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- Han, Shengqian, et al.
(author)
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Channel norm-based user scheduler in coordinated multi-point systems
- 2009
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In: GLOBECOM - IEEE Global Telecommunications Conference. - 9781424441488
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Conference paper (peer-reviewed)abstract
- In this paper, we address the problem of user scheduling in downlink coordinated multi-point transmission (CoMP) systems, where multiple users are selected and then served with zero forcing beamformer simultaneously by several cooperative base stations (BSs). To reduce the enormous overhead led by obtaining full channel state information at the transmitter, a low-feedback user scheduling method called channel norm-based user scheduler (NUS), is proposed by exploiting the asymmetric channel feature of CoMP systems. Simulation results show that the channel norm provides sufficient information for user scheduling when each BS has one antenna, where the performance gap between the NUS and the greedy user selection (GUS) is negligible with respect to both the cell average throughput and the cell edge throughput. When each BS has multiple antennas, NUS is inferior to GUS, but still significantly outperforms the uncoordinated systems.
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- Shariati, Nafiseh
(author)
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Robust Transmit Signal Design and Channel Estimation for Multiantenna Systems
- 2014
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Doctoral thesis (other academic/artistic)abstract
- In the development of advanced signal processing techniques, dealing with both uncertainties and computational burden is essential. Taking the uncertainties into consideration is required in order to guarantee a certain level of performance, even when the system is designed based on imperfect prior knowledge. Addressing the computational issues is required due to the great popularity of large-scale systems in recent years, where low-complexity signal processing techniques are not only desirable but required.The thesis considers robustness and complexity as two lines of analysis for problems related to parameter estimation and spatial power distribution. In particular, the robustness is realized by means of designing transmit signals using optimization frameworks which account for uncertainties. The complexity issue, on the other hand, is addressed by proposing methods to reduce the computational load for the Bayesian parameter estimation. The common theme throughout the thesis is the use of array with multiple elements which is applied to different applications: wireless communication systems, radars and hyperthermia therapy.The first part of the thesis focuses on the multiple-input multiple-output (MIMO) channel estimation problem. First, a worst-case robust design framework is introduced to deal with uncertain model parameters. This framework is developed specifically to provide robust training sequences for minimum mean square error (MMSE) channel estimation in MIMO communication systems. Second, low-complexity estimators are proposed for large-scale MIMO systems in order to be applied instead of the optimal MMSE estimator which suffers from high computational complexity.The second part of the thesis deals with spatial power distribution using a multi-element array. The distribution of power in space can be controlled in an optimal manner thanks to waveform diversity offered by the MIMO technology. Robust waveforms are designed by using a worst-case robust optimization framework which is tailored specifically for radar and hyperthermia therapy applications, taking uncertain array steering vectors into account.
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- Svedman, Patrick, 1975-
(author)
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Cross-layer Aspects in OFMDA Systems : Feedback, Scheduling and Beamforming
- 2007
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Doctoral thesis (other academic/artistic)abstract
- This thesis mainly studies the downlink of a wireless multiuser system, where the transmitter has limited knowledge about the communication channels of the users. Key techniques to improve the performance of such systems are, for instance, multiple antennas, multiuser diversity and orthogonal frequency-division multiple access (OFDMA). Common for these techniques is that, to exploit them fully, a cross-layer approach has to be adopted. This means that the scheduling and the signal designs are done jointly and based on parameters from several communication layers. Multiuser diversity can be used to significantly increase system throughput in wireless communication systems. The idea is to schedule users when they experience good channel conditions and let them wait when the channels are weak. In this thesis, several aspects of OFDMA systems that exploit multiuser diversity are investigated. An adaptive reduced feedback scheme for OFDMA is proposed. It significantly reduces the total feedback overhead while maintaining a multiuser diversity gain. The scheme uses clusters of sub-carriers as feedback units and feeds back information about the fading peaks only. It adapts to the number of users so that less feedback per user is required if there are many users in the system. With such a selective feedback scheme, there is a risk that the scheduler has no instantaneous channel quality information for some parts of the spectrum. Better uses for these sub-carriers are investigated. In addition, an alternative based on the the channel quality feedback of some uniformly spaced sub-carriers is proposed. The scheduler estimates the channel quality on the other sub-carriers. Channel-aware scheduling is necessary in order to exploit multiuser diversity. A modified proportional fair (PF) scheduler is proposed. It incorporates individual target bit-rates and delays and a tunable fairness level. An opportunistic beamforming scheme for clustered OFDMA is presented and evaluated. A key aspect of the opportunistic beamforming scheme is that it induces artificial frequency selectivity for users with relatively flat channels. Several aspects of the proposed system are evaluated by means of simulations. In the simulations, the clustered beamforming with the modified PF scheduler performs better than three comparison systems. The modified PF scheduler manages to divide the resources according to the user targets, while at the same time exploiting the multiuser diversity as well as the standard PF algorithm. In many scenarios, the largest gains from having multiple antennas at the basei station come from space-division multiple access (SDMA). In the downlink, this means that data is transmitted to several users simultaneously by using several beams. Opportunistic space-division OFDMA is proposed and evaluated. An enhancement that exploits temporal channel correlation is able to boost the throughput significantly. SDMA based on subspace packings is proposed and evaluated. A set of beamforming matrices (a subspace packing) is made a priori available at the base-station and at all users. In each block, one of the matrices is used for multi-beam transmission. The users pick and feed back the index of one preferred column (beam) from one of the matrices, and the corresponding SINR, which includes all potential inter-beam interference. This enables scheduling of spatially compatible users and accurate rate adaptation, with relatively little feedback. Three different subspace packings are considered and evaluated with simulations. For the i.i.d. Rayleigh fading channel, Grassmannian subspace packings were the best choice. Moreover, a method to further reduce the feedback for large packings is proposed and evaluated. It is based on the arrangement of beams in a graph and the feedback of a neighbor index. Numerical results show that the feedback can be significantly reduced with only small performance losses, even for relatively fast fading channels.
