Bahattin Karakaya

An adaptive channel interpolator based on Kalman filter for LTE uplink in high Doppler spread environments

Long-Term Evolution (LTE) systems will employ single carrier frequency division multiple access (SC-FDMA) for the uplink. Similar to the Orthogonal frequency-division multiple access (OFDMA) technology, SC-FDMA is sensitive to frequency offsets leading to intercarrier interference (ICI). In this paper, we propose a Kalman filter-based approach in order to mitigate ICI under high Doppler spread scenarios by tracking the variation of channel taps jointly in time domain for LTE uplink systems. Upon acquiring the estimates of channel taps from the Kalman tracker, we employ an interpolation algorithm based on polynomial fitting whose order is changed adaptively. The proposed method is evaluated under four different scenarios with different settings in order to reflect the impact of various critical parameters on the performance such as propagation environment, speed, and size of resource block (RB) assignments. Results are given along with discussions.

Low complexity channel estimation for 3GPP LTE downlink MIMO OFDM systems

Multiple Input Multiple Output - Orthogonal Frequency Division Multiplexing (MIMO-OFDM) is a promising technique for reaching high data rates targeted in the 3rd Generation Partnership Project - Long Term Evolution (3GPP-LTE). MIMO-OFDM channel estimation schemes play a very important role on achieving this aim. However, the complexity of the estimators increases exponentially due to the structure of the MIMO systems. This causes an increase in the computational burden of the transceivers. As a result, the complexity of the channel estimators is becoming an important issue in real world MIMO-OFDM applications. In this paper, the performance of the low complexity Minimum Mean Square Error (MMSE) channel estimator scheme based on Karhunen-Loéve (KL) series expansion coefficients for the 3GPP-LTE Downlink MIMO-OFDM systems is examined. System level simulations are accomplished to compare the performances of the estimators under the spatially correlated channel coefficient variations.

Channel estimation for MIMO-OFDM systems in fixed broadband wireless applications

Systems employing multiple transmit and receive antennas, known as multiple input multiple output (MIMO) systems can be used with OFDM to improve the resistance to channel impairments. Thus the technologies of OFDM and MIMO are equipped in fixed wireless applications with attractive features, including high data rates and robust performance. However, since different signals are transmitted from different antennas simultaneously, the received signal is the superposition of these signals, which implies new challenges for channel estimation. In this paper we propose a time domain MMSE based channel estimation approach for MIMO-OFDM systems. The proposed approach employs a convenient representation of the discrete multipath fading channel based on the Karhunen-Loeve (KL) orthogonal expansion and finds MMSE estimates of the uncorrelated KL series expansion coefficients. Based on such an expansion, no matrix inversion is required in the proposed MMSE estimator. Also the performance of the proposed approach is studied through the evaluation of minimum Bayesian MSE.

Time and Frequency Synchronization with Channel Estimation for SC-FDMA Systems Over Time-Varying Channels

The constant amplitude zero autocorrelation sequence based synchronization and its usage in the block-type channel estimation for the single carrier frequency division multiple access (SC-FDMA) systems are vulnerable to the time-varying channels. Therefore, the channel estimation errors limit the performance of SC-FDMA systems in fast fading environments and result an irreducible error floor. In this paper, cyclic prefix based maximum likelihood estimator of time/frequency synchronization with comb-type channel estimation for the SC-FDMA systems are proposed to track the channel variations. Moreover, the residual time/frequency offsets calculations are derived for SC-FDMA systems. Simulation results confirm and illustrate that the proposed receiver is capable of tracking fast fading channel parameters and improving the overall performance as compared with the conventional receiver.

Full length article: Channel interpolation for LTE uplink systems with high mobility using Slepian sequences

Single Carrier Frequency Division Multiple Access (SC-FDMA) has been adopted for 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) uplink communication since it combines the low peak to average power ratio feature of single-carrier transmission systems with the multipath resistance and flexible frequency allocation of orthogonal frequency-division multiple access (OFDMA). Unfortunately, SC-FDMA transmission over time-varying multipath fading channels for very high speed environments the subcarrier orthogonality is destroyed giving rise to intercarrier interference (ICI) due to channel variation within an SC-FDMA symbol. This paper is, therefore, focused on the use of time-domain channel interpolator for LTE uplink systems in large Doppler spread scenarios to improve channel tracking performance. The resulting interpolation algorithm is based on the discrete prolate spheroidal wave functions (DPSWF) which is particularly well suited to represent the rapidly time-varying fading channel due to optimum finite time and frequency support feature. The variations of the channel taps are tracked first by a Kalman filter in time domain during training symbols. Then, a DPSWF based channel interpolator is applied to recover the time variation of channel taps between training symbols within two consecutive slots in a single subframe. The performance of the proposed interpolator is compared with a polynomial interpolator whose order is adjusted adaptively to achieve the best fit under particular user mobilities. Moreover, the analytical mean square error (MSE) performance of DPSWF is derived and compared with the simulations. The results show that the DPSWF represents the time-varying fading channel very effectively and the proposed algorithm has excellent channel estimation performance in high SNR even with a very small number of channel interpolation parameters employed in the algorithm.

Time synchronization and channel estimation for cooperative SC-FDMA systems over time varying channels

In this paper, cyclic prefix based time synchronization and comb type channel estimation are studied for a SC-FDMA systems with AF relaying cooperative communication technique over time varying channels. Preamble based synchronization and its usage in the block type channel estimation for SC-FDMA systems are vulnerable to time varying channels. Therefore, undeductible channel estimation errors in time varying channels limit the performance of SC-FDMA systems. For this reason, in this paper comb type channel estimation is proposed to track the channel variations. Simulation results demonstrate that the cyclic prefix based time synchronization and comb type channel estimation have better bit error rate performance over time varying channels.

Cyclic prefix based time synchronization and comb type channel estimation for SC-FDMA systems over time-varying channels

In the long term evolution (LTE) uplink systems, reference signals based on Zadoff-Chu sequences are employed for time synchronization and block type channel estimation because the pilots are inserted into all the subcarriers of one single carrier-frequency division multiple access (SC-FDMA) symbol with a certain period. The block-type channel estimation can be adopted for slowly fading channel while the channel can be considered as stationary within a certain period of SC-FDMA symbols. However, in practice, a wideband radio channel is time-variant and the channel estimation errors based on block-type channel estimation will significantly degrade the performance of SC-FDMA systems in fast fading environments. Therefore, in this paper, the maximum likelihood (ML) time synchronization and comb-type channel estimation for SC-FDMA systems is presented. We demonstrated that the proposed system has significant bit error rate (BER) advantages over fast varying environments.

Leakage reduction in DFT-based channel estimation for LTE Uplink

Channel estimation is an integral part of receiver designs in coherent OFDM systems. Moreover channel estimation is a challenging problem in wireless systems, due to time variation of the multipath channel. In this paper, a time domain channel estimation method for rapidly time varying channel in LTE Uplink, which we have been proposed previously is further studied in the presence of guard band and multiple access. Since the presence of guard band and multiple access results in leakage, we therefore propose a new approach to reduce leakage on the number of taps which are to be tracked by Kalman Filter.

Two-way relay underwater acoustic communication channels with distributed space-time block coding

In this paper we study the performance of two-way relaying (TWR) over underwater acoustic (UWA) channels in conjunction with distributed space-time block coding (D-STBC). In particular, we consider the communication between two sources via relay nodes. The underlying channels are characterized as doubly selective channels. Orthogonal frequency division multiplexing (OFDM) is used to combat frequency selectivity of the channels, while front-end multiple resampling (MR) combined with frequency-domain equalization is used to combat intercarrier interference (ICI) resulting from time selectivity of the channel caused by the relative motion between the transceivers. Simulation results show the superiority of MR over its single resampling (SR) counterpart. Also, under total power constraint, AF-D-STBC (when only one source is activated at a time) outperforms AF-TWR-D-STBC, however, at the expense of less bandwidth efficiency. Also, AF-TWR-D-STBC outperforms AF-TWR (when one relay is activated) even though the former contributes more interference. Finally, to further boost the performance, successive interference cancellation (SIC) is used to extract the spatial diversity offered by the relays.

A Simulation Study of Underwater Acoustic Communications in the North Field of Qatar

Abstract Qatar is a leading natural gas producer and exporter in the world. Most of the natural gas (and oil) of Qatar is extracted from offshore wells, and then it is transferred to onshore for processing. In addition, Qatar is connected to UAE by one of the worlds longest underwater pipelines (managed by Dolphin Energy), to transfer processed gas from the offshore north field to the UAE. Security of such critical offshore infrastucture against threats along with the environmental and preventive maintenance monitoring (e.g., pollution, leakage) are of utmost importance. A wireless underwater sensor network can be deployed for the security and safety of underwater pipelines. However, underwater acoustic communication brings its own challenges such as limited transmission range, low data rates and link unreliability. In this paper, we propose “cooperative communication” as an enabling technology to meet the challenging demands in underwater acoustic communication (UWAC). Specifically, we consider a multi-carrier and multi-relay UWAC system and investigate relay (partner) selection rules in a cooperation scenario. For relay selection, we consider different selection criteria, which rely either on the maximization of signal-to-noise ratio (SNR) or the minimization of probability of error (PoE). These are used in conjunction with so-called per-subcarrier, allsubcarriers, or subcarrier grouping approaches in which one or more relays are selected. In our simulation study, we choose an offshore area in the North Eastern side of Qatar (which coincides with the North Field) and conduct an extensive Monte Carlo simulation study for the chosen location to demonstrate the performance of the proposed UWAC system. Our channel model builds on an aggregation of both large-scale path loss and small-scale fading. For acoustic path loss modeling, we use the ray-tracing algorithm Bellhop software to precisely reflect the characteristics of the simulation location such as the sound speed profile, sound frequency, bathymetry, type of bottom sediments, depths of nodes, etc (See Fig.1 ). Our simulation results for the error rate performance have demonstrated significant performance improvements over direct transmission schemes and highlighted the enhanced link reliability made possible by cooperative communications (See Fig.2 ).