To meet the high demand for mobile data, the Third Generation Partnership Project (3GPP) established a set of standards known as 5G New Radio (5G NR). The architecture of 5G NR includes a flexible radio access network and a core network. 3GPP has also been working on a new radio access technology, called 5G NR Unlicensed (5G NR-U), which aims at extending 5G NR to unlicensed bands. In this paper, we give an overview of the most recent 5G NR-U design elements and discuss potential concerns, including fair coexistence with other unlicensed technologies such as Wi-Fi. We use simulations to study coexistence between Wi-Fi and 5G NR-U systems. Our evaluation indicates that NR-U often achieves higher throughput and lower delay than Wi-Fi (802.11ac). The two systems experience different buffer occupancies and spectrum utilization statistics. We also discuss the improvements that NR-U offers over LTE Licensed Assisted Access (LTE-LAA). I. INTRODUCTION Next-generation wireless networks will support applications with widely diverse performance requirements. In its International Mobile Communications (IMT)-2020 recommendations, the International Telecommunications Union (ITU) specifies three use cases for next-generation wireless networks: Enhanced mobile broadband (eMBB), ultra-reliable and low latency communication (URLLC), and massive machine-type communication (mMTC). While these use cases embody different performance requirements, they all share the need for more spectrum. In its effort to extend 5G cellular operation to unlicensed spectrum, 3GPP is initially targeting the Unlicensed National Information Infrastructure (UNII) bands at 5 GHz and 6 GHz. Future specifications will address unlicensed millimeter wave (mmWave) bands at 60 GHz. Wireless systems can operate over unlicensed bands as long as they comply with spectrum regulations, which are intended to ensure harmonious coexistence of various incumbents that operate on the same band. The ubiquity of Wi-Fi networks makes achieving harmonious 5G NR-U and Wi-Fi coexistence a key objective for NR-U designers. To ensure fairness in channel access, NR-U should not impact an existing Wi-Fi system more than the impact of another Wi-Fi system [1]. Early works surveying 5G NR-U can be found in [2]-[5]. These works focused on pre-standard NR-U operation at sub-6 GHz and/or mmWave frequencies and discussed the feasibility of utilizing the channel access procedures of 'further enhanced' LTE LAA (feLAA) in 5G networks. The effectiveness of unlicensed bands for IoT applications was investigated in [6], where the authors studied challenges
Abstract-We propose multiple antenna transmission methods for fast fading channel conditions. Proposed methods allocate alternative multiple antenna transmission modes depending on the SNR, modulation order and Doppler frequency to increase reliability, i.e., decrease bit-error-rate (BER). A major difference of our approach from previous works is to allow different transmission modes during a single frame by considering the channel variations within the frame for high mobility scenarios. First method effectively makes use of available channel knowledge by allocating beamforming (BF) as long as channel knowledge is not outdated and switches to space frequency block coding (SFBC) afterwards. Approximate BER expressions for BF and SFBC that are functions of SNR, modulation order, Doppler frequency and initial channel knowledge are derived to be used as a decision metric for mode allocation throughout the frame. Second method adapts rate and transmission mode across symbols in a frame by considering that average channel power is monotonically decreasing as a function of time within the frame when multiuser diversity is exploited. First method performs as good as the better of BF and SFBC over all SNR values. Second method provides additional performance gain over the first method due to more efficient use of better channel conditions. Index Terms-MIMO, OFDMA, time varying channel, nonuniform channel knowledge, statistical bit loading, transmission mode allocation.
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