Listen-before-talk Research Articles

NR-U and IEEE 802.11 Technologies Coexistence in Unlicensed mmWave Spectrum: Models and Evaluation

New Radio-based access to Unlicensed spectrum (NR-U) intends to expand the applicability of 5G NR access technology to support operation in unlicensed bands by adhering to Listen-Before-Talk (LBT) requirement for accessing the channel. As the NR-U specification is being developed, simulations to assess the performance of NR-U and IEEE 802.11 technologies coexistence in unlicensed spectrum bands are crucial. In this paper, we report on extensions to a popular and open source network simulator, ns-3, to build an NR-U system-level simulator and to model the NR-U and IEEE 802.11 technologies coexistence in the currently available unlicensed spectrum bands. The proposed NR-U model capitalizes on an NR Release-15 based model that has been extended to operate in unlicensed bands, while meeting its regulatory requirements. For the coexistence analysis, we pay particular attention to the millimeter-wave bands and provide a complete set of simulation campaigns evaluating the coexistence of NR-U and IEEE 802.11ad Wireless Gigabit (WiGig) in the 60 GHz bands. In particular, we focus on determining whether NR-U fulfills its coexistence objective in terms of a fairness criterion by testing different NR-U parameters, such as the numerology, the bandwidth, the channel access scheme, the energy detection threshold, and the beamforming method.

Hybrid channel access mechanism based on coexistence scenario of NR-unlicensed

With the rapid development of 5G NR (New Radio), the explosive increment of traffic amount is calling the utilization of unlicensed band. 3GPP has proposed LAA (Licensed Assisted Access) to use LTE in unlicensed band and pointed out that NR-U (NR-Unlicensed) can reuse most designs of it. However, the existing channel access mechanism of LAA is conservative under the coexistence scenario of NR-U, which leads to the waste of time resource. To address the problem this paper proposes a hybrid channel access mechanism to take advantage of the LBT (Listen-Before-Talk) mechanism of LAA when channel is quite busy and transmit directly with reduced power when it is relatively idle. The channel busy degree is judged by a series of periodically updated adaptive thresholds. System-level simulation verifies that under the coexistence scenario of NR-U the proposed mechanism can achieve higher UPT (User Perceived Throughput) and lower delay than other channel access mechanisms.

Group-Wise Listen-Before-Talk Protocol for Dynamic Spectrum Sharing: A New Framework for Full Frequency Reuse

In existing dynamic spectrum sharing (DSS) systems, each node arbitrates channel access independently based on carrier-sensing mechanisms such as the listen-before-talk (LBT) protocol. Owing to the uncoordinated channel access between all nodes, the channel is occupied in a random pattern. This makes it difficult to reduce the mismatch in channel quality indicator (CQI) while increasing the spatial reuse gain between the different nodes; therefore, the areal capacity gain obtained by adding nodes is much lower in existing DSS systems than in the cellular system. In this paper, we propose a different means of improving the areal capacity for downlink DSS systems. It is a group-wise DSS approach that ensures full frequency reuse in each group of base stations (BSs) by performing LBT only between the representative BSs, each of them selected by each group. Once a channel is secured by each group, all its member BSs share a channel simultaneously. This approach makes closed-loop feedback-based link adaptation practical while boosting spatial reuse gain even in the DSS environment. To implement the group-wise DSS, we propose a single unified framework that employs elementary algorithms for BS grouping and carrier-sensing threshold adjustment. Our system-level simulation results demonstrate that the proposed framework boosts the areal capacity gain by approximately 4.42 times as much as the conventional approach.

