Downlink Harq Enhancement For Listen-Before-Talk Based Lte In Unlicensed Spectrum

In order to extend the available spectrum to deal with the dramatically increasing data traffic, unlicensed spectrum, e.g., 5 GHz, is considered by the mobile operators as a complement in addition to licensed spectrum. The licensed-assisted access using Long Term Evolution (LAA-LTE) in unlicensed spectrum has been studied in the 3rd Generation Partnership Project (3GPP) Release 13 to achieve more efficient usage of unlicensed spectrum and guarantee fair coexistence with other systems. According to European regulatory requirements, listen-before-talk (LBT) function should be introduced for LAA-LTE. However, after LBT is introduced for time division duplexing (TDD) LAA-LTE, conventional feedback timing may be broken when the channel is busy and corresponding subframes cannot be transmitted. Therefore, downlink hybrid automatic repeat request (HARQ) enhancement needs to be considered for LBT based TDD LAA-LTE. In this paper, we mainly focus on the downlink HARQ timing issue. Three methods are proposed to defer the feedback to following available subframes by new HARQ timing design. Numerical results show that the proposed methods can largely reduce the retransmission probability and achieve high transmission efficiency.