Abstract:
Apparatus and method for adjusting at a user equipment a power level of a first signal type transmitted over a set of resource elements (REs). The method includes transmitting the first signal type with a first power level over a first set of REs, and transmitting the first signal type with a second power level over a second set of REs, wherein the second power level is higher than the first power level and the second set of REs is a sub-set of the first set of REs.

Description:
PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. § 119 to a United States Provisional Application filed in the United States Patent and Trademark Office on Feb. 4, 2008 and assigned Ser. No. 61/025,925, the entire disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention is directed, in general, to wireless communication systems and, more specifically, to the adaptation of transmission power adjustments in Single-Carrier Frequency Division Multiple Access (SC-FDMA) communication systems. 
         [0004]    2. Description of the Related Art 
         [0005]    The invention considers transmission power adjustments in SC-FDMA communication systems and is further considered in the development of the 3 rd  Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE). The invention assumes the UpLink (UL) communication corresponding to the signal transmission from mobile User Equipments (UEs) to a serving base station (Node B). A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a wireless modem card, etc. A Node B is generally a fixed station and may also be called a base transceiver system (BTS), an access point, or some other terminology. 
         [0006]    The UL physical channel carrying the transmission of data information signals from UEs is referred to as the Physical Uplink Shared CHannel (PUSCH). In addition to data information signals, the PUSCH carries the transmission of Reference Signals (RS), also known as pilot signals. The PUSCH may also carry the transmission of Uplink Control Information (UCI) signals. The UCI signals may include any combination of positive or negative acknowledgement signals (ACK or NAK, respectively), Channel Quality Indicator (CQI) signals, Precoding Matrix Indicator (PMI) signals, and Rank Indicator (RI) signals. In the absence of data information signals, UCI signal transmission is through the Physical Uplink Control CHannel (PUCCH). 
         [0007]    The ACK or NAK signal is associated with the use of Hybrid Automatic Repeat reQuest (HARQ) and is in response to the respective correct or incorrect data packet reception in the DownLink (DL) of the communication system corresponding to signal transmission from the serving Node B to a UE. The CQI signal from a UE is intended to inform the serving Node B of the channel conditions the UE experiences for signal reception in the DL, enabling the Node B to perform channel-dependent scheduling of DL data packets. The PMI/RI signals from a UE are intended to inform the serving Node B how to combine the transmission of a signal to the UE from multiple Node B antennas in accordance with the Multiple-Input Multiple-Output (MIMO) principle. Any one of the possible combinations of ACK/NAK, CQI, PMI, and RI signals may be transmitted by a UE in the same Transmission Time Interval (TTI) with data signal transmission or in a separate TTI without data transmission. 
         [0008]    UEs are assumed to transmit UCI and/or data signals in the PUSCH over a TTI corresponding to a sub-frame. 
         [0009]      FIG. 1  illustrates the sub-frame structure assumed in the exemplary embodiment of the invention. The sub-frame  110  includes two slots. Each slot  120  further includes N symb   UL =7 transmission symbols, for example, and each transmission symbol  130  further includes a cyclic prefix (CP) for mitigating interference due to channel propagation effects. The PUSCH transmission in the two slots may be in the same part or it may be at two different parts of the operating BandWidth (BW). RS are transmitted in the middle transmission symbol of each slot  140  and in the same BW as the data signal. The RS are primarily used to provide channel estimation enabling coherent demodulation of the UCI or data signal (DM RS). The PUSCH transmission BW includes frequency resource units, which may be referred to as Resource Blocks (RBs). In an exemplary embodiment, each RB includes N sc   RB =12 Resource Elements (REs), or sub-carriers, and a UE is allocated M PUSCH  consecutive RBs  150  for its PUSCH transmission. 
