Method and apparatus for transmission power control for a sounding reference signal

PRIORITY

0001The present application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application No. 61/539,701 which was filed in the United States Patent and Trademark Office on Sep. 27, 2011, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

00021. Field of the Invention

0003The present invention relates generally to wireless communication systems and, more specifically, to transmission power control of sounding reference signals.

00042. Description of the Art

0005A communication system includes a DownLink (DL) that conveys signals from one or more base stations (NodeBs) to User Equipments (UEs), and an UpLink (UL) that conveys signals from UEs to one or more NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile, and may be a device such as a wireless device, a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, etc.

0006A communication system also supports several signal types of transmissions including data signals conveying information content, control signals enabling proper processing of data signals, and Reference Signals (RS), also known as pilots, enabling coherent demodulation of data signals or control signals or providing Channel State Information (CSI) corresponding to an estimate of a channel medium experienced by their transmission.

0007UL data information is conveyed through a Physical UL Shared CHannel (PUSCH). UL Control Information (UCI) is conveyed through a Physical UL Control CHannel (PUCCH), unless a UE also transmits a PUSCH, in which case the UE may convey at least some UCI in a PUSCH. UCI includes ACKnowledgment (ACK) information associated with a Hybrid Automatic Repeat reQuest (HARM) ACK (HARQ-ACK) process and is transmitted in response to receiving, by a UE, data Transport Blocks (TBs). UCI also includes DL CSI that informs a NodeB of a channel medium experienced by a signal transmission to a UE. An UL RS can be used to demodulate data or control signals, in which case the UL RS is referred to as DeModulation RS (DMRS), or to sound an UL channel medium and provide NodeBs with UL CSI, in which case the UL RS is referred to as Sounding RS (SRS).

0008DL data information is conveyed through a Physical DL Shared CHannel (PDSCH). DL Control Information (DCI) is conveyed through respective Physical DL Control CHannels (PDCCHs). A PDCCH can convey a Scheduling Assignment (SA) for PUSCH transmission from a UE (UL SA) or for PDSCH reception by a UE (DL SA).

0009<figref idref="DRAWINGS">FIG. 1</figref> is a diagram illustrating a conventional PUSCH structure over a Transmission Time Interval (TTI).

0010Referring to <figref idref="DRAWINGS">FIG. 1</figref>, a TTI includes one subframe <b>110</b>, which includes two slots. Each slot <b>120</b> includes N_symb^ULsymbols used to transmit data information, UCI, or RS. Each symbol <b>130</b> includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The transmission in one slot may be at a same or at a different BandWidth (BW) than the transmission in the other slot. Some PUSCH symbols in each slot may be used to transmit a DMRS <b>140</b>. The transmission BW includes frequency resource units referred to as Resource Blocks (RBs). Each RB includes N_sc^RBor Resource Elements (REs). A UE is allocated M_PUSCHRBs <b>150</b> for a total of M_sc^PUSCH=M_PUSCH·N_sc^RBREs for a PUSCH transmission BW.

0011In <figref idref="DRAWINGS">FIG. 1</figref>, the last subframe symbol may be used to transmit SRS <b>160</b> from at least one UE. Then, N_symb^PUSCH=2·(N_symb^UL−1)−N_SRSsubframe symbols are available for data/UCI/DMRS transmissions, where N_SRS=1 if the last subframe symbol is used to transmit SRS, and N_SRS=0 otherwise. An SRS transmission from a UE may be sent periodically at predetermined subframes with transmission parameters configured to a UE by higher layer signaling, such as Radio Resource Control (RRC) signaling, or it may be sent aperiodically and triggered by an UL SA or a DL SA.

0012A PUSCH transmission power is determined so that the associated signals are received with a desired Signal to Interference and Noise Ratio (SINR) at serving NodeBs while controlling interference to neighboring cells thereby achieving reception reliability targets and ensuring proper network operation. Open-Loop (OL) Transmission Power Control (TPC) with cell-specific and UE-specific parameters is typically used together with Closed Loop (CL) corrections through TPC commands from one or more serving NodeBs.

0013If a PUSCH is scheduled by an UL SA, a respective TPC command is included in the UL SA. If a PUSCH is scheduled according to Semi-Persistent Scheduling (SPS), where a UE is configured by higher layer signaling of a set of parameters for periodic PUSCH transmissions, a TPC command is provided by a separate PDCCH that provides TPC commands to potentially multiple UEs. For each PDCCH, a type of the PDCCH is identified by a scrambling applied to a Cyclic Redundancy Check (CRC) included in a PDCCH codeword. For DL SAs or UL SAs, a CRC is scrambled with a Cell Radio Network Temporary Identifier (C-RNTI). For a PDCCH providing PUSCH TPC commands, a CRC is scrambled by a TPC-PUSCH-RNTI. A PDCCH may also provide PUCCH TPC commands and a respective CRC is then scrambled by a TPC-PUCCH-RNTI. The scrambling operation may be an exclusive OR (XOR) operation, defined by: XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0.

0014<figref idref="DRAWINGS">FIG. 2</figref> is a block diagram illustrating a conventional transmitter block diagram for data in a PUSCH.

0015Referring to <figref idref="DRAWINGS">FIG. 2</figref>, data bits (and possibly UCI data bits) <b>210</b> are provided to a Discrete Fourier Transform (DFT) unit <b>220</b>A mapper <b>230</b>, which performs sub-carrier mapping, maps the output of the DFT unit to REs of a PUSCH transmission BW as indicated by selection unit <b>240</b>, which controls transmission BW. An Inverse Fast Fourier Transform (IFFT) is subsequently performed by IFFT unit <b>250</b>, a CP is inserted by CP insertion unit <b>260</b> and the resulting signal is filtered by filter <b>270</b> for time widowing. Finally, a transmission power (P_PUSCH,c) is applied by power amplifier (PA) <b>280</b> and the resulting signal <b>290</b> is transmitted.

