25 research outputs found
Construction of MIMO MAC Codes Achieving the Pigeon Hole Bound
This paper provides a general construction method for multiple-input
multiple-output multiple access channel codes (MIMO MAC codes) that have so
called generalized full rank property. The achieved constructions give a
positive answer to the question whether it is generally possible to reach the
so called pigeon hole bound, that is an upper bound for the decay of
determinants of MIMO-MAC channel codes.Comment: 5 pages, nofigures, conferenc
MIMO-OFDM communication systems: channel estimation and wireless location
In this new information age, high data rate and strong reliability features our wireless communication systems and is becoming the dominant factor for a successful deployment of commercial networks. MIMO-OFDM (multiple input multiple output-orthogonal frequency division multiplexing), a new wireless broadband technology, has gained great popularity for its capability of high rate transmission and its robustness against multi-path fading and other channel impairments. A major challenge to MIMO-OFDM systems is how to obtain the channel state information accurately and promptly for coherent detection of information symbols and channel synchronization. In the first part, this dissertation formulates the channel estimation problem for MIMO-OFDM systems and proposes a pilot-tone based estimation algorithm. A complex equivalent base-band MIMO-OFDM signal model is presented by matrix representation. By choosing equally-spaced and equally-powered pilot tones from sub-carriers in one OFDM symbol, a down-sampled version of the original signal model is obtained. Furthermore, this signal model is transformed into a linear form solvable for the LS (least-square) estimation algorithm. Based on the resultant model, a simple pilot-tone design is proposed in the form of a unitary matrix, whose rows stand for different pilot-tone sets in the frequency domain and whose columns represent distinct transmit antennas in the spatial domain. From the analysis and synthesis of the pilot-tone design in this dissertation, our estimation algorithm can reduce the computational complexity inherited in MIMO systems by the fact that the pilot-tone matrix is essentially a unitary matrix, and is proven an optimal channel estimator in the sense of achieving the minimum MSE (mean squared error) of channel estimation for a fixed power of pilot tones. In the second part, this dissertation addresses the wireless location problem in WiMax (worldwide interoperability for microwave access) networks, which is mainly based on the MIMO-OFDM technology. From the measurement data of TDOA (time difference of arrival), AOA (angle of arrival) or a combination of those two, a quasi-linear form is formulated for an LS-type solution. It is assumed that the observation data is corrupted by a zero-mean AWGN (additive white Gaussian noise) with a very small variance. Under this assumption, the noise term in the quasi-liner form is proved to hold a normal distribution approximately. Hence the ML (maximum-likelihood) estimation and the LS-type solution are equivalent. But the ML estimation technique is not feasible here due to its computational complexity and the possible nonexistence of the optimal solution. Our proposed method is capable of estimating the MS location very accurately with a much less amount of computations. A final result of the MS (mobile station) location estimation, however, cannot be obtained directly from the LS-type solution without bringing in another independent constraint. To solve this problem, the Lagrange multiplier is explored to find the optimal solution to the constrained LS-type optimization problem
MIMO Systems
In recent years, it was realized that the MIMO communication systems seems to be inevitable in accelerated evolution of high data rates applications due to their potential to dramatically increase the spectral efficiency and simultaneously sending individual information to the corresponding users in wireless systems. This book, intends to provide highlights of the current research topics in the field of MIMO system, to offer a snapshot of the recent advances and major issues faced today by the researchers in the MIMO related areas. The book is written by specialists working in universities and research centers all over the world to cover the fundamental principles and main advanced topics on high data rates wireless communications systems over MIMO channels. Moreover, the book has the advantage of providing a collection of applications that are completely independent and self-contained; thus, the interested reader can choose any chapter and skip to another without losing continuity
A Matter of Perspective: Reliable Communication and Coping with Interference with Only Local Views
This dissertation studies interference in wireless networks. Interference results from multiple simultaneous attempts to communicate, often between unassociated sources and receivers, preventing extensive coordination. Moreover, in practical wireless networks, learning network state is inherently expensive, and nodes often have incomplete and mismatched views of the network. The fundamental communication limits of a network with such views is unknown.
To address this, we present a local view model which captures asymmetries in node knowledge. Our local view model does not rely on accurate knowledge of an underlying probability distribution governing network state. Therefore, we can make robust statements about the fundamental limits of communication when the channel is quasi-static or the actual distribution of state is unknown: commonly faced scenarios in modern commercial networks. For each local view, channel state parameters are either perfectly known or completely unknown. While we propose no mechanism for network learning, a local view represents the result of some such mechanism.
