Enabling Frequency Reuse for Licensed-Assisted Access with Listen-before-talk in Unlicensed Bands †
Hua Wang† , Markku Kuusela‡ , Claudio Rosa , and Antti Sorri‡ Wireless Communication Networks, Aalborg University, Aalborg, Denmark ‡ Nokia Networks, Espoo, Finland Nokia Networks, Aalborg, Denmark Email: †
[email protected]
Abstract—Licensed-Assisted Access (LAA) which allows LTE to be deployed in unlicensed spectrum is a promising technique currently under standardization in 3GPP Rel-13. To guarantee a peaceful coexistence of LAA with the incumbent systems such as WiFi, listen-before-talk (LBT) which requires the node to sense the medium before data transmission is introduced. On the other hand, LBT also prevents simultaneous transmission from neighbouring nodes of the same LAA operator. In order to increase the spectral efficiency of LAA, frequency reuse for transmission by neighbouring nodes of the same operator has been agreed to be one target of LAA design. In this paper we evaluate the performance of LAA in an indoor scenario assuming both one and two LAA networks deployed by different operators. It is shown that LBT is indeed needed to ensure good performance with multiple LAA operators, especially at high load. And frequency reuse between neighbouring nodes of the same LAA operator can further improve the performance.
I. I NTRODUCTION The exploration of LTE deployment in unlicensed spectrum is a promising solution due to the limited availability of licensed spectrum. It is also known as Licensed-Assisted Access (LAA) using LTE [1]. LAA has been approved as a Rel-13 work item in 3GPP and is expected to be one of the key features of Rel-13. The focus of LAA is using the existing carrier aggregation framework to aggregate carriers of a primary cell on licensed spectrum and a secondary cell on unlicensed spectrum to boost downlink data rate. LAA using LTE is attractive because of the large spectrum availability in the frequency band between 5 and 6 GHz. However, since other wireless systems such as WiFi are already deployed in this band, significant care must be taken to ensure the co-existence of LTE and WiFi. The system performance with plain coexistence of LTE and WiFi was studied in [2][3]. Simulation results show that without any coexistence schemes WiFi performance is seriously affected by LAA while LAA performance is only slightly affected. Therefore appropriate coexistence schemes are needed in order to protect the performance of incumbent WiFi system from the impacts of LAA. In [4][5], the LTE muting scheme was evaluated in which the LTE eNBs mute some of the subframes to increase WiFi transmission opportunity. In [6][7], the use of listen-before-talk (LBT) was proposed and analyzed, which requires all LTE eNBs to sense the medium before transmission. A comparison of different coexistence schemes between LAA and WiFi systems is studied in [8]. Besides,
coexistence of LAA with other LAA operators using LBT is considered in [9]. Finally, a comprehensive co-existence study of LAA with both WiFi and other LAA systems is included in [10]. It was generally agreed that LBT is among the functionalities identified to be required for an LAA system to coexist with WiFi and other LAA systems. However, the LBT procedure can prevent frequency reuse operation between LAA eNBs belonging to the same LAA network. An LAA eNB already transmitting on a channel can simply block neighbouring eNBs of the same LAA network from simultaneously accessing the channel. On the other hand, frequency reuse operation within the cells of the same network is one of the key mechanisms for achieving high spectral efficiency in LTE, as LTE is designed to cope with interference. In this paper, we discuss possible design options for enabling frequency reuse one with LBT. The performance both with and without frequency reuse one is evaluated in an indoor scenario with single/mutiple LAA operators. As coexistence between LAA and WiFi has been evaluated extensively in the open literature, we focus on LAA performance and coexistence between two LAA networks belonging to different operators in this paper. The rest of the paper is organized as follows: Section II provides an overview of LAA, and the various LBT schemes with/without frequency reuse. The considered network model is presented in Section III. Performance results are analyzed in Section IV. Finally, conclusions are drawn in Section V. II. L ICENSED -A SSISTED ACCESS To allow coexistence with WiFi and other LAA systems, LTE nodes operating with LAA in unlicensed spectrum must fulfill the LBT rules defined by ETSI [11]. The basic idea of LBT is that prior to data transmission, an LAA eNB has to sense the channel for a period of time and if no ongoing transmission is observed, the eNB can start its transmission. There are two approaches defined by ETSI: Frame Based Equipment (FBE), and Load Based Equipment (LBE) [11]. LBE is a good solution for LTE LAA downlink operation because it allows for maximized downlink throughput and minimized scheduling delays. Therefore LBE has been agreed as baseline for LAA [10], and we mainly focus on LBE in this paper. The different LBT operation schemes we considered are described in the following subsections.
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