Cenk Toker

Closed-loop quasi-orthogonal STBCs and their performance in multipath fading environments and when combined with turbo codes

Quasi-orthogonal space-time block codes (QO-STBCs) achieve full code rate at the expense of loss in diversity gain. We propose two feedback methods for QO-STBCs to achieve full diversity and full code rate. In the first method, signals radiated from various antennas are rotated by phasors according to feedback from the receiver, whereas the second method is based upon antenna weighting/selection. For high to moderate feedback error rates, it is demonstrated that the proposed methods outperform the quantized transmit beamformer. The performance improvement is also investigated for these closed-loop methods when the transmitted signal is error control coded.

Closed-loop space time block coding techniques for OFDM broadband wireless access systems

We propose a closed loop space time block coding technique using four antennae at the transmitter for the enhancement of link layer performance of orthogonal frequency division multiplexing (OFDM) based broadband wireless access systems such as wireless local area networks (WLANs) and wireless metropolitan area networks (WMANs). Since full code rate and complex valued space-time block codes (STBCs) do not exist for more than two transmit antennae, we propose two feedback based STBC schemes. In the first scheme, phases of certain symbols in each sub carrier are rotated according to the feedback from the receiver. In the second scheme, best two out of four antennae are activated for transmission based on the associated channel gains in each subcarrier. In both schemes, the channel perceived at the receiver becomes orthogonal and the transmit diversity gain is maximized. One of the major challenges associated with the closed loop STBC schemes in OFDM systems as compared to CDMA systems is the requirement for extensive channel state information feedback as channel orthogonalization is performed in each subcarrier. However, by exploiting the feedback correlation among subcarriers, we have proposed a group based quantization technique to reduce feedback overhead significantly, making this scheme very attractive to broadband wireless access systems.

Space-time block coding for four transmit antennas with closed loop feedback over frequency selective fading channels

Orthogonal space-time block coding is a transmit diversity method that has the potential to enhance forward capacity. For a communication system with a complex alphabet, full diversity and full code rate space-time codes are available only for two antennas, and for more than two antennas full diversity is achieved only when the code rate is lower than one. A quasi-orthogonal code could provide full code rate, but at the expense of loss in diversity, which results in degradation of performance. We propose a closed loop feedback scheme for quasi-orthogonal codes which provides full diversity while achieving the full code rate. We investigate, in particular, the performance of this scheme, when the feedback information is quantised and when the fading of the channel is frequency-selective.

Joint Transceiver Design for MIMO Channel Shortening

Channel shortening equalizers can be employed to shorten the effective impulse response of a long intersymbol interference (ISI) channel in order, for example, to decrease the computational complexity of a maximum-likelihood sequence estimator (MLSE) or to increase the throughput efficiency of an orthogonal frequency-division multiplexing (OFDM) transmission scheme. In this paper, the issue of joint transmitter-receiver filter design is addressed for shortening multiple-input multiple-output (MIMO) ISI channels. A frequency-domain approach is adopted for the transceiver design which is effectively equivalent to an infinite-length time-domain design. A practical space-frequency waterfilling algorithm is also provided. It is demonstrated that the channel shortening equalizer designed according to the time-domain approach suffers from an error-floor effect. However, the proposed techniques are shown to overcome this problem and outperform the time-domain channel shortening filter design. We also demonstrate that the proposed transceiver design can be considered as a MIMO broadband beamformer with constraints on the time-domain multipath length. Hence, a significant diversity gain could also be achieved by choosing strong eigenmodes of the MIMO channel. It is also found that the proposed frequency-domain methods have considerably low computational complexity as compared with their time-domain counterparts.

Blind, Adaptive Channel Shortening Equalizer Algorithm Which Can Provide Shortened Channel State Information (BACS-SI)

Channel shortening equalization plays an important role in multicarrier modulation (MCM) systems. In this paper, we propose a blind channel shortening equalizer structure named blind, adaptive channel shortening equalizer which can provide the shortened channel state information (BACS-SI). The algorithm depends on the minimization of a cost function defined as the sum-squared difference of the autocorrelations of the shortened channel impulse response (CIR) and a target impulse response. The surface is proven to be multimodal; however, minima are shown to be related to each other in a certain way. A two-phase approach is proposed. In the first phase, the cost function is minimized by a stochastic gradient descent algorithm in order to find an arbitrary minimum. In the second phase using the relation between minima, genetic algorithms are employed to find the best minimum according to a fitness function. The algorithm can both successfully shorten the channel and also explicitly provides shortened CIR which is a necessary information for the proper operation of a MCM receiver, in contrast to many other algorithms proposed in the literature which cannot directly provide this information.

