73 research outputs found
Detection for a Statistically-Known, Time-Varying Dispersive Channel
Detection for the statistically known channel (SKC) is aimed at obtaining good performance in situations where our statistical knowledge of a time-varying channel is good, and where other equalization/detection schemes are either too complex to implement, or their performance is limited due to the rapidity of channel fading, or where we are simply unable to perform channel estimation. By using a statistical characterization of the channel, we develop a new detector that performs maximum-likelihood sequence estimation (MLSE) (given the channel model) on blocks of N symbols. Both symbol-spaced and fractionally spaced samples are used, to obtain two different detectors, that are generalizations of those devised for optimal block schemes on nondispersive channels. The detector that uses fractionally spaced samples is shown to outperform the detector that uses symbol-spaced samples. The performance of both appears to approach that of the corresponding known channel (KC) detector as the block length increases. We also numerically evaluate the SKC detector performance under conditions where the channel parameters (statistics) are incorrectly estimated, and show that the fractionally spaced detector is fairly robust to modeling errors. Finally, we devise a sliding block algorithm, for use when transmitting more than N symbols
Visible Light Communication (VLC)
Visible light communication (VLC) using light-emitting diodes (LEDs) or laser diodes (LDs) has been envisioned as one of the key enabling technologies for 6G and Internet of Things (IoT) systems, owing to its appealing advantages, including abundant and unregulated spectrum resources, no electromagnetic interference (EMI) radiation and high security. However, despite its many advantages, VLC faces several technical challenges, such as the limited bandwidth and severe nonlinearity of opto-electronic devices, link blockage and user mobility. Therefore, significant efforts are needed from the global VLC community to develop VLC technology further. This Special Issue, “Visible Light Communication (VLC)”, provides an opportunity for global researchers to share their new ideas and cutting-edge techniques to address the above-mentioned challenges. The 16 papers published in this Special Issue represent the fascinating progress of VLC in various contexts, including general indoor and underwater scenarios, and the emerging application of machine learning/artificial intelligence (ML/AI) techniques in VLC
Simulator for the Performance Analysis of CPM Schemes in an Indoor Wireless Environment
A software simulator for characterising Continuous Phase Modulation (CPM) schemes in an indoor multipath environment has been developed using SIMULINK and MATLAB. The simulator is capable of simulating a wide range of CPM schemes to determine bandwidth efficiency and robustness to additive white Gaussian noise (AWGN) and Rician fading. Initial trials of the simulator indicate that the simulator is functioning correctly. Eventually, the simulator will be used to determine the most suitable modulation scheme for the development of an actual indoor wireless system
Estimation and detection techniques for doubly-selective channels in wireless communications
A fundamental problem in communications is the estimation of the channel.
The signal transmitted through a communications channel undergoes distortions
so that it is often received in an unrecognizable form at the receiver.
The receiver must expend significant signal processing effort in order to be
able to decode the transmit signal from this received signal. This signal processing
requires knowledge of how the channel distorts the transmit signal,
i.e. channel knowledge. To maintain a reliable link, the channel must be
estimated and tracked by the receiver.
The estimation of the channel at the receiver often proceeds by transmission
of a signal called the 'pilot' which is known a priori to the receiver.
The receiver forms its estimate of the transmitted signal based on how this
known signal is distorted by the channel, i.e. it estimates the channel from
the received signal and the pilot. This design of the pilot is a function of the
modulation, the type of training and the channel. [Continues.
Detection of Linear Modulations in the Presence of Strong Phase and Frequency Instabilities
Noncoherent sequence detection algorithms, recently proposed by the authors, have a performance which approaches that of coherent detectors and are robust to phase and frequency instabilities. These schemes exhibit a negligible performance loss in the presence of a frequency offset, provided this offset does not exceed an order of 1 % of the signaling frequency. For higher values, the performance rapidly degrades. In this paper, detection schemes are proposed, characterized by high robustness to frequency offsets and capable of tolerating offset values up to 10 % of the signaling frequency. More generally, these detection schemes are very robust to rapidly varying phase and frequency instabilities. The general case of coded linear modulations is addressed, with explicit reference to-ary phase shift keying and quadrature amplitude modulation
Analysis of and techniques for adaptive equalization for underwater acoustic communication
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2011Underwater wireless communication is quickly becoming a necessity for applications
in ocean science, defense, and homeland security. Acoustics remains the only practical
means of accomplishing long-range communication in the ocean. The acoustic
communication channel is fraught with difficulties including limited available bandwidth,
long delay-spread, time-variability, and Doppler spreading. These difficulties
reduce the reliability of the communication system and make high data-rate communication
challenging. Adaptive decision feedback equalization is a common method to
compensate for distortions introduced by the underwater acoustic channel. Limited
work has been done thus far to introduce the physics of the underwater channel into
improving and better understanding the operation of a decision feedback equalizer.
