Tommy Öberg

Experimental evaluation of an adaptive antenna for a TDMA mobile telephony system

Measurements (at 1.8 GHz) of an adaptive DCS-1800 base station antenna, used in uplink only, are presented and analysed. Both laboratory measurements and outdoor field trials have been performed. The antenna improves the C/I ratio more than 30 dB. Calculations using measured data, assuming downlink performance equal to that of the uplink, show a spectral efficiency gain of 6 over present base station systems.

Robust detection in digital communications

This paper deals with the problem of communicating through unspecified noise. Detectors, robust against variations in the probability density function of the noise, are developed and discussed. The paper covers three issues. First, the relation between a distance measuring receiver and a correlating receiver in a general case is shown. Second, a theoretical method for the computation of an upper limit for the probability of symbol error is presented. This computation fits into the ordinary framework for computation of the error probability by changing the inverted noise density 2/N/sub 0/ to efficacy, /spl epsiv/. Efficacy is defined in the paper. Third, detectors based on M-, i.e., maximum likelihood type, and R-, i.e., rank, statistics are tested and compared for GMSK and /spl pi//4-shifted DQPSK. From numerical comparisons of the upper bounds and their simulated estimates for robust detectors, it is concluded that the loss in Gaussian noise is very small compared to the optimum quadratic detector. The gain, compared to a nonrobust receiver optimized to Gaussian noise, is around 0.5 to 2 dB for large SNR and around 2 to 4 dB for low SNR in impulsive noise. This offers new methods of significantly improving communication when the noise is unknown. >

A Circuit for Contact Monitoring in Electrocardiography

A circuit for contact impedance monitoring in ECG is presented. An ac current in the same frequency band as the ECG signal is used for the measurement. This makes the measurement independent of polarization potentials and gives the correct weight to the impedances in the skin-to-electrode junction. The measurement of the contact impedance is made continuously during ECG monitoring and causes no interference in the ECG signal.

An adaptive antenna for the NMT 900 mobile telephony system

An adaptive antenna for the receiver base station of the NMT 900-system has been designed at the Circuits and Systems Group, Uppsala University. The properties of the antenna have been evaluated by Telia Research AB. The work involved in the antenna design and the field tests is reported.<<ETX>>

Auto-calibrating adaptive array for mobile telecommunications

A major concern for adaptive antennas is the calibration of hardware. Here we consider the calibration of an analog beamformer (ABF), which calculates the weights in a digital signal processor (DSP), but weights the RF signals using a hardware beamformer. Two novel methods are presented of performing a calibration of this adaptive antenna during normal operation. This will mitigate temperature drift and aging of active components. Both methods uses a feedback of the beamformer output signal, to calculate the error in the beamformer output or to identify the parameters in the temperature drift. The algorithms are transparent to normal antenna operation and are computationally simple. Simulations show that when using these auto-calibration methods, the performance is independent of variation in hardware parameters when a realistic temperature drift is introduced. The proposed algorithms are intended for use in a time-division multiple access (TDMA) mobile telephone system, but the methods are applicable in radar systems also.

Effects of nonlinear distortion on switched multibeam FDMA systems

The effects of using a multicarrier amplifier (MCPA) in the transmit chain of a cellular frequency-division multiple-access system utilizing switched multibeam base-station antennas is investigated. By combining several carriers prior to amplification, the signal envelope will be time varying and the MCPA will introduce nonlinear distortion of the amplified waveforms. It is shown how the main beam direction and frequency of any intermodulation product of any order can be calculated. A frequency allocation scheme is presented that reduces the intermodulation distortion for the mobile users. By Monte Carlo simulations, the probability distribution function of the received intermodulation distortion power is estimated, assuming a GSM system, as a function of the number of antenna elements and the number of active users. Comparisons with a one-element reference antenna are made, and it is shown that the received intermodulation distortion power for the users in the system is substantially reduced when the number of beams is increased or the user activity is reduced.

Intermodulation distortion in switched multibeam antennas for cellular radio systems

The performance of switched multibeam antennas for cellular radio systems is investigated when the transmit amplifier is of a multicarrier (MCPA) type. The MCPA nonlinearity generates intermodulation distortion in the transmitted signal that is spatially filtered by the array radiation pattern. Hence, the intermodulation distortion becomes direction-dependent, and the direction of the generated intermodulation products of any order is derived. It is shown by using Monte Carlo simulations how the downlink carrier to interference ratio depends on the frequency reuse distance and on the MCPA linearity measured as its noise power ratio (NPR). For reuse factor one systems, which is made possible by use of multibeam antennas, the linear co-channel interference dominates the interference. On the contrary, for a reuse factor seven system it is shown that a weak nonlinearity directly gives a degradation in carrier to interference ratio (CIR) due to intermodulation distortion. However the reuse factor seven system has a large CIR overhead due to the increase in CIR using a multibeam antenna, and the outage probability is thereby only slightly affected.