Patent Description:
The current fourth generation (<NUM>) wireless access within the 3rd generation partnership project (3GPP) long-term evolution (LTE) is based on orthogonal frequency-division multiplexing (OFDM) in downlink and discrete Fourier transform (DFT) spread OFDM, also known as single carrier frequency division multiple access (SC-FDMA), in uplink.

A candidate for a fifth generation (<NUM>) air interface is to scale the current LTE air interface, which is limited to <NUM> bandwidth, N times in bandwidth with <NUM>/N times shorter transmission-time duration. A typical value may be N=<NUM> so that the carrier has <NUM> bandwidth and <NUM> millisecond slot lengths. With this approach many functions in LTE can remain the same, which would simplify the standardization effort and allow for a reuse of technology components.

The carrier frequency for an anticipated <NUM> system could be higher than current <NUM> systems. Values in the range of <NUM>-<NUM> have been discussed. At such high frequencies, it is suitable to use an array antenna to achieve beamforming gain. Since the wavelength is small, e.g., less than <NUM>, an array antenna with a large number of antenna elements can be fitted into an antenna enclosure with a size comparable to <NUM> and <NUM> base station antennas of today.

<FIG> is a block diagram illustrating a radio network <NUM> that includes one or more wireless devices 110A-C, network nodes 115A-C (shown in <FIG> as base stations), radio network controller <NUM>, and packet core network <NUM>.

A wireless device <NUM> may communicate with a network node <NUM> over a wireless interface. For example, wireless device <NUM> may transmit wireless signals to network node <NUM> and/or receive wireless signals from network node <NUM>. The wireless signals may contain voice traffic, data traffic, control signals, and/or any other suitable information.

<FIG> is a block diagram illustrating a network <NUM> that includes three transmission points (TP) <NUM> for communicating with wireless device <NUM> or other user equipment (UE) via array antennas generating multiple beams <NUM>. A transmission point may include any network node such as network node <NUM> shown in <FIG>.

The beams generated by array antennas may typically be highly directive and give beamforming gains of <NUM> dB or more due to that a large number of antenna elements participate in forming a beam. This means that each beam is relatively narrow in angle, a half-power beam width (HPBW) of <NUM> degrees is not unlikely. Hence, a sector of a network node, such as a base station, must be covered with a large number of beams.

Where a system such as system <NUM> of <FIG> includes multiple transmission nodes, each node may have an array antenna capable of generating many beams <NUM> with small HPBW. These nodes may then, for instance, use one or multiple carriers, so that a total transmission bandwidth of multiples of hundreds of MHz can be achieved leading to downlink (DL) peak user throughputs reaching as high as <NUM> Gbit/s or more.

In LTE access procedures, a wireless device, or UE, first searches for a cell using a cell search procedure, where unique primary and secondary synchronization signals (PSS and SSS, respectively) are transmitted from each network node, or eNodeB in the context of LTE. When a cell has been found, the wireless device can proceed with further steps to become associated with this cell, which is then known as the serving cell for this wireless device. After the cell is found, the wireless device can read system information (transmitted on the physical broadcast channel), known as the master information block (MIB), which is found in a known time-frequency position relative to the PSS and SSS locations. After MIB is detected, the system frame number (SFN) and the downlink system bandwidth are known.

In LTE, as in any communication system, a mobile terminal may need to contact the network without having a dedicated resource in the uplink (UL) from wireless device to network node, or base station. To handle this, a random-access procedure is available where a wireless device that does not have a dedicated UL resource may transmit a signal to the base station.

<FIG> is a block diagram illustrating random-access preamble transmission <NUM>. The first message of this procedure is typically transmitted on a special communication resource reserved for random access, a physical random-access channel (PRACH). This channel can for instance be limited in time and/or frequency (as in LTE).

The communication resources available for PRACH transmission are provided to the wireless devices as part of the broadcasted system information in system information block two (SIB-<NUM>) or as part of dedicated radio-resource control (RRC) signaling in case of, e.g., handover.

The resources consist of a preamble sequence and a time/frequency resource. In each cell, there are <NUM> preamble sequences available. Two subsets of the <NUM> sequences are defined, where the set of sequences in each subset is signaled as part of the system information.

