WO2019061082A1

WO2019061082A1 - Handover procedures for wireless networks using beamforming

Info

Publication number: WO2019061082A1
Application number: PCT/CN2017/103724
Authority: WO
Inventors: Jing He; Haitao Li; Li Zhang; Chunhai Yao
Original assignee: Nokia Shanghai Bell Co., Ltd.; Nokia Solutions And Networks Oy; Nokia Technologies Oy
Priority date: 2017-09-27
Filing date: 2017-09-27
Publication date: 2019-04-04
Also published as: CN111165059A; EP3689083A1; EP3689083A4; CN111165059B

Abstract

Various communication systems may benefit from an improved method for handovers when beamfoming is used. For example, in case of intra-network handovers, or inter-network handovers, it may be beneficial to use a method, which comprises receiving a dedicated random access channel configuration associated with at least one first downlink beam, determining that the apparatus is not in a coverage area of the at least one first downlink beam and transmitting, based on the determination, a random access message for at least one second downlink beam using the dedicated random access channel configuration associated with the at least one first downlink beam.

Description

HANDOVER PROCEDURES FOR WIRELESS NETWORKS USING BEAMFORMING

TECHNICAL FIELD

The present invention relates generally to apparatuses， methods and computer programs for handover procedures for wireless networks using beamforming.

BACKGROUND

Embodiments of the invention relate to wireless or mobile communications networks， such as， but not limited to， the Global System for Mobile Communications， GSM， Wideband Code Division Multiple Access， WCDMA， Long Term Evolution， LTE， and/or 5G radio access technology， which may also be called as new radio access technology， NR. Since its inception， LTE has seen extensive deployment in a wide variety of contexts involving the communication of data and 3^rd Generation Partnership Project， 3GPP， still develops LTE. Similarly， 3GPP also develops the standard for 5G/NR. The goal of the 3GPP is， in general， to further develop and improve wireless cellular systems.

One of the topics in the 3GPP discussions related to LTE and 5G/NR is mobility. It may be necessary to hand over a mobile terminal from one base station to another to maintain connectivity when the mobile terminal moves. Such handovers may be needed within a network (intra-network handover) or between networks (inter-network handover) . In general， it is desibrable to improve handover procedures to enable efficient operation.

Beamforming may be used in wireless networks to steer the transmission and/or reception to a certain direction to achieve better performance compared to an omnidirectional transmission and/or reception. Nevertheless， continuous connectivity should be maintained if a user equipment moves， even if beamforming would be used.

Similar enhancements may also be employed in other wireless cellular systems such as GSM and WCDMA for example. In addition to different wireless cellular systems， these enhancements may be utilized in relation to， or in combination with， several other wireless systems， such as， Wireless Local Area Network， WLAN， and Worldwide Interoperability for Microwave Access， WiMAX， systems as well.

SUMMARY

According to certain embodiments， an apparatus may comprise at least one processor and at least one memory including computer program code. The at least one memory and the computer program code may be configured to， with the at least one processor， cause the apparatus at least to receive a dedicated random access channel configuration associated with at least one first downlink beam， determine that the apparatus is not in a coverage area of the at least one first downlink beam and transmit， based on the determination， a random access message for at least one second downlink beam using the dedicated random access channel configuration associated with the at least one first downlink beam.

According to certain embodiments， a first method may comprise receiving a dedicated random access channel configuration associated with at least one first downlink beam， determining that the apparatus is not in a coverage area of the at least one first downlink beam and transmitting， based on the determination， a random access message for at least one second downlink beam using the dedicated random access channel configuration associated with the at least one first downlink beam.

According to certain embodiments， an apparatus may comprise at least one processor and at least one memory including computer program code. The at least one memory and the computer program code may be configured to， with the at least one processor， cause the apparatus at least to transmit， to a user equipment， a dedicated random access channel configuration associated with at least one first downlink beam and receive， from the user equipment， a random access message for at least one second downlink beam on the dedicated random access channel associated with the at least one first downlink beam.

According to certain embodiments， a second method may comprise transmitting， to a user equipment， a dedicated random access channel configuration associated with at least one first downlink beam and receiving， from the user equipment， a random access message for at least one second downlink beam on the dedicated random access channel associated with the at least one first downlink beam.

According to certain embodiments， a computer program product may be configured to control an apparatus to perform a process according to the first or the second method.

