WO2017045695A1

WO2017045695A1 - Method, system and apparatus for switching between d2d and cellular communications

Info

Publication number: WO2017045695A1
Application number: PCT/EP2015/070939
Authority: WO
Inventors: Richa Gupta; Suresh Kalyanasundaram; Ajith Kumar P R; Shalini Gulati
Original assignee: Nokia Solutions And Networks Oy
Priority date: 2015-09-14
Filing date: 2015-09-14
Publication date: 2017-03-23

Abstract

There is provided a method comprising determining, for a resource in a transmission time interval, a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity and scheduling said at least one device-to- device communication capable user equipment to operate in said determined mode on the resource in the transmission time interval.

Description

Title

METHOD, SYSTEM AND APPARATUS FOR SWITCHING BETWEEN D2D AND CELLULAR

COM MUNICATIONS

Field

The present application relates to a method, apparatus, system and computer program and in particular but not exclusively to device-to-device (D2D) communication.

Background

A communication system can be seen as a facility that enables communication sessions between two or more entities such as user terminals, base stations and/or other nodes by providing carriers between the various entities involved in the communications path. A communication system can be provided for example by means of a communication network and one or more compatible communication devices. The communication sessions may comprise, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and/or content data and so on. Non-limiting examples of services provided comprise two-way or multi-way calls, data communication or multimedia services and access to a data network system, such as the Internet.

In a wireless communication system at least a part of a communication session between at least two stations occurs over a wireless link. Examples of wireless systems comprise public land mobile networks (PLMN), satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). The wireless systems can typically be divided into cells, and are therefore often referred to as cellular systems.

A user can access the communication system by means of an appropriate communication device or terminal. A communication device of a user is often referred to as user equipment (UE). A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling communications, for example enabling access to a communication network or communications directly with other users. The communication device may access a carrier provided by a station, for example a base station of a cell, and transmit and/or receive communications on the carrier. The communication system and associated devices typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. Communication protocols and/or parameters which shall be used for the connection are also typically defined. An example of attempts to solve the problems associated with the increased demands for capacity is an architecture that is known as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. The LTE is being standardized by the 3rd Generation Partnership Project (3GPP). The various development stages of the 3GPP LTE specifications are referred to as releases. Certain releases of 3GPP LTE (e.g., LTE Rel-1 1 , LTE Rel-12, LTE Rel-13) are targeted towards LTE-Advanced (LTE- A). LTE-A is directed towards extending and optimising the 3GPP LTE radio access technologies. A goal of LTE-A is to provide significantly enhanced services by means of higher data rates and lower latency with reduced cost. Summary

In a first aspect there is provided a method comprising determining, for a resource in a transmission time interval, a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity and scheduling said at least one device-to-device communication capable user equipment to operate in said determined mode on the resource in the transmission time interval.

If the determined mode is cellular, no further user equipments of the plurality of user equipments in cellular mode, or cellular user equipments, may be scheduled to operate on the resource in the transmission time interval.

If the at least one device-to-device communication capable user equipment is scheduled in one of the direct and cellular mode, the at least one device-to-device communication capable user equipment may no longer be considered for scheduling in the other of the direct and cellular mode for another resource in the transmission time interval.

The determining may comprise determining expected instantaneous data rate in the resource for a given transmission time interval for each user equipment of the plurality of user equipments. The method may comprise, if the user equipment is a device-to-device capable user equipment, determining expected instantaneous data rate for the given transmission time interval if the user equipment is scheduled in direct mode and expected instantaneous data rate for the given transmission time interval if the device is scheduled in cellular mode.

Determining may comprise determining a proportional fairness metric for each user equipment of the plurality of user equipments in dependence on the respective expected instantaneous data rate, determining which user equipment of the plurality of user equipments maximises the sum proportional fairness metric, and the associated mode and scheduling the determined user equipment to operate in the associated mode on the resource in the transmission time interval.

The proportional fairness metric may be determined in dependence on interference from any user equipments of the plurality of user equipments scheduled on the same resource in the transmission time interval.

In a second aspect there is provided a method comprising determining a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity, in dependence on an expected instantaneous data rate and scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource.

The method may comprise scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource in every TTI.

Determining may comprise determining whether to switch a mode of operation from one of the direct mode and cellular mode to the other of the direct mode and cellular mode.

Determining whether to switch a mode of operation may comprise determining a switching metric in dependence on the expected instantaneous data rate.

The switching metric may be dependent on interference at the plurality of user equipments from the at least one device-to-device communication capable user equipment if the device- to-device communication capable user equipment switches modes from one of the direct mode and cellular mode to the other of the direct mode and cellular mode. Determining a switching metric may comprise determining a ratio of the expected instantaneous data rate in the other of the direct mode and cellular mode to the expected instantaneous data rate in the current mode of operation and switching the mode of operation if the switching metric is greater than 1 .

Determining whether to switch a mode of operation may comprise determining that the at least one device-to-device communication capable user equipment has a maximum switching metric of the plurality of user equipments.

The method may comprise determining the mode of operation when device to device traffic commences and/or when a change in channel conditions reaches a threshold.

The method may comprise determining the mode of operation every transmission time interval or every plurality of transmission time intervals.

The mode may be determined per resource, wherein the resource comprises the allocated resource. The method may comprise determining the expected instantaneous data rate in dependence on the number of resources the UE may be allocated in the respective mode under the system load conditions.

The cellular mode may use both UL and DL resources. The direct mode may use only UL resources.

The number of resources a UE may be allocated may be dependent on the reuse of resources between direct mode and cellular mode transmission. In a third aspect, there is provided an apparatus, said apparatus comprising means for determining, for a resource in a transmission time interval, a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity and means for scheduling said at least one device-to-device communication capable user equipment to operate in said determined mode on the resource in the transmission time interval. If the determined mode is cellular, no further user equipments of the plurality of user equipments in cellular mode, or cellular user equipments, may be scheduled to operate on the resource in the transmission time interval. If the at least one device-to-device communication capable user equipment is scheduled in one of the direct and cellular mode, the at least one device-to-device communication capable user equipment may no longer be considered for scheduling in the other of the direct and cellular mode for another resource in the transmission time interval. The means for determining may comprise means for determining expected instantaneous data rate in the resource for a given transmission time interval for each user equipment of the plurality of user equipments.

The apparatus may comprise means for, if the user equipment is a device-to-device capable user equipment, determining expected instantaneous data rate for the given transmission time interval if the user equipment is scheduled in direct mode and expected instantaneous data rate for the given transmission time interval if the device is scheduled in cellular mode.

