Tamás Krébesz

Theory and Application of Software Defined Electronics: Design Concepts for the Next Generation of Telecommunications and Measurement Systems

The analog signal processing is substituted everywhere by its digital counterpart because of its higher accuracy and flexibility, its much lower cost and because in digital signal processing there is no need for regular calibration as it is required in the analog systems. The most important feature of digital approach is that the HW and SW components can be fully separated and the same HW, referred to as a universal HW device, can be used to implement very different applications. Although digital signal processing has been around everywhere in the low frequency applications for many years, until this time it could not satisfy the requirements of RF and microwave engineering. The main challenges in RF and microwave radio communications and measurements are: (i) implementation of ultra wide dynamic range (limited by the quantization noise and linearity) and (ii) minimize sampling rate required. In our time the situation is changing rapidly. Software Defined Radio, Universal Software Radio Peripheral and Virtual Instrumentation all mean that a universal HW device is used to extract the complex envelope of an RF bandpass signal to be demodulated or analyzed, and the implementation of radio receiver or signal analyzer is implemented entirely in SW. The complex envelope, processed in SW, carries all information available in the RF signal and assures the minimum sampling rate which is determined by the bandwidth of RF signal. The complex envelope represents fully the RF bandpass signal without any distortion and every RF bandpass signal can be reconstructed from its complex envelope without any distortion. This tutorial surveys the theory of complex envelopes, shows how the baseband equivalent models of RF systems can be derived and demonstrates how different radio transceivers and test equipment can be implemented by means of the same universal RF HW device in the 2.4-GHz ISM frequency band.

Feasibility of UWB radio: Impulse radio versus chaos-based approach

Both the peak power and the average power of emission are limited in UWB systems in order to keep the interference caused in an already existing narrowband radio system low enough. It is frequently overlooked that these power limits have to be checked not over the transmitted UWB signal but at the output of a bandpass filter specified in the FCC regulations. A further constraint exists in the handheld cheap devices where the low supply voltage reduces even further the attainable energy available to carry one bit information. The bit energy determines the coverage, the larger the bit energy, the larger the coverage. Introducing a novel approach an exact link budget calculation is proposed here for UWB impulse radio. Since the NLOS coverage of impulse radio obtained from the link budget is less than 2 m, the implementation of WLAN systems is not feasible with UWB impulse radio devices. To increase the bit energy, the UWB carrier-based approach, where the duty cycle is considerably increased, is proposed here. More than 16-dB improvement in bit energy has been achieved by increasing the UWB carrier duration to 300 ns.

Improving the noise performance of energy detector based UWB systems by optimizing the receiver parameters

In an Ultra-WideBand (UWB) Low-Rate (LR) Wireless Personal Area Network (WPAN) application low-cost, low-power and low-complexity systems are expected to meet the requirements of the typical applications. These demands can only be satisfied by simple modulation schemes and easy to implement noncoherent receiver configurations. However performed field tests showed that the noise performance of such systems is quite poor. Therefore noise performance improvement is a must. This paper investigates and compares the Transmitted Reference (TR) modulation scheme with autocorrelation receiver, the On-Off Keying- (OOK) and the Pulse Position Modulation (PPM) schemes with energy detector for UWB impulse radio (IR) systems. The contribution provides an efficient method for the improvement of the noise performance of the UWB IR systems by matching the parameters of the receiver to the UWB pulse. Using this technique almost 8-dB noise performance improvement can be achieved. The Bit Error Rate (BER) of the three systems will be compared and an analytical expression will be provided for the estimation of the BER of each system.

Use of UWB Impulse Radio Technology in In-Car Communications: Power Limits and Optimization

In-car wireless data communications systems require a short-range unlicensed radio communications technology that causes a very low level of interference in the other, already deployed radio links and networks, offers low and medium data rate, can reuse the already occupied radio-frequency (RF) bands, and assures low probability of message collisions. Ultrawide band (UWB) impulse radio employs RF pulses with very short duration to carry the information; consequently, it is an optimal candidate for the in-car wireless communications and intravehicular wireless sensor networks. Data rate and shape of RF carrier pulse determine the performance of a UWB radio link. To limit the interference caused, the maximum power radiated by an UWB device is restricted by the Federal Communications Commission (FCC) in the U.S. Introducing a new mathematical model and starting from the FCC regulations, analytical expressions for the calculation of FCC power limits are derived here. It is shown that the low- and high-rate UWB impulse radio systems are peak and average power limited, respectively. The relationship between the mathematical model and the parameters of an UWB carrier pulse used in a built UWB radio is established. The performances of RF carrier pulses known from the literature are evaluated and compared. All expressions derived are verified by measurements.

