Patent Application
20120178502
This invention relates generally to antenna selection in wireless networks, and more particularly to selecting antennas in wireless networks.
OFDM
Orthogonal frequency division multiplexing (OFDM) is a multi-carrier communication technique, which employs multiple orthogonal sub-carriers to transmit parallel data streams. Due to the relatively low symbol-rate on each of the sub-carriers, OFDM is robust to severe channel conditions, such as frequency attenuation, narrowband interference, and frequency-selective fading. By prepending a cyclic prefix (CP) in front of each symbol, OFDM can eliminate inter-symbol interference (ISI) when the delay spread of the channel is shorter than the duration of CP. OFDM can also simplify frequency-domain channel equalization because the multiple sub-carriers are orthogonal to each other to eliminate inter-carrier interference (ICI).
OFDMA
When OFDM is combined with a multiple access mechanism, the result is orthogonal frequency division multiplexed access (OFDMA). OFDMA allocates different sub-carriers or groups of sub-carriers to different transceivers (user equipment (UE)). OFDMA exploits both frequency and multi-user diversity gains. OFDMA is included in various wireless communication standards, such as IEEE 802.16 also known as Wireless MAN. Worldwide Interoperability for Microwave Access (WiMAX) based on 802.16 and the 3rd generation partnership project (3GPP) long-term evolution (LTE), which has evolved from Global System for Mobile Communications (GSM), also use OFDMA.
SC-FDMA Structure in LTE Uplink
The basic uplink (UL) transmission scheme in 3GPP LTE is described in 3GPP TR 25.814, v7.1.0, “Physical Layer Aspects for Evolved UTRA,” incorporated herein by reference. That structure uses a single-carrier FDMA (SC-FDMA) with cyclic prefix (CP) to achieve uplink inter-user orthogonality and to enable efficient frequency-domain equalization at the receiver side. This allows for a relatively high degree of commonality with the downlink OFDM scheme such that the same parameters, e.g., clock frequency, can be used.
Antenna Selection
The performance of the system can be enhanced by multiple-input-multiple-output (MIMO) antenna technology. MIMO increases system capacity without increasing system bandwidth. MIMO can be used to improve the transmission reliability and to increase the throughput by appropriately utilizing the multiple spatially diverse channels.
While MIMO systems perform well, they may increase the hardware cost, signal processing complexity, power consumption, and component size at the transceivers, which limits the universal application of MIMO technique. In particular, the RF chains of MIMO systems are usually expensive. In addition, the signal processing complexity of some MIMO methods also increases exponentially with the number of antennas.
While the RF chains are complex and expensive, antennas are relatively simple and cheap. Antenna selection (AS) reduces some of the complexity drawbacks associated with MIMO systems. In an antenna selection system, a subset of an set of the available antennas is adaptively selected by a switch, and only signals for the selected subset of antennas are processed by the available RF chains, R1-063089, “Low cost training for transmit antenna selection on the uplink,” Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#47, R1-063090, “Performance comparison of training schemes for uplink transmit antenna selection,” Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#47, R1-063091, “Effects of the switching duration on the performance of the within TTI switching scheme for transmit antenna selection in the uplink,” Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#47, and R1-051398, “Transmit Antenna Selection Techniques for Uplink E-UTRA,” Institute for Infocomm Research (I2R), Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#43, R1-070524, “Comparison of closed-loop antenna selection with open-loop transmit diversity (antenna switching between TTIs),” Mitsubishi Electric, 3GPP RAN1#47bis, R1-073067, “Adaptive antenna switching with low sounding reference signal overhead,” Mitsubishi Electric, 3GPP RAN1#49bis, R1-073068, “Impact of sounding reference signal loading on system-level performance of adaptive antenna switching,” Mitsubishi Electric, 3GPP RAN1#49bis, all incorporated herein by reference.