The means for determining may comprise means for determining a proportional fairness metric for each user equipment of the plurality of user equipments in dependence on the respective expected instantaneous data rate, means for determining which user equipment of the plurality of user equipments maximises the sum proportional fairness metric, and the associated mode and means for scheduling the determined user equipment to operate in the associated mode on the resource in the transmission time interval.

The proportional fairness metric may be determined in dependence on interference from any user equipments of the plurality of user equipments scheduled on the same resource in the transmission time interval. In a fourth aspect there is provided an apparatus comprising means for determining a mode of operation selected from a direct mode and a cellular mode, for at least one device-to- device communication capable user equipment of a plurality of user equipments associated with a scheduling entity, in dependence on an expected instantaneous data rate and means for scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource. The apparatus may comprise means for scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource in every TTI. Means for determining may comprise means for determining whether to switch a mode of operation from one of the direct mode and cellular mode to the other of the direct mode and cellular mode.

Means for determining whether to switch a mode of operation may comprise means for determining a switching metric in dependence on the expected instantaneous data rate.

The switching metric may be dependent on interference at the plurality of user equipments from the at least one device-to-device communication capable user equipment if the device- to-device communication capable user equipment switches modes from one of the direct mode and cellular mode to the other of the direct mode and cellular mode.

Means for determining the switching metric may comprise means for determining a ratio of the expected instantaneous data rate in the other of the direct mode and cellular mode to the expected instantaneous data rate in the current mode of operation and means for switching the mode of operation if the switching metric is greater than 1 .

Means for determining whether to switch a mode of operation may comprise means for determining that the at least one device-to-device communication capable user equipment has a maximum switching metric of the plurality of user equipments.

The apparatus may comprise means for determining the mode of operation when device to device traffic commences and/or when a change in channel conditions reaches a threshold.

The apparatus may comprise means for determining the mode of operation every transmission time interval or every plurality of transmission time intervals.

The mode may be determined per resource, wherein the resource comprises the allocated resource. The apparatus may comprise means for determining the expected instantaneous data rate in dependence on the number of resources the UE may be allocated in the respective mode under the system load conditions. The cellular mode may use both UL and DL resources. The direct mode may use only UL resources. The number of resources a UE may be allocated may be dependent on the reuse of resources between direct mode and cellular mode transmission.

In a fifth aspect there is provided an apparatus, said apparatus comprising at least one processor and at least one memory including a computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to determine, for a resource in a transmission time interval, a mode of operation selected from a direct mode and a cellular mode, for at least one device- to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity and schedule said at least one device-to-device communication capable user equipment to operate in said determined mode on the resource in the transmission time interval.

The apparatus may be configured to determine expected instantaneous data rate in the resource for a given transmission time interval for each user equipment of the plurality of user equipments.

The apparatus may be configured to, if the user equipment is a device-to-device capable user equipment, determine expected instantaneous data rate for the given transmission time interval if the user equipment is scheduled in direct mode and expected instantaneous data rate for the given transmission time interval if the device is scheduled in cellular mode.

The apparatus may be configured to determine a proportional fairness metric for each user equipment of the plurality of user equipments in dependence on the respective expected instantaneous data rate, determine which user equipment of the plurality of user equipments maximises the sum proportional fairness metric, and the associated mode and schedule the determined user equipment to operate in the associated mode on the resource in the transmission time interval.

The proportional fairness metric may be determined in dependence on interference from any user equipments of the plurality of user equipments scheduled on the same resource in the transmission time interval. In a sixth aspect, there is provided an apparatus, said apparatus comprising at least one processor and at least one memory including a computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to determine a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity, in dependence on an expected instantaneous data rate and schedule the at least one device- to-device communication capable user equipment to operate in said determined mode on an allocated resource. The apparatus may be configured to schedule the at least one device-to-device

communication capable user equipment to operate in said determined mode on an allocated resource in every TTI.

The apparatus may be configured to determine whether to switch a mode of operation from one of the direct mode and cellular mode to the other of the direct mode and cellular mode.

The apparatus may be configured to determine a switching metric in dependence on the expected instantaneous data rate. The switching metric may be dependent on interference at the plurality of user equipments from the at least one device-to-device communication capable user equipment if the device- to-device communication capable user equipment switches modes from one of the direct mode and cellular mode to the other of the direct mode and cellular mode.

The apparatus may be configured to determine a ratio of the expected instantaneous data rate in the other of the direct mode and cellular mode to the expected instantaneous data rate in the current mode of operation and switch the mode of operation if the switching metric is greater than 1.

The apparatus may be configured to determine that the at least one device-to-device communication capable user equipment has a maximum switching metric of the plurality of user equipments.

The apparatus may be configured to determine the mode of operation when device to device traffic commences and/or when a change in channel conditions reaches a threshold.

The apparatus may be configured to determine the mode of operation every transmission time interval or every plurality of transmission time intervals.

The mode may be determined per resource, wherein the resource comprises the allocated resource.

The apparatus may be configured to determine the expected instantaneous data rate in dependence on the number of resources the UE may be allocated in the respective mode under the system load conditions.

The number of resources a UE may be allocated may be dependent on the reuse of resources between direct mode and cellular mode transmission.

In a seventh aspect, there is provided a computer program embodied on a non-transitory computer-readable storage medium, the computer program comprising program code for controlling a process to execute a process, the process comprising determining, for a resource in a transmission time interval, a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity and scheduling said at least one device-to-device communication capable user equipment to operate in said determined mode on the resource in the transmission time interval. If the determined mode is cellular, no further user equipments of the plurality of user equipments in cellular mode, or cellular user equipments, may be scheduled to operate on the resource in the transmission time interval. If the at least one device-to-device communication capable user equipment is scheduled in one of the direct and cellular mode, the at least one device-to-device communication capable user equipment may no longer be considered for scheduling in the other of the direct and cellular mode for another resource in the transmission time interval. The determining may comprise determining expected instantaneous data rate in the resource for a given transmission time interval for each user equipment of the plurality of user equipments.

The process may comprise, if the user equipment is a device-to-device capable user equipment, determining expected instantaneous data rate for the given transmission time interval if the user equipment is scheduled in direct mode and expected instantaneous data rate for the given transmission time interval if the device is scheduled in cellular mode.

