In this paper, we propose a general beamforming criterion for pre-FFT processing in orthogonal frequency division multiplexing (OFDM) systems with multiple transmit and receive antennas. The proposed criterion depends on a single parameter α, which establishes a tradeoff between the energy of the equivalent SISO channel (after Tx-Rx beamforming) and its spectral flatness. The proposed cost function embraces most reasonable criteria for designing Tx-Rx pre-FFT beamformers. Hence, for particular values of α the proposed criterion reduces to the minimization of the mean square error (MSE), the maximization of the system capacity, or the maximization of the received signal-to-noise ratio (SNR). In general, the proposed criterion results in a non convex optimization problem. However, we show that the problem can be approximately solved by semidefinite relaxation (SDR) techniques. Additionally, since the computational cost of SDR for this problem is rather high, we propose a simple yet efficient gradient search algorithm which provides satisfactory solutions with a moderate computational cost for OFDM-based WLAN standards such as 802.11a. Finally, the good performance of the proposed technique is illustrated by means of some numerical results.

In this paper, we study beamforming schemes for a novel MIMO transceiver, which performs adaptive signal combining in the radio-frequency (RF) domain. Assuming perfect channel knowledge at the receiver side, we consider the problem of designing the transmit and receive RF beamformers under orthogonal frequency division multiplexing (OFDM) transmissions. In particular, a general beamforming criterion is proposed, which depends on a single parameter ¿. This parameter establishes a tradeoff between the energy of the equivalent SISO channel (after Tx-Rx beamforming) and its spectral flatness. The proposed cost function embraces most reasonable criteria for designing analog Tx-Rx beamformers. Hence, for particular values of ¿ the proposed criterion reduces to the minimization of the mean square error (MSE), the maximization of the system capacity, or the maximization of the received signal-to-noise ratio (SNR). In general, the proposed criterion results in a nonconvex optimization problem. However, we show that the problem can be rewritten as a convex cost function subject to a couple of rank-one constraints, and hence it can be approximately solved by semidefinite relaxation (SDR) techniques. Since the computational cost of SDR for this problem is rather high, and building on the observation that the minima of the original problem must be solutions of a pair of coupled eigenvalue problems, we propose yet another simple and efficient gradient search algorithm which, in practice, provides satisfactory solutions with a moderate computational cost. Finally, several numerical examples show the good performance of the proposed technique for both uncoded and 802.11a-coded transmissions.

We suggest a method for determining the efficiency of the power amplifier of an OFDM broadcast system. We present how far the efficiency of conventional power amplifiers can be increased by applying envelope following. The dependency of the efficiency on the probability of clipping is derived. In this context all three possibilities of operating point adjustment are investigated. Finally, we show that by adjusting only the operating point voltage of the power amplifier the efficiency of OFDM broadcast systems can be doubled.

In this paper, we review channel access concepts for two competitive radiolocation systems, High-Precision Location System (HPLS) and Local Positioning Radar (LPR), which share similar physical layers, but operate with different bandwidths. After a short review of the advantages of ultra-wideband signaling as employed by LPR, we spotlight tradeoffs in the servicing of multiple terminals. While HPLS supports both dynamic and static channel access, LPR is limited to static frequency multiplexing. The characteristics of these approaches are highlighted with theoretical analysis and simulation results.

The transmission phase variations versus gain in common emitter and common base amplifiers are analyzed revealing that these stages can be tuned to yield opposite phase characteristics versus gain. By cascading these two stages, e.g., on the basis of a cascode, and optimizing added feedback elements, it is possible to compensate these phase variations. A universal analysis based on bipolar transistors is derived. However, the insights can be mapped to other transistors such as field-effect transistors. The analysis is verified by implementation of a low-noise cascode amplifier in 0.25-mum silicon germanium heterojunction bipolar transistors. At 50-Omega terminations, 1.6-V supply voltage, 1-mA current consumption, and a gain of 7 dB plusmn 0.25 dB, a noise figure of less than 3.2 dB, and a third-order output intercept point of -3 dBm are measured within a frequency range from 5.2 to 5.9 GHz. For a gain control range of 12 and 20 dB, the transmission phase variations are reduced to 3deg and 6deg, respectively, which is around a factor of 7 better than for a conventional noncompensated cascode topology. The fully integrated circuit is well suited for wireless local area network systems applying adaptive antenna combining and operating in accordance to the 802.11 a/n standards.

In this work, performance aspects of a high-precision radiolocation system are evaluated. An integrated simulation environment is introduced, which allows the co-evaluation of both physical layer parameters, such as measurement error, as well as network-related performance figures like the time-to-fix and throughput. Simulation results for different strategies and algorithms are presented and analyzed for their practicability.

In this paper, we study transmission schemes for a novel OFDM-based MIMO system which performs adaptive signal combining in radio-frequency (RF). Specifically, we consider the problem of selecting the linear precoder and the transmit and receive RF weights (or beamformers) for minimizing the bit error rate (BER) under the assumption of perfect channel knowledge and linear receivers. Firstly, it is shown that the optimal precoder amounts to uniformly distribute the overall mean square error (MSE) among the information symbols. Secondly, we propose a gradient search algorithm to obtain the optimal pair of beamformers. Interestingly, in the case of low signal to noise ratios (SNR), the proposed beamforming criterion is equivalent to the maximization of the received SNR. However, for moderate and high SNRs, part of the received SNR is sacrificed in order to improve the channel response of the worst subcarriers, which translates into significant advantages over other previously proposed approaches. Finally, the performance of the proposed scheme is illustrated by means of some numerical examples.

