User equipment assistance for interference mitigation in unlicensed frequency bands

A first base station selects a subset of channels of an unlicensed frequency band based on indications of energy received over the channels. The first base station transmits one or more messages instructing user equipment to monitor the subset of channels and report information indicating whether at least one second base station is transmitting over the subset of channels. User equipment monitors the subset of channels of the unlicensed frequency band in response to receiving the one or more messages from a first base station indicating the subset of channels. The user equipment transmits one or more messages including information indicating whether one or more second base stations are transmitting over the subset of channels.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No. 14/476,029, entitled “INTERFACE FOR INTERFERENCE MITIGATION IN UNLICENSED FREQUENCY BANDS” and filed on even date herewith, the entirety of which is incorporated by reference herein.

BACKGROUND

Field of the Disclosure

The present disclosure relates generally to wireless communication systems and, more particularly, to unlicensed frequency bands in wireless communication systems.

Description of the Related Art

Unlicensed frequency bands are portions of the radiofrequency spectrum that do not require a license for use and may therefore be used by any device to transmit or receive radiofrequency signals. For example, the Unlicensed National Information Infrastructure (UNII) is formed of portions of the radio spectrum that include frequency bands in the range of 5.15 GHz to 5.825 GHz. For another example, the industrial, scientific, and medical (ISM) radio bands are portions of the radio spectrum that are reserved internationally for unlicensed communication. The ISM radio bands include bands with a center frequency of 2.4 GHz and a bandwidth of 100 MHz, a center frequency of 5.8 GHz and a bandwidth of 150 MHz, and a center frequency of 24.125 GHz and a bandwidth of 250 MHz, among other frequency bands. Unlicensed frequency bands can be contrasted to licensed frequency bands that are licensed to a particular service provider and may only be used for wireless communication that is authorized by the service provider. Wireless communication devices that transmit or receive signals in licensed or unlicensed frequency bands are typically referred to as nodes, which may include Wi-Fi access points that operate according to IEEE 802.11 standards in the unlicensed spectrum or base stations that operate in licensed spectrum according to standards such as Long Term Evolution (LTE) standards defined by the Third Generation Partnership Project (3GPP). Base stations that operate according to LTE may also implement supplementary downlink (SDL) channels in the unlicensed spectrum to provide additional bandwidth for downlink communications to user equipment that are also communicating with the base station using channels in a licensed frequency band.

SUMMARY OF EMBODIMENTS

The following presents a summary of the disclosed subject matter in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

In some embodiments, a method is provided for user equipment-assisted interference mitigation in unlicensed frequency bands of a wireless communication system. The method includes selecting, at a first base station, a subset of channels of an unlicensed frequency band based on indications of energy received over the channels. The method also includes transmitting, from the first base station, one or more messages instructing user equipment to monitor the subset of channels and report information indicating whether one or more second base stations are transmitting over the subset of channels.

In some embodiments, a method is provided for assisting interference mitigation in unlicensed frequency bands of a wireless communication system. The method includes monitoring, at user equipment, a subset of channels of an unlicensed frequency band in response to receiving at least one message from a first base station indicating the subset of channels. The method also includes transmitting, from the user equipment, one or more messages including information indicating whether one or more second base stations are transmitting over the subset of channels

In some embodiments, a base station is provided for user equipment-assisted interference mitigation in unlicensed frequency bands of a wireless communication system. The base station includes a processor to select a subset of channels of an unlicensed frequency band based on indications of energy received over the channels. The base station also includes a transceiver to transmit one or more messages instructing user equipment to monitor the subset of channels and report information indicating whether one or more second base stations are transmitting over the subset of channels.

In some embodiments, user equipment is provided for assisting interference mitigation in unlicensed frequency bands of a wireless communication system. The user equipment includes a processor to monitor a subset of channels of an unlicensed frequency band in response to receiving at least one message from a first base station indicating the subset of channels. The user equipment also includes a transceiver to transmit one or more messages including information indicating whether one or more base stations are transmitting over the subset of channels.

