Gerhard Schultes

Irreducible error performance of a digital portable communication system in a controlled time-dispersion indoor channel

The authors investigated experimentally, analytically, and by simulation the irreducible errors such as error floor for high signal-to-noise ratio that are introduced by time dispersion in a slowly fading Rayleigh channel. A multipath indoor propagation environment with controlled delay spread was created to validate this approach. The system is close to the Digital European Cordless Telecommunications (DECT) standard with 1 Mb/s GMSK-modulated signals being transmitted in FDM/TDMA/TDD format and received by a differential direct-conversion receiver. The measured and simulated error probability floor is Pe approximately=5*10/sup -3/ as a consequence of delay spread that is 10% of the bit length. This value is approximated by a closed-form analytical description of phase jitter due to large group delay bursts. Simulation results obtained previously by others for comparable cases are partially corroborated and partially disproved. >

A new incoherent direct conversion receiver

A novel type of noncoherent homodyne receiver is presented. It is designed for low-cost mobile communication applications in the upper UHF area. The receiver operates on digital angle-modulated signals at high data rates in GMSK and in most other two- and four-state modulation schemes of the proposed European DECT-standard, as well as one more frequency-efficient linear modulation techniques. The design of the RF and mixing circuits, as well as the baseband filtering and the vector-operation-based demodulation circuit are presented. Some computer simulations of the operating performance of this receiver system are described.<<ETX>>

DECT transceiver architectures: Superheterodyne or direct conversion?

A conventional heterodyne transceiver and a direct conversion transceiver for application in the DECT standard are compared. The authors describe how both systems master the frequency domain multiple (access)/time domain duplex/time domain multiple access (FDM(A)/TDD/TDMA) transmission of DECT, and describe the implementation problems with respect to complexity and integrability.

Personal communications transceiver architectures for monolithic integration

The proper combination of architecture and integration concept will decide the success of a transceiver realization. The authors discuss three realization concepts of monolithic integrated transceivers: (i) a modular multichip solution, (ii) a dedicated few chip solution and (iii) a single chip solution of full custom library modules. The authors compare them with respect to size, power consumption, and production cost. Applied to heterodyne and direct conversion transceiver architectures, they find that direct conversion is more complex but superior for integration and cost. They present first-time experimental results of a prototype 1.5 GHz fully integrated direct conversion transceiver chip that uses a VCO at twice the required LO frequency for reduction of DC offset. A novel, mainly digital high-dynamic-range limiting baseband detector complements a direct conversion solution.

Error floor mechanisms in indoor digital portable communications

By analytical derivation, supported by simulation, the authors develop a closed form theory of the error floor probability of differentially detected MSK in a slowly fading Rayleigh channel with small delay spread. As the major error source they identify the phase jitter generated by group delay bursts coinciding with the frequency deviation of the modulated signal, rather than the timing jitter following the instantaneous mean delay. Such errors turn up in the fades even when the channel is short-term invariant and the receiver is stationary. For experimental verification they apply controlled delay spreads (over the air) in a DECT-like test bed. The results confirm that the error probability is, to first order, a function of the delay spread to bit-length ratio only and varies with the square of this ratio.<<ETX>>

A testbed for DECT physical- and medium access-layer

The next generation of cordless telephones in Europe will conform to the DECT (Digital European Cordless Telecommunications) standard. This standard makes a change from todays analog FDMA (Frequency Domain Multiple Access) transmission to the fully digital FDM/TDD/TDMA (Frequency Domain Multiplex/Time Domain Duplex/Time Domain Multiple Access) system. Also, channels will be assigned dynamically at request of the mobile. For the performance of this transmission and access method under indoor conditions no experimental results have been published. Only a few computer simulations of system behavior have been carried out. The authors have set up a testbed for (i) verification of the DECT standards physical- and medium access-layer specifications, (ii) testing of DECT equipment under typical indoor conditions, and (iii) finding the limitations of a DECT link for indoor and outdoor applications especially in a slightly dispersive radio channel. They present the principle of the DECT testbed, its practical realisation and a few illustrative measurement examples.<<ETX>>

Design principles and limitations of integrated silicon bipolar transistor amplifiers for mobile communications

A numerical analysis of Class-C amplifiers on the basis of a simple model of the bipolar junction transistor is presented. The trade-off between collector efficiency, power added efficiency, gain, and input impedance of a 250-mW Class-C amplifier on one side and power supply voltage on the other is evaluated. Results are obtained using nonlinear time-domain simulation and optimization without recourse to graphical methods invoking the concept of conduction angle or to piecewise-linear approximations. It is found that, in contrast to other mobile terminal components, a reduction of the Class-C amplifier supply voltage causes lower gain and an increase of the required battery power at constant RF-output power.

Fractional-bit detection with advanced sampling to combat multipath-induced error floor

In digital mobile communications, the error floor caused by time dispersion of the channel limits data rate even in low delay spread situations. The errors originate in the transition regions between adjacent symbols when deep fades occur. A detection method to reduce these errors, i.e., fractional bit differential detection with sample timing which is advanced from the average mean excess delay, avoiding the transition regions, is analyzed. Assuming a minimum shift keying (MSK) modulation format, the error probability is calculated and verified by a Monte Carlo software simulation. The approach increases the tolerable delay spread in an unequalized receiver, depending on the details of the delay power profile. These are accounted for by introducing an effective delay spread. Limiting the bandwidth tends to smear the transition regions over the entire bit duration, thus offsetting part of the methods benefit.