Low-Complexity Algorithms for Channel Estimation in Optimised Pilot-Assisted Wireless OFDM Systems

Orthogonal frequency division multiplexing (OFDM) has recently become a dominant transmission technology considered for the next generation fixed and mobile broadband wireless communication systems. OFDM has an advantage of lessening the severe effects of the frequency-selective (multipath) fading due to the band splitting into relatively flat fading subchannels, and allows for low-complexity transceiver implementation based on the fast Fourier transform algorithms. Combining OFDM modulation with multilevel frequency-domain symbol mapping (e.g., QAM) and spatial multiplexing (SM) over the multiple-input multiple-output (MIMO) channels, can theoretically achieve near Shannon capacity of the communication link. However, the high-rate and spectrumefficient system implementation requires coherent detection at the receiving end that is possible only when accurate channel state information (CSI) is available. Since in practice, the response of the wireless channel is unknown and is subject to random variation with time, the receiver typically employs a channel estimator for CSI acquisition. The channel response information retrieved by the estimator is then used by the data detector and can also be fed back to the transmitter by means of in-band or out-of-band signalling, so the latter could adapt power loading, modulation and coding parameters according to the channel conditions. Thus, design of an accurate and robust channel estimator is a crucial requirement for reliable communication through the channel, which is selective in time and frequency. In a MIMO configuration, a separate channel estimator has to be associated with each transmit/receive antenna pair, making the estimation algorithm complexity a primary concern. Pilot-assisted methods, relying on the insertion of reference symbols in certain frequencies and time slots, have been found attractive for identification of the doubly-selective radio channels from both the complexity and performance standpoint. In this dissertation, a family of the reduced-complexity estimators for the single and multiple-antenna OFDM systems is developed. The estimators are based on the transform-domain processing and have the same order of computational complexity, irrespective of the number of pilot subcarriers and their positioning. The common estimator structure represents a cascade of successive small-dimension filtering modules. The number of modules, as well as their order inside the cascade, is determined by the class of the estimator (one or two-dimensional) and availability of the channel statistics (correlation and signal-to-noise power ratio). For fine precision estimation in the multipath channels with statistics not known a priori, we propose recursive design of the filtering modules. Simulation results show that in the steady state, performance of the recursive estimators approaches that of their theoretical counterparts, which are optimal in the minimum mean square error (MMSE) sense. In contrast to the majority of the channel estimators developed so far, our modular-type architectures are suitable for the reconfigurable OFDM transceivers where the actual channel conditions influence the decision of what class of filtering algorithm to use, and how to allot pilot subcarrier positions in the band. In the pilot-assisted transmissions, channel estimation and detection are performed separately from each other over the distinct subcarrier sets. The estimator output is used only to construct the detector transform, but not as the detector input. Since performance of both channel estimation and detection depends on the signal-to-noise power vi ratio (SNR) at the corresponding subcarriers, there is a dilemma of the optimal power allocation between the data and the pilot symbols as these are conflicting requirements under the total transmit power constraint. The problem is exacerbated by the variety of channel estimators. Each kind of estimation algorithm is characterised by its own SNR gain, which in general can vary depending on the channel correlation. In this dissertation, we optimise pilot-data power allocation for the case of developed low-complexity one and two-dimensional MMSE channel estimators. The resultant contribution is manifested by the closed-form analytical expressions of the upper bound (suboptimal approximate value) on the optimal pilot-to-data power ratio (PDR) as a function of a number of design parameters (number of subcarriers, number of pilots, number of transmit antennas, effective order of the channel model, maximum Doppler shift, SNR, etc.). The resultant PDR equations can be applied to the MIMO-OFDM systems with arbitrary arrangement of the pilot subcarriers, operating in an arbitrary multipath fading channel. These properties and relatively simple functional representation of the derived analytical PDR expressions are designated to alleviate the challenging task of on-the-fly optimisation of the adaptive SM-MIMO-OFDM system, which is capable of adjusting transmit signal configuration (e.g., block length, number of pilot subcarriers or antennas) according to the established channel conditions