DETAILED DESCRIPTION

Base stations perform carrier sensing to select channels for downlink transmission in unlicensed frequency bands. For example, a base station may measure energy received in channels in the unlicensed frequency bands to identify a “clean” channel, e.g., an average of the received energy from other LTE base stations or Wi-Fi access points on the channel is below a threshold value. The base station may then use the clean channel for downlink transmissions. If the base station is unable to identify a clean channel, the base station has to share the channel with one or more other transmitting nodes. The channels that are available for sharing may be limited by constraints imposed by different radio access technologies (RATs). For example, Wi-Fi access points use a primary channel and a secondary channel to communicate with receiving nodes. The primary channel carries a beacon signal that conveys system information to the receiving nodes. The receiving nodes may not be able to successfully receive (i.e., demodulate and decode) the primary channel if there is interference from downlink transmissions on the primary channel by an LTE base station. Failure to receive the beacon signal on the primary channel may cause the receiving nodes to lose their association with the Wi-Fi access point. Consequently, LTE base stations should not select the primary channel of a Wi-Fi access point for downlink transmissions. However, LTE base stations do not typically include functionality that allows them to identify primary or secondary channels of a Wi-Fi access point using signals received from the Wi-Fi access point over the primary or secondary channels.

Channel sharing may be further complicated by the fact that nodes such as Wi-Fi access points and LTE base stations are prone to a “hidden node problem.” If two nodes are within range of the user equipment, but are too far apart to be aware of each other, the two nodes are “hidden” from each other. Nodes that are hidden from each other cannot coordinate transmission and reception of packets, e.g., to force time-sharing between the two nodes or to prevent one node from transmitting on the primary channel of another node. Packets transmitted by nodes that are hidden from each other may therefore collide at a receiving node, which can only decode one packet at a time. Consequently, packets intended for the receiving node may be missed or lost if they collide with other packets transmitted by a hidden node. For example, two or more base stations transmitting over the same channel in the unlicensed frequency band using their respective LTE supplementary downlink channel may interfere with each other if they use the same on/off pattern to avoid interference with one or more Wi-Fi access points during the gating cycle. For another example, LTE base stations may cause nodes to lose their association with a hidden Wi-Fi access point if the LTE base station chooses a primary channel of the hidden Wi-Fi access point for downlink transmission.

Interference between nodes that are hidden from each other may be reduced by instructing user equipment to monitor a subset of channels of an unlicensed frequency band and report information indicating whether one or more nodes are transmitting over the subset of channels. The subset of channels may be selected based on measurements of energy received by a first node over the channels of the unlicensed frequency band. For example, the first node may rank the channels so that the highest ranked channels correspond to lower measured values of received energy (which indicates a lower probability that another node is transmitting on the channel) and the lowest ranked channels correspond to higher measured values of received energy (which indicates a higher probability that another node is transmitting on the channel). The first node may then instruct the user equipment to monitor a subset that includes the highest ranked channels of the unlicensed frequency band, e.g., during a measurement gap. Some embodiments of the user equipment may report channel activity information for the subset of the channels and the first node may use the reported channel activity information to select one or more channels from the subset for downlink transmissions. Some embodiments of the user equipment may identify channels used by nodes that operate according to different RATs (such as primary and secondary channels used by Wi-Fi access points), or by another hidden LTE-U base station, so that the first node can bypass downlink transmissions on one or more of the identified channels. Some embodiments of the first node may also use the information reported by the user equipment to negotiate time-sharing of the channel with other nodes, e.g., using messages exchanged over a backhaul interface.