(ReLBT): A Reinforcement learning-enabled listen before talk mechanism for LTE-LAA and Wi-Fi coexistence in IoT

The emergence of Internet of Things (IoT) has increased number of connected devices and consequently transmitted traffic over the Internet. In this regard, Long Term Evolution (LTE) is growing its utilization in unlicensed spectrum as well, and Licensed Assisted Access (LAA) technology is one of the examples. However, unlicensed spectrum is already occupied by other wireless technologies, such as Wi-Fi. The diverse and dissimilar physical layer and medium access control (MAC) layer configurations of LTE-LAA and Wi-Fi lead to coexistence challenges in the network. Currently, LTE-LAA uses a listen-before-talk (LBT) mechanism, and Wi-Fi uses a carrier sense multiple access with collision avoidance (CSMA/CA) as a channel access mechanism. LBT and CSMA/CA are moderately similar channel access mechanisms. However, there is an efficient coexistence issue when these two technologies coexist. Therefore, this paper proposes a Reinforcement Learning-enabled LBT (ReLBT) mechanism for efficient coexistence of LTE-LAA and Wi-Fi scenarios. Specifically, ReLBT utilizes a channel collision probability as a reward function to optimize its channel access parameters. Simulation results show that the proposed ReLBT mechanism efficiently enhances the coexistence of LTE-LAA and Wi-Fi as compared to the LBT, thus improves fairness performance.

Wi-Fi and LTE-LAA coexistence problems, challenges and features in 5GHz unlicensed bandwidth

The demand for high data rate and stable Quality of Service (QoS) from cellular networks’ subscribers is a huge challenge for cellular operators. To overcome this issue (providing high data rate and stable QoS) it requires more bandwidth. The lack of licensed bandwidth and also its high cost make it hard for operators to overcome this issue. To find a fair solution and expand the capacity of cellular networks, operators started deployment of Long Term Evolution (LTE) in unlicensed bandwidth, exactly utilizing 5GHz unlicensed bandwidth in small cell scenarios. The extension of LTE to the 5GHz unlicensed bandwidth where it is already in used by Wi-Fi will create huge coexistence problems between Wi-Fi and LTE. It results from that Wi-Fi and LTE have different channel access mechanisms. This paper presents the channel access mechanisms of Wi-Fi and LTE-Licensed Assisted Access (LTE-LAA), coexistence problems, challenges, privileges, coexistence features of LTE and Wi-Fi and Finally, we conducted the simulation for LAA based on Listen Before Talk (LBT) mechanism and Wi-Fi to find that how LAA with LBT mechanism is friendly to the Wi-Fi networks when they share the same bandwidth. The simulation results illustrate that LAA based on LBT channel access mechanism by alone cannot bring a fair coexistence between these two technologies, it needs further improvement.

An IoT Architecture for Continuous Livestock Monitoring Using LoRa LPWAN

The Internet of Things (IoT) architecture is quickly becoming popular even outside of its originating scenario of home automation. This paper reports the design, implementation, and performance of an IoT hardware and software architecture conceived for the continuous monitoring of livestock located in barns and during grazing. We have adopted the LoRa low power wide area network (LPWAN) technology to cover the diverse environments, and a suitable configuration of web services to perform data storage, analysis, and visualization. Since the LoRa LPWAN (LoRaWAN) medium access control (MAC) layer does not provide a listen-before-talk (LBT) mechanism, we propose a custom MAC layer with LBT-based carrier-sense multiple access with collision avoidance (CSMA/CA). The devised system has been implemented using off-the-shelf hardware, and its performance has also been estimated with the help of a C++ event-based simulator. The preliminary results of our HW implementation on the field confirm the stability of the conceived system and its reliability.

Evaluation of Listen Before Talk (LBT) mechanism fairness at LTE-Licensed Assisted Access (LAA) against Wi-Fi 5 GHz

LTE Release 13 allow the use of unlicensed frequency variations called Licensed Assisted Access (LAA). Combination of licensed LTE is used as primary cells to carry information and transport traffic data. Whereas the unlicensed spectrum is used as secondary cells that are proposed to increase capacity and distribute the spectrum fairly. The simulation results of scenario 1 produce a fairness index of 64 users for FTP over UDP in scenario 1 of 0.998, scenario 2 is 0.857 and scenario 3 is 0.997. Whereas for FTP over TCP in scenario 1 is 0.779, scenario 2 is 0.639 and scenario 3 is 0.74.

