Method and apparatus for selecting a beam combination in a MIMO wireless communication system

A method and apparatus for selecting a beam combination of beam switched antennas in a multiple-input multiple-output (MIMO) wireless communication system including a first node and a second node. The first node sends a plurality of modulation and coding scheme (MCS) requests to the second node. Each of the plurality of MCS requests is sent using a particular beam combination. The second node receives the MCS requests and generates MCS feedback signals for each of the MCS requests. Each MCS feedback signal includes an MCS recommendation for the particular beam. The first node selects a beam combination for communicating with the second node based on the MCS recommendations.

FIELD OF INVENTION

The present invention is related to a wireless communication system. More particularly, the present invention is related to a method and apparatus for selecting a beam combination of beam switched antennas in a multiple-input multiple-output (MIMO) wireless communications system.

BACKGROUND

Among many emerging technologies developed to meet the increasing demand of high speed data transfer, MIMO is one of the most promising technologies. Unlike traditional techniques, such as a diversity technique which tries to mitigate multipaths, MIMO takes advantage of the existence of multipaths.

In a prior art MIMO system, multiple omni-directional antennas are typically placed at a transmitter and a receiver. To improve the MIMO system performance, multiple beam switched subscriber based smart antennas (SBSAs) are provided to replace omni-directional antennas at the transmitter, receiver, or both. The beam switched SBSAs may be switched parasitic antennas (SPAs), such as Trident or Delta type antennas, or phase-shift based beam selection antennas, such as those using a Butler matrix or a fast Fourier transform (FFT) matrix.

To support SBSA MIMO, physical layer information, such as a signal-to-noise ratio (SNR) and channel state information (CSI) is available. Even though this information is accessible from the physical layer, it requires internal bandwidth to exchange this information between the physical layer and a medium access control (MAC) layer. Therefore, it is desirable to use some explicit information at the MAC layer to support the beam selection in the SBSA MIMO system.

SUMMARY

The present invention is related to a method and apparatus for selecting a beam combination of beam switched antennas in a MIMO wireless communication system including a first node and a second node. The first node sends a plurality of modulation and coding scheme (MCS) requests to the second node. Each of the plurality of MCS requests is sent using a particular beam combination. The second node receives the MCS requests and generates MCS feedback signals for each of the MCS requests. Each MCS feedback signal includes an MCS recommendation for the particular beam. The first node selects a beam combination for communicating with the second node based on the MCS recommendations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, the terminology “wireless transmit/receive unit” (WTRU) includes but is not limited to a user equipment, 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 “access point (AP)” includes but is not limited to a Node-B, a site controller, a base station 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.

In accordance with the present invention, the wireless communication system includes two nodes and at least one of the nodes includes multiple switched beam antennas.

FIG. 1shows an SBSA MIMO system100including two nodes110,120. Each node110,120includes a beam controller112,122and a plurality of SPAs1141-114N,1241-124N. The beam controllers112,122select a beam combination of the beams by selecting at least one beam from each of the SPAs1141-114N,1241-124N. One beam or a subset of beams of each of the SPAs1141-114N,1241-124Nis activated in accordance with a control signal from the beam controller112. The node110maps multiple data streams1111-111Nto the SPAs1141-114Nand transmits the data streams1111-111Nvia the selected beam combination.

FIG. 2shows an SBSA MIMO system200including two nodes210,220. Each node210,220includes a phase selection switch212,222, a phase shift matrix214,224and an omni-directional antenna array216,226comprising a plurality of antenna elements2161-216N,2261-226N, respectively. Each of the phase shift matrixes214,224receives input from the respective omni-directional antenna array216,226and forms an output including a plurality of fixed antenna beams. Each node210,220employs the phase selection switch212,222to select a subset of fixed antenna beams for communications between the two nodes210,220upon command and under control of a beam controller (not shown). Input data streams2111,2112of the node210are mapped to one or more of the beams and transmitted using the generated beams in accordance with the activation signal of the phase selection switch212.