FIG. 1is a diagram of a first example of a wireless communication system100according to some embodiments. The wireless communication system100includes a plurality of wireless communication nodes101,102,103,104,105(collectively referred to herein as “the nodes101-105”). Embodiments of the nodes101-103may be wireless transceivers such as user equipment, mobile units, mobile terminals, stations, access terminals, and the like. In the interest of clarity, the nodes101-103are referred to herein as user equipment101-103. Embodiments of the nodes104,105may be devices for providing wireless connectivity within corresponding geographic areas that are conventionally referred to as cells110,115. The nodes104,105may also be referred to as base stations, eNodeBs, access points, access serving networks, macrocells, microcells, metrocells, femtocells, picocells, and the like. In the interest of clarity, the nodes104,105are referred to herein as the base stations104,105. The base stations104,105may transmit signals over a downlink (or forward link) to the user equipment101-103. The user equipment101-103may transmit signals over an uplink (or reverse link) to the base stations104,105.

The user equipment101-103and the base stations104,105may be configured to communicate over an air interface in licensed frequency bands or unlicensed frequency bands. As used herein, the phrase “unlicensed frequency band” will be understood to refer to a portion of the radiofrequency spectrum that does not require a license for use and may therefore be used by any of the user equipment101-103or the base stations104,105to transmit or receive radiofrequency signals. For example, the Unlicensed National Information Infrastructure (UNII) includes portions of the radio spectrum in frequency bands that range from 5.15 GHz to 5.825 GHz. For another example, the industrial, scientific, and medical (ISM) radio bands are reserved internationally for unlicensed communication. The ISM radio bands include bands with a center frequency of 2.4 GHz and a bandwidth of 100 MHz, a center frequency of 5.8 GHz and a bandwidth of 150 MHz, and a center frequency of 24.125 GHz and a bandwidth of 250 MHz, among other frequency bands. As used herein, the phrase “licensed frequency band” will be understood to refer to a portion of the radiofrequency spectrum that is licensed to a particular service provider or providers and may only be used for wireless communication by the user equipment101-103or the base stations104,105that are authorized by the service provider. For example, the United States Federal Communication Commission (FCC) licenses the frequency bands 698-704 MHz and 728-734 MHz to Verizon Wireless and the frequency bands 710-716 MHz and 740-746 MHz to AT&T.

The unlicensed frequency bands support a plurality of channels that may be used for downlink transmissions from the base stations104,105to the user equipment101-103. For example, the 5 GHz unlicensed frequency band allocated to the UNII may be divided into a predetermined number of 20 MHz channels. Some embodiments of the base stations104,105may use the channels in the unlicensed frequency band to support local area wireless radio access technologies (RATs) such as Wi-Fi or other communications according to the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. Some embodiments of the base stations104,105may use the channels in the unlicensed frequency band to supplement downlink transmissions over channels of a licensed frequency band. For example, a base station that operates according to LTE may transmit best effort data on a supplemental downlink channel in the unlicensed frequency band concurrently with transmitting data, control information, or other critical information on a channel of the licensed frequency band.

The base stations104,105may use a channel selection algorithm to choose one or more of the unlicensed frequency band channels for downlink transmission. Some embodiments of the base stations104,105may select unlicensed channels based on measurements of energy received over one or more of the channels for a predetermined time interval (e.g., long-term energy detection), detection of preambles such as Wi-Fi preambles received over the channels, detection of overhead broadcast channels from neighboring nodes, and the like. However, the base stations104,105may not be able to detect each other over the air interface. For example, the base station104is not encompassed by the boundary of the cell115and the base station105is not encompassed by the boundary of the cell110. Consequently, the base stations104,105may not be able to detect each other's downlink transmissions on channels of unlicensed frequency bands. The base stations104,105are therefore “hidden” from each other.

User equipment101-103may assist the base stations104,105by informing the base stations104,105of interfering downlink transmissions on channels of the unlicensed frequency bands. The user equipment101-103may monitor channels of the unlicensed frequency bands based on information received from one or more of the base stations104,105. For example, the base station104may measure signal strengths for transmissions received on a set of channels of the unlicensed frequency band and rank the channels based on the measured signal strength. The base station104may then select a subset of the channels as candidates for downlink transmissions, with channels having the lowest measured signal strengths getting the highest ranking. The number of channels in the subset may range from a single channel to the number of channels in the unlicensed frequency band.