According to certain embodiments， an apparatus may comprise means for performing the first or the second method.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of example embodiments of the present invention， reference is now made to the following descriptions taken in connection with the accompanying drawings in which：

Fig. 1 illustrates a regular procedure for handovers；

Fig. 2 illustrates an example of a handover procedure in accordance with embodiments of invention；

Fig. 3 illustrates a process in accordance with some embodiments of the invention；

Fig. 4 illustrates a flowchart of a method in accordance with embodiments of the invention；

Fig. 5 illustrates a flowchart of a method in accordance with embodiments of the invention；

Fig. 6 illustrates an apparatus in accordance with embodiments of the invention.

DETAILED DESCRIPTION

Embodiments of the invention relate to handover procedures for wireless networks using beamforming and provide an improved solution for handovers， when beamforming is used.

In case of wireless communications， radio resources may be allocated in two ways. Dedicated resources may be used only by one device and hence， collisions can be avoided. On the other hand， shared resources may be shared between multiple devices， which may lead to a situation wherein collisions may occur.

Random access channels， RACHs， may be exploited for example to access a wireless communication system or when a mobile station is handed over from one base station to another， within a network or between networks. Both， dedicated and shared， resources may be used for random access depending on the situaton. Contention-free random access， CFRA， refers to the use of dedicated resources while contention-based random accees， CBRA， refers to the use of shared resources. RACH resource may be， for example， a preamble， a sequence， a frequency resource or a time resource. For example， a preamble may comprise a Cyclic Prefic， CP， a sequence and a guard time. The sequence may be a Zadoff-Chu sequence， or another suitable sequence. Moreover， a RACH resource may also refer to a set of resources， such as， two or more preambles， two or more sequences， two or more frequency resources or two or more time resources. In some embodiments a RACH resource may be a Physical Random Access Channel， PRACH， resource or a set of PRACH resources.

Dedicated and shared resources may be exploited together with beamforming as well. By using beamforming， it is possible to direct transmission and/or reception to optimize communication by adapting the radiation pattern of an antenna array on the transmitter and/or on the receiver side. Hence beamforming may be bi-directional. Beamforming may be seen， for example， as steering a lobe of power to a certain direction. In case of beamforming a RACH resource or set of resources may be associated with a downlink beam or a set of downlink beams.

If beamforming， or any other similar technology， is used it may be possible for a user equipment， UE， to report measurement information per beam， or a set of beams， to a base station. The reported information may be based on a synchronization signal， SS， and/or channel state indication reference signal， CSI-RS. In some embodiments the information may be transmitted to a base station， BS， in a measurement report. Received information may be used， for example， at the BS to adjust resource allocation accordingly.

Fig. 1 illustrates a procedure for inter-BS handovers， i.e.， intra-network handovers. The present invention is described in the context of this architecture， but may be employed in any suitable alternative network architecture， such as， for inter-network handovers. As a starting point， an UE may be in a connected state， for example， in the RRC-CONNECTED state. In case of an upcoming handover for an UE， both， source and target， base stations may exploit the reported information related to a beam， or set of beams， as well. The source BS may receive reported information related to the beam， or the set of beams， from the UE， for example in a measurement info message (110) .

Moreover， the source BS may transmit， or forward， the information related to the beam， or the set of beams， to a target BS， for example in a handover request message (120) . Upon reception of the information， the target BS may perform admission control and select the most appropriate beam for the UE. The target BS may also allocate a dedicated random access channel resource， associated with the most appropriate beam， for the UE accordingly. If a set of RACH resources is allocated， those may be associated with a set of downlink beams.

After that， the target BS may transmit a message， for example， an acknowledgment message (130) related to the upcoming handover to the source BS. The target BS may also produce an RRCConnectionReconfiguration message for the UE， which may be a part of the message. The RRCConnectionReconfiguration message may comprise an information element related to mobilityControlInfo as well.

The source BS may transmit the information included in the received message to the UE， possibly in the form of a handover command (140) . That is to say， the message may be transmitted from the target BS to the UE via the source BS. In some embodiments the handover command (140) may comprise at least a cell identity of the target BS. Alternatively， or in addition， the handover command (140) may comprise information that is needed for accessing the target BS. Such information may be sufficient for the UE to access the target BS. For example， in some cases， the handover command (140) may comprise information required for CBRA and/or CFRA. In case ofbeamforming， the handover command (140) may also comprise information that is related to a beam， or a set of beams.

For example， the target BS may provide a common RACH configuration， which is a shared resource， and/or a dedicated RACH configuration， which is a dedicated resource， to the UE. In some embodiments， the common RACH configuration may be a common RACH resource associated with SS blocks that all UEs may use. In addition， in some embodiments， the dedicated RACH configuration may be a dedicated RACH resource associated to dedicated SS Blocks and/or associated to dedicated CSI-RSs that only one UE may use.

