The technical field of this invention is protocols in wireless communications.
A number of advanced features are being considered to enhance the cell throughput and/or the cell-edge throughput of the Evolved Universal Terrestrial Radio Access (E-UTRA) during the long term evolution (LTE) study stage. In coordinated multi-point reception (also know as macro-diversity), received signals from multiple cooperating units are combined and processed at a single location. This single location may be different for different user equipments (UEs). In different instances, cooperating units can be separate e-NodeBs, remote-radio units (RRUs), relays etc. The main concern is that coordinated multi-point reception requires coordinated multi-point synchronization.
Coordinated multi-point reception has a problem in different signal propagation delays from the UE to different cooperating units.
Proper receiver operation in any communication system requires appropriate timing synchronization with the transmitter. Multi-point reception requires the UE to be simultaneously synchronized to both cooperating unit receivers. In certain cases, this would be practically impossible to achieve. Whenever |τ1-τ2| exceeds the cyclic prefix duration this goal is practically impossible to achieve. However, it is possible to achieve simultaneous synchronization to different cooperating units in most other cases.
As in any other Orthogonal Frequency Division Multiplexing (OFDM) based system, the receiver timing can be regarded as a reference point for synchronization. All UEs talking to the receiver should adjust their transmission timing so that their signals arrive approximately simultaneously within the Cyclic Prefix (CP) tolerance at the receiver. Timing advance (TA) commands are sent to each UE to compensate for the propagation delay in the channel. Upon reception of a timing advance command, the UE adjusts its uplink transmission timing for Physical Uplink Control CHannel (PUCCH), Physical Uplink Shared CHannel (PUCCH) and sounding reference signals (SRS). The timing advance command indicates the change of uplink timing relative to the current uplink timing in multiples of a frame time constant. This propagation delay is commonly understood to be the first arriving path. For the example illustrated in
This invention concerns overcoming some issues with coordinated multi-point reception. This invention also presents an initial study of throughput enhancements through Macro-diversity via cooperating receiving units.
In this invention at least two cooperating base station units measure the propagation delay τ1 between the user equipment and a first cooperating unit and the propagation delay τ2 between the user equipment and a second cooperating unit. These propagation delays are used to compute a timing advance amount to the user equipment to enable coordinated multi-point reception. In a first embodiment the second cooperating unit computes function f(τ2) of the propagation delay τ2 which it transmits to the first cooperating unit. The first cooperating unit computes the timing advance amount and transmits a corresponding timing advance command to the user equipment. In a second embodiment a central unit receives respective functions f(τ1) and f(τ2) and computes the timing advance amount. This central unit them transmits the corresponding timing advance command to the user equipment. The function f(τ) may be quantization of the propagation delay τ or a suggested timing advance amount.
These and other aspects of this invention are illustrated in the drawings, in which:
In block 404 second cooperating unit 212 computes the function f(τ2). As noted above function f(τ2) could be either a quantization of the measured propagation delay τ2 or a TA amount suggestion. In block 405 second cooperating unit 212 transmits this function f(τ2) to first cooperating unit 211.
In block 406 first cooperating unit 211 computes the function g(f(τ2), τ1). In block 407 first cooperating unit 211 transmits a TA command to UE 201 corresponding to the computed function g(f(τ2), τ1). This TA command is selected to enable coordinated multi-point reception at UE 201. In block 408 UE 201 operates according to this TA command.
The technique described in relation to
A second embodiment to achieve network synchronization is a symmetric option illustrated in
In block 608 central unit 220 computes the function amount needed in a TA command to UE 201. This TA command amount is selected to enable coordinated multi-point reception at UE 201. In block 609 central unit 220 transmits a TA command to UE 201 corresponding to the computed TA amount. In block 610 UE 201 operates according to this TA command.
In general, when Macro-diversity is used in the uplink (UL) with signals processed at a central location, the system capacity can be extremely large. A portion of the network can be treated a single very large Multiuser, Multiple Input, Multiple Output (MU-MIMO) system becoming almost purely Adaptive White Gaussian Noise (AWGN) limited. However, the computational complexity of this technique could become very large, especially when using more advanced receiver techniques.
The following is an example of how the network computes the timing advance. In
This scheme is generally applicable to K cooperating units, with K greater than or equal to 2. The set of cooperating units for a UE can be chosen based on multiple criteria such as reference signal received power (RSRP), signal to interference plus noise ratio (SINR) and propagation delay. An example is as follows. The UE sequentially transmits SRS to K base stations, where the RSRP of the Kth base station is within a defined tolerance of the RSRP of the serving base station of the UE. Alternatively, the UE sequentially transmits SRS to K base stations where the uplink SINR seen by the Kth base station is within a defined tolerance of the SINR seen by the serving base station of the UE. The Kth base station measures the propagation delay τk and sends f(τk) to a network decision unit. The network decision unit determines a set of M cooperating units from the K (M≦K) base stations such that for the Mth base station |τ0-τm| is within a tolerable fraction of its cyclic prefix, where τ0 is a network reference time. The network then sends the appropriate timing advance command to the UE.
The following describes a simulation of the invention in a 19-site system having a matched filter receiver combining signals across cellular sites and different cells. This effectively combines the signal to interference plus noise ratios (SINRs) of any given user at multiple cellular sites. Table 1 lists the assumptions employed in this simulation.
Table 2 lists the UE power settings examined in this simulation.
It is possible to further enhance the system throughput using a minimum mean square error (MMSE) receiver.
Even using simple SINR combining at a central processing location, the cell-edge throughput can be substantially enhanced. Depending on the operating point of cell-average throughput, this enhancement can be a factor of a few to several. Furthermore, using more advanced MIMO processing at the receiver further improves the extent of potential gains. Multi-point synchronization Therefore thus enable macro-diversity reception.
This application is a continuation of prior application Ser. No. 12/611,547, filed Nov. 3, 2009, which claims priority under 35 U.S.C. 119(e)(1) to U.S. Provisional Application No. 61/110,685 filed Nov. 3, 2008.
Number | Name | Date | Kind |
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20070298802 | Kaminski | Dec 2007 | A1 |
20110007707 | Kazmi | Jan 2011 | A1 |
Entry |
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3GPP R1-082468, “Carrier Aggregation in LTE-Advanced”, Ericsson, Warsaw, Poland, Jun. 30-Jul. 4, 2008. |
3GPP R1-082469, LTE-Advanced—Coordinated Multipoint Transmission/Reception, Ericsson, Warsaw, Poland, Jun. 30-Jul. 4, 2008. |
3GPP R1-082575, “Proposals for LTE-Advanced Technologies”, NTT DoCoMo, Inc., Warsaw, Poland, Jun. 30-Jul. 4, 2008. |
3GPP R1-083059, LTE-Advanced—Coordinated Multipoint Transmission/Reception, Ericsson, Jeju, Korea, Aug. 18-22, 2008. |
Number | Date | Country | |
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20130155956 A1 | Jun 2013 | US |
Number | Date | Country | |
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61110685 | Nov 2008 | US |
Number | Date | Country | |
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Parent | 12611547 | Nov 2009 | US |
Child | 13767040 | US |