The present invention is related to a wireless communication system including at lease one wireless transmit/receive unit (WTRU), at least one Node-B and a plurality of cells. More particularly, the present invention is related to a method and apparatus for selecting a serving cell and Node-B in a single carrier frequency division multiple access (SC-FDMA) system.
The third generation partnership project (3GPP) and 3GPP2 are currently considering a long term evolution (LTE) of the universal mobile telecommunication system (UMTS) terrestrial radio access (UTRA). Currently, SC-FDMA is being considered for the uplink of the evolved UTRA.
In an SC-FDMA system, a plurality of orthogonal subcarriers are transmitted simultaneously. The subcarriers are divided into a plurality of subcarrier blocks, (also known as “resource blocks”). A subcarrier block may be a localized subcarrier block or a distributed subcarrier block. The localized subcarrier block is defined as a set of consecutive subcarriers and the distributed subcarrier block is defined as a set of non-consecutive subcarriers.
For uplink macro diversity, (either inter-Node-B or intra-Node-B macro diversity), soft handover or fast cell selection may be performed. In a conventional wideband code division multiple access (WCDMA), a serving cell/Node-B selection for soft handover or fast cell selection is based on channel quality indicator (CQI) measurements of the uplink. One CQI measurement of the entire bandwidth in the uplink for each cell/Node-B is used to make the serving cell/Node-B selection. However, in the SC-FDMA system, there is one CQI per subcarrier block. Therefore, there are multiple CQIs for the uplink per cell. In addition, a WTRU usually does not transmit data using the whole bandwidth. Therefore, it is desirable to provide an improved method for serving cell/Node-B selection in SC-FDMA.
The present invention is related to serving cell and Node-B selection in a SC-FDMA system. For intra-Node-B serving cell selection, a serving Node-B measures CQIs of each subcarrier block in an uplink of each cell controlled by the serving Node-B and selects a new serving cell based on the CQIs. The serving Node-B reports the selected new serving cell to a WTRU. For inter-Node-B serving cell selection, each of a plurality of Node-Bs measures a CQI of each of a plurality of subcarrier blocks in an uplink transmission in each cell controlled by each Node-B and forwards the CQIs to a serving cell selection entity. The serving cell selection entity selects a new serving cell/Node-B based on the CQIs. The serving cell selection entity may be a centralized control-plane access gateway, a current serving Node-B or a WTRU.
When referred to hereafter, the terminology “WTRU” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, the terminology “Node-B” includes but is not limited to a base station, a site controller, an access point (AP) or any other type of interfacing device in a wireless environment.
The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.
Where only one Node-B, such as Node-B 204a, is involved in a serving cell/Node-B selection and two or more cells, such as cells 208a-208c, are controlled by the Node-B, such as Node-B 204a, an intra-Node-B cell selection is performed. There are two possible macro diversity schemes. One is soft handover and the other is fast cell selection. In soft handover, a transmission from a WTRU 202 is received and processed by several cells, (e.g., cells 208a-208c), controlled by the same Node-B, (e.g., Node-B 204a). Among those cells, one cell is designated as a serving cell, (e.g., cell 208a). In fast cell selection, a transmission from the WTRU 202 is received and processed only by the serving cell 208a, and the WTRU 202 may switch from one cell to another very quickly to achieve a “best” radio link.
For both handover and fast cell selection, the same serving cell selection procedure is implemented in accordance with the present invention. The major difference between the serving cell selection in soft handover and fast cell selection is the frequency that the serving cell selection is performed. The uplink serving cell selection for soft handover may be performed as fast as one per several transmission time intervals (TTIs); whereas the uplink fast cell selection may be performed as fast as one per TTI or per several TTIs, which should be faster than the intra-Node-B soft handover. The time interval that the uplink serving cell selection may be performed is called uplink intra-Node-B serving cell selection interval.
