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A Fully Coordinated New Radio-Unlicensed System for Ultra-Reliable Low-Latency Applications
... Major distinctive features of LBT are that it meets regulatory requirements, as well as it is accepted by both cellular nodes and WiFi APs, because of its CSMA/CA-like operations [6]. 3GPP adopts LBT for LAA and NR-U technologies [71]. ...
... It is rather asynchronous and driven by demand [73][74]. Once the data is available for transmission, loadbased LBT nodes perform the channel access procedure immediately [71]. In doing so, load-based LBT nodes carry out the following steps [71]. ...
... Once the data is available for transmission, loadbased LBT nodes perform the channel access procedure immediately [71]. In doing so, load-based LBT nodes carry out the following steps [71]. Nodes execute a random backoff algorithm with variable contention window size. ...
This paper provides a comprehensive survey on the coexistence of cellular and IEEE 802.11 standards from a holistic viewpoint that takes into account the coexistence of all existing and future cellular and IEEE 802.11 standards in all the available unlicensed spectrum bands. Unlike existing survey works focusing mostly on any unlicensed band and/or standard, we start by giving an overview of unlicensed spectrum bands, including 2.4 GHz, 5 GHz, 6 GHz, and 60 GHz. We then review the operation of cellular technologies, namely Long-Term Evolution Unlicensed (LTE-U), Licensed Assisted Access (LAA), and New Radio Unlicensed (NR-U), worldwide in the unlicensed spectrum bands. Further, we summarize scenarios and categories of coexistence mechanisms, conditions for a fair coexistence, and coexistence-related features. An extensive study on the coexistence mechanisms, deployment scenarios, as well as standardization efforts for the coexistence between cellular and IEEE 802.11 standards, is carried out. Finally, we highlight the coexistence challenges and open problems, the convergence of the Third Generation Partnership Project (3GPP) and IEEE standards, as well as future research directions. Moreover, to provide insights on the relative measures, we also carry out comparative studies of several key concerns with regard to the coexistence, namely unlicensed spectrum band, regulatory requirement, coexistence mechanism, and cellular standardization effort. Each study presents a comparison among potential features of one of these concerns in tabular forms. Finally, we summarize key lessons that are learned and discussed throughout the paper.
... Park et al. [26] provide an analytical model of FBE to determine latency. Maldonado et al. [27] consider a fully coordinated FBE and LBE approach, where each FBE node calculates its uplink/downlink ratio during idle periods and communicates this to a central controller, which calculates an appropriate frame structure for the connected FBE nodes. Le et al. [28] propose dynamic switching between LBE and FBE in which the transmitting nodes switch the channel access mechanisms based on frame priority. ...
... Another interesting research topic is the coexistence with networks using LBE-based channel access (including Wi-Fi). Finally, our implementation can be extended by an additional controller module, which seems to be an inevitable element of such networks [27]. ...
Smart grid operators seeking to extend their wireless network capacity can use unlicensed bands. However, devices in these shared bands must follow rules such as Listen Before Talk (LBT), standardized by ETSI. In this paper, we focus on the performance of the frame-based equipment (FBE) version of LBT channel access. We design, implement, and validate a fully functional FBE channel access simulator. Next, we conduct an extensive performance analysis of the FBE variants encountered in the literature, focusing on channel efficiency and fairness in upper-bound and coexistence scenarios. Our study leads to several conclusions about the operation of FBE-based devices, including the need for proper configuration of channel access parameters to ensure fairness and optimal performance. We also observe generally poor coexistence among FBE variants: the highest Jain’s fairness index was only 0.88, with an average normalized channel efficiency of 0.76. Therefore, we identify several open research areas in the field, such as the need for further development of parameter adaptation algorithms, the deployment of an external controller to update channel access parameters, and new FBE designs with better coexistence qualities.
... On the other hand, FBE is an attractive channel access mode in controlled environments, where interference is exclusively caused by devices of the same network. Further details about both channel access schemes are found in [12], [13]. ...
... To cope with potential TDD frame selection misalignment and channel access failures, we adopt a fixed TDD frame structure during each gNB occupancy period. More details about the coordinated solution and its performance are documented in [13]. Aligning the downlink-to-uplink and uplink-to-downlink transitions within the TDD frame removes the uncertainty introduced by the channel access procedures. ...
