LTE Licensed Assisted Access (LTE-LAA) is a promising solution for harmonious coexistence with WiFi in unlicensed spectrum. Although LTE-LAA employs a listen-before-talk approach similar to the distributed coordination function in IEEE 802.11, it uses different parameters and varying transmission durations . As a result, achieving fair coexistence between LTE-LAA and WiFi (by any definition) remains an open question, which in a pragmatic framework, devolves to: how LTE-LAA should select its parameters. To address this issue, a multi-group model is proposed for LTE-LAA and WiFi coexistence, as a function of respective initial backoff window sizes, sensing durations, maximum backoff stages, retry limits and transmission opportunities. The network steady-state point in saturated conditions is obtained, based on which the node airtime and total network airtime are derived as functions of system parameters of LTE-LAA and WiFi networks. The analysis shows that LTE-LAA can maintain proportional fairness with WiFi by either tuning its initial backoff window size or sensing duration. In particular, the optimal initial backoff window size and number of sensing slots of LTE-LAA are both derived and verified by simulation. The significance of our analysis is two-fold: a) it exposes the result that the current standard-proposed parameter settings will not generically achieve fairness and thereafter b) suggests optimal settings whereby LTE-LAA and WiFi nodes can achieve equal per-node airtime. It is further revealed that the initial backoff window size tuning of LTE-LAA could be a preferable option for achieving fairness between LTE-LAA and WiFi in practical scenarios as it requires less system information and achieves better precision.

Achieving Proportional Fairness for LTE-LAA and Wi-Fi Coexistence in Unlicensed Spectrum

Growing requirements for cellular network capacity made 3GPP consortium introduce License-Assisted Access (LAA), which implies unlicensed spectrum exploitation. Since these frequencies are already occupied by other technologies, such as Wi-Fi, it is vital to assure fair and efficient sharing of the unlicensed bands by LAA and Wi-Fi devices. Hence, LAA uses Listen Before Talk, the principle of which is similar to CSMA/CA implemented in Wi-Fi. Specifically, similar to Wi-Fi, an LAA base station performs the backoff procedure before every transmission attempt. However, the base station can only start LAA data transmission at licensed spectrum slot boundaries, which results in an extremely long vulnerable period from the end of backoff procedure till the slot boundary. During such a vulnerable period other devices, e.g., Wi-Fi stations, can occupy the channel, canceling the LAA transmission. To protect LAA transmissions, many developers propose sending a reservation signal during the whole vulnerable period, which in turn blocks Wi-Fi stations, occupying the channel with the useless signal and resulting in the channel resource sharing unfairness. To solve this problem, this paper designs a novel mechanism, which allows the LAA base station to detect collisions of Wi-Fi transmissions with the reservation signal. An analytical model for various LAA implementations is developed. Using the model, it is shown how LAA performance can be improved with the proposed mechanism without Wi-Fi performance degradation.

New Collision Detection Method for Fair LTE-LAA and Wi-Fi Coexistence

The fifth-generation (5G) and beyond networks are designed to efficiently utilize the spectrum resources to meet various quality of service (QoS) requirements. The unlicensed frequency bands used by WiFi are mainly deployed for indoor applications and are not always fully occupied. The cellular industry has been working to enable cellular and WiFi coexistence. In particular, 5G New Radio in unlicensed channel spectrum (NR-U) supports the uplink and downlink transmission on the maximum channel occupation time (MCOT) duration. In this paper, we consider maximizing the total throughput of both downlink and uplink in NR-U by jointly optimizing the time and power allocation during MCOT while ensuring fair coexistence with WiFi. Fairness is guaranteed in two steps: 1) tuning the access related parameters of NR-U to achieve proportional fairness, and 2) including 3GPP fairness from the throughput perspective as a constraint in NR-U throughput maximization. Numerical analysis and simulation have demonstrated the superior performance of the proposed resource allocation algorithm compared to conventional deployment strategies.

Joint Time and Power Allocation for 5G NR Unlicensed Systems

To improve the capacity of cellular systems without additional expenses on licensed frequency bands, the 3GPP consortium has designed cellular technologies that use the unlicensed spectrum. The important peculiarity of the usage models of these technologies, the latest of which is New Radio Unlicensed (NR-U), is the coexistence with Wi-Fi networks deployed in the same frequency band. That is why NR-U uses channel access methods similar to those of Wi-Fi. However, the performance of Wi-Fi networks notably deteriorates in coexistence scenarios. One of the reasons is the slotted structure of transmissions of cellular base stations in the unlicensed spectrum, which may imply the use of a reservation signal. The paper proposes two novel channel access methods for NR-U, in which an NR-U base station randomly stops sending the reservation signal to listen to the channel to detect and resolve collisions. Analytical models are developed to validate the proposed methods and evaluate their efficiency, taking into account important features of NR-U networks, such as flexible numerology and mini-slot transmissions. The obtained numerical results show that the proposed methods significantly improve the performance of the Wi-Fi or NR-U network without degradation of the throughput of the other technology in coexistence scenarios.

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Enhanced Collision Resolution Methods With Mini-Slot Support for 5G NR-U

Utilizing the unlicensed spectrum for the Long Term Evolution (LTE), enables the service providers to significantly increase the capacity of the network. However, LTE has to coexist with other network technologies and especially Wi-Fi. LTE employs the Licensed Assisted Access (LAA) protocol and applies the Listen Before Talk (LBT) technique to efficiently share the communication channel with Wi-Fi. Nonetheless, there is still a big gap in throughput performance between the two technologies, with the LAA-LTE considerably outperforming Wi-Fi. This unfair behavior becomes more severe in a saturation mode scenario, where the two networks are fully loaded. In this paper, we try to ensure the fair coexistence between Wi-Fi and LTE by proportionally changing the maximum packet length for each network by applying an optimization and a heuristic approach. The ultimate goal is to equalize the overall throughput for Wi-Fi and LAA or equalize the individual throughput attained for each communicating device. An admission control scheme is also studied to allocate incoming users between the two networks, while trying to balance the throughput distribution. Our results show that by changing the ratio of packet length between Wi-Fi and LAA-LTE a fair share of the medium can be achieved.

Ioannis Marmokos

Papers

Analyzing the Coexistence of Wi-Fi and LAA-LTE Towards a Proportional Throughput Fairness