0016A UE can derive a PUSCH transmission power P_PUSCH,c(i), in deciBels per milliwatt (dBm), in a serving cell c during subframe i, such as in Equation (1), where for simplicity it is assumed that a UE does not transmit both PUSCH and PUCCH in a same subframe.

0017<maths id="MATH-US-00001" num="00001"><math overflow="scroll"><mtable><mtr><mtd><mrow><mrow><msub><mi>P</mi><mrow><mi>PUSCH</mi><mo>,</mo><mi>c</mi></mrow></msub><mo>⁡</mo><mrow><mo>(</mo><mi>i</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mi>min</mi><mo>⁢</mo><mrow><mrow><mo>{</mo><mtable><mtr><mtd><mrow><mrow><msub><mi>P</mi><mrow><mi>CMAX</mi><mo>,</mo><mi>c</mi></mrow></msub><mo>⁡</mo><mrow><mo>(</mo><mi>i</mi><mo>)</mo></mrow></mrow><mo>,</mo></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mn>10</mn><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mrow><msub><mi>log</mi><mn>10</mn></msub><mo>⁡</mo><mrow><mo>(</mo><mrow><msub><mi>M</mi><mrow><mi>PUSCH</mi><mo>,</mo><mi>c</mi></mrow></msub><mo>⁡</mo><mrow><mo>(</mo><mi>i</mi><mo>)</mo></mrow></mrow><mo>)</mo></mrow></mrow></mrow><mo>+</mo><mrow><msub><mi>P</mi><mrow><mi>O_PUSCH</mi><mo>,</mo><mi>c</mi></mrow></msub><mo>⁡</mo><mrow><mo>(</mo><mi>j</mi><mo>)</mo></mrow></mrow><mo>+</mo></mrow></mtd></mtr><mtr><mtd><mrow><mrow><mrow><msub><mi>α</mi><mi>c</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>j</mi><mo>)</mo></mrow></mrow><mo>·</mo><msub><mi>PL</mi><mi>c</mi></msub></mrow><mo>+</mo><mrow><msub><mi>Δ</mi><mrow><mi>TF</mi><mo>,</mo><mi>c</mi></mrow></msub><mo>⁡</mo><mrow><mo>(</mo><mi>i</mi><mo>)</mo></mrow></mrow><mo>+</mo><mrow><msub><mi>f</mi><mi>c</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>i</mi><mo>)</mo></mrow></mrow></mrow></mtd></mtr></mtable><mo>}</mo></mrow><mo>⁡</mo><mrow><mo>[</mo><mi>dBm</mi><mo>]</mo></mrow></mrow></mrow></mrow></mtd><mtd><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow></mtd></mtr></mtable></math></maths><br /> wherein Equation (1): <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0018">P_CMAX,c(i) is a maximum UE transmit power configured to a UE by higher layer signaling and may depend on a UE power amplifier class.</li><li id="ul0002-0002" num="0019">M_PUSCH,c(i) is a PUSCH transmission BW expressed in number of RBs.</li><li id="ul0002-0003" num="0020">P_O_{<sub2>_</sub2>}_PUSCH,c(j) controls a mean received SINR at serving NodeBs and is the sum of a cell-specific component P_O_{<sub2>_</sub2>}_NOMINAL_{<sub2>_</sub2>}_PUSCH,c(j) and a UE-specific component P_O_{<sub2>_</sub2>}_UE_{<sub2>_</sub2>}_PUSCH,c(j) that are provided to a UE by higher layer signaling for j=0 and j=1.</li><li id="ul0002-0004" num="0021">For j=0 or j=1, α_c∈{0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1} is a 3-bit parameter provided to a UE by higher layer signaling. For j=2, α_c(j)=1. Fractional TPC applies for α_c&lt;1 as the Path Loss (PL) is not fully compensated. For α_c=1, pure CL TPC is provided.</li><li id="ul0002-0005" num="0022">PL_cis a DL PL estimate, in dBs, a UE calculates as PL_c=referenceSignalPower−higher layer filtered RSRP, where referenceSignalPower is provided to a UE by higher layer signaling and corresponds to a transmission power of a DL RS a UE measures to determine a PL. RSRP corresponds to a RS Received Power (RSRP) a UE measures using a filter configuration provided by higher layer signaling. A serving cell c for a PL measurement is configured to a UE by higher layer signaling using a parameter pathlossReferenceLinking.</li><li id="ul0002-0006" num="0023">Δ_TF,c(i) either equals 0 or is determined by a spectral efficiency of a PUSCH transmission. Further details are omitted for brevity, as they are not material to the present invention.</li><li id="ul0002-0007" num="0024">f_c(i)=f_c(i−1)+δ_PUSCH,cwhere δ_PUSCH,crepresents a TPC command included in an UL SA scheduling a PUSCH transmission or in a separate PDCCH providing TPC commands to a group of UEs. The TPC command may be accumulative or absolute.</li></ul></li></ul>

0025For SRS transmission in a serving cell c during subframe i, an SRS transmission power P_SRS,cfollows a PUSCH transmission power, such as in Equation (2): <br />P_SRS,c(i)=min{P_CMAX,c(i),P_SRS_{<sub2>_</sub2>}_OFFSET,c(m)+10 log₁₀(M_SRS,c)+P_O_{<sub2>_</sub2>}_PUSCH,c(j)+α_c(j)·PL_c+f_c(i)}[dBm].  (2)

0026In Equation 2: <ul id="ul0003" list-style="none"><li id="ul0003-0001" num="0000"><ul id="ul0004" list-style="none"><li id="ul0004-0001" num="0027">P_SRS_{<sub2>_</sub2>}_OFFSET,c(m) is a 4-bit parameter configured to a UE by higher layer signaling where for periodic SRS transmission m=0 and for aperiodic SRS transmission m=1.</li><li id="ul0004-0002" num="0028">M_SRS,cis a SRS transmission BW expressed in number of RBs.</li></ul></li></ul>

