Channel estimation is useful in millimeter wave (mmWave) MIMO communication systems. Channel state information allows optimized designs of precoders and combiners under different metrics such as mutual information or signal-to-interference-noise (SINR) ratio. At mmWave, MIMO precoders and combiners are usually hybrid, since this architecture provides a means to trade-off power consumption and achievable rate. Channel estimation is challenging when using these architectures, however, since there is no direct access to the outputs of the different antenna elements in the array. The MIMO channel can only be observed through the analog combining network, which acts as a compression stage of the received signal. Most of prior work on channel estimation for hybrid architectures assumes a frequencyflat mmWave channel model. In this paper, we consider a frequency-selective mmWave channel and propose compressed-sensing-based strategies to estimate the channel in the frequency domain. We evaluate different algorithms and compute their complexity to expose trade-offs in complexity-overheadperformance as compared to those of previous approaches.

Configuring the hybrid precoders and combiners in a millimeter wave (mmWave) multiuser (MU) multiple-input multiple-output (MIMO) system is challenging in frequency selective channels. In this paper, we develop a system that uses compressive estimation on the uplink to configure precoders and combiners for the downlink (DL). In the first step, the base station (BS) simultaneously estimates the channels from all the mobile stations (MSs) on each subcarrier. To reduce the number of measurements required, compressed sensing techniques are developed that exploit common support on the different subcarriers. In the second step, exploiting reciprocity and the channel estimates, the base station designs hybrid precoders and combiners. Two algorithms are developed for this purpose, with different performance and complexity tradeoffs: 1) a factorization of the purely digital solution, and 2) an iterative hybrid design. Extensive numerical experiments evaluate the proposed solutions comparing to state-of-the-art strategies, and illustrating design tradeoffs in overhead, complexity, and performance.

No abstract

Amputation of the auricle is a periodic occurrence leading to disfigurement if not treated properly. Venous stasis is a common complication in reattachments and requires decongestant and anticoagulant treatment. Today, leech therapy is the treatment of choice. Common problems are that it is not available everywhere and that it is usually contraindicated in anticoagulated patients. The peculiarities of leech therapy and the various aspects of surgical management are reviewed. A case of a partial amputation of the auricle in a patient under concomitant anticoagulation therapy with warfarin is presented. The amputated part was reattached in another hospital without microvascular anastomosis. The patient presented to our department with early signs of venous congestion. Leech therapy was started 35 hours after trauma, and the patient continued his anticoagulation therapy. With this treatment, 90% of the amputated part was rescued. The anticoagulation therapy of the patient may have played an important role in the first hours after reattachment, preventing capillary thrombosis and in consequence facilitating the minimal oxygenation necessary. The claim that anticoagulation therapy is a contraindication to leeching should be questioned in cases of reattachments in well-controllable locations without arterial anastomosis.