The proportional fairness metric may be determined in dependence on interference from any user equipments of the plurality of user equipments scheduled on the same resource in the transmission time interval. In an eighth aspect, there is provided a computer program embodied on a non-transitory computer-readable storage medium, the computer program comprising program code for controlling a process to execute a process, the process comprising determining a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity, in dependence on an expected instantaneous data rate and scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource. The process may comprise scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource in every TTI.

Determining the switching metric may comprise determining a ratio of the expected instantaneous data rate in the other of the direct mode and cellular mode to the expected instantaneous data rate in the current mode of operation and switching the mode of operation if the switching metric is greater than 1 .

The process may comprise determining the mode of operation when device to device traffic commences and/or when a change in channel conditions reaches a threshold.

The process may comprise determining the mode of operation every transmission time interval or every plurality of transmission time intervals.

The mode may be determined per resource, wherein the resource comprises the allocated resource. The process may comprise determining the expected instantaneous data rate in

dependence on the number of resources the UE may be allocated in the respective mode under the system load conditions. The cellular mode may use both UL and DL resources. The direct mode may use only UL resources. The number of resources a UE may be allocated may be dependent on the reuse of resources between direct mode and cellular mode transmission.

In a ninth aspect there is provided a computer program product for a computer, comprising software code portions for performing the steps the method of the first aspect and/or second when said product is run on the computer.

In the above, many different embodiments have been described. It should be appreciated that further embodiments may be provided by the combination of any two or more of the embodiments described above.

Description of Figures

Embodiments will now be described, by way of example only, with reference to the accompanying Figures in which: Figure 1 shows a schematic diagram of an example communication system comprising a plurality of base stations and a plurality of communication devices;

Figure 2 shows a schematic diagram of an example mobile communication device; Figure 3 shows a flowchart of an example method of mode selection and resource allocation;

Figure 4 shows a flowchart of an example method of mode selection and resource allocation;

Figure 5 shows a schematic diagram of interference caused by D2D direct mode transmission of UE2 on other UEs;

Figure 6 shows a schematic diagram of interference caused by cellular mode transmission of UE1 on other UEs; Figure 7 shows a flowchart of an example algorithm for mode selection and resource allocation;

Figure 8 shows the geometric mean of UEs' throughput gains for joint mode selection and resource allocation;

Figure 9 shows geometric mean of UE throughput gains for joint scheduling and mode selection and slow and fast scale mode selection; Figure 10 shows a schematic diagram of an example control apparatus;

Detailed description

Before explaining in detail the examples, certain general principles of a wireless communication system and mobile communication devices are briefly explained with reference to Figures 1 to 2 to assist in understanding the technology underlying the described examples.

In a wireless communication system 100, such as that shown in figure 1 , mobile communication devices or user equipment (UE) 102, 104, 105 are provided wireless access via at least one base station or similar wireless transmitting and/or receiving node or point. Base stations are typically controlled by at least one appropriate controller apparatus, so as to enable operation thereof and management of mobile communication devices in communication with the base stations. The controller apparatus may be located in a radio access network (e.g. wireless communication system 100) or in a core network (CN) (not shown) and may be implemented as one central apparatus or its functionality may be distributed over several apparatus. The controller apparatus may be part of the base station and/or provided by a separate entity such as a Radio Network Controller. In Figure 1 control apparatus 108 and 109 are shown to control the respective macro level base stations 106 and 107. The control apparatus of a base station can be interconnected with other control entities. The control apparatus is typically provided with memory capacity and at least one data processor. The control apparatus and functions may be distributed between a plurality of control units. In some systems, the control apparatus may additionally or alternatively be provided in a radio network controller.

LTE systems may however be considered to have a so-called "flat" architecture, without the provision of RNCs; rather the (e)NB is in communication with a system architecture evolution gateway (SAE-GW) and a mobility management entity (MME), which entities may also be pooled meaning that a plurality of these nodes may serve a plurality (set) of (e)NBs. Each UE is served by only one MME and/or S-GW at a time and the (e)NB keeps track of current association. SAE-GW is a "high-level" user plane core network element in LTE, which may consist of the S-GW and the P-GW (serving gateway and packet data network gateway, respectively). The functionalities of the S-GW and P-GW are separated and they are not required to be co-located.

In Figure 1 base stations 106 and 107 are shown as connected to a wider communications network 1 13 via gateway 1 12. A further gateway function may be provided to connect to another network.

The smaller base stations 1 16, 1 18 and 120 may also be connected to the network 1 13, for example by a separate gateway function and/or via the controllers of the macro level stations. The base stations 1 16, 1 18 and 120 may be pico or femto level base stations or the like. In the example, stations 1 16 and 1 18 are connected via a gateway 1 1 1 whilst station 120 connects via the controller apparatus 108. In some embodiments, the smaller stations may not be provided. Smaller base stations 1 16, 1 18 and 120 may be part of a second network, for example WLAN and may be WLAN APs.

A possible mobile communication device will now be described in more detail with reference to Figure 2 showing a schematic, partially sectioned view of a communication device 200. Such a communication device is often referred to as user equipment (UE) or terminal. An appropriate mobile communication device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples comprise a mobile station (MS) or mobile device such as a mobile phone or what is known as a 'smart phone', a computer provided with a wireless interface card or other wireless interface facility (e.g., USB dongle), personal data assistant (PDA) or a tablet provided with wireless communication capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and so on. Users may thus be offered and provided numerous services via their communication devices. Non-limiting examples of these services comprise two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. Users may also be provided broadcast or multicast data. Non-limiting examples of the content comprise downloads, television and radio programs, videos, advertisements, various alerts and other information. The mobile device 200 may receive signals over an air or radio interface 207 via appropriate apparatus for receiving and may transmit signals via appropriate apparatus for transmitting radio signals. In Figure 2 transceiver apparatus is designated schematically by block 206. The transceiver apparatus 206 may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device.

A mobile device is typically provided with at least one data processing entity 201 , at least one memory 202 and other possible components 203 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access systems and other communication devices. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 204. The user may control the operation of the mobile device by means of a suitable user interface such as key pad 205, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 208, a speaker and a microphone can be also provided. Furthermore, a mobile communication device may comprise appropriate connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto.