0029A UE transmits DMRS or SRS by transmitting a respective Zadoff-Chu (ZC) sequence. For a UL system BW including N_RB^max,ULRBs, a sequence r_u,v^(α)(n) can be defined by a Cyclic Shift (CS) α of a base sequence <o ostyle="single">r</o>_u,v(n) according to r_u,v^(α)(n)=e^jαn<o ostyle="single">r</o>_u,v(n), 0≤n&lt;M_sc^RS, where M_sc^RS=mN_sc^RBis the length of the sequence, 1≤m≤N_RB^max,UL, and <o ostyle="single">r</o>_u,v(n)=x_q(n mod N_ZC^RS), where a q^throot ZC sequence is defined by

0030<maths id="MATH-US-00002" num="00002"><math overflow="scroll"><mrow><mrow><mrow><msub><mi>x</mi><mi>q</mi></msub><mo>⁡</mo><mrow><mo>(</mo><mi>m</mi><mo>)</mo></mrow></mrow><mo>=</mo><mrow><mi>exp</mi><mo>⁡</mo><mrow><mo>(</mo><mfrac><mrow><mrow><mo>-</mo><mi>j</mi></mrow><mo>⁢</mo><mstyle><mspace width="0.3em" height="0.3ex" /></mstyle><mo>⁢</mo><mrow><mi>πqm</mi><mo>⁡</mo><mrow><mo>(</mo><mrow><mi>m</mi><mo>+</mo><mn>1</mn></mrow><mo>)</mo></mrow></mrow></mrow><msubsup><mi>N</mi><mi>ZC</mi><mi>RS</mi></msubsup></mfrac><mo>)</mo></mrow></mrow></mrow><mo>,</mo></mrow></math></maths><br /> 0≤m≤N_ZC^RS−1 with q given by q=└<o ostyle="single">q</o>+½┘+v·(−1)^{└2<o ostyle="single">q</o>┘} and <o ostyle="single">q</o> given by <o ostyle="single">q</o>=N_ZC^RS·(u+1)/31. The length N_ZC^RSof a ZC sequence is given by the largest prime number such that N_ZC^RS&lt;M_sc^RS. Multiple RS sequences can be defined from a single base sequence through different values of α. A UE implicitly determines a ZC sequence to use for DMRS or for SRS transmission through an identity N_ID^TPof a respective TP.

0031<figref idref="DRAWINGS">FIG. 3</figref> is a block diagram illustrating a conventional transmitter block diagram for a ZC sequence.

0032Referring to <figref idref="DRAWINGS">FIG. 3</figref>, a mapper <b>320</b>, which performs sub-carrier mapping, maps a ZC sequence of length M_sc^RS<b>310</b> to REs of an assigned transmission BW as the Res are indicated by RE selection unit <b>330</b>, which controls of transmission BW. The mapping can map the ZC sequence to consecutive REs for a DMRS or to alternate REs for a SRS thereby creating a comb-type spectrum. Subsequently, an IFFT is performed by IFFT unit <b>340</b>, a CS is applied to the output by CS unit <b>350</b>, a CP is inserted by CP insertion unit <b>360</b>, and the resulting signal is filtered by filter <b>370</b> for time windowing. Finally, a transmission power (P_PUSCH,cfor a DMRS or P_SRS,cfor a SRS) is applied by PA <b>380</b> and the resulting RS transmission signal <b>390</b> is transmitted.

0033Improving coverage and cell-edge throughput are key objectives in communication systems. Coordinated Multi-Point transmission/reception (CoMP) is an important technique utilized to achieve these objectives. CoMP operation relies on the fact that when a UE is in a cell-edge region, it may be able to reliably receive signals from a first set of NodeBs (DL CoMP) and reliably transmit signals to a second set NodeBs (UL CoMP). DL CoMP schemes can range from simple schemes involving interference avoidance, such as coordinated scheduling, to more complex schemes requiring accurate and detailed CSI such as joint transmission from multiple NodeBs. UL CoMP schemes can also range from simple schemes where PUSCH scheduling is performed by a single NodeB to more complex schemes where received signal characteristics and generated interference at multiple NodeBs are considered. Herein, NodeBs for DL CoMP are referred to as Transmission Points (TPs) while NodeBs for UL CoMP are referred to as Reception Points (RPs).

0034<figref idref="DRAWINGS">FIG. 4</figref> is a diagram illustrating a conventional DL CoMP operation.

0035Referring to <figref idref="DRAWINGS">FIG. 4</figref>, TP<b>0</b><b>410</b> and TP<b>1</b><b>420</b> are connected through a fiber optic link <b>430</b>, which enables information exchange with negligible latency. A UE <b>440</b> receives a first signal <b>450</b> from TP<b>0</b><b>410</b> and a second signal <b>460</b> from TP<b>1</b><b>420</b>, where the first and second signals <b>450</b> and <b>460</b> convey same information. Depending on a DL CoMP scheme, a combination of these two signals at a UE may be transparent or non-transparent. <figref idref="DRAWINGS">FIG. 4</figref> may also be used to describe UL CoMP operation where TPs are instead viewed as RPs, transmissions of signals from TPs are instead viewed as receptions of signals from RPs, and a signal reception from a UE is instead viewed as a signal transmission.

0036Support of UL CoMP introduces new TPC requirements for PUSCH, PUCCH, and SRS. SRS transmission power control is also relevant for DL CoMP in Time Division duplex (TDD) systems where, due to the DL/UL channel reciprocity, a SRS may be used to obtain more accurate CSI over conventional CSI feedback from a UE based on DL RS measurements.

0037Therefore, there is a need to define an SRS transmission power control method to support UL CoMP or DL CoMP. There is also a need to decouple a PUSCH TPC process and a SRS TPC process to support UL CoMP or DL CoMP. Finally, there is also a need to provide different TPC commands for a PUSCH transmission power and for a SRS transmission power to support UL CoMP or DL CoMP.

