Journal Papers

Overview

• U.-C. Fiebig, M. Schnell, "Correlation Properties of Extended m-Sequences", IEE Electronics Letters, Vol. 29, No. 20, September 1993, pp. 1753-1755

• K. Fazel, S. Kaiser, M. Schnell, "A Flexible and High Performance Cellular Mobile Communications System Based on Orthogonal Multi-Carrier SSMA", Wireless Personal Communications, Vol. 2, Nos. 1&2, 1995, pp. 121-144

• M. Schnell, A.J. Han Vinck, "A Low Complexity Equalization Concept for Frequency-Selective Mobile Radio Channels", AEÜ International Journal of Electronics and Communications, Vol. 50, No. 6, November 1996, pp. 343-352

• M. Schnell, U.-C. Fiebig, "Fade Slope Statistics of 40 GHz Beacon Signals", IEE Electronics Letters, Vol. 33, No. 21, October 1997, pp. 1819-1821

• M. Schnell, D. von Hugo, "Fade Countermeasures and Adaptive Resource Sharing for an Experimental TDMA Satellite Communications System Operating at K_a-Band", European Transactions on Telecommunications (ETT), Vol. 9, No. 6, November/December 1998, pp. 561-568

• M. Schnell, I. De Broeck, U. Sorger, "A Promising New Wideband Multiple-Access Scheme for Future Mobile Communications Systems", European Transactions on Telecommunications (ETT), Vol. 10, No. 4, July/August 1999, pp. 417-427

• E. Haas, H. Lang, M. Schnell, "Development and Implementation of an Advanced Airport Data Link Based on Multi-Carrier Communications", European Transactions on Telecommunications (ETT), Vol. 13, No. 5, September/October 2002, pp. 447-454

Abstracts

• Correlation Properties of Extended m-Sequences: In this article the correlation properties of extended m-sequences are treated and compared to those of m-sequences. Extended m-sequences are obtained from m-sequences inserting an additional element per period yielding periods 2ⁿ where n is an integer. It turns out that extended m-sequences provide good autocorrelation but poor crosscorrelation properties. This behaviour is similar to that of m-sequences.
  
  
• A Flexible and High Performance Cellular Mobile Communications System Based on Orthogonal Multi-Carrier SSMA: The conventional Direct-Sequence Spread-Spectrum-Multiple-Access (DS-SSMA) system with RAKE-receiver is an interesting approach in mobile communications in order to combat the frequency selectivity of the channel. The main advantages of DS-SSMA are well known, but its capacity is limited by other-user interference. Another interesting SSMA technique that combats the frequency selectivity of the channel and achieves high spectral efficiency is based on Orthogonal-Frequency-Division-Multiplexing (OFDM). This new SSMA technique combines the principle of DS-SSMA with OFDM and is called Multi-Carrier Spread-Spectrum-Multiple-Access (MC-SSMA). By providing high frequency diversity, this combination enables the possibility to perform a maximum-likelihood detection (MLD) resulting in high performance/capacity, to use the spectrum in an efficient way and to retain many advantages of the conventional DS-SSMA system. In addition, it allows both simple cell-separation by using frequency hopping and simple hardware realization. An example for the downlink of a mobile communications system, i.e. the transmission from the base station to a mobile station, using MC-SSMA with Walsh-Hadamard code spreading is studied. Different coherent detection algorithms such as conventional detection, MLD, and iterative detection are analyzed. The analytical performance of MLD is evaluated. It is also shown analytically and by simulations that the MC-SSMA system with MLD outperforms the DS-SSMA system with RAKE-receiver. Up to 64 active users can be transmitted in a 1.25 MHz bandwidth. Each user has a rate of 16.0 kbit/s by using BPSK modulation. At a signal to noise ratio (SNR) of 13 dB a bit error probability of P_b=10^-3 can be guaranteed without channel coding. This results in a spectral efficiency of about 0.8 bit/s/Hz. Under the same conditions the conventional DS-SSMA system results in a spectral efficiency of about 0.15 bit/s/Hz. Hence, using MC-SSMA for the downlink of a cellular mobile communications system is a promising approach.
  