The communication devices 102, 104, 105 may access the communication system based on various access techniques, such as code division multiple access (CDMA), or wideband CDMA (WCDMA). Other non-limiting examples comprise time division multiple access (TDMA), frequency division multiple access (FDMA) and various schemes thereof such as the interleaved frequency division multiple access (IFDMA), single carrier frequency division multiple access (SC-FDMA) and orthogonal frequency division multiple access (OFDMA), space division multiple access (SDMA) and so on. An example of wireless communication systems are architectures standardized by the 3rd Generation Partnership Project (3GPP). A latest 3GPP based development is often referred to as the long term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. The various development stages of the 3GPP specifications are referred to as releases. More recent developments of the LTE are often referred to as LTE Advanced (LTE-A). The LTE employs a mobile architecture known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). Base stations of such systems are known as evolved or enhanced Node Bs (eNBs) and provide E-UTRAN features such as user plane Packet Data Convergence/Radio Link Control/Medium Access Control/Physical layer protocol (PDCP/RLC/MAC/PHY) and control plane Radio Resource Control (RRC) protocol terminations towards the communication devices. Other examples of radio access system comprise those provided by base stations of systems that are based on technologies such as wireless local area network (WLAN) and/or WiMax (Worldwide Interoperability for Microwave Access). A base station can provide coverage for an entire cell or similar radio service area.

Device-to-device (D2D) communication as an underlying network co-existing with cellular networks has been proposed as a means of increasing spectrum utilization. However, reuse of resources between cellular and device-to-device direct communication may increase interference in a system if the resources are not optimally allocated. D2D communication capable UEs (D2D UEs) may transmit data to each other in one of two modes: direct mode and cellular mode. In direct mode, data is transmitted over sidelink (direct link) between D2D UEs. In cellular mode, transmission is done via infrastructure, i.e., legacy mode of data transmission that includes an uplink transmission followed by a downlink transmission using two-hop communication.

Cellular resources are reused in the direct mode of communication between a pair of D2D devices. A pair of D2D devices which is in close proximity to one another may achieve high throughput by transmitting data on direct link, instead of using cellular mode transmission; the spectral efficiency may be larger on the direct link and there may be larger resource availability due to resource reuse. The reuse of spectrum may improve system throughput but may create interference for co-scheduled cellular users. The interference introduced on cellular links may be large depending on the distance between direct mode D2D UEs and/or the distance between the D2D transmitter and the cellular receiver.

The cellular mode of communication between a pair of D2D devices requires two hop data transmission. If the communicating D2D devices are far apart, then cellular mode

communication may provide larger throughput and cause less interference to neighboring UEs when compared to direct mode communication, because smaller transmit power and fewer resources are used to transmit to the eNB.

In addition, the number of resources (or PRBs) the UE is allocated in cellular and direct modes will also affect the throughput the UE can expect in these two modes. Mode selection for D2D communication may be performed via coalition where UEs randomly pick a coalition given that it satisfies their rate constraint. An algorithm tries to find a stable coalition, which may not provide a globally optimal solution. This method uses an objective of minimizing total power subject to meeting the rate requirements of the users.

Some proposals aim to provide sum-rate maximization rather than sum-utility maximization.

An efficient mode selection scheme is thus desirable for effective D2D communication. It may be desirable that resources among the cellular UEs and D2D UEs using either cellular mode or direct mode are allocated such that the overall network-wide utility is improved. Mode selection for D2D UEs and resource allocation for all the UEs in the system, i.e., cellular and D2D UEs may be considered together.

In the following, schemes for D2D mode selection, i.e., for a given (source, destination) pair, whether the communication should happen in the direct mode or cellular mode, are provided.

Three options are provided. In a first option, referred to as Slow D2D Mode Selection, the D2D Mode is selected on slow time-scale, e.g., when D2D traffic starts between a (source, destination) pair, and when the channel conditions have changed significantly. Once D2D mode is selected, then scheduling, resource allocation, and power control happen in every TTI based on the selected mode.

In a second option, referred to as Fast D2D Mode Selection, in every TTI or every few TTIs (e.g. 5ms), D2D mode is re-evaluated. This may allow the D2D mode to be adapted to current load and channel conditions. Once D2D mode is selected, then scheduling, resource allocation, and power control may happen in every TTI based on the selected mode.

In Fast and Slow D2D Mode selection, an algorithm may allocate resources based on a proportional fairness metric, which is a metric that may achieve a good trade-off between efficiency and fairness among UEs. Resource allocation, and power control happens in every TTI based on the selected mode.

In a third option, referred to as Joint D2D mode selection and scheduling, D2D mode and UEs to be scheduled is jointly decided in every TTI. The third option may be more complex than the first and second option, but because there is no estimate required before the allocation, and because there is no apriori determination of the mode, better performance may be obtained (as discussed with reference to Figures 8 and 9).

Figure 3 shows a flowchart of a method of mode selection for a D2D capable UE. The method comprises, in a first step 320, determining a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity, in dependence on an expected instantaneous data rate. In a second step 340, the method comprises scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource.

The determining may comprise determining whether to switch a mode of operation from one of the direct mode and cellular mode to the other of the direct mode and cellular mode. Determining whether to switch a mode of operation comprises determining a switching metric in dependence on the expected instantaneous data rate.

The method may comprise determining the mode of operation when device to device traffic commences and/or when a change in channel conditions reaches a threshold (referred to as slow or slow scale mode selection). Alternatively or in addition, the method may comprise determining the mode of operation every transmission time interval or every plurality of transmission time intervals (referred to as fast or fast scale mode selection).

In D2D fast and slow scale mode selection, for each D2D UE, the throughput improvement if the UE switched its mode is estimated. In its simplest form, the switching metric is the ratio of the UE's throughput after mode switch to its throughput before mode switch (the ratio of the expected instantaneous data rate in the other of the direct mode and cellular mode to the expected instantaneous data rate in the current mode of operation. More generally, the overall system utility (or geometric mean of all UEs' throughputs) if a D2D UE switched its mode is estimated. The ratio of the system performance if the UE switched its mode to that of the system performance if the UE did not switch the mode is taken. This ratio is known as the switching metric. Switching metric_k = System performance if UE k switched its mode/System performance if UE k did not switch its mode.

Determining whether to switch a mode of operation may comprise determining that at least one device-to-device communication capable user equipment has a maximum switching metric of the plurality of user equipments. Determining the switching metric may comprise determining a ratio of the expected instantaneous data rate in the other of the direct mode and cellular mode to the expected instantaneous data rate in the current mode of operation and switching the mode of operation if the switching metric is greater than 1 . At every switching instance, among all UEs in the scope of the switching entity, the UE switches UE k^* = arg max{switching metric_k}, provided switching metric_k* > 1. Thus the UE that provides the largest improvement in the system performance is switched, provided there is an improvement from such a switch. Once an optimal mode has been selected for a UE, appropriate CSI information more applicable to the currently chosen mode is reported by the UE. For instance, if the cellular mode is selected for the UE, the source UE may be required to transmit PHR and uplink SRS, assuming the transmissions need to be received at the eNB, and similarly the destination UE transmits the CSI for the eNB to destination UE link.