SUMMARY OF THE INVENTION

0038Accordingly, the present invention has been provided to address at least the aforementioned limitations and problems. An aspect of the present invention provides methods and apparatus for performing SRS transmission power control in support of UL CoMP or DL CoMP and for introducing a separate SRS transmission power control process supporting DL CoMP from a conventional SRS transmission power control process supporting UL CoMP.

0039According to an aspect of the present invention, a method for transmitting a reference signal from a User Equipment (UE) that receives signaling from at least one transmission point and transmitting signaling to at least one reception point is provided. The method includes transmitting a first reference signal using a sequence determined from an identity of the at least one transmission point and transmitting a second reference signal with a transmission power having an offset with a first maximum value configured by higher layer signaling from the at least one transmission point; and transmitting a first reference signal using a sequence configured to the UE by higher layer signaling from the at least one transmission point, transmitting a second reference signal with a transmission power having an offset with a second maximum value configured by higher layer signaling from the at least one transmission point, wherein the second maximum value is larger than the first maximum value, and the first reference signal and the second reference signal are either identical or non-identical.

0040According to another aspect of the present invention, a method for transmitting a reference signal from a User Equipment (UE) in response to an indication by a field included in a Downlink Control Information (DCI) format conveyed to the UE in a physical downlink control channel transmitted from at least one transmission point is provided. The method includes transmitting the reference signal with a power determined from a first set of parameters if the DCI format, schedules data transmission to the UE; and transmitting the reference signal with a power determined from a second set of parameters if the DCI format schedules data transmission from the UE.

0041According to another aspect of the present invention, a method for providing Transmission Power Control (TPC) commands to a User Equipment (UE) for adjusting a transmission power of at least one of a first reference signal and a second reference signal, the TPC commands are provided by a Downlink Control Information (DCI) format that includes Cyclic Redundancy Check (CRC) bits that are scrambled by a Radio Network Temporary Identifier (RNTI) is provided. The method includes adjusting the transmission power of the first reference signal if the UE receives a first TPC command among the TPC commands in a DCI format with CRC scrambled by a first RNTI or if the UE receives a TPC command in first information bits of a DCI format; and adjusting the transmission power of the second reference signal if the UE receives a second TPC command among the TPC commands in a DCI format with CRC scrambled by a second RNTI or if the UE receives a TPC command in second information bits of a DCI format, wherein the first TPC command and the second TPC command are either identical or non-identical.

0042According to another aspect of the present invention, a User Equipment (UE) for transmitting a reference signal, the UE receiving signaling from at least one transmission point and transmitting signaling to at least one reception point is provided. The UE includes a receiver for receiving higher layer signaling for the at least one transmission point; and a transmitter for transmitting a first reference signal with a sequence determined from an identity of the at least one transmission point and a second reference signal with transmission power having an offset with a first maximum value configured by higher layer signaling from the at least one transmission point and for transmitting a first reference signal with a sequence configured by higher layer signaling from the at least one transmission point a second reference signal with a transmission power having an offset with a second maximum value configured by higher layer signaling from the at least one transmission point wherein the second maximum value is larger than the first maximum value, wherein the first reference signal and the second reference signal are either identical or non-identical.

0043According to another aspect of the present invention, a User Equipment (UE) for transmitting a reference signal in response to an indication by a field included in a Downlink Control Information (DCI) format conveyed in a physical downlink control channel transmitted from at least one transmission point is provided. The UE includes a receiver for receiving DCI formats in respective physical downlink control channels; and a transmitter for transmitting a reference signal with a power determined from a first set of parameters if a first DCI format among the DCI formats schedules data transmission to the UE apparatus and for transmitting a reference signal with a power determined from a second set of parameters if a second DCI format among the DCI formats schedules data transmission from the UE apparatus, wherein and the first DCI and the second DCI are either identical or non-identical.

0044According to another aspect of the present invention, a User Equipment (UE) for obtaining Transmission Power Control (TPC) commands and adjusting a transmission power of at least one of a first reference signal and a second reference signal, the TPC commands provided by a Downlink Control Information (DCI) format that includes Cyclic Redundancy Check (CRC) bits that are scrambled by a Radio Network Temporary Identifier (RNTI) is provided. The UE includes a receiver for receiving DCI formats in respective physical downlink control channels; and a transmission power controller for adjusting a transmission power of a first reference signal if a first TPC command among the TPC commands is received in a DCI format with CRC scrambled by a first RNTI or if a second TPC command among the TPC commands is received in first information bits of a DCI format and for adjusting the transmission power of a second reference signal if a TPC command is received in a DCI format with CRC scrambled by a second RNTI or if a TPC command is received in second information bits of a DCI format, wherein the first TPC command and the second TPC command are either identical or non-identical.

BRIEF DESCRIPTION OF THE DRAWINGS

0045The above and other aspects, features, and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

0046<figref idref="DRAWINGS">FIG. 1</figref> is a diagram illustrating a conventional PUSCH structure over a Transmission Time Interval;

0047<figref idref="DRAWINGS">FIG. 2</figref> is a block diagram illustrating a conventional transmitter for data in a PUSCH;

0048<figref idref="DRAWINGS">FIG. 3</figref> is a block diagram illustrating a conventional transmitter for a ZC sequence;

0049<figref idref="DRAWINGS">FIG. 4</figref> is a diagram illustrating a conventional DL CoMP operation;

0050<figref idref="DRAWINGS">FIG. 5</figref> is a diagram illustrating two different PLs for a signal at two different RPs according to an embodiment of the present invention;

0051<figref idref="DRAWINGS">FIG. 6</figref> is a diagram illustrating a comparison between a conventional SRS TPC and an enhanced SRS TPC according to an embodiment of the present invention;

