Patent Grant
8565825
The present invention relates to a controller for selecting an antenna for use in a multiple-input/multiple-output communication (MIMO) transmission system for performing mobile radio communications using a plurality of antennas for transmissions and receptions.
A MIMO transmission is used to improve a transmission rate with limited frequency resources, and different pieces of data are transmitted from a plurality of correlated antennas, there by performing spatial multiplexing. Thus, a transmission rate can be improved without increasing the frequency bands. The MIMO technology is expected to be applied to a next-generation mobile radio communication system mainly for a high-speed data communication such as LTE (long term evolution), WiMax (worldwide interoperability for microwave access), etc.
The base station apparatus illustrated in
Each branch of sectors 1 through 3 is configured by the following combination of an antenna and a transmission/reception unit.
Since the MIMO transmission can be performed in an area where a plurality of antennas can receive data, the following conditions are considered to prevent the MIMO transmission between the base station apparatus 101 and a mobile station 102 in the cell 103.
(1) Cell Edge
The correlation of antennas cannot be recognized when the mobile station 102 is apart from the base station apparatus 101.
(2) Sector Boundary
The MIMO transmission cannot be selected by handover when the mobile station 102 is approaching the boundary of sectors.
The mobile station 102 normally performs the MIMO transmission by selecting a plurality of antennas in good reception statuses. However, when the selected antennas belong to respective different sectors, the MIMO transmission is not selected, and a fast cell selection (FCS) or a soft handover (SHO) is selected because a control target in the conventional scheduling is assigned to a sector and the MIMO transmission over sectors is not defined.
For example, when the mobile station 102 is located around the center of the sector 1 the signal-to-interference ratios (SIRs) of the branches Br1 and Br2 of the sector 1 are sufficiently large as illustrated for the case C1 in
Next, when the mobile station 102 moves in the vicinity of the boundary between the sectors 1 and 2, the SIRs of the branch Br2 of the sector 1 and the branch Br1 of the sector 2 are large, but the SIRs of the branch Br1 of the sector 1 and the branch Br2 of the sector 2 are small as illustrated for the case C2. Therefore, it is hard to perform the 2×2 MIMO transmission, and the FCS or the SHO is normally applied.
Next, when the mobile station 102 moves in the vicinity of the center of the sector 2, the SIRs of the branches Br1 and Br2 of the sector 2 are large as illustrated for the case C3, and the 2×2 MIMO transmission is performed using the antennas 113 and 114.
Thus, when the mobile station 102 moves from the sector 1 to the sector 2, it is necessary to switch the connection of the user data by handover. As illustrated in
If the transmission/reception units 201 through 204 are connected to the baseband processing unit 211 through the bus 231 or a mesh as illustrated in
On the other hand, as designed for the HSDPA (high speed downlink packet access), if the baseband processing units 211 and 212 are respectively assigned to the sectors 1 and 2, and the mobile station 102 moves to the sector 2, then the user data 401 is moved to the baseband processing unit 212 as illustrated in
While the mobile station 102 exists around the boundary of the sectors land 2, the FCS is applied as illustrated in
As described above, while the mobile station 102 exists around the center of the sector 1 or 2, user data can be transmitted at a high speed in the 2×2 MIMO transmission. However, while the mobile station 102 exists around the boundary of the sectors 1 and 2, control is passed to the FCS/SHO. Therefore, the transmission rate is decreased, and the maximum transmission rate may not be attained.
The following patent document 1 relates to a communication system capable of switching selection diversity/MIMO transmission using a plurality of antennas, and the patent document 2 relates to a system of dividing base station antennas into a plurality of array groups and controlling a directional beam in each array group. The patent document 3 relates to a system of transmitting data in a multiple diversity transmission mode.
Patent Document 1: Japanese Laid-open Patent Publication No. 2005-333443
Patent Document 2: Japanese Laid-open Patent Publication No. 2003-338781
Patent Document 3: Japanese Translation of PCT International Application No. 2005-531219
An object of the present invention is to improve average throughput in a cell of a mobile radio communication system by increasing the area where the MIMO transmission can be realized in the cell.
The first base station apparatus according to the present invention performs radio communications in a MIMO transmission with a mobile station in a cell having a plurality of sectors, and includes one or more antennas provided for each sector and a control unit. When a mobile station moves in the vicinity of the boundary of two sectors, the control unit selects an antenna from among the antennas provided for each of the sectors, and selects the MIMO transmission using the selected antennas.