The expected instantaneous data rate may be determined in dependence on the number of resources the UE may be allocated in the respective mode under the system load conditions. The cellular mode may use both UL and DL resources. The direct mode may use only UL resources.

The following provides a detailed method by which the switching metric of a given UE may be determined. The target SINR value for a UE in uplink may be determined as follows

target J ₊ ]f

(standardized by 3GPP for LTE Uplink) ^{T 8} g =—

where g is the reciprocal of UE's pathloss, i.e., ^ , where PL is pathloss between the transmitter and the receiver

For fast time scale D2D mode selection, a better channel gain estimate (instead of pathloss) can also be obtained using SRS measurements, a is the fractional power control pathloss compensation coefficient. I is the inter-cell interference measured at the receiver, and N_T is the thermal noise in the system. SI NR_max corresponds to the highest allowed MCS and would be an upper bound for target SINR. Hence, the target SINR can be further refined as shown below: SINR _lget = min

I + N_{T g}

Estimate of downlink throughput for a UE served by a cell having N_DL active UEs may be as follows:

where R_cur is the instantaneous data rate of UE if it is scheduled at the current MCS over the entire bandwidth.

Estimate of uplink throughput for a UE served by a cell having NUL active UEs may be follows

Tput _ Est_UL = min N_RB_k Aog 1 Tput_h

-where N_RB_k is number of RBs that can be allocated to this UE based on the power/buffer limit of this and other UEs. The method to determine N_RB_k is illustrated below. For convenience, we have dropped the taking of the minimum with SINR_max in the determination of the target SINR. g_k is the channel gain of the transmitter UE to the serving cell of the transmitter UE. n_Reuse is the reuse factor of PUSCH RBs. n_Reuse >= 1 and can be estimated by running an IIR filter on the current ratio of sum of the number of used RBs to the number of PUSCH RBs available. Tput_NTBs is the max throughput the UE can obtain if the UE cannot handle N_RB_k RBs due to its power limitation

Estimate of throughput for a UE served on direct D2D link may be as follows:

Tput _ Est_mD ,Tput Nmc ,direct

where g_D2D is the channel gain from the transmitter to the receiver UE over the direct link, PO_D2D is the P₀ value for the direct link, N_RB_k,_direct and TputN_TBs, direct denote the same parameters as before, just that they now correspond to that of the direct link.

The cellular mode uses UL resources to transmit packets from the transmitter UE to its serving eNB, and the serving eNB of the receiver transmits the received packets to the receiver using the DL spectrum. The throughput perceived by the UE would be the minimum of the UL and DL throughputs. Throughput estimate for cellular mode may be computed as follows

Throughput cellular e

where N_RB_k is max number of RBs that can be allocated to this UE based on the power/buffer limit of this and other UEs. The method to determine this quantity is shown at the end of this section.

The direct mode might use only uplink resources to transmit data to receiver. The throughput estimate for direct mode may be as follows:

Throughput direct e mm N RB k, direct Tput ,direct

where N_RB_k,_direct is number of RBs in direct mode that can be allocated to this UE based the power/buffer limit of this and other UEs. The method to determine N_RB_k is illustrated at the end of this section. Note that the l+N_T measurement for the cellular mode is made at the eNB and for the direct mode, it is made at the receiver device.

The switching metric for switching a UE k from direct mode to cellular mode may be given by

ThrOUghput_ce||_u|_ar__mode,k*ninterference,k,direct->cellular/ThrOUghputcii_rect__mode,k

The term r)interference,k,direct->ceiiuiar accounts for the impact on other UEs due to the change in interference when UE k switches from direct to cellular mode, and is given by (1-

Ci/R_Cur,DL,k)^*(1-C2(1/Rcur,uL,k-1/Rcur,direct,k)), where Ci and c₂ are some configurable constants

The switching metric for switching a UE k from cellular to direct mode may be given by

ThrOUghputdirect_mode,k^*n interference,k,cellular->direct/ThrOUghput_ce||_u|_ar__mode,k

The term ni_nterference,k,ceiiuiar->direct accounts for the impact on other UEs due to the change in interference when UE k switches from cellular to direct mode, and is given by

(1 +Ci/R_curiDL,k)^*(1-C2(1/Rcur,direct,k-1/Rcur,UL,k)) The number of PRBs per TTI that a UE can get allocated may be estimated as follows:

N_RBPUSCH = Total RBs for a bandwidth - PUCCH RBs

n_Reuse is the number of UEs reusing a PRB and is determined by MR filtering the ratio of sum over all UEs of PRBs assigned per TTI/N_RB_PUSCH

The UEs are sorted in increasing order of number of PRBs that the UE can handle given their power limit/pathloss

1 . Initialize n_UEs = nu_L ⁺n_D2D, i=1

2.

N_RBPUSCH * riReuse nUEs

4. If NTBSJ >=N_RBi, then UE i is allocated N_RB_r =N_RB, RBs, else N_RB_r =

N_TBs,i is allocated to UE i

- N_RBj*

7. Increment i and go back to step 3 Figure 4 shows a flowchart of an example method for jointly selecting the communication mode for D2D devices and allocating resources for all UEs that are in the scope of a decision entity every TTI. The method comprises, in step 420, determining, for a resource in a transmission time interval, a mode of operation selected from a direct mode and a cellular mode, for at least one device to device communication capable user equipment of a plurality of user equipments associated with a scheduling entity. The plurality of user equipments may comprise at least one user equipment capable of operating in direct mode or cellular mode (D2D UE) and at least one user equipment capable of operating only in cellular mode (a cellular UE).

In step 440, the method comprises scheduling said at least one device-to-device

communication capable user equipment to operate in said determined mode on the resource in the transmission time interval. Any number of D2D UE (min 0) and any number of cellular UE (min 0) may be considered for scheduling on a resource, with the provision that maximum one cellular mode transmission is possible.

A method for resource allocation for cellular and D2D UEs along with mode selection such as that shown in Figure 4 may maximize system utility. Performing mode selection and resource allocations jointly, by determining the mode for a UE to operate in for a respective resource, avoids the need for a decision to be made upfront on whether or not a D2D UE should be in direct mode or cellular mode, and thus would improve system utility. The method provides an algorithm where transmission mode and resource allocation for all the UEs is determined jointly every TTI based on proportional fairness metric that accounts for channel condition, co-scheduling with other UEs, interference in the system, reuse of spectrum, fairness, etc. The joint allocation computes the metrics in real time based on actual interference and realized throughput. The algorithm determines the optimal allocation maximizing system utility considering the impact of resource reuse, number of resources it is expected to receive, interference caused on others due to the allocation, channel conditions of the link, etc. Since there is no estimate required before the allocation and because there is no apriori determination of the mode, better performance may be obtained (as discussed with reference to Figures 8 and 9).