The base station104may transmit one or more messages120,125to instruct the user equipment101,102to measure one or more indicators of downlink transmissions on the subset of channels of the unlicensed frequency band. For example, the base station104may transmit messages120,125that include information that can be used to configure the user equipment101,102to monitor the indicators during a measurement gap. The user equipment101,102temporarily suspend transmission or reception with the serving base station104to monitor signals from other nodes during the configured measurement. For example, the user equipment102may monitor an interfering downlink signal130produced when the base station105transmits a downlink signal135to the user equipment103. The monitored signals can be used to measure indicators such as received energy, received signal strengths, signal-to-noise ratios (SNRs), signal-to-interference-plus-noise ratios (SINRs), and the like. The measured indicators may be used to determine whether other base stations are transmitting on the monitored subset of channels. For example, the user equipment102may detect the presence of the base station105if a received signal strength indicator (RSSI) associated with the downlink signal130on one of the subset of channels exceeds a threshold value. The user equipment101may monitor the requested subset of channels but may not detect the presence of other base stations. For example, the monitored RSSI for the subset of channels monitored by the user equipment101may be below the threshold value.

Some embodiments of the user equipment101,102may demodulate or decode the monitored signals to determine characteristics of the channels. For example, if the base station105transmits the downlink signals130,135on an unlicensed channel according to the Wi-Fi standards, the user equipment101,102may use the demodulated or decoded signals130,135to identify a primary channel used to transmit beacon signals or a secondary channel associated with the primary channel. Examples of primary channels include channels in the unlicensed frequency band that have a bandwidth of 20 MHZ or 40 MHZ channels, as defined according to 802.11n, and channels in the unlicensed frequency band that have a bandwidth of 40 MHZ, 80 MHZ or 160 MHZ, as defined according to 802.11ac. The primary and the secondary channels may be contiguous 20 MHZ channels, i.e. the secondary channel is either one before or one after the primary channel number. The primary and secondary channel association may be represented as (36, 1), in which case the primary channel is channel 36 and the secondary is channel 40, or (44, −1), in which case the primary channel is 44 and the secondary channel is 40. The user equipment101,102may transmit one or more messages140,145reporting the results of the measurements such as values of the indicators or information identifying the primary or secondary channels.

The base station104may select a clear channel if the messages140,145received from the user equipment101,102indicate that one or more of the channels in the subset is clear. As used herein, the term “clear” is understood to indicate that a measured value of an indicator of the presence of downlink transmissions in the unlicensed frequency band (such as a received signal strength indicator or other parameters discussed herein) is below a threshold value indicating that the unlicensed frequency band is clear of transmissions by other nodes and packets transmitted over a channel of the unlicensed frequency band are unlikely to collide with packets transmitted by other nodes. For example, the base station104may use a channel of the unlicensed frequency band for downlink transmissions if the user equipment102measures a received signal strength for the downlink transmissions130from the node105on the channel that is below a threshold value.

However, the base station104may have to share the channel with the hidden base station105if the message145received from the user equipment102indicates that the channels are not clear, e.g., due to interfering downlink transmissions130from the hidden base station105. The base stations104,105may therefore select or negotiate time intervals for downlink transmissions by the base stations104,105over the shared channel of the unlicensed frequency band. Some embodiments of the base stations104,105select a first portion of a gating cycle for transmission over a channel of the unlicensed frequency band in response to determining that the channel is clear of transmissions from other nodes and a second portion of the gating cycle that is time division multiplexed with the first portion in response to determining that the channel is shared with at least one other node. For example, the base station105may select the first half of the gating cycle if the base station104is not transmitting on a channel and the base station104may subsequently (e.g. in response to the message145) select the second half of the gating cycle for transmission in response to determining that the base station105is already transmitting on the channel.