0052<figref idref="DRAWINGS">FIG. 7</figref> is a diagram illustrating a DL CoMP operation according to an embodiment of the present invention;

0053<figref idref="DRAWINGS">FIG. 8</figref> is a diagram illustrating alternatives for assigning a TPC command δ_SRS,cfor an enhanced SRS transmission according to an embodiment of the present invention;

0054<figref idref="DRAWINGS">FIG. 9</figref> is a diagram illustrating a process for a UE to determine an enhanced SRS transmission power according to an embodiment of the present invention; and

0055<figref idref="DRAWINGS">FIG. 10</figref> is a schematic diagram illustrating a structure for a UE according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

0056Various embodiments of the present invention are described as follows with reference to the accompanying drawings. This present invention may, however, be embodied in many different forms and is not limited to the embodiments set forth herein. Although embodiments of the present invention are described herein with reference to Discrete Fourier Transform (DFT)-spread Orthogonal Frequency Division Multiplexing (OFDM), embodiments of the present invention also are applicable to all Frequency Division Multiplexing (FDM) transmissions in general and to OFDM in particular.

0057An embodiment of the present invention that considers a SRS transmission power control in support of UL CoMP is described as follows.

0058For UL CoMP, an SRS transmitted from a UE should be received with sufficient SINR at RPs so that accurate CSI can be obtained and PUSCH transmissions can be scheduled in consideration of a joint SINR at RPs in order to maximize UL throughput. This consideration is not necessarily required for PUSCH (data/UCI/DMRS) transmissions, as the respective signals at RPs can be soft-combined to produce a single signal and, unlike the SRS, the signals do not need to be individually processed at each RP. As additional RPs associated with UL CoMP will typically receive a SRS with a lower SINR than a single RP when there is no UL CoMP, it is beneficial to enable increasing an SINR of an SRS reception so that a smallest SINR observed by a RP is sufficiently high to obtain accurate CSI from the SRS.

0059<figref idref="DRAWINGS">FIG. 5</figref> is a diagram illustrating two different PLs for a signal at two different RPs according to an embodiment of the present invention.

0060Referring to <figref idref="DRAWINGS">FIG. 5</figref>, in order to provide accurate CSI, an SRS transmission power may account for the PL, PL<b>1</b><b>510</b>, to RP<b>1</b><b>520</b>, which for single RP operation would be the selected RP based on the criterion of smallest PL experienced by a UE, as well as account for the larger PL, PL<b>2</b><b>530</b>, to RP<b>2</b><b>540</b>. The RPs may be assumed to be connected through a fast link such as a fiber optics link (not shown). Therefore, a larger PL for a set of UL CoMP RPs may be used for SRS transmission, as compared to when there is no UL CoMP. Conversely, for data/UCI/DMRS transmission in a PUSCH, a transmission power may account only the smallest PL (corresponding to transmissions to RP<b>1</b>) in order to avoid creating additional interference.

0061The additional interference generated from transmitting an SRS with additional power to support UL CoMP may not be a detrimental factor. For example, a network can configure a same subframe to be used for SRS transmissions from UEs participating in UL CoMP for a set of RPs and such UEs can be further assigned orthogonal SRS resources. For example, orthogonal SRS multiplexing in a same subframe (last symbol) for all RPs can be achieved either by using different combs or by using different cyclic shifts when SRS transmissions have a same BW size and BW position and use a same ZC sequence that can be provided to each UE through higher layer signaling (instead of being implicitly derived by the TP identity as for a conventional single TP or single RP operation). A ZC sequence used by a UE to transmit SRS may be different than a ZC sequence used by a UE to transmit PUCCH or DMRS associated with a PUSCH, which can also be provided to each UE through higher layer signaling (instead of being implicitly derived by the TP identity as for a conventional single TP or single RP operation). For PUSCH (data/UCI/DMRS) transmissions, such orthogonal multiplexing is not possible, as orthogonal multiplexing cannot be achieved (especially for data/UCI transmissions) without reserving a respective BW set to be unavailable for PUSCH transmissions from other UEs.

0062Based on Equation (2), in order to enable enhanced SRS TPC for a UE participating in UL CoMP, parameters that may be modified include P_SRS_{<sub2>_</sub2>}_OFFSET,c(m), α_c(j), PL_c, and f_c(i) for periodic and aperiodic SRS transmissions.

0063In order to compensate for the larger PL, an SRS transmission from a UE to an RP may experience, other than the RP for which the UE measures a PL, PL_c, the UE may be signaled by higher layers to apply a PL offset, ΔPL_c^SRS, to its SRS transmission. This signaling avoids requiring a UE to measure a PL for each RP and provides a network with the flexibility to assign a desired PL compensation ΔPL_c^SRS. A network may determine ΔPL_c^SRSbased on a received SRS SINR at different RPs.

0064Due to the assignment of orthogonal resources for SRS transmissions from UEs having a same set of RPs, fractional PL compensation may not be necessary in order to control inter-RP interference, and α_c(j) may be independently assigned by higher layer signaling for SRS and for PUSCH (data/UCI/DMRS) transmissions. A respective parameter for SRS transmissions is denoted by α_SRS,c(j) and it's a value of the parameter can be selected from the same set of values as for α_c(j). For example, while α_SRS,c(j)=1 while α_c(j)&lt;1.

0065Assuming that RPs for SRS are same as the RPs for data/UCI/DMRS, a decoupling of a same TPC command that is applicable to both SRS and data/UCI/DMRS transmissions in order to support UL CoMP may not be necessary. However, for SRS transmission power control in support of DL CoMP in TDD systems, separate TPC commands for SRS transmissions and for PUSCH transmissions may be necessary, thereby requiring the introduction of a separate (enhanced) SRS TPC process other than the conventional process that must be associated with a respective PUSCH. A TPC command for an enhanced SRS transmission (not applicable to a PUSCH transmission) is denoted as δ_SRS,cand a respective power control loop based on a TPC command for an enhanced SRS transmission is f_SRS,c(i)=f_SRS,c(i−1)+δ_SRS,c(which is separate from the power control loop based on a TPC command for a PUSCH transmission as it was previously described).