Signaling and Protocol Design for Antenna Selection
A signaling format for indicating a selected antenna is described in R1-070860, “Closed loop antenna switching in E-UTRA uplink,” NTT DoCoMo, Institute for Infocomm Research, Mitsubishi Electric, NEC, Sharp, Toshiba Corporation, 3GPP RAN1#48, incorporated herein by reference. In order to indicate one antenna out of two possible antennas (A and B), that scheme uses 1 of bit information, either explicitly or implicitly, into an “uplink scheduling grant” message, which indicates the antenna selection decision, 0 means antenna A, and 1 indicates antenna B.
In the prior art, antenna selection is typically performed using pilot signals. Furthermore, antenna selection has been performed only for small-range indoor wireless LANs (802.11n), and where only a single user is on a wideband channel at any one time, which greatly simplifies antenna selection.
In the prior art, sounding reference signals (SRS) and data demodulation (DM) reference signals are only used for frequency dependent scheduling.
A protocol and exact message structure for performing antenna selection for large-range, outdoor OFDMA 3GPP networks is not known at this time. It is desired to provide this protocol and message structure for performing antennas selection for an uplink of an OFDMA 3GPP wireless network.
One embodiment of invention discloses a method for transmitting data from a user equipment (UE) including a transceiver having a set of available antennas. The method selects a subset of antennas from the set of available antennas; and transmits the data by the transceiver using the subset of antennas. The selecting subset of antennas includes transmitting sounding reference signals (SRSs) according to specified times, frequencies, and antennas; and receiving, in response to the transmitting the SRSs, information indicative of a selection of the subset of antennas.
Usually, the SRS are transmitted in response to receiving information indicative of the specific times, frequency and antennas. Also, the subset of antennas can be selected by a base station based on the transmitted SRSs, such that the UE receives the selection from the base station.
Another embodiment discloses a user equipment (UE) forming at least part of a wireless network. The UE includes a set of available antennas; at least one RF chain, wherein a number of available antennas of the UE is greater than a number of RF chains; and a means for transmitting the SRSs according to specified times, frequencies, and antennas, and for receiving, in response to the transmitting the SRSs, information indicative of a selection of the subset of antennas, wherein the RF chain transmits data using the selected subset of antennas. In one variation of this embodiment, the means for transmitting the SRS and for receiving the information is a transceiver including a processor.
Yet another embodiment discloses a method for transmitting data from a transceiver having a set of available antennas and at least one RF chain, wherein a number of available antennas in the set is greater than a number of RF chains, wherein the transceiver is a part of a wireless network including a base station and a plurality of transceivers. The method includes steps of transmitting, to the base station, sounding reference signals (SRSs) according to specified times, frequencies, and antennas; receiving, from the base station in response to the transmitting the SRSs, information indicative of a selection of the subset of antennas; and transmitting the data to the base station using the subset of antennas.
LTE System Overview
The base station is called an evolved Node B (eNodeB) in the 3GPP LTE standard. The eNodeB 110 manages and coordinates all communications with the transceivers in a cell using connections 101, 102, 103. Each connection can operate as a downlink from the base station to the UE or an uplink from the UE to the base station. Because the transmission power available at the base station is orders of magnitude greater than the transmission power at the UE, the performance on the uplink is much more critical.
To perform wireless communication, both the eNodeB and the transceivers are equipped with at least one RF chain and one antenna. Normally, the number of antennas and the number RF chains are equal at the eNodeB. The number of antennas at the base station can be quite large, e.g., dozens. However, due to the limitation on cost, size, and power consumption, UE transceivers usually have less RF chains than antennas 115. The number of antennas available at the UE is relatively small, e.g., two or four, when compared with the base station. Therefore, antenna selection as described is applied at the transceivers. However, the base station can also perform the antenna selection as described herein.
Generally, antennas selection selects a subset of antennas from a set of available antennas at the transceivers.
LTE Frame Structure
Method
The base station selects 170 a subset of antennas 181 based on the received SRSs 161. The base station then indicates 180 the selected subset of antenna 181 to the transceiver. Subsequently, the transceiver 101 can transmit 190 data 191 using the selected subset of antennas 181. The transceiver can also use the same subset of antennas for receiving data from the base station.
