4,861 research outputs found
DOA Estimation for MC-CDMA Uplink Transmissions
No full textWe consider the uplink of an MC-CDMA network and assume that the base station is endowed with a uniform linear array. Transmission takes place over a multipath channel and the goal is the estimation of the directions of arrival (DOAs) of the signal from the active users. In a multi-user scenario difficulties are due primarily to the large number of parameters involved in the estimation of the DOAs which makes this problem much more challenging than in single-user transmissions. The solution we propose allows estimating the DOAs of different users independently, thereby leading to significant reduction in the system complexity. In the presence of multipath propagation, however, estimating the DOAs of a given user through maximum likelihood methods remains a formidable task since it involves a search over a multidimensional domain. Therefore, we look, for simpler solutions and discuss two alternative schemes based on the SAGE and ESPRIT algorithms
Parameters Estimation for Interference Cancellation in Power-Controlled MC-CDMA Downlink Transmissions
No full textWe consider the downlink of a power-controlled MC-CDMA network in which the base station assigns the power to each user according to its location within the cell. In particular, more power is assigned to the users near the cell boundaries in order to improve the geographical fairness in data reception. This results in a near-far effect for the users located close to the base station, which enhances the multiple-access interference (MAI) and limits the performance of conventional single-user receivers. Interference cancellation (IC) schemes may be employed to mitigate the detrimental effects of MAI. However, they require knowledge of the channel responses as well as of the noise power and strengths of the interfering signals. In this paper, we address the problem of estimating all of the above parameters using a maximum likelihood (ML) approach and exploiting pilot blocks multiplexed with the transmitted data. The estimates are then used by a non-linear detector in which an interfering signal is cancelled provided that its estimated power exceeds a given threshold. Simulation results show that the proposed scheme outperforms other existing IC-based techniques
A Unified Framework for Tomlinson-Harashima Pre-Coding in MC-CDMA and OFDMA Downlink Transmissions
No full textWe consider a unified framework comprising both multicarrier code-division multiple-access (MC-CDMA) and orthogonal frequency-division multiple-access (OFDMA), and discuss nonlinear prefiltering for downlink transmissions based on Tomlinson-Harashima precoding. The base station (BS) is equipped with multiple transmitting antennas and channel state information is assumed to be available at the transmit side. We design the prefiltering matrices so as to minimize the sum of the mean square errors at all mobile terminals when a conventional single-user data detector is employed at the receiver side. In this way, most of the computational burden is moved to the BS, where power consumption and computational resources are not critical requirements. Computer simulations are used to assess the performance of the proposed scheme under different operating scenarios. It turns out that OFDMA outperforms MC-CDMA when the system resources (subcarriers and/or spreading codes) are optimally assigned to the active users according to the channel quality
Channel Estimation in the Presence of Timing Offsets for MC-CDMA Uplink Transmissions with Combined Equalization
No full textIn this paper we consider the uplink of a multi-carrier code-division multiple-access (MC-CDMA) system employing combined equalization and we address the problem of channel estimation in the presence of residual timing offsets. In particular, we propose a method based on the maximum-likelihood (ML) principle that jointly estimates the pre-equalized channel response and timing offset of each user. The resulting scheme operates in the frequency-domain and exploits some training blocks (carrying known symbols) placed at the beginning of the uplink time-slot. Numerical results are given to highlight the effectiveness of the proposed method and to make comparisons with other existing solutions
Channel Acquisition and Tracking for MC-CDMA Uplink Transmissions
No full textThis paper deals with channel acquisition and tracking
in the uplink of a multicarrier code division multiple access
(MC-CDMA) system. The transmission medium is characterized
by multipath propagation, and the goal is to estimate the channel
frequency response and the noise-power of each active user.
Channel acquisition is performed jointly with the noise-power
estimation following two different approaches. The first assumes
independently faded subcarriers, while the second exploits the
fading correlation across the signal bandwidth to improve the
system performance. Both schemes are based on the maximum
likelihood (ML) criterion and exploit some training blocks carrying
known symbols. Channel tracking is pursued through least
mean square (LMS) techniques using data decisions provided by a
partial parallel interference cancellation (PPIC) receiver.
Numerical results are given to highlight the effectiveness of
the proposed methods and to present comparisons with other
existing solutions
A novel pre-filtering technique for downlink transmissions in TDD MC-CDMA systems
No full textWe discuss a prefiltering technique for interference mitigation in the downlink of a time division duplex (TDD) multicarrier code-division multiple access (MC-CDMA) system. The base station (BS) is equipped with multiple transmit antennas, and channel state information (CSI) is obtained at the transmitter side by exploiting the channel reciprocity between uplink and downlink transmissions. The prefiltering coefficients are designed so as to minimize a proper cost function that depends on the signal-to-interference-plus-noise ratios (SINRs) at the mobile terminals (MTs). The resulting scheme allows using a simple despreading receiver, thereby eliminating the need for channel estimation and equalization. Numerical results show the advantages of the proposed scheme over some existing solutions
Channel Estimation for MC-CDMA Uplink Transmissions with Combined Equalization
No full textCombined equalization has recently been proposed to enhance the error rate performance of conventional multicarrier code-division multiple-access (MC-CDMA) systems. This technique applies pre-equalization at the transmitter in conjunction with post-equalization at the receiver, thereby splitting the overall equalization process into two separate parts. In this way, efficient power allocation over the available subcarriers is possible at the transmitter, while leaving the interference cancellation task at the receiver. In this paper, we consider the uplink of an MC-CDMA system employing combined equalization. As the users transmit from different locations, the uplink signals arrive at the base station after passing through different multipath channels and the goal is to estimate the pre-equalized channel frequency response of each user. This is pursued following two different approaches. The first operates in the frequency-domain and treats the channel gains over adjacent subcarriers as independent unknown parameters. The second operates in the time-domain and achieves better performance by reducing the number of unknown parameters. Both schemes are based on maximum-likelihood reasoning and require knowledge of the transmitted symbols. Numerical examples are given to highlight the effectiveness of the proposed methods
Dynamics of Network Formation Processes in the Co-Author Model
No full textThis article studies the dynamics in the formation processes of a mutual consent network in game theory setting: the Co-Author Model. In this article, a limited observation is applied and analytical results are derived. Then, 2 parameters are varied: the number of individuals in the network and the initial probability of the links in the network in its initial state. A simulation result shows a finding that is consistent with an analytical result for a state of equilibrium while it also shows different possible equilibria.Dynamics, Network, Game Theory, Model,Simulation, Equilibrium, Complexity
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