Low latency physical layer design for contention-based uplink channels

Certain aspects relate to methods and apparatus for latency reduction for UEs in a RRC connected mode. During contention-based uplink access by groups of UEs within a subframe, an eNB may decode the received uplink transmission based, at least in part, on the assigned group of resources assigned to the UE and used for transmission. Additional orthogonalization techniques such as reduced TTI size can be used to reduce collisions among different users performing contention-based transmissions. Furthermore, when the eNB fails to successfully decode the uplink transmission, the eNB may identify the UE that sent the uplink transmission based on a detected reference signal and may transmit an uplink assignment to the identified UE.

FIELD

The present disclosure relates generally to wireless communications and, more specifically, to methods and apparatus for low-latency contention-based access within an uplink subframe.

BACKGROUND

A wireless communication network may include a number of base stations (BSs) that can support communication for a number of user equipments (UEs). A UE may communicate with a base station (BS) via the downlink and uplink. The downlink (or forward link) refers to the communication link from the BS to the UE, and the uplink (or reverse link) refers to the communication link from the UE to the BS. A BS may transmit data and control information on the downlink to a UE and/or may receive data and control information on the uplink from the UE.

Currently, a UE transmits uplink data after receiving a grant from a BS for uplink transmissions. Unnecessary delays in transmission of uplink data may occur during low uplink traffic scenarios, as the UE waits for the uplink grant. It is desirable to reduce delays for uplink transmissions.

SUMMARY

Certain aspects of the present disclosure provide a method for wireless communication by an evolved Node B (eNB). The method generally includes assigning different groups of resources, within an uplink subframe, to different groups of one or more user equipments (UEs), wherein each UE selects resources from its assigned group for contention-based access within the uplink subframe, and decoding uplink transmissions received from the UEs in the subframe based, at least in part, on the assigned group of resources.

Certain aspects of the present disclosure provide an apparatus for wireless communication. The apparatus generally includes means for assigning different groups of resources, within an uplink subframe, to different groups of one or more user equipments (UEs), wherein each UE selects resources from its assigned group for contention-based access within the uplink subframe, and means for decoding uplink transmissions received from the UEs in the subframe based, at least in part, on the assigned group of resources.

Certain aspects of the present disclosure provide an apparatus for wireless communication. The apparatus generally includes at least one processor and a memory coupled to the at least one processor with instructions stored thereon. The at least one processor may be configured to assign different groups of resources, within an uplink subframe, to different groups of one or more user equipments (UEs), wherein each UE selects resources from its assigned group for contention-based access within the uplink subframe, and decode uplink transmissions received from the UEs in the subframe based, at least in part, on the assigned group of resources.

Certain aspects of the present disclosure provide a computer readable medium for wireless communication having instructions stored thereon. The instructions are executable by one or more processors for assigning different groups of resources, within an uplink subframe, to different groups of one or more user equipments (UEs), wherein each UE selects resources from its assigned group for contention-based access within the uplink subframe, and decoding uplink transmissions received from the UEs in the subframe based, at least in part, on the assigned group of resources.

Numerous other aspects are provided including apparatus, systems and computer program products. Various aspects and features of the disclosure are described in further detail below.

DETAILED DESCRIPTION

In LTE networks, a UE may transmit a scheduling request (SR) or a message on a random access channel (RACH) to an eNB when it has uplink (UL) data to transmit. In response, the UE may receive, from an eNB, a grant for UL transmissions. Thereafter, the UE may transmit UL data.

In an effort to reduce latency, according to aspects of the present disclosure, the eNB may schedule different groups of resources, within an uplink subframe, to different groups of UEs. The UEs may select resources from its assigned group of resources, for contention-based uplink access.

Using techniques described herein, when there is no contention, the eNB may receive uplink contention-based transmissions on an uplink channel (e.g., PUSCH) with less delay, as compared to conventional means (e.g., as compared to when a UE performs UL transmissions after receiving an UL grant). However, when uplink collisions occur, an increased delay, relative to conventional means, may occur. For example, when uplink collisions occur, the eNB may decode uplink data with increased latency as compared to if the UE had transmitted a SR or a message on a RACH, received an UL grant, and transmitted uplink data in response to the received grant.