0066Mechanisms for a UE to obtain a TPC command, δ_PUSCH,c, for a PUSCH transmission and a separate TPC command, δ_SRS,c, for an enhanced SRS transmission according to embodiments of the present invention are subsequently described herein. For an enhanced SRS TPC process, a UE may use either δ_SRS,cor both δ_SRS,cand δ_PUSCH,c.

0067In order to more flexibly accommodate a larger SRS transmission power when there are multiple RPs, larger values for the parameter P_SRS_{<sub2>_</sub2>}_OFFSET,c(m) may be supported and the respective parameter is denoted by P_SRS_{<sub2>_</sub2>}_OFFSET,c^extend(m).

0068With all the above modifications, a SRS transmission power P_SRS,ccan be expressed as in Equation (3): <br />P_SRS,c(i)=min{P_CMAX,c(i),P_SRS_{<sub2>_</sub2>}_OFFSET,c^extend(m)+10 log₁₀(M_SRS,c)+P_O_{<sub2>_</sub2>}_PUSCH,c(j)+α_SRS,c(j)·(PL_c+ΔPL_c^SRS)+f_SRS,c(i)}[dBm]  (3)

0069<figref idref="DRAWINGS">FIG. 6</figref> is a diagram illustrating a comparison between a conventional SRS TPC and an enhanced SRS TPC according to an embodiment of the present invention.

0070Referring to <figref idref="DRAWINGS">FIG. 6</figref>, for a conventional SRS TPC <b>610</b>, such as an SRS TPC operating according to Equation (2), a UE is signaled by higher layers a P_SRS_{<sub2>_</sub2>}_OFFSET,c(m) value <b>620</b> having a first maximum value. For an enhanced SRS TPC <b>615</b>, such as an SRS TPC operating according to Equation (3), a UE is signaled by higher layers a P_SRS_{<sub2>_</sub2>}_OFFSET,c^extend(m) value <b>625</b> having a second maximum value that is larger than the first maximum value. Therefore, P_SRS_{<sub2>_</sub2>}_OFFSET,c^extend(m) has a greater range than the P_SRS_{<sub2>_</sub2>}_OFFSET,c(m).

0071For a conventional SRS TPC <b>610</b>, a UE measures a PL <b>630</b> based on a DL RS transmitted from a TP associated with a RP of SRS reception. For an enhanced SRS TPC <b>615</b>, a UE measures a PL based on a DL RS transmitted from a TP associated with an RP of SRS reception and combines the measured PL with a parameter ΔPL_c^SRSsignaled by higher layers <b>635</b>. Finally, for a conventional SRS TPC <b>610</b>, a UE is signaled a single parameter α_c(j) <b>640</b>, which the UE applies for PL compensation for both data/UCI/DMRS and conventional SRS transmissions. Meanwhile, for an enhanced SRS TPC <b>615</b>, a UE is signaled a first parameter α_c(j), which the UE applies for PL compensation for TPC of data/UCI/DMRS transmissions and a second parameter α_SRS,c(j)=1 which it applies for TPC of enhanced SRS transmission <b>645</b>. A UE does not simultaneously transmit a SRS with conventional TPC and a SRS with enhanced TPC in a same cell.

0072Another embodiment of the present invention considers an SRS TPC in support of DL CoMP in TDD systems where multiple TPs need to simultaneously receive SRS to estimate a respective CSI. These TPs may be different than the RP(s).

0073<figref idref="DRAWINGS">FIG. 7</figref> is a diagram illustrating a DL CoMP operation according to an embodiment of the present invention.

0074Referring to <figref idref="DRAWINGS">FIG. 7</figref>, a UE receives DL transmissions from two TPs, TP<b>1</b><b>710</b> and TP<b>2</b><b>720</b> while, due to the smaller transmission power of the UE, UL reception from the UE is only at 1 RP (TP<b>1</b><b>710</b> also serves as the RP). The transmission from each TP is associated with a different PL, PL<b>1</b><b>730</b> and PL<b>2</b><b>740</b>, respectively.

0075Conventional SRS TPC links an SRS transmission power to a PL, PL_c, that a UE measures based on an RS transmitted from a TP of serving cell c. This TP is assumed to be associated with a RP of serving cell c. In general, such TP/RP collocation may not always exist, particularly in heterogeneous networks that employ high transmission power nodes and low transmission power nodes. Then, due to the imbalance of transmission powers from different nodes, a UE connect to a high transmission power node in the DL (larger RSRP), as well as connect to a low transmission power node in the UL (smaller PL) as the geographical border between the high and low power nodes is closer to the low power node in terms of RSRP and closer to the high power node in terms of PL.

0076For PUSCH TPC, when a UE is connected to a different node in the DL than in the UL, the UE can be signaled by higher layers a PL offset parameter, ΔPL_c^PUSCH, and compute a PL as PL_c+ΔPL_c^PUSCH. A PL offset parameter for a PUSCH transmission ΔPL_c^PUSCHmust be different than a PL offset parameter for an enhanced SRS transmission ΔPL_c^SRS, as the PL offset parameter for the PUSCH transmission may need to have a negative value while the PL offset parameter for the enhanced SRS transmission may need to have a zero or a positive value. In <figref idref="DRAWINGS">FIG. 5</figref>, although a PL for computing a PUSCH transmission power may be based for reception only at the RP, the PL for computing an enhanced SRS transmission power should consider reception at both TPs and use PL_c(or PL_c+ΔPL_c^SRS) instead of PL_c+ΔPL_c^PUSCH.