         [0010]    In order for a Node B to determine the RBs where to schedule PUSCH transmissions and the associated Modulation and Coding Scheme (MCS), a CQI estimate, such as a Signal-to-Interference and Noise Ratio (SINR) estimate, is needed over the UL scheduling BW. Typically, this UL CQI estimate is obtained through the separate transmission of an RS sounding the UL scheduling BW (Sounding RS, or SRS). The SRS is transmitted from a UE in a transmission symbol  160  of some UL sub-frames, over M SRS  consecutive RBs  170 , replacing the transmission of data or control information from the same or from a different UE. The SRS may be transmitted from UEs not having PUSCH transmission in the reference sub-frame and its transmission power is independent of the PUSCH transmission power. 
         [0011]    An exemplary block diagram of the transmitter functions for SC-FDMA signaling is illustrated in  FIG. 2 . Coded CQI bits and/or PMI bits  205  and coded data bits  210  are multiplexed  220  through rate matching. If ACK/NAK bits are also multiplexed, coded data bits are punctured to accommodate ACK/NAK bits  230 . The Discrete Fourier Transform (DFT) of the combined data bits and UCI bits is then obtained  240 , the sub-carriers  250  for the assigned transmission BW are selected  255 , the Inverse Fast Fourier Transform (IFFT) is performed  260 , the CP  270  is inserted, and the selection  280  of the power amplification level  285  is applied to the transmitted signal  290 . For brevity, additional transmitter circuitry, such as digital-to-analog converter, analog filters, and transmitter antennas are not shown. Also, the encoding process for data bits and CQI and/or PMI bits, as well as the modulation process, are omitted for brevity. 
         [0012]    At the receiver, the reverse (complementary) transmitter operations are performed. This is conceptually illustrated in  FIG. 3  where the reverse operations of those illustrated in  FIG. 2  are performed. After an antenna receives the radio-frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters), which are not shown for brevity, the digital received signal  310  has its CP removed  320 . Subsequently, the receiver unit applies an FFT  330 , selects  345  the sub-carriers  340  used by the transmitter, applies an Inverse DFT (IDFT)  350 , extracts the ACK/NAK bits and places respective erasures for the data bits  360 , and de-multiplexes  370  the data bits  380  and CQI/PMI bits  390 . As for the transmitter, well known receiver functionalities such as channel estimation, demodulation, and decoding are not shown for brevity. 
         [0013]    Initial transmission of a Transport Block (TB) from a UE may be configured by the Node B through the transmission of a Scheduling Assignment (SA) or through higher layer signaling to the reference UE. In the former case, the PUSCH transmission parameters, including the transmission power and the Modulation and Coding Scheme (MCS) may be adapted by the SA and the respective PUSCH transmission is referred to as adaptive. 
         [0014]    PUSCH HARQ retransmissions of a TB are assumed to be synchronous. If the initial transmission occurs in UL sub-frame n, a retransmission will occur at UL sub-frame n+M, where M is a known integer such as, for example, M=8. It is assumed that the Node B transmits to the reference UE a NAK signal in a DL sub-frame n+L prior to the n+M UL sub-frame, where L is a known integer with L&lt;M (for example, L=4). 
         [0015]    PUSCH HARQ retransmissions may be adaptive (configured by a SA) or non-adaptive (occurring without SA). Non-adaptive HARQ retransmissions use the same parameters as the initial transmission for the same TB. In particular, the transmission power adjustment and the PUSCH RBs are the same as the ones used for the initial transmission of the same TB. 
         [0016]    When UCI or SRS is multiplexed in the PUSCH, it is possible that either the initial transmission includes UCI or SRS while the HARQ retransmission does not, or the reverse. In either case, for non-adaptive HARQ retransmissions, the amount of REs available for data information is different between the initial transmission and the HARQ retransmission for the same TB since the number of RBs remains the same. The UE autonomously performs rate matching or puncturing of coded data symbols to accommodate UCI or SRS transmission. However, the PUSCH transmission power remains the same. This results in a different BLock Error Rate (BLER) for the data between the initial transmission and a HARQ retransmission for the same TB due to the different effective data coding rate. The same may also occur for PUSCH transmissions configured by higher layer signaling (non-adaptive). 