We apply the local view model to study the two-user Gaussian interference channel: the smallest building block of any interference network. All seven possible local views are studied, and we find that for five of the seven, there exists no policy or protocol that universally outperforms time-division multiplexing (TDM), justifying the orthogonalized approach of many deployed systems. For two of the seven views, TDM-beating performance is possible with use of opportunistic schemes where opportunities are revealed by the local view.
We then study how message cooperation --- either at transmitters or receivers --- increases capacity in the local view two-user Gaussian interference channel. The cooperative setup is particularly appropriate for modeling next-generation cellular networks, where costs to share message data among base stations is low relative to costs to learn channel coefficients. For the cooperative setting, we find: (1) opportunistic approaches are still needed to outperform TDM, but (2) opportunities are more abundant and revealed by more local views.
For all cases studied, we characterize the capacity region to within some known gap, enabling computation of the generalized degrees of freedom region, a visualization of spatial channel resource usage efficiency
High speed energy efficient incoherent optical wireless communications
The growing demand for wireless communication capacity and the overutilisation of the conventional
radio frequency (RF) spectrum have inspired research into using alternative spectrum
regions for communication. Using optical wireless communications (OWC), for example, offers
significant advantages over RF communication in terms of higher bandwidth, lower implementation
costs and energy savings. In OWC systems, the information signal has to be
real and non-negative. Therefore, modifications to the conventional communication algorithms
are required. Multicarrier modulation schemes like orthogonal frequency division multiplexing
(OFDM) promise to deliver a more efficient use of the communication capacity through adaptive
bit and energy loading techniques. Three OFDM-based schemes – direct-current-biased OFDM
(DCO-OFDM), asymmetrically clipped optical OFDM(ACO-OFDM), and pulse-amplitude modulated
discrete multitone (PAM-DMT) – have been introduced in the literature.
The current work investigates the recently introduced scheme subcarrier-index modulation OFDM
as a potential energy-efficient modulation technique with reduced peak-to-average power ratio
(PAPR) suitable for applications in OWC. A theoretical model for the analysis of SIM-OFDMin a
linear additive white Gaussian noise (AWGN) channel is provided. A closed-form solution for the
PAPR in SIM-OFDM is also proposed. Following the work on SIM-OFDM, a novel inherently
unipolar modulation scheme, unipolar orthogonal frequency division multiplexing (U-OFDM), is
proposed as an alternative to the existing similar schemes: ACO-OFDMand PAM-DMT. Furthermore,
an enhanced U-OFDMsignal generation algorithm is introduced which allows the spectral
efficiency gap between the inherently unipolar modulation schemes – U-OFDM, ACO-OFDM,
PAM-DMT – and the conventionally used DCO-OFDM to be closed. This results in an OFDM-based
modulation approach which is electrically and optically more efficient than any other
OFDM-based technique proposed so far for intensity modulation and direct detection (IM/DD)
communication systems.
Non-linear distortion in the optical front-end elements is one of the major limitations for high-speed
communication in OWC. This work presents a generalised approach for analysing nonlinear
distortion in OFDM-based modulation schemes. The presented technique leads to a closed-form
analytical solution for an arbitrary memoryless distortion of the information signal and has
been proven to work for the majority of the known unipolar OFDM-based modulation techniques
- DCO-OFDM, ACO-OFDM, PAM-DMT and U-OFDM.
The high-speed communication capabilities of novel Gallium Nitride based μm-sized light emitting
diodes (μLEDs) are investigated, and a record-setting result of 3.5Gb/s using a single 50-μm
device is demonstrated. The capabilities of using such devices at practical transmission distances
are also investigated, and a 1 Gb/s link using a single device is demonstrated at a distance of up
to 10m. Furthermore, a proof-of-concept experiment is realised where a 50-μm LED is successfully
modulated using U-OFDM and enhanced U-OFDM to achieve notable energy savings in
comparison to DCO-OFDM
NOVEL OFDM SYSTEM BASED ON DUAL-TREE COMPLEX WAVELET TRANSFORM
The demand for higher and higher capacity in wireless networks, such as cellular,
mobile and local area network etc, is driving the development of new signaling
techniques with improved spectral and power efficiencies. At all stages of a
transceiver, from the bandwidth efficiency of the modulation schemes through highly
nonlinear power amplifier of the transmitters to the channel sharing between different
users, the problems relating to power usage and spectrum are aplenty. In the coming
future, orthogonal frequency division multiplexing (OFDM) technology promises to
be a ready solution to achieving the high data capacity and better spectral efficiency in
wireless communication systems by virtue of its well-known and desirable
characteristics.
Towards these ends, this dissertation investigates a novel OFDM system based on
dual-tree complex wavelet transform (D