WO2013026184A1 - Apparatus and method for random access channel resource selection - Google Patents

Abstract

A method is provided that includes various operations from the perspective of a user equipment. The method includes receiving a plurality of selection probabilities for a respective plurality of component carriers in a carrier aggregation communication scheme, with the selection probabilities being quantitative values received from a base station via a paging message on a paging channel. The method also includes selecting a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities. And the method includes causing participation in a random-access procedure with the base station, including causing transmission of a random access preamble to the base station using the selected random-access channel resource. A corresponding method from the perspective of the base station is also provided.

Description

APPARATUS AND METHOD FOR RANDOM ACCESS CHANNEL RESOURCE SELECTION

TECHNOLOGICAL FIELD

Embodiments of the present invention relate generally to wireless

communication technology and, more particularly, relate to an apparatus and for random access channel resource selection.

BACKGROUND

Carrier aggregation is a combination of two or more component carriers (CCs) operating at different frequencies in order to provide a broader transmission bandwidth for a user equipment (UE). Depending upon its capabilities, a UE may simultaneously receive or transmit on one or more of the CCs. The CCs aggregated in accordance with carrier aggregation include a primary CC and one or more secondary CCs. The primary CC is one in which the UE either performs an initial connection establishment procedure or initiates the connection re-establishment procedure, or one that was indicated as the primary CC in a handover procedure. Conversely, a secondary CC is one that may be configured once radio resource control (RRC) is established and may be used to provide additional radio resources.

Although the focus to date has been principally upon frequency division duplex (FDD) networks, time division duplex (TDD) networks that support carrier aggregation must also be considered. Indeed, in a TDD network, the primary CC and the secondary CCs may have respective TDD uplink (UL)/downlink (DL) subframe configurations. Each configuration divides a frame into a number of subframes each of which is reserved for UL transmission, DL transmission or a special subframe. In Long Term Evolution (LTE) release 10, UEs that support TDD signaling are required to operate in accordance with a TDD UL-DL subframe configuration that is aligned and consistent across the primary and secondary CCs that are to be aggregated.

For UEs configured in accordance with LTE release 1 1 , however, the primary and secondaiy CCs are permitted to have different TDD UL-DL subframe

configurations. These different TDD UL-DL subframe configurations may be useful in order to allow a CC to be compatible with a neighbor legacy TDD system, such as a Time Division Synchronous Code Division Multiple Access (TD-SCDMA) system, for which multiple CCs having aligned TDD UL-DL subframe configurations are not available. Additionally, primary and secondary CCs having different TDD UL-DL subframe configurations may provide different amounts of resources and a different DL/UL ratio for the UE. For example, the use of different TDD UL-DL subframe configurations by the primary and secondary CCs in LTE release 1 1 may permit different ones of the CCs to provide different coverage by, for example, enabling more UL subframes in a lower frequency CC to enlarge the coverage. The ability to provide different amounts of resources and a different DL UL ratio may be of particular importance to a UE that utilizes carrier aggregation since carrier aggregation is oftentimes utilized in an instance in which the UE has a relatively large amount of data to transmit. And this may increase the importance of defining the TDD UL-DL subframe configurations of the primary and secondary CCs so as to provide a suitable DL/UL ratio and to otherwise efficiently utilize the communication resources.

A UE that desires to establish communication with an evolved Node B (eNB) has to search for and select a cell, and establish downlink synchronization

(frequency/timing) with the selected cell (i.e., camp on the selected cell). The UE must then receive and decode system information including operating parameters for the cell on which the UE is camping. The UE may then use the system information to perform a random access procedure - at times referred to by the random access channel (RACH) as a RACH procedure - to establish uplink synchronization with the eNB. During the RACH procedure, the UE may transmit a random access (or RACH) preamble to the eNB on a physical random access channel (PRACH), and receive a random access response (RAR) from the eNB on a physical downlink shared channel (PDSCH). This process is typically followed by the UE on starting up, and the RACH procedure portion of the process is also followed by the UE during handover or transition from an idle state to a connected state. The RACH procedure may be performed in a contention-based or contention-free manner.

The system information received by the UE may be broadcast by the eNB, such as in a master information block (MIB) and a number of system information blocks (SIBs). The system information may include or otherwise indicate, for example, an assigned TDD UL-DL subframe configuration (e.g., TDD-config) delivered in a first SIB (SIB1), and a RACH configuration specifying an available set of resources that can be used by the UE (e.g., prach-Configurationlndex) delivered in a second SIB (SIB2). In LTE, a number of RACH configurations (random access preamble mappings in time and frequency) are defined in table 5.7.1-4 of 3GPP TS 36.211, Physical Channels and Modulation, v.10.0.0 (Dec. 2010).

BRIEF SUMMARY OF EXAMPLE EMBODIMENTS

In case of CC-specific TDD UL-DL subframe configurations, for a given

RACH configuration, different RACH resources may be available for each CC. The UE may equally select the component carriers, and then select RACH resources according to the RACH configuration. Latency experienced by the UE may be affected by the UE's selection of component carriers and RACH resources. In this regard, latency may increase in instances in which the UE selects a CC having a configuration with fewer UL subframes, or experiencing a higher RACH load (which may also result in increased RACH collisions). In this regard, RACH load may be affected by conditions such as (i) different path loss (e.g., CC-1 on a lower-frequency band for higher coverage, and CC-2 on a higher-frequency band for higher capacity), and/or (ii) different interference from neighbor-cell attached UE UL transmissions.

In view of the foregoing, example embodiments of the present invention provide an apparatus and method of selecting component carriers and RACH resources in a manner that reduces latency in the control plane, and that additionally or alternatively increases successful RACH preamble probability (reduces collisions). According to one example aspect, a method is provided that includes various operations from the perspective of a user equipment (UE). The method includes receiving a plurality of selection probabilities for a respective plurality of component earners in a carrier aggregation communication scheme, with the selection

probabilities being quantitative values received from a base station (e.g., eNB) via a paging message on a paging channel. The method also includes selecting a random- access channel resource on a component carrier of the plurality of component earners based on the plurality of selection probabilities. And the method includes causing participation in a random-access procedure with the base station, including causing transmission of a random access preamble to the base station using the selected random-access channel resource.

