  Deployment of Long Term Evolution (LTE) in unlicensed spectrum has been a
candidate feature to meet the explosive growth of traffic demand since 3GPP
release 13. To further explore the advantage of unlicensed bands, in this
context the operation of both uplink and downlink has been supported and
studied in the subsequent releases. However, it has been identified that
scheduled uplink transmission performance in unlicensed spectrum is
significantly degraded due to the double listen-before-talk (LBT) requirements
at both eNB when sending the uplink grant, and at the scheduled UEs before
transmission. In this paper, in order to overcome this issue, a novel uplink
transmission scheme, which does not require any grant, is proposed, and the
details regarding the system design are provided. By modeling the dynamics in
time of the LBT for both a system that employs a conventional uplink scheme, as
well as the proposed scheme, it is verified through analytical evaluation that
the double LBT scheme for uplink transmission greatly reduces the channel
access probability for the UE, and leads consequently to performance loss,
while the proposed scheme is able to alleviate this issue. System level
simulation results, compliant with the LTE standard, show that the proposed
scheme can achieve a significant performance gain in terms of throughput with
negligible performance loss for the downlink, and other technologies operating
in the same spectrum.