The present invention provides an MCS feedback mechanism which exchanges an MCS request and an MCS feedback signal between the nodes, and the nodes select a beam combination for MIMO communication based on the MCS feedback signal.

FIG. 3is a flow diagram of a process300for selecting a beam combination for MIMO in accordance with the present invention. Referring toFIGS. 1-3, a first node110,210sends MCS requests to the second node120,220while switching a beam combination for each MCS request among a plurality of beam combinations generated by switched beam antennas1141-114N,2161-216N(step302). The second node120,220receives the MCS requests and generates an MCS feedback signal for each of the MCS requests (step304). Each MCS feedback signal includes an MCS recommendation for the corresponding beam combination. The MCS recommendation is generated based on several factors, such as a channel condition, a signal-to-noise ratio (SNR), or the like. The first node110,210receives the MCS feedback signals (step306). The first node110,210then selects a beam combination for communication with the second node120,220based on the MCS recommendations and other factors, (e.g., modulation order, received signal strength indication (RSSI), or the like) (step308).

The first node110,210may select a beam combination corresponding to the MCS recommendation providing the highest data rate since the MCS leading to the highest data rate implies the best channel and signal condition. If there is more than one MCS recommendation generating the highest data rate, an MCS recommendation having the simplest modulation scheme may be selected. The first node110,210preferably measures an RSSI on the MCS feedback signals. If there is more than one MCS recommendation having the simplest modulation scheme, the first node110,210may select a beam combination associated with the highest RSSI. If there is more than one beam combination associated with the highest RSSI, the first node110,210may select a beam combination with a lowest beam combination number.

The MCS requests and the MCS feedback signals may be exchanged using any signaling mechanism. For example, the MCS requests and the MCS feedback signals may be included in a mode request frame and a mode response frame, which are defined in the IEEE 802.11n standard. Alternatively, the MCS requests and the MCS feedback signals may be exchanged using an initiator aggregation control (IAC) MAC protocol data unit (MPDU) and a responder aggregation control (RAC) MPDU, which are defined in IEEE 802.11 standards. The beam switched antennas can be any type of antennas including, but not limited to, an SPA type, (such as trident and delta antennas) or a phase-shift type, (such as Buttler or FFT matrix based beam selection antenna). The present invention may be applied to selecting only subsets of the antennas.

FIG. 4is a block diagram of a system400including an AP402and a WTRU404. The AP402may include a switched beam antenna403, a beam combination selector406in a transceiver407and a beam controller408and the WTRU404may also include a switched beam antenna405, a beam combination selector409in a transceiver410and a beam controller411. The switched beam antenna403,405may be an SPA inFIG. 1or a phased antenna array inFIG. 2. At least one of the WTRU404and the AP402is configured to select a beam combination in accordance with the present invention. Specific embodiments of the present invention are explained hereinafter with reference to a wireless communication system including the AP402and the WTRU404. The beam combination selectors406,409support medium access control (MAC) layer procedures for beam combination selection which will be described in detail hereinafter.

In accordance with a first embodiment of the present invention, only one of the WTRU404and the AP402includes switched beam antennas. Assume that antennas403are N omni-directional antennas and antennas405are M switched beam antennas. The process is basically the same for the case where only the AP402includes the switched beam antennas. The number of beam combinations, NUM, is dependent on the type of switched beam antennas to be used. For example, when N trident antennas are used, there are NUM=3Nbeam combinations. If two (2) trident antennas are used, there are 9 beam combinations. The MAC layer procedures described hereinafter are to select the most appropriate beam combination among the NUM beam combinations for the data transmission. The MAC layer procedures to support the beam selection in accordance with the present invention reside at both the AP402and the WTRU404.

FIG. 5is a flow diagram of a process500for selecting a beam in accordance with the first embodiment of the present invention. Whenever a WTRU404enters a basic service set (BSS), the WTRU404begins a procedure to associate with an AP402(step502). All the handshakes for the association procedure are performed using an omni-mode signal (which may be referred to as “beam combination #0”) at both the AP402and the WTRU404. After association, the WTRU404selects a beam combination (step504). The WTRU404then sends an MCS request using the selected beam combination (step506). The MCS request may be sent using a mode request frame, an IAC MPDU, or any other signaling.