In one example, the random-access channel resource may be selected from an available set of random-access channel resources based on operating parameters of the base station. In this example, the operating parameters may include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

In one example, the plurality of selection probabilities may be a plurality of unequal component earner-selection probabilities. In this example, selecting the random-access channel resource may include selecting a component carrier of the plurality of component carriers with the plurality of unequal component carrier- selection probabilities. And selecting the random-access channel resource may include selecting the random-access channel resource from the available set of random-access channel resources on the selected component carrier with equal probability.

In one example, the plurality of selection probabilities may be a plurality of unequal resource-selection probabilities. In this example, selecting the random-access channel resource may include selecting the random-access channel resource from the available set of random-access channel resources on any of the plurality of component carriers with the plurality of unequal resource-selection probabilities.

According to another example aspect, a method is provided that includes various operations from the perspective of a base station (e.g., eNB). The method of this aspect includes determining a plurality of selection probabilities for a respective plurality of component carriers in a earner aggregation communication scheme, with the selection probabilities being quantitative values determined based on random- access channel loads on the respective component carriers. The method also includes causing transmission of the plurality of selection probabilities to a user equipment via a paging message on a paging channel. The user equipment is thereby enabled to select a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities. And the method includes causing participation in a random-access procedure with the user equipment, including detecting or otherwise receiving a random access preamble from the user equipment using the selected random-access channel resource.

In one example, the plurality of selection probabilities are determined selectively based on or independent of the random-access channel loads. In this regard, the selection may be based on a number of attached user equipment or detection of random-access channel collisions of attached user equipment. In one example, the method may further include causing transmission of operating parameters to the user equipment that is thereby enabled to select the random-access channel resource from an available set of random-access channel resources based on the operating parameters. In this example, the operating parameters may include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

In one example, the plurality of selection probabilities is a plurality of unequal component carrier-selection probabilities. In this example, transmission of the plurality of selection probabilities is caused to enable the user equipment to select a component carrier of the plurality of component carriers with the plurality of unequal component carrier-selection probabilities. And the user equipment is enabled to select the random-access channel resource from the available set of random-access channel resources on the selected component carrier with equal probability.

In one example, the plurality of selection probabilities is a plurality of unequal resource-selection probabilities. In this example, transmission of the plurality of selection probabilities is caused to enable the user equipment to select the random- access channel resource from the available set of random-access channel resources on any of the plurality of component carriers with the plurality of unequal resource- selection probabilities.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) Having thus described embodiments of the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

FIG. 1 illustrates one example of a communication system according to an example embodiment of the present invention;

FIG. 2 illustrates a block diagram showing an apparatus in accordance with an example embodiment of the present invention;

FIG. 3 illustrates an example of two CCs (CC-1 and CC-2) having two different UL-DL subframe configurations, in accordance with an example

embodiment of the present invention; and FIGS. 4 and 5 are flowcharts illustrating various operations in methods according to example embodiments of the present invention.

DETAILED DESCRIPTION

Some embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, various embodiments of the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like reference numerals refer to like elements throughout.

As used in this application, the term "circuitry" refers to any or all of the following: (a) hardware- only circuit implementations (such as implementations in only analog and/or digital circuitry); (b) combinations of circuits and software (and/or firmware), such as (as applicable): (i) a combination of processor(s) or (ii) to portions of processor(s)/software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions); and (c) to circuits, such as a microprocessor(s) or a portion of a microprocessor(s), that require software or firmware for operation, even if the software or firmware is not physically present.

This definition of "circuitry" applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term "circuitry" would also cover an implementation of merely a processor (or multiple processors) or portion of a processor and its (or their) accompanying software and/or firmware. The term "circuitry" would also cover, for example and if applicable to the particular claim element, a baseband integrated circuit or applications processor integrated circuit for a mobile phone or a similar integrated circuit in server, a cellular network device, or other network device.

FIG. 1 illustrates a first communication device (e.g., user equipment (UE) 10) that is capable of communication with a network 12 (e.g., a core network) via a base station (e.g., an evolved Node B (eNB) 14). While the network may be configured in accordance with LTE or LTE- Advanced (LTE-A), other networks may support the method, apparatus and computer program product of embodiments of the present invention including those configured in accordance with wideband code division multiple access (W-CDMA), CDMA2000, global system for mobile communications (GSM), general packet radio service (GPRS) and/or the like.

The network 12 may include a collection of various different nodes, devices or functions that may be in communication with each other via corresponding wired and/or wireless interfaces, For example, the network may include one or more eNBs 14, each of which may serve a coverage area divided into one or more cells. The eNBs or other communication node could be, for example, part of one or more cellular or mobile networks or public land mobile networks (PLMNs). In turn, other devices such as processing devices (e.g., personal computers, server computers or the like) may be coupled to the UE and/or other communication devices via the network.

A communication device, such as the UE 10 (also known as a mobile terminal), may be in communication with other communication devices or other devices via the eNB 14 and, in turn, the network 12. In some cases, the

communication device may include an antenna for transmitting signals to and for receiving signals from an eNB.

In some example embodiments, the UE 10 may be a mobile communication device such as, for example, a mobile telephone, portable digital assistant (PDA), pager, laptop computer, or any of numerous other hand held or portable

communication devices, computation devices, content generation devices, content consumption devices, or combinations thereof. As such, the UE may include one or more processors that may define processing circuitry either alone or in combination with one or more memories. The processing circuitry may utilize instructions stored in the memory to cause the UE to operate in a particular way or execute specific functionality when the instructions are executed by the one or more processors. The UE may also include communication circuitry and corresponding hardware/software to enable communication with other devices and/or the network 12,

In an example embodiment, components of the system of FIG. 1 may include or otherwise employ an apparatus configured according to an example embodiment of the present invention. FIG, 2 illustrates a schematic block diagram of such an apparatus 20 that may be configured to function as one or more of the components of the system, such as a UE 10 or eNB 14, according to an example embodiment of the present invention. FIG. 2 illustrates certain elements of the apparatus, but it should be understood that the components, devices or elements described below may not be mandatory and thus some may be omitted in certain embodiments. Additionally, some embodiments may include further or different components, devices or elements beyond those shown and described herein.