With the above-mentioned configuration, the MIMO transmission can be performed at the boundary of sectors, and the area in a cell where the MIMO transmission can be performed increases. The control unit of the first base station apparatus corresponds to, for example, a scheduler 951 illustrated in
The second base station apparatus according to the present invention performs radio communications in a MIMO transmission with a mobile station in a cell having no sector configuration, and includes a plurality of antennas provided corresponding to a cell and a control unit. The control unit selects two or more antennas from among the plurality of antennas when the mobile station moves, and selects the MIMO transmission using the selected antennas.
With the above-mentioned configuration, the MIMO transmission can be performed in any position in the cell, and the area in a cell where the MIMO transmission can be performed increases. The control unit of the second base station apparatus corresponds to, for example, a scheduler 1951 illustrated in
The best modes for embodying the present invention are described below in detail with reference to the attached drawings.
The base station apparatus illustrated in
Each branch of the sectors 1 through 3 is configured by the combination of the following antennas and transmission/reception units.
The transmission/reception units 901 through 906 perform the signal processing for each antenna (for each branch) The transmission/reception unit 901 includes a radio unit (RF) 921 and a modulation/demodulation unit (Mod/Dem) 931. Similarly, the transmission/reception units 902 and 906 respectively include radio units 922 through 926 and modulation/demodulation units 932 through 936.
The baseband processing units 911 through 913 perform the signal processing for each user. The baseband processing unit 911 includes a coder/decoder 941 and the scheduler 951. Similarly, the baseband processing units 912 and 913 respectively include the coder/decoder 941 and 942, and schedulers 952 and 953.
Schedulers 951 through 953 are implemented using, for example, a CPU (central processing unit) and a memory and select an antenna, a modulation system, etc. for the MIMO transmission by performing scheduling control according to the quality information about a signal transmitted/received through the antennas 811 through 816.
With the above-mentioned configuration, the baseband processing unit processing user data can be connected to any antenna of any sector although a mobile station 802 moves.
In
In
Next, when the mobile station 802 moves in the vicinity of the boundary between the sectors 1 and 2, the SIRs of the branch Br2 of the sector 1 and the branch Br1 of the sector 2 are large as illustrated for the case C2. Therefore, the 2×2 MIMO transmission is performed using the antennas 812 and 813. In this case, as illustrated in
Next, when the mobile station 802 moves in the vicinity of the center of the sector 2, the SIRs of the branches Br1 and Br2 of the sector 2 are large as illustrated for the case C3. Therefore, the 2×2 MIMO transmission is performed using the antennas 813 and 814.
Thus, the MIMO transmission can be performed at the maximum transmission rate at the sector boundary by removing the signal processing assigned to a sector and flexibly selecting antennas used in the MIMO transmission. Therefore, depending on the connection state of the mobile station 802, the FCS or the MIMO transmission can be selected in the downlink, or the SHO (including the number of selected sectors) or the MIMO transmission can be selected in the uplink.
As illustrated in
In the conventional system illustrated in
Next, the scheduling control in the base station apparatus 801 is described below with reference to
The configuration of the scheduling control can be the distributed system of implementing the schedulers 951 through 953 respectively for the baseband processing units 911 through 913 as illustrated in
In the distributed configuration, the assignment of all branches in the base station apparatus 801 is managed as distributed for each baseband processing unit. The managing method can be a method of restricting the number of branches managed for each baseband processing unit, a method of providing a master scheduler in any of the baseband processing units, etc.
In the centralized configuration, the assignment of all branches in the base station apparatus 801 is managed by the scheduler 1301. Since the assignment status of all branches is managed by the scheduler 1301, the assignment of a destination branch can be performed at a high speed.
In the downlink, a signal (pilot signal) of a shared channel is constantly transmitted from each antenna of each sector. The mobile station 802 receives these signals and recognizes to which base station and sector it belongs, observes the reception status of the signal of each branch, and collects the CQI (channel quality indicator) information indicating the reception status. The CQI information can be, for example, an SIR, a Doppler frequency, a delay spread, etc.
When the mobile station 802 is connected, the CQI information is included in the uplink signal and fed back to the base station apparatus 801. The scheduler 951 of the base station apparatus 801 that has received the fed back information first recognizes the reception status of the mobile station 802, and then selects the sector, an antenna, and a transmitting method (MIMO, FCS, modulation system, coding rate, etc.) to be used in the transmission in the downlink. The factors for determining these selection items can be the amount of down transmission, the number of users in the sector, etc.
