116 research outputs found
Receiver design for nonlinearly distorted OFDM : signals applications in radio-over-fiber systems
Tese de doutoramento. Engenharia Electrotécnica e de Computadores. Universidade do Porto. Faculdade de Engenharia. 201
High-performance signal acquisition algorithms for wireless communications receivers
Due to the uncertainties introduced by the propagation channel, and RF and
mixed signal circuits imperfections, digital communication receivers require efficient
and robust signal acquisition algorithms for timing and carrier recovery, and interfer-
ence rejection.
The main theme of this work is the development of efficient and robust signal
synchronization and interference rejection schemes for narrowband, wideband and
ultra wideband communications systems. A series of novel signal acquisition schemes
together with their performance analysis and comparisons with existing state-of-the-
art results are introduced. The design effort is first focused on narrowband systems,
and then on wideband and ultra wideband systems.
For single carrier modulated narrowband systems, it is found that conventional
timing recovery schemes present low efficiency, e.g., certain feedback timing recov-
ery schemes exhibit the so-called hang-up phenomenon, while another class of blind
feedforward timing recovery schemes presents large self-noise. Based on a general re-
search framework, we propose new anti-hangup algorithms and prefiltering techniques
to speed up the feedback timing recovery and reduce the self-noise of feedforward tim-
ing estimators, respectively.
Orthogonal frequency division multiplexing (OFDM) technique is well suited for
wideband wireless systems. However, OFDM receivers require high performance car-rier and timing synchronization. A new coarse synchronization scheme is proposed for
efficient carrier frequency offset and timing acquisition. Also, a novel highly accurate
decision-directed algorithm is proposed to track and compensate the residual phase
and timing errors after the coarse synchronization step. Both theoretical analysis
and computer simulations indicate that the proposed algorithms greatly improve the
performance of OFDM receivers.
The results of an in-depth study show that a narrowband interference (NBI) could
cause serious performance loss in multiband OFDMbased ultra-wideband (UWB) sys-
tems. A novel NBI mitigation scheme, based on a digital NBI detector and adaptive
analog notch filter bank, is proposed to reduce the effects of NBI in UWB systems.
Simulation results show that the proposed NBI mitigation scheme improves signifi-
cantly the performance of a standard UWB receiver (this improvement manifests as
a signal-to-noise ratio (SNR) gain of 9 dB)
SatCom Today in Canada: Significant Research: Broadband Satellite Communications List of CITR related Publications (1998-2003)
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Canadian Space Agency Recent Publication
On Efficient Signal Processing Algorithms for Signal Detection and PAPR Reduction in OFDM Systems
The driving force of the study is susceptibility of LS algorithm to noise. As LS algorithm is simple to implement, hence it’s performance improvement can contribute a lot to the wireless technology that are especially deals with high computation. Cascading of AdaBoost algorithm with LS greatly influences the OFDM system performance. Performance of Adaptive Boosting based symbol recovery was investigated on the performance of LS, MMSE, BLUE were also compared with the performance of AdaBoost algorithm and MMSE has been found the higher computational complexity. Furthermore, MMSE also requires apriori channel statistics and computational complexity O(5N3) of the MMSE increases exponentially as the number of carrier increases. For the Adaboost case the computational complexity calculation is little different.Therefore, in the training stage of the AdaBoost algorithm, the computational complexity is only O(nT M) Furthermore, as it is a classification algorithm so in the receiver side we will require a separate de-mapper (or decoder) to get the desired data bits, i.e., a. SAS aided DCT based PAPR reduction 1326 and b. SAS aided DCT based PAPR reduction. A successive addition subtraction preprocessed DCT based PAPR reduction technique was proposed. Here, the performance of proposed method was compared with other preexisting techniques like SLM and PTS and the performance of the proposed method was seen to outperform specially in low PAPR region. In the proposed PAPR reduction method, the receiver is aware of the transmitted signal processing, this enables a reverse operation at the receiver to extract the transmit data. Hence the requirement of sending extra information through extra subcarrier is eliminated. The proposed method is also seen to be spectrally efficient. In the case of PTS and SLM it is inevitable to send the side information to retrieve the transmit signal. Hence, these two methods are spectrally inefficient. Successive addition subtraction based PAPR reduction method was also applied to MIMO systems. The performance of the SAS based PAPR reduction method also showed better performance as compared to other technique. An extensive simulation of MIMO OFDM PAPR reduction was carried out by varying the number of subcarriers and number of transmitter antennas. A detailed computational complexity analysis was also carried out. BATE aided SDMA multi user detection. A detailed study of SDMA system was carried out with it’s mathematical analysis.Many linear and non linear detectors like ML, MMSE, PIC, SIC have been proposed in literature for multiuser detection of SDMA system. However, except MMSE every receivers other are computational extensive. So as to enhance the performance of the MMSE MUD a meta heuristic Bat algorithm was incorporated in cascade with MMSE
Multiuser detection in CDMA using blind techniques
Thesis (Master)--Izmir Institute of Technology, Electronics and Communication Engineering, Izmir, 2004Includes bibliographical references (leaves: 63-65)Text in English; Abstract: Turkish and Englishxiv, 69 leavesIn code division multiple access (CDMA) systems, blind multiuser detection (MUD) techniques are of great importance, especially for downlinks, since in practice, it may be unrealistic for a mobile user to know the spreading codes of other active users in the channel. Furthermore, blind methods remove the need for training sequences which leads to a gain in the channel bandwidth. Subspace concept in blind MUD is an alternative process to classical and batch blind MUD techniques based on principle component analysis, or independent component analysis (ICA) and ICA-like algorithms, such as joint approximate diagonalization of eigen-matrices (JADE), blind source separation algorithm with reference system, etc. Briefly, the desired signal is searched in the signal subspace instead of the whole space, in this type of detectors. A variation of the subspace-based MUD is reduced-rank MUD in which a smaller subspace of the signal subspace is tracked where the desired signal is contained in. This latter method leads to a performance gain compared to a standard subspace method. In this thesis, blind signal subspace and reduced-rank MUD techniques are investigated, and applied to minimum mean square error (MMSE) detectors with two different iterative subspace tracking algorithms. The performances of these detectors are compared in different scenarios for additive white Gaussian noise and for multipath fading channels as well. With simulation results the superiority of the reduced-rank detector to the signal subspace detector is shown. Additionally, as a new remark for both detectors, it is shown that, using minimum description length criterion in subspace tracking algorithm results in an increase in rank-tracking ability and correspondingly in the final performance. Finally, the performances of these two detectors are compared with MMSE, adaptive MMSE and JADE detectors
Nonlinear receivers for DS-CDMA
The growing demand for capacity in wireless communications is the driving force behind improving
established networks and the deployment of a new worldwide mobile standard. Capacity
calculations show that the direct sequence code division multiple access (DS-CDMA)
technique has more capacity than the time division multiple access technique. Therefore, most
3rd generation mobile systems will incorporate some sort of DS-CDMA.