<FIG> is a signaling diagram illustrating a contention-based random-access procedure used in LTE. The wireless device <NUM> starts the random-access procedure by randomly selecting one of the preambles available for contention-based random access. At step <NUM>, the wireless device <NUM> transmits a random-access preamble (MSG1) on the physical random-access channel (PRACH) to network node <NUM>.

At step <NUM>, the radio-access network (RAN) acknowledges any preamble it detects by transmitting, from network node <NUM>, a random-access response (MSG2) including an initial grant to be used on the uplink shared channel, a radio network temporary identifier (TC-RNTI), and a time alignment (TA) update. When receiving the response, the wireless device <NUM> uses the grant to transmit a scheduled transmission message (MSG3) to network node <NUM> at step <NUM>.

The procedure ends with the RAN resolving any preamble contention that may have occurred for the case that multiple wireless devices transmitted the same preamble at the same time. This can occur since each wireless device <NUM> randomly selects when to transmit and which preamble to use. If multiple wireless devices select the same preamble for the transmission on PRACH, there will be contention that needs to be resolved through a contention resolution message (MSG4), which may be transmitted in a step <NUM>.

<FIG> also illustrates transmissions of hybrid automatic repeat request acknowledgement messages (HARQ ACK).

<FIG> is a block diagram illustrating contention-based random access, where there is contention between two wireless devices. Specifically, two wireless devices 110A, 110B, transmit the same preamble, p<NUM>, at the same time. A third wireless device 110C also transmits at the same time, but since it transmits with a different preamble, p<NUM>, there is no contention between this wireless device and the other two wireless devices.

A wireless device <NUM> can also perform non-contention based random access. <FIG> is a flowchart illustrating the procedure for a wireless device <NUM> to perform contention-free random access based on reception of a random access (RA) order message from network node <NUM>. Non-contention based random access is typically used in handover between two network nodes, such as any two of the network nodes 115A, 115B, 115C illustrated in <FIG>. In this case, the order for a non-contention based random access is transmitted from a source network node while the random access preamble (MSG <NUM>) is received at another target network node, which also transmits the random access response (MSG <NUM>). Similar to the contention-based random access, the random-access response (MSG2) is transmitted in the downlink (DL) to the wireless device <NUM> following successful detection of a random-access preamble (MSG1).

A prior art example is <CIT>. Other prior art examples are: <CIT>; and <CIT>.

Particular realizations of the invention are defined in the appended dependent claims.

In a beam-based radio-access system, it is a problem for the network side, i.e., network node <NUM>, to select in which beam to transmit the random-access responses, i.e., MSG2, to the wireless device <NUM>.

Furthermore, it is a complexity problem for the network side to detect random-access preambles, i.e., MSG1, in a beam-based radio-access system since the network node does not know which receive beam is the best to receive the preamble, and thus the network node <NUM> needs to repeat the search in each beam. Using the uplink received best beam to also transmit downlink signals to the same wireless device requires well-calibrated uplink and downlink radio-frequency chains (RF) in the network in order to ensure that the advantageous reception conditions over the uplink received best beam is reflected also over downlink, which is costly to implement.

An object of the present disclosure is to provide at least a wireless device, a network node, and methods for random access, which seek to mitigate, alleviate, or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination.

This object is obtained by: a method performed by a wireless device according to claim <NUM>; a method performed by a network node according to independent claim <NUM>; a wireless device according to independent claim <NUM>; and a network node according to independent claim <NUM>.

Hereby, since the network, i.e., the network node, knows the beam to use for random-access response, the coverage of random-access responses is improved. Also, the random-access procedure can be completed earlier, which improves latency and reduces interference in the network.

Another technical advantage may be that there is no need to have calibrated and aligned RF for uplink and downlink which reduces implementation cost and power consumption.

The object is also obtained by a method in a network node for supporting random access from a wireless device. The method comprises transmitting a set of beam-specific reference signals (BRS), and detecting a preamble in a signal received from the wireless device. The preamble detection indicates a BRS preferred by the wireless device. The method also comprises transmitting a random-access response in the same beam, and/or beam direction, and/or with the same beamforming weights, as the preferred BRS indicated by the preamble detection.

Again, since the network, i.e., the network node, knows the beam to use for random-access response, the coverage of random-access responses is improved. Also, the random-access procedure can be completed earlier, which improves latency and reduces interference in the network.