Some embodiments of the base stations104,105may operate according to the same RAT and may be connected by an interface120such as a backhaul interface. One example of a backhaul interface is the X2 interface defined by the Third Generation Partnership Project (3GPP) standards, which may be used to connect LTE base stations104,105. The base stations104,105may therefore exchange messages over the interface120to negotiate the time intervals that are used by the base stations104,105for downlink transmissions. AlthoughFIG. 1depicts a single pair of base stations104,105that are connected by the interface120, some embodiments of the wireless communication system100may include larger numbers of base stations that are interconnected by additional interfaces that may be used to negotiate time intervals for sharing channels of the unlicensed frequency band. Furthermore, some embodiments of the base stations104,105may operate according to different RATs and consequently may not be connected by an interface. For example, the base stations104,105may not be connected by an interface if the base station104operates according to LTE and the base station105operates according to Wi-Fi.

FIG. 2is a diagram of a second example of a wireless communication system200according to some embodiments. The wireless communication system200includes a base station205that may support wireless connectivity, e.g., to a node such as user equipment210. Some embodiments of the base station205or the user equipment210may correspond to one or more of the user equipment101-103or the base stations104,105shown inFIG. 1. The base station205and the user equipment210may communicate over one or more uplink channels215and one or more downlink channels220in a licensed frequency band. The base station205and the user equipment210may also communicate over a supplementary downlink channel225in an unlicensed frequency band.

The base station205includes a transceiver230that is coupled to an antenna235. The transceiver230may transmit messages or signals over the downlink channels220in the licensed frequency band or the supplementary downlink channel225in the unlicensed band. The transceiver230may also receive signals over the uplink channels215. The base station205includes memory240for storing information such as processor instructions, data for transmission, received data, and the like. A processor245may be used to process information for transmission, process received information, or perform other operations as discussed herein, e.g., by executing instructions stored in the memory240.

The user equipment210includes a transceiver250that is coupled to an antenna255. The transceiver250may transmit signals over the uplink channel215in the licensed frequency band. The transceiver250may receive signals over the downlink channel220in the licensed frequency band and the supplementary downlink channel225in the unlicensed frequency band. The user equipment210also includes a processor260and a memory265. The processor260may be used to process information for transmission, process received information, or perform other operations as discussed herein, e.g., by executing instructions stored in the memory265.

The processor245may be used to select subsets of carriers of the unlicensed frequency band for monitoring by the user equipment210. For example, the transceiver230may provide measured values of a received signal strength indicator (RSSI) to the processor245, which may use the measured values to generate a ranking270of the channels of the unlicensed frequency band. The ranking270may rank the channels so that the highest ranked channels have the lowest RSSI (which indicates the lowest probability that another base station is transmitting on the channel) and the lowest ranked channels have the highest RSSI (which indicates the highest probability that another base station is transmitting on the channel). The processor245may then select a subset of the highest ranked channels for monitoring by the user equipment210. For example, the processor245may select channels 32 (−80 dBm), 44 (−75 dBm), and 36 (−72 dBm) and the processor245may not select channel 40 (−42 dBm) or any lower ranked channels that have higher measured values of the RSSI.

Some embodiments of the transceiver230may be used to transmit messages that include information indicating a candidate list of the subset of channels to be monitored by the user equipment210during a measurement gap. For example, the transceiver230may transmit a message including information indicating the selected channels 32, 44, and 36 over the downlink channel220of the licensed frequency band. The transceiver230may also be used to transmit messages including information for configuring the measurement gap used by the user equipment210. Some embodiments of the message may include parameters such as a gap pattern identifier, a measurement gap length, a measurement gap repetition period, a minimum available time for measurements during a predetermined time interval, a purpose of the measurement, and the like.

Table 1 provides examples of gap configurations for different gap identifiers.

