via Power Domain Channel Estimation
A novel technique is proposed which enables each transmitter to acquire
global channel state information (CSI) from the sole knowledge of individual
received signal power measurements, which makes dedicated feedback or
inter-transmitter signaling channels unnecessary. To make this possible, we
resort to a completely new technique whose key idea is to exploit the transmit
power levels as symbols to embed information and the observed interference as a
communication channel the transmitters can use to exchange coordination
information. Although the used technique allows any kind of {low-rate}
information to be exchanged among the transmitters, the focus here is to
exchange local CSI. The proposed procedure also comprises a phase which allows
local CSI to be estimated. Once an estimate of global CSI is acquired by the
transmitters, it can be used to optimize any utility function which depends on
it. While algorithms which use the same type of measurements such as the
iterative water-filling algorithm (IWFA) implement the sequential best-response
dynamics (BRD) applied to individual utilities, here, thanks to the
availability of global CSI, the BRD can be applied to the sum-utility.
Extensive numerical results show that significant gains can be obtained and,
this, by requiring no additional online signaling
On the Degrees of Freedom of time correlated MISO broadcast channel with delayed CSIT
We consider the time correlated MISO broadcast channel where the transmitter
has partial knowledge on the current channel state, in addition to delayed
channel state information (CSI). Rather than exploiting only the current CSI,
as the zero-forcing precoding, or only the delayed CSI, as the Maddah-Ali-Tse
(MAT) scheme, we propose a seamless strategy that takes advantage of both. The
achievable degrees of freedom of the proposed scheme is characterized in terms
of the quality of the current channel knowledge.Comment: 7 pages, 1 figure, submitted to ISIT 2012, extended version with
detailed proof
Communications over fading channels with partial channel information : performance and design criteria
The effects of system parameters upon the performance are quantified under the assumption that some statistical information of the wireless fading channels is available. These results are useful in determining the optimal design of system parameters. Suboptimal receivers are designed for systems that are constrained in terms of implementation complexity.
The achievable rates are investigated for a wireless communication system when neither the transmitter nor the receiver has prior knowledge of the channel state information (CSI). Quantitative results are provided for independent and identically distributed (i.i.d.) Gaussian signals. A simple, low-duty-cycle signaling scheme is proposed to improve the information rates for low signal-to-noise ratio (SNR), and the optimal duty cycle is expressed as a function of the fading rate and SNR. It is demonstrated that the resource allocations and duty cycles developed for Gaussian signals can also be applied to systems using other signaling formats.
The average SNR and outage probabilities are examined for amplify-and-forward cooperative relaying schemes in Rayleigh fading channels. Simple power allocation strategies are determined by using knowledge of the mean strengths of the channels.
Suboptimal algorithms are proposed for cases that optimal receivers are difficult to implement. For systems with multiple transmit antennas, an iterative method is used to avoid the inversion of a data-dependent matrix in decision-directed channel estimation. When CSI is not available, two noncoherent detection algorithms are formulated based on the generalized likelihood ratio test (GLRT). Numerical results are presented to demonstrate the use of GLRT-based detectors in systems with cooperative diversity
Transmitter precoding for multi-antenna multi-user communications
Emerging wireless sensor networks and existing wireless cellular and ad hoc networks motivate the design of low-power receivers. Multi-user interference drastically reduces the energy efficiency of wireless multi-user communications by introducing errors in the bits being detected at the receiver. Interference rejection algorithms and multiple antenna techniques can significantly reduce the bit-error-rate at the receiver. Unfortunately, while interference rejection algorithms burden the receiver with heavy signal processing functionalities, thereby increasing the power consumption at the receiver, the small size of receivers, specifically in sensor networks and in downlink cellular communications, prohibits the use of multiple receive antennas. In a broadcast channel, where a central transmitter is transmitting independent streams to decentralized receivers, it is possible for the transmitter to have a priori knowledge of the interference. Multiple antennas can be used at the transmitter to enhance energy efficiency. In some systems, the transmitter has access to virtually an infinite source of power. A typical example would be the base station transmitter for the downlink of a cellular system. The power consumption at receivers can be reduced if some of the signal processing functionality of the receiver is moved to the transmitter.;In this thesis, we consider a wireless broadcast channel with a transmitter equipped with multiple antennas and having a priori knowledge of interference. Our objective is to minimize the receiver complexity by adding extra signal processing functions to the transmitter. We need to determine the optimal signal that should be transmitted so that interference is completely eliminated, and the benefits that can be obtained by using multiple transmit antennas can be maximized. We investigate the use of linear precoders, linear transformations made on the signal before transmission, for this purpose
Analysis and optimization of pilot symbol-assisted Rake receivers for DS-CDMA systems
The effect of imperfect channel estimation (CE) on the performance of pilot-symbol-assisted modulation (PSAM) and MRC Rake reception over time- or frequency-selective fading channels with either a uniform power delay profile (UPDP) or a nonuniform power delay profile (NPDP) is investigated. For time-selective channels, a Wiener filter or linear minimum mean square error (LMMSE) filter for CE is considered, and a closed-form asymptotic expression for the mean square error (MSE) when the number of pilots used for CE approaches infinity is derived. In high signal-to-noise ratio (SNR), the MSE becomes independent of the channel Doppler spectrum. A characteristic function method is used to derive new closed-form expressions for the bit error rate (BER) of Rake receivers in UPDP and NPDP channels. The results are extended to two-dimensional (2-D) Rake receivers. The pilot-symbol spacing and pilot-to-data power ratio are optimized by minimizing the BER. For UPDP channels, elegant results are obtained in the asymptotic case. Furthermore, robust spacing design criteria are derived for the maximum Doppler frequency
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