The Listen-Before-Talk (LBT) is the main procedure for Licensed Assisted Access (LAA) to accomplish fair and friendly coexistence with other operators or technologies operating over unlicensed spectrum. However, in LBT, the lack of coordination with other existing systems brings challenges in sustaining performance in the coexistence of LAA and WiFi networks. Specifically, the hidden node problem (HNP) and exposed node problem (ENP) cannot be effectively handled when both LAA and WiFi nodes attempt to access the unlicensed spectrum at the same time. Thus, transmission failure might occur and the network performance would be degraded. In order to mitigate the influences caused by HNP and ENP, based on LBT, we firstly analyze HNP and ENP by means of mathematical approach. The analytical results surprisingly reveal that the hidden node and exposed node probabilities are as high as 41% and 39.33%, respectively. Then, a Give And p-persistent Take (GAT) mechanism with the Listen-Before-Receive (LBR) procedure, namely LBR-GAT, is proposed to cope with LBT to reduce the collision caused by HNP as well as to retrieve the bandwidth sacrificed by the ENP. With LBR-GAT, the LAA sender conditionally gives up or takes back transmission opportunities, and thus the unlicensed spectrum could be efficiently shared between LAA and WiFi. Evaluation results show that the proposed LBR-GAT could conditionally obtain better network performance comparing to legacy LBT.

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The existing architectures used in the multiparty audio conferencing systems are typically categorized as either centralized or decentralized. These architectures expose a trade-off between processing latency and system capacity, namely the number of participants. This paper proposes a multiparty audio conferencing system for mobile users to improve the processing latency and system capacity. Instead of using the pure centralized or decentralized architecture, the proposed system adopts a novel cooperation-based architecture, in which only some participants are selected as the central controllers to deal with the tasks such as acoustic echo cancellation, encoding, decoding, mixing and de-mixing. The proposed system also uses a buffer reordering scheme to solve the problems of network jitters and out-of-order packets. This study analyzed the processing latency of the multiparty audio conferencing systems using the existing and proposed architectures. We also implemented these systems on diverse mobile platforms to compare the processing latency and number of participants. Performance evaluation results confirmed that the proposed cooperation-based architecture can not only reduce the processing latency but also support more participants, compared to the existing architectures.