Minimumavailabletime forMeasure-inter-frequencyMeasure-mentand inter-RATmentGapmeasurementsGapRepetitionduring 480GapLengthPeriodms periodMeasurementPattern Id(MGL, ms)(MGRP, ms)(Tinter1, ms)Purpose064060Inter-Frequency E-UTRAN FDDand TDD,UTRAN FDD,GERAN,LCR TDD,HRPD,CDMA2000 1x168030Inter-Frequency E-UTRAN FDDand TDD,UTRAN FDD,GERAN,LCR TDD, HRPD,CDMA2000 1x2TBDTBDTBDIEEE 802.11Beacon RSSImeasurement.
Messages that include the gap pattern ID2are used to configure the user equipment210to perform RSSI measurements of beacon signals transmitted according to IEEE 802.11, e.g., on channels of the unlicensed frequency band. The measurement gap length may be in the range 1-10 ms, the measurement gap repetition period may be in the range 10-100 ms, and the minimum available time may be in the range 10-100 ms. Some embodiments of the messages may also be used to configure the user equipment210to identify a primary channel or a secondary channel, such as a primary or secondary channel used by a Wi-Fi access point. For example, the user equipment210may be configured to demodulate or decode signals received over the channel during the configured measurement gap and use information in the demodulated or decoded signals such as a preamble transmitted in the beacon signal to identify the primary or secondary channel used by a Wi-Fi access point.

The transceiver250may receive (over the downlink channel220) one or more messages requesting that the user equipment210monitor the subset of channels in the unlicensed frequency band. The one or more messages may also include gap configuration information, as discussed herein. The processor260may configure the user equipment210to perform measurements during the measurement gap indicated in the message. The transceiver250and the antenna255may be used to perform measurements of energy, RSSI, SNR, SINR, and the like for the subset of channels. The processor260may generate one or more messages including measurement reports based on the measurements and the transceiver250may report the results of monitoring the subset of channels by transmitting the messages over the uplink channel215of the licensed frequency band. For example, the transceiver250may use radio resource control (RRC) layer signaling to convey the monitoring results over the uplink channel215.

FIG. 3is a flow diagram of a method300of requesting and acquiring measurement reports from user equipment for a subset of channels of an unlicensed frequency band according to some embodiments. The method300may be implemented in some embodiments of the base stations104,105shown inFIG. 1and some embodiments of the base station205shown inFIG. 2. At block305, a base station measures energy on one or more channels of an unlicensed frequency band. For example, the base station may measure the energies received over each of the channels during a predetermined time interval. At block310, the base station ranks the channels based on the measured energies so that higher rankings indicate lower energies (and lower probabilities that another base station is transmitting on the channel) and lower rankings indicate higher energies (and higher probabilities that another base station is transmitting on the channel). At block315, the base station selects a subset of the channels based on the rankings. For example, the base station may select a predetermined number (such as three) of the highest ranked channels.

At block320, the base station instructs one or more user equipment to monitor the subset of channels of the unlicensed frequency band. Some embodiments of the base station may transmit a message including a candidate list that indicates the subset of channels. The base station may also transmit a message including information for configuring a measurement gap during which the user equipment can monitor the subset of channels. At block325, the base station receives one or more messages from the user equipment including a report on the monitored subset of channels. The received messages may include information such as a measured RSSI, SNR, or SINR of one or more of the channels. The receive messages may also include information indicating whether one or more of the channels is a primary channel or a secondary channel, such as a primary or secondary channel of a Wi-Fi access point.

FIG. 4is a flow diagram of a method400for negotiating transmission time intervals of a shared channel of an unlicensed frequency band according to some embodiments. The method400may be implemented in some embodiments of the base stations104,105shown inFIG. 1and some embodiments of the base station205shown inFIG. 2. At block405, the base station receives a report from one or more user equipment for a monitored subset of channels. In the illustrated embodiment, the base station receives one or more messages including the report, which may include information such as a measured RSSI, SNR, or SINR of one or more of the channels.

