BACKGROUND

Technical Field

The present invention relates to a radio base station, a user terminal and a radio communication method in next-generation mobile communication systems.

Background Art

In the UMTS (Universal Mobile Telecommunications System) network, the specifications of long term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1). The specifications of LTE-advanced (Rel. 10 to 12) have been drafted for the purpose of achieving further broadbandization and higher speeds beyond LTE, and, in addition, for example, a successor system of LTE—referred to as “5G” (5th generation mobile communication system)—is under study.

In LTE of Rel. 8 to 12, the specifications have been drafted assuming exclusive operations in frequency bands that are licensed to operators—that is, licensed bands. As licensed bands, for example, 800 MHz, 2 GHz and/or 1.7 GHz are used.

User traffic has been increasing steeply following the spread of high-performance user terminals/user equipment (referred to as “UE”) such as smart-phones and tablets. Although more frequency bands need to be added to meet this increasing user traffic, licensed bands have limited spectra (licensed spectra). Consequently, a study is in progress to enhance the frequencies of LTE systems by using bands of unlicensed spectra (hereinafter referred to as “unlicensed bands”) that are available for use apart from licensed bands (see non-patent literature 2).

For unlicensed band, for example, 2.4 GHz, which is the same as in Wi-Fi (registered trademark), or the 5 GHz band and/or the like may be used. With Rel. 13 LTE, a study is in progress to execute carrier aggregation (CA) between licensed bands and unlicensed bands. Communication that is carried out by using unlicensed bands with licensed bands like this is referred to as “LAA” (License-Assisted Access). In the future, dual connectivity (DC) between licensed bands and unlicensed bands and stand-alone in unlicensed bands may become the subject of study under LAA.Non-Patent Literature 1: 3GPP TS 36. 300 “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall Description; Stage 2”Non-Patent Literature 2: AT&T, Drivers, Benefits and Challenges for LTE in Unlicensed Spectrum, 3GPP TSG-RAN Meeting #62 RP-131701

SUMMARY

A study is in progress to introduce interference control functionality to unlicensed bands, in order to allow co-presence with other operators' LTE, Wi-Fi or different systems. In Wi-Fi, LBT (Listen Before Talk), which is based on CCA (Clear Channel Assessment), is used as an interference control function within the same frequency.

Consequently, when unlicensed bands are configured in LTE systems, UL transmission and/or DL transmission may be controlled by implementing “listening” (for example, LBT) as an interference control function. In this case, there is a demand to enable both efficient and fair co-presence with other systems (for example, Wi-Fi) and other LTE operators, and efficient operation of frequencies.

The present invention has been made in view of the above, and one or more embodiments of the present invention provide a radio base station, a user terminal and a radio communication method that can allow co-presence with other systems, and that can furthermore achieve improved spectral efficiency in cells (for example, unlicensed bands) in which pre-transmission listening is employed.

According to the present invention, a radio base station has a transmission section that transmits a DL signal, a control section that performs listening before the DL signal is transmitted, and controls DL transmission, and a receiving section that receives a delivery acknowledgement signal (ACK/NACK) in response to the DL transmission, and the control section controls a contention window size to apply to listening based on the number of NACKs in response to a predetermined period of DL transmission.

According to the present invention, it is possible to allow co-presence with other systems, and, furthermore, achieve improved spectral efficiency in cells in which pre-transmission listening is employed (for example, unlicensed bands).

