To achieve higher throughputs in cellular networks, 3GPP proposes to use unlicensed frequency bands and develops technologies — the latest one is NR-U — allowing a cellular base station to transmit in unlicensed bands, which are already occupied by Wi-Fi networks. To enable fair channel sharing between two technologies, the base station uses a sort of CSMA/CA with binary exponential backoff similar to Wi-Fi. However, the base station can start data transmission only at strictly periodic time moments. Many papers propose sending a reservation signal between the end of the backoff procedure and such a moment to prevent nearby devices from accessing the channel. However, this approach significantly reduces Wi-Fi performance. The paper proposes a novel method CR-LBT of transmitting a reservation signal that greatly decreases channel resource waste caused by collisions and improves channel resource sharing fairness. With developed analytical models and simulations, it is shown that CR-LBT may simultaneously increase the throughput of both NR-U and Wi-Fi networks. The effect is more noticeable for the Wi-Fi network, the throughput of which may rise three times compared with the traditional method of sending the reservation signal. Finally, the influence of various factors on CR-LBT performance is studied.

The volume of traffic transmitted over cellular networks is growing significantly every year. To improve LTE network throughput, 3GPP has proposed License-Assisted Access (LTE-LAA), which allows utilizing the unlicensed 5GHz spectrum already used by Wi-Fi and other technologies. To enable fair resource sharing, LTE-LAA adopts CSMA/CA-based channel access with binary exponential backoff similar to Wi-Fi, However, because of the LTE-LAA technology constraints, the LTE-LAA base station may start data transmission only at rather rare licensed carrier slot boundaries. The essential question, what the base station should do between the end of the backoff procedure and the closest licensed slot boundary, is left implementation dependent. In the paper, we have developed analytical models to study the performance of different LTE-LAA implementations and analyze channel sharing fairness between LTE-LAA and Wi-Fi networks in the coexistence scenario. Moreover, we investigate how the performance of LTE-LAA channel access can be improved using New Radio flexible numerology, which is one of the main features of currently developed 5G systems.

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.

Nowadays multi-hop wireless networks have become a popular topic with various applications, including smart grids and sensor networks aimed at monitoring and quality control of industrial systems. In such networks, the amount of generated data is not high, but this data should be delivered with low delay. However, random access methods commonly used in multi-hop networks usually adopt a backoff procedure aimed at collision avoidance, which significantly increases packet delivery time in case of multi-hop transmission. In this paper, to reduce delays in multi-hop networks, we propose an extension to the well-known slotted ALOHA channel access method, which allows performing the first forwarding attempt instantly, without any backoff procedure. We have developed an analytical model to find the stability region of the proposed channel access method, as well as to find its optimal parameter values. In addition, we prove the high efficiency of the proposed method with simulation.

In order to meet the continuously increasing demands for high throughput in wireless networks, IEEE 802 LAN/MAN Standard Committee is developing IEEE 802.11ax: a new amendment for the Wi-Fi standard. This amendment provides various ways to improve the efficiency of Wi-Fi. The most revolutionary one is OFDMA. Apart from obvious advantages, such as decreasing overhead for short packet transmission at high rates and improving robustness to frequency selective interference, being used for uplink transmission, OFDMA can increase power spectral density and, consequently, user data rates. However, the gain of OFDMA mainly depends on the resource scheduling between users. The peculiarities of OFDMA implementation in Wi-Fi completely change properties of classic schedulers used in other OFDMA systems, e.g. LTE. In the paper, we consider the usage of OFDMA in Wi-Fi for uplink transmission. We study peculiarities of OFDMA in Wi-Fi, adapt classic schedulers to Wi-Fi, explaining why they do not perform well. Finally we develop a novel scheduler, MUTAX, and evaluate its performance with simulation.

In the paper, we address the influence of Head-of-line (HOL) blocking on real-time video streaming in wireless networks. We consider short channel failures when the probability of successful packet transmission to a station abruptly drops to a near zero value. In wireless networks, the duration of such channel failure intervals may reach some hundred milliseconds, making packets staying in the same queue and destined for other stations be blocked and experience large delays. Because of delay sensitivity of real-time traffic, the quality of the received videos degrades. In the paper, we propose a feasible for implementation queue management algorithm to avoid video quality degradation. We prove by simulation that our algorithm outperforms existing solutions significantly, minimizing video quality degradation for the streams destined for stations with channel failures while avoiding blocking streams destined for other stations.