At decision block410, the base station determines whether a clear channel has been detected for downlink transmissions. For example, the base station may determine that a clear channel has been detected if the measurement report from the user equipment indicates that one of the channels has a measured received energy or RSSI that is below a threshold. If a clear channel has been detected, the base station may transmit downlink signals on the clear channel at block415. If the base station does not detect a clear channel based on the measurement report received from the user equipment, then the base station may have to share one of the channels in the unlicensed frequency band with one or more other nodes. The other nodes may or may not operate according to the same RAT. For example, the node may operate according to a first RAT such as LTE and the other nodes may operate according to the first RAT or a second RAT such as Wi-Fi.

If the base station selects a shared channel for downlink transmissions, then the base station may negotiate (at block420) with the other base stations using the shared channel for time division multiplexing (TDM) intervals in portions of a repeating gating cycle. If the base station determines (using the reports from the user equipment) that the other base stations are transmitting over the shared channel according to the first RAT and none of the other base stations are transmitting according to a different (second) RAT, the base station may negotiate for time intervals that may span the entire gating cycle. If the base station determines that one or more of the other base stations are transmitting according to a different (second) RAT, the base station may reserve a predetermined time interval in the gating cycle for downlink transmission according to the second RAT. The base station may bypass transmission during the reserved predetermined time interval in the gating cycle. The base stations that operate according to the first RAT may then negotiate TDM time intervals in the unreserved portion of the gating cycle. At block425, the base stations may transmit downlink signals over the shared channel of the unlicensed frequency band during the negotiated TDM time intervals.

FIG. 5is a diagram showing allocation of time intervals in a gating cycle500for downlink transmissions by two base stations on a channel of an unlicensed frequency band according to some embodiments. The gating cycle500may repeat indefinitely or for a predetermined amount of time. A first allocation505indicates time intervals in the gating cycle500that are allocated to a first base station (such as the base station104shown inFIG. 1) and a second allocation510indicates time intervals in the gating cycle500that are allocated to a second base station (such as a base station105shown inFIG. 1). The horizontal axes indicate time increasing from left to right. The first and second base stations operate according to the same radio access technology (RAT) and so they can share the entire gating cycle500. For example, the first and second base stations may transmit downlink signals on the channel of the unlicensed frequency band according to LTE.

The time interval515in the gating cycle500, as well as the time interval520in the subsequent gating cycle in a series of repeating gating cycles, maybe allocated to the first base station for downlink transmissions on the channel of the unlicensed frequency band. The time interval525in the gating cycle500may be allocated to the second base station for downlink transmissions on the channel of the unlicensed frequency band. Consequently, downlink transmissions by the first and second base stations may not interfere with each other during the gating cycle500. As discussed herein, the second base stations may select the time interval525in response to determining that the first base station is already transmitting on the channel. Some embodiments of the first and second base stations may negotiate for the time intervals515,520,525by exchanging messages over an interface such as the interface120shown inFIG. 1.

FIG. 6is a diagram showing allocation of time intervals in a gating cycle600for downlink transmissions by three base stations on a channel of an unlicensed frequency band according to some embodiments. The gating cycle600may repeat indefinitely or for a predetermined amount of time. A first allocation605indicates time intervals in the gating cycle600that are allocated to a first base station (such as the base station104shown inFIG. 1), a second allocation610indicates time intervals in the gating cycle600that are allocated to a second base station (such as the base station105shown inFIG. 1), and a third allocation615indicates time intervals in the gating cycle600that are allocated to a third base station. The horizontal axes indicate time increasing from left to right. The first, second, and third base stations operate according to the same RAT and so they can share the entire gating cycle600. For example, the first, second, and third base stations may transmit downlink signals on the channel of the unlicensed frequency band according to LTE.