After receiving the MCS request from the WTRU404, the AP402performs the post-processing and generates an MCS recommendation for the following communication with the WTRU404(step508). The AP402generates an MCS feedback signal including the MCS recommendation and sends the MCS feedback signal to the WTRU404(step510). The MCS feedback signal may be sent using a mode response frame, an RAC MPDU, or any other signaling. After receiving the MCS feedback signal from the AP402, the WTRU404stores the MCS recommendation and preferably measures an RSSI of the MCS feedback signal (step512).

The WTRU404then determines whether there is another beam combination remaining (step514). If so, the WTRU404selects the next beam combination at step516and the process500returns to step506. The steps506-514are repeated while the WTRU404switches the beam combination until all the NUM beam combinations are exhausted. When all the beam combinations are exhausted, the WTRU404receives NUM MCS recommendations from the AP402. The WTRU404selects a beam combination for the following communication with the AP402based on the MCS recommendations (step518).

The WTRU404preferably selects an MCS recommendation that provides the highest data rate and the beam combination associated with the highest data rate MCS is selected for the following communications with the AP402. If there are several MCS recommendations providing the highest data rate, the WTRU404may select an MCS recommendation with the simplest modulation scheme and corresponding beam combination. If there is still more than one beam combination to be selected, the beam combination with the highest RSSI may be selected. If there is more than one beam combination having the same RSSI, then the beam combination with the lowest beam combination number may be selected. The foregoing description is provided as an example, and any other selection criteria may be used.

The MAC procedure in the first embodiment requires the WTRU404and the AP402to exchange the same number of messages, (e.g., IAC/RAC MPDUs), as the potential beam combinations. When the number of beam combinations increases due to more beam switched antennas, the procedure will consume a lot of bandwidth. Therefore, for bandwidth efficiency, the WTRU404may send NUM MCS requests consecutively while switching beam combinations. The IAC-MPDUs including the MCS requests may be aggregated with other data MPDUs.

When the AP402receives each of the MCS requests with different beam combinations from the WTRU404, the AP402decides a proper MCS for each of the beam combinations. However, the AP402does not respond to each MCS request, but waits for all MCS requests. After the AP402receives all NUM MCS requests, based on the decision of MCS recommendation for each beam combination, the AP402sends out an aggregated MCS feedback signal. Each MCS feedback signal includes an MCS recommendation for each beam combination. After the WTRU404receives the aggregated MCS feedback signal, the WTRU404makes the decision for which beam combination to be chosen as stated hereinabove.

If only the AP402includes switched beam antennas, the process is substantially the same as the foregoing process500. After association, the AP402sends MCS requests while switching beam combinations and receives MCS feedback signals from the WTRU404and makes a decision on a proper beam combination based on the MCS recommendation made by the WTRU404. The AP402maintains a table for this beam combination selection for the WTRU404, and updates the table when a link quality between the AP402and the WTRU404drops below a predetermined threshold.

Alternatively, as explained above, the AP402may send NUM MCS requests consecutively while switching beam combinations and the WTRU404may send an aggregated MCS feedback signal with NUM MCS recommendations.

In accordance with a second embodiment of the present invention, MAC layer procedures support beam combination selection when switched beam antennas are provided at both the AP402and the WTRU404. The antennas403are N switched beam antennas and the antennas405are M switched beam antennas. The number of beam combinations at the AP402is NUMAPand the number of beam combinations at the WTRU404is NUMWTRU, which are dependent on the type of switched beam antennas as mentioned hereinabove. The MAC layer procedures in accordance with the present invention is to select the most appropriate beam combination among all beam combinations, (NUMWTRU×NUMAP), for the data transmission at both the AP402and the WTRU404. The MAC layer procedures for MCS request and MCS feedback signal reside at both the AP402and the WTRU404in order to make the beam selection process work properly.