New carrier generation techniques and their influence on bit energy in UWB radio

In low-rate UWB impulse radio the carrier pulses have very short duration that limits the energy per bit in an extremely low value. The low bit energy results in an unacceptable short radio coverage. In the paper new UWB carrier pulse generation techniques are proposed to increase the pulse duration considerably. In order to get an ultra wideband carrier a spectrum widening signal is used. Using this approach the energy per bit could be increased even by 17.6 dB when the pulse duration has been set to 70 ns. The coverage of low rate UWB impulse radio is increased considerably due to the increased energy per bit. The novel UWB carriers are all constant envelope signals, therefore, they may be amplified even by nonlinear power amplifiers.

Chaotic Communications with Autocorrelation Receiver: Modeling, Theory and Performance Limits

Chaotic signals are ultra-wideband signals that can be generated with simple circuits in any frequency bands at arbitrary power level. The ultra-wideband property of chaotic carriers is beneficial in indoor and mobile applications where multipath propagation limits the attainable bit error rate. Another possible application is the ultra-wideband (UWB) radio, where the spectrum of transmitted signal covers an ultra-wide frequency band (a few GHz) and the power spectral density of transmitted UWB signal is so low that it does not cause any noticeable interference in the already existing conventional telecommunications systems sharing the same RF band. The UWB technology makes the reuse of the already assigned frequency bands possible. This chapter provides a unified framework for modeling, performance evaluation and optimization of UWB radios using either impulses or chaotic waveforms as carrier. The Fourier analyzer concept introduced provides a mathematical framework for studying the UWB detection problem. The autocorrelation receiver, the most frequently used UWB detector, is discussed in detail and an exact closed-form expression is provided for the prediction of its noise performance. Conditions assuring the best bit error rate with chaotic UWB radio are also given.

UWB Radio: Digital Communication with Chaotic and Impulse Wavelets

Radio communications via channels already occupied by traditional telecommunication systems can be achieved by using ultra-wideband (UWB) radio where extremely wideband wavelets are used in order to reduce the power spectral density (psd) of transmitted signal. Since the recovery of these UWB carriers is not feasible, noncoherent demodulation techniques have to be used. The letter evaluates and compares the noise performances of the feasible noncoherent UWB modulation schemes, namely, that of the noncoherent pulse polarity modulation and the transmitted reference system.

Application of universal software defined PXI platform for the performance evaluation of FM-DCSK communications system

A specific method for the application of a universal software defined wireless platform is presented to determine system performance of communications system based on real field tests. On the platform every application can be implemented: the transmitter and the receiver establishing a wireless link, the communications channel and the algorithm to determine system performance. Implementation is done in software that offers flexible, efficient and cheap solution for performance testing. Every physical RF analog signals generated or processed in software can be recovered on the PXI hardware platform used in our tests. Since the analog RF output and input of the transceiver are available, real field tests can be performed or even the system performance can be evaluated in a real operating network. Using the PXI platform, based on real field tests the evaluation of system performance of a 2.4-GHz ISM band wireless communication system with FM-DCSK modulation is presented here. A systematic method for the validation of system implemented on the PXI HW is provided.

Ultra-wideband impulse radio based on pulse compression technique

The energy transmitted per bit limits the radio coverage. In impulse radio the UWB pulses used carry a very little energy since they are extremely short. As a consequence the radio coverage is unacceptable short. A solution to increase the energy per bit is the enlargement of the duration of UWB carrier pulse, however, this solution cannot be used because the correlation of received pulse envelope with a reference pulse defined in IEEE Std. 802.15.4a has to exceed a prescribed value. This problem can be overcome if the pulse compression approach is used. This technique, where the duration of radiated UWB carrier pulse is enlarged considerably to get enough energy per bit and the duration of received UWB pulse is compressed by a matched filter at the receiver, is introduced in this contribution. The increased bit energy increases the radio coverage and the envelope of compressed UWB pulse satisfies the requirements of IEEE Std. 802.15.4a. The gains in energy per bit are about 18 dB and 22 dB when the UWB pulse durations are set to 100 ns and 300 ns, respectively.

From simulations to field tests: PXI-based software defined wireless platform for performance evaluation of FM-DCSK

A universal software defined wireless platform and a specific method for its application is proposed here to perform field test measurements for performance evaluation of communications system. The platform implements every application, starting from the transmitter and receiver of wireless link, the virtual measurement instruments for the baseband signals and the performance evaluation algorithm, purely in software which solution offers high level of flexibility, and it provides an easy and cheap solution for performance testing. Since a PXI-based HW platform is also included in the solution proposed here, every physical RF analog signals generated or processed in software can be recovered. Both the analog RF output and input of the transceiver are available, therefore real field tests can be performed, even more, the performance of a communications system can be evaluated in a real operating network. In this paper the performance of FM-DCSK wireless communications system, operated in the 2.4-GHz ISM band, is evaluated based on real field tests using PXI platform. A systematic method for validation of system implemented on the PXI HW is provided.