The gating cycle600is subdivided into time slots620(only one indicated by a reference numeral in the interest of clarity) that can be allocated to the first, second, or third base stations for downlink transmissions on the channel of the unlicensed frequency band. The first, second, and third base stations may therefore negotiate to partition the timeslot620among themselves, e.g., by exchanging information over interfaces between the first, second, and third base stations. The negotiation protocol is a matter of design choice. In the illustrated embodiment, the first, second, and third base stations have negotiated to allocate a subset625of the timeslots in the gating cycle600, as well as a subset630of the timeslots in the subsequent gating cycle, to the first base station. The dotted lines indicate time slots that are not allocated to the first base station. As a result of the negotiations, the subset635in the gating cycle600, as well as the timeslot640in the subsequent gating cycle, are allocated to the second base station and the subset645is allocated to the third base station.

FIG. 7is a diagram showing allocation of time intervals in a gating cycle700for downlink transmissions by two base stations that operate according to a first RAT and a third base station that operates according to a second RAT according to some embodiments. The gating cycle700may repeat indefinitely or for a predetermined amount of time. First and second base stations that operate according to a first RAT have detected the presence of a third base station that operates according to a second RAT. For example, the first and second base stations may operate according to LTE and the third base station may operate according to Wi-Fi. The third base station is transmitting downlink signals over a shared channel of an unlicensed frequency band. The first and second base stations therefore reserve a predetermined time interval705for downlink transmissions by the third base station. For example, the predetermined time interval705may correspond to a 50% duty cycle. The first and second base stations bypass downlink transmissions on the shared channel during the predetermined time interval705.

The first and second base stations negotiate allocation of the unreserved portion of the gating cycle700, e.g., using messages transmitted over an interface between the first and second base stations. As a result of the negotiation, the first base station is allocated a time interval710in unreserved portion of the gating cycle700, as well as the time interval715in an unreserved portion of the subsequent gating cycle, for downlink transmissions over the shared channel of the unlicensed frequency band. The second base station is allocated a time interval720in the unreserved portion of the gating cycle700, as well as the time interval725in an unreserved portion of the subsequent gating cycle, for downlink transmission over the shared channel. In some embodiments, the time intervals710,715,720,725may include one or more timeslots such as the time slots620shown inFIG. 6. Timeslots in the unreserved portion of the gating cycle700may therefore be allocated to more than two base stations that share the channel of the unlicensed frequency band and operate according to the first RAT.

FIG. 8is a flow diagram of a method800for identifying and bypassing transmission over a primary channel of an unlicensed frequency band according to some embodiments. The method800may be implemented in some embodiments of the base stations104,105shown inFIG. 1and some embodiments of the base station205shown inFIG. 2. At block805, the base station receives a report from one or more user equipment for a monitored subset of channels. In the illustrated embodiment, the base station receives one or more messages including the report, which may include information such as a measured RSSI, SNR, or SINR of one or more of the channels. The one or more messages also include information indicating whether one or more of the channels is a primary channel or a secondary channel used by a base station that operates according to a RAT such as Wi-Fi or other IEEE 802.11 standards. As discussed herein, user equipment may determine whether one or more of the channels is a primary channel or a secondary channel using information (such as a beacon signal preamble) in the modulated or decoded signals received by the user equipment over the channel during a measurement gap.

At decision block810, the base station uses information in the measurement report to determine whether the user equipment detected a beacon signal transmitted on a primary channel. Some embodiments of a beacon signal may convey system information to user equipment that user equipment need to maintain their association with the base station or access point that transmitted the beacon signal. The base stations should not select the primary channel for downlink transmissions because user equipment may not be able to successfully receive (i.e., demodulate and decode) the primary channel if there is interference from downlink transmissions on the primary channel by the base station. Thus, if the measurement report indicates that no beacon signals or primary channels were detected by the user equipment, the base station may transmit (at block815) on a clear channel from the subset of channels. However, if the measurement report indicates that the user equipment detected a beacon signal or a primary channel, the base station may identify the primary channel using the information in the measurement report at block820. The base station may then bypass transmission on the primary channel at block825. Bypassing transmission on the primary channel may include selecting a different channel from the subset of channels and transmitting on the selected channel or it may include bypassing transmission altogether until a clear, non-primary channel has been detected.