Common RACH configuration may be equivalent to a configuration information broadcasted in a system information block. Hence the UE may use the common RACH configuration for performing CBRA. On the other hand， the dedicated RACH configuration may be allocated to only one， specific UE for performing CFRA.

If the target BS uses beamforming， the dedicated RACH resource configuration may be associated with one beam， or a set of beams. The configuration may be associated with SS block (s) and/or CSI-RS (s) . In some embodiments one RACH resource may be allocated to the UE for performing CFRA while in some embodiments a set of RACH resources may be allocated. If a set of RACH resources is allocated， the dedicated RACH resource configuration may comprise one RACH resource per beam. For example， if three dedicated RACH resources are configured associated with three beams， the dedicated RACH configuration may comprise one resource per beam.

Upon receiving the handover command， the UE may switch to， or access， the target BS. For this， the UE may exploit the information received in the handover command (140) ， such as， for example， the information required for CBRA and/or CFRA. Once the UE has decided to switch to the target BS， it may send a message， for example a handover complete message (150) to the target BS， to access the target BS. Transmission of the message may be done by exploiting the information required for CBRA and/or CFRA.

However， one issue may be that ifbeamforming is used at the target BS， at least in some cases the UE may move after it has reported the beam measurement information. In such a case it may happen that the indicated beam/beams in the handover command (140) is/are not suitable for accessing the target BS anymore， i.e.， the indicated beam/beams may be invalid. For example， the target BS may allocate a dedicated RACH resource related to a first downlink beam， or first set of downlink beams， for the UE. However， if the UE has moved， the quality of the first downlink beam， or the first set of downlink beams， may have degraded due to the movement of the UE. On the other hand， it may happen that the quality of a second downlink beam， or a second set of downlink beams， has improved and at this point the second downlink beam， or the second set of downlink beams， would be better for the UE.

So if in this scenario the UE would use the configured， dedicated RACH resource related to the first downlink beam (s) for sending a message to the target BS， the target BS would respond by sending a message using the first downlink beam (s) . Consequently the UE would not receive the message from the target BS correctly. Hence the random access process would continue and after several attemps the UE would fall back to CBRA. This would introduce significant delays to the handover procedure. One possible solution may be to configure the UE so that it falls back to CBRA right away once it notices that the first downlink beam (s) is not suitable for communication. However， in general the use of CBRA should be avoided because it is a shared resource so a collision may happen， which would again increase the delay. That is to say， the use of a dedicated RACH resource， i.e.， CFRA， should be preferred over CBRA to provide better success rate and lower latency.

In view of the above-described shortcomings， certain embodiments of the invention may improve the handover procedure， especially when beamforming is used at the target BS. Embodiments of the invention enable faster random access for moving UEs. Furthermore， embodiments of the invention also provide a robust solution， wherein an UE may not have to fall back to CBRA even if it has moved from the coverage area of the first downlink beam (s) to the coverage area of the second downlink beam (s) .

For example， if the UE has moved away from the coverage area of the first downlink beam (s) ， associated with the indicated dedicated RACH resource， the UE may continue to use the dedicated RACH resource， e.g.， if the target BS can receive omnidirectionally. For this， the target BS may indicate to the UE that it can use the dedicated RACH resource even if it has moved out from the coverage area of the first downlink beam (s) . In such a case the UE may indicate an index of the second downlink beam (s) to the target BS so that the target BS may send a response message to the UE using the second downlink beam (s) .

Alternatively， the UE may quickly fall back to use CBRA for the second downlink beam (s) if it knows that the target BS is able to only receive the dedicated RACH resource in a certain direction. This may beneficial， for example， if uplink is beamed in the target BS， i.e.， the target BS uses beamforming in reception. For this option the target BS may indicate to the UE that it cannot use the dedicated RACH resource even if it has moved out from the coverage area of the first downlink beam (s) . That is to say， the target BS may indicate to the UE whether it can use the dedicated RACH resource even if it has moved out from the coverage area of the first downlink beam (s) .

