In order to allow for cost efficient and flexible deployment solutions, within the third generation partnership project (3GPP) relaying is investigated as one of the new technologies for Long Term Evolution (LTE) networks and in particular for Long Term Evolution Advanced (LTA-Advanced) networks. It has been shown that with the usage of a relay node (RN) or more relay nodes (RNs) the spatial coverage and/or the capacity of a base station (BS) can be significantly increased. Further, areas can be covered which without using RN would suffer from bad radio conditions. Such areas are located typically at the edge of a cell being served by a particular BS.
Apart from this main goal of coverage extension, introducing relay concepts can also help in (a) providing a high-bit-rate coverage in high shadowing environments, (b) reducing average radio-transmission power at a user equipment (UE), thereby leading to long battery life, (c) enhancing the cell capacity and effective throughput, e.g., increasing cell-edge capacity and balancing cell load and (d) enhancing the overall performance and deployment cost of a Radio Access Network (RAN).
Also the IEEE standardization bodies such as the IEEE 802.11 and IEEE 802.16 group notice and investigate the potential of relaying technology. In this respect it is mentioned that the specification IEEE 802.16 is influenced for instance by pre-standardization activities such as for instance Wireless World Initiative New Radio (WINNER) project (see http://www.ist-winner.org/), wherein investigations regarding RN are carried out. This means that telecommunication networks relying on RN are achieving the level of maturity that is needed in ongoing standardization activities. The best evidence of this maturity is the IEEE 802.16j standardization where RNs are added on top of the IEEE 802.16e standard. This recent development has increased the pressure to consider RN also in LTE standardization.
There are many kinds of relay systems proposed starting from the simplest amplify/forward RN, which is applied e.g. in single frequency Digital Video Broadcasting-Handhelds (DVB-H) networks ending up to the most complex one, which utilizes a network coding to improve the overall performance. The most common type of RN that is proposed for use of RN in cellular networks (cellular relaying) is a decode/forward type of RN, where an input signal is detected and typically decoded and retransmitted using the same procedure as in the original transmission. Such an approach is assumed in this document.
Cellular relaying can be realized at the different layers of a protocol stack, which layers are described by the well known Open Systems Interconnection Reference Model (OSI model). A simple amplify and forward relaying can be realized at the Layer 1 of the protocol stack where the RN is required to have only (some part of) the PHY layer. Layer 2 RNs, which include the protocol stack up to the Media Access Control (MAC)/Radio Link Control (RLC) layers, enable the possibility of doing decentralized radio resource management. Layer 3 or higher layer RNs could almost be considered as wireless base stations and support all the protocol layers of normal base stations. Layer 3 or higher layer relaying is assumed in this document for the sake of simplicity in notations. However, the described radio resource partitioning procedure can easily be extended for other types of relays (e.g. layer 2) as well.
In order to make LTE-Advanced economically viable, it is required to be as much backward compatible with 3GPP Release 8 as possible. This is especially important for the UE side, as it will allow users to benefit from relaying with their Release 8 terminals. Based on previous 3GPP experiences it is herein assumed that full backward compatibility is required from UE side, i.e. Release 8 and LTE-Advanced UEs should work equally well in Release 8 and LTE-Advanced networks. At the network side software and even hardware updates between standard releases may be possible but preferably they should be as small as possible. Hence, from the viewpoint of a UE the serving network node respectively the current access point should function in exactly the same way as a Release 8 BS, which is called enhanced NodeB (eNB). Due to this requirement the reduction of BS functionalities when defining a RN will be difficult and RNs need to support all main eNB functions of a BS respectively of an eNB. Due to this fact it is often assumed that RNs, which will be employed in future telecommunication networks, will be capable of flexible resource sharing with the eNB that controls them. Moreover, it is often assumed that (a) the telecommunication network will allow at maximum 2 hops (BS-UE and BS-RN-UE) and (b) the network topology has a tree design (no connections between different RNs are allowed), but again the described resource partitioning procedure also works in the general case without these restrictions, indeed it will be applicable to intermediate RNs as well.