A further technical advantage may be that computational complexity in a network node, such as an eNodeB, is reduced by the present teaching. A random-access preamble detector in a network node only needs to search for a sub-set of preamble sequences in each uplink receiver direction. This subset equals to those random-access sequences that are mapped to the same downlink transmission beam (or spatial direction) as the receiver uplink beam (or spatial direction).

Some embodiments may benefit from some, none, or all of the above-mentioned advantages. Other technical advantages may be readily ascertained by one of ordinary skill in the art.

For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:.

Particular embodiments are described below with reference to <FIG> of the drawings, like numerals being used for like and corresponding parts of the various drawings.

<FIG> illustrates a system <NUM> that includes a wireless device <NUM> (shown in <FIG> as a 'terminal') operable to select a beam <NUM> based on received signal strength in the downlink (DL), according to certain embodiments. As depicted, system <NUM> includes multiple network nodes 115A, 115B, each transmitting unique reference signals per beam. In a particular embodiment, the two network nodes 115A, 115B may be two transmission points (TP) capable of performing multi-beam transmissions, in the same cell (same physical cell ID), or it may be nodes belonging to different cells.

In a particular embodiment, wireless device <NUM> can detect the preferred downlink beam (and eventually network node). In the depicted example, wireless device <NUM> has detected a beam-specific reference signal (BRS-<NUM>-<NUM>) from network node <NUM>. Wireless device <NUM> may then select a PRACH signal to transmit in the uplink so that the network gets information about which BRS is the "best" for the wireless device <NUM> and thus the network knows which downlink beam to use for subsequent messages such as the random-access channel (RACH) response. Note that the preambles associated with network node 115A can also be detected by network node 115B, if the two network nodes are coordinated.

Thus, a PRACH signal to transmit in the uplink is selected by the UE or wireless device <NUM> based on transmission conditions in the downlink from the network node <NUM> to the wireless device <NUM>.

As mentioned in the background section, the PRACH resources consist of a preamble sequence and a time/frequency resource. A PRACH resource can be taken from a subset of the set of all available preambles and/or the wireless device can transmit the preamble in a certain frequency band within the system bandwidth. When the network has detected the preamble transmitted from the wireless device, it knows which downlink beam is the preferred to be used for downlink transmissions such as the following RACH response.

Thus, the preamble and/or the time/frequency resource used for transmitting the preamble, is selected by the UE, or wireless device <NUM>, based on transmission conditions in the downlink from the network node <NUM> to the wireless device <NUM>.

<FIG> illustrates a flowchart depicting exemplary method steps performed for the selection of a preferred downlink beam, according to certain embodiments. Specifically, the right hand side of the flowchart depicts the steps that may be performed by wireless device <NUM>, and the left hand side depicts the steps that may be performed by network node <NUM>, according to certain embodiments.

More specifically, the right hand side illustrates a method in a wireless device <NUM> for performing random access to a network node <NUM>. The method comprises receiving <NUM> a set of downlink beam-specific reference signals, BRS, from the network node <NUM>. The method also comprises determining <NUM> a preferred BRS based on the received signal power for each BRS, as well as selecting <NUM>, based on the preferred BRS, a random-access resource to be used for transmitting a random-access attempt to the network node <NUM>. The method further comprises using <NUM> the selected random-access resource when transmitting a random-access attempt to the network node <NUM>, whereby the selection of random-access resource indicates to the network node which downlink beam is preferred by the wireless device to be used for downlink transmissions.

The left hand side of the flowchart shown in <FIG> illustrates a method in a network node <NUM> for supporting random access from a wireless device <NUM>. The method comprises transmitting <NUM> a set of beam-specific reference signals (BRS). The method also comprises detecting <NUM> a preamble in a signal received from the wireless device <NUM>, said preamble detection indicating a BRS preferred by said wireless device. The method further comprises transmitting <NUM> a random-access response in the same beam, and/or beam direction, and/or with the same beamforming weights, as the preferred BRS indicated by the preamble detection.

Of course, the network node <NUM> will attempt to detect more than one single preamble during a given time duration, and thus will sequentially or in parallel attempt to detect all relevant preambles in the communication system.