New Radio Beam-Based Access to Unlicensed Spectrum: Design Challenges and Solutions

This paper elaborates on the design challenges, opportunities, and solutions for New Radio-based access to Unlicensed spectrum (NR-U) by taking into account the beam-based transmissions and the worldwide regulatory requirements. NR-U intends to expand the applicability of 5th generation New Radio access technology to support operation in unlicensed bands by adhering to Listen-Before-Talk (LBT) requirements for accessing the channel. LBT was already adopted by different variants of 4th generation Long Term Evolution (LTE) in unlicensed spectrum, i.e., Licensed-Assisted Access and MulteFire, to guarantee fair coexistence among different radio access technologies. In the case of beam-based transmissions, the NR-U coexistence framework is significantly different as compared to LTE in unlicensed spectrum due to the use of directional antennas, which enhance the spatial reuse but also complicate the interference management. In particular, beam-based transmissions are needed in the unlicensed spectrum at millimeter-wave (mmWave) bands, which is an attractive candidate for NR-U due to its large amount of allocated spectrum. As a consequence, some major design principles need to be revisited to address coexistence for beam-based NR-U. In this paper, different problems and the potential solutions related to channel access procedures, frame structure, initial access procedures, re-transmission procedures, and scheduling schemes are discussed. A simulation evaluation of different LBT-based channel access procedures for NR-U/Wi-Fi indoor mmWave coexistence scenarios is also provided.

An Optimal Channel Occupation Time Adjustment Method for LBE in Unlicensed Spectrum

Load Based Equipment (LBE) mechanism is a category of Listen-Before-Talk (LBT) protocol adopted by LTE to access unlicensed channels to realize fair coexistence between LTE and Wi-Fi networks. However, most LBE optimization methods fix Channel Occupation Time (COT) in LBE neglecting its influence on network throughput and fairness. In the paper, considering the influence, we propose an optimal COT adjustment method for the LBE mechanism to maximize the throughput of LTE while satisfying the LTE users data rate demands and ensuring the fairness coexistence between LTE and Wi-Fi networks. To this end, we first propose a method to separately calculate the throughput of LTE network and Wi-Fi network on the coexistence unlicensed spectrum considering the synchronous of LTE on licensed and unlicensed spectrum, and then utilize a virtual Wi-Fi network construction method to assure the used fairness criterion. Based on the aforementioned methods the problem of throughput maximization of LTE is formulated as a constrained non-linear optimization problem solved by an optimal algorithm. Simulation results prove our proposed throughput calculation method is valid, and then demonstrate the LBE mechanism using our proposed method optimizes the throughput of the SBS network and ensures fairness.

Performance Modeling and Analysis of the LAA Category-4 LBT Procedure

This paper studies the modeling of an LTE LAA category-4 listen before talk (LBT) procedure for downlink data transmission in an unlicensed spectrum band, and analyzes the performance of the LTE LAA category-4 LBT procedure based on the modeling. A 3-D Markov model is first developed to describe the category-4 LBT procedure of an LAA eNodeB. The 3-D Markov model takes into account the GAP period and transmission priority in the category-4 LBT procedure, which are not considered in all existing models for LTE LAA LBT procedures. Based on the 3-D Markov model, a performance model is derived for analyzing the steady-state transmission probability, normalized throughput, and mean access delay of an LAA eNodeB, respectively. The derived performance models are justified through simulation results. The impacts of the GAP period and transmission priority on the system performance are investigated through numerical results, respectively.

