Andreas Ahrens

Decentralized Interference Cancellation in Mobile Radio Networks

Various interference cancellation techniques for overcoming the interference limitations inherent to cellular mobile radio systems were proposed. A joint processing of received and transmitted signals of neighboring access points turns out to be very beneficial from a performance point of view. Such a system can be considered a multiuser MIMO system with the entirety of the antennas of all mobile terminals at one end and the antennas of the access points at the other end of the communication link. Unfortunately, the signal processing techniques proposed so far require centralized signal processing units and high speed communication links between these central units and the access points. In the present paper decentralized interference cancellation techniques are presented. They are based on distributed signal processing in the access points. The only prerequisite are efficient communication links between neighboring access points. The presented techniques could be applied, e.g., in future TDD based OFDM mobile radio networks.

Decentralized Interference Management in Mobile Radio Networks

Abstract – Interference management will play an important role in future mobile radio systems with higher requirements of data rate and quality of service (QoS). The decentralized iterative partial joint detection (JD) in the uplink (UL) and joint transmission (JT) in the downlink (DL) are very beneficial approaches to eliminate or avoid interferences with good system performance and moderate implementation complexity. In this paper, the system performance of the iterative decentralized signal processing with partial channel-state information (CSI) is analytically assessed with the help of a novel analysis based on the channel matrix formalism of the significant useful channels and the significant interference channels. A comparison of the system performance between the UL and the DL is performed based on the same available CSI in the UL and in the DL.

SVD-Aided, Iteratively Detected Spatial Division Multiplexing Using Long-Range Channel Prediction

In this contribution iteratively detected spatial division multiplexing is investigated under the constraint of a fixed data throughput. Existing bit loading and transmit power allocation techniques are often optimized for maintaining both a fixed transmit power and a fixed target bit-error rate, while attempting to maximize the overall data-rate, albeit delay-critical real-time interactive applications, such as voice or video transmission, may require a fixed data rate. As an alternative design option, in addition to sophisticated joint bit- and power loading, in this contribution we invoke both coded modulation as well as channel prediction and identify the most beneficial number of modulation signalling levels, while minimizing the bit-error ratio under the constraints of a given fixed throughput. Our performance results show the superiority of bit-interleaved coded modulation using iterative decoding (BICM-ID) against turbo trellis-coded modulation (TTCM), regardless of using idealistic perfect or realistic imperfect channel state information (CSI).

BER Minimization in SVD equalized Wavelet-based Multicarrier Systems using Transmit Power Allocation

In this contribution, transmit power allocation schemes for BER performance improvements in a wavelet-based multicarrier transmission system with SVD (singular value decomposition) equalization are investigated under the constraint of a limited total transmit power. Contrary to other contributions, here the transmit power is not only adapted to the subchannels but rather to each symbol of the SVD equalized data block individually. Based on the Lagrange multiplier method different solutions are investigated. In order to obtain numerical results, the schemes are exemplarily applied to different radio channel characteristics including Rayleigh and Rice distributed path coefficients

Modulation-mode and power assignment for MIMO-BICM schemes

In this contribution we jointly optimize the number of activated MIMO layers and the number of bits per symbol along with the appropriate allocation of the transmit power under the constraint of a given fixed data throughput and integrity. In analogy to bit-interleaved coded irregular modulation, we introduce a MIMO-BICM scheme, where different signal constellations and mappings were used within a single codeword. EXIT charts are used for analyzing and optimizing the convergence behaviour of the iterative demapping and decoding.

Optimal Power Allocation in SVD Equalized Multicarrier Systems

Multicarrier transmission is an established technique for radio transmission systems and it can be considered as a promising approach for next generation wireless systems. In channel situations with high delay-spreads, OFDM requires large guard interval lengths which decrease the spectral efficiency significantly. Therefore, alternative multicarrier transmission systems are of great practical interest, which guarantee a reliable transmission without a guard interval. In multicarrier systems the subcarriers are weighted in a quite different way by the frequency selective channel, based on the inherent structure of the multicarrier transmission system. If the transmitter knows the channel, the subchannels transmission parameters such as transmit power or modulation alphabets can be adapted. In this contribution, transmit power allocation schemes for BER performance improvements in a wavelet-based multicarrier transmission system with SVD equalization (singular value decomposition) are investigated under the constraints of a limited total transmit power and a given data rate. Thereby the differences between the optimal but highly complex solution and suboptimal but complexity-reduced solutions are analyzed. Numerical results are obtained by simulations assuming different time-variant channel characteristics, with delays larger than the multicarrier symbol duration.