LTE Frame Structure
Sounding Reference Signal (SRS)
Except for the 4th and the 11th LBs, the other LBs are used for transmitting control and data signals, as well as uplink sounding reference signals (SRS). For instance, the first LB can carry the SRS. The SRS is usually a wideband or variable bandwidth signal. The SRS enables the base station to estimate the frequency response of the entire system bandwidth or a portion thereof. This information enables the base station to perform resource allocation such as uplink frequency-domain scheduling.
According to the embodiment of the invention, the SRSs are also used for antenna selection.
Another option considered for 3GPP LTE is a frequency-hopping (FH) based SRS. Specifically, a hopping SRS, with a bandwidth smaller than the system bandwidth, is transmitted based on a pre-determined hopping pattern. The hopped SRSs, over multiple transmissions, span a large portion of the system bandwidth or even the entire system bandwidth. With frequency hopping the probability that transceivers interfere with each other during sounding is decreased.
In 3GPP LTE, the eNodeB can enable and disable SRS transmission by the UE transceiver. Moreover, when antenna selection is enabled, the eNodeB can specify the SRS parameters to the transceiver, including transmission bandwidth, starting or ending bandwidth position, transmission period, cyclic shift hopping sequence, transmission sub-frame, repetition factor for indicating the density of the pilot subcarriers in the SRS LB, duration of SRS transmission, symbol position of SRS within a sub-frame, and hopping SRS related parameters, among others. Furthermore, to support antenna selection by using SRS, the same SRS is used by all antennas. Thus, the eNodeB knows in advance, which antenna is sending the SRS.
In one embodiment of the invention, we describe a format and protocol for antenna selection by using SRS in the 3GPP LTE wireless network. When SRS are used for antenna selection, the SRS is called an antenna selection SRS (A-SRS). Otherwise, the SRS is called a regular SRS (R-SRS). Making the A-SRS protocol compatible with the R-SRS protocol ensures that extra signaling overhead associated with A-SRS is as low as possible.
Signaling for Antenna Selection
In general, our invention comprises three levels of messages, namely, Level-A registration signaling, Level-B slow signaling, and Level-C fast signaling, all or some of which can be used for antenna selection. A summary of the possible signaling messages for enabling antenna selection is shown in Table 1A and Table 1B, where the two tables correspond to two slightly different signaling options: Option1 and Option2.
The major difference between Option1 and Option2 is the “SRS start/stop” message. The “SRS start/stop” is a Level-B message in Option1 and a Level-C message in Option2. In the following, we first describe Option1 in details. Then, we describe Option2 by mainly focusing on the differences between the two options.
Signaling Description for [Option1]
As shown in Table 1A, the Level-A registration signaling indicates whether both the transceiver and the eNodeB support uplink (UL) antenna selection. If the eNodeB does not support antenna selection but the transceiver does, the transceiver can use open-loop antenna selection, which does not require any support from the eNodeB. This information is exchanged between the transceiver and the eNodeB at the beginning of the communications, for example, when the transceiver registers with the wireless network upon entry.
Level-B is a layer 3 (or radio resource control (RRC) layer) signaling that is used to set up AS training parameters for the SRS. Level-B is a slow form of signaling that is used infrequently. The eNodeB uses Level-B signaling to stop and start the transceiver to send the A-SRS, or to change the A-SRS parameters.
Level-C is fast signaling that is used by the eNodeB to communicate to the transceiver its antenna selection decisions, and to enable the antenna selection to track short-term variations due to channel fading.
In the uplink (UL), only the Level-A message is needed from the transceiver to notify the eNodeB of its capability of supporting AS. In the downlink (DL), some or all of the three levels of messages may be necessary.
Level-A Signaling
The Level-A registration signaling is used to indicate if both the transceiver and the eNodeB support uplink antenna selection. This information is exchanged between the transceiver and the eNodeB when the transceiver enters the network and before beginning data communications.