A Chaotic Q-learning-Based Licensed Assisted Access Scheme Over the Unlicensed Spectrum

To meet the ever-increasing demand for mobile data traffic, mobile operators are seeking to utilize unlicensed spectrum as a supplement to the licensed spectrum. The harmonious and spectrum-efficient coexistence scheme between LTE and incumbent users on the unlicensed spectrum is thus mandatory. Currently, advanced intelligent technologies are being expected to play the crucial role in the future communication system. We thus introduce the Q-learning (QL) framework into LTE licensed assisted access (LAA) scheme in the paper, thereby forming a QL based LAA scheme. We first redefine the fairness in the sharing of unlicensed spectrum and then divide the state space into six states based on the predefined throughput and fairness thresholds, followed by the definition of the action set and reward function. In the proposed scheme, based on the convergent Q table, where each element is used to evaluate the pros and cons of taking an action, the agent can repeatedly interact with the environment until it reaches the terminal state, i.e. selects the optimal action (i.e. contention window size). Additionally, the chaotic motion with ergodicity, regularity and randomness is first introduced into the action-decision strategy to accelerate the training velocity with the balance consideration of exploration and exploitation. The simulation results prove that the proposed $\epsilon$ -chaotic greedy selection strategy has faster convergence velocity compared with other methods such as $\epsilon$ -greedy, pure greedy, Bolzmann and random selection strategy, and that the proposed chaotic QL LAA scheme outperforms the other LAA schemes such as the 3GPP, linear, fixed LAA and Listen Before Talk (LBT) adaptive schemes in terms of throughput, collision probability, fairness and delay.

Learning-Theoretic Multi-Channel Spectrum Sensing and Access in Full-Duplex Cognitive Radio Networks with Unknown Primary User Activities

The majority of the opportunistic spectrum access schemes in cognitive radio networks (CRNs) rely on the Listen-Before-Talk (LBT) model due to the half-duplex nature of conventional wireless radios. In this paper, we consider the problem of optimal opportunistic multi-channel spectrum sensing and access using full-duplex (FD) radios in the presence of uncertain primary user (PU) activity statistics. A joint learning and spectrum access scheme is proposed. To optimize its throughput, the SU sensing period has to be carefully tuned. However, in absence of exact knowledge of the PU activity statistics, the PU's performance may be adversely affected. To address this problem, we formulate a robust optimization problem. Our analysis shows that under some non-restrictive simplifying assumptions, the robust optimization problem is convex. We analyze the impact of the sensing period on the PU collision probability and the SU throughput, and find the optimal sensing period via convex optimization. We show that sublinear regrets can be attained by the proposed estimation and robust optimization strategy. Simulation studies also demonstrate that the resulting robust solution provides a good trade-off between optimizing the SU's throughput and protecting the PU.

Hybrid Adaptive Channel Access for LTE-U Systems

To alleviate the shortage of spectrum resource on licensed bands, long-term evolution in unlicensed bands (LTE-U) has been proposed. To facilitate the fair coexistence between LTE-U and the legacy WiFi networks on unlicensed bands, two well-known channel access mechanisms named as duty-cycle muting (DCM) and listen-before-talk (LBT) have been proposed and studied extensively. Accordingly, in this paper, a unified hybrid adaptive channel access scheme is proposed, which takes advantages of both the DCM and LBT mechanisms. Our proposal can realize the flexible handoff between the DCM and LBT mechanisms to meet the requirements of different markets. It can also adaptively adjust the important parameters, such as the back-off window size and the duty-cycle time fraction based on the WiFi traffic and the available licensed spectrum resource while guaranteeing the fair coexistence between the WiFi and LTE-U systems. Moreover, joint spectrum and transmission power allocation is also studied to improve the spectra efficiency (SE) on both licensed and unlicensed bands. Finally, analytical and numerical results are demonstrated to verify the effectiveness of the proposed scheme.

Enhancement of Opportunistic Co-Occurrence of U-LTE and Wi-Fi/Iot in 5 GHz Using Cognitive Radio