The serving Node-B 204a selects a new serving cell for the WTRU 202 based on the CQIs (step 304). For example, the Node-B 204a may simply select a new serving cell that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks. Alternatively, the Node-B 204a may select the new serving cell for the WTRU 202 by considering both CQIs, (i.e., CQIs of the WTRU 202 and other WTRUs in the cells 208a-208c controlled by the Node-B 204a), and scheduling strategy. For example, an appropriate scheduling strategy may balance the cell loads based on the number of WTRUs transmitting in the cells and their data rate and channel conditions, (e.g., uplink CQIs).
The serving Node-B 204a then reports the selected new cell to the WTRU 202, (preferably via a downlink shared control channel), (step 306). The process 300 is repeated every uplink serving cell selection interval.
Where several Node-Bs are involved with the serving cell/Node-B selection, an inter-Node-B cell/Node-B selection is performed. The inter-Node-B cell/Node-B selection decision is made by a serving cell selection entity. The serving cell selection entity may be a centralized aGW 206, a current serving Node-B 204a, a WTRU 202 or any other entity in the network, depending on the network architecture.
There are two possible macro diversity schemes for the inter-Node-B cell/Node-B selection. One scheme is soft handover and the other is fast cell selection. In soft handover, a transmission from the WTRU 202 is received and processed by several cells 208a-208i controlled by different Node-Bs 204a-204c. Among those cells, one cell is designated as a serving cell, (e.g., cell 208a). A Node-B, (e.g., Node-B 204a), that controls the serving cell is called a serving Node-B. The WTRU 202 may receive scheduling information, (i.e., at which subcarrier blocks to transmit), only from the serving Node-B 204a. In fast cell selection, a transmission from the WTRU 202 is received and processed by cells 208a-208c controlled by the serving Node-B 204a. The WTRU 202 may switch from one Node-B to another very quickly to get the “best” radio link.
For both handover and fast cell selection, the same serving cell selection procedure is implemented in accordance with the present invention. The major difference between the serving cell/Node-B selection for soft handover and fast cell selection is the frequency that the serving cell/Node-B selection may be performed. Basically, the fast cell selection may be performed faster than the serving cell/Node-B selection in soft handover. The time interval that uplink serving cell/Node-B selection is performed is called uplink inter-Node-B serving cell/Node-B selection interval.
The Node-Bs 204a-204c report the CQIs to a centralized aGW 206 (step 404). The centralized aGW 206 connects several Node-Bs via a high-speed link 210. Each Node-B 204a-204c preferably reports CQIs of the best K subcarrier blocks of each cell controlled by the Node-B 204a-204c. The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell. To reduce the signaling overhead, each Node-B 204a-204c may report CQIs of the cell that has the best K CQIs, (e.g., in terms of average CQI), among the cells controlled by the Node-B 204a-204c.
The centralized aGW 206 then selects a new cell/Node-B based on the CQIs (step 406). For example, the centralized aGW 206 may simply select the cell/Node-B that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks. Alternatively, the centralized aGW 206 may select a cell/Node-B by considering both CQIs of the WTRU 202 and other WTRUs in the cells 208a-208i controlled by the Node-Bs 204a-204c and scheduling strategy.
The centralized aGW 206 sends messages to the current serving Node-B 204a, the new Node-B, (e.g., Node-B 204b), which controls the selected new cell and optionally other Node-Bs, (e.g., Node-B 204c), to report the selected new serving cell/Node-B (step 408). The current serving Node-B 204a sends a message to the WTRU 202 to report the selected new serving cell/Node-B, (preferably via a downlink shared control channel), (step 410). The process 400 is repeated every uplink inter-Node-B serving cell/Node-B selection interval.
Non-serving Node-Bs 204b-204c report the CQIs to a current serving Node-B 204a (step 504). Each non-serving Node-B 204b-204c preferably reports CQIs of the best K subcarrier blocks of each cell 208d-208i controlled by the non-serving Node-B 204b-204c. The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell. To reduce the signaling overhead, each non-serving Node-B 204b-204c may report CQIs of the cell that has the best K CQIs, (e.g., in terms of average CQI), among the cells controlled by the non-serving Node-B 204b-204c.
