Ernesto Zimmermann

A simple cooperative extension to wireless relaying

In this paper, we propose and analyze a simple protocol that simultaneously exploits two potentials offered by wireless relay systems: diversity gains and pathloss savings. An intermediate decode-and-forward relay assists transmission from source to destination, and the destination combines the signals it receives from source and relay. The key feature of the proposed system is that the relay node decides independently whether or not to forward information to the destination, thereby minimizing the risk of error propagation while providing truly constructive diversity gains. In combination with pathloss savings, this leads to significant gains over both direct transmission and conventional relaying. The results are obtained from an analysis of the end-to-end bit error rate, which are confirmed by simulation.

On the performance of cooperative relaying protocols in wireless networks

Cooperative relaying recently emerged as a viable option for future wireless networks. By simultaneously exploiting path loss savings known from relaying scenarios and the diversity inherent to any scheme involving spatially separated transmitters, this technique is able to leverage gains from both relaying and spatial diversity techniques. In this paper, we study different cooperative relaying protocols and compare their performance with that of direct transmission and conventional relaying. We investigate under which conditions the developed techniques provide gains over other approaches. Our results confirm that cooperative relaying is an effective means of enhancing the performance of wireless systems whenever temporal and frequency diversity is scarce.

Cooperative multi-hop transmission in wireless networks

We consider various relaying strategies for wireless networks by comparatively examining direct transmission, conventional relaying, and the novel concepts of cooperative relaying. The latter build on two inherent benefits of relaying systems: the spatial diversity offered by the relay channel, and the ability to exploit the broadcast nature of the wireless medium. Studied cooperative protocols include adaptive decode-and-forward schemes as a simple extension of conventional store-and-forward relaying systems, and more complex decode-and-reencoding schemes that realize distributed coding strategies. We provide a unifying analysis for the tractable two-hop case, before extending the consideration to multi-hop scenarios. The analysis is conducted from the perspective of communication over fading channels under limited bandwidth, energy, and end-to-end delay; main parameters include propagation loss, network geometry, and targeted end-to-end spectral efficiency. Main results indicate that (i) cooperative relaying provides attractive benefits for wireless systems whenever temporal and frequency diversity are scarce or not exploited, (ii) using just two hops is reasonable for many practical scenarios, and (iii) the advantages of the studied relaying schemes decrease for higher desired end-to-end spectral efficiency.

On the performance of cooperative diversity protocols in practical wireless systems

The concepts of cooperative diversity promise to offer the benefits of spatial diversity gains to handheld wireless devices with single antennas. The information-theoretic bounds that have been established recently serve as basic guidelines; yet, the performance of such protocols should additionally be examined for more realistic assumptions. Towards this end, we study cooperative diversity protocols for systems employing limited modulation alphabets and realistic receiver structures regarding the knowledge of channel state information. Our findings imply that under these conditions full second order diversity can only be achieved by using adaptive versions of cooperative protocols. As with other diversity schemes (e.g. space time block codes), our results for uncoded transmission can easily be combined with FEC techniques to obtain excellent error rate performance.

The impact of cooperation on diversity-exploiting protocols

Cooperative relaying is a recently developed concept that allows for providing single-antenna devices with gains from spatial diversity. So far, the performance of those schemes has mainly been investigated in comparison to conventional multiple-antenna systems and conventional relaying techniques. Yet, the cooperation of mobile terminals offers another important enhancement. Whenever other sources of diversity are scarce, the transmission over a statistically independent relay path can provide a significant amount of spatial diversity then to be exploited by error correction techniques to effectively combat fading effects. We therefore examine the performance of cooperative relaying protocols in slow and fast fading regimes, in comparison to approaches that exploit temporal diversity. Our results imply that user cooperation is a powerful means of enhancing link level performance in environments where temporal diversity is limited and delay constraints preclude the use of larger interleavers.

A Fixed-Complexity Smart Candidate Adding Algorithm for Soft-Output MIMO Detection

We present a soft-output multiple-input multiple-output (MIMO) detection algorithm that achieves near max-log optimal error rate performance with low- and fixed-computational complexity. The proposed smart ordering and candidate adding (SOCA) algorithm combines a smart-ordered QR decomposition with smart candidate adding and a parallel layer-by-layer search of the detection tree. In contrast to prior algorithms that use smart candidate adding, the proposed algorithm has fixed computational complexity, and it never visits a node more than once. Results indicate that the SOCA algorithm has an attractive performance-complexity profile for both fast and slow fading 4 × 4 and 8 × 8 MIMO channels with quadrature amplitude modulation (QAM) inputs.

Linear Mimo Receivers vs. Tree Search Detection: A Performance Comparison Overview

Next generation wireless systems use a combination of large system bandwidths and multiple antennas (MIMO) to deliver very high data rate services. The efficient demodulation of MIMO signals at the receiver can be a challenging task. In this paper, we study the gains achievable by using capacity-approaching tree search MIMO detection algorithms, for differently parameterized OFDM system setups and channel scenarios. Our results indicate that the high amount of diversity typically available in broadband MIMO-OFDM systems together with the use of low rate channel coding allows achieving very good performance using even simple linear MMSE detection. In iterative receiver approaches, the combination of initial linear MMSE detection and subsequent soft interference cancellation performs within 1 - 2 dB of the respective performance bound. The complexity involved in tree search detection techniques provides benefits mainly in very low-diversity environments, or if high rate channel coding is used together with higher order modulation

Iterative Phase Noise Mitigation in MIMO-OFDM Systems with Pilot Aided Channel Estimation

The use of multiple transmit and receive antennas in combination with multicarrier modulation, e.g. MIMO-OFDM, is a very promising technique for future wireless communication systems. In this work we investigate preamble based channel estimation under the presence of phase noise. Neglecting the influence of phase noise in the design of the preamble will lead to a significant loss in accuracy of the channel estimation. The solution involves an analysis of the mean square error approximation of the channel estimation and incorporating the result in the phase noise mitigation and data detection step.

Adaptive vs. Hybrid Iterative MIMO Receivers Based on MMSE Linear and Soft-SIC Detection

In this paper, we propose the use of a combination of linear MMSE and soft successive interference cancellation based detectors for the application in (turbo-)MIMO receivers. In an adaptive setup we switch between the two detector types based on the quality of the received signal, while in a hybrid setup, we use a linear MMSE in the first and a SoftSIC in all subsequent iterations. We show that for a number of scenarios, both techniques show performance close to that of more sophisticated detection techniques, at a fraction of the complexity

Exploiting Phase Noise Properties in the Design of MIMO-OFDM Receivers

Phase noise (PN) is a serious challenge for multi- carrier systems as it can drastically decrease the system performance. In order to tackle this problem, recent contributions propose iterative approaches which compensate both the common phase error and the intercarrier interference (ICI) resulting from PN. However, the fact that the ICI is not Gaussian distributed has so far not been taken into account in the calculation of detector soft output. Furthermore, the ICI is typically approximated by a truncated Fourier series, implicitly assuming a periodicity of the PN trajectory - which is in general not the case. In this paper we will address these problems and show that taking the PN properties properly into account allows to significantly improve the system performance.