Therefore, aspects of the present disclosure also provide a design in which advanced receiver algorithms, at an eNB, separate transmissions from UEs even when uplink collisions exist. As will be described in more detail herein, even when uplink transmissions from UEs may not be decoded (for example, due to uplink collisions), the eNB may identify a UE that transmitted the unsuccessfully decoded transmission, and may transmit an uplink grant to the identified UE. In this manner, the identified UE may transmit its uplink data, contention-free, on an uplink channel.

FIG. 1shows a wireless communication network100(e.g., an LTE network), in which the techniques described herein may be practiced. For example, the techniques may be utilized for communications between groups of UEs120and an eNB110. As will be described in more detail herein, the eNB110(e.g., eNB110a, eNB110b, eNB110c) may assign groups of resources to groups of one or more UEs120for contention-based access within an uplink subframe. Further, the eNB110may decode, uplink transmissions received from the UEs120in the subframe based, at least in part, on the assigned group of resources.

As illustrated, the wireless network100may include a number of evolved Node Bs (eNBs)110and other network entities. An eNB may be a station that communicates with user equipment devices and may also be referred to as a BS, a Node B, an access point (AP), etc. Each eNB110may provide communication coverage for a particular geographic area. The term “cell” may refer to a coverage area of an eNB and/or an eNB subsystem serving this coverage area, depending on the context in which the term is used.

An eNB may provide communication coverage for a macro cell, a pico cell, a femto cell, and/or other types of cell. A macro cell may cover a relatively large geographic area (e.g., several kilometers in radius) and may allow unrestricted access by UEs with service subscription. A pico cell may cover a relatively small geographic area and may allow unrestricted access by UEs with service subscription. A femto cell may cover a relatively small geographic area (e.g., a home) and may allow restricted access by UEs having association with the femto cell (e.g., UEs in a Closed Subscriber Group (CSG), UEs for users in the home, etc.). An eNB for a macro cell may be referred to as a macro eNB. An eNB for a pico cell may be referred to as a pico eNB. An eNB for a femto cell may be referred to as a femto eNB or a home eNB. In the example shown inFIG. 1, eNBs110a,110b, and110cmay be macro eNBs for macro cells102a,102b, and102c, respectively. eNB110xmay be a pico eNB for a pico cell102x. eNBs110yand110zmay be femto eNBs for femto cells102yand102z, respectively. An eNB may support one or multiple (e.g., three) cells.

The wireless network100may also include relay stations. A relay station is a station that receives a transmission of data and/or other information from an upstream station (e.g., an eNB or a UE) and sends a transmission of the data and/or other information to a downstream station (e.g., a UE or an eNB). A relay station may also be a UE that relays transmissions for other UEs. In the example shown inFIG. 1, a relay station110rmay communicate with eNB110aand a UE120rin order to facilitate communication between eNB110aand UE120r. A relay station may also be referred to as a relay eNB, a relay, etc.

The wireless network100may be a heterogeneous network that includes eNBs of different types, e.g., macro eNBs, pico eNBs, femto eNBs, relays, etc. These different types of eNBs may have different transmit power levels, different coverage areas, and different impact on interference in the wireless network100. For example, macro eNBs may have a high transmit power level (e.g., 20 watts) whereas pico eNBs, femto eNBs, and relays may have a lower transmit power level (e.g., 1 watt).

The wireless network100may support synchronous or asynchronous operation. For synchronous operation, the eNBs may have similar frame timing, and transmissions from different eNBs may be approximately aligned in time. For asynchronous operation, the eNBs may have different frame timing, and transmissions from different eNBs may not be aligned in time. The techniques described herein may be used for both synchronous and asynchronous operation.

A network controller130may couple to a set of eNBs and provide coordination and control for these eNBs. The network controller130may communicate with the eNBs110via a backhaul. The eNBs110may also communicate with one another, e.g., directly or indirectly via wireless or wireline backhaul.