Even though the allocated dedicated RACH resource may be a set of resources， one issue to consider is how many resources should be included in the set. This leads to a tradeoff between RACH resource consumption and the possibility for the UE to use CFRA. The dedicated RACH resource， once configured to the UE， cannot be used by any other UEs for a while. Hence one network implementation could be to configure a small number of dedicated RACH resources， e.g. only one. However， in that case the likelihood the UE has moved outside of the coverage of the dedicated RACH resources increases， compared to a case wherein a large number of RACH resources would be allocated. In practice RACH resources are limited and thus， it may not be desirable to reserve many dedicated RACH resources for a handover of a single UE. Certain embodiments of the invention enable resource savings of dedicated RACH resources， because less resources may be allocated for one UE， while latency is also minimized.

The following provides examples of how this could be achieved. Fig. 2 represents an example of a network scenario according to embodiments of the invention. A network may comprise a core network， two or more base stations， BSs (210， 220) ， and at least one user equipment， UE (230) . The BS (210) may be a source BS for a handover while the BS (220) may be a target BS for the handover.

In the context of 5G/NR and intra-network handovers， the BSs (210， 220) may be called as gNBs. On the other hand， in the context of LTE and intra-network handovers， the BSs (210， 220) may be called as eNBs. In case of inter-network handovers the BS (210) may be a gNB while the BS (220) may be an eNB， or vice versa. Nevertheless， the present invention is not restricted to any specific definition of the BSs， that is， a person skilled in the art will understand how to apply the invention in different wireless systems that may have various names for the BSs (210， 220) . Similarly， even though the invention is described using the UE (230) as an example， other namings may be used as well， such as， for example， a mobile station or a wireless terminal.

As a starting point， the UE (230) may be associated with a BS (210) . In some embodiments the UE may be in the RRC_CONNECTED state. The UE (230) may perform measurements related to one or more beams and report the information related to the measurements to the BS (210) . The UE (230) may report， for example， measurements related to the first downlink beam (240) if a received power of the first downlink beam (240) is above a threshold (A4 event) or an offset better than a received power of a serving beam (A3 event) ， wherein the serving beam is associated with the BS (210) .

Upon detecting a need for a handover， the BS (210) may transmit a message to the BS (220) via a fixed， backhaul connection. The message may comprise the information related to the beam measurements reported by the UE (230) . Based on this information the BS (220) may determine a dedicated RACH resource that the UE (230) may use for accessing the BS (220) . The dedicated RACH resource may be for a beam， or a set of beams. With reference to Fig. 2， the dedicated RACH resource may be associated with a first downlink beam (240) .

Then， the BS (220) may transmit a message comprising information about the dedicated RACH to the BS (210) . In some embodiments the message may be in the form of an RRCConnectionReconfiguration message， possibly comprising a mobilityControlInfo information element. The message may also comprise an indication， or instruction， about whether the UE (230) should continue to use the dedicated RACH resource， even if it finds out that it has moved away from a coverage area of the first downlink beam (240) ， which is associated with the dedicated RACH resource. The BS (210) may transmit， or forward， the message to the UE (230) . That is to say， the message is transmitted from the BS (220) to the UE (230) via the BS (210) .

With reference to Fig. 2 again， the UE may have moved along the trajectory (260) during the handover process. Thus， the UE (230) may determine whether the first downlink beam (240) associated with the dedicated RACH is still suitable for communication， or not. In other words， the UE (230) determines whether it has moved away from the coverage area of the first downlink beam (240) . The determination may be based on latest measurement results.

For example， the UE (230) may compare the received power of the first downlink beam (240) to the threshold or the offset between the received power of the first downlink beam (240) and the serving beam. For example， if the received power (e.g.， Reference Signal Received Power， RSRP) is above the threshold， the first downlink beam (240) may be determined as suitable for communication. However， if the received power is below the threshold， the first downlink beam (240) may be determined as not suitable for communication.

In some embodiments， the received power of the first downlink beam (240) may be compared to a received power of a second downlink beam (250) . For example， if the received power of the first downlink beam (240) is much lower than the received power of the second downlink beam (250) ， it may be determined that the first downlink beam (240) is not suitable for communication while the second downlink beam (250) is.

If it is determined that the first downlink beam (240) is not suitable for communication but the second downlink beam (250) is， the UE (230) may indicate this by transmitting a first message to the BS (220) ， for accessing the BS (220) . The first message may comprise information related to the second downlink beam (250) . Such information may comprise， for example， measured information related to the second downlink beam (250) ， an index related to the second downlink beam (250) ， or a measurement report related to the second downlink beam (250) . The measurement report may comprise information related to one or more other beams as well. After transmission of the first message to the BS (220) the UE (230) may start monitoring for， or receive， a random access response message from the BS (220) on the second downlink beam (250) .