Determining a mode of operation may comprise determining expected instantaneous data rate in the resource for a given TTI for each user equipment of the plurality of user equipments. The plurality of UEs associated with a scheduling entity may be a plurality of UEs in the scope of a scheduling entity, e.g. all the UEs in a cell.

If the user equipment is a D2D UE, determining may comprise determining expected instantaneous data rate if the user equipment is scheduled in direct mode and expected instantaneous data rate if the device is scheduled in cellular mode.

In an embodiment, the method may comprise, in each TTI for a resource, for each of a plurality of UEs, computing a scheduling metric, such as a proportional fairness (PF) metric, for each UE. The PF metric may be computed considering the interference from co- scheduled UEs in the same subframe. Note that initially there are no other UEs scheduled in that cell hence no additional interference needs to be accounted due to other links (sidelinks or cellular links). For a cellular UE, the PF metric is denoted as PF_ceii_Ui_ar. For a D2D UE, two PF metrics are computed, PF_D2D_direct if the D2D UE is to be scheduled in direct mode and PF_D2D__Ceiiuiar if the D2D UE is to be scheduled in cellular mode.

The expected instantaneous data rate for a cellular UE in a given TTI may be computed using the channel gain for the given link between UE and serving cell and interference introduced due to the co-scheduled UEs within the cell and inter-cell interference from other cells. Other cellular UEs and D2D UEs that are scheduled in cellular mode do not cause any interference for the given cellular UE as these resources are orthogonal to each other (assuming no multi-user MIMO operation). Only the co-scheduled D2D UEs using the same resources in direct mode cause interference to the given cellular UE due to resource reuse.

In the following, a given D2D pair is referred to by the same identity in the below description, e.g., PF metric computed for a pair of D2D UE k is meant for the link that has UE k as transmitter and its corresponding D2D pair UE k as receiver. Although the following is described with reference to a case where D2D communication on direct link uses uplink resources, the same solution may be applicable to the case where D2D direct mode communication uses downlink resources.

PF metric for a cellular UE k may be computed in theory using Shannon's capacity formula as tx&k,BS

1 + -

N₀ BS + ^BS + tx D2D£ j

d=\

M k, cellular

Ravg_k

where g_k:BS is the channel gain between transmitter UE k and receiver cell for UL link.

Ptx is the transmit power of cellular UE that can be based on fractional power control (FPC).

Ptx = mid -> P - max in case of FPC, where P₀ is conveyed from cell to UEs. . Here a is the fractional power control pathloss compensation coefficient that is conveyed from cell to UEs. P_max is the maximum power that the user can use for transmission in serving cell. l_Bs is the inter-cell interference measured at the receiver, and N_0,BS is the thermal noise in the system. y_d e {0,1} where 0 indicates that the D2D UE d is not scheduled in direct mode in this TTI and 1 indicates that D2D UE d is scheduled in direct mode in this TTI. g_d:BS is the channel gain between transmitter D2D UE d and the victim receiver, i.e., the eNB in this case. More generally, higher rank transmissions, beamformers, and practical limitations such as use of quantized MCS levels may be taken into account by inserting suitable approximation constants in the above equation.

Ptx_D2D is the transmit power of D2D UE in direct mode. P_TX_D2D = in

case of FPC, where value of P₀_D2D and a_D2D (fractional power control pathloss compensation coefficient for D2D transmission) is transmitted by the cell to UEs. Ravg_k is the average throughput of the cellular UE k thus far.

Figure 5 shows a schematic diagram of interference caused by D2D direct mode

transmission of UE2 on UE 1 and UE3. In practice, the computation of the throughput and the achievable rate may account for the allowed set of MCS levels, outer-loop link adaptation (OLLA) offsets, SRS measurement errors, etc.

The achievable rate for a D2D UE in cellular mode in a given TTI may be computed using the channel gain for the given link between UE and serving cell and interference introduced due to the co-scheduled UEs within the cell and inter-cell interference from other cells.

Cellular UEs and other D2D UEs that are scheduled using cellular mode do not cause any interference for the given D2D UE to be scheduled in cellular mode as these resources are orthogonal to each other (assuming no multi-user MIMO operation). Only the co-scheduled D2D UEs using the same resources in direct mode may cause interference to the given UE due to resource reuse.

PF metric for a D2D UE k to be scheduled in cellular mode, may be computed as

M

k,D2D cellularMode

Ravg_k

, where the notations correspond to those defined previously. The assumption above is that the UL throughput that the UE k is less than the DL throughput that the D2D UE pair k would receive. Otherwise, the numerator in the above term would be replaced by the minimum of the UL throughput and DL throughput that the D2D UE pair would get.

The achievable rate for a D2D UE in direct mode in a given TTI may be computed using the spectral efficiency for the given link between pair of D2D UE, i.e., using the channel gain between the transmitter and receiver D2D UE and the interference introduced due to the co- scheduled UEs. Cellular UEs and D2D UEs that are scheduled using cellular mode and the co-scheduled D2D UEs in direct mode may cause interference to the given UE due to resource reuse. Figure 6 shows a schematic diagram of interference caused by cellular mode transmission (cellular UE or D2D UE) of UE1 on UE2 and UE3.

PF metric for a D2D UE k to be scheduled in direct mode, may be computed

M k,D2D directMode

Ravg_k

where g_k:k is the channel gain between pair of transmitter and receiver D2D UE k, l_DR is the inter-cell interference measured at the receiver D2D UE k, and N_{o D} is the thermal noise at the D2D receiver UEs.

Interference from a co-scheduled cellular UE c or D2D UE d scheduled in cellular mode may be determined based on P_tx transmit power and channel gain between transmitting UE and receiver D2D UE k, i.e., g_c,k Or g_d:k as appropriate. Interference from a co-scheduled D2D UE d scheduled in direct mode may be determined based on

transmit power and channel gain between transmitting UE d and receiver UE k g_{d k}. Other notations correspond to those defined previously. Considering the change in interference caused due to a resource allocation for the selected mode may be important to achieve the desired gains.