This thesis examines how to use physical models to improve the reliability and reduce
the computational complexity of the decision feedback equalizer. The specific topics
covered by this work are: how to handle channel estimation errors for the time varying
channel, how to use angular constraints imposed by the environment into an array
receiver, what happens when there is a mismatch between the true channel order and
the estimated channel order, and why there is a performance difference between the
direct adaptation and channel estimation based methods for computing the equalizer
coefficients. For each of these topics, algorithms are provided that help create a more
robust equalizer with lower computational complexity for the underwater channel.This work would not have been possible without support from the O ce of Naval
Research, through a Special Research Award in Acoustics Graduate Fellowship (ONR
Grant #N00014-09-1-0540), with additional support from ONR Grant #N00014-05-
10085 and ONR Grant #N00014-07-10184
Channel Estimation and Equalization for Cooperative Communication
The revolutionary concept of space-time coding introduced in the last decade has demonstrated that the deployment of multiple antennas at the transmitter allows for simultaneous increase in throughput and reliability because of the additional degrees of freedom offered by the spatial dimension of the wireless channel. However, the use of antenna arrays is not practical for deployment in some practical scenarios, e. g. , sensor networks, due to space and power limitations. A new form of realizing transmit diversity has been recently introduced under the name of user cooperation or cooperative diversity. The basic idea behind cooperative diversity rests on the observation that in a wireless environment, the signal transmitted by the source node is overheard by other nodes, which can be defined as "partners" or "relays". The source and its partners can jointly process and transmit their information, creating a "virtual antenna array" and therefore emulating transmit diversity. Most of the ongoing research efforts in cooperative diversity assume frequency flat channels with perfect channel knowledge. However, in practical scenarios, e. g. broadband wireless networks, these assumptions do not apply. Frequency-selective fading and imperfect channel knowledge should be considered as a more realistic channel model. The development of equalization and channel estimation algorithms play a crucial element in the design of digital receivers as their accuracy determine the overall performance. This dissertation creates a framework for designing and analyzing various time and frequency domain equalization schemes, i. e. distributed time reversal (D-TR) STBC, distributed single carrier frequency domain (D-SC-FDE) STBC, and distributed orthogonal frequency division multiplexing (D-OFDM) STBC schemes, for broadband cooperative communication systems. Exploiting the orthogonally embedded in D-STBCs, we were able to maintain low-decoding complexity for all underlying schemes, thus, making them excellent candidates for practical scenarios, such as multi-media broadband communication systems. Furthermore, we propose and analyze various non-coherent and channel estimation algorithms to improve the quality and reliability of wireless communication networks. Specifically, we derive a non-coherent decoding rule which can be implemented in practice by a Viterbi-type algorithm. We demonstrate through the derivation of a pairwise error probability expression that the proposed non-coherent detector guarantees full diversity. Although this decoding rule has been derived assuming quasi-static channels, its inherent channel tracking capability allows its deployment over time-varying channels with a promising performance as a sub-optimal solution. As a possible alternative to non-coherent detection, we also investigate the performance of mismatched-coherent receiver, i. e. , coherent detection with imperfect channel estimation. Our performance analysis demonstrates that the mismatched-coherent receiver is able to collect the full diversity as its non-coherent competitor over quasi-static channels. Finally, we investigate and analyze the effect of multiple antennas deployment at the cooperating terminals assuming different relaying techniques. We derive pairwise error probability expressions quantifying analytically the impact of multiple antenna deployment at the source, relay and/or destination terminals on the diversity order for each of the relaying methods under consideration
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