As shown, the apparatus 20 may include or otherwise be in communication with processing circuitry 22 that is configurable to perform actions in accordance with example embodiments described herein. The processing circuitry may be configured to perform data processing, application execution and/or other processing and management services according to an example embodiment of the present invention. In some embodiments, the apparatus or the processing circuitry may be embodied as a chip or chip set. In other words, the apparatus or the processing circuitry may include one or more physical packages (e.g., chips) including materials, components and/or wires on a structural assembly (e.g., a baseboard). The structural assembly may provide physical strength, conservation of size, and/or limitation of electrical interaction for component circuitry included thereon. The apparatus or the processing circuitry may therefore, in some cases, be configured to implement an embodiment of the present invention on a single chip or as a single "system on a chip." As such, in some cases, a chip or chipset may constitute means for performing one or more operations for providing the functionalities described herein.

In an example embodiment, the processing circuitry 22 may include a processor 24 and memory 26 that may be in communication with or otherwise control a device interface 28 and, in some cases, a user interface 30. As such, the processing circuitry may be embodied as a circuit chip (e.g., an integrated circuit chip)

configured to (e.g., with hardware, alone or in combination with software) perform operations described herein. However, in some embodiments, the processing circuitry may be embodied as a portion of a server, computer, workstation or other fixed or mobile computing device. In situations where the processing circuitry is embodied as a server or at a remotely located computing device, the user interface may be disposed at another device (e.g., at a computer terminal or client device) that may be in communication with the processing circuitry via the device interface and/or a network.

The processor 24 may be embodied in a number of different ways. For example, the processor may be embodied as various processing means such as one or more of a microprocessor or other processing element, a coprocessor, a controller or various other computing or processing devices including integrated circuits such as, for example, an ASIC (application specific integrated circuit), an FPGA (field programmable gate array), or the like. In an example embodiment, the processor may be configured to execute instructions stored in the memory 26 or otherwise accessible to the processor. As such, whether configured by hardware or by a combination of hardware and software, the processor may represent an entity (e.g., physically embodied in circuitry - in the form of processing circuitry 22) capable of performing operations according to embodiments of the present invention while configured accordingly. Thus, for example, when the processor is embodied as an ASIC, FPGA or the like, the processor may be specifically configured hardware for conducting the operations described herein. Alternatively, as another example, when the processor is embodied as an executor of software instructions, the instructions may specifically configure the processor to perform the operations described herein.

In an exemplary embodiment, the memory 26 may include one or more non- transitory memory devices such as, for example, volatile and/or non-volatile memory that may be either fixed or removable. The memory may be configured to store information, data, applications, instructions or the like for enabling the apparatus 20 to carry out various functions in accordance with exemplary embodiments of the present invention. For example, the memory could be configured to buffer input data for processing by the processor 24. Additionally or alternatively, the memory could be configured to store instructions for execution by the processor. As yet another alternative, the memory may include one of a plurality of databases that may store a variety of files, contents or data sets. Among the contents of the memory, applications may be stored for execution by the processor in order to carry out the functionality associated with each respective application. In some cases, the memory may be in communication with the processor via a bus for passing information among components of the apparatus.

The device interface 28 may include one or more interface mechanisms for enabling communication with other devices and/or networks. In some cases, the device interface may be any means such as a device or circuitry embodied in either hardware, or a combination of hardware and software that is configured to receive and/or transmit data from/to a network and/or any other device or module in communication with the processing circuitry 22. In this regard, the device interface may include, for example, an antenna (or multiple antennas) and supporting hardware and/or software for enabling communications with a wireless communication network and/or a communication modem or other hardware/software for supporting 7 communication via cable, digital subscriber line (DSL), universal serial bus (USB), Ethernet or other methods,

The user interface 30 (if implemented) may be in communication with the processing circuitry 22 to receive an indication of a user input at the user interface and/or to provide an audible, visual, mechanical or other output to the user. The user interface may include, for example, a keyboard, a mouse, a

joystick, a display, a touch screen, a microphone, a speaker, and/or other

input/output mechanisms. The user interface may also include user interface circuitry and user interface software configured to facilitate user control of at least some functions of the apparatus through use of a display. In an

exemplary embodiment in which the apparatus is embodied at a server or other network device (e.g., the eNB 14), the user interface may be fully implemented, limited, remotely located or eliminated.

Returning to FIG. 1 , a UE 10 that desires to establish communication with an eNB 14 has to search for and select a cell, and establish downlink synchronization (frequency/timing) with the selected cell (i.e., camp on the selected cell). The UE must then receive and decode system information including operating parameters for the cell on which the UE is camping. The system information may include or otherwise indicate, for example, an assigned TDD UL-DL subframe configuration (e.g., TDD-config) delivered in a first SIB (SIB1), and a RACH configuration specifying an available set of resources that can be used by the UE (e.g., prach- Configurationlndex) delivered in a second SIB (SIB2). The UE may then use the system information to perform a RACH procedure to establish uplink synchronization with the eNB. During the RACH procedure, the UE may transmit a RACH preamble to the eNB on a PRACH, and receive a RACH response from the eNB on a PDSCH. This process is typically followed by the UE on starting up, and the RACH procedure portion of the process is also followed by the UE during handover or transition from an idle state to a connected state. The RACH procedure may be performed in a contention-based or contention-free manner.

As explained in the above summary section, in case of CC-specific TDD UL-

DL subframe configurations, for a given RACH configuration, different RACH resources may be available for each CC. The UE 10 may equally select the component carriers, and then select RACH resources according to the RACH configuration. Latency experienced by the UE may be affected by the UE's selection of component carriers and RACH resources. In this regard, latency may increase in instances in which the UE selects a CC having a configuration with fewer UL subframes, or experiencing a higher RACH load (which may also result in increased RACH collisions). In this regard, RACH load may be affected by conditions such as (i) different path loss (e.g., CC-1 on a lower- frequency band for higher coverage, and CC-2 on a higher-frequency band for higher capacity), and/or (ii) different

interference from neighbor-cell attached UE UL transmissions.