The UEs120may be dispersed throughout the wireless network100, and each UE may be stationary or mobile. A UE may also be referred to as a terminal, a mobile station, a subscriber unit, a station, etc. A UE may be a cellular phone, a smart phone, a personal digital assistant (PDA), a wireless modem, a wireless communication device, a handheld device, a laptop computer, a netbook, a smartbook, an ultrabook, a cordless phone, a wireless local loop (WLL) station, a tablet, a position location device, a gaming device, a camera, a wearable device (e.g., smart glasses, smart goggles, smart bracelet, smart watch, smart band, smart ring, smart clothing), a drone, a robot, etc. A UE may be able to communicate with macro eNBs, pico eNBs, femto eNBs, relays, etc. InFIG. 1, a solid line with double arrows indicates desired transmissions between a UE and a serving eNB, which is an eNB designated to serve the UE on the downlink and/or uplink. A dashed line with double arrows indicates interfering transmissions between a UE and an eNB.

LTE utilizes orthogonal frequency division multiplexing (OFDM) on the downlink and single-carrier frequency division multiplexing (SC-FDM) on the uplink. OFDM and SC-FDM partition the system bandwidth into multiple (K) orthogonal subcarriers, which are also commonly referred to as tones, bins, etc. Each subcarrier may be modulated with data. In general, modulation symbols are sent in the frequency domain with OFDM and in the time domain with SC-FDM. The spacing between adjacent subcarriers may be fixed, and the total number of subcarriers (K) may be dependent on the system bandwidth. For example, K may be equal to 128, 256, 512, 1024, or 2048 for system bandwidth of 1.25, 2.5, 5, 10, or 20 megahertz (MHz), respectively. The system bandwidth may also be partitioned into subbands. For example, a subband may cover 1.08 MHz, and there may be 1, 2, 4, 8, or 16 subbands for system bandwidth of 1.25, 2.5, 5, 10, or 20 MHz, respectively.

A UE may be within the coverage of multiple eNBs. One of these eNBs may be selected to serve the UE. The serving eNB may be selected, for example, based on various criteria such as received power, received quality, path loss, signal-to-noise ratio (SNR), etc.

A UE may operate in a dominant interference scenario in which the UE may observe high interference from one or more interfering eNBs. A dominant interference scenario may occur due to restricted association. For example, inFIG. 1, UE120ymay be close to femto eNB110yand may have high received power for eNB110y. However, UE120ymay not be able to access femto eNB110ydue to restricted association and may then connect to macro eNB110cwith lower received power (as shown inFIG. 1) or to femto eNB110zalso with lower received power (not shown inFIG. 1). UE120ymay then observe high interference from femto eNB110yon the downlink and may also cause high interference to eNB110yon the uplink.

A dominant interference scenario may also occur due to range extension, which is a scenario in which a UE connects to an eNB with lower path loss and lower SNR among all eNBs detected by the UE. For example, inFIG. 1, UE120xmay detect macro eNB110band pico eNB110xand may have lower received power for eNB110xthan eNB110b. Nevertheless, it may be desirable for UE120xto connect to pico eNB110xif the path loss for eNB110xis lower than the path loss for macro eNB110b. This may result in less interference to the wireless network for a given data rate for UE120x.

In an aspect, communication in a dominant interference scenario may be supported by having different eNBs operate on different frequency bands. A frequency band is a range of frequencies that may be used for communication and may be given by (i) a center frequency and a bandwidth or (ii) a lower frequency and an upper frequency. A frequency band may also be referred to as a band, a frequency channel, etc. The frequency bands for different eNBs may be selected such that a UE can communicate with a weaker eNB in a dominant interference scenario while allowing a strong eNB to communicate with its UEs. An eNB may be classified as a “weak” eNB or a “strong” eNB based on the relative received power of signals from the eNB received at a UE (e.g., and not based on the transmit power level of the eNB).