Optimum Uplink Power Control under Power and Interference Constraints

This paper considers optimum allocation of transmission powers in next generation networks. The proposed framework is general enough to cover both emerging heterogeneous network (HetNet) architectures and cognitive radio (CR) networks. There are two types of users in our model. Type 1 users (T1U) represent either femtocell users in a HetNet, or secondary users in a CR network. Type 2 users (T2U) represent either macrocell users in a HetNet, or primary users in a CR network. T1Us share the same frequency band with T2Us, and they form an uplink to their intended base station (i.e., either the femtocell base station or the secondary base station) while causing interference to T2Us. The optimum power allocation strategy maximizing the aggregate communication rate of T1Us is found under individual transmission power constraints and a total interference power constraint at T2Us. It is shown that the optimum power allocation exhibits a binary structure, which means links are either quot;onquot; or quot;offquot;, up to at most one exceptional fractional power level. Further, it is shown that T1Us transmitting at positive power correspond to the ones having better quot;jointquot; power and interference channel gains. Applications of these results are illustrated for well-known fading models such as Rayleigh, Rician-K, and Nakagami-m fading.

Suboptimal recursive optimisation framework for adaptive resource allocation in spectrum-sharing networks

The authors propose a suboptimal algorithm for adaptive subcarrier, bit and power allocation for orthogonal frequency division multiple access-based spectrum-sharing networks. This problem in its original form is non-convex and may be solved using greedy algorithms or integer linear programming (ILP) techniques. However, the computational complexity of the latter techniques is quite high, while the suboptimal greedy algorithms are not very well suited for spectrum-sharing networks because of multiple constraints on the transmitted power, interference leakage and individual user data rate. Therefore the authors propose a novel recursion-based linear optimisation framework that provides a solution that is very close to the optimal one and that has the ability to perform adaptive subcarrier, bit and power allocation for multiple users in the presence of multiple individual user constraints. Owing to the convexity of the proposed algorithm at each recursion, its overall complexity is substantially lower than that of the ILP-based solution.

A low complexity resource allocation algorithm for OFDMA systems

In a multiuser system, the users can share the common channel using the OFDMA (Orthogonal Frequency Division Multiple Access) technique. In this scenario, the issue of allocating the shared channel resources (subcarrier and power) among the users is called the resource allocation problem. This problem, which is nonlinear due to its nature, also has a discrete structure. In this paper, a fast algorithm based on linear programming is proposed. The algorithm is composed of two phases. In the first one, the original integer programming problem is first linearized and then after relaxing the integer constraints a solution is obtained by the aid of the Simplex Algorithm. In the second phase, by exploiting the sub-carrier allocation obtained in first phase, bits are allocated to each user independently using the Greedy Algorithm. The simulation results have demonstrated that, the proposed algorithm can achieve a performance very close to that of the integer programming techniques while being significantly faster. Computational complexity analysis of the algorithm revealed that it can be implemented real-time on a standard microprocessor/DSP.

Generalized coordinated port selection in a multi-cell distributed antenna system using semidefinite relaxation

The downlink of a coordinated multi-cell distributed antenna system is considered. In [1], coordinated port selection was shown to achieve significant performance gains. However, in the system considered therein, the ports in each cell were constrained to transmit only to user terminals (UTs) in that cell. In this work, we consider a generalization of the problem considered in [1] by alleviating this constraint. We formulate the problem of determining the ports that maximize the minimum signal to interference plus noise ratio observed by the UTs as a binary-constrained optimization problem. Observing that it is NP-hard, we propose a semidefinite relaxation and Gaussian randomization based technique to obtain close-to-optimal solutions. Our simulation results show that the performance achieved by the proposed technique approaches that of the optimal solution. It is also shown that the proposed technique outperforms the one in [1], particularly for cell-edge UTs, albeit with an increased computational complexity.

Space weather activities of IONOLAB group using TNPGN GPS Network

Characterization and constant monitoring of variability of the ionosphere is of utmost importance for the performance improvement of HF communication, Satellite communication, navigation and guidance systems, Low Earth Orbit (LEO) satellite systems, Space Craft exit and entry into the atmosphere and space weather. Turkish National Permanent GPS Network (TNPGN) is the Reference Station Network of 146 continuously-operating GNSS stations of which are distributed uniformly across Turkey and North Cyprus Turkish Republic since May 2009. IONOLAB group is currently investigating new techniques for space-time interpolation, and automatic mapping of TEC through a TUBITAK research grant. It is utmost importance to develop regional stochastic models for correction of ionospheric delay in geodetic systems and also form a scientific basis for communication link characterization. This study is a brief summary of the efforts of IONOLAB group in monitoring of space weather, and correction of geodetic positioning errors due to ionosphere using TNPGN.