5,764 research outputs found
Optical Time-Frequency Packing: Principles, Design, Implementation, and Experimental Demonstration
Time-frequency packing (TFP) transmission provides the highest achievable
spectral efficiency with a constrained symbol alphabet and detector complexity.
In this work, the application of the TFP technique to fiber-optic systems is
investigated and experimentally demonstrated. The main theoretical aspects,
design guidelines, and implementation issues are discussed, focusing on those
aspects which are peculiar to TFP systems. In particular, adaptive compensation
of propagation impairments, matched filtering, and maximum a posteriori
probability detection are obtained by a combination of a butterfly equalizer
and four 8-state parallel Bahl-Cocke-Jelinek-Raviv (BCJR) detectors. A novel
algorithm that ensures adaptive equalization, channel estimation, and a proper
distribution of tasks between the equalizer and BCJR detectors is proposed. A
set of irregular low-density parity-check codes with different rates is
designed to operate at low error rates and approach the spectral efficiency
limit achievable by TFP at different signal-to-noise ratios. An experimental
demonstration of the designed system is finally provided with five
dual-polarization QPSK-modulated optical carriers, densely packed in a 100 GHz
bandwidth, employing a recirculating loop to test the performance of the system
at different transmission distances.Comment: This paper has been accepted for publication in the IEEE/OSA Journal
of Lightwave Technolog
New Adaptive Data Transmission Scheme Over HF Radio
Acceptable Bit Error rate can be maintained by adapting some of the design parameters such as modulation, symbol rate, constellation size, and transmit power according to the channel state.<br />An estimate of HF propagation effects can be used to design an adaptive data transmission system over HF link. The proposed system combines the well known Automatic Link Establishment (ALE) together with variable rate transmission system. The standard ALE is modified to suite the required goal of selecting the best carrier frequency (channel) for a given transmission. This is based on measuring SINAD (Signal plus Noise plus Distortion to Noise plus Distortion), RSL (Received Signal Level), multipath phase distortion and BER (Bit Error Rate) for each channel in the frequency list. Channel condition evaluation is done by two arrangements. In the first an FFT analysis is used where a pilot signal is transmitted over the channel, while the data itself is used in the second arrangement. Passive channel assessment is used to avoid bad channels hence limiting the frequency pool size to be used in the point to point communication and the time required for scanning and linking. An exchange of channel information between the transmitting and receiving stations is considered to select the modulation scheme for transmission. Mainly MPSK and MFSK are considered with different levels giving different data rate according to the channel condition. The results of the computer simulation have shown that when transmitting at a fixed channel symbol rate of 1200 symbol/sec, the information rate ranges from 2400 bps using 4FSK up to 3600 bps using 8PSK for SNR ranges from 11dB up to 26dB.<br /
Selective Combining for Hybrid Cooperative Networks
In this study, we consider the selective combining in hybrid cooperative
networks (SCHCNs scheme) with one source node, one destination node and
relay nodes. In the SCHCN scheme, each relay first adaptively chooses between
amplify-and-forward protocol and decode-and-forward protocol on a per frame
basis by examining the error-detecting code result, and () relays will be selected to forward their received signals to the
destination. We first develop a signal-to-noise ratio (SNR) threshold-based
frame error rate (FER) approximation model. Then, the theoretical FER
expressions for the SCHCN scheme are derived by utilizing the proposed SNR
threshold-based FER approximation model. The analytical FER expressions are
validated through simulation results.Comment: 27 pages, 8 figures, IET Communications, 201
SGD Frequency-Domain Space-Frequency Semiblind Multiuser Receiver with an Adaptive Optimal Mixing Parameter
A novel stochastic gradient descent frequency-domain (FD) space-frequency (SF) semiblind multiuser receiver with an adaptive optimal mixing parameter is proposed to improve performance of FD semiblind multiuser receivers with a fixed mixing parameters and reduces computational complexity of suboptimal FD semiblind multiuser receivers in SFBC downlink MIMO MC-CDMA systems where various numbers of users exist. The receiver exploits an adaptive mixing parameter to mix information ratio between the training-based mode and the blind-based mode. Analytical results prove that the optimal mixing parameter value relies on power and number of active loaded users existing in the system. Computer simulation results show that when the mixing parameter is adapted closely to the optimal mixing parameter value, the performance of the receiver outperforms existing FD SF adaptive step-size (AS) LMS semiblind based with a fixed mixing parameter and conventional FD SF AS-LMS training-based multiuser receivers in the MSE, SER and signal to interference plus noise ratio in both static and dynamic environments
Digital data transmission over an HF channel
The thesis is concerned with detection, estimation techniques and a method of the
adaptive adjustment of the equaliser, for use in a 4800bit/sec synchronous digital
transmission system operating over a voice-band time-varying HF channel. Two main
impairments are additive Gaussian noise and inter-symbol interference (ISI), which can
be very severe at times. All techniques considered here are algorithms or processes that
operate on sequences of sample values. Modern digital modems normally operate in this
way, and the techniques described are of direct application to practical systems, and
could be implemented using the new technology of high speed real-time digital signal
processing (DSP). The performance of the various systems that employ the above
techniques are obtained using the computer simulated model of three types of HF
channels.
The ionospheric propagation medium, the characteristics of HF channel and the
signal distortion introduced by the channel are first described. The thesis then presents a suitable base-band model of the HF channel for computer simulation of quadrature
amplitude modulation systems. A suitable method for the adjustment of the receiver is
described next. This method is suitable both for the adjustment of a conventional
decision feedback equaliser (DFE), and also for the adjustment of a linear feedforward
filter that is employed ahead of a near-maximum likelihood (NML) detector. This
method uses a minimum phase (root-finding) algorithm (MPA) to convert the channel
response from being non-minimum phase to at least approximately minimum phase. The
results of computer simulation tests of this algorithm are then presented over different
types of HF channel models. The results demonstrate the algorithm's capability to make
the channel response minimum (or near-minimum) phase.
Various NML detectors, derived from the Viterbi detector, are discussed. Each
detector is here preceded by an adaptive linear filter that is adjusted adaptively using an
MPA. The performance of these detectors is compared with the conventional DFE,
whose tap-gains are adjusted adaptively using an MPA, and the detector which gives the
best compromise between performance and complexity is selected for combined
receivers. These results are obtained using perfect estimation of the channel
response.
The estimation techniques studied in this thesis include both new and conventional
estiniators, which are based on the least- mean-square (LMS) algorithm or recursive least-square(RLS) algorithm. The estimator provides an estimate of the sampled impulse response (SIR) of the channel, necessary for the NML detector or MPA. The
performances of these estimators are compared using computer simulation tests. The
results also demonstrate that the simpler LMS algorithm with adaptive step size gives a
comparable level of accuracy with the more complex RLS algorithm.
Finally the most promising of the detectors and estimators are connected with an
adaptive equaliser, using an MPA, to form a new combined receiver. The details of the
combined system structure with its computational complexity are given. Extensive
computer simulation tests have been carried out on the different arrangements of the
combined system including DFE, when all the functions of detection, estimation and
MPA are present, in order to find the most cost effective system in terms of performance
and complexity. A considerable reduction in the equipment complexity can be achieved
by allowing a long period between successive adjustment of the adaptive filter and
estimator
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