This application is related to wireless communications.
Closed loop multiple-input multiple-output (MIMO) is an important operation mode in future long term evolution (LTE) networks. Under such a mode, a wireless transmit/receive unit (WTRU) feeds back a rank index (RI) and a precoding matrix index (PMI) to a base station, (i.e., an enhanced eNodeB (eNodeB)), along with channel quality indicator (CQI) information. In general, the base station responds to the WTRU feedback and sends downlink (DL) data accordingly. However, in certain circumstances, the base station may decide to override the RI feedback, and transmit DL data with a different rank than indicated by the WTRU feedback. Such an operation is referred to as rank overriding (RO).
For RO to perform properly, two conditions must be met:
1) The base station must derive a new precoding matrix for the newly selected rank; and
2) The base station must derive new CQI values for the newly derived precoding matrix so that proper modulation and coding schemes (MCS) may be assigned to each layer of MIMO transmission.
To meet the first condition, the LTE codebook forces a “nested property.” When the base station overrides the WTRU feedback rank with a lower rank, the “nested property” allows the base station to use a subset of the original precoding matrix as a new precoding matrix. According to the current LTE specification, it is difficult to derive an accurate CQI after the base station performs RO. Therefore, throughput after RO is reduced.
The following is an example illustrating the problem of current LTE codebook with four (4) transmit antennas and overriding operation. According to current channel conditions, the WTRU determined rank-4 can be accommodated, and the best precoding matrix (out of 16) is W0. The WTRU then sends feedback PMI=0, and RI=3 (rank 4) to the base station. In the meantime, the WRTU also calculates CQI under the assumption RI=3 (rank 4), and PMI=0. According to the LTE specification, two codewords will be used for rank-4. Therefore, two CQI values must be calculated: CQI1 and CQI2. CQI1 is the channel quality indicator for the first codeword (CW1), which is split into first and second layers. CQI2 is the channel quality indicator for the second codeword (CW2), which is split into third and forth layers.
The channel matrix is H, and the effective channel vector is
{tilde over (H)}n=HW0{n}. Equation (1)
Although the exact formula to calculate CQI values may vary depending on the type of WTRU receivers, the channel quality of the first codeword (CQI1) is proportional to the average strength of {tilde over (H)}1 and {tilde over (H)}2, and the channel quality of the second codeword (CQI2) is proportional to the average strength of {tilde over (H)}3 and {tilde over (H)}4.
In this example, if the base station decides to transmit DL data with rank-3, (which is different than the WTRU feedback), it would select rank-3 precoding matrix Wo{124} as the new precoding matrix. According to the codeword to layer mapping rule, the first codeword (CW1) is mapped to the layer corresponding to the effective channel {tilde over (H)}1, and the second codeword (CW2) is mapped to the two layers corresponding to {tilde over (H)}2 and {tilde over (H)}4. The base station would then require a pair of new CQIs corresponding to the new precoding matrix. The new CQI values should be such that CQI1_RO is proportional to the strength of the effective channel {tilde over (H)}1, and CQI2_RO is proportional to the average strength of the effective channels {tilde over (H)}2 and {tilde over (H)}4. The CQI1_RO is different than the original WTRU feedback CQI1, and the CQI2_RO is different than the original WTRU feedback CQI2. Consequently, with the current LTE codebook and codeword to layer mapping rule, it would be difficult for the base station to calculate CQI1_RO and CQI2_RO according to CQI1 and CQI2. Therefore, the base station will likely assign an improper MCS to each codeword, resulting inefficient transmission.
This application is related to an apparatus and method of generating an LTE codebook and performing rank overriding. Reordering rules are presented, whereby a second column vector of each rank-4 precoding matrix will not appear in column vectors of a rank-3 precoding matrix, and the first column vector of each rank-4 precoding matrix is identical to the first column vector of the corresponding rank-3 precoding matrix. Furthermore, precoder hopping between two precoding matrices corresponding to a particular PMI is implemented, whereby a first one of the two precoding matrices comprises a first subset of column vectors of an original precoding matrix that corresponds to the particular PMI, and a second one of the two precoding matrices comprises a second subset of column vectors of the original precoding matrix. The precoder hopping is performed in time and/or frequency domain.
A more detailed understanding may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein:
When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment.
When referred to hereafter, the terminology “base station” includes but is not limited to an evolved or E-UTRAN Node-B (eNodeB), a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment.
One method of improving the data transmission after rank overriding is to change the order of the column vector in a rank-4 precoding matrix.
New reordering rules are proposed herein, whereby the second column vector of each rank-4 precoding matrix will not appear in the column vectors of the rank-3 precoding matrix, and the first column vector of each rank-4 precoding matrix is identical to the first column vector of the corresponding rank-3 precoding matrix.
Using the example above, the CQI1 calculated by the WTRU is proportional to the average strength of {tilde over (H)}1 and {tilde over (H)}3, and the CQI2 calculated by the WTRU is proportional to the average strength of {tilde over (H)}2 and {tilde over (H)}4. In this example, the CQI2 is consistent with CQI2_RO. Therefore, the base station can use the WTRU feedback on CQI2, without modification, in assigning a MCS to the second codeword, without causing performance degradation to CW2. However, the CQI1 definition differs from CQI1_RO, even after modification of rank-4 precoding matrices.
Precoding hopping after rank overriding will now be described. Under the current LTE codebook definition and codeword to layer mapping, rank overriding is performed by arbitrarily removing one or more column vector(s) from the original precoding matrix fed back by WTRU. Therefore, it is then possible that column vectors corresponding to satisfactory channel quality are removed. This also causes CQI discrepancy between the WTRU and base station. In the proposed precoding hopping scheme, all column vectors of the original precoding matrix are used to precode DL data, even after rank overriding. Since the number of column vectors is larger than the rank, the base station switches the precoding matrix in either time and/or frequency domain.