FIG. 6is a flow diagram of a process600for selecting a beam combination in accordance with the second embodiment of the present invention. Whenever a WTRU404enters a BSS, the WTRU404begins a procedure to associate with an AP402(step602). All the handshakes during the association procedure are performed using an omni-mode at both the AP402and the WTRU404.

After association, the AP402and the WTRU404select a first beam combination, respectively (step604). The AP402then sends an MCS request using the selected beam combination (step606). After receiving the MCS request from the AP402, the WTRU404performs a post-processing and generates an MCS recommendation for the beam combination, (i.e., the combination of the AP's first beam combination and the WTRU's first beam combination) (step608). The WTRU404then generates an MCS feedback signal including the MCS recommendation and sends the MCS feedback signal to the AP402(step610). After sending out the MCS feedback signal, the WTRU404also switches to the next beam combination for the next MCS request reception from the AP402. After receiving the responded MCS feedback signal from the WTRU404, the AP402stores the MCS recommendation and preferably measures the RSSI on the MCS feedback signal (step612).

The WTRU and the AP determine whether there is a remaining beam combination among the NUMWTRUbeam combinations at the WTRU (step614). If so, the WTRU selects the next beam combination at step616and the process600returns to step606and the AP and the WTRU repeats the steps606-614while the WTRU switches the beam combination until the NUMWTRUbeam combinations at the WTRU404are exhausted.

If the NUMWTRUbeam combinations are exhausted at the WTRU404, it is determined whether there is a remaining beam combination among NUMAPbeam combinations at the AP (step618). If so, the AP402selects the next beam combination and the WTRU404selects the first beam combination at step620and the process returns to step606and the AP402and the WTRU404repeats steps606-618until the NUMAPbeam combinations at the AP402are exhausted.

After completion, the AP402obtains NUM (NUMWTRU×NUMAP) MCS recommendations. Among the NUM MCS recommendations the one leads to the highest data rate is chosen, and the beam combination corresponding to this highest data rate MCS is selected as a preferred beam combination for the following communications between the AP402and the WTRU404(step622). The AP402and the WTRU404exchange the beam selection information using an omni-mode signal at both sides (step624).

The AP402maintains a table for mapping the beam combination to the WTRU404. The AP402updates the table when the link quality between the AP402and the WTRU404drops below a predetermined threshold.

If there are several MCS recommendations providing the highest data rate, the AP402may select an MCS recommendation with the simplest modulation scheme and corresponding beam combination. If there is more than one beam combination to be selected, the beam combination with the highest RSSI may be selected. If both MCS and RSSI are tied up, then the beam combination with the lowest number may be selected. The foregoing description is provided as an example, and any other selection criteria may be used.

Alternatively, for bandwidth efficiency, the AP402may send NUMAPMCS requests consecutively while switching beam combinations. During the transmission of all NUMAPMCS requests, the WTRU404receives the MCS requests using the first beam combination and does not respond to each MCS request, but waits for all NUMAPMCS requests. The MCS requests may be included in IAC MPDUs, (or mode request frames), and the IAC-MPDUs can be aggregated with other data MPDUs. When the WTRU404receives each MCS request from the AP402, the WTRU404decides a proper MCS for each beam combination and stores it. After the WTRU404receives all NUMAPMCS requests, based on the decision of MCS recommendation for each beam selection, the WTRU404sends out an aggregated MCS feedback signal, (e.g., using an RAC-MPDU or a mode response frame), with NUMAPMCS feedback signals aggregated together using the first beam combination. Each MCS feedback signal has an MCS recommendation for each beam combination. After sending out MCS feedback signals, the WTRU404switches to beam combination 2 for the next MCS request reception from the AP402.

After receiving the MCS feedback signals from the WTRU404, the AP402stores the MCS recommendations and may measure an RSSI on each MCS feedback signal. Then, the AP402and the WTRU404repeats the previous steps by NUMWTRUtimes until NUMWTRUbeam combinations at the WTRU404are exhausted. After completion of the steps, the AP402obtains all MCS recommendations from the WTRU404and selects a proper beam combination as explained hereinabove.

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.