Analytical approach to design of proportional-to-the-absolute-temperature current sources and temperature sensors based on heterojunction bipolar transistors

Embedded temperature sensors based on proportional-to-the-absolute-temperature (PTAT) current sources have the potential to lay the foundation for low-cost temperature-aware integrated circuit architectures if they meet the requirements of miniaturization, fabrication process match, and precise estimation in a wide range of temperatures. This paper addresses an analytical approach to the minimum-element PTAT circuit design capitalizing on the physics-based modeling of the heterojunction bipolar transistor (HBT) structures. It is shown that a PTAT circuit can be implemented on only two core HBT elements with good accuracy. Derived parametric relations allow a straightforward specification of the thermal gain at the design stage, which affects sensor sensitivity. Further derived current-to-temperature mapping expresses a temperature estimate based on the measured PTAT output current. Numerical examples indicate attainable estimation accuracy of 0.43% in case of a measurement instance taken in the absence of measurement noise.The National Research Foundation of South Africa under Grant UID:74041http://ieeexplore.ieee.org/hb2013ai201

Modelling and analysis of RMS-DC solid state thermal converter

Abstract: The Root Mean Square (RMS) measurements of alternating signal are frequently done by means of thermal transfer instruments. The principle of these thermal transfer instruments is to transfer the heating energy of resistive load to a temperature-sensing element. The DC output of the sensor gauges the amount of electrical power (AC or DC) applied to the input. The paper examines performance of an RMS-DC thermal converter of a solid state technology type at various frequencies such as LT1088CD integrated circuit. The theoretical and simulation analysis is carried out to outline the factors influencing the transfer characteristics of the converter device in the DC and AC input regimes

Protection against transient overvoltage in precision AC-DC measurement system

Abstract: Reduction in size and improvement of sensitive margins in modern semiconductor devices makes them increasingly susceptible to stress and subsequent damage caused by the overvoltage. A fast responding and higher energy absorbing overvoltage protection is desired. A hybrid solution is proposed. Moderate energy absorbing device is connected in parallel with a fast responding device to complement each other. Nonlinear current-voltage characteristics of these devices enables them to least interfere with the performance of the precision AC measurement system while reducing the effects of transient overvoltage on the system. In this paper, the causes of transient overvoltage in AC-DC measurement system are analysed and a hybrid semiconductor devices voltage limiting solution is proposed

ESD Stress Analysis and Suppression in a Single-Junction Thermal Converter

This article presents an outline of Elec- tric Transient Disturbances (ETDs), represented by the ElectroStatic Discharge (ESD) in accordance with the Human-Body Model (HBM), on the AC-DC trans- fer measurement standard, represented by the Single- Junction Thermal Converter (SJTC) Thermal Ele- ment (TE). Mitigation technique against the power dissipation build-up, higher than the operational mar- gins recommended by a manufacturer, on the TE were proposed and modelled using Laplace Transform (LT) analysis. A mathematical model and an optimiza- tion algorithm were developed to determine the equiva- lent circuit model parameters of a Transient Overload Protection Module (TOPM) that would offer adequate protection against destructive power dissipation levels build-up on the TE. The mathematical model was de- veloped using an 8 kV ESD, which was expected to de- liver short-circuit current with a peak value of approx- imately 5.33 A through a load impedance of approxi- mately 1 mΩ. The ESD stress signal was injected into the TOPM connected in parallel with the TE. The ac- tive power dissipated by the SJTC TE per period of transient response was calculated from the current and voltage obtained from the mathematical analysis, and the results indicate a power dissipation of 10 mW by the TE. From the algorithm, the model parameter that noticeably influences the power dissipation capabilities of the TOPM is the inductance and it must be smaller than 1.2 nH. A CAD based simulation model was devel- oped and analysed. The simulation results agreed with the mathematical model