Kanalkapazität vielpaariger symmetrischer Kupferkabel, Teil II: Ausnutzung der Fernnebensprechwege

In den vielpaarigen symmetrischen Kupferkabeln der Ortsnetze begrenzt das Nebensprechen wesentlich die erreichbare ¨ Ubertragungskapazitat. In einem zu- gehorigen ersten Teil dieses Beitrages wurde die Kapazitat von symmetrischen Leiter- paaren innerhalb vielpaariger Kabel bei Storung durch Nah- und Fernnebensprechen in bidirektional betriebenen Kabeln sowie bei alleiniger F ernnebensprechstin unidirektional genutzten Kabeln berechnet. In diesem zweiten Teil des Beitrages wird fur unidirektional betriebene Kabel der Blickwinkel verandert und die Kanalkapazitat dafur berechnet, dass die ¨ uber die Fernnebensprechpfade auf benachbarte Aderpaa- re ¨ ubergekoppelten Signalanteile ebenso wie das direkt ¨ u bertragene Signal als Nutz- signal betrachtet werden. Als Ergebnis zeigt sich, dass die Kanalkapazitat von Ka- belbundeln durch die Ausnutzung des Fernnebensprechens gegenuber der Kapazitat von (fiktiven) Bundeln mit perfekt geschirmten Einzelpaar en gesteigert werden kann. Noch weitaus deutlicher wird der erzielbare Kapazitatsgewinn gegenuber der prak- tisch relevanten Kapazitat von Kabelbundeln mit nicht geschirmten Aderpaaren bei gegenseitiger Fernnebensprechstorung. Mit zunehmender Anzahl in einem Bundel zu- sammengefasster Aderpaare werden die erzielbaren Kapazit¨ atsgewinne groser, wobei insbesondere bei kurzen Kabeln die Steigerung der Kanalkapazitat durch Ausnutzung des Fernnebensprechens besonders deutlich ist.

Power allocation in service area interference channels

Adaptive power allocation is an effective means for interference management. This paper addresses transmit power allocation in interference channels with a total power constraint. In particular, realistic service area networks which can be modeled as interference channels are considered for investigation. Various power allocation strategies in service area interference channels are discussed, among which the proposed iterative waterfilling algorithm is verified to be a favorable approach in terms of sum capacity, especially for universal frequency reuse.

Cheannel Capacity of Twisted Wire Pairs in Multi-Pair Symmetric Copper Cables

Crosstalk is caused by electromagnetic coupling between neighbouring wire pairs in multi-pair copper cables. In this contribution the channel capacity of a twisted wire pair in an unidirectional driven multi-pair cable is calculated, where only far-end crosstalk is present. Thereby two scenarios are considered: Firstly the far-end crosstalk from neighbouring wire pairs is interpreted as disturbance, which affects the transmission in the considered wire pair. Secondly the far-end crosstalk paths are viewed as additional transmission paths, which together with the wanted signal path convey the signal from the near to the far cable end. In the former case the channel capacity is decreased compared to the transmission with only noise disturbance, whereas in the latter case the channel capacity is increased due to the utilization of additional transmission paths. In particular for short cables the effect of far-end crosstalk is very strong and with rising number of neighbouring wire pairs the far-end crosstalk impact increases

Equalization of intersymbol and far-end crosstalk interference

Electromagnetic coupling between neighbouring wire pairs in copper cables causes crosstalk. In this contribution, linear equalization techniques are compared on a unidirectional driven cable, where only far-end crosstalk occurs. The performance of a conventional equalizer per wire pair only compensates the intersymbol interference and is limited by the crosstalk disturbance, which essentially depends on the number of disturbing wire pairs. Furthermore, a multidimensional equalizer is presented, which is able to compensate both the intersymbol and the far-end crosstalk interference. It is shown that, using this multidimensional equalizer, the bit-error rate can be improved compared to conventional equalizers, which only compensate the cable frequency response. The possible gains depend on the cable length and the number of disturbing wire pairs.