The basic procedure between transceiver and eNodeB to exchange the registration information is shown in
In one embodiment of the invention, we include the 1-bit uplink Level-A signaling in the “UE capability information” message 303 sent by the transceiver, and include the 1-bit downlink Level-A signaling into “UE capability information confirm” message 304 sent by the eNodeB.
The “UE capability information” contains a “radio access capability” field. The “radio access capability” field further comprises a “physical channel capability” field. Similar to the “UE MIMO support” already included in the “physical channel capability”, a 1-bit “UE AS support” field is added into the “physical channel capability” to indicate the UE's antenna selection capability.
It is also possible to include the above Level-A signaling information into other messages. Depending on how the radio resource control (RRC) protocol is designed in 3GPP LTE, the Level-A signaling can be adjusted accordingly.
Level-B Signaling
The frame structure for Level-B message [Option1] is shown in Table 2. The Level-B signaling is used to set up AS parameters. This information is required when the eNodeB requests the transceiver to start or stop sending the SRS, or to change A-SRS parameters. R-SRS and A-SRS share the same Level-B signaling message, except that two fields (i.e., “A-SRS Enable” and “Period2” shown in boldface in Table 2) are for A-SRS. It should be noted that all the message format descriptions provided herein are only examples and variations are possible within the scope of this invention.
A-SRS Enable
Period2
The field “SRS Start/Stop”, when set to 1, indicates the request from the eNodeB to start sending SRS (for both A-SRS and R-SRS cases). Otherwise, when this bit is set to 0, then the eNodeB requests the transceiver to stop sending SRS.
The field “A-SRS Enable”, when set to 1, indicates that the A-SRS is enabled. Then, all the other fields of this message are used for setting up A-SRS parameters. The meaning of each field is described in the “Comment” column of Table 2. If “A-SRS Enable” is set to 0, then R-SRS is enabled. Thus, the other fields (except “Period2”) of this message are used for setting up R-SRS parameters. By sharing parameter fields with R-SRS, the overhead for enabling A-SRS is low.
The field “Period1” indicates the interval (in terms of number of TTIs) between any two consecutive SRSs, which is used for both A-SRS and R-SRS. The field “Period2”, on the other hand, is only used for periodic A-SRS, which indicates the interval between two consecutive A-SRSs as well as the pattern of transmission of the A-SRS. By using “Period2”, the eNodeB can dynamically adjust the portion of the SRSs that are sent from the unselected antenna, achieving a tradeoff between the performance and the antenna-switching overhead. The value “Period2” should be no less than 2. If Period2=2, then the SRS is alternatively transmitted from the selected antenna and the unselected antenna.
Upon receiving the Level-B message, the transceiver first checks the “SRS Start/Stop” field. If “SRS Start/Stop=0”, then the transceiver stops sending SRS. The other fields of this message are omitted. Otherwise, if “SRS Start/Stop=1”, then the transceiver is told to start sending SRS according to the format (e.g., either A-SRS or R-SRS; either periodic or adaptive, etc) defined in the parameter list.
A number of variations for the structure of the above Level-B message are possible. First, all the fields need not be sent together at the same time. Depending on the function categories, the Level-B message might be split into sub-messages and sent separately. Second, the 1-bit field “A-SRS Enable” can be inside another field of this message. Depending on how R-SRS signaling is designed in 3GPP LTE, A-SRS signaling may need to be adjusted accordingly in compliance with R-SRS.
Level-C Signaling
The frame structure for Level-C message [Option1] is shown in Table 3. The Level-C fast signaling message is used to signal the transceiver about which antenna to use for data transmission. For selecting one antenna out of two possible candidates, a 1-bit information field suffices. One option is to include this 1-bit information in the “uplink scheduling grant” message. It should be noted that all the message format descriptions provided herein are only examples.
The “uplink scheduling grant” is used by the eNodeB to make an uplink scheduling decision for a transceiver specified by the “ID” field. In the “resource assignment” field, the eNodeB notifies the transceiver which RBs are assigned for its data transmission. The 1-bit antenna selection decision can be created in this field. Thus, when antenna selection is enabled, the “resource assignment” field indicates a joint scheduling and antenna selection decision.