In view of the foregoing, the UE 10 of example embodiments of the present invention is configured to select component carriers and RACH resources in a manner that reduces latency in the control plane, and that additionally or alternatively increases successful RACH preamble probability (reduces collisions). In accordance with example embodiments of the present invention, the eNB 14 is configured to signal UE CC-selection probabilities (pi) or UE RACH resource-selection

probabilities (r,) via a paging message on a shared or paging channel. More particularly, for example, the eNB may transmit the probabilities in a UE-dedicated paging message on the PDSCH, such as from the paging channel (PCH) downlink transport channel specified in Section 6.2.2 of TS 36.331 (e.g., using one or more spare bits or re-interpreting one or more bits). In one example, the eNB is configured to signal pi or n based on an average RACH load experienced on each CC-;^' and random-access-preamble time-frequency mapping parameters ¾Α⁽¹⁾ and ¾A^(¾

configured for each CC by a higher layer.

The UE 10 may be configured to receive the paging message including p or r_i} and select an available RACH resource in any UL subframe based on the probabilities, and based on an available set of RACH resources selected or otherwise determined based on the operating parameters. More particularly as to the operating parameters, after RACH triggering, the UE may be configured to select or otherwise determine an available set of RACH resources as indicated by ¾Α^(¾· This available set of resources may be in the half of the radio frame containing the RACH triggering, or in the next half of the radio frame in case no UL subframes contain available RACH resources, as indicated by ¾A^{( 1 )}.

The UE may therefore select any of the available RACH resources in any of the UL subframes indicated by ¾A^{( 1 )}, ¾A⁽²⁾ with probabilities p_{ or r_f signaled to the UE via the paging message. In an instance in which the eNB 14 signals the

probabilities n to UE instead of the probabilities p_h the UE may effectively aggregate CC-1 and CC-2 as a combined TDD carrier and select an available RACH resource on CC-i with un-equal probabilities n. Otherwise, in an instance in which the eNB signals the probabilities p_i} UE may first select CC-z with un-equal probabilities /?,-, and then select an available RACH resource with equal probabilities. Example embodiments may therefore optimize control plane latency in UE-triggered RACH or eNB-triggered, contention-based RACH in case multiple component carriers configured with different TDD UL/DL subframe configurations.

Under the indicated selection probabilities, the UE 10 may be configured to select the available PRACH resource according to the time slot in which the RACH is triggered, and also the available set of PRACH resources on each carrier. It could therefore be that the UE sees the aggregated TDD carriers as one combined TDD carrier, and selects the nearest PRACH resources considering the probabilities indicated by the eNB 14 via its paging message.

One example of two CCs (CC-1 and CC-2) having two different UL-DL subframe configurations is shown in FIG. 3. As shown, following RACH triggering in subframe #3 on CC-2, the UE 10 may check the configured (7RA^{C 1} \ ¾A^C¾) values for CC-1 and CC-2. Based on these values, the UE may determine the RACH

transmission opportunity in subframes #7 and #8 on a RACH resource indicated as '4,' '5' or '6' on CC-1. Here, a RACH resource may be a physical resource block (PRB), and a group of PRBs may be referred to as a chunk of PRBs. The

probabilities p\, P₂ or n, r₂ may not be used since CC-2 does not include RACH resources in the second half of the radio frame.

In an instance in which the RACH trigger occurred in subframe #0 on CC-1 or CC-2, the UE 10 may transmit a RACH preamble in subframe #1 or #2 at the earliest. The UE may select CC-1 with probability p_\ or CC-2 with probability /¾. If CC-1 , the UE may select with equal probability, a RACH resource indicated as Ί,' '2' or '3' to transmit a RACH preamble; or if CC-2, the UE may select with equal probability a RACH resource indicated as Ί ,' '2,' '3,' '4,' '5' or '6' to transmit the RACH preamble. Alternatively, the UE may select any of the available RACH resources on CC- 1 and CC-2 in subframes #1 or #2 based on probabilities r_\ and r₂.

In various instances, example embodiments may further mitigate RACH collision probability, which may be particularly useful in instances in which UE CC selection or RACH resource selection with un-equal probabilities otherwise results in an increase in collision probability depending on RACH load and number of attached UEs. In this regard, the collision probability may be mitigated by the eNB 14 via reconfiguration of the probabilities p or n. For example, the eNB may configure pi or n without using the RACH load (i.e., equal probability), or provide a safety margin by setting the minimum and maximum values of pi and η.

In a more particular example, Table 5.7.1-4 of TS 36.21 1 lists the mapping to physical resources for the different random access opportunities needed for a certain PRACH density value, D_RA. Each quadruple of the format (/¾_A, ¾_A ⁽⁰⁾, W ^¾) indicates the location of a specific random access resource. In the above, RA is a frequency resource index within the considered time instance. The variable ¾Α⁽⁰⁾ = 0, 1 , 2 indicates whether the resource is reoccurring in all radio frames, in even radio frames, or in odd radio frames, respectively. The variable ¾A⁽¹⁾ = 0, 1 indicates whether the random access resource is located in first half frame or in second half frame, respectively. And ¾_A ⁽²⁾ is the uplink subframe number where the preamble starts, counting from 0 at the first uplink subframe between 2 consecutive downlink- to-uplink switch points.

Also in this example, )] and pi denote the UE CC-selection probabilities for CC-1 and CC-2, respectively, and r_\ and r₂ denote the probabilities of selecting a RACH resource chunk on CC-1 and CC-2, respectively. In one example, p_\, p₂ and r_\, r₂ may be defined as follows:

pi x 1/ u_\ = r

p₂ ^x 1/ U2 ~ !'2

P\ +P2 ⁼ U] x r\ + u₂ x r₂ = 1

Without load information, one may assume equal probabilities ρ_\ =ρι = 1/2.

