Selective relay-activation for conditional DF relaying

S. H. Song, Q. T. Zhang, Khaled Letaief

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1 Citation (Scopus)


This paper considers a conditional decode-and-forward (DF) based cooperative system where a source (S) with multiple (M) antennas transmits information to a single-antenna destination (D) with the help of multiple (L ≤ M-1) single-antenna relays (Ri). The optimal transmit weighting vector at the source is not available in the literature due to the non-linear conditional DF operation, which renders the problem non-convex. To solve this problem, we first show that the optimal transmit vector for the single-relay system can be determined by comparing the S-D beamformer (maximum-ratio-transmit beamforming vector for the S-D link) with the one that utilizes just sufficient energy to activate the relay-link. However, it is difficult to directly apply the above idea to the multi-relay system, due to the fact that the S-R and S-D links are normally not orthogonal. To tackle this issue, we propose to utilize basis functions that are orthogonal to the S-D and S-R links, respectively, which enables the activating of one S-R link without considering the S-D link and the other S-R links. We then apply the new basis functions to the multi-relay system and propose a selective relay-activation algorithm, where the optimal solution is obtained by comparing the S-D beamformer with schemes that selectively activate different combinations of the relay-links. The selective relay-activation algorithm is different from the conventional water-filling in the sense that the energy is filled to discrete levels to activate the S-R links, a unique feature arising from the conditional DF operation.

Original languageEnglish
Article number6742723
Pages (from-to)888-899
Number of pages12
JournalIEEE Transactions on Communications
Issue number3
Publication statusPublished - 2014
Externally publishedYes



  • Cooperative system
  • decode-and-forward
  • selective water-filling

ASJC Scopus subject areas

  • Electrical and Electronic Engineering

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