According to aspects, and as will be described in more detail herein, an eNB110may assign different groups of resources to groups of UEs120. Each of the UEs120may select resources from its assigned group of resources for contention-based access within an uplink subframe. According to the techniques described herein, the eNB110may decode uplink transmissions based, at least in part, on the assigned group of resources. Furthermore, when an uplink transmission is not successfully decoded by the eNB110, the eNB may identify the UE that transmitted the uplink transmissions, for example, based on a detected demodulation reference signal (DMRS). Regardless of the receiver algorithms used by the eNB to identify the UE, upon identification, the eNB may transmit an uplink grant to the identified UE.

FIG. 2shows a frame structure used in LTE. For example, eNB110may communicate on the downlink (DL) using the illustrated frame structure.

The transmission timeline for the downlink may be partitioned into units of radio frames. Each radio frame may have a predetermined duration (e.g., 10 milliseconds (ms)) and may be partitioned into 10 subframes with indices of 0 through 9. Each subframe may include two slots. Each radio frame may thus include 20 slots with indices of 0 through 19. Each slot may include L symbol periods, e.g., L=7 symbol periods for a normal cyclic prefix (as shown inFIG. 2) or L=6 symbol periods for an extended cyclic prefix. The 2L symbol periods in each subframe may be assigned indices of 0 through 2L−1. The available time frequency resources may be partitioned into resource blocks. Each resource block may cover N subcarriers (e.g., 12 subcarriers) in one slot.

In LTE, an eNB may send a primary synchronization signal (PSS) and a secondary synchronization signal (SSS) for each cell in the eNB. The primary and secondary synchronization signals may be sent in symbol periods 6 and 5, respectively, in each of subframes 0 and 5 of each radio frame with the normal cyclic prefix (CP), as shown inFIG. 2. The synchronization signals may be used by UEs for cell detection and acquisition. The eNB may send a Physical Broadcast Channel (PBCH) in symbol periods 0 to 3 in slot 1 of subframe 0. The PBCH may carry certain system information.

The eNB may send a Physical Control Format Indicator Channel (PCFICH) in the first symbol period of each subframe, as shown inFIG. 2. The PCFICH may convey the number of symbol periods (M) used for control channels, where M may be equal to 1, 2 or 3 and may change from subframe to subframe. M may also be equal to 4 for a small system bandwidth, e.g., with less than 10 resource blocks. The eNB may send a Physical HARQ Indicator Channel (PHICH) and a Physical Downlink Control Channel (PDCCH) in the first M symbol periods of each subframe (not shown inFIG. 2). The PHICH may carry information to support hybrid automatic repeat request (HARQ). The PDCCH may carry information on resource allocation for UEs and control information for downlink channels. The eNB may send a Physical Downlink Shared Channel (PDSCH) in the remaining symbol periods of each subframe. The PDSCH may carry data for UEs scheduled for data transmission on the downlink.

A number of resource elements may be available in each symbol period. Each resource element (RE) may cover one subcarrier in one symbol period and may be used to send one modulation symbol, which may be a real or complex value. Resource elements not used for a reference signal in each symbol period may be arranged into resource element groups (REGs). Each REG may include four resource elements in one symbol period. The PCFICH may occupy four REGs, which may be spaced approximately equally across frequency, in symbol period 0. The PHICH may occupy three REGs, which may be spread across frequency, in one or more configurable symbol periods. For example, the three REGs for the PHICH may all belong in symbol period 0 or may be spread in symbol periods 0, 1, and 2. The PDCCH may occupy 9, 18, 36, or 72 REGs, which may be selected from the available REGs, in the first M symbol periods, for example. Only certain combinations of REGs may be allowed for the PDCCH.

FIG. 2Ashows an exemplary format200A for the uplink in LTE. As described herein, an eNB may assign groups of uplink resources to groups of one or more UEs for contention-based access within an uplink subframe. The eNB may decode uplink transmissions received from the UEs in the subframe based, at least in part, on the assigned group of resources.

