In the following we describe some recent results to illustrate our areas of interest in this field: we have introduced two-way relaying schemes, which double the spectral efficiency of half duplex relay networks. We have suggested a relay based solution to the "rank deficient channel" problem of MIMO Wireless. Our work on nonlinear MIMO detection is fundamental for the application of MIMO in low complexity/low power nodes.

In the following we describe some recent results to illustrate our areas of interest in this field: we have introduced cooperative relaying protocols (coherent multiuser relaying), which achieve a distributed spatial multiplexing gain even if all participating nodes have only single antennas. We have shown, that a minimum of N(N-1)+1 amplify&forward relays is required to achieve a spatial multiplexing gain N. The relaying protocols have been verified in our RACooN cooperative relaying testbed.
In the area of non-coherent relaying we have introduced cooperative diversity protocols which achieve full diversity. They have very low requirements on the relay synchronization and information exchange. In low mobility scenarios it is hard to achieve multiuser diversity under a fairness constraint. Our solution is based on a combination of the cooperative diversity schemes with opportunistic scheduling.

Localization: Due to the high bandwidth of UWB signals, the spatial configuration of scatterers around source and destination translates into a location-specific structure of the channel impulse response (CIR). On the other hand the CIR decorrelates over a node displacement of several cm. We have invented location fingerprinting algorithms, which despite this fact are able to group a set of CIR into clusters, which correspond to spatially adjacent nodes.

WBAN: We identified UWB as the most promising radio technology for Ultra Low Power Wireless Body Area Networks (WBAN). The high UWB peak data rate together with the moderate required throughput allows a low duty cycle operation, which dramatically reduces the average current consumption. Operation with high peak data rate introduces intersymbol interference (ISI) whose impact has been reduced by introducing a new family of low complexity symbol estimators for PPM impulse radio. Further features of our WBAN design are a cognitive MAC to cope with burst interferers and a novel robust burst and symbol synchronization. The estimated current consumption is 1mA@1.2V.

Dispersed Wireless: in the long term we anticipate a major paradigm shift, as wireless systems become more dense and node separation is no longer much larger than the wavelength: the functionality of a classical wireless node will be dispersed into spatially separate subnodes, which communicate with each other through the wireless medium (including near field coupling effects). The capabilities of this community of subnodes depend on their (random) aggregation and we have to design signaling and protocols in such a way, that evolutionary growth of collective capabilities is enabled. One application of these technologies are wireless NanoNets.

Future MIMO: Wireless systems operate at ever increasing carrier frequencies. This has two adverse effects: (i) increased path loss and (ii) more antennas are possible in form factor constrained mobile nodes. In current systems the number of multipath components is typically larger than the number of antennas, that can be accommodated in a mobile. As a result they operate in the rich scattering/poor array regime. With increasing carrier frequency this relation reverses and we converge to the rich array/poor scattering regime (i.e. the number of antennas considerably exceeds the number of relevant multipath components). We anticipate, that this is the upcoming challenge in MIMO wireless. We are investigating cooperative signaling schemes and protocols in the rich array/poor scattering regime and we study hardware-efficient signal processing algorithms.