In this thesis DS-CDMA receiver structures are investigated from the view point of pattern
recognition which leads to new DS-CDMA receiver structures. It is known that the optimum
DS-CDMA receiver has a nonlinear structure with prohibitive complexity for practical implementation.
It is also known that the currently implemented receiver in 2nd generation DSCDMA
mobile handsets has poor performance, because it suffers from multiuser interference.
Consequently, this work focuses on sub-optimum nonlinear receivers for DS-CDMA in the
downlink scenario.
First, the thesis reviews DS-CDMA, established equalisers, DS-CDMA receivers and pattern
recognition techniques. Then the new receivers are proposed. It is shown that DS-CDMA can
be considered as a pattern recognition problem and hence, pattern recognition techniques can be
exploited in order to develop DS-CDMA receivers. Another approach is to apply known equaliser
structures for DS-CDMA. One proposed receiver is based on the Volterra series expansion
and processes the received signal at the chip rate. Another receiver is a symbol rate radial
basis function network (RBFN) receiver with reduced complexity. Subsequently, a receiver is
proposed based on linear programming (LP) which is especially tailored for nonlinearly separable
scenarios. The LP based receiver performance is equivalent to the known decorrelating
detector in linearly separable scenarios. Finally, a hybrid receiver is proposed which combines
LP and RBFN and which exploits knowledge gained from pattern recognition. This structure
has lower complexity than the full RBF and good performance, and has a large potential for
further improvements.
Monte-Carlo simulations compare the proposed DS-CDMA receivers against established linear
and nonlinear receivers. It is shown that all proposed receivers outperform the known linear receivers.
The Volterra receiver’s complexity is relatively high for the performance gain achieved
and might not suit practical implementation. The other receiver’s complexity was greatly reduced
but it performs nearly as well as an optimum symbol by symbol detector.
This thesis shows that DS-CDMA is a pattern recognition problem and that pattern recognition
techniques can simplify DS-CDMA receiver structures. Knowledge is gained from the DSCDMA
signal patterns which help to understand the problem of a DS-CDMA receiver. It
should be noted that from the large number of known techniques, only a few pattern recognition
techniques are considered in this work, and any further work should look at other techniques.
Pattern recognition techniques can reduce the complexity of existing DS-CDMA receivers
while maintaining performance, leading to novel receiver structures
Wavelet-based multi-carrier code division multiple access systems
EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Robust frequency-domain turbo equalization for multiple-input multiple-output (MIMO) wireless communications
This dissertation investigates single carrier frequency-domain equalization (SC-FDE) with multiple-input multiple-output (MIMO) channels for radio frequency (RF) and underwater acoustic (UWA) wireless communications. It consists of five papers, selected from a total of 13 publications. Each paper focuses on a specific technical challenge of the SC-FDE MIMO system. The first paper proposes an improved frequency-domain channel estimation method based on interpolation to track fast time-varying fading channels using a small amount of training symbols in a large data block. The second paper addresses the carrier frequency offset (CFO) problem using a new group-wise phase estimation and compensation algorithm to combat phase distortion caused by CFOs, rather than to explicitly estimate the CFOs. The third paper incorporates layered frequency-domain equalization with the phase correction algorithm to combat the fast phase rotation in coherent communications. In the fourth paper, the frequency-domain equalization combined with the turbo principle and soft successive interference cancelation (SSIC) is proposed to further improve the bit error rate (BER) performance of UWA communications. In the fifth paper, a bandwidth-efficient SC-FDE scheme incorporating decision-directed channel estimation is proposed for UWA MIMO communication systems. The proposed algorithms are tested by extensive computer simulations and real ocean experiment data. The results demonstrate significant performance improvements in four aspects: improved channel tracking, reduced BER, reduced computational complexity, and enhanced data efficiency --Abstract, page iv
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