Generally, when the MIMO transmission is applied, a certain level of SIR is required. If the SIR does not reach a predetermined value, it can be advantageous that the FCS is selected with an error rate and the frequency of retransmissions taken into account. If a high-speed transmission is requested in the downlink, and there are two or more antennas having an SIR equal to or exceeding a threshold x (a relatively higher quality) as illustrated in the case C2, the scheduler 951 selects the MIMO transmission. On the other hand, if the SIR is equal to or exceeds a threshold y and does not reach the threshold x as illustrated for the case C4, it selects the FCS.
When the antenna of the sector 2 is used in a determined transmitting method, it is necessary to confirm the resource assignment state of the sector 2. Therefore, a control signal about scheduling is transmitted/received between the schedulers 951 and 952. Then, after the resource assignment and the adjustment of the transmission timing are completed between the schedulers, the MIMO transmission is started.
Afterwards, the MIMO transmission is continued between the baseband processing unit 911 and the mobile station 802 through the transmission/reception units 901 and 902 of the sector 1 (step 1503). In the meantime, as illustrated for the case C1 in
Next, the scheduler 951 of the baseband processing unit 911 performs a threshold judgment for the SIR contained in the received CQI information (step 1505). In this example, as illustrated for the case C2 in
Then, the MIMO transmission using the branch Br2 of the sector 1 and the branch Br1 of the sector 2 is selected, and the resource assignment status of the sector 2 is inquired of the scheduler 952 of the baseband processing unit 912 (step 1506). Then, the scheduler 952 returns a reply message that there is available resource in the branch Br1 of the sector 2 (step 1507).
Next, the scheduler 951 transmits a setting change notice message to the mobile station 802, and notifies the station that the antenna is to be changed after the transmission of a predetermined number of frames (step 1508). Then, the mobile station 802 returns a reply message (step 1509).
Next, the scheduler 951 transmits a connection release message to the transmission/reception unit 901 (step 1510), and the transmission/reception unit 901 returns a reply message (step 511). The scheduler 951 transmits a connection setting message to the transmission/reception unit 903 (step 1512), and the transmission/reception unit 903 returns a reply message (step 1513).
Next, the scheduler 951 transmits a transmission resumption message to the mobile station 802 (step 1514). The baseband processing unit 911 MIMO transmits user data to the mobile station 802 through the transmission/reception unit 902 of the sector 1 and the transmission/reception unit 903 of the sector 2 (step 1515), and the mobile station 802 receives the user data and a pilot signal (step 1516).
Afterwards, the MIMO transmission is continued between the baseband processing unit 911 and the mobile station 802 through the transmission/reception unit 902 of the sector 1 and the transmission/reception unit 903 of the sector 2 (step 1517). Then, the mobile station 802 transmits the CQI information about each branch of each sector to the baseband processing unit 911 (step 1518).
If the mobile station 802 further moves and enters the state as illustrated for the case C3 in
A radio propagation environment constantly changes by the moving speed of the mobile station 802 and the influence of a multipath system by the reflecting object in the vicinity. The moving speed can be estimated by the mobile station 802 measuring the Doppler frequency. The influence of the multipath system can be digitized by obtaining the delay spread.
The delay spread refers to a standard deviation of a power delay profile indicating the spread of the power distribution with respect to the delay time. When the power delay profile of the reception wave (direct wave or delay wave) at time τ is represented by a function p(τ), the delay spread Tm is obtained by the following equation.
where the summation symbols in the equations (2) and (3) indicate the summation with respect to a direct wave and a plurality of delay waves.
The scheduler 951 changes the threshold of the SIR using a Doppler frequency and/or a delay spread included in the received CQI information as necessary in step 1505 to consider the influence of the moving speed and the multi-pass system.
If the SIRs of the branch Br2 of the sector 1 and the branch Br1 of the sector 2 are SIR12 and SIR21 respectively, a normal antenna selection logic is defined as follows.
1. SIR12≧x, SIR21≧x
2. SIR12≧x, x>SIR21≧y
3. x>SIR12≧y, SIR21≧x
4. x>SIR12≧y, x>SIR21≧y
5. x>SIR12≧y, y>SIR21
6. y>SIR12, x>SIR21≧y
7. y>SIR12, y>SIR21
On the other hand, when the Doppler frequency fd and the delay spread σ are considered, the thresholds x and y are adjusted by the following equations, and an antenna is selected on the basis of the above-mentioned logic using the adjusted thresholds x′ and y′.
x′=x+α+β (4)
y′=y+α+β (5)
“α” in the equations (4) and (5) is a parameter set depending on the value of the delay spread σ and “β” is a parameter set depending on the value of the Doppler frequency fd. The correspondence between σ and α and the correspondence between fd and β are assigned in, for example, a table format to the scheduler 951.
