391 research outputs found
Improving QPSK Transmission In Band-Limited Channels With Interchannel Interference Through Equalization
This paper describes the use of equalization in conjunction with channel filtering to improve QPSK transmission subject to both InterSymbol interference (ISI) and interchange interference (ICI). Performance bounds are computed using the nonclassical Gauss-quadrature rule (GQR) method. The signal-to-noise ratio (SNR) gain due to linear equalization over Non equalization is thereby obtained and presented. The performance of a linear equalizer thus obtained is compared with the Viterbi algorithm sequence estimator (VASE). In the absence of bounds for the VASE receiver under the channel conditions considered, simulation results are used to make the comparison. With a possible difference in the accuracies of the performance thus obtained it is shown that the VASE provides improved performance over the linear equalizer under the channel conditions considered. Copyright © 1977 by The Institute of Electrical and Electronics Engineers, Inc
Digital data transmission over mobile radio channels
The aim of this work is to study data transmission over a microwave
digital mobile radio channel at 900 MHz, where the channel is subjected
to multipath fading. Besides the fading, the other impairments assumed
here are additive noise, co-channel interference and adjacent channel
interference. Two modulation techniques are investigated in this work,
namely Quadrature-Amplitude-Modulation (QAM) and Quadrature-Phase-Shift-
Keying (QPSK). The channel is characterised digitally, assuming multipath
Rayleigh fading in the presence of noise. The detection process
studied here are near-maximum likelihood schemes: non-linear equalisation
methods are also considered in detail.
The thesis is also concerned with carrier synchronisation and channel
estimation under conditions of Rayleigh fading. Since the carrier
syncn,honisation is a most important requirement in mobile radio, a
Digital Phase Locked Loop (DPLL) technique has been designed and investigated
in the form of a feedback digital synchronisation system. Two
types of channel estimation technique, namely feedforward and feedback
estimators, are also investigated in this work. The feedback estimator
is modified by the addition of a digital control system, in order to
reduce its delay, and to cope with rapidly fading signals. Successful
carrier synchronisation is demonstrated by the use of space diversity.
The study was completed using models of the component parts of the system,
and by the use of extensive computer simulations to analyse the system
under various operating conditions
Low-complexity soft-decision feedback turbo equalization for multilevel modulations
This dissertation proposes two new decision feedback equalization schemes suitable for multilevel modulation systems employing turbo equalization. One is soft-decision feedback equalization (SDFE) that takes into account the reliability of both soft a priori information and soft decisions of the data symbols. The proposed SDFE exhibits lower signal to noise ratio (SNR) threshold that is required for water fall bit error rate (BER) and much faster convergence than the near-optimal exact minimum mean square error linear equalizer (Exact-MMSE-LE) for high-order constellation modulations. The proposed SDFE also offers a low computational complexity compared to the Exact-MMSE-LE. The drawback of the SDFE is that its coefficients cannot reach the matched filter bound (MFB) and therefore after a large number of iterations (e.g. 10), its performance becomes inferior to that of the Exact-MMSE-LE. Therefore, soft feedback intersymbol interference (ISI) canceller-based (SIC) structure is investigated. The SIC structure not only exhibits the same low complexity, low SNR threshold and fast convergence as the SDFE but also reaches the MFB after a large number of iterations. Both theoretical analysis and numerical simulations demonstrate why the SIC achieves MFB while the SDFE cannot. These two turbo equalization structures are also extended from single-input single-output (SISO) systems to multiple-input multiple-output (MIMO) systems and applied in high data-rate underwater acoustic (UWA) communications --Abstract, page iv
Unbiased MMSE vs. Biased MMSE Equalizers
[[abstract]]We systematically analyze the biased and unbiased minimum mean square error (MMSE) equalizers of finite as well as infinite length, with and without decision feedback sections. New closed-form expressions of optimum equalizer weights, the MMSE, and symbol error probabilities (SEP), solely in terms of channel response parameters and noise power, are derived for the above receivers. These new expressions have not appeared in the literature and should be included for completeness. We also prove analytically that the biased and unbiased MMSE equalizers have the same optimum weights and that an infinitely long unbiased MMSE equalizer approaches the optimum minimum error probability equalizer. Performance curves are presented and compared for all the receivers discussed. Moreover, for all the infinite length equalizers presented, alternative error probability expressions are provided to best suit computer simulations.[[notice]]補æ£å®Œç•¢[[incitationindex]]EI[[booktype]]ç´™
Iterative receivers and multichannel equalisation for time division multiple access systems
The thesis introduces receiver algorithms improving the performance of TDMA mobile radio systems. Particularly, we consider receivers utilising side information, which can be obtained from the error control coding or by having a priori knowledge of interference sources. Iterative methods can be applied in the former case and interference suppression techniques in the latter.
Convolutional coding adds redundant information into the signal and thereby protects messages transmitted over a radio channel. In the coded systems the receiver is usually comprised of separate channel estimation, detection and channel decoding tasks due to complexity restrictions. This suboptimal solution suffers from performance degradation compared to the optimal solution achieved by optimising the joint probability of information bits, transmitted symbols and channel impulse response. Conventional receiver utilises estimated channel state information in the detection and detected symbols in the channel decoding to finally obtain information bits. However, the channel decoder provides also extrinsic information on the bit probabilities, which is independent of the received information at the equaliser input. Therefore it is beneficial to re-perform channel estimation and detection using this new extrinsic information together with the original input signal.
We apply iterative receiver techniques mainly to Enhanced General Packet Radio System (EGPRS) using GMSK modulation for iterative channel estimation and 8-PSK modulation for iterative detection scheme. Typical gain for iterative detection is around 2Â dB and for iterative channel estimation around 1Â dB. Furthermore, we suggest two iteration rounds as a reasonable complexity/performance trade-off. To obtain further complexity reduction we introduce the soft trellis decoding technique that reduces the decoder complexity significantly in the iterative schemes.
Cochannel interference (CCI) originates from the nearby cells that are reusing the same transmission frequency. In this thesis we consider CCI suppression by joint detection (JD) technique, which detects simultaneously desired and interfering signals. Because of the complexity limitations we only consider JD for two binary modulated signals. Therefore it is important to find the dominant interfering signal (DI) to achieve the best performance. In the presence of one strong DI, the JD provides major improvement in the receiver performance.
The JD requires joint channel estimation (JCE) for the two signals. However, the JCE makes the implementation of the JD more difficult, since it requires synchronised network and unique training sequences with low cross-correlation for the two signals.reviewe
Decision-feedback equalization for digital communication over dispersive channels.
ESD-TR-67-466.Bibliography: p.85.Contract AF 19(628)-5167
The application of iterative techniques to adaptive detection processes
In a synchronous, serial, data-transmission system using orthogonal
groups of signal elements, the received signal elements of a group
are detected simultaneously in an iterative process, a separate detection
process being used for each received group of elements. At transmission
rates of up to 9600 bits-per-second over voice frequency channels, a
particularly cost-effective detection process operates by solving the
appropriate set of simultaneous equations. Many different iterative
techniques may be used to solve these equations, and the first
objective of the research project is to determine which of the available techniques achieves the best compromise between the
rate of convergence to the required solution and the equipment
complexity involved. [Continues.
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