On the performance gain of STFC-LDPC concatenated coding scheme for MIMO-WiMAX

In mobile communications, using multiple transmit and receive antennas has shown considerable improvement over single antenna systems. The performance increase can be characterized by more reliable throughput obtained through diversity and the higher achievable data rate through spatial multiplexing. The combination of multiple-input multiple-output (MIMO) wireless technology with the IEEE 802.16e-2005 (WiMAX) standard has been recognized as one of the most promising technologies with the advent of next generation broadband wireless communications. The dissertation introduces a performance evaluation of modern multi-antenna coding techniques on a MIMO-WiMAX platform developed to be capable of simulating space-selective, time-selective and frequency-selective fading conditions, which are known as triply-selective fading conditions. A new concatenated space-time-frequency low-density parity check (LDPC) code is proposed for high performance MIMO systems, where it is shown that the newly defined coding technique outperforms a more conventional approach by concatenating space-time blocks with LDPC codes. The analysis of the coding techniques in realistic mobile environments, as well as the proposed STFC-LDPC code, can form a set of newly defined codes, complementing the current coding schemes defined in the WiMAX standard.Dissertation (MEng)--University of Pretoria, 2009.Electrical, Electronic and Computer Engineeringunrestricte

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

Capacity Enhancement in WLAN using MIMO

Increasing demand for high-performance 4G broadband wireless is enabled by the use of multiple antennas at both transmitter and receiver ends. Multiple antenna technologies enable high capacities suited for Internet and multimedia services, and also dramatically increase range and reliability. The combination of multiple-input multiple-output (MIMO) signal processing with orthogonal frequency division multiplexing (OFDM) is regarded as a promising solution for enhancing the data rates of next-generation wireless communication systems operating in frequencyselective fading environments. In this paper ,we focus mainly on Internet users in hotspots like Airport etc., requiring high data rate services. A high data rate WLAN system design is proposed using MIMO-OFDM. In the proposed WLAN system, IEEE 802.11a standard design is adopted but the results prove a data rate enhancement from the conventional IEEE 802.11

PIP-OFDM System Design and Application for Cognitive Radio Communications

This thesis defines a new Orthogonal Frequency Division Multiplexing (OFDM) system with Precoded In-band Pilots (PIP) tailored for cognitive radio (CR) communications. The motivation, principle, system design, implementation architecture, and CR application specific considerations of proposed PIP-OFDM system are investigated in this thesis. Principles and limitations of existing spectrum sensing techniques for cognitive radio communications are first analyzed and compared, with a focus on implementation challenges of pilot-based spectrum sensing for OFDM signals due to its robust performance in low signal-to-noise ratio (SNR) conditions. Several technical difficulties which haven’t been well addressed in previous pilot-based OFDM spectrum sensing studies, including impact of cyclic prefix, frequency offset between transmitter and spectrum sensing device, and noise uncertainty in the sensing threshold design, are taken into consideration in the analysis. Considering the poor performance of existing spectrum sensing techniques on user identification in cognitive radio network, where multiple secondary users may coexist, a precoded in-band pilots design is proposed in this thesis to enhance the user identification capabilities at low SNRs. The pilots in proposed PIP-OFDM system consist of uniform pilots and identification pilots. Each secondary user is associated with a unique identification pilot signal for identification purpose. Encoding of identification pilots is investigated, which will be used at the spectrum sensing device to identify the active user on the frequency band of interest. 111 Abstract To demodulate/decode identification pilots for user identification purpose, synchronization between transmitter and spectrum sensing device needs to be established. The synchronization in PIP-OFDM system, which is different from that in traditional OFDM systems, is subsequently investigated. Coarse time and frequency synchronization are achieved by correlation respectively in time and frequency domain. Through phase shift estimation in time domain, fine frequency synchronization is reached using a modified maximum likelihood estimation algorithm exploiting the redundancy in cyclic prefix. Based on this observation, a fine time synchronization algorithm is proposed in this thesis using redundant information on specifically designed uniform pilots. A multiple OFDM symbols processing strategy is used to improve the synchronization performance of PIP-OFDM system considering the poor performance of synchronization at low SNR. With the developed synchronization strategies, channel estimation in PIP-OFDM system is achieved using well developed estimation techniques in frequency domain. User identification is subsequently realized through demodulating the identification pilots. Theoretical performance and simulation results of user identification in PIP-OFDM system are provided to further confirm the effectiveness of the proposed design

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