0077Another issue related to the use of SRS to obtain CSI for DL CoMP is the application of a same TPC command δ_PUSCH,cfor SRS transmission and for data/UCI/DMRS transmission in a PUSCH (as described in Equation (1) and Equation (2)). For conventional operations, TPC commands for SRS transmissions are always linked to the commands for data/UCI/DMRS transmission in a PUSCH and are provided only through UL SAs or through PDCCH providing TPC commands for PUSCH transmissions to a group of UEs. As accurate TP association and accurate CSI are key components for achieving throughput gains from DL CoMP, and as the channel between a UE and the different TPs may be independent than the channel between a UE and different RP(s), decoupling the TPC commands used for enhanced SRS transmission and for a PUSCH transmission is generally necessary.

0078A first approach to providing a separate TPC command δ_SRS,cfor an enhanced SRS transmission according to an embodiment of the present invention, in contrast to the TPC command δ_PUSCH,cconventionally used for both SRS and for data/UCI/DMRS transmissions in a PUSCH, is to allow for TPC commands for enhanced SRS transmissions to be included in DL SAs (as TPC commands in UL SAs are associated with conventional PUSCH/SRS TPC). In particular, a TPC command field can be included in DL SAs scheduling PDSCH transmissions from TPs to a UE in order for the UE to adjust an enhanced SRS transmission power (but not a PUSCH transmission power).

0079A second approach to providing a separate TPC command δ_SRS,cfor an enhanced SRS transmission according to an embodiment of the present invention, in contrast to the TPC command δ_PUSCH,cconventionally used for both SRS and data/UCI/DMRS transmissions in a PUSCH, is to provide TPC commands to a group of UEs using a RNTI, TPC-SRS-RNTI, to scramble the CRC in a PDCCH conveying TPC commands for enhanced SRS transmission to a group of UEs that is different from the TPC-PUSCH-RNTI. This allows a UE to distinguish the applicability of a TPC command either for an enhanced SRS transmission (TPC-SRS-RNTI) or for a PUSCH transmission (TPC-PUSCH-RNTI).

0080A third approach to providing a separate TPC command δ_SRS,cfor an enhanced SRS transmission according to an embodiment of the present invention, in contrast to the TPC command δ_PUSCH,cconventionally used for both SRS and for data/UCI/DMRS transmissions in a PUSCH, is to use higher layer signaling to inform a UE that, for an existing PDCCH providing TPC commands using for example a TPC-PUSCH-RNTI (or a TPC-PUCCH-RNTI) to scramble its CRC, enhanced SRS TPC commands are provided by one set of bits while PUSCH (or PUCCH) TPC commands are provided by another set of bits.

0081Although one of the primary motivations for decoupling a TPC command δ_SRS,cfor an enhanced SRS transmission from a TPC command δ_PUSCH,cfor a PUSCH transmission is to support DL CoMP, this decoupling can also provide a more generic functionality that is provided to all UEs (especially if UE-transparent DL CoMP operation is used). For example, for aperiodic SRS triggering by a DL SA, the conventional method for providing a TPC command through a respective UL SA is not applicable, and the only possibility for adjusting the transmission power of the aperiodic SRS is to include a respective TPC command in a DL SA, either by an explicit field in the DL SA or by a respective field in the configuration of parameters for the aperiodic SRS transmission.

0082<figref idref="DRAWINGS">FIG. 8</figref> is a diagram illustrating alternatives for assigning a TPC command δ_SRS,cfor an enhanced SRS transmission according to an embodiment of the present invention.

0083Referring to <figref idref="DRAWINGS">FIG. 8</figref>, δ_SRS,cmay be assigned either by a DL SA, or by a PDCCH with CRC scrambled by a TPC-SRS-RNTI, or by a PDCCH with CRC scrambled by TPC-PUSCH-RNTI (or by TPC-PUCCH-RNTI). In a first approach to providing a TPC command for an enhanced SRS transmission that is decoupled from the TPC command for a PUSCH transmission, a TPC field is included in a DL SA <b>810</b>. The DL SA according to an embodiment of the present invention includes conventional fields, such existing DL SA fields <b>812</b>, and also includes a SRS TPC field <b>814</b>. In an alternative approach according to another embodiment of the present invention, a PDCCH with CRC scrambled by a TPC-SRS-RNTI is exclusively dedicated to providing TPC commands for enhanced SRS transmissions <b>820</b> from a group of UEs and each individual field <b>822</b>, <b>824</b>, <b>826</b>, provides a TPC command for an enhanced SRS transmission by a respective UE. A UE is configured by higher layers to monitor PDCCH with a specific TPC-SRS-RNTI and a specific field providing a TPC command for an enhanced SRS transmission. In another approach according to an embodiment of the present invention, an existing PDCCH with CRC scrambled with a TPC-PUSCH-RNTI (or with a TPC-PUCCH-RNTI) is used to also provide TPC commands for enhanced SRS transmissions <b>830</b>. In addition to conventional TPC commands for PUSCH (or PUCCH) transmissions by respective UEs, <b>832</b>, <b>836</b>, a PDCCH with CRC scrambled with a TPC-PUSCH-RNTI (or with a TPC-PUCCH-RNTI) also provides TPC commands for enhanced SRS transmissions <b>834</b>. A UE is configured according to higher layers to monitor a specific field of such PDCCH for a TPC command adjusting its enhanced SRS transmission power. In all previous approaches, the TPC commands can be represented by 2 bits, for example.

0084<figref idref="DRAWINGS">FIG. 9</figref> is a diagram illustrating a process for a UE to determine an enhanced SRS transmission power according to an embodiment of the present invention.