In the following there will be considered by way of example a common handover situation with reference to FIG. 2. A user equipment UE is originally located within source coverage area CA1 being spanned by a source relay node RN1. CA1 and RN1 are located within a source cell, which is spanned by a source base station BS1. The UE is connected to the source relay node RN1 via a radio access link. The source relay node RN1 is connected to the source base station BS1 via a so called backhaul link.
The UE is moving into a target coverage area CA2, which is located within a target cell of the telecommunication network. The movement of the UE is indicated in FIG. 2 with an arrow origination from the UE. The target coverage area CA2 is spanned by a target relay node RN2c, which is connected to a target base station BS2. The target base station BS2 spans the target cell. Apart from the target relay node RN2c there are further relay nodes RN2a and RN2b connected to the target base station BS2. Each of these further relay nodes RN2a and RN2b may serve further UE's, which for the sake of clarity are not depicted in FIG. 2. In addition, further UE's may be connected directly to the target base station BS2. Therefore, the radio transmission resources, which are in total available for the target cell, have to be distributed among various radio links (access links between a UE and a RN, backhaul links between a RN and the target base station and/or direct links between a UE and the target base station).
Further relay nodes may be connected to the source base station BS1 and may serve further UE's and further UE's may be connected directly to the base station BS1. For sake of clarity they are not depicted in FIG. 2. Also in the source cell available resources have to be distributed among various radio links.
However, the handover from RN1 to RN2c may fail or at least result in bad performance, if the resource partitioning is not properly done by the target base station BS2. For example, if assuming that in the non depicted coverage areas of RN2a and RN2b there have occurred high data traffic dynamics, most of the available radio transmission resources may be assigned to backhaul and access links of RN2a and RN2b. If RN1 wants to handover the UE to RN2c, the handover (HO) could fail because RN2c does not have enough radio transmission resources to accommodate the additional UE that RN1 wants to handover to RN2c. Generally speaking, a HO of UEs to a target RN may fail because (i) there are not enough available radio transmission resources for the target RN access link and/or (ii) there are not enough available resources for target RN backhaul link. Therefore, an appropriate radio transmission resource partitioning may be necessary in order to improve the success probability of a HO procedure and therefore improve the overall network performance. If the handover is executed despite an insufficient availability of resources, then the performance of the UE that was handed over, or the other UEs, or both, will be degraded.
A radio transmission resource partitioning configuration is a slower procedure than a HO. When a HO is initiated the HO procedure needs to be performed quickly while the configuration of a new radio transmission resource partitioning could require longer time. Therefore, reconfiguring the resource portioning at the target BS when a HO request towards the target RN is received may not be sufficient in order to ensure the success of a HO process.
A new radio transmission resource partitioning configuration may require signalling on the Broadcast Control Channel (BCCH) because a radio transmission resource partitioning may involve a Multi-Media Broadcast over a Single Frequency Network (MBSFN) sub-frame allocation. These MBSFN subframes have been proposed initially to make UEs aware that normal transmission from the access point is suspended for that subframe and instead a so called Single Frequency Network (SFN) transmission, which is a coordinated transmission from several access points, is performed in that subframe. However instead of a SFN transmission any other operation can be done instead, including a backhauling transmission from a mother base station to its assigned RN.
As a consequence of a new radio transmission resource partitioning configuration a new MBSFN sub-frame allocation may be needed in order to provide radio transmission resources for backhauling RNs to the respective mother BS. However, signalling on the BCCH occurs infrequently (for instance every 80 ms). This may introduce a significant delay between the decision for a new radio transmission resource partitioning and its configuration. In this respect it is mentioned that the signaling of a resource partitioning can be done over different control channels, e.g. the Physical Downlink Control Channel (PDCCH) for the scheduling, the BCCH for the configuration of MBSFN sub-frames, etc.
There may be a need for improving a partitioning of radio transmission resources in connection with a handover of a user equipment.