In this modern era, Unlicensed Long-Term Evolution (U-LTE) is the most interested research area to enable its access in 5GHz ISM unlicensed band for increasing data rate and spectral efficiency. The major constraint for accessing these frequencies is being able to coexist with other Wi-Fi/IoT users. Such constraint has been tackled by developing Wi-Fi/IoT by using Cognitive Radio Network (CRN) with two objectives such as deploying the Listen-Before-Talk (LBT) regulatory requirement of radio communication in U-LTE and enhancing their co-existence with Wi-Fi/IoT users in a non-interference manner. However, uplink data transmission was not considered in the unlicensed spectrum usage and also the spectrum efficiency was less. Hence, in this paper, the co-occurrence between LTE and Wi-Fi/IoT using CRN is further enhanced and realized. Initially, the co-occurrence between LTE and Wi-Fi in the unlicensed spectrum is enhanced by proposing efficient spectrum utilization based on Conflict-Tolerant Channel Allocation (CTCA) algorithm that reduces the channel inconsistencies efficiently. Moreover, this algorithm is enhanced to increase the spectrum efficiency and simultaneous transmissions in the similar channel based on an enhanced Cell ON/OFF mechanism which optimizes the resource allocation. Finally, the simulation results show that the performance efficiency of the proposed system compared to the existing in terms of spectrum efficiency, throughput and transmission time delay.

Multi-Carrier Listen Before Talk With Power Leakage Awareness for LTE-LAA in Unlicensed Spectrum

Licensed-assisted access (LAA) is a new feature of 3GPP LTE that utilizes unlicensed spectrum as a means of providing additional bandwidth. For fair coexistence with other incumbent systems such as Wi-Fi, LAA runs a listen-before-talk (LBT) procedure before transmission, which is designed to support multi-carrier operation as well. However, inherent power leakage to adjacent carriers ruins multi-carrier LBT and deteriorates aggregation capacity considerably. To solve this power leakage problem, we propose a hybrid design of the two LBT types of the specification to have the advantages of both, which is realized by carrier grouping, and develop an algorithm that determines division of carriers into multiple groups with a guard band in between to avoid power leakage among groups and selects the primary carrier of each group so as to maximize LAA’s carrier aggregation capacity. Through extensive simulation, we demonstrate that the proposed algorithm enhances system throughput considerably over conventional mechanisms while coexisting fairly with Wi-Fi systems. We also investigate the impact of the depth of group splitting and transmit power on performance.

A dual threshold listen-before-talk for unlicensed LTE systems

The insufficient wireless frequency bands are likely to arise with the increase in mobile traffic demand. Many mobile operators suffer from a high annual increase in the cost of licensed wireless bands. Unlicensed bands present a potential opportunity for mobile operators. A single 5-GHz band with a large bandwidth provides faster transmission for each user. However, interference caused by existing wireless transmissions, such as Wi-Fi in unlicensed bands, is a serious issue in the effective utilization of unlicensed bands by mobile operators. The Listen-Before-Talk (LBT) was proposed as a promising method to ensure the coexistence of Wi-Fi and commercial mobile services such as LTE. We focus on effective control of the contention window (CW) size to maximize the operability of LBT. Control of the CW size depends on the repetition of extended clear channel assessment (ECCA). We introduce a dual threshold system that counts the repetition of ECCA. The newly developed NS-3-based unlicensed band network simulator shows the effectiveness of the proposed control mechanism. The results indicated that the advantage of the proposed method improves 4.3–7.6% than traditional LBT method. We develop a composite metric of throughput and fairness to evaluate the performance of LTE-LAA access.

Augmenting LoRaWAN Performance With Listen Before Talk

Standard LoRaWANs leverage pure ALOHA at the medium access control layer, which is proved to be a performance bottleneck as the network size scales up. Stimulated by this fact, this paper studies the applicability and the performance of listen before talk (LBT) medium access schemes in the context of LoRaWANs. We consider two different implementations of LBT: physical layer LBT based on energy detection only and MAC layer LBT based on layer 2 frame decoding, and we propose a Markovian framework to evaluate the performance of LoRaWANs under such setting in terms of data extraction rate and average delay experienced by transmitted uplink messages. The proposed framework is also leveraged to assess the performance of “mixed” LoRaWAN scenarios, where some devices access the channel according to the standard-compliant ALOHA protocol, while other devices transmit according to LBT.

Slot Jamming Effect and Mitigation Between LTE-LAA and WLAN Systems With Heterogenous Slot Durations.