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- Tumula V. K., Chaitanya, 1982-
(author)
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HARQ Systems: Resource Allocation, Feedback Error Protection, and Bits-to-Symbol Mappings
- 2013
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Doctoral thesis (other academic/artistic)abstract
- Reliability of data transmission is a fundamental problem in wireless communications. Fading in wireless channels causes the signal strength to vary at the receiver and this results in loss of data packets. To improve the reliability, automatic repeat request (ARQ) schemes were introduced. However these ARQ schemes suffer from a reduction in the throughput. To address the throughput reduction, conventional ARQ schemes were combined with forward error correction (FEC) schemes to develop hybrid-ARQ (HARQ) schemes. For improving the reliability of data transmission, HARQ schemes are included in the present wireless standards like LTE, LTE-Advanced and WiMAX.Conventional HARQ systems use the same transmission power and the same number of channel uses in different ARQ rounds. However this is not optimal in terms of minimizing the average transmit power or the average energy spent for successful transmission of a data packet. We address this issue in the first part of the dissertation, where we consider optimal resource allocation in HARQ systems with a limit on the maximum number of allowed transmissions for a data packet. Specifically, we consider the problem of minimizing the packet drop probability (PDP) under an average transmit power constraint or equivalently minimizing the average transmit power under a fixed PDP constraint. We consider both incremental redundancy (IR)-based and Chase combining (CC)-based HARQ systems in our work. For an IR-HARQ system, for the special case of two allowed transmissions for each packet, we provide a solution for the optimal number of channel uses and the optimal power to be used in each ARQ round. For a CC-HARQ system, we solve the problem of optimal power allocation in i.i.d. Rayleigh fading channels as well as correlated Rayleigh fading channels. For the CC-HARQ case, we also provide a low complexity geometric programming (GP) solution using an approximation of the outage probability expression.HARQ systems conventionally use one bit acknowledgement (ACK)/negative ACK (NACK) feedback from the receiver to the transmitter. In the 3GPP-LTE systems, one method for sending these HARQ acknowledgement bits is to jointly code them with the other control signaling information using a specified Reed-Muller code consisting of 20 coded bits. Even though the resources used for sending this control signaling information can inherently provide a diversity gain, the Reed-Muller code with such a short block size is not good at extracting all of the available diversity. To address this issue, in the second part of this dissertation, we propose two new methods: i) based on complex-field coding (CFC), and ii) using repetition across frequency bands, to extract the inherent diversity available in the channel resources and improve the error protection for the HARQ acknowledgement bits along with the other control signaling information. In the second part of the dissertation, we also propose a new signal space diversity (SSD) scheme, which results in transmit signals having constant envelope (CE). The proposed CE-SSD scheme results in a better overall power efficiency due to the reduced back-off requirements on the radio frequency power amplifier. Moreover, the proposed CE-SSD technique can be useful for application scenarios involving transmission of small number of information bits, such as in the case of control signaling information transmission.In conventional HARQ systems, during the retransmission phase, the channel resources are exclusively used for the retransmitted data packet. This is not optimal in terms of efficient resource utilization. For efficient utilization of channel resources during the retransmissions, a superposition coding (SPC) based HARQ scheme was proposed in the literature. In an SPC based HARQ system, an erroneous packet is transmitted together with a new data packet by superposition in the Euclidean space. In the final part of this dissertation, we study performance of different bits-to-symbol mappings for such an SPC based HARQ system.
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| 8. |
- Wang, Anyue, et al.
(author)
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Joint Optimization of Beam-Hopping Design and NOMA-Assisted Transmission for Flexible Satellite Systems
- 2022
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In: IEEE Transactions on Wireless Communications. - : Institute of Electrical and Electronics Engineers (IEEE). - 1536-1276 .- 1558-2248. ; , s. 1-1
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Journal article (peer-reviewed)abstract
- Next-generation satellite systems require more flexibility in resource management such that available radio resources can be dynamically allocated to meet time-varying and non-uniform traffic demands. Considering potential benefits of beam hopping (BH) and non-orthogonal multiple access (NOMA), we exploit the time-domain flexibility in multi-beam satellite systems by optimizing BH design, and enhance the power-domain flexibility via NOMA. In this paper, we investigate the synergy and mutual influence of beam hopping and NOMA. We jointly optimize power allocation, beam scheduling, and terminal-timeslot assignment to minimize the gap between requested traffic demand and offered capacity. In the solution development, we formally prove the NP-hardness of the optimization problem. Next, we develop a bounding scheme to tightly gauge the global optimum and propose a suboptimal algorithm to enable efficient resource assignment. Numerical results demonstrate the benefits of combining NOMA and BH, and validate the superiority of the proposed BH-NOMA schemes over benchmarks.
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