In this example, an antenna is selected on the basis of the SIR of each branch, but it is also possible to select an antenna on the basis of other quality information indicating the quality of a signal of each branch. In this case, other quality information is transmitted as the CQI information from the mobile station 802.
In addition, with the configuration illustrated in
In the MIMO transmission system illustrated in
The base station apparatus illustrated in
Each branch is configured by a combination of the following antennas and transmission/reception units.
The transmission/reception units 1901 through 1906 perform signal processing for each antenna (for each branch). The transmission/reception unit 1901 includes a radio unit (RF) 1921 and a modulation/demodulation unit (Mod/Dem) 1931. Similarly, the transmission/reception units 1902 through 1906 respectively include radio units 1922 through 1926 and modulation/demodulation units 1932 through 1936.
The baseband processing units 1911 through 1913 perform the signal processing for each user. The baseband processing unit 1911 includes a coder/decoder 1941 and schedulers 1951 and 1952. Similarly, the baseband processing unit 1912 includes a coder/decoder 1942 and schedulers 1953 and 1954, and the baseband processing unit 1913 includes a coder/decoder 1943 and schedulers 1955 and 1956.
The schedulers 1951 through 1956 manage the resources of the branches Br1 through Br6 respectively, and performs the scheduling control for each branch.
In
Next, when the mobile station 1802 moves to the position P2, the SIRs of the branches Br2 and Br3 are large as illustrated for the case C2, the 2×2 MIMO transmission is performed using the antennas 1812 and 1813.
Next, when the mobile station 1802 further moves ahead, the SIRs of the branches Br3 and Br4 are large as illustrated for the case C3. Therefore, the 2×2 MIMO transmission is performed using the antennas 1813 and 1814.
With the above-mentioned configuration, since the FCS/SHO are not applied at the sector boundary, the load of the scheduling control is lighter than in the case of the sector configuration.
Then, the MIMO transmission is continued between the baseband processing unit 1911 and the mobile station 1802 through the transmission/reception units 1901 and 1902 (step 2103). In the meantime, as illustrated for the case C1 in
Next, the scheduler 1951 of the baseband processing unit 1911 performs a threshold judgment for the SIR included in the received CQI information (step 2105). In this example, as illustrated for the case C2 in
The MIMO transmission using the branches Br2 and Br3 is selected, and the resource assignment status of the branch Br3 is inquired of the scheduler 1953 of the baseband processing unit 1912 (step 2106). Then, the scheduler 1953 returns a reply message that there is available resource in the branch Br3 (step 2107).
Next, the scheduler 1951 transmits a setting change notice message similar to the message illustrated in
Next, the scheduler 1951 transmits a connection release message to the transmission/reception unit 1901 (step 2110), and the transmission/reception unit 1901 returns a reply message (step 2111). Then, the scheduler 1951 notifies the scheduler 1952 of a change of the management source of the user data from the branch Br1 (scheduler 1951) to the branch Br2 (scheduler 1952) (step 2112).
Next, the scheduler 1952 transmits a connection setting message to the transmission/reception unit 1903 (step 2113), and the transmission/reception unit 1903 returns a reply message (step 2114).
Next, the scheduler 1952 transmits a transmission resumption message to the mobile station 1802 (step 2115). Then, the baseband processing unit 1911 MIMO transmits user data to the mobile station 1802 through the transmission/reception units 1902 and 1903 (step 2116), and the mobile station 1802 receives the user data and a pilot signal (step 2117).
Then, the MIMO transmission is continues between the baseband processing unit 1911 and the mobile station 1802 through the transmission/reception units 1902 and 1903 (step 2118). The mobile station 1802 transmits the CQI information about each branch to the baseband processing unit 1911 (step 2119).
If the mobile station 1802 further moves and enters the state illustrated for the case C3 in
In the above-mentioned scheduling control, the number of schedulers is larger than in the case illustrated in
If the SIRs of the branches Br2 and Br3 are respectively SIR2 and SIR3, the scheduler 1951 selects an antenna in the following logic in step 2105.
1. SIR2≧x, SIR3≧x
2. SIR2≧x, x>SIR3≧y
3. x>SIR2≧y, SIR3≧x
4. x>SIR2≧y, x>SIR3≧y
5. x>SIR2≧y, y>SIR3
6. y>SIR2, x>SIR3≧y
7. y>SIR2, y>SIR3
The thresholds x and y can also be changed into thresholds x′ and y′ in the above-mentioned adjusting method. In addition, with the configuration illustrated in
This application is a continuation application of International PCT Application No. PCT/JP2006/324979 which was filed on Dec. 14, 2006.
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