To improve spectrum sharing between long-term evolution (LTE) license assisted access (LAA) and incumbent systems such as wireless local area networks (WLANs) in unlicensed spectrum, listen before talk (LBT) has been proposed as a candidate for LAA channel access. To allow for a robust spectrum sensing performance, LBT may use a backoff-slot duration that is substantially larger than its WLAN counterpart. There is potential for an unknown backoff slot-jamming (SJ) effect, which may significantly decrease channel access probability (CAP) and throughput of LAA-LBT links. In this paper, we study the SJ effect and propose an effective anti-SJ (ASJ) LBT scheme. To gain theoretical insight, we develop a new performance analysis approach on coexisting systems with different slot durations. We model the LAA backoff process with super-counters, provide an in-depth analysis of the backoff process, and derive key performance indicator (KPI) statistics. These KPIs include backoff hold time, successful transmission probability, CAP, and throughput. Simulation results thoroughly validate our analytical results, and show that the ASJ-LBT scheme is effective in mitigating the SJ effect. These results fill a major technical gap in spectrum sharing research and may be extended to support system optimization and coexistence analysis of other heterogeneous systems.

Null-While-Talk: Interference nulling for improved inter-technology coexistence in LTE-U and WiFi networks

A recent proposal known as unlicensed LTE offers cost-effective capacity extension to the cellular network operators in which LTE operators bundle the unlicensed spectrum in 5GHz UNII bands with their licensed spectrum via carrier aggregation. But, unlicensed spectrum requires coexistence among the networks operating in the same spectrum, e.g., IEEE 802.11 (WiFi) networks at 5GHz. While WiFi implements Listen-Before-Talk (LBT) and is therefore coexistence-friendly, LTE-Unlicensed (LTE-U) lacks such capability as it is not designed with shared spectrum access in mind. Hence, LTE has a potential to seriously harm WiFi. Prior works suggest forcing LTE-U to separate its transmission in either frequency, time, or space, and without directly collaborating with the WiFi networks. Contrary to these schemes, we introduce an explicit cooperation between neighboring LTE-U and WiFi networks. We propose Null-While-Talk(NWT) which suggests that LTE-U BSs employ MIMO signal processing to create coexistence gaps in space domain in addition to the time domain gaps by means of cross-technology interference nulling towards WiFi nodes in the interference range. In return, LTE-U can increase its own airtime utilization while trading off slightly its gain from MIMO. First, we present simulation results indicating that such cooperation offers benefits to both networks, WiFi and LTE-U, in terms of improved throughput and decreased channel access delay. Moreover, we present Xzero which implements NWT in a practical setting where the LTE-U BS lacks channel state and location information about the WiFi stations to be nulled. Xzero overcomes this challenge by performing an intelligent null search guided by constant feedback from the WiFi node on the null directions being tested. Our Xzero prototype implemented on SDR and COTS WiFi hardware shows the feasibility of our proposal.

Experiencing LoRa Network Establishment on a Smart Energy Campus Testbed

The fundamental properties of long-range (LoRa) performance have been revealed by previous research, but advanced issues remain unresolved. This paper tackles three technical challenges that are confronted when establishing a LoRa network on a smart energy campus testbed in Korea. First, the communication range of LoRa in a combined indoor and outdoor environment has yet to be determined. To address this problem, this study builds a LoRa testbed from which we measure the propagation properties of radio signals in a combined environment. Then, we establish a simplified path loss model for LoRa PHY and compute the communication range. Next, national regulations enforce the use of the Listen-Before-Talk (LBT) scheduling scheme, but its performance has never been examined. This paper enumerates the impacts of LBT, threshold values, back-off algorithms, and application parameters on LoRa Medium Access Control (MAC) performance. Last, the application parameters required by the smart energy scenario influence LoRa performance, but their impacts have been scarcely reported. Based on the application parameters and experimental results, this paper proposes a strategy to deploy LoRa gateway(s) and nodes on the campus. To the best of the authors’ knowledge, this is the first work that considers performance issues when establishing a LoRa network on a real-world application testbed.