Patrick Herhold

Relay-based deployment concepts for wireless and mobile broadband radio

In recent years, there has been an upsurge of interest in multihop-augmented infrastructure-based networks in both the industry and academia, such as the seed concept in 3GPP, mesh networks in IEEE 802.16, and converge extension of HiperLAN/2 through relays or user-cooperative diversity mesh networks. This article, a synopsis of numerous contributions to the working group 4 of the wireless world research forum and other research work, presents an overview of important topics and applications in the context of relaying. It covers different approaches to exploiting the benefits of multihop communications via relays, such as solutions for radio range extension in mobile and wireless broadband cellular networks (trading range for capacity), and solutions to combat shadowing at high radio frequencies. Furthermore, relaying is presented as a means to reduce infrastructure deployment costs. It is also shown that through the exploitation of spatial diversity, multihop relaying can enhance capacity in cellular networks. We wish to emphasize that while this article focuses on fixed relays, many of the concepts presented can also be applied to systems with moving relays.

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.

Relaying in CDMA networks: pathloss reduction and transmit power savings

Relaying has emerged as a field of growing interest for wireless systems. The use of intermediate nodes for relaying information from a source to its destination promises improvements on various levels, ranging from increased connectivity and reduced transmit powers to diversity gains. We examine various propagation models and network parameters and show to which extent the pathloss in cellular wireless systems can be reduced by the use of relay nodes in a two-hop scenario. Having highlighted these potentials, we discuss by means of numerical analysis and system-level simulation under which conditions these savings can be turned into a transmit power reduction for CDMA FDD systems. It becomes evident that the overall performance of the relay system depends on the node density and the relative load.