The methods illustrated in <FIG> may begin at step <NUM> when network node <NUM> (eNB, base station) may transmit a set of beam-specific reference signals in the downlink. The signals may be received by the wireless device at step <NUM>. Wireless device <NUM> may then perform measurements on these different (preferably orthogonal) reference signals and then determine a preferred BRS at step <NUM>. This can be done by measuring reference signal received power (RSRP). The reference signal can be beamformed synchronization signals (Primary Synchronization Signal PSS/ Secondary Synchronization Signal SSS), beamformed channel state information reference signals (CSI-RS), beamformed discovery signals or it can be newly designed beam-specific reference signal (BRS) sequences. Herein, we denote and classify the beam-specific reference signals as BRS, for simplicity.

The beam-specific reference signals are assumed known by e.g. specification or from (broadcasted) system information, before the wireless device can start measuring and identifying the preferred downlink beam. However, in one embodiment, configuration signaling takes place prior to the identification but on non-beam-based legacy system such as LTE. In practice, the wireless device detects a preferred beam-specific reference signal from a set of beam-specific reference signals, so the wireless device is not aware of the actual beam direction of beam radiation pattern, or beam forming weights used by the transmitter side which is entirely implementation specific.

At step <NUM>, wireless device <NUM> selects random-access resource for transmitting random-access attempt to the network node <NUM>.

According to some embodiments, the selecting <NUM> comprises selecting 808a a preamble, from a set of preambles, to be used for transmitting the random-access attempt.

According to some embodiments, the selecting <NUM> comprises selecting 808b a time and/or frequency resource to be used for transmitting the random-access attempt. According to such embodiments, the PRACH resource (which potentially is one of multiple resources distributed in time or frequency) to use when transmitting the preamble depends on the detected preferred BRS. Hence, the network will know which BRS was preferred from the wireless device, or UE, side from which band and/or time-domain location the network has detected the preamble in the uplink. And thus, the network knows the direction in which to transmit the random-access response (MSG <NUM>) since it is the same as the preferred BRS. This embodiment can be combined with the previous, comprising selecting a preamble, so that both a subset of preambles and a certain frequency band and/or subframe, is used to transmit the preamble.

According to further aspects, the selecting <NUM> comprises selecting 808c the random-access resource based on pre-defined association rules, known at the wireless device.

In one embodiment, after determining a preferred downlink BRS, the wireless device uses a function or look-up table, specified in a manual or standard or given by prior broadcast signaling or configured by dedicated signaling (such as RRC signaling) on an assisting legacy network, to select 808d a random-access preamble from a set of preambles. The wireless device then uses this selected preamble in its random-access attempt in step <NUM>.

The network can then from detecting the PRACH preamble (at step <NUM>) determine which downlink beam the wireless device has found to be strongest and it will thus preferably use this when transmitting the random-access response message(s) at step <NUM>. The network has several choices in selecting the beam-forming weights for the random-access response messages. It can simply choose the same beamforming weights as was used when forming the beam to transmit the BRS that was preferred by the wireless device.

According to some embodiments, the network node transmits 814a the random-access response according to one or more pre-defined association rules known at the network node <NUM>.

Alternatively, a wider beam or more narrow beam or a beam with lower side lobes can be generated by using different beamforming weights for the following random-access response than for the BRS transmission. It may be so that BRS are transmitted with larger HPBW and physical downlink shared channel (PDSCH) beams (like random-access responses) are transmitted in beams with smaller HPBW. In any case, the beam direction of the beam of the preferred BRS gives the network information of the pointing direction of the following random-access response beam (even though the beamforming weights are not exactly the same).

According to some aspects, the method illustrated in <FIG> further comprises selecting <NUM> an uplink beam for detecting <NUM> the preamble according to one or more pre-defined association rules between preambles and uplink beams known at the network node <NUM>.

According to some further aspects, the method further comprises selecting 813a a time and/or frequency resource for preamble detection according to one or more pre-defined association rules between preambles and time/frequency resources known at the network node <NUM>.

In some embodiments, the set of preambles and resources are divided into groups, where each group is associated with a beam-specific reference signal (BRS). The association between BRS and preamble may be given by standard specification. The wireless device selects 808e, randomly or otherwise, a preamble from the associated group to use in its random-access attempt. The group could for example be all available preamble sequences using one PRACH resource.