  
• A Low Complexity Equalization Concept for Frequency-Selective Mobile Radio Channels: In this paper, a low complexity equalization concept applicable to frequency-selective mobile radio channels is proposed. The equalization concept is a concatenated scheme combining low complexity equalizers with an efficient coding scheme in order to achieve good performance at low overall complexity. The coding scheme consists of a simple single-parity-check code and allows for maximum-likelihood decoding at the receiver. Thus, high coding gains with only little additional complexity are achieved.
Different suboptimum, low complexity equalization schemes are investigated and their performance losses with respect to the optimum equalizer are determined using Monte-Carlo simulations. Appropriate low complexity equalizer structures are identified and further investigated as part of the proposed equalization concept. Simulation results show that the performance losses introduced through low complexity equalization can easily be overcompensated applying the proposed equalization concept. Typically, the performance losses in signal-to-noise ratio at a bit error rate of 10^-4 are less than 1 dB, whereas the coding gains are more than 2 dB. Thus, the proposed equalization concept achieves better performance than the optimum equalizer at a lower overall complexity.
 
• Fade Slope Statistics of 40 GHz Beacon Signals: Fade slope statistics for negative and positive slopes are presented which are obtained by propagation measurements of the 40 GHz beacon signal of the Italian satellite ITALSAT. Especially, the fade slope and the hereby introduced 3 dB time duration are evaluated. During the measurements fade slopes up to 3.2 dB/sec have been recorded. However, in 99.99% of the fading event time the fade slopes are less than 1.9 dB/sec.
  
  
• Fade Countermeasures and Adaptive Resource Sharing for an Experimental TDMA Satellite Communications System Operating at K_a-Band: Satellite communications systems operating at K_a-band suffer from severe rain fading. Thus, in order to achieve an acceptable availability fade countermeasures have to be applied which are efficiently implemented using adaptive resource sharing. In this letter, the results from a measurement campaign during which an experimental time-division multiple-access satellite communications system applying fade countermeasures has been operated at K_a-band are reported and discussed. The results show that fade countermeasures can be used efficiently leading to a considerable improvement in performance and/or availability.
  
  
• A Promising New Wideband Multiple-Access Scheme for Future Mobile Communications Systems: Recently, "Interleaved Frequency-Division Multiple-Access" (IFDMA) has been introduced as a new spread-spectrum multiple-access scheme for mobile communications which is suitable for both downlink and uplink applications. IFDMA combines spread-spectrum multicarrier transmission with frequency-division multiple-access and, therefore, is capable of achieving frequency diversity while multiple-access interference is avoided. Moreover, compared to other wideband multiple-access schemes IFDMA shows several additional advantages. The cost to achieve these advantages is to allow for intersymbol interference. However, the effects of intersymbol interference can be reduced significantly by applying an equalizer at the IFDMA receiver.
In this paper, the principle of IFDMA is reviewed and it is shown that IFDMA is a promising new wideband multiple-access scheme suitable for future mobile communications systems. The frequency diversity achievable with IFDMA is determined as well as performance results for uncoded and coded transmission taking into account different equalization techniques. As a result, uncoded IFDMA applying optimum equalization performs even better than uncoded multicarrier code-division multiple-access applying maximum-likelihood multi-user detection. In the case of coded IFDMA transmission coding gains up to 6.8 dB at a bit-error-rate of 10^-4 are achieved using a standard convolutional code. An additional performance gain of up to 2.2 dB is possible if equalization and decoding are considered jointly.
 
• Development and Implementation of an Advanced Airport Data Link Based on Multi-Carrier Communications: In this paper, both the concept and the demonstrator implementation of an advanced airport data link is described which is used for data exchange between pilots and controllers in future surface movement guidance and control systems on airports. To fulfil future airport capacity requirements, the proposed data link has to be a high capacity, high rate data link. Therefore, the data link design is based on multi-carrier code-division multiple-access which is a flexible and highly efficient multiple-access transmission technique suitable for mobile communications in fading conditions. Both simulation results with typical aeronautical channel scenarios and bit error rate measurements using the demonstrator hardware show the suitability of multi-carrier code-division multiple-access for aeronautical communications within the airport and its vicinity. Moreover, implementation effects which degrade the performance are identified and countermeasures are proposed and investigated. The results show that a reduction of the implementation loss of up to 2 dB is achievable by applying appropriate countermeasures.