The following example describes rank 4 to rank 3 overriding to illustrate the concept of precoder hopping after rank overriding. The W0{1324} is the original rank-4 precoding matrix fed back by the WTRU. To override the rank to 3, the current LTE specification would use Wo{124} as the rank-3 precoding matrix in all orthogonal frequency division multiplexing (OFDM) symbols and all subcarriers.
Similarly, the precoder hopping can be done in time domain, as shown in
In addition, the precoder hopping can be performed in both time and frequency domain simultaneously as shown in
All of the precoder hopping patterns shown in
Rank overriding is not limited to only rank-4 to rank-3 overriding. FIG. 6 shows a table that summarizes the precoder hopping pattern for other rank overriding scenarios. As shown in
The order of the column vectors of rank-4 precoding matrices may be changed, which maintains the current codeword to layer mapping, or the rank-4 precoding matrices can remain unchanged, while changing the fixed rank-4 codeword to layer mapping to PMI dependent mapping, as shown in
In the original mapping shown in
The processor 815 may be configured to assign a first column vector to each of the precoding matrices in the rank-1 column, assign a first column vector and a second column vector to each of the precoding matrices in the rank-2 column, assign a first column vector, a second column vector and a third column vector to each of the precoding matrices in the rank-3 column, and assign a first column vector, a second column vector, a third column vector and a fourth column vector to each of the precoding matrices in the rank-4 column. Either the second or third column vector of each precoding matrix in the rank-3 column that corresponds to a particular PMI is the same as the second column vector in a precoding matrix in the rank-2 column that also corresponds to the particular PMI. The last two column vectors of each precoding matrix in the rank-4 column that corresponds to a particular PMI are the same as the last two column vectors in the rank-3 column for the particular PMI. The second column vector of any precoding matrix in the rank-4 column that corresponds to a particular PMI is not included in a precoding matrix in the rank-3 column that also corresponds to the particular PMI.
The WTRU 800 may also be configured to perform rank overriding using frequency domain precoder hopping in an LTE codebook having a rank-1 column, a rank-2 column, a rank-3 column and a rank-4 column. Each column includes a plurality of precoding matrices having column vectors assigned thereto. Each precoding matrix corresponds to a respective PMI. The processor 815 may be configured to alternate between the use of two precoding matrices corresponding to a particular PMI. A first one of the two precoding matrices comprises a first subset of column vectors of an original precoding matrix that corresponds to the particular PMI, and a second one of the two precoding matrices comprises a second subset of column vectors of the original precoding matrix. The alternation between the use of two precoding matrices is implemented by precoder hopping that is performed in time domain and/or frequency domain.
In one scenario, the first one of two precoding matrices is applied on odd subcarriers of each OFDM symbol, and the second one of two precoding matrices is applied on even subcarriers of each OFDM symbol.
In another scenario, the first one of the two precoding matrices may be applied on all subcarriers of odd orthogonal OFDM symbols, and the second one of the two precoding matrices is applied on all subcarriers of even OFDM symbols.
In yet another scenario, the first one of the two precoding matrices may be applied on all odd subcarriers of odd OFDM symbols, and on all even subcarriers of even OFDM symbols. The second one of the two precoding matrices may be applied on all even subcarriers of odd OFDM symbols, and on all odd subcarriers of even OFDM symbols.
The processor 915 may be configured to assign a first column vector to each of the precoding matrices in the rank-1 column, assign a first column vector and a second column vector to each of the precoding matrices in the rank-2 column, assign a first column vector, a second column vector and a third column vector to each of the precoding matrices in the rank-3 column, and assign a first column vector, a second column vector, a third column vector and a fourth column vector to each of the precoding matrices in the rank-4 column. The second column vector of any precoding matrix in the rank-4 column that corresponds to a particular PMI is not included in a precoding matrix in the rank-3 column that also corresponds to the particular PMI. The base station 900 may also be configured to perform rank overriding using frequency domain precoder hopping in an LTE codebook having a rank-1 column, a rank-2 column, a rank-3 column and a rank-4 column. Each column includes a plurality of precoding matrices having column vectors assigned thereto. Each precoding matrix corresponds to a respective PMI. The processor 915 may be configured to alternate between the use of two precoding matrices corresponding to a particular PMI. A first one of the two precoding matrices comprises a first subset of column vectors of an original precoding matrix that corresponds to the particular PMI, and a second one of the two precoding matrices comprises a second subset of column vectors of the original precoding matrix. The alternation between the use of two precoding matrices is implemented by precoder hopping that is performed in time domain and/or frequency domain.
In one scenario, the first one of two precoding matrices is applied on odd subcarriers of each OFDM symbol, and the second one of two precoding matrices is applied on even subcarriers of each OFDM symbol.
In another scenario, the first one of the two precoding matrices may be applied on all subcarriers of odd orthogonal OFDM symbols, and the second one of the two precoding matrices is applied on all subcarriers of even OFDM symbols.
In yet another scenario, the first one of the two precoding matrices may be applied on all odd subcarriers of odd OFDM symbols, and on all even subcarriers of even OFDM symbols. The second one of the two precoding matrices may be applied on all even subcarriers of odd OFDM symbols, and on all odd subcarriers of even OFDM symbols.
Although the features and elements are described in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. The methods or flow charts provided may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs).
Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine.
A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit/receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) module.
This application claims the benefit of U.S. Provisional Application No. 60/986,651 filed Nov. 9, 2007, which is incorporated by reference as if fully set forth.
Number | Date | Country | |
---|---|---|---|
60986651 | Nov 2007 | US |