If the set of available preambles are divided into too many smaller groups, such that the number of preambles in each group is small, this may lead to larger probability of RACH collision. In a related embodiment, a set (more than one) of BRS are all associated with a group of PRACH preambles. The network can then use the set of BRS associated with the same group of PRACH preambles in adjacent downlink beams (adjacent in downlink transmitted beam direction). In case there are many BRS, then the set of available PRACH preambles associated with the detected best BRS is rather large so the preamble collision probability (in case of contention-based random access) is maintained low.

In yet a further variant of this embodiment, some BRS and preambles can be associated to multiple groups. The beam direction can be partly overlapping between two groups. If the preamble belongs to the two groups, the network node should use the overlapping beam directions between these two groups to transmit the DL RACH response.

In a further network embodiment, the network only searches 820a the subset of preambles in each uplink beam, for which the associated BRS is transmitted in downlink. Each BRS points out the subset of preambles to be used in PRACH preamble receiver. Hence, there is a reduction in network preamble detection complexity. This solution requires however that the relation between uplink receive beam and downlink transmit beam is known, by e.g. RF calibration at the network side.

In yet another embodiment, preamble sequences and PRACH resources are reused for BRSs associated with beams with sufficient angular separation so that they can be discriminated by using different uplink beams.

According to some aspects, the wireless device receives the random-access response from the network node at step <NUM>.

Wireless device <NUM> and network node <NUM> illustrated, e.g., in <FIG>, may use any suitable radio-access technology, such as long-term evolution (LTE), LTE-Advanced, universal mobile telecommunications system (UMTS), high speed packet access (HSPA), global system for mobile communications (GSM), cdma2000, WiMax, WiFi, another suitable radio-access technology, or any suitable combination of one or more radio-access technologies. For purposes of example, various embodiments may be described within the context of certain radio-access technologies. However, the scope of the disclosure is not limited to the examples and other embodiments could use different radio-access technologies. Each of wireless device <NUM>, network node <NUM>, radio network controller <NUM>, and packet core network <NUM> may include any suitable combination of hardware and/or software. Examples of particular embodiments of wireless device <NUM>, network node <NUM>, and network nodes (such as radio network controller <NUM> or packet core network <NUM>) are described with respect to <FIG>, <FIG>, and <FIG> below, respectively.

<FIG> is a block diagram illustrating certain embodiments of a UE or wireless device <NUM>. Examples of wireless device <NUM> include a mobile phone, a smart phone, a personal digital assistant (PDA), a portable computer, e.g., laptop, tablet, a sensor, a modem, a machine type (MTC) device / machine-to-machine (M2M) device, laptop-embedded equipment (LEE), laptop-mounted equipment (LME), universal serial bus (USB) dongles, a device-to-device capable device, or another device that can provide wireless communication. Wireless device <NUM> may also be a radio communication device, target device, device to device UE, machine type UE or wireless device capable of machine-to-machine communication, a sensor equipped with wireless device, iPad, tablet, mobile terminals, smart phone, laptop-embedded equipped (LEE), laptop-mounted equipment (LME), USB dongles, customer premises equipment (CPE), etc..

Though the terms UE and wireless device <NUM> are used predominantly herein, the equipment may also be referred to as a station (STA), a device, or a terminal in some embodiments. As depicted, wireless device <NUM> includes transceiver <NUM>, processor <NUM>, and memory <NUM>.

In some embodiments, transceiver <NUM> facilitates transmitting wireless signals to and receiving wireless signals from network node <NUM>, e.g., via an antenna, processor <NUM> executes instructions to provide some or all of the functionality described above as being provided by wireless device <NUM>, and memory <NUM> stores the instructions executed by processor <NUM>.

Processor <NUM> may include any suitable combination of hardware and software implemented in one or more modules to execute instructions and manipulate data to perform some or all of the described functions of wireless device <NUM>. In some embodiments, processor <NUM> may include, for example, one or more computers, one or more central processing units (CPUs), one or more microprocessors, one or more applications, and/or other logic.

Memory <NUM> is generally operable to store instructions, such as a computer program, software, an application including one or more of logic, rules, algorithms, code, tables, etc. and/or other instructions capable of being executed by a processor. Examples of memory <NUM> include computer memory (for example, random access memory (RAM) or read only memory (ROM)), mass storage media (for example, a hard disk), removable storage media (for example, a compact disk (CD) or a digital video disk (DVD)), and/or or any other volatile or non-volatile, non-transitory computer-readable and/or computer-executable memory devices that store information.