Based on the received information related to the second downlink beam (250) ， the BS (220) may then select the second downlink beam (250) as the appropriate beam， instead of the first downlink beam (240) ， and inform the UE (230) accordingly. The BS (220) may also transmit subsequent downlink messages to the UE (230) by using the selected beam. That is to say， upon reception of the random access response message the UE (230) may use it to configure for accessing the BS (220) and receive transmissions accordingly.

The measured， new information in the first message may be related to one or more downlink beams that the UE measures， or other assistant information that the target BS may use to identify the current location of the UE (230) and a best beam， and/or to an offset between an allocated beam index and the best beam index. In the scenario of Fig. 2， the best beam may be the second downlink beam (250) while the allocated beam index is related to the first downlink beam (240) .

Transmission of the first message， from the UE (230) to the BS (220) ， may depend on the indication about whether the UE (230) should continue to use the dedicated RACH resource， even if it finds out that it has moved away from the coverage area of the first downlink beam (240) . For example， the UE (230) may be instructed， by the BS (220) ， not to use the dedicated RACH resource. In such a case， the UE (230) may start the random access process using CBRA right away， i.e.， the UE (230) may be instructed， by the BS (220) to use CBRA ifit finds out that it has moved away from the coverage area of the first downlink beam (240) . Hence the UE may use CBRA with a common RACH resource associated with the beam it measured as suitable， for example， the second downlink beam (250) in Fig. 2. In this case the BS (220) would not be able to correctly receive the first message anyway， if the dedicated RACH resource associated with the first downlink beam (240) would be used， because the BS would be receiving with beamforming.

Moreover， the UE (230) may be instructed， by the BS (220) to continue the use of the dedicated RACH resource， even if it finds out that it has moved away from the coverage area of the first downlink beam (240) . In such a case， the UE (230) may use the dedicated RACH resource for transmitting a random access message for a suitable beam， e.g.， second downlink beam (250) in Fig. 2. The UE may indicate in the first message e.g. an index of the suitable beam or some other information related to the suitable beam. In some embodiments， if the UE (220) is using transmit beamforming， it may form a beam according to the downlink beam it measured as suitable.

Fig. 3 illustrates a process in accordance with some embodiments of the invention. During a handover process， or before， a UE may receive a dedicated RACH configuration associated with a first downlink beam (s) ， at step 310. In addition， the UE may receive an instruction related to the use of the dedicated RACH configuration in case the UE finds out that it has moved away from a coverage area of the first downlink beam (s) ， at step 320. In some embodiments， the instruction may be received using dedicated signaling. However， such an instruction may be optional though. In some embodiments the UE may be configured to use the dedicated RACH configuration in case the UE finds out that it has moved away from a coverage area of the first downlink beam (s) by default， for example， based on its own capability. In this case the UE may receive the information in a broadcast message， such as， for example， in System Information Block， SIB.

After that， at step 330， the UE may determine whether it is in the coverage area of the first downlink beam (s) . If it is， the UE may， at step 340， transmit a random access， RA， message for the first downlink beam (s) using the dedicated RACH configuration， associated with the first downlink beam. After this the UE may start monitoring for， receive， a random access response message on the first downlink beam (s) .

However， if the UE determines that it is not in the coverage area of the first downlink beam (s) ， the process then may proceed to step 350， wherein the UE may determine whether the dedicated RACH configuration may be used for a second downlink beam (s) ， for example， based on the received instruction related to the use of the dedicated RACH configuration in case the UE finds out that it has moved away from a coverage area of the first downlink beam (s) . If yes， the UE may transmit a random access message for the second downlink beam (s) using the dedicated RACH configuration， at step 360. On the other hand， if it is determined that the dedicated RACH configuration may not be used for a second downlink beam (s) ， the UE may directly transmit a RA message for the second downlink beam using CBRA， at step 370. After this the UE may start monitoring for， receive， a random access response message on the second downlink beam (s) . Once the UE has received the random access response message， it may use the information therein for receiving subsequent downlink transmissions.

Fig. 4 then illustrates a method according to certain embodiments. In some embodiments a UE may perform the method. As shown in Fig. 4， the method may include， at 410， receiving a dedicated random access channel configuration associated with at least one first downlink beam. In some embodiments the dedicated random access channel configuration may be received from a BS， by the UE. In case of a handover， the BS may be a source BS for the handover. The at least one first downlink beam and/or the at least one second downlink beam may comprise one downlink beam or a set of downlink beams of a target BS， but not all.