0085Referring to <figref idref="DRAWINGS">FIG. 9</figref>, δ_SRS,cexclusively applies for an enhanced SRS transmission and is received by a UE either in a DL SA, or in a PDCCH with CRC scrambled by a TPC-SRS-RNTI, or in a PDCCH with CRC scrambled by TPC-PUSCH-RNTI (or by TPC-PUCCH-RNTI). A UE first receives a TPC command δ_SRS,c<b>910</b> in UL subframe i. A UE does not consider this TPC command as applicable for a PUSCH or for a PUCCH transmission. Subsequently, a UE incorporates the TPC command δ_SRS,cfor enhanced SRS transmission in the respective power control loop <b>920</b> as f_SRS,c(i)=f_SRS,c(i−1)+δ_SRS,c(that is independent from the conventional one used for PUSCH transmissions). Finally, a UE transmits an enhanced SRS with a power adjusted based on the received TPC command δ_SRS,c<b>930</b>, for example as in Equation (3).

0086The δ_SRS,cmay be applicable for both types of SRS (periodic and aperiodic) or only for one SRS type. For example, if δ_SRS,cis received in a DL SA activating aperiodic SRS transmission it may apply only for the aperiodic SRS while if it is received in a PDCCH with CRC scrambled by a TPC-SRS-RNTI it may apply only for the periodic SRS.

0087<figref idref="DRAWINGS">FIG. 10</figref> is a schematic diagram illustrating a structure for a UE according to an embodiment of the present invention. In the example according to <figref idref="DRAWINGS">FIG. 10</figref>, the UE receives signaling from at least one transmission point and transmitting signaling to at least one reception point.

0088Referring to <figref idref="DRAWINGS">FIG. 10</figref>, UE <b>1000</b> includes a receiver <b>1002</b>, a transmission power controller <b>1004</b>, and a transmitter <b>1006</b>.

0089Operations performed by components of the UE <b>1000</b> according to a first example according to an embodiment of the present invention are described as follows. The receiver <b>1002</b> receives higher layer signaling for the at least one transmission point.

0090The transmitter <b>1006</b> transmits a first reference signal with a sequence determined from an identity of the at least one transmission point and a second reference signal with a transmission power having an offset with a first maximum value configured by higher layer signaling from the at least one transmission point, and the transmitter <b>1006</b> transmits a second reference signal with a transmission power having an offset with a second maximum value configured by higher layer signaling from the at least one transmission point wherein the second maximum value is larger than the first maximum value. The first reference signal is one of a demodulation reference signal, which is associated either with a physical uplink shared channel or with a physical uplink control channel, and a sounding reference signal. The second reference signal is defined as a sounding reference signal and the transmission power offset is relative to a transmission power of a signal other than sounding reference signals, in a physical uplink shared channel. The sounding reference signals may be transmitted periodically or dynamically.

0091The transmission power controller <b>1004</b> applies a first compensation factor to a first path loss estimate for determining a first transmission power for a sounding reference signal having a power offset with a first configured maximum value and a second compensation factor to a second path loss estimate for determining a second transmission power for a sounding reference signal having a power offset with a second configured maximum value. The sequence is a Zadoff-Chu sequence.

0092Operations performed by components of the UE <b>1000</b> according to another example according to embodiment of the present invention are described as follows. The UE <b>1000</b> transmits a reference signal in response to an indication by a field included in a Downlink Control Information (DCI) format conveyed in a physical downlink control channel transmitted from at least one transmission point. In particular, the receiver <b>1002</b> receives DCI formats in respective physical downlink control channels. The transmitter <b>1006</b> transmits a reference signal with a power determined from a first set of parameters if a first DCI format among the DCI formats schedules data transmission to the UE apparatus and transmits a reference signal with a power determined from a second set of parameters if a second DCI format among the DCI formats schedules data transmission from the UE apparatus, wherein and the first DCI and the second DCI may be either identical or non-identical. Each of the first and second sets of parameters includes a power control command, a power control command included in the first set and a power control command included in the second set may be either identical or non-identical. Each of the first and second sets of parameters includes a power offset. A power offset included in the first set and a power offset included in the second set may be either identical or non-identical.

0093The transmission power controller <b>1004</b> controls the power determined from the first set not associated with the power for transmission of signals in a physical uplink shared channel and the power determined from the second set associated with the power for transmission of signals in a physical uplink shared channel.

0094Operations performed by components of the UE <b>1000</b> according to further another example according to an embodiment of the present invention are described as follows. The UE <b>1000</b> obtains Transmission Power Control (TPC) commands and adjusts a transmission power of at least one of a first reference signal and a second reference signal, where the TPC commands are provided by a Downlink Control Information (DCI) format that includes Cyclic Redundancy Check (CRC) bits that are scrambled by a Radio Network Temporary Identifier (RNTI).

0095The receiver <b>1002</b> receives DCI formats in respective physical downlink control channels. The transmission power controller <b>1004</b> adjusts a transmission power of a first reference signal if a first TPC command among the TPC commands is received in a DCI format with CRC scrambled by a first RNTI or if a second TPC command among the TPC commands is received in first information bits of a DCI format. The transmission power controller <b>1004</b> also adjusts the transmission power of a second reference signal if a TPC command is received in a DCI format with CRC scrambled by a second RNTI or if a TPC command is received in second information bits of a DCI format. The first TPC command and the second TPC command may be either identical or non-identical.

0096For example, the reference signal may be a Sounding Reference Signal (SRS), while the first RNTI is a TPC-PUSCH-RNTI and the TPC command in a respective DCI format that also adjusts a transmission power of data signals and a transmission power of other reference signals that the UE apparatus transmits in a Physical Uplink Shared CHannel (PUSCH). The second RNTI is a TPC-SRS-RNTI and the TPC command in a respective DCI format that is discarded by the UE apparatus as a result of adjusting a PUSCH transmission power.

0097According to another example, the reference signal may be a Sounding Reference Signal (SRS), while the first information bits also adjust a transmission power of data signals and of other reference signals that the UE apparatus transmits in a Physical Uplink Shared CHannel (PUSCH), and the second information bits are discarded by the UE apparatus as a result of adjusting a PUSCH transmission power.

0098While the present invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims and their equivalents.