A UE may be assigned resource blocks in the control section to transmit control information to an eNB. The UE may also be assigned resource blocks in the data section to transmit data to the Node B. The UE may transmit control information in a Physical Uplink Control Channel (PUCCH)210a,210bon the assigned resource blocks in the control section. The UE may transmit data or both data and control information in a Physical Uplink Shared Channel (PUSCH)220a,220bon the assigned resource blocks in the data section. An uplink transmission may span both slots of a subframe and may hop across frequency as shown inFIG. 2A.

FIG. 3illustrates a block diagram of a design of a BS/eNB110and a UE120in the wireless communication network100. In certain aspects, the BS/eNB110may be one of the BSs/eNBs illustrated inFIG. 1and the UE120may be one of the UEs illustrated inFIG. 1. The BSs/eNBs and UEs described herein may include one or more modules as shown inFIG. 3. The BS/eNB110may be configured to perform the operations described herein, and as detailed inFIG. 6.

For a restricted association scenario, the eNB110may be macro eNB110cinFIG. 1, and UE120may be UE120yinFIG. 1. The eNB110may also be a BS of some other type. The eNB110may be equipped with T antennas334athrough334t, and the UE120may be equipped with R antennas352athrough352r, where in general T≥1 and R≥1.

At the eNB110, a transmit processor320may receive data from a data source312and control information from a controller/processor340. The control information may be for the PBCH, PCFICH, PHICH, PDCCH, etc. The data may be for the PDSCH, etc. The transmit processor320may process (e.g., encode and symbol map) the data and control information to obtain data symbols and control symbols, respectively. The transmit processor320may also generate reference symbols, e.g., for the PSS, SSS, and cell-specific reference signal. A transmit (TX) multiple-input multiple-output (MIMO) processor330may perform spatial processing (e.g., precoding) on the data symbols, the control symbols, and/or the reference symbols, if applicable, and may provide T output symbol streams to T modulators (MODs)332athrough332t. Each modulator332may process a respective output symbol stream (e.g., for OFDM, etc.) to obtain an output sample stream. Each modulator332may further process (e.g., convert to analog, amplify, filter, and upconvert) the output sample stream to obtain a downlink signal. T downlink signals from modulators332athrough332tmay be transmitted via T antennas334athrough334t, respectively.

At the UE120, antennas352athrough352rmay receive the downlink signals from the eNB110and may provide received signals to demodulators (DEMODs)354athrough354r, respectively. Each demodulator354may condition (e.g., filter, amplify, downconvert, and digitize) a respective received signal to obtain input samples. Each demodulator354may further process the input samples (e.g., for OFDM, etc.) to obtain received symbols. A MIMO detector356may obtain received symbols from all R demodulators354athrough354r, perform MIMO detection on the received symbols, if applicable, and provide detected symbols. A receive processor358may process (e.g., demodulate, deinterleave, and decode) the detected symbols, provide decoded data for the UE120to a data sink360, and provide decoded control information to a controller/processor380.

On the uplink, at the UE120, a transmit processor364may receive and process data (e.g., for the PUSCH) from a data source362and control information (e.g., for the PUCCH) from the controller/processor380. The transmit processor364may also generate reference symbols for a reference signal. The symbols from the transmit processor364may be precoded by a TX MIMO processor366if applicable, further processed by modulators354athrough354r(e.g., for SC-FDM, etc.), and transmitted to the eNB110. At the eNB110, the uplink signals from the UE120may be received by antennas334, processed by demodulators332, detected by a MIMO detector336if applicable, and further processed by a receive processor338to obtain decoded data and control information sent by the UE120. The receive processor338may provide the decoded data to a data sink339and the decoded control information to the controller/processor340.

The controller/processor340,380may direct the operation at the eNB110and the UE120, respectively. For example, the controller/processor340and/or other processors and modules at the BS/eNB110may perform or direct operations described below with reference toFIG. 6and/or other processes for the techniques described herein. The memory342may store data and program codes for the eNB110. The memory382may store data and program codes for the UE120. A scheduler344may schedule and/or assign groups of resources, within an uplink subframe to different groups of one or more UEs. One or more antennas334and demodulators/modulators332may decode received UL transmissions from the UEs, based at least in part on the assigned group of resources, receive a buffer status report (BSR) in an uplink transmission from at least one of the UEs, and/or transmit an UL grant.

