The present invention is related to a wireless communication system. More particularly, the present invention is related to a method and apparatus for sending downlink control information in an orthogonal frequency division multiple access (OFDMA) 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). OFDMA is adopted for the downlink of the evolved UTRA.
In an OFDMA system, data is transmitted simultaneously over a plurality of orthogonal subcarriers. The subcarriers are divided into a plurality of subcarrier blocks. A localized subcarrier block is a basic resource unit in an OFDMA system. The localized subcarrier block includes a set of consecutive subcarriers.
One or more subcarriers blocks are assigned to wireless transmit/receive units (WTRUs) by a Node-B. In assigning the subcarrier blocks, the Node-B may implement frequency and time domain channel-dependent scheduling or frequency diversity-based scheduling.
WTRUs with high data rate requirements may be assigned to several subcarrier blocks. For example, WTRU A, that has a high data rate requirement, is assigned to subcarrier blocks 1, 3 and 5 in TTI 1, and is assigned to subcarrier blocks 1 and 3-5 in TTI 2. Transmissions to WTRUs with low data rate requirements may be multiplexed into one subcarrier block in one TTI in a time division multiplexing (TDM) manner. For example, WTRUs B-E, that have a low data rate requirement, are assigned to subcarrier block 7 in TTI 2, and the transmissions to WTRUs B-E are multiplexed within TTI 2 in a TDM manner.
In the prior art, the downlink control signaling only covers the case where localized subcarrier blocks are used, (i.e., frequency and time domain channel-dependent scheduling), and a WTRU uses all OFDM symbols of its assigned subcarrier blocks within a TTI.
In order for the WTRUs to receive and decode downlink transmissions, the Node-B sends downlink control information to the WTRUs via a downlink control channel. Therefore, it is desirable to provide an efficient method for sending the downlink control information to support operations in an OFDMA system.
The present invention is related to a method and apparatus for sending downlink control information in an OFDMA system. A Node-B allocates at least one subcarrier block to each of a plurality of WTRUs for transmission of downlink user data via an OFDMA downlink data channel in accordance with a scheduling mode. The Node-B compiles downlink control information based on the scheduling mode. The Node-B sends the downlink control information to the WTRUs via an OFDMA downlink control channel. The WTRUs receive and process the downlink user data based on the downlink control information.
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 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.
The control information includes at least one of scheduling information, demodulation information, hybrid automatic repeat request (H-ARQ) information and a scheduling mode indicator (optional). The scheduling information includes at least one of WTRU identity, a frequency domain location of assigned subcarrier block(s), and a time domain location of scheduled downlink transmissions to each WTRU. The demodulation information includes at least one of a data modulation scheme, a transport block size and a coding rate (optional). The H-ARQ information includes at least one of an H-ARQ process identity, a redundancy version (RV) and a new data indicator. The H-ARQ process identity indicated the H-ARQ process that the current transmission is addressing. The RV is to support incremental redundancy in soft combining. The new data indicator indicates that the current transmission is a new transmission so that a soft buffer is cleared.
When the Node-B implements frequency and time domain channel-dependent scheduling, the Node-B dynamically assigns at least one subcarrier block to each of the WTRUs at each TTI based on the channel condition. The frequency domain location of the assigned subcarrier block(s) is signaled to each of the WTRUs separately (or jointly).
The Node-B may also perform time domain scheduling of downlink transmissions and sends the time schedule to the WTRUs via the time domain location 614a-614n in the control packet 600. The time domain scheduling is performed based on data rate requirements of WTRUs, (or buffer occupancy). For a WTRU with a low data rate requirement, (or low buffer occupancy), transmissions to such WTRUs may be multiplexed on a TTI basis or within a TTI as shown in
When the transmissions to WTRUs are multiplexed within one TTI on one subcarrier block, (i.e., data to a particular WTRU is transmitted at one or several OFDMA symbols within the TTI), the symbol location for each WTRU for each assigned subcarrier is indicated by the time domain location field 614a-614n.
Alternatively, in order to reduce the amount of signaling, the Node-B may assign the same symbol location(s) within the TTI at each of its assigned subcarrier blocks. That is, the time domain location is the same for the WTRU in all its assigned subcarrier blocks.
The Node-B may send a special indication to notify the WTRU that the transmissions to the WTRU are not multiplexed with transmissions to other WTRUs. Alternatively, such indication may be indicated implicitly by omitting the time domain location in the control packet. Alternatively, an invalid symbol location value may be used for such notification.
When the Node-B implements frequency and time domain channel-dependent scheduling, a data modulation scheme and transport block size information, (i.e., the number of information bits) for each subcarrier block are signaled separately in the modulation scheme field 616a-616n and the transport block size field 618a-618n in the second part 604a-604n of the control packet 600, as shown in
The coding rate may be derived from the data modulation scheme, the number of allocated subcarriers, and the transport block size. Therefore, the coding rate field 620a-620n may not be included in the control packet 600.
When the Node-B implements frequency diversity-based scheduling, multiple subcarrier blocks are assigned to multiple WTRUs and transmissions to the WTRUs are multiplexed on the assigned subcarrier blocks. In accordance with the present invention, the Node-B assigns multiple equally spaced subcarrier blocks to multiple WTRUs. Therefore, the Node-B needs to signal only the location of the first subcarrier block and the distance between two adjacent subcarrier blocks in frequency domain via the scheduling information.
In accordance with the present invention, when the Node-B implements frequency diversity-based scheduling, one common time domain location, one common data modulation scheme and one common transport block size are assigned for all subcarrier blocks. Therefore, only one time domain location field 806, one modulation scheme field 808, one transport block size field 810 are necessary in the control packet 800 and the signaling overhead is much lower than that in the frequency and time domain channel-dependent scheduling.
It is not efficient to use the same control packet format for the frequency diversity-based scheduling and the frequency and time domain channel-dependent scheduling. Preferably, the scheduling mode is indicated by the scheduling mode indicator and a different control packet format is used for the frequency diversity-based scheduling and the frequency and time domain channel-dependent scheduling. Alternatively, the scheduling mode may not be explicitly indicated by the scheduling mode indicator, but may be indicated implicitly. Alternatively, the same control packet format may be used for both the frequency diversity-based scheduling and the frequency and time domain channel-dependent scheduling.
Alternatively, the same control packet format may be used for both frequency and time domain channel-dependent scheduling and frequency diversity-based scheduling. For example, the control packet 700 shown in
Although the features and elements of the present invention are described in the preferred embodiments 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. 60/707,874 filed Aug. 12, 2005, which is incorporated by reference as if fully set forth.
Number | Date | Country | |
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60707874 | Aug 2005 | US |