Determining a mode of operation may comprise determining which user equipment maximises a sum scheduling metric (for example, PF metric) when co-scheduled with other UEs on this resource and the associated mode and scheduling said user equipment to operate in said mode on the resource in the transmission time interval. The sum scheduling metric is the summation of the scheduling metric of all the UEs scheduled on this resource. In an embodiment, determining a mode of operation may comprise determining a proportional fairness metric for each user equipment of the plurality of user equipments in dependence on the respective expected instantaneous data rate, determining which user equipment of the plurality of user equipments maximises the sum proportional fairness metric, and the associated mode in which the UE maximises the sum PF if the UE is a D2D UE and scheduling the determined user equipment to operate in the mode that maximises the sum PF on the resource in the transmission time interval.

The objective in the method is to maximize the network-wide utility of users, which is defined as the sum of the utilities achieved by each user, where each user's utility may be defined as the logarithm of user's achieved throughput. The final scheduling on a resource is such that any number of direct mode UEs can be scheduled (including 0), and at most one cellular mode UE (assuming no multi-user MIMO operation) can be scheduled (min can be 0) based on the allocation that maximizes sum PF. In each TTI, for a single resource, embodiments may provide an algorithm which is a greedy heuristic to the following utility maximization resource allocation problem:

subject to

where M_CfieMar, M_dD2D_ceiiuiarMode, M_d,_D2D_directMode is the PF metric for the cellular UE, D2D UE d in cellular mode, and D2D UE d in direct mode communication, respectively and C is the total number of cellular UEs and D is the total number of D2D UEs in the system. Y_c e{0,l}Vc, where 0 indicates that cellular UE c is not scheduled in this TTI and 1 indicates that cellular UE c is scheduled in this TTI. y_d

where 0 indicates that D2D UE d is not scheduled in cellular mode in this TTI and 1 indicates that D2D UE d is scheduled in cellular mode in this TTI. y_d G {0,1} W , where 0 indicates that D2D UE d is not scheduled in direct mode in this TTI and 1 indicates that D2D UE d is scheduled in direct mode in this TTI.

A D2D UE may be scheduled in either direct mode or cellular mode but not both, i.e.

There may be at most one cellular mode UE scheduled on a resource (assuming no multi-

C D

user MIMO), i.e.,∑r_c + γ_ά = 1

c=l d=l

In an embodiment, the UEs are sorted by their associated PF in descending order.

The UE that has max PF(max_pf) is determined. If the UE with max_pf is a D2D UE, the mode in which it has max PF is selected.

This UE is removed from the candidate list, scheduled for that resource and mode allocation is assigned.

If a D2D UE is scheduled using one of the modes, the scheduled D2D UE should not be considered for the other mode. If said determined mode is cellular, assuming no multi-user MIMO operation, no further user equipments may be scheduled to operate in cellular mode on the resource in the

transmission time interval. For example, if a cellular UE is scheduled, or if a D2D UE is scheduled in cellular mode, all other cellular UEs are removed, and all other D2D UEs in cellular mode are removed from further consideration in this TTI.

This process may be continued until the sum PF can no longer be improved and/or when all UEs have been considered. While embodiments have been described with reference to resource allocation of a single resource per TTI, an extension of this scheme for multiple resources/frequency-selective scheduling may be performed in a similar fashion by doing a "cherry picking" over all UEs and resource block group(s). Note that depending on whether a given user's transmission needs to be contiguous or not in a TTI, slightly different methods would be needed.

If the D2D UE is scheduled in one of the direct and cellular modes, it may no longer be considered for scheduling in the other of the direct and cellular mode for another resource in the transmission time interval.

Figure 7 shows a flowchart of an algorithm which may be used for performing a method such as that described above.

Figure 8 shows simulation results for the proposed joint mode selection and resource allocation scheme. A single cell of radius 866 m (3GPP case 3, ISD 1732m) is considered and cellular UEs and D2D UEs are distributed randomly in the cell. The D2D UEs are dropped such that the distance between a pair of receiver and transmitter D2D UEs is uniformly distributed in the range 3m to a maximum defined distance, which is a parameter we set in the range (30m to 500m). As the interference due to scheduling in neighbor cells is less when compared to the intra-cell interference due to resource reuse, we consider a fixed inter-cell interference value. The simulations are also done for various values of the proportion of D2D UEs as a fraction of all the UEs in the cell.

The results of Figure 8 show the gains in the geometric mean of UE throughputs with respect to the case where all D2D UEs use cellular mode. Across the different cases of 10%, 20% and 40% of D2D UEs in the system, the total number of UEs in the simulation is fixed at 50. The results show that the maximum gains are at lower distances between D2D UE pairs. This is because the ratio of D2D UEs transmitting in direct mode decreases with increase in distance between D2D UEs transmitter and receiver pair (results for the same are not shown here). This is because with increasing distance between the transmitter and receiver both the spectral efficiency of the D2D direct link decreases and there is a larger interference impact on other UEs due to the D2D direct link. All the D2D UEs transmit in direct mode for very small distances between D2D UE pairs Another observation from the results is that, as the density of D2D UEs increases in the system, joint mode selection and scheduling provides larger performance improvements because there is a larger scope for resource reuse with larger fraction of D2D UEs. Figure 9 shows simulation results showing the gains of geometric mean of UE throughputs for joint scheduling and mode selection with respect to fast and slow scale mode selection schemes. As the re-evaluation time for selecting transmission mode (D2D v/s cellular) for D2D UEs increases, the gains of joint mode selection and scheduling over fast and slow- scale mode selection scheme followed by resource allocation increases. Another

observation from the results is that most of the gains of joint mode selection and scheduling or fast scale mode selection over slow scale mode selection scheme are at around 200 m distance between D2D UEs pair. This is because at 200m distance between D2D devices, due to fast fading, a small fluctuation in the channel condition may require mode switching for D2D UEs which gets delayed in case of slow scale mode selection. At smaller distances between D2D UEs, we use direct mode almost all the time for all D2D UE pairs and at larger distances the use of cellular mode increases. There is a mix of small and large distances D2D UE pairs for the scenario corresponding to large distances. Hence the gains from joint mode selection and scheduling are more at larger distances compared to lower distances.

It should be understood that each block of the flowchart of the Figures and any combination thereof may be implemented by various means or their combinations, such as hardware, software, firmware, one or more processors and/or circuitry.

