3GPP cellular communications in unlicensed spectrum allow transmission only after completing listen-before-talk (LBT) operation. For downlink, the LBT operation helps cellular traffic to coexist well with Wi-Fi traffic. However, cellular uplink transmission is attempted only at the time specifically determined by the base station after having a successful LBT and the user equipment (UE) may suffer transmission failure and delayed transmission due to Wi-Fi interference. As a result, cellular uplink traffic does not coexist well with Wi-Fi traffic. This paper mathematically analyzes the problem of unfairness between cellular and Wi-Fi for uplink channel access. To address the coexistence problem in unlicensed spectrum, we propose a standard-compliant approach, termed UpChance, which allows the UE to use a minimum length of uplink reservation signal and the base station to determine the optimal timing for the UE's uplink transmission. Through ns-3 simulation, we verify that UpChance improves the performance of fairness and random access completion time by up to 88% and 99%, respectively.
We show the inline selection of transmission or reflection spectrum of one of two fiber Bragg gratings (FBGs) with different Bragg wavelengths by incorporating a polarization-diversity loop without reconfiguring the filter structure. The proposed filtering apparatus consists of a fiber-pigtailed polarization beam splitter, two FBGs, and three quarter-wave plates (QWPs). Without optical switches and couplers, the proposed filter can flexibly choose the transmission or reflection spectrum of each FBG through proper control of the QWPs contained in the filter. The fabricated filter shows an average insertion loss of ∼4.43 dB, average band rejection ratio of ∼17.92 dB, and average side-mode suppression ratio of ∼19.73 dB.
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