         [0017]    For example, a transmission power adjustment Δ TF  in sub-frame i may be computed as Δ TF (i)=10 log 10 (2 K(TBS/N     RE     ) −1) where K is a constant, TBS is the TB Size, and N RE =2·M PUSCH ·N sc   RB ·(N symb   UL −1) is the number of REs per sub-frame excluding REs used for DM RS transmission (one transmission symbol per slot is used for DM RS transmission in the exemplary structure in  FIG. 1 ). Therefore, the PUSCH transmission power remains the same in non-adaptive HARQ retransmissions regardless of the presence or absence of UCI or SRS. 
         [0018]    The data BLER difference between the initial transmission and a non-adaptive HARQ retransmission for the same TB is not deterministic and depends on the difference in the REs available for the data information (difference in data coding rate). If more REs are allocated to data information in the initial transmission (for example, there is no UCI or SRS transmission) than in a HARQ retransmission (for example, there is UCI or SRS transmission), the puncturing or rate matching of data information needed in the HARQ retransmission results in increased data BLER. The reverse applies if fewer REs are allocated to data information in the initial transmission and the data BLER in the non-adaptive HARQ retransmission may be lower than needed for achieving the desired system throughput. 
         [0019]    When the previously described variability exists in the number of REs available for data information in the initial transmission and in a non-adaptive HARQ retransmission for the same TB, or for transmissions configured by higher layer signaling, the conventional approach considers that the PUSCH power adjustment in a HARQ retransmission remains the same as in the initial transmission. For example, as it was previously described, Δ TF (i) does not account for UCI or SRS transmission. Then, two distinct cases exist: 
         [0020]    (A) UCI or SRS is not included in the initial transmission but is included in a non-adaptive HARQ retransmission. Then, to decode the data, the Node B receiver may insert erasures in REs where UCI or SRS replaces data or account for the different rate matching performed at the UE. The data BLER increases, due to the increase in the effective coding rate that is not compensated by a respective increase in transmission power, and the system throughput decreases. 
         [0021]    (B) UCI or SRS is included in the initial transmission but is not included in a non-adaptive HARQ retransmission. Then, during the HARQ retransmission, the UE may transmit additional bits in the REs where UCI or SRS were located in the initial transmission, thereby decreasing the effective code rate with rate matching. The data BLER unnecessarily decreases, due to the decrease in the effective code rate that is not compensated by a respective decrease in transmission power, and the interference management is sub-optimal due to the unnecessarily high transmission power. 
         [0022]    Therefore, there is a need to adjust the transmission power depending on the amount of control signals or reference signals in a PUSCH transmission. 
         [0023]    There is another need to determine the appropriate adaptation of the transmission power adjustment depending on the amount of control signals or reference signals in a PUSCH transmission. 
       SUMMARY OF THE INVENTION 
       [0024]    Accordingly, the invention has been designed to solve the above-mentioned problems occurring in the prior art, and embodiments of the invention provide apparatus and method for enabling a user equipment to autonomously adapt the power of a non-adaptive signal transmission in response to variations in the presence of control signal transmission or reference signal transmission in the same transmission time interval. 
         [0025]    In one aspect of the invention, a method for adjusting at a user equipment a power level of a first signal type transmitted over a set of resource elements (REs) includes transmitting the first signal type with a first power level over a first set of REs, and transmitting the first signal type with a second power level over a second set of REs, wherein the second power level is higher than the first power level and the second set of REs is a sub-set of the first set of REs. 
         [0026]    In another aspect of the invention, an apparatus for adjusting at a user equipment a power level of a first signal type transmitted over a set of resource elements (REs) includes an amplification unit to provide power for transmitting the first signal type, and a power control unit to apply a first power level to the amplification unit to transmit the first signal type over a first set of REs and a second power level to the amplification unit to transmit the first signal type over a second set of REs, wherein the second power level is higher than the first power level and the second set of REs is a sub-set of the first set of REs. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The 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: 
           [0028]      FIG. 1  is a block diagram illustrating an exemplary sub-frame structure for the SC-FDMA communication system; 
           [0029