The method may further include， at 420， determining that the UE is not in a coverage area of the at least one first downlink beam. Moreover， the method may include， for example， determining that the UE is not in the coverage area of the at least one first downlink beam by checking a measurement result related to the at least one first downlink beam. Alternatively， or additionally， the method may include determining that the UE is not in a coverage area of the at least one first downlink beam by comparing a received power of the at least one first downlink beam to a threshold. Similarly， the method may also include determining that the UE is in the coverage area of at least one second downlink beam， which may be done by using the same procedures as for determining that the UE is not in the coverage area of the at least one first downlink beam.

In some embodiments， the method may also include receiving an instruction to use the dedicated random access channel configuration for transmitting the random access message for the at least one second downlink beam if， or when， it is determined that the apparatus is not in the coverage area of the at least one first downlink beam， i.e.， the at least one first downlink beam is not suitable for communication.

Additionally， or alternatively， the method may also include receiving an instruction to not to use the dedicated random access channel configuration related to at least one second downlink beam if， or when， it is determined that the UE is not in the coverage area of the at least one first downlink beam. In other words， the method may also include receiving an instruction to use contention based random access related to the at least one second downlink beam if， or when， it is determined that the UE is not in the coverage area of the at least one first downlink beam.

The method may also include， at step 430， transmitting， based on the determination， a random access message for at least one second downlink beam using the dedicated random access channel configuration associated with the at least one first downlink beam. In case of a handover， the method may include receiving the dedicated random access channel configuration for the at least one first downlink beam from a source base station and transmitting the random access message to a target base station for the handover.

The method may also include transmitting the random access message using the dedicated random access channel based on the received instruction. In some embodiments， the method may also include starting monitoring for， or receiving， a random access response message from a base station on the at least one second downlink beam after the transmission of the random access message. Moreover， the method may include receiving subsequent downlink transmissions based on information contained in the random access response message.

Alternatively， or in addition， the method may include transmitting the random access message using contention based random access. Decision to transmit the random access message using contention based random access may be based on the received instruction to not to use the dedicated random access channel configuration for transmitting the random access message for the at least one second downlink beam.

The random access message may comprise a preamble associated with the dedicated random access channel configuration and information related to the at least one second downlink beam. The information may comprise， for example， an index associated with the at least one second downlink beam， any other information that may be used to calculate the index associated with the at least one second beam， measured information related to the at least one second downlink beam， or a new measurement report.

Fig. 5 illustrates another method according to certain embodiments. In some embodiments the method may be performed by a base station. For example， in case of a handover the method may be performed by a target base station. The method may include， at 510， transmitting， to a user equipment， a dedicated random access channel configuration associated with at least one first downlink beam. Alternatively， or in addition， the method may include transmitting an instruction to not to use the dedicated random access channel configuration for transmitting the random access message for at least one second downlink beam. Such instructions may be given， for example， for a scenario wherein the user equipment may determine that the UE is not in the coverage area of the at least one first downlink beam.

At step 520， the method may include， receiving， from the user equipment， a random access message for at least one second downlink beam on the dedicated random access channel associated with the at least one first downlink beam， for example， if the instruction to use the dedicated random access channel configuration for transmitting the random access message for at least one second downlink beam has been transmitted， and the user equipment has determined that it is not in the coverage area of the at least one first downlink beam. Similarly， the method may further include receiving the random access message using contention based random access， for example， if the instruction to not to use the dedicated random access channel configuration for transmitting the random access message for at least one second downlink beam has been transmitted， and the user equipment has determined that it is not in the coverage area of the at least one first downlink beam.

Fig. 6 illustrates an apparatus (10) according to embodiments of the invention. Apparatus (10) may be a wireless device， such as a user equipment， for example. In other embodiments， apparatus (10) may be a base station， access point， software defined network， SDN controller， cloud base station controller， or centralized base station controller， for example.

A wireless device or user equipment may be a mobile station， MS， such as a mobile phone or smart phone or multimedia device， a computer， such as a tablet， provided with wireless communication capabilities， personal data or digital assistant， PDA， provided with wireless communication capabilities， portable media player， digital camera， pocket video camera， navigation unit provided with wireless communication capabilities or any combinations thereof. The wireless device or user equipment may be a sensor or smart meter， or other device that may usually be configured for a single location. Additionally， the wireless device or user equipment may be a device-to-device user equipment or a device for machine-type-communications.