According to certain aspects, a UE or eNB may support a low latency (“LL” or ultra low latency “ULL”) capability. As used herein, the term ultra low latency capability generally refers to the capability to perform certain procedures with low latency relative to devices that lack the capability (e.g., so called “legacy” devices). In one implementation, the ULL capability may refer to the ability to support transmission time interval (TTI) periods around 0.1 ms or less (e.g., 20 μs) (with 0.1 ms or 20 μs corresponding to a conventional LTE subframe duration). However, it should be noted that, in other implementations, the ULL capability may refer to other low latency periods. Some examples of TTI considered for LL or ULL include: TTI spanning one slot (½ of a subframe), TTI spanning one symbol ( 1/14thof a subframe), or TTI spanning 1/10thof a subframe.

As described above, in current LTE networks, a UE transmits a SR or a message on a RACH prior to transmitting uplink data to an eNB. In response, the eNB transmits an uplink grant to the UE and the UE transmits uplink data according to its received grant. Accordingly, the delay in the uplink transmission may include, for example, the SR delay (e.g., depending on periodicity), the time to receive an uplink grant, and the time to transmit the uplink data. Aspects of the present disclosure decrease this delay.

For contention-based uplink access (e.g., on the PUSCH), the eNB may provide persistent uplink (PUSCH) assignments to multiple UEs. For example, multiple UEs may be assigned overlapping resources. The UEs may directly transmit on the uplink channel according to their pre-assigned resources.

Several considerations regarding contention-based uplink channel access exist. For example, an eNB may control how uplink resources are assigned to UEs. The eNB may pre-assign exact resources to each of the UEs. The eNB may overload different users sharing the same time-frequency resources. For contention-based uplink access, the UE may transmit when it has data. When the UE does not have uplink data to transmit, the UE may enter a power saving mode (e.g., discontinuous transmission (DTx), sleep mode, idle mode, etc.).

According to another example, for contention-based uplink access, the eNB may assign UEs a region of resources, as opposed to exact resources. In response, each UE may randomly select the resources within the assigned region to use for uplink transmission.

To decode uplink transmissions, the eNB may need to separate transmissions received from different UEs. Data transmitted on the PUSCH may have a different cell radio network temporary identity (C-RNTI) based scrambling. Additionally or alternatively, a user may use different sequence and/or shifts for its demodulation reference signals (DMRS). The sequences or shifts may be assigned to the UE or may be randomly selected, for example, by the UE. Additionally or alternatively, transmissions from different UEs may be separated using spatial separation (e.g., multiple antenna processing).

As described herein, advanced receiver processing, at the eNB, with interference cancellation may be used to separate contention-based transmissions received from different UEs. Further, code division multiplexing (CDM) or Walsh covers may be applied to the DMRS and/or PUSCH transmissions across different transmission time intervals (TTIs). In this manner, UEs may be separated by orthogonal codes in contention-based transmission scenarios.

FIG. 4illustrates an example400subframe with collision avoidance with reduced TTI, according to aspects of the present disclosure. Two slots, Slot 0 and Slot 1, of a subframe400are illustrated. Conventionally, a single user may be assigned to both uplink slots, as shown at402and404.

According to aspects of the present disclosure, an eNB may assign, to a group of UEs, resources corresponding to a TTI with a duration that is less than a subframe. For example, a group of UEs may be assigned to at least one group of resources406, which is less than a subframe. Another group of UEs may be assigned to at least one group of resources408, which is also less than a subframe.

