We are interested in the effect of multi-user interference (MUI) on IR-UWB Networks. We argue that in a real system MUI inevitably occurs and consequently has to be dealt with. First, there may be other co-existing IR-UWB networks that are not under your control but still interfere with your network. Second, from a networking perspective one potential benefit of UWB over narrowband networks is the possibility to allow parallel transmissions. The resulting MAC protocols do not try to prevent MUI by enforcing mutual exclusion, but rather allow concurrent transmissions and thus to some extent the occurence of MUI.

The effect of MUI on system performance is generally less detrimental in IR-UWB networks than in narrowband networks. Nevertheless, the resulting performance degradation can still be significant, especially in the case of nearby interferers (near-far effect). MUI thus has to be taken into account in system design and schemes to mitigate its effect need to be developed.

Our current research on the topic covers a wide range of areas. It includes the physical as well as the MAC layer, synchronization and data decoding, ranging and localization.

• We propose a new timing acquisition and synchronization scheme that is robust to MUI, even in the hard case of heterogeneous power levels (near-far effect). Our method solves the problem without any additional complexity overhead, e.g. for a digital receiver, it employs the same sampling frequency and number of operations as a conventional detection method.
• We show that mitigation of MUI on the physical layer can be achieved with a receiver that employs a combination of statistical interference modeling and thresholding. In contrast to existing schemes, our receiver accounts for the multipath nature of UWB channels. We find that in a multipath environment the proposed receiver significantly outperforms existing receiver designs that either completely neglect the effect of MUI or only use a simple threshold to reject samples from interfering users. To model MUI we consider a hidden Markov model (HMM) and a Gaussian mixture model (GMM). We find that the HMM models interference better than the GMM. However, the resulting performance difference is not huge and comes at the cost of increased receiver complexity.
• We evaluate the performance of an IEEE 802.15.4a ultra-wideband (UWB) physical layer with an energy-detection receiver in the presence of MUI. In our simulations we consider a complete packet based system, taking into account packet detection and timing acquisition, the estimation of the power delay profile of the channel and the recovery of the encoded payload. Our analysis shows that an 802.15.4a compliant energy detector shows only very limited capture effect and thus anihilates one of UWBs most appealing benefits, namely the possibility to allow concurrent transmissions.
• We propose a MAC protocol that takes advantage of the possiblity to mitigate interference on the physical layer.
• Due to its fine timing resolution, UWB has a huge potential in ranging and localization applications. Also in these applicaitons, MUI is an issue and we thus study the impact of MUI on ranging and localization.

For more information on this topic, the following selected publications are available:

• M. Flury, R. Merz and J.-Y. Le Boudec. Managing Impulsive Interference in Impulse Radio UWB Networks. ST Journal of Research, 2007.
• A. El Fawal and J.-Y. Le Boudec. A Power Independent Detection Method for UltraWide Band (UWB) Impulse Radio Networks. IEEE International Conference on Ultra-Wideband (ICU 2005), 2005.
• A. El Fawal and J.-Y. Le Boudec. Synchronizing Method for Impulse Radio Network. P-26-526-US, October 2004.
• M. Flury and J.-Y. Le Boudec Interference Mitigation by Statistical Interference Modeling in an Impulse Radio UWB Receiver. IEEE International Conference on Ultra-Wideband (ICUWB 2006), 2006.
• M. Flury, R. Merz, J.-Y. Le Boudec and J. Zory. Performance Evaluation of an IEEE 802.15.4a Physical Layer with Energy Detection and Multi-User Interference. IEEE International Conference on Ultra-Wideband (ICUWB 2007), 2007.
• M. Flury, R. Merz, J.-Y. Le Boudec. An Energy Detection Receiver Robust to Multi-User Interference for IEEE 802.15.4a Networks. IEEE International Conference on Ultra-Wideband (ICUWB 2008), 2008.
• M. Flury, R. Merz and J.-Y. Le Boudec Robust Non-Coherent Timing Acquisition in IEEE 802.15.4a IR-UWB Networks Personal, Indoor and Mobile Radio Communications, 2009. PIMRC 2009. IEEE 20th International Symposium on, 2009.
• J.-Y. Le Boudec, R. Merz, B. Radunovic, and J. Widmer. DCC-MAC: A decentralized MAC protocol for 802.15.4a-like UWB mobile ad-hoc networks based on dynamic channel coding. In First International Conference on Broadband Networks (BroadNets 2004), San Jose, CA, October 2004.
• H. Zhan, J. Ayadi, J. Farserotu and J.-Y. Le Boudec. High-Resolution Impulse Radio Ultra Wideband. IEEE International Conference on Ultra-Wideband (ICUWB 2007), 2007.