Apparatus (10) may comprise a processor (22) for processing information and executing instructions or operations. Processor (22) may be any type of general or specific purpose processor. While a single processor (22) is shown in Fig. 6， multiple processors may be utilized according to other embodiments. Processor (22) may also comprise one or more of general-purpose computers， special purpose computers， microprocessors， digital signal processors， DSPs， field-programmable gate arrays， FPGAs， application-specific integrated circuits， ASICs， and processors based on a multi-core processor architecture， as examples.

Apparatus (10) may further comprise a memory (14) ， coupled to processor (22) ， for storing information and instructions that may be executed by processor (22) . Memory (14) may be one or more memories and of any type suitable to the local application environment， and may be implemented using any suitable volatile or nonvolatile data storage technology such as a semiconductor-based memory device， a magnetic memory device and system， an optical memory device and system， fixed memory， and removable memory. For example， memory (14) may be comprised of any combination of random access memory， RAM， read only memory， ROM， static storage such as a magnetic or optical disk， or any other type of non-transitory machine or computer readable media. The instructions stored in memory (14) may comprise program instructions or computer program code that， when executed by processor (22) ， enable the apparatus (10) to perform tasks as described herein.

Apparatus (10) may also comprise one or more antennas (not shown) for transmitting and receiving signals and/or data to and from apparatus (10) . Apparatus (10) may further comprise a transceiver (28) that modulates information on to a carrier waveform for transmission by the antenna (s) and demodulates information received via the antenna (s) for further processing by other elements of apparatus (10) . In other embodiments， transceiver (28) may be capable of transmitting and receiving signals or data directly.

Processor (22) may perform functions associated with the operation of apparatus (10) comprising， without limitation， precoding of antenna gain/phase parameters， encoding and decoding of individual bits forming a communication message， formatting of information， and overall control of the apparatus (10) ， comprising processes related to management of communication resources.

In certain embodiments， memory (14) stores software modules that provide functionality when executed by processor (22) . The modules may comprise an operating system (15) that provides operating system functionality for apparatus (10) . The memory may also store one or more functional modules (18) ， such as an application or program， to provide additional functionality for apparatus (10) . The components of apparatus (10) may be implemented in hardware， or as any suitable combination of hardware and software.

The described features， advantages， and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances， additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention.

Moreover， one having ordinary skill in the art will readily understand that the invention as discussed above may be practiced with steps in a different order， and/or with hardware elements in configurations which are different than those which are disclosed. Therefore， although the invention has been described based upon these preferred embodiments， it would be apparent to those of skill in the art that certain modifications， variations， and alternative constructions would be apparent， while remaining within the spirit and scope of the invention.

In an exemplary embodiment， an apparatus， such as a user equipment or base station， may comprise means for carrying out embodiments described above and any combination thereof.

In another exemplary embodiment， an apparatus， such as a user equipment or base station， may comprise at least one processor； and at least one memory including computer program code， the at least one memory and the computer program code may be configured， with the at least one processor， to cause the apparatus at least to carry out embodiments described above and any combination thereof.

In another exemplary embodiment， a computer program product may be configured to control an apparatus to perform a process according to embodiments described above and any combination thereof. The computer program product may be embodied on a non-transitory computer readable medium.

Claims

An apparatus， comprising：

at least one processor； and

at least one memory including computer program code，

the at least one memory and the computer program code configured， with the at least one processor， to cause the apparatus at least to，

receive a dedicated random access channel configuration associated with at least one first downlink beam；

determine that the apparatus is not in a coverage area of the at least one first downlink beam； and

transmit， based on the determination， a random access message for at least one second downlink beam using the dedicated random access channel configuration associated with the at least one first downlink beam.
The apparatus according to claim 1， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

determine that the apparatus is in a coverage area of the at least one second downlink beam.
The apparatus according to claim 1 or claim 2， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

receive an instruction to use the dedicated random access channel configuration for transmitting the random access message for the at least one second downlink beam when it is determined that the apparatus is not in the coverage area of the at least one first downlink beam.
The apparatus according to claim 3， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

transmit the random access message using the dedicated random access channel based on the received instruction.
The apparatus according to any of the preceding claims， wherein the random access message comprises a preamble associated with the dedicated random access channel configuration and information related to the at least one second downlink beam.
The apparatus according to any of the preceding claims， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

start monitoring for， or receive， a random access response message from a base station on the at least one second downlink beam after the transmission of the random access message.
The apparatus according to any of the preceding claims， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

receive an instruction to use contention based random access when it is determined that the apparatus is not in the coverage area of the at least one first downlink beam.
The apparatus according to any of the preceding claims， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