The “AS Decision” bit, when set to 1, indicates that the transceiver should switch to a different transmit antenna to transmit data. If this field is set to 0, then the transceiver uses the same antenna to transmit data. Upon receiving this message, the transceiver continues to use the same antenna, or switches to a different antenna, according to the decision made by the eNodeB. The above method corresponds to a “relative antenna index” based approach. That is, the eNodeB does not know exactly which antenna is used. Instead, the eNodeB just notifies the transceiver to either “switch” or “not switch” the subset of selected antennas. It is also possible to use an “absolute antenna index” based approach to indicate the antenna selection decision, where the eNodeB notifies the transceiver either to use the antenna or the 2nd antenna, or otherwise designated subsets.
It should be noted that it is also possible to include the AS decision information in other fields (e.g., “TF” field) of the uplink scheduling grant message, or even inside other message.
Signaling Description for [Option2]
As shown in Table 1B, [Option2] is similar to [Option1] except for the “SRS start/stop” message, which is a Level-B message in [Option1] and a Level-C message in [Option2]. The advantage of [Option2] is that the SRSs (both R-SRS and A-SRS) can be configured quickly to start/stop (especially stop) for granting a priority to other transceivers. However, the disadvantage is the slightly larger payload of the Level C messages.
In [Option1], the A-SRS parameters are combined together with SRS request (either to start or stop). In [Option2], the A-SRS parameters and SRS request are sent separately. Therefore, in [Option2], the Level-B message does not include “SRS start/stop” field (i.e., the first field in Table 2). Meanwhile, 2 bits are added to the Level-C message in order to achieve the same “SRS start/stop” function. Thus, a total of 3 bits are required for Level-C message in [Option2].
The fields that constitute a Level-C message [Option2] are shown in Table 4. The Level-C message is used to indicate A-SRS request start or stop and antenna selection decision. In one embodiment of the invention, we include this 3-bit information in “uplink scheduling grant” message. It should be noted that all the message format descriptions provided herein are only examples.
Upon receiving the Level-C message, the transceiver checks “SRS start” and “SRS stop” bits. If either bit is set to 1, then this message contains the eNodeB's request to either start or stop sending SRS. When “SRS start=1”, the transceiver is told to start sending SRS based on the Level-B parameters. It is assumed that the transceiver has already obtained the Level-B parameters in advance in a separate message (or transceiver can store a set of default Level-B parameters). When “SRS stop=1”, then the transceiver stops sending the SRS. However, it is possible that both bits are 0. In this case, the transceiver keeps its current SRS status, until either “SRS start” or “SRS stop” is set to 1.
The transceiver also checks the “AS Decision” bit. The responses to “AS Decision” bit are the same as [Option1] at transceiver.
It should be noted that it is also possible to include “SRS Start” and “SRS Stop” information inside another field (e.g., “TF” field) of the uplink scheduling grant message, or even inside other message. Also, the “SRS Start” and “SRS Stop” can be at a separate message from the “AS Decision”. In this case, the “SRS Start” and “SRS Stop” can be combined together into 1 bit, just as that in [Option1]. However, A-SRS and R-SRS share the same SRS request. Depending on how R-SRS signaling will be designed in 3GPP LTE, A-SRS signaling may need to be adjusted accordingly in compliance with R-SRS.
Protocol for Antenna Selection
In one embodiment of the invention, our protocol utilizes the sounding reference signal (SRS) 161 for uplink transmit antenna selection, R1-073067, “Adaptive antenna switching with low sounding reference signal overhead,” Mitsubishi Electric, 3GPP RAN1#49bis, R1-073068, “Impact of sounding reference signal loading on system-level performance of adaptive antenna switching,” Mitsubishi Electric, 3GPP RAN1#49bis. The antenna switching is performed within a TTI, but we do not preclude between TTI switching, R1-063089, “Low cost training for transmit antenna selection on the uplink,” Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#47, R1-063090, “Performance comparison of training schemes for uplink transmit antenna selection,” Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#47, and U.S. patent application Ser. No. 11/620,105, “Method and System for Antenna Selection in Wireless Networks” filed by Mehta et al. on Jan. 5, 2007, incorporated herein by reference.
