The present invention concerns a base station in a mobile telecommunication access network such as the UMTS terrestrial radio access network (UTRAN). The UTRAN is illustrated in FIG. 1 and comprises at least one Radio Network System 100 connected to the Core Network (CN) 200. The CN is connectable to other networks such as the Internet, other mobile networks e.g. GSM systems and fixed telephony networks. The RNS 100 comprises at least one Radio Network Controller 110. Furthermore, the respective RNC 110 controls a plurality of Node-Bs 120,130 that are connected to the RNC by means of the Iub interface 140. Each Node B, also referred to as base station, covers one or more cells and is arranged to serve the User Equipment (UE) 300 within said cell. Finally, the UE 300, also referred to as mobile terminal, is connected to one or more Node Bs over the Wideband Code Division Multiple Access (WCDMA) based radio interface 150.
3GPP Release 6 has recently been updated with the “Enhanced Uplink” concept, which includes a new uplink transport channel, E-DCH. A new scheduler located in the Node B that performs transmission resource management based on the utilisation of the allocated radio resources is introduced as an alternative or complement to the existing packet scheduler located in the RNC. Combined with fast L1 HARQ schemes, the proposed algorithm can provide a cell throughput gain. Since the packet scheduler functionality is located in the Node B, a fast scheduling concept is introduced. Fast scheduling from the Node B denotes the possibility for the Node B to control when a UE is transmitting and at what data rate. The Node Bs can assign scheduling grants to the UEs, where these grants are based on both the transmission resource availability and the requested need for transmission resources, i.e. the scheduling requests from the UEs.
HARQ is a more advanced form of an ARQ retransmission scheme. In conventional ARQ schemes the receiver checks if a packet is received correctly. If it is not received correctly, the erroneous packet is discarded and a retransmission is requested. With HARQ the erroneous packet is not discarded. Instead the packet is kept and combined with a result of the retransmission. That implies that even if both the first transmission and the retransmission are erroneous, they may be combined to a correct packet. This means that fewer retransmissions are required.
On E-DCH the Node B scheduler has no direct information about the data that to be transmitted from the UEs. Thus the UEs are required to indicate the amount of data available, the priority of the data, the transmitter power available etc. to the Node B through scheduling requests. When the Node B has received the scheduling request from the UE and has decided to schedule the UE based on the received scheduling requests, it may transmit a grant, also denoted scheduling grant indicator herein, to the UE, indicating the amount of data or actually with which power the UE is allowed to transmit.
A particular aspect of relevance for the present invention is the fact that enhanced uplink, i.e. the E-DCH supports soft-handover. Soft-handover implies that a UE is connected to multiple base stations simultaneously. Thus, a UE in soft handover is power-controlled from multiple cells, and supported by data reception at multiple cells (i.e. macro diversity is utilized). Power control from multiple cells is needed to limit the inter-cell interference, while macro-diversity gains can be achieved by receiving data at multiple cells. FIG. 2 illustrates a scenario when a UE is in soft handover in a UMTS network as shown in FIG. 1. The network comprises base stations connected to a RNC 208, wherein the RNC 208 is further connected to a CN 210. The UE 202 is connected to the base stations 204 and 206 simultaneously.
In soft-handover in enhanced uplink, one cell is selected by the RNC to act as the Serving Cell, and the Node B in control of the Serving Cell is here denoted the Serving E-DCH Node B. The cells connected to the same UE is referred to as the E-DCH active set. The Serving E-DCH Node B controls the transmission resources of the UE, i.e. it is allowed to grant requested transmission resources. The Non-serving E-DCH Node Bs (i.e. Node Bs not in control of the serving cell) are not able to monitor the grants given by the E-DCH serving Node B and has thus no knowledge of the granted transmission resources. The Serving E-DCH Node B should typically be the Node B with the strongest uplink from the UE, but the E-DCH serving Node B may also be chosen differently. One alternative is to tie the E-DCH serving cell to the downlink HS-DSCH serving cell. Tying the E-DCH serving cell selection to the HS-DSCH serving cell may increase the likelihood that the strongest uplink in the active set is governed by a non-serving E-DCH Node B.
The present invention considers problems arising from such configuration that may result in that the uplink of the serving Node B is not the strongest link in the active set, i.e. another Node B of the active set receives the strongest signal from the UE in soft handover.
The E-DCH scheduling is mainly controlled from the serving E-DCH Node B, which can assign Absolute Grants (AG) to the UE's. These Absolute Grants limit the maximum transmission resources, e.g. power, the UE is allowed to use. Within this restriction, the final selection of data-rate is then performed by the UE itself, based on the data available in its buffers and on the available UE power. Alternatively, the serving E-DCH Node B can use Relative Grants to control the transmission rates of the UEs. The Relative Grants from the serving E-DCH cell can take three values: Up, Hold, and Down. However, the non serving Node Bs do not receive any information of limitations indicated by the absolute grant or relative grant from the serving E-DCH cell and is therefore not aware of the future processing resource need of the UE.
The scheduling control from non-serving E-DCH Node Bs is mainly intended for inter-cell interference suppression and system stability control. The Relative Grants that can be sent from the non-serving E-DCH Node Bs therefore take only two values: Hold and Down. With these relative grants, a Node B can reduce the interference contribution from UEs, which are not primarily controlled from this Node B.
Thus, the present invention deals with the processing resource allocation problem of the Node B that depends on that the non-serving Node Bs are not aware of the amount of transmission resources that is granted from the Node B controlling the serving cell.
From a Node B internal hardware allocation point of view, there is a significant difference between the serving E-DCH Node B and the non-serving Node Bs. The serving E-DCH Node B has information about the scheduling grant sent to the UE and therefore knowledge about the maximum amount of hardware resources needed for processing transmissions from this particular UE. However, a Node B that is not in control of the Serving E-DCH cell has much more limited means to predict the processing resources needed for the UE in question. This resource allocation problem is further complicated by the fact that the Node B processing resource allocation typically takes some time (e.g. 10-50 ms), meaning that a predictive processing resource allocation would be necessary to ensure that the reception of a transmission from a UE can be successfully completed.
Below are two existing solutions to this problem described:
According to a first example, processing resources are over-allocated and rate-limitation of terminals in soft handover may also occur. In this brute-force solution, sufficient Node B processing resources for the highest possible data rate are always allocated from the non-serving Node B. To reduce the need of hardware resources, the maximum bit rate of UEs in soft-handover may have to be limited.
According to a second example, the non-serving Node B could under-allocate processing resources, knowing that it may not be able to decode the first few Transmission Time Intervals (TTIs) of a UE transmission, in case the UE starts at a rate higher than estimated. Once the UE starts to transmit at a high data rate, the non-serving Node B can reallocate processing resources to this UE, assuming that it will continue to transmit for some time. Non-serving Node Bs may also try to listen to the scheduling requests from the UE to the serving cell to get some information about the amount of transmission resources the UE may need.
Both approaches are, however, equipped with obvious drawbacks:
The solution according to the first alternative may result in low utilization of available hardware resources, costly deployments with a necessity to deploy large pools of Node B processing resources and/or tight bit-rate restrictions for users in soft handover. This is neither desirable nor acceptable.
The opportunistic approach in the solution according to the second example may result in loss of the macro-diversity gain, since the first few HARQ transmission attempts at a non-serving Node B are lost. In addition, the second solution may interact with Outer-Loop Power-Control, which typically is operated on the number of HARQ transmissions. An increase in the number of HARQ transmissions due to hardware limitations may result in an unnecessary increase in the uplink SIR target with a capacity loss as a consequence.