The current serving Node-B 204a then selects a new cell/Node-B based on the CQIs (step 506). For example, the serving Node-B 204a may simply select the cell/Node-B that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks. Alternatively, the serving Node-B 204a may select a cell/Node-B by considering both CQIs of the WTRU 202 and other WTRUs in the cells controlled by the Node-Bs 204a-204c and scheduling strategy.
The current serving Node-B 204a sends a message to a centralized aGW 206 to report the selected new cell/Node-B (step 508). The centralized aGW 206 connects several Node-Bs 204a-204c via a high-speed link. The centralized aGW 206 then forwards the messages to the selected new Node-B, (e.g., Node-B 204b), which controls the selected new cell and optionally other Node-Bs, (e.g., Node-B 204c), to report the selected new serving cell/Node-B (step 510).
The current serving Node-B 204a sends a message to the WTRU 202 to report the selected new serving cell/Node-B, (preferably via a downlink shared control channel), (step 512). The process 500 is repeated every uplink inter-Node-B serving cell/Node-B selection interval.
Non-serving Node-Bs 204b-204c report the CQIs to a current serving Node-B 204a (step 604). Each non-serving Node-B 204b-204c preferably reports CQIs of the best K subcarrier blocks of each cell controlled by the non-serving Node-B 204b-204c. The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell. To reduce the signaling overhead, each non-serving Node-B 204b-204c may report CQIs of the cell that has the best K CQIs, (e.g., in terms of average CQI), among the cells controlled by the non-serving Node-B 204b-204c.
The current serving Node-B 204a then selects a new cell/Node-B based on the CQIs (step 606). For example, the serving Node-B 204a may simply select the cell/Node-B that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks. Alternatively, the serving Node-B 204a may select a cell/Node-B by considering both CQIs of the WTRU 202 and other WTRUs in the cells controlled by the Node-Bs 204a-204c and scheduling strategy.
The current serving Node-B 204a sends messages to the selected new Node-B, (e.g., Node-B 204b), which controls the selected new cell and optionally other Node-Bs, (e.g., Node-B 204c), to report the selected new serving cell/Node-B via a high-speed link 212 connecting the Node-Bs 204a-204c to each other (step 608). The current serving Node-B 204a sends a message to the WTRU 202 to report the selected new serving cell/Node-B, (preferably via a downlink shared control channel), (step 610). The process 600 is repeated every uplink inter-Node-B serving cell/Node-B selection interval.
The Node-Bs 204a-204c report the CQIs to the WTRU 202 (step 704). Each Node-B 204a-204c preferably reports CQIs of the best K subcarrier blocks of each cell 208a-208i controlled by the Node-B 204a-204c. The K subcarrier blocks are those that have the K best CQIs among all N subcarrier blocks within a cell 208a-208i. To reduce the signaling overhead, each Node-B 204a-204c may report CQIs of the cell that has the best K CQIs, (e.g., in terms of average CQI), among the cells 208a-208i controlled by the Node-B 204a-204c.
The WTRU 202 then selects a new cell/Node-B based on the CQIs (step 706). For example, the WTRU 202 may simply select the cell/Node-B that has the best average or weighted average CQI of M subcarrier blocks out of the K subcarrier blocks.
The WTRU 202 sends messages to the current serving Node-B 204a, the selected new Node-B, (e.g., Node-B 204b), which controls the selected new cell and optionally other Node-Bs, (e.g., Node-B 204c), to report the selected new serving cell/Node-B (step 708). The process 700 is repeated every uplink inter-Node-B serving cell/Node-B selection interval.
Although the features and elements of the present invention are described in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.
This application claims the benefit of U.S. Provisional Application No. 60/711,592 filed Aug. 26, 2005, which is incorporated by reference as if fully set forth.
Number | Date | Country | |
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60711592 | Aug 2005 | US |