In this paper a broadband IFA antenna is presented for WLAN applications in the 5.6 GHz band. An improvement in the design of this IFA antenna is analyzed to achieve a radiating element more stable when changes are done in the input microstrip line or/and in the dimensions of the ground plane. This fact helps to integrate the antenna with the other parts of the transceiver using microstrip line. A comparison between arrays (MIFA array and IFA array) is done too to present the improvements of the new IFA antenna.

In this paper, a CMOS vector modulator designed for smart antenna arrays at 5.5 GHz is presented. A quadrature all pass filter and sign select switches yield two orthogonal signal paths. Two variable gain amplifiers with strongly reduced phase shift of only ±6 ° are used to weight these paths. It has a phase control range of 360 ° and a gain range of −20 dB to 2.8 dB. The input-referred IP3 is −7 dBm at maximum gain. The current drawn from a 1.5 V supply amounts 12 mA. Using a 180 nm technology, the chip core area amounts 1.2×0.8 mm².

Next generation wireless radios demand for better spectral efficiencies and for high energy efficient (green) handhelds, and multiple-input multiple-output (MIMO) air interfaces represent a popular approach to address these challenges. In this work, spatial signal processing techniques in MIMO radios at different domains are evaluated with respect to their achievable performance, their power consumption and system sizes. Here, adaptive weighting in the analogue domain reveals several benefits compared to digital approaches. The obtained results are based on a four antennas receiver implementation in an 0.25µm SiGe BiCMOS process.

Local positioning systems significantly enhance the value of wireless sensor networks (WSN). This paper gives an overview of local positioning system topologies for WSNs of the European project RESOLUTION, which operate in the unlicensed ISM band at 5.8 GHz with 150 MHz bandwidth and allowed 25 mW effective isotropic radiated transmit power. The achievable 3D-localization accuracy is analyzed with respect to the performance of the designed hardware components and the indoor environment. It is further shown that accuracy as a primary attribute of local positioning systems also affects secondary quality-of-service (QoS) features such as update-rates or latency. This investigation allows an optimization of both attributes simultaneously, primary and secondary QoS aspects.

This letter presents a hardware-efficient phase estimation algorithm that can replace the common complex estimators for a coherent quadrature phase-shift keying transmission system.

This paper presents an integrated synthesizer for frequency ramp generation in local positioning systems using frequency modulated continuous wave (FMCW) radar in the ISM band at 5.8 GHz. The synthesizer is designed and implemented as a fractional-N phase locked loop (PLL) in a 0.18 mum SiGe-BiCMOS technology controlled by a MASH 1-1-1 SigmaDelta-modulator, which is integrated on an FPGA. Besides preventing spurs in the output spectrum, this digital control allows a flexible programming of the synthesizer with respect to its frequency ramp generation and output spectrum respectively. The complete fractional-N synthesizer was integrated into a single system PCB and measurements regarding phase noise and tuning range were performed. Moreover, an FMCW radar system using this synthesizer achieved an accuracies of 13 cm in indoor environments.

Using radial basis function networks for function approxima- tion tasks suffers from unavailable knowledge about an adequate network size. In this work, a measuring technique is proposed which can control the model complexity and is based on the correlation coefficient between two basis functions. Simulation results show good performance and, therefore, this technique can be integrated in the RBF training procedure.

Neural networks are intended to be used in future nanoelectronics since these architectures seem to be robust against malfunctioning elements and noise. In this paper we analyze the robustness of radial basis function networks and determine upper bounds on the mean square error under noise contaminated weights and inputs.

RESOLUTION aims at developing a wireless three-dimensional (3-D) local positioning system with measurement accuracy in the centimetre regime and real-time ability. A novel frequency modulated continuous wave (FMCW) radar principle with pulsed active reflector is employed. This High-Precision-Localisation-System (HPLS) will be implemented together with common WLAN systems that are used for data communication purposes. Due to its high data rate capabilities and large potential bandwidth, the 802.11a/n standard allocated bandwidth around 5.5 GHz is applied. Special emphasis is given to the system's reconfigurability by efficiently using inherent synergies between the WLAN system and the HPLS approach. To allow multifunctional tasks, highly integrated system on chip (SoC) frontends will be designed on advanced CMOS or BiCMOS technology. Smart power and adaptive performance control will be applied to minimise the power consumption according to application needs. In order to enhance performance and coverage range, the transceiver features adaptive antenna combining (AAC) in the radio frequency (RF) receiver. AAC significantly decreases the power consumption, size and costs, since the number of multiple components is reduced to a minimum. Because of the high 3-D resolution and real-time ability, which can be achieved in indoor environments with strong multipath effects and fading, novel local positioning applications, e.g. for smart factories, robotics, interactive guiding, object tracking and augmented reality are presumably leading to a large economic potential.

Artificial neural networks are intended to be used in emerging technologies as information processing systems because their biological equivalents seem to be tolerant to internal failures of computational elements. In this paper, we introduce a measurement which can identify significant neurons of the local cluster neural network and can be used to increase the fault tolerance of this network architecture. Furthermore, it show that this technique can control the network's complexity. Moreover, by this quality different parameter sets of the network and training techniques can be judged with respect to their fault tolerant properties.

Artificial neural networks are able to solve a great variety of different applications, e.g. classification or approximation tasks. To utilize their advantages in technical systems various hardware realizations do exist. In this work, the impact of shrinking device sizes on the activation function of neurons is investigated with respect to area demands, power consumption and the maximum resolution in their information processing. Furthermore, analog and digital implementations are compared in emerging silicon technologies beyond 100 nm feature size.

Artificial neural networks are intended to be used in future nanoelectronics since their biological examples seem to be robust to noise. In this paper, we analyze the robustness of Local Cluster Neural Networks and determine upper bounds on the mean square error for noise contaminated weights and inputs.