The use of wireless communications in Industrial Internet of Things (IIoT) enables unparalleled levels of flexibility and instantaneous reconfiguration for autonomous industrial processes. In this paper, the focus is on optimizing and evaluating Wi-Fi 6 and 5G New Radio (NR) licensed and unlicensed wireless networks for meeting the packet latency and reliability requirements of critical IIoT applications. The study is based on extensive system simulations using a 3GPP-defined IIoT indoor factory framework and application traffic models. Each radio technology is individually optimized leveraging the pros and cons of that technology to maximize the carried load in the network while fulfilling the delay requirements at a specified reliability level of 99.999 %. In addition to a performance comparison, the paper also provides deployment guidance for applying each radio technology in the considered IIoT setting. With proposed latency aware scheduling and when operated in interference free spectrum, Wi-Fi 6 can support <1 ms applications at a very low load, whereas the performance gap with respect to 5G NR reduces as delay requirements are relaxed to 10-100 ms. Conditioned on the fulfilment of the application latency and reliability requirements, unlicensed 5G NR shows nearly 2x the spectral efficiency of Wi-Fi 6 in all available configurations. Licensed 5G NR shows generally the best performance, especially for delay requirement <1 ms, supporting 2-4x the spectral efficiency achievable by unlicensed technologies.
... In [45], a central entity is used in a fully coordinated FBE approach to configure a common Time Division Duplex (TDD) configuration among the nodes in the system so that the UE's UL transmission to a gNB is not blocked by the neighbor gNB DL transmission due to the misalignment of UL and DL slots among the gNBs. A common TDD configuration among the gNB nodes might not satisfy the specific requirements of each gNB network about the ratio of DL and UL transmissions. ...
The advent of new use cases and new applications such as augmented/virtual reality, industrial automation, autonomous vehicles, etc. in 5G has made the Third Generation Partnership Project (3GPP) specify Ultra-reliable low-latency communications (URLLC) as one of the service categories. To support URLLC with the strict requirements of reliability and latency, 3GPP Release 15 and Release 16 have specified the URLLC features in licensed spectrum. The ongoing 3GPP Release 17 extends the URLLC features to unlicensed spectrum to target the new use cases in the industrial scenario. In the first part of the thesis from Chapter 2 to Chapter 4, we focus on the URLLC in licensed spectrum. The first study deals with the problem of ensuring the configured number of uplink (UL) configured-grant (CG) repetitions of a transport block. Secondly, we study the collisions of an eMBB UL transmission of a user equipment (UE) and an URLLC UL transmission of another UE on the CG resources. Thirdly, the focus of this study is the downlink (DL) transmission where the feedback of the DL semi-persistent scheduling transmission is dropped due to the conflict of the DL/UL symbols. In the second part from Chapter 5 to Chapter 8, we focus on URLLC operation in unlicensed spectrum. In unlicensed spectrum, a 5G device is required to access to a channel by using load based equipment (LBE) or frame based equipment (FBE). The uncertainty of obtaining channel access through LBE or FBE can impede the achievement of the URLLC latency requirements. Therefore, the study of impact of LBE and FBE on URLLC transmission and the enhancements of LBE and FBE are needed.
... In [17], a central entity is used in fully coordinated FBE approach to configure a common Time Division Duplex (TDD) configuration among the nodes in the system so that the UE UL transmission to a gNB is not blocked by the neighbor gNB DL transmission due to the misalignment of UL and DL slots among the gNBs. A common TDD configuration among the gNB nodes might not satisfy the specific requirements of each gNB network about the ratio of DL and UL transmissions. ...
Ultra-reliable low-latency communication (URLLC) is defined in 5G New Radio to support the applications such as Industry 4.0, remote surgery, etc. URLLC features have initially been defined for licensed spectrum. However, due to an increase of traffic demand in licensed spectrum, URLLC operation is currently being extended to unlicensed spectrum in the ongoing Release 17, although the uncertainty of the required listen before talk affects URLLC operation. This paper focuses on frame based equipment (FBE) channel access mechanism. The operation of FBE in the controlled unlicensed environment is analyzed through a Markov chain to show that URLLC latency requirement limits the FBE performance causing a decrease of URLLC transmission's reliability. Based on the analysis, two schemes are proposed to improve the URLLC performance in FBE. The first scheme allows the transmitters to use multiple fixed frame period (FFP) configurations to sense and obtain the channel in URLLC latency budget. The second scheme configures the FFP's starting point of each transmitter based on its priority so that a high priority transmitter's transmission is not blocked by a low priority transmitter's transmission in all cases. The simulations show the benefits of these schemes to URLLC transmission compared to the conventional scheme.