In terms of functionality, the protocol is flexible and applicable to different antenna selection scenarios. First, both periodic antenna selection and adaptive antenna selection are supported. In particular, the protocol can switch between different periodic AS (with different sounding intervals), or between different adaptive AS (with different sounding intervals), or between periodic and adaptive AS, or even allow them together, as dictated by the eNodeB. Second, both non-hopping SRS based and hopping SRS based antenna selections are supported. The protocol can also switch between them as dictated by eNodeB. Third, the protocol supports antenna selection based on different SRSs, including wideband SRS, variable bandwidth SRS, and narrow-band SRS. Fourth, the protocol supports antenna selection for packet retransmission in both asynchronous HARQ and synchronous HARQ modes.
The current protocol focuses on 1 out of 2 antenna selection, while the extension to multiple antenna selection is possible with a cost of additional signaling overhead.
Protocol Description for [Option1]
For clarity Level-A signaling exchange is omitted herein. It should be noted that all the protocols herein are only examples.
No Frequency Hopping—Wideband SRS and Variable BW SRS
Periodic SRS:
Because “Period2=3”, one out of every 3 SRSs is sent from the unselected antenna. As shown in
For comparison purpose,
In
It should be noted that in the example protocols, we assume a certain delay for the eNodeB to make AS and scheduling decision, and a certain delay for the transceiver to react to the eNodeB's instruction. The delay depends on the standard specification, and the values provided herein are only examples.
Adaptive SRS:
Frequency Hopping—Narrow-band SRS
Periodic SRS:
As shown in
Adaptive SRS:
Protocol Description for [Option2]
Similar to
Switching Between Different SRS Patterns
In order to switch between different SRS patterns (e.g., periodic vs. adaptive, hopping vs. non-hopping, etc), a Level-B slower signaling from the eNodeB to the transceiver is required for both [Option1] and [Option2] to set up different SRS parameters. In addition, for [Option2], an “SRS Start” from the eNodeB to the transceiver is also needed.
It should be noted that under the current protocol, the eNodeB can possibly send SRS request and the AS decision in the same TTI. It should also be noted that when the number of hops (i.e., the “Num_Hops” field in parameter list) is larger than 2, different hopping patterns that jointly span the frequency-space domain can be designed. The pattern can be either signaled by the eNodeB or is chosen from a pre-determined set. In
Antenna Selection Protocol for HARQ
Asynchronous HARQ
If the system operates in an asynchronous HARQ mode, then the eNodeB indicates to the transceiver when, which RBs, and with what MCS (modulation and coding scheme) to retransmit the packet. Because the eNodeB has complete control over the packet retransmission in asynchronous HARQ, the eNodeB can also signal the transceiver whether or not to switch the antenna for retransmission. It can also indicate to the transceiver to send an aperiodic or a periodic A-SRS. In this case, the eNodeB makes a joint AS and scheduling decision for the retransmitted packet, similar to that for a normal packet.
Synchronous HARQ
If the system operates in synchronous HARQ mode, then the transceiver knows a priori exactly when to retransmit the packet when it does not receive an ACK from the eNodeB after a pre-specified number of TTIs. In this case, the transceiver uses the same resource block (RB) and same MCS for the retransmission. Because the transceiver has complete control over the packet retransmission in synchronous HARQ, whenever retransmission occurs, the transceiver can automatically switch to another subset of antennas to retransmit (using the same RB and MCS). This is to avoid the scenario that the channel quality of the previously selected subset of antennas is poor.
The embodiments of the invention provide signaling and protocol for antenna selection in the uplink of OFDM 3GPP wireless network between the transceiver and the eNodeB.
Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications may be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.
This application is a Continuation of prior U.S. patent application Ser. No. 11/834,345, filed Aug. 6, 2007, by Mehta et al.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11834345 | Aug 2007 | US |
| Child | 13301519 | US |