According to aspects, the TTI may have a duration that is less than one time slot of the subframe. For example, a group of UEs may be assigned to at least one group of resources410, which is less than one slot of the subframe. Other TTIs412,414, and416, which also have a duration that is less than one slot of a subframe, may be assigned to different groups of one or more UEs. According to aspects, a reduced TTI (e.g., TTI less than one subframe) duration may be used for UEs with packets to transmit that are small enough to fit into the smaller TTI. For example, metered, machine-type communication (MTC) UEs may be assigned a reduced TTI duration. MTC UEs may communicate with a BS/eNB, another remote device, or some other entity. Machine type communications may involve one or more entities that do not necessarily need human interaction. Examples of MTC devices include various wireless sensors, monitors, detectors, meters, or other type data monitoring, generating, or relaying devices that may be expected to operate (possibly unattended) for years on a single battery charge. MTC devices may also include drones, robots, and other forms of automated or autonomous devices. MTC UEs may operate in a Cellular Internet of Things (CIOT), whereby UEs may collect and transmit data.

According to aspects, a reduced TTI duration may be assigned to UEs to transmit a buffer status report (BSR) along with small packet data (e.g., MTC data). The eNB may provide one or more grants for subsequent uplink transmission based, at least in part, on the BSR.

According to aspects, in an effort to avoid uplink collisions in contention-based uplink access, an eNB with a multi-user detection (MUD) receiver may assign UEs with different cyclic shifts and/or different root sequences for DMRS and overlapping assigned resources on the uplink channel (e.g., PUSCH).

When the UE has UL data to transmit, the UE may transmit on the uplink channel according to its persistent assignment, using the assigned shift and/or root sequence. The UE may refrain from transmitting when it does not have UL data.

The eNB may separate users using the received DMRS and may attempt to decode the uplink channel (e.g., PUSCH) with interference cancellation at the eNB receiver. For example, the eNB may use the assigned cyclic shifts and/or different assigned root sequences to identify different UEs and separate contention-based uplink transmissions. In this manner, according to aspects, the reduced shifts provide an additional dimension to increase the ability of an eNB to decode contention-based uplink transmissions.

FIG. 5illustrates an example of collision avoidance500, according to aspects of the present disclosure.502aillustrates example user assignments for UEs within a group of UEs.502billustrates examples of actual uplink transmissions by users with assignments as shown in502a.

As described above, different groups of UE may be assigned different groups of resources. Each UE within the group of UEs may be assigned cyclic shifts and/or root sequences as illustrated in502a. For example, as shown at504a, two users of the group may each be assigned different cyclic shifts and/or different root sequences, illustrated by the two shaded regions. As shown at506a, three users of another group of UEs may each be assigned different cyclic shifts and/or different root sequences, as shown by the three shaded regions. Similarly, groups of four, six, and twelve users may each be assigned different cyclic shifts and/or different root sequences as shown at508a,510a, and512a, respectively.

Example uplink transmissions by UEs of a group of UEs are illustrated at502b. At504b, only one user of the group of two users (e.g., at504a) transmitted using its assigned cyclic shift and/or root sequence, as is shown by the single shaded region. At506b, while three users of a group were each assigned different cyclic shifts and/or different root sequences (e.g., at506a), only two users, shown by the two shaded regions, transmitted using their assigned cyclic shifts and/or different root sequences. Similarly, while four, six, and twelve users were each assigned different cyclic shifts and/or different root sequences as shown at508a,510a, and512a, only two, three, and five users, respectively, actually transmitted as shown by the corresponding two shaded regions of508b, three shaded regions of510b, and five shaded regions of512bin502b. As described above, the eNB receiving contention-based uplink transmissions may attempt to separate users using the shift and/or sequence of the received uplink transmission.

As described above, collision handling at an eNB is important in contention-based uplink channel access, so as not to increase latency as compared to conventional means. In other words, it is desirable to have collision handling such that when an eNB does not successfully decode a detected DMRS from at least one uplink transmission, latency is not worse than if the UE had transmitted a SR, received an uplink grant in response to the SR, and transmitted data according to the received grant.

According to aspects of the present disclosure, the eNB may rely on DMRS design and receiver processing to identify the transmitting UE when decoding is unsuccessful. For example, the eNB may configure unique sequences (root and/or cyclic shift sequences) for each UE. When the eNB decodes the uplink channel and detects the presence of a UE's transmission, it may acknowledge the transmission and/or assign further resources depending on a received BSR.