In this tutorial we present recipes for dynamic systemlevel simulations (SLSs) of 5G and beyond cellular radio systems. A key ingredient for such SLSs is selection of proper models to make sure that the performance determining effects are properly reflected to ensure output of realistic radio performance results. We therefore present a significant number of SLS models and related methodologies for a variety of use cases. Our focus is on generally accepted models that are largely supported by academia and industrial players and adopted by 3GPP as being realistic. Among others, we touch on deployment models, traffic models, non-terrestrial cellular networks with satellites, SLS methodologies for Machine Learning (ML) enabled air-interface solutions, and many more. We also present several recommendations for best practices related to preparing and running detailed SLS campaigns, and agile software engineering considerations. Throughout the article we use the 3GPP defined 5G and 5G-Advanced systems to illustrate our points, extending it also into the 6G-era that is predicted to build on alike SLS methodologies and best practices.
In this paper, the achievable latency-reliability performance of a standalone cellular network over the 5 GHz unlicensed spectrum is analysed. Fulfilling strict latency-reliability requirements comes with significant challenges for unlicensed operation, especially due to mandatory channel access procedures. Using MulteFire as the reference system-model, an analysis of a highly realistic multi-cell network with bi-directional traffic shows that latency of 23 ms with a reliability level of 99.99% is achievable for low-loads, while latency is increased to 79 ms at high-loads. Different techniques are described to improve the system performance. First, a pre-emptive scheme to cope with continuous uplink listen before talk (LBT) failures for uplink control transmissions is proposed. It provides a latency reduction of 24% at low-loads with two transmission opportunities and 11% for high-loads with three opportunities. Secondly, the possibility of skipping LBT performance under given conditions is evaluated. This results in a lower uplink LBT failure rate which translates to a latency reduction of 8% for low-loads and up to 14% for high-loads, at 99.99% reliability. Thirdly, as an alternative to grant-based uplink, grant-free uplink is evaluated. Grant-free uplink achieves better performance than grant-based uplink at low-loads, offering 50% lower uplink latency. At high-loads, the gain of grant-free uplink decreases due to the high number of simultaneous transmissions.
In this article, we aim to address the question of how to exploit the unlicensed spectrum to achieve URLLC. Potential URLLC PHY mechanisms are reviewed and then compared via simulations to demonstrate their potential benefits to URLLC. Although a number of important PHY techniques help with URLLC, the PHY layer exhibits an intrinsic trade-off between latency and reliability, posed by limited and unstable wireless channels. We then explore MAC mechanisms and discuss multi-channel strategies for achieving low-latency LTE unlicensed band access. We demonstrate, via simulations, that the periods without access to the unlicensed band can be substantially reduced by maintaining channel access processes on multiple unlicensed channels, choosing the channels intelligently, and implementing RTS/CTS.
In wireless networks with smaller cell sizes and dense network deployments the spectral efficiency is limited by inter-cell interference. To improve signal-to-noise-plus-interference-ratio, interference can be mitigated using advanced receivers like an interference rejection combining (IRC) receiver. The performance of a practical IRC-receiver is dependent on the quality of the channel and interference covariance estimation. Interference covariance can be estimated for example by using reference signals. As the interference structure is dependent on several components, such as scheduler decisions made and precoders used by other cells, receiver evaluations are performed on system level. Because of full network system level simulations are very complex and highly computationally intensive, simplifications in modeling are needed. Typically time-frequency resolution can be reduced and only the fast-fading coefficients of the interfering links are generated. The lack of symbol samples requires a model to estimate the losses of realistic IRC receivers. In this paper, we show system level results for practical IRC algorithms.
In this study we analyze the downlink OFDMA system level performance for three different channel quality indicator (CQI) reporting schemes. The effect of terminal measurement and estimation errors, quantization from formatting and compression, and uplink reporting delays and detection errors are included. We find that a simple threshold-based CQI scheme provides an attractive trade-off between downlink system level performance and uplink CQI signaling overhead, as compared to using a best-M scheme. When applied to the UTRAN LTE system in a 10 MHz bandwidth, we find that a frequency domain packet scheduling gain of 40% is achievable with a CQI word size of only 30-bits. Finally, the effect of applying a so-called outer loop link adaptation algorithm is reported.
MulteFire Release 1.0 Technical Paper: A New Way to Wireless
- M Alliance