The method may be implemented on a mobile device as described with respect to figure 2 or control apparatus as shown in Figure 10. Figure 10 shows an example of a control apparatus for a communication system, for example to be coupled to and/or for controlling a station of an access system, such as a RAN node, e.g. a base station, (e) node B or 5G AP, or a node of a core network such as an MME or S-GW, a scheduling entity, or a server or host. The method may be implanted in a single control apparatus or across more than one control apparatus. The control apparatus may be integrated with or external to a node or module of a core network or RAN. In some embodiments, base stations comprise a separate control apparatus unit or module. In other embodiments, the control apparatus can be another network element such as a radio network controller or a spectrum controller. In some embodiments, each base station may have such a control apparatus as well as a control apparatus being provided in a radio network controller. The control apparatus 300 can be arranged to provide control on communications in the service area of the system. The control apparatus 300 comprises at least one memory 301 , at least one data processing unit 302, 303 and an input/output interface 304. Via the interface the control apparatus can be coupled to a receiver and a transmitter of the base station. The receiver and/or the transmitter may be implemented as a radio front end or a remote radio head. For example the control apparatus 300 can be configured to execute an appropriate software code to provide the control functions. Control functions may comprise determining, for a resource in a transmission time interval, a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity and scheduling said at least one device-to-device communication capable user equipment to operate in said determined mode on the resource in the transmission time interval.

Alternatively, or in addition, control functions may comprise determining a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device

communication capable user equipment of a plurality of user equipments associated with a scheduling entity, in dependence on an expected instantaneous data rate and scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource.

It should be understood that the apparatuses may comprise or be coupled to other units or modules etc., such as radio parts or radio heads, used in or for transmission and/or reception. Although the apparatuses have been described as one entity, different modules and memory may be implemented in one or more physical or logical entities.

It is noted that whilst embodiments have been described in relation to LTE/LTE-A similar principles can be applied in relation to other networks and communication systems, for example, 5G networks. Therefore, although certain embodiments were described above by way of example with reference to certain example architectures for wireless networks, technologies and standards, embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein.

It is also noted herein that while the above describes example embodiments, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention.

In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects of the invention may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

The embodiments of this invention may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Computer software or program, also called program product, including software routines, applets and/or macros, may be stored in any apparatus-readable data storage medium and they comprise program instructions to perform particular tasks. A computer program product may comprise one or more computer- executable components which, when the program is run, are configured to carry out embodiments. The one or more computer-executable components may be at least one software code or portions of it.

Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD. The physical media is a non-transitory media.

The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may comprise one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), FPGA, gate level circuits and processors based on multi core processor architecture, as non-limiting examples.

Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate. The foregoing description has provided by way of non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims. Indeed there is a further embodiment comprising a combination of one or more embodiments with any of the other embodiments previously discussed.

Claims

1. A method comprising:

determining, for a resource in a transmission time interval, a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity; and

scheduling said at least one device-to-device communication capable user equipment to operate in said determined mode on the resource in the transmission time interval.

2. A method according to claim 1 , wherein if the determined mode is cellular, no further user equipments of the plurality of user equipments in cellular mode, or cellular user equipments, are scheduled to operate on the resource in the transmission time interval.

3. A method according to any preceding claim, wherein if the at least one device-to- device communication capable user equipment is scheduled in one of the direct and cellular mode, the at least one device-to-device communication capable user equipment is no longer considered for scheduling in the other of the direct and cellular mode for another resource in the transmission time interval.

4. A method according to any preceding claim, wherein the determining comprises determining expected instantaneous data rate in the resource for a given transmission time interval for each user equipment of the plurality of user equipments.

5. A method according to claim 4, comprising, if the user equipment is a device-to- device capable user equipment, determining expected instantaneous data rate for the given transmission time interval if the user equipment is scheduled in direct mode and expected instantaneous data rate for the given transmission time interval if the device is scheduled in cellular mode.

6. A method according to any one of claims 4 and 5, wherein determining comprises determining a proportional fairness metric for each user equipment of the plurality of user equipments in dependence on the respective expected instantaneous data rate;

determining which user equipment of the plurality of user equipments maximises the sum proportional fairness metric, and the associated mode; and scheduling the determined user equipment to operate in the associated mode on the resource in the transmission time interval.

7. A method according to claim 6, wherein the proportional fairness metric is

determined in dependence on interference from any user equipments of the plurality of user equipments scheduled on the same resource in the transmission time interval.

8. A method comprising:

determining a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity, in dependence on an expected

instantaneous data rate; and

scheduling the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource.

9. A method according to claim 8, wherein determining comprises determining whether to switch a mode of operation from one of the direct mode and cellular mode to the other of the direct mode and cellular mode.

10. A method according to claim 9, wherein determining whether to switch a mode of operation comprises determining a switching metric in dependence on the expected instantaneous data rate.

1 1 . A method according to claim 10, wherein the switching metric is dependent on interference at the plurality of user equipments from the at least one device-to-device communication capable user equipment if the device-to-device communication capable user equipment switches modes from one of the direct mode and cellular mode to the other of the direct mode and cellular mode.

12. A method according to claim 10 or claim 1 1 , wherein determining the switching metric comprises determining a ratio of the expected instantaneous data rate in the other of the direct mode and cellular mode to the expected instantaneous data rate in the current mode of operation and switching the mode of operation if the switching metric is greater than 1.

13. A method according any one of claims 10 to 12, wherein determining whether to switch a mode of operation comprises determining that the at least one device-to-device communication capable user equipment has a maximum switching metric of the plurality of user equipments.

14. A method according to any one of claims 8 to 13, comprising determining the mode of operation when device to device traffic commences and/or when a change in channel conditions reaches a threshold.

15. A method according to any one of claims 8 to 14, comprising determining the mode of operation every transmission time interval or every plurality of transmission time intervals.

16. A method according to claim 8, wherein the mode is determined per resource, wherein the resource comprises the allocated resource.

17. An apparatus comprising means for performing a method according to any one claims 1 to 16.

18. A computer program product for a computer, comprising software code portions for performing the steps of any of claims 1 to 16 when said product is run on the computer.

19. An apparatus comprising:

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

determine, for a resource in a transmission time interval, a mode of operation selected from direct mode and a cellular mode, for at least one device-to-device

communication capable user equipment of a plurality of user equipments associated with a scheduling entity; and

schedule the at least one device-to-device communication capable user equipment to operate in said determined mode on the resource in the transmission time interval.

20. An apparatus comprising:

determine a mode of operation selected from a direct mode and a cellular mode, for at least one device-to-device communication capable user equipment of a plurality of user equipments associated with a scheduling entity, in dependence on an expected instantaneous data rate; and

schedule the at least one device-to-device communication capable user equipment to operate in said determined mode on an allocated resource.