Other embodiments of wireless device <NUM> may include additional components beyond those shown in <FIG> that may be responsible for providing certain aspects of the wireless device's functionality, including any of the functionality described above and/or any additional functionality (including any functionality necessary to support the solution described above).

<FIG> is a block diagram illustrating certain embodiments of a network node <NUM>. Examples of network node <NUM> include an eNodeB, a node B, a base station, a wireless access point (e.g., a Wi-Fi access point), a low power node, a base transceiver station (BTS), transmission points, transmission nodes, remote RF unit (RRU), remote radio head (RRH), etc. Network nodes <NUM> may be deployed throughout network <NUM> as a homogenous deployment, heterogeneous deployment, or mixed deployment. A homogeneous deployment may generally describe a deployment made up of the same (or similar) type of network nodes <NUM> and/or similar coverage and cell sizes and inter-site distances. A heterogeneous deployment may generally describe deployments using a variety of types of network nodes <NUM> having different cell sizes, transmit powers, capacities, and inter-site distances. For example, a heterogeneous deployment may include a plurality of low-power nodes placed throughout a macro-cell layout. Mixed deployments may include a mix of homogenous portions and heterogeneous portions.

Network node <NUM> may include one or more of transceiver <NUM>, processor <NUM>, memory <NUM>, and network interface <NUM>. In some embodiments, transceiver <NUM> facilitates transmitting wireless signals to and receiving wireless signals from wireless device <NUM> (e.g., via an antenna), processor <NUM> executes instructions to provide some or all of the functionality described above as being provided by a network node <NUM>, memory <NUM> stores the instructions executed by processor <NUM>, and network interface <NUM> communicates signals to backend network components, such as a gateway, switch, router, Internet, public switched telephone network (PSTN), packet core network <NUM>, radio network controllers <NUM>, etc..

Processor <NUM> may include any suitable combination of hardware and software implemented in one or more modules to execute instructions and manipulate data to perform some or all of the described functions of network node <NUM>. In some embodiments, processor <NUM> may include, for example, one or more computers, one or more central processing units (CPUs), one or more microprocessors, one or more applications, and/or other logic.

In some embodiments, network interface <NUM> is communicatively coupled to processor <NUM> and may refer to any suitable device operable to receive input for network node <NUM>, send output from network node <NUM>, perform suitable processing of the input or output or both, communicate to other devices, or any combination of the preceding. Network interface <NUM> may include appropriate hardware (e.g., port, modem, network interface card, etc.) and software, including protocol conversion and data processing capabilities, to communicate through a network.

Other embodiments of network node <NUM> may include additional components beyond those shown in <FIG> that may be responsible for providing certain aspects of the network node's functionality, including any of the functionality described above and/or any additional functionality (including any functionality necessary to support the solution described above). The various different types of network nodes may include components having the same physical hardware but configured (e.g., via programming) to support different radio access technologies, or may represent partly or entirely different physical components.

Also in some embodiments generic terminology, "network node" or simply "network node (NW node)", may be used. The terms may refer to any kind of network node which may comprise of base station, radio base station, base transceiver station, base station controller, network controller, evolved Node B (eNB), Node B, RNC, relay node, positioning node, E-SMLC, location server, repeater, access point, radio access point, remote radio unit (RRU), remote radio head (RRH), multistandard radio (MSR) radio node such as MSR BS nodes in distributed antenna system (DAS), SON node, O&M, OSS, MDT node, Core network node, MME etc..

<FIG> is a block diagram illustrating certain embodiments of a radio network controller <NUM> or node in packet core network <NUM>. Examples of network nodes can include a mobile switching center (MSC), a serving GPRS support node (SGSN), a mobility management entity (MME), a radio network controller (RNC), a base station controller (BSC), and so on. The network node includes processor <NUM>, memory <NUM>, and network interface <NUM>. In some embodiments, processor <NUM> executes instructions to provide some or all of the functionality described above as being provided by the network node, memory <NUM> stores the instructions executed by processor <NUM>, and network interface <NUM> communicates signals to a suitable node, such as a gateway, switch, router, Internet, public switched telephone network (PSTN), network nodes <NUM>, radio network controllers <NUM>, node in packet core network <NUM>, etc..