Otherwise, RACH load information may be obtained as L_\ and L₂ for CC-1 and CC-2, respectively, with value [0, 1] in which 1 indicates 100% RACH load. In this instance, P\ and p₂ may be defined in terms of the load information as follows:

p_] = (l - L_]) / [(l - L_i) + (l - L₂)]

p₂ = (l - L₂) / [(l - L_l) + (l ~ L₂)]

In one example, eNB-triggered contention-based random access triggering may be accomplished via downlink control information (DCI) format 1 A, such as in the manner specified by 3GPP TS 36.212, Multiplexing and Channel Coding, v.10.0.0 (Dec. 2010); and 3 GPP TS 36.321, Medium Access Control, v.10.0.0 (Dec. 2010). In an instance in which the transmission mode is TDD, the PRACH Mask Index is equal to zero and the ra-Preamblelndex is explicitly signaled as 000000, the UE 10 may randomly select with equal probability one RACH resource (6 PRBs) from the RACH resources available in the determined subframe and the next two consecutive subframes.

As one example of RACH resource selection, assume a RACH configuration index #18, where CC-1 and CC-2 have TDD UL-DL configurations #1 and #5, respectively, as specified in TS 36.21 1. Assume RACH triggering occurs in the first half of a radio subframe (e.g., subframe #0), and that UE 10 may further determine that there are RACH chunks of

= 3 PRBs and w₂ = 6 PRBs available in CC-1 and CC-2, respectively, in the example as shown in FIG. 3. In this example, without load information, one may assume equal probabilities p_\ =pi = 1/2, with r_\ = 1/6 and r₂ = 1/12 based on the above equations and using u_\ - 3 and w₂ - 6. In various instances, one may achieve the following probabilities with respective loads:

Example 1 :

L, = 0.4, £2 = 0.8

^_t - (1 - 0.4) / [(1 - 0.4) + (1 0.8)]

j¾ = (1 - 0.8) / [(1 - 0.4) + (1 0.8)]

Π = 3/4 1/3 = 1/4

r₂ = 1/4 1/6 - 1/24 Example 2:

= 0.9, £₂ = 0.1

p = (1 - 0.9) / [(1 - 0.9) + (1 - 0.1)3 = 1/10

P2 = (1 - 0.1) / [(1 - 0.9) + (1 - 0.1)] = 9/10

/-_! = 1/10 1/3 = 1/30

r₂ = 9/10 x 1/6 - 9/60

Example 3:

= 0.1, L₂ = 0.9

pi = (1 - 0.1) / [(1 - 0.1) + (1 - 0.9)] - 9/10

/¾ = 0 - 0.9) / [(1 - 0.1) + (1 - 0.9)] = 1/10

n - 9/10 1/3 - 9/30

r₂ = 1/10 1/6 - 1/60

The UE 10 may select a RACH resource based on probabilities p, or r,- in any a number of different manners. In one example, the UE may randomly or pseudo- randomly select a CC, with a probability p_\ of selecting CC-1, and with a probability P₂ of selecting CC-2. A RACH resource on the selected CC may then be randomly or pseudo-randomly selected from an available set of RACH resources on the selected CC with equal probability. In this regard, the available set of RACH resources may be determined or otherwise selected based on tRA^{( 1 )}, tRA⁽²⁾, subject to restriction based on where the triggering occurred (as mentioned above).

In the above example with p = 3/4 and p₂ = 1/4, the UE 10 may randomly or pseudo-randomly select a CC, with a probability p_\ = 3/4 that the selected CC is CC-1 , and with a probability p₂ = 1/4 that the selected CC is CC-2. Analogously, consider a bag with

= 3 red balls (CC-1), and B₂ = 1 black ball (CC-2). In this example, a ball is randomly or pseudo-randomly drawn from the bag. If the drawn ball is red, the UE selects CC-1 ; otherwise, if the drawn ball is black, the UE selects CC-2.

In another example, there may be unequal probabilities of selecting a RACH resource on CC-1 or CC-2. In this example, the UE 10 may use as the probability of selecting a RACH resource on CC-;^'. There are u_\ RACH resources on CC-1 each one of which may be selected with probability r_\. The probability of UE selecting any RACH resource on CC-1 may then be u_\ ^x r < 1. Likewise, there are u₂ resources on CC-2 each one of which may be selected with probability r_\. The probability of UE selecting any RACH resource on CC-2 may then be u₂ ^x r₂ < 1 · The probability that UE selects a RACH resource on CC-1 or CC-2 may then be the sum of probabilities of selecting RACH resource on CC-z, so u_\ * r_\ + u₂ ^x r₂ = 1 (as given in one of the above equations).

In the above, the UE 10 may randomly or pseudo-randomly select a RACH resource, with a probability u_\ x r_\ that the selected resource is on CC-1 , and with a probability u₂ ^x r₂ that the selected resource is on CC-2. As indicated above, the RACH resource may be selected according to the unequal probabilities, but also further based on an available set of RACH resources on the selected CC with equal probability. Again, the available set of RACH resources may be determined or otherwise selected based on tRA^{( 1 )}, tRA⁽²⁾, subject to restriction based on where the triggering occurred (as mentioned above),

In an analogy according to this example, a bag of balls similar to that described above may include B_\ =u_\lr₂ red balls (RACH resource on CC-1), and B₂ =u₂ ~_\ black balls (RACH resource on CC-2). And similar to before, a ball may be randomly or pseudo-randomly drawn from the bag to select a RACH resource on CC- 1 or CC-2. Applying this analogy to the above example with u_\ - 3, ui - 6, n = 1/4 and i'₂ = 1/24, one may get B_\ - 72 red balls, and B₂ = 24 black balls.

Although described in the context of two CCs above, the selection of a RACH resource (directly or via first selecting a CC) may be readily extended to include more than two CCs. In the case of three CCs, for example, the UE may select a resource on CC-1 , CC-2 or CC-3 with probabilities pi, p2,pi or n, r₂, r₃, and further based on an available set of resources on the respective CCs; or analogously, the UE may make a bag with red, black, and blue balls.

