| 1. |
- Ahsin, Tafzeel ur Rehman, 1979-
(författare)
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Link Reliability in Cooperative Relaying Using Network Coding
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Demand for high data rates is increasing rapidly for future wireless systems.This trend is due to the increase in the number of mobile subscribers that need bandwidth hungry multimedia applications anywhere, anytime. Fourth generation cellular systems like IMT-advanced are being developed to meet these requirements. The unreliable nature of the wireless medium is one of the main hinderance in providing high data rates. Cooperative communication in cellular networks is emerging as a new paradigm to deal with the channel impairments. User cooperation via fixed relays in cellular systems form multiple access relay channels (MARCs) and provide an effective and cost efficient solution to achieve spatial diversity gains. Network resources can be utilized efficiently by using network coding at cooperating nodes. A lot of research work has focused on highlighting the gains achieved by using network coding in MARCs. However, there are certain areas that are not fully explored yet. For instance, the kind of the detection scheme used at the base station receiver and its impact on the link performance has not been addressed. In most cases, the outage probability has been used as a performance measure of MARCs. However, it is well known that the outage probability gives information about the signal availability, but it does not give the complete picture about the reliability of the link and the achieved quality of service. This thesis work looks at the link performance, in terms of symbol error probability, of multiple access relay channels that employ network coding at the relay node. Different types of detection schemes are considered and their performance is compared under different link conditions. Analytical expressions for the average symbol error probability of the cooperating users are derived. Focusing on the uplink of cellular systems, certain rules are devised on how to group users at relay node to ensure mutual benefit for the cooperating users. As a way of improving the link performance of multiple access relay channels and their robustness, the thesis considers constellation selection for the different branches. This method takes advantage of the redundancy between the transmitted symbols created by network coding and the augmented signal space obtained at the base station receiver. The obtained results show that, with a proper selection of the constellation sets, the link performance of MARCs can be improved. The thesis further looks at the interaction between the channel coding schemes of the cooperating users and network coding. It is shown that joint channel-network coding in MARCs can be seen as a product code. This new representation provides considerable flexibility in selecting efficient decoding algorithms at the base station receiver and gives the possibility to use more powerful network coding schemes for MARCs.
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| 2. |
- Aronsson, Daniel, 1976-
(författare)
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Channel Estimation and Prediction for MIMO OFDM Systems : Key Design and Performance Aspects of Kalman-based Algorithms
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wireless broadband systems based on Orthogonal Frequency Division Multiplexing (OFDM) are being introduced to meet demands for high data transfer rates. In multiple users systems, the available bandwidth has to be shared efficiently by several users. The radio channel quality will fluctuate, or fade, as users move. Fading complicates the resource allocation, but channel prediction may alleviate this problem. A flexible and computationally inexpensive state space representation of fading channels is here used in conjunction with a Kalman filter, operating on special-purpose reference signals, to track and predict fading OFDM channels. The thesis investigates key design and performance aspects of such estimators. Taking a probabilistic approach, we interpret the output of the Kalman filter as a full representation of a state of knowledge about the fading channels, given whatever information is at hand. For systems analysis, this permits conclusions to be drawn about channel estimation and prediction performance based on only vague information about the fading characteristics of the channel rather than on actual channel measurements. This is an alternative to conducting classic simulation studies. Various reference signal designs are studied and good design choices are recommended. Superimposed reference signal schemes are also proposed for and evaluated in cases where multiple signals are received, e.g. in multi-user (MU), multi-input multi-output (MIMO), or coordinated multi-point (CoMP) settings. By using time-varying reference signals, channel estimation and prediction performance is shown to be improved considerably in crowded frequency bands. The variation of prediction performance with prediction range and Doppler spectrum characteristics is investigated. For link adaptation, we derive the appropriate metric on which adaptation decisions should be used. The probability density function for this metric is derived for general MIMO channels. Link adaptation is studied for a single link system when channel prediction and estimation errors are present, both for uncoded systems and systems using large block codes with soft decoders. Various aspects of channel model acquisition are addressed by conducting studies on measured channels. Owing to the use of special matrix structures and fast convergence to time-invariant or periodic solutions, we find the Kalman filter complexity to be reasonable for future implementation. Finally, expressions for the impact of modelling errors are derived and used to study the impact of modelling errors on channel prediction performance in some example cases.
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| 3. |
- Edfors, Ove, et al.
(författare)
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Is orbital angular momentum (OAM) based radio communication an unexploited area?
- 2012
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Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X. ; 60:2, s. 1126-1131
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Tidskriftsartikel (refereegranskat)abstract
- We compare the technique of using the orbital angular momentum (OAM) of radio waves for generating multiple channels in a radio communication scenario with traditional multiple-in-multiple-out (MIMO) communication methods. We demonstrate that, for certain array configurations in free space, traditional MIMO theory leads to eigen-modes identical to the OAM states. From this we conclude that communicating over the sub-channels given by OAM states is a subset of the solutions offered by MIMO, and therefore does not offer any additional gains in capacity.
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| 4. |
- Foroughi, Farzad, et al.
(författare)
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Low complexity channel estimation for LTE in fast fading environments for implementation on multi-standard platforms
- 2010
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Ingår i: IEEE 72ND VEHICULAR TECHNOLOGY CONFERENCE FALL 2010. - 1090-3038.
