Time Domain Signal Processing for Broadband Cooperative Relaying

Cooperative relay network

The use of diversity in the spatial and temporal dimension to mitigate the effects of fading and therefore to increase the reliability of radio links in wireless networks is a well known technique for systems with co-located antennas (space-time coding).

Recently a new form of realizing spatial diversity has been introduced in [1] called cooperative diversity or user cooperation diversity. The main idea is to use multiple single antenna nodes as a virtual macro antenna array, realizing spatial diversity in a distributed fashion. In such a network several, maybe idle, nodes serve as relays for an active source-destination pair. Relays can be classified as either decode-and-forward (DF) or amplify-and-forward (AF) relays. AF relays, which are considered in this project, only retransmit an amplified version of their received signals. This leads to low-complexity relay transceivers and to lower power consumption since there is no signal processing for decoding procedures. Moreover, AF relays are transparent to adaptive modulation techniques which may be employed by the source. One disadvantage of AF relays is, that they also amplify their own noise.

In this project the advantage of transparency of AF relays should be maintained even in the case of broadband communication between source and destination. In this case the broadband source signals are transmitted by means of OFDM. It assumed that the parameters of the OFDM, (e.g., number of frequency bins or length of cyclic prefix) are only known to source and destination. The relays do not have this knowledge about these parameters. They only operate in the same bandwidth and employ time-domain signal processing techniques (e.g., filtering) to the received signals. They don't process the data in the frequency domain by using FFT and IFFT.

The goal of the project is to explore the trade-offs with respect to performance and implementation complexity. As reference scenario serves the in [2] proposed cooperative diversity scheme.

[1] J. N. Laneman, D. N. Tse, and G. W. Wornell, ''Cooperative diversity in wireless networks: Efficient protocols and outage behavior,'' submitted to IEEE Trans. Inform. Theory, 2002.
[2] I. Hammerstroem, M. Kuhn, and A. Wittneben, ''Cooperative diversity by relay phase rotations in block fading environments,'' in Proc. SPAWC, Lisbon, Portugal, July, 2004.

Type Student project (Semesterarbeit)
Semester Summer 2005
Subject area Wireless relay networks, cooperative diversity, OFDM
Type of work 50% Theory, 25% Software, and 25% Simulations
Student Stefan Zwicky
Supervisor Dr. Ingmar Hammerström
Professor Prof. Dr. Armin Wittneben