Processor <NUM> may include any suitable combination of hardware and software implemented in one or more modules to execute instructions and manipulate data to perform some or all of the described functions of the network node. In some embodiments, processor <NUM> may include, for example, one or more computers, one or more central processing units (CPUs), one or more microprocessors, one or more applications, and/or other logic.

In some embodiments, network interface <NUM> is communicatively coupled to processor <NUM> and may refer to any suitable device operable to receive input for the network node, send output from the network node, perform suitable processing of the input or output or both, communicate to other devices, or any combination of the preceding. Network interface <NUM> may include appropriate hardware (e.g., port, modem, network interface card, etc.) and software, including protocol conversion and data processing capabilities, to communicate through a network.

Other embodiments of the network node may include additional components beyond those shown in <FIG> that may be responsible for providing certain aspects of the network node's functionality, including any of the functionality described above and/or any additional functionality (including any functionality necessary to support the solution described above).

<FIG> illustrates a flowchart depicting exemplary method steps performed for the selection of a preferred downlink beam, according to certain embodiments. Specifically, the right side of the flowchart depicts the steps that may be performed by UE <NUM>, and the left side depicts the steps that may be performed by a network node <NUM>, according to certain embodiments.

The method may begin at step <NUM> when network node <NUM> (eNB, base station) may transmit a set of beam formed reference signals in the downlink. The signals may be received by the UE at step <NUM>. UE <NUM> may then perform measurements on these different (preferably orthogonal) reference signals and then determine a preferred downlink beam at step <NUM>. This can be done by measuring the received signal power (RSRP) for each beam. The reference signal can be beam formed synchronization signals (PSS/SSS), beam formed channel state information signals (CSI-RS), beam formed discovery signals (DSS) or it can be newly designed beam reference signal sequences (BRS). In the following, we denote and classify the beam specific reference signals as BRS, for simplicity.

The beam specific reference signals are assumed known by specification or from broadcasted system information so that no dedicated configuration signaling is needed between network node <NUM> and UE <NUM>, before the UE can start measuring and identifying the preferred downlink beam. However, in one embodiment, configuration signaling takes place prior to the identification but on non-beam-based legacy system such as LTE. (In practice, the UE detects a preferred beam specific RS from a set of beam specific RS, so the UE is not aware of the actual beam direction of beam radiation pattern, or beam forming weights used by the transmitted side which is entirely implementation specific).

At step <NUM>, terminal <NUM> selects random access response resource. In one embodiment, after determining a preferred downlink beam RS, the UE uses a function or look-up table, specified in a manual or standard or given by prior broadcast signaling or configured by dedicated signaling (such as RRC signaling) on an assisting legacy network, to select a random-access preamble from a set of preambles. The UE than uses this selected preamble in its random-access attempt in step <NUM>.

In a further embodiment, the PRACH resource (which is one of multiple resources distributed in time or frequency) to use when transmitting the preamble depends on the detected preferred BRS. Hence, the network will know which BRS was preferred from the UE side from which band the network has detected the preamble in the uplink. And thus, the network knows the direction in which to transmit the random-access response (MSG <NUM>) since it is the same as the preferred BRS. This embodiment can be combined with the previous, so that both a subset of preambles and a certain frequency band (and/or subframe), is used to transmit the preamble.

The network can then from detecting the PRACH preamble (at step <NUM>) determine which downlink beam the UE has found to be strongest and it will thus preferably use this when transmitting the random-access response message(s) at step <NUM>. The network has several choices in selecting the beam forming weights for the random access response messages. It can simply choose the same beamforming weights as was used when forming the beam to transmit the BRS that was preferred by the UE. Alternatively, a wider beam or more narrow beam or a beam with lower side lobes can be generated by using different beam forming weights for the following random-access response than for the BRS transmission. It may be so that BRS are transmitted with larger HPBW and PDSCH beams (like random-access responses) are transmitted in beams with smaller HPBW. In any case, the beam direction of the beam of the preferred BRS gives the network information of the pointing direction of the following random-access response beam (even though the beamforming weights are not exactly the same).