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Konferensbidrag (refereegranskat)abstract
- In this paper a number of low-complexity channel estimator structures tailored to the needs of LTE terminals have been investigated. The focus has been on the algorithms exhibiting reduced complexity while providing an acceptable level of performance. To design these estimators other factors such as interoperability with another OFDM-based standard, DVB-H, in a multi-standard environment have been taken into account. The key underlying parameter has been exploiting the pilot pattern in LTE providing the possibility of minimal filtering in time domain while exhibiting an acceptable level of performance in fast fading environments.
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| 5. |
- Foroughi, Farzad, et al.
(författare)
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On coefficient memory co-optimization for channel estimation in a multi-standard environment (LTE and DVB-H)
- 2011
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Ingår i: Proc. of the 8th International Workshop on Multi-Carrier Systems & Solutions (MC-SS).
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Konferensbidrag (refereegranskat)abstract
- This paper presents a novel memory optimization technique to exploit the analogy between correlation coefficients in MMSE-based channel estimators across multiple standards. The need for coefficient storage for robust MMSE channel estimators is expensive in terms of on-chip memory. Besides, the memory requirements grow linearly by integrating more standards onto one platform. Thus, it becomes inevitable to address the increased on-chip memory problem for such estimators. In this paper, we have shown that by exploiting the inherent similarities between LTE and DVB-H design parameters, a three-fold memory optimization may be achieved with minimal performance loss.
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| 6. |
- Gangarajaiah, Rakesh, et al.
(författare)
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A High-Speed QR Decomposition Processor for Carrier-Aggregated LTE-A Downlink Systems
- 2013
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Konferensbidrag (refereegranskat)abstract
- This paper presents a high-speed QR decomposition (QRD) processor targeting the carrier-aggregated 4 × 4 Long Term Evolution-Advanced (LTE-A) receiver. The processor provides robustness in spatially correlated channels with reduced complexity by using modifications to the Householder transform, such as decomposing-target redefinition and matrix real-valued decomposition. In terms of hardware design, we extensively explore flexibilities in systolic architectures using a high-level synthesis tool to achieve area-power efficiency. In a 65 nm CMOS technology, the processor occupies a core area of 0.77mm2 and produces 72MQRD per second, the highest reported throughput. The power consumed in the proposed processor is 219mW.
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| 7. |
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| 8. |
- Gao, Xiang, et al.
(författare)
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Channel behavior for very-large MIMO systems - initial characterization
- 2012
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Wireless communication using very-large multiple-input multiple-output (MIMO) antennas is a new research field, where base stations are equipped with a very large number of antennas as compared to previously considered systems. Unlike the conventional small and compact antenna arrays, the very-large arrays may span many wavelengths in space, thus they can experience large-scale fading over the array. This power variation over the antenna array may be critical to algorithm design and performance evaluations for very-large MIMO systems, and it is thus important that it is included in channel models. Based on channel measurements using a 128-antenna linear array in a semi-urban area, we characterize and model the large-scale fading properties. We focus on cluster-based modeling which can be seen as an extension of the current COST 2100 channel model. In the paper, we report our latest channel behavior modeling progress for very-large MIMO systems.
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| 9. |
- Gao, Xiang, et al.
(författare)
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Large antenna array and propagation environment interaction
- 2014
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Ingår i: Proc. 48th Asilomar Conference on Signals, Systems and Computers, ACSSC 2015. - 9781479982974 ; , s. 666-670
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Konferensbidrag (refereegranskat)abstract
- In conventional MIMO, propagation conditions are often considered wide-sense stationary over the entire antenna array. In massive MIMO systems, where arrays can span over large physical dimensions, the situation is quite different. For instance, significant variations in signal strength, due to shadowing, can be experienced across a large array. These effects vary with propagation environment in which the array is placed, and influence achievable sum-rates. We characterize these variations for several measured propagation scenarios in the 2.6 GHz frequency range and illustrate how power variations and correlation properties change along the array.
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| 10. |
- Gao, Xiang, et al.
(författare)
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Linear pre-coding performance in measured very-large MIMO channels
- 2011
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Ingår i: Proc. of the 74th IEEE Vehicular Technology Conference.
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Konferensbidrag (refereegranskat)abstract
- Wireless communication using very-large multiple-input multiple-output (MIMO) antennas is a new research field, where base stations are equipped with a very large number of antennas as compared to previously considered systems. In theory, as the number of antennas increases, propagation properties that were random before start to become deterministic. Theoretical investigations with independent identically distributed (i.i.d.)\ complex Gaussian (Rayleigh fading) channels and unlimited number of antennas have been done, but in practice we need to know what benefits we can get from very large, but limited, number of antenna elements in realistic propagation environments. In this study we evaluate properties of measured residential-area channels, where the base station is equipped with 128 antenna ports. An important property to consider is the orthogonality between channels to different users, since this property tells us how advanced multi-user MIMO (MU-MIMO) pre-coding schemes we need in the downlink. We show that orthogonality improves with increasing number of antennas, but for two single-antenna users there is very little improvement beyond 20 antennas. We also evaluate sum-rate performance for two linear pre-coding schemes, zero-forcing (ZF) and minimum mean squared-error (MMSE), as a function of the number of base station antennas. Already at 20 base station antennas these linear pre-coding schemes reach 98\% of the optimal dirty-paper coding (DPC) capacity for the measured channels.
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