When the eNB is unable to decode the uplink channel, for example based on detecting the UE's DMRS, the eNB may identify the UE that sent the uplink transmission which was not successfully received, and transmit an uplink assignment to the identified UE. Thus, when collisions occur on the contention-based uplink channel within a subframe, the eNB may use collision handling techniques to identify a user that transmitted the unsuccessfully decoded uplink transmission. The eNB may subsequently transmit an uplink grant to the identified user. In this manner, aspects of the present disclosure provide techniques for contention-based uplink channel access wherein the latency may not be worse than conventional means.

Furthermore, DMRS designs may be improved for better UE identification. For example, more DMRS symbols may be used and/or additional root sequences may be employed. Additionally or alternatively, the eNB may assign users different resources if collisions occur frequently.

As described above, the user may be identified in contention-based uplink access based, at least in part, on the detected DMRS and assigned root and/or shift sequence. The eNB may perform interference cancelation based on the partial decoding of an uplink transmission received from at least one UE to decode an uplink transmissions received from another UE. With interference cancellation, the eNB may have knowledge of what UEs are assigned on what resources, which may improve demodulation performance and may support more users for contention-based uplink channel access. According to aspects, interference cancellation may work with users with different coding rates/different rate distribution (e.g., according to rate region), HARQ termination targets, number of retransmissions in a HARQ process, and/or traffic needs. Thus, an eNB may assign different groups of resources to different groups of UEs taking at least one of these factors into account.

FIG. 6illustrates example operations600performed by an eNB, such as eNB110ofFIG. 1with one or more components of the eNB110illustrated inFIG. 3, according to aspects of the present disclosure. For example, modulator/demodulator,332, antenna334, schedule344, one or more controller/processors340, and memory342may perform the operations ofFIG. 6and the methods described herein.

At602, the eNB may assign different groups of resources, within an uplink subframe, to different groups of one or more UEs. Each UE may select resources from its assigned group for contention-based access within the uplink subframe.

At604, the eNB may decode uplink transmissions received from the UEs in the subframe based, at least in part, on the assigned group of resources.

As described with reference toFIG. 4, the eNB may assign, to a group of one or more UEs, at least one group of resources corresponding to a TTI with a duration that is less than a subframe400. According to aspects, the TTI may have a duration that is less than one time slot of a subframe.

As described with reference toFIG. 5, the eNB may assign different cyclic shifts or root sequences to each UE within a group of UEs. The eNB may decode uplink transmissions by distinguishing different UEs using the different cyclic shifts or root sequences. Further, the eNB may perform interference cancelation, based, at least in part, on an uplink transmission from at least one of the distinguished UEs, to decode an uplink transmission from another UE.

As described herein, the eNB generally decodes the uplink transmissions received from the UEs in the subframe by performing interference cancelation. The interference cancellation is based, at least, on a partial decoding of an uplink transmission received from at least one UE, to decode an uplink transmission from another UE. Furthermore, an eNB may generally assign different groups of resources to different groups of UEs based on least one of coding rates or traffic needs of the different UEs.

As described above, aspects of the present disclosure provide for latency reduction for UEs in a RRC connected mode. During contention-based uplink access by groups of UEs within a subframe, an eNB may decode the received uplink transmission based, at least in part, on the resources used for transmission. Furthermore, when the eNB fails to successfully decode the uplink transmission, the eNB may identify the UE that sent the uplink transmission based on, for example, a detected reference signal, and may transmit an uplink assignment to the identified UE. Using such collision handling, latency may be no worse than conventional means.

The various operations of methods described above may be performed by any suitable means capable of performing the corresponding functions. The means may include various hardware and/or software/firmware component(s) and/or module(s), including, but not limited to a circuit, an application specific integrated circuit (ASIC), or processor (e.g., controller/processor340, transmit processor320, transmit MIMO processor330, receive processor338, modulator/demodulator332, antenna334, controller/processor380, transmit processor364, transmit MIMO processor366, MIMO detector356, receive processor358, modulator/demodulator354, antenna352).