In one embodiment, the set of preambles and resources are divided into groups, where each group is associated with a beam specific reference signal (BRS). The association between BRS and preamble is thus given by standard specification. The UE randomly selects a preamble from the associated group to use in its random-access attempt. The group could for example be all available preamble sequences using one PRACH resource.

It may be a problem if the set of available preambles are divided into too many smaller groups, such that the number of preambles in each group is small since this may lead to larger probability of RACH collision. In a related embodiment, a set (more than one) of BRS are all associated with a group of PRACH preambles. At step <NUM>, the network can then use the set of BRS associated with the same group of PRACH preambles in adjacent downlink beams (adjacent in downlink transmitted beam direction). In case there are many BRS, then the set of available PRACH preambles associated with the detected best BRS is rather large so the preamble collision probability (in case of contention based random-access) is maintained low.

In yet a further variant of this embodiment, some BRS and preambles can be associated to multiple groups. The beam direction can be partly overlapped between two groups. If the preamble belong to the two groups are selected, the network node should use the overlapped BRS between these two groups to transmit the DL RACH response.

In a further network embodiment, the network only searches the subset of preambles in each uplink beam, for which the associated BRS is transmitted in downlink. Each BRS points out the subset of preambles to be used in PRACH preamble receiver. Hence, there is a reduction in network preamble detection complexity. This solution requires however that the relation between uplink receive beam and downlink transmit beam are known, by e.g. RF calibration at the network side.

There is furthermore disclosed herein various additional example embodiments. Some such embodiments propose solutions for selecting a physical random access channel based on the strongest beam received in the downlink. In one example embodiment, user equipment may perform steps of:.

In another example embodiment, the network node may perform the steps of:.

Other implementations may include a wireless communication device and/or access node configured to implement the described method, or a wireless communication system in which a wireless communication device and/or access node implement the described method.

Some embodiments of the disclosure may provide one or more technical advantages. For example, in certain embodiments, since the network knows the beam to use for the random access channel response, the coverage of the random access channel responses is improved. Another technical advantage may be that, the random access channel procedure can be completed earlier, which improves latency and reduces interference in the network. Another technical advantage may be that there is no need to have calibrated and aligned RF for uplink and downlink which reduces implementation cost and power consumption.

A further technical advantage may be that computational complexity in eNode is reduced. The physical random access channel preamble detector in eNode B only needs to search for a sub-set of sequences in each uplink receiver direction. This subset equals to those physical random access channel sequences that are mapped to the same downlink transmission beam (or spatial direction) as the receiver uplink beam (or spatial direction).

Some embodiments may benefit from some, none, or all of these advantages. Other technical advantages may be readily ascertained by one of ordinary skill in the art.

In particular example implementations, the proposed solutions may provide methods for random-access selection of a preferred downlink beam. In one example embodiment, user equipment may perform steps of:.

Modifications, additions, or omissions may be made to the systems and apparatuses disclosed herein without departing from the scope of the invention, without departing from the scope of the appended claims.

The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. Additionally, operations of the systems and apparatuses may be performed using any suitable logic comprising software, hardware, and/or other logic. As used in this document, "each" refers to each member of a set or each member of a subset of a set.

Modifications, additions, or omissions may be made to the methods disclosed herein without departing from the scope of the invention, without departing from the scope of the appended claims.

Claim 1:
A method performed by a wireless device (<NUM>) for performing random access to a network node (<NUM>), the method comprising:
- receiving (<NUM>) a set of downlink beam-specific reference signals, BRS, from the network node (<NUM>);
- determining (<NUM>) a preferred BRS based on the received signal power for each BRS in the set;
- selecting (<NUM>), based on the preferred BRS, a random-access resource to be used for a random-access attempt to the network node (<NUM>) according to one or more pre-defined association rules defining an association between a random access resource and a BRS; and
- using (<NUM>) the selected random-access resource for the random-access attempt to the network node (<NUM>), whereby the selected random-access resource indicates to the network node (<NUM>) which downlink beam is preferred by the wireless device (<NUM>) to be used for downlink transmissions,
wherein the random access resource comprises a random-access preamble, wherein the selecting (<NUM>) further comprises selecting (808a) a random-access preamble from a set of random-access preambles to be used for the random-access attempt, and
wherein the selecting (<NUM>) further comprises selecting (808d) the random-access preamble from the set of random-access preambles using a function specified in a standard.