In a number of the above examples, only RACH load information used by the eNB 14 to determine p_\ and ₂. There could be scenario where eNB sets p high and j¾ very low, such as in the above example in which p\ = 9/10 and p% = 1/10. Many attached-UEs may effectively send their RACH preambles on CC-1 since p_\ > p₂, but only three RACH chunks are available on CC-1 based on tRA^(¾. In instances in which the number of attached UEs is high, the system may be more susceptible to RACH collisions. In these instances (or even more generally), the eNB may

instead configure p_\ and p₂ without using the RACH load, or may provide a safety margin by setting the minimum and maximum values of p_\ and j¾ (still having p_\ + p2 = 1). For example, in an instance in which the number of attached UEs exceeds a threshold or RACH collisions are otherwise detected, the eNB may instead set p_\ = 6/10 and p₂ = 4/10 instead of ?, = 9/10 and p₂ = 1/10.

As explained above, the eNB 14 may transmit to the UE 10 an indication of pi or I-, via a UE-dedicated paging message on the PDSCH, such as from the paging channel (PCH) downlink transport channel. In one example, the indication may be included in the message as a new nonCriticalExtension subfield (e.g., RACH- Probability) in a paging message such as that specified in Section 6.2.2 of TS 36.331 , as shown below.

Paging message

- ASN1 START

Paging ::= SEQUENCE {

pagingRecordList PagingRecordList OPTIONAL, - Need ON systemlnfoModification ENUMERATED {true} OPTIONAL, - Need

ON

etws-Indication ENUMERATED {true} OPTIONAL, - Need

ON nonCriticalExtension Paging-v890-IEs OPTIONAL

non CriticalExtension RACH-Probability OPTIONAL

}

RACH-Probability ::= {

p r_;}

Paging-v890-IEs ::= SEQUENCE {

lateNonCriticalExtension OCTET STRING

OPTIONAL, -- Need OP

nonCriticalExtension Paging-v920-IEs

OPTIONAL Paging- v920-IEs ::= SEQUENCE {

cmas-Indication-r9 ENUMERATED {true}

OPTIONAL, - Need ON

nonCriticalExtension SEQUENCE {}

OPTIONAL - Need OP

}

PagingRecordList ::= SEQUENCE (SIZE (L.maxPageRec)) OF PagingRecord

PagingRecord ::= SEQUENCE {

ue-Identity PagingUE-Identity,

cn-Domain ENUMERATED {ps, cs},

}

PagingUE-Identity ::= CHOICE {

s-TMSI S-TMSI,

imsi IMSI,

}

IMSI ::= SEQUENCE (SIZE (6..21)) OF IMSI-Digit IMSI-Digit ::= INTEGER (0..9) -- ASN1 STOP

In current specifications, the UE 10 may receive system information on the cell it is camping on while in the RRC_IDLE state. This may include the SIB 1 -linked TDD_config, and the SIB2-linked prach-Configurationlndex. In LTE, TDD UEs camping on a cell may also receive the SIB2-linked configuration and SIB-1 linked TDD configuration on the other cells if they support carrier aggregation with the above RACH resource selection mechanisms. Hence, these UEs may be configured to read and update the SIB1 and SIB2 messages broadcasted on each cell.

The paging message transmitted on a single CC may indicate the

probabilities p and/or η for a portion or all of the CC-i used for the carrier aggregation, with t - 1, 2 in the given example. The network may assume that the UE has received the SIB1 and SIB2 messages, so it may accordingly

include p r,- in the paging message.

FIGS. 4 and 5 are flowcharts of a system, method and program product according to example embodiments of the invention. It will be understood that each block of the flowcharts, and combinations of blocks in the flowcharts, may be implemented by various means, such as hardware, firmware, processor, circuitry and/or other device associated with execution of software including one or more computer program instructions. For example, one or more of the procedures described above may be embodied by computer program instructions. In this regard, the computer program instructions which embody the procedures described above may be stored by a memory device of an apparatus employing an embodiment of the present invention and executed by a processor in the apparatus. As will be appreciated, any such computer program instructions may be loaded onto a computer or other programmable apparatus (e.g., hardware) to produce a machine, such that the resulting computer or other programmable apparatus provides for implementation of the functions specified in the flowcharts block(s). These computer program instnactions may also be stored in a non-transitory computer-readable storage memory that may direct a computer or other programmable apparatus to function in a particular manner, such that the instructions stored in the computer-readable storage memory produce an article of manufacture the execution of which implements the function specified in the flowcharts block(s). The computer program instructions may also be loaded onto a computer or other programmable apparatus to cause a series of operations to be performed on the computer or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide operations for implementing the functions specified in the flowcharts block(s).

Accordingly, blocks of the flowcharts support combinations of means for performing the specified functions and combinations of operations for performing the specified functions. It will also be understood that one or more blocks of the flowcharts, and combinations of blocks in the flowcharts, can be implemented by special purpose hardware-based computer systems which perform the specified functions, or combinations of special purpose hardware and computer instructions.

FIG. 4 illustrates various operations in a method according to example embodiments of the present invention from the perspective of a UE 10. As shown at block 40, the method includes receiving a plurality of selection probabilities for a respective plurality of component carriers in a carrier aggregation communication scheme, with the selection probabilities being quantitative values received from a base station (e.g., eNB 14) via a paging message on a paging channel. The method also includes selecting a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities, as shown at block 42. And the method includes causing participation in a random-access procedure with the base station, including causing transmission of a random access preamble to the base station using the selected random-access channel resource, as shown at block 44.

In one example, the random-access channel resource may be selected from an available set of random-access channel resources based on operating parameters of the base station. In this example, the operating parameters may include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

In one example, the plurality of selection probabilities may be a plurality of unequal component carrier-selection probabilities. In this example, selecting the random-access channel resource may include selecting a component carrier of the plurality of component carriers with the plurality of unequal component carrier- selection probabilities. And selecting the random-access channel resource may include selecting the random-access channel resource from the available set of random-access channel resources on the selected component carrier with equal probability.

In one example, the plurality of selection probabilities may be a plurality of unequal resource-selection probabilities. In this example, selecting the random-access channel resource may include selecting the random-access channel resource from the available set of random-access channel resources on any of the plurality of component carriers with the plurality of unequal resource-selection probabilities.