transmit the random access message using contention based random access.
The apparatus according to any of the preceding claims， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

receive the dedicated random access channel configuration for the at least one first downlink beam from a source base station； and

transmit the random access message to a target base station for a handover.
The apparatus according to any of the preceding claims， wherein the random access message comprises measurement information related to the at least one second downlink beam.
The apparatus according to any of the preceding claims， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

determine that the apparatus is not in the coverage area of the at least one first downlink beam by comparing a received power of the at least one first downlink beam to a threshold.
An apparatus， comprising：

at least one processor； and

at least one memory including computer program code，

the at least one memory and the computer program code configured， with the at least one processor， to cause the apparatus at least to，

transmit， to a user equipment， a dedicated random access channel configuration associated with at least one first downlink beam； and

receive， from the user equipment， a random access message for at least one second downlink beam on the dedicated random access channel associated with the at least one first downlink beam.
The apparatus according to claim 12， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

transmit an instruction to use the dedicated random access channel configuration for transmitting the random access message for the at least one second downlink beam when it is determined that the user equipment is not in the coverage area of the at least one first downlink beam.
The apparatus according to claim 12 or 13， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

transmit an instruction to not to use the dedicated random access channel configuration for transmitting the random access message for the at least one second downlink beam when it is determined that the user equipment is not in the coverage area of the at least one first downlink beam.
The apparatus according to any claims 12 to 14， wherein the at least one memory and the computer program code are configured， with the at least one processor， to further cause the apparatus at least to，

receive the random access message using contention based random access.
The apparatus according to any of claims 12 to 15， wherein the apparatus is a target base station for a handover.
A method， comprising，

receiving a dedicated random access channel configuration associated with at least one first downlink beam；

determining that the apparatus is not in a coverage area of the at least one first downlink beam； and

transmitting， based on the determination， a random access message for at least one second downlink beam using the dedicated random access channel configuration associated with the at least one first downlink beam.
The method according to claim 17， further comprising，

determining that the apparatus is in a coverage area of the at least one second downlink beam.
The method according to claim 17 or claim 18， further comprising，

receiving an instruction to use the dedicated random access channel configuration for transmitting the random access message for the at least one second downlink beam when it is determined that the apparatus is not in the coverage area of the at least one first downlink beam.
The method according to claim 19， further comprising，

transmitting the random access message using the dedicated random access channel based on the received instruction.
The method according to any of claims 17-20， wherein the random access message comprises a preamble associated with the dedicated random access channel configuration and information related to the at least one second downlink beam.
The method according to any of claims 17-21， further comprising，

starting monitoring for， or receiving， a random access response message from a base station on the at least one second downlink beam after the transmission of the random access message.
The method according to any of claims 17-22， further comprising，

receiving an instruction to use contention based random access when it is determined that the apparatus is not in the coverage area of the at least one first downlink beam.
The method according to any of claims 17-23， further comprising，

transmitting the random access message using contention based random access.
The method according to any of claims 17-24， further comprising，

receiving the dedicated random access channel configuration for the at least one first downlink beam from a source base station； and

transmitting the random access message to a target base station for a handover.
The method according to any of claims 17-25， wherein the random access message comprises measurement information related to the at least one second downlink beam.
The method according to any of claims 17-26， further comprising，

determining that the apparatus is not in the coverage area of the at least one first downlink beam by comparing a received power of the at least one first downlink beam to a threshold.
A method， comprising，

transmitting， to a user equipment， a dedicated random access channel configuration associated with at least one first downlink beam； and

receiving， from the user equipment， a random access message for at least one second downlink beam on the dedicated random access channel associated with the at least one first downlink beam.
The method according to claim 28， further comprising，

transmitting an instruction to use the dedicated random access channel configuration for transmitting the random access message for the at least one second downlink beam when it is determined that the user equipment is not in the coverage area of the at least one first downlink beam.
The method according to claim 28 or claim 29， further comprising，

transmitting an instruction to not to use the dedicated random access channel configuration for transmitting the random access message for the at least one second downlink beam when it is determined that the user equipment is not in the coverage area of the at least one first downlink beam.
The method according to any claims 28 to 30， further comprising，

receiving the random access message using contention based random access.
The method according to any of claims 28 to 31， wherein the apparatus is a target base station for a handover.
An apparatus， comprising means for performing the method according to claims 17-27 or 28-32.
A computer program product configured to control an apparatus to perform a process according to the method of claims 17-27 or 28-32.