Abbas Yongacoglu

Differential detection of GMSK using decision feedback

Differential detector structures that use decision feedback to partially remove the effect of destructive intersymbol interference from minimum-shift keying (MSK) are introduced. In comparison to the conventional two-bit differential detection of GMSK (Gaussian MSK), the proposed structures provide about 4-dB E/sub b//N/sub 0/ improvement when the premodulation filter time-bandwidth product is 0.25. As the premodulation filter becomes narrower, the advantages of the proposed receivers are more pronounced. Thus the bit-error-rate performance of differential detection is brought closer to coherent detection while avoiding the problems associated with the carrier recovery. >

Sparse channel estimation using orthogonal matching pursuit algorithm

Sparse channels are encountered in several communication applications. Exploiting the sparsity, a channel estimate can be obtained by using a matching pursuit (MP) algorithm. Previously, it was demonstrated that the MP based channel estimation outperforms the conventional least squares (LS) estimation algorithm for sparse channels. In this paper, we propose to use the orthogonal matching pursuit (OMP) algorithm for channel estimation. Using OMP, the convergence problem in MP algorithm based on re-selection of the basis vectors is eliminated. It is also verified that by avoiding the re-selection problem more accurate channel estimates can he obtained by using the OMP algorithm. The performance of decision feedback equalizers based on the channel estimates obtained by using the MP and OMP algorithms are compared, verifying that the OMP outperforms the MP, with a comparable computational complexity.

IEEE 802.11a Throughput Performance with Hidden Nodes

Analytic performance of IEEE 802.11 distributed coordination function that takes into account hidden node problem and unsaturated traffic condition is presented for symmetric networks. This enables us to estimate the performance of practical wireless local area networks more accurately. It is shown that the presence of hidden nodes barely affects the network performance in low traffic conditions, but it causes 33% performance loss in moderate traffic scenarios. Analytical results presented in the paper are backed by simulation results.

On the Number and 3D Placement of Drone Base Stations in Wireless Cellular Networks

Using drone base stations (drone-BSs) in wireless networks has started attracting attention. Drone-BSs can assist the ground BSs in both capacity and coverage enhancement. One of the important problems about integrating drone-BSs to cellular networks is the management of their placement to satisfy the dynamic system requirements. In this paper, we propose a method to find the positions of drone-BSs in an area with different user densities using a heuristic algorithm. The goal is to find the minimum number of drone-BSs and their 3D placement so that all the users are served. Our simulation results show that the proposed approach can satisfy the quality-of-service requirements of the network.

A sequential decoder for differential detection of trellis coded PSK signals

A soft-decision sequential decoder is presented for the differential detection of the trellis-coded phase-shift keyed (PSK) signal. The decoder is based on use of the redundancy introduced by the differential encoder. This is done by joint utilization of several differential detectors. The performance of the decoder is evaluated using the trellis-coded 4-PSK and 8-PSK schemes. Simulation results indicate that for Gaussian and Rician channels, the sequential decoder provides significant performance improvements.<<ETX>>

RAKE receiver structures for differential and pilot symbol-assisted detection of DS-CDMA signals in frequency-selective Rayleigh fading channels

The capacity of a CDMA system is a function of the bit error rate performance of individual users. Therefore it is important to optimize the individual links before proceeding to system level analysis. This is particularly true for operating in a fading channel where the performance without diversity reception is rather poor. This paper compares the BER performance of differential detection and pilot symbol-assisted coherent detection of a direct-sequence spread spectrum signal on a frequency-selective Rayleigh fading channel using RAKE reception. Both equal gain and maximal ratio combining are considered, and the effect of convolutional coding with interleaving is studied.<<ETX>>

Application of noisy-independent component analysis for CDMA signal separation

We propose a noisy-independent component analysis (ICA) based CDMA receiver for multiple access communication channels. ICA is a statistical method for transforming an observed multidimensional random vector into components that are statistically as independent from each other as possible. We apply noisy-ICA as a post processor attached to a subspace based CDMA receiver in the presence of Gaussian noise. The proposed algorithm reduces the bias caused by channel noise in ordinary ICA algorithms and further decreases the noise by dimension reduction. The downlink CDMA channel is investigated and we assume that only the code of the wanted mobile user is known (i.e., blind symbol separation). We compare the proposed receiver with noisy-ICA ability to the conventional matched filter, well-known linear MMSE multiuser detector and ordinary (noise free) ICA based receivers. Numerical simulations indicate that the performance of the noisy-ICA based receiver is superior to conventional detectors, and comparable to exact-MMSE (i.e., all user codes are known) detection performance in a synchronous multiple access CDMA channel. The performance of the ordinary ICA based CDMA receiver is improved with noise bias removal and principal component analysis (PCA) based dimension reduction.

Novel receiver structures for systems using differential detection

Novel receiver structures are introduced which improve the bit error rate performance of differentially detected MSK and QPSK systems. These new receiver structures are based on 1) combining with feedback, 2) a simple Viterbi decoder which uses one- and two-bit detector outputs for maximum-likelihood sequence estimation, and 3) using the correlation of noise for the partial removal of its effect prior to decision. The new receiver structures presented are useful in mobile radio and mobile satellite communications where power efficiency, synchronization, and implementation complexity are of primary concern.

A novel association algorithm for congestion relief in IEEE 802.11 WLANs

Many wireless local area network (WLAN) performance estimations are done with the assumption of uniformly distributed stations (STAs). In practice, on the contrary STAs are distributed unevenly among access points (APs), causing hot-spots and under utilized APs in a wireless network. Considering a WLAN is made up of multiple APs, having some APs carrying excessive loads (i.e. hot-spots) degrades both the considered APs as well as the overall network performance. The system performance can be improved by associating incoming STAs effectively throughout the network, in a sense to balance the network load evenly between APs and relieve the hot-spot congestion. Currently employed user association method in IEEE 802.11 WLANs considers only the received signal strength of APs at STAs, and associates STAs to the closest (in signal strength sense) AP, ignoring its load and interference value.Novel user association algorithms are required for congestion relief and network performance improvement. In this work, a new distributed association algorithm taking into consideration not only the received signal strength of the APs at STAs but also AP loadings and interference is proposed. A new AP load calculation method acknowledging the interference between STAs and APs is presented. Our simulations demonstrate that the proposed algorithm can improve the overall system throughput performance more than 50% and offers a better load distribution across the network compared to conventional association algorithm.

A comparison of interference cartography generation techniques in cognitive radio networks

Interference cartographs can be utilized to determine no-transmit zones and to set the interference-free transmit power for secondary users in cognitive radio networks. This paper provides a performance comparison of common interpolation techniques for generating interference cartographs. The performances of natural-neighbor, thin-plate spline and kriging interpolations are evaluated in terms of primary emitter localization accuracy and RF field strength estimation efficiency. The simulation results suggest that the natural-neighbor interpolation technique achieves the same level of accuracy and provides more desirable features suitable for cognitive radio networks.