FIG. 5 illustrates various operations in a method according to example embodiments of the present invention from the perspective of a base station (e.g., eNB 14). As shown at block 48, the method of these example embodiments includes determining a plurality of selection probabilities for a respective plurality of component carriers in a carrier aggregation communication scheme, with the selection probabilities being quantitative values determined based on random-access channel loads on the respective component carriers. The method also includes causing transmission of the plurality of selection probabilities to a UE 10 via a paging message on a paging channel, as shown in block 50. The UE is thereby enabled to select a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities. And the method includes causing participation in a random-access procedure with the UE, including detecting or otherwise receiving a random access preamble from the UE using the selected random-access channel resource, as shown in block 52.

In one example, the plurality of selection probabilities are determined selectively based on or independent of the random-access channel loads. In this regard, the selection may be based on a number of attached UEs 10 or detection of random-access channel collisions of attached UEs.

In one example, the method may further include causing transmission of operating parameters to the UE 10 that is thereby enabled to select the random-access channel resource from an available set of random-access channel resources based on the operating parameters, as shown in block 46. In this example, the operating parameters may include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

In one example, the plurality of selection probabilities is a plurality of unequal component carrier-selection probabilities. In this example, transmission of the plurality of selection probabilities is caused to enable the UE 10 to select a component carrier of the plurality of component carriers with the plurality of unequal component earner-selection probabilities. And the UE is enabled to select the random-access channel resource from the available set of random-access channel resources on the selected component carrier with equal probability.

In one example, the plurality of selection probabilities is a plurality of unequal resource-selection probabilities. In this example, transmission of the plurality of selection probabilities is caused to enable the UE 10 to select the random-access channel resource from the available set of random-access channel resources on any of the plurality of component carriers with the plurality of unequal resource-selection probabilities.

Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Moreover, although the foregoing descriptions and the associated drawings describe example

embodiments in the context of certain example combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative embodiments without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

WHAT IS CLAIMED IS:

1. 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 being configured to, with the at least one processor, cause the apparatus at least:

receive a plurality of selection probabilities for a respective plurality of component carriers in a carrier aggregation communication scheme, the selection probabilities being quantitative values received from a base station via a paging message on a paging channel;

select a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities; and cause participation in a random-access procedure with the base station, including cause transmission of a random access preamble to the base station using the selected random-access channel resource,

2. The apparatus of Claim 1, wherein the random-access channel resource is selected from an available set of random-access channel resources based on operating parameters of the base station.

3. The apparatus of Claim 2, wherein the operating parameters include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

4. The apparatus of either of Claims 2 or 3, wherein the plurality of selection probabilities is a plurality of unequal component carrier-selection probabilities, and wherein the apparatus being caused to select the random-access channel resource includes being caused to:

select a component carrier of the plurality of component carriers with the plurality of unequal component carrier-selection probabilities; and select the random-access channel resource from the available set of random- access channel resources on the selected component carrier with equal probability.

5. The apparatus of either of Claims 2 or 3, wherein the plurality of selection probabilities is a plurality of unequal resource-selection probabilities, and wherein the apparatus being caused to select the random-access channel resource includes being caused to:

select the random-access channel resource from the available set of random- access channel resources on any of the plurality of component earners with the plurality of unequal resource-selection probabilities.

6. 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 being configured to, with the at least one processor, cause the apparatus at least:

determine a plurality of selection probabilities for a respective plurality of component carriers in a earner aggregation communication scheme, the selection probabilities being quantitative values determined based on random-access channel loads on the respective component carriers;

cause transmission of the plurality of selection probabilities to a user equipment via a paging message on a paging channel, the user equipment thereby being enabled to select a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities; and

cause participation in a random-access procedure with the user equipment, including detect a random access preamble from the user equipment using the selected random-access channel resource. 7. The apparatus of Claim 6, wherein the plurality of selection probabilities are determined selectively based on or independent of the random-access channel loads, the selection being based on a number of attached user equipment or detection of random-access channel collisions of attached user equipment.

8. The apparatus of either of Claims 6 or 7, the at least one memory and the computer program code are further configured to, with the at least one processor, cause the apparatus to further:

cause transmission of operating parameters to the user equipment that is thereby enabled to select the random-access channel resource from an available set of random-access channel resources based on the operating parameters.

9. The apparatus of Claim 8, wherein the operating parameters include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

10. The apparatus of either of Claims 8 or 9, wherein the plurality of selection probabilities is a plurality of unequal component carrier-selection probabilities, and wherein transmission of the plurality of selection probabilities is caused to enable the user equipment to:

select a component carrier of the plurality of component carriers with the plurality of unequal component carrier-selection probabilities; and

select the random-access channel resource from the available set of random- access channel resources on the selected component carrier with equal probability.

1 1. The apparatus of either of Claims 8 or 9, wherein the plurality of selection probabilities is a plurality of unequal resource-selection probabilities, and wherein transmission of the plurality of selection probabilities is caused to enable the user equipment to:

select the random-access channel resource from the available set of random- access channel resources on any of the plurality of component earners with the plurality of unequal resource-selection probabilities.

12. A method comprising:

receiving a plurality of selection probabilities for a respective plurality of component carriers in a carrier aggregation communication scheme, the selection probabilities being quantitative values received from a base station via a paging message on a paging channel;

selecting a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities; and causing participation in a random-access procedure with the base station, including causing transmission of a random access preamble to the base station using the selected random-access channel resource.

13. The method of Claim 12, wherein selecting the random-access channel resource and causing participation in the random-access procedure are performed by an apparatus including a processor and memory including computer program code, the memory and computer program code being configured to, with the processor, cause the apparatus at least select the random-access channel resource and cause participation in the random-access procedure.

14. The method of either of Claims 12 or 13, wherein the random-access channel resource is selected from an available set of random-access channel resources based on operating parameters of the base station.

15. The method of Claim 14, wherein the operating parameters include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

16. The method of either of Claims 14 or 15, wherein the plurality of selection probabilities is a plurality of unequal component carrier-selection probabilities, and wherein selecting the random-access channel resource comprises: selecting a component carrier of the plurality of component carriers with the plurality of unequal component carrier-selection probabilities; and

selecting the random-access channel resource from the available set of random-access channel resources on the selected component carrier with equal probability.

17. The method of either of Claims 14 or 15, wherein the plurality of selection probabilities is a plurality of unequal resource-selection probabilities, and wherein selecting the random-access channel resource comprises:

selecting the random-access channel resource from the available set of random-access channel resources on any of the plurality of component carriers with the plurality of unequal resource-selection probabilities.

18. A method comprising:

determining a plurality of selection probabilities for a respective plurality of component carriers in a carrier aggregation communication scheme, the selection probabilities being quantitative values determined based on random-access channel loads on the respective component carriers;

causing transmission of the plurality of selection probabilities to a user equipment via a paging message on a paging channel, the user equipment thereby being enabled to select a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities; and

causing participation in a random-access procedure with the user equipment, including detecting a random access preamble from the user equipment using the selected random-access channel resource.

19. The method of Claim 18, wherein determining the plurality of selection probabilities is performed by an apparatus including a processor and memory including computer program code, the memory and computer program code being configured to, with the processor, cause the apparatus at least determine the plurality of selection probabilities.

20. The method of either of Claims 18 or 19, wherein the plurality of selection probabilities are determined selectively based on or independent of the random-access channel loads, the selection being based on a number of attached user equipment or detection of random-access channel collisions of attached user equipment.

21. The method of any of Claims 18, 19 or 20 further comprising:

causing transmission of operating parameters to the user equipment that is thereby enabled to select the random-access channel resource from an available set of random-access channel resources based on the operating parameters.

22. The method of Claim 21 , wherein the operating parameters include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

23. The method of either of Claims 21 or 22, wherein the plurality of selection probabilities is a plurality of unequal component carrier-selection probabilities, and wherein transmission of the plurality of selection probabilities is caused to enable the user equipment to:

select a component carrier of the plurality of component carriers with the plurality of unequal component carrier-selection probabilities; and

select the random-access channel resource from the available set of random- access channel resources on the selected component carrier with equal probability.

24. The method of either of Claims 21 or 22, wherein the plurality of selection probabilities is a plurality of unequal resource-selection probabilities, and wherein transmission of the plurality of selection probabilities is caused to enable the user equipment to:

select the random-access channel resource from the available set of random- access channel resources on any of the plurality of component carriers with the plurality of unequal resource-selection probabilities.

25. An apparatus comprising:

means for receiving a plurality of selection probabilities for a respective plurality of component carriers in a carrier aggregation communication scheme, the selection probabilities being quantitative values received from a base station via a paging message on a paging channel; means for selecting a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities; and

means for causing participation in a random-access procedure with the base station, including causing transmission of a random access preamble to the base station using the selected random-access channel resource.

26. The apparatus of Claim 25, wherein the means for selecting the random-access channel resource and means for causing participation in the random- access procedure comprise a processor and memory including computer program code, the memory and computer program code being configured to, with the processor, cause the apparatus at least select the random-access channel resource and cause participation in the random-access procedure. 27. The apparatus of either of Claims 25 or 26, wherein the random-access channel resource is selected from an available set of random-access channel resources based on operating parameters of the base station.

28. The apparatus of Claim 27, wherein the operating parameters include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts. 29. The apparatus of either of Claims 27 or 28, wherein the plurality of selection probabilities is a plurality of unequal component carrier-selection

probabilities, and wherein selecting the random-access channel resource comprises: selecting a component carrier of the plurality of component carriers with the plurality of unequal component carrier-selection probabilities; and

selecting the random-access channel resource from the available set of random-access channel resources on the selected component carrier with equal probability.

30. The apparatus of either of Claims 27 or 28, wherein the plurality of selection probabilities is a plurality of unequal resource-selection probabilities, and wherein selecting the random-access channel resource comprises:

selecting the random-access channel resource from the available set of random-access channel resources on any of the plurality of component carriers with the plurality of unequal resource-selection probabilities.

31. An apparatus comprising;

means for determining a plurality of selection probabilities for a respective plurality of component carriers in a carrier aggregation communication scheme, the selection probabilities being quantitative values determined based on random-access channel loads on the respective component carriers;

means for causing transmission of the plurality of selection probabilities to a user equipment via a paging message on a paging channel, the user equipment thereby being enabled to select a random-access channel resource on a component carrier of the plurality of component carriers based on the plurality of selection probabilities; and

means for causing participation in a random-access procedure with the user equipment, including detecting a random access preamble from the user equipment using the selected random-access channel resource.

32. The apparatus of Claim 31 , wherein determining the plurality of selection probabilities is performed by an apparatus including a processor and memory including computer program code, the memory and computer program code being configured to, with the processor, cause the apparatus at least determine the plurality of selection probabilities.

33. The apparatus of either of Claims 31 or 32, wherein the plurality of selection probabilities are determined selectively based on or independent of the random-access channel loads, the selection being based on a number of attached user equipment or detection of random-access channel collisions of attached user equipment.

34. The apparatus of any of Claims 31 , 32 or 33 further comprising:

means for causing transmission of operating parameters to the user equipment that is thereby enabled to select the random-access channel resource from an available set of random-access channel resources based on the operating parameters.

35. The apparatus of Claim 34, wherein the operating parameters include a first indication of the available set of random-access channel resources being located in a first half or second half of a frame divided into a number of subframes, and a second indication of an uplink one of the subframes in which transmission of the random access preamble starts.

36. The apparatus of either of Claims 34 or 35, wherein the plurality of selection probabilities is a plurality of unequal component earner-selection probabilities, and wherein transmission of the plurality of selection probabilities is caused to enable the user equipment to:

select a component carrier of the plurality of component carriers with the plurality of unequal component carrier-selection probabilities; and

select the random-access channel resource from the available set of random- access channel resources on the selected component carrier with equal probability.

37. The apparatus of either of Claims 34 or 35, wherein the plurality of selection probabilities is a plurality of unequal resource-selection probabilities, and wherein transmission of the plurality of selection probabilities is caused to enable the user equipment to:

select the random-access channel resource from the available set of random- access channel resources on any of the plurality of component carriers with the plurality of unequal resource-selection probabilities.

38. A computer program which, when executed, causes the method of any one or more of Claims 12-17 to be performed.

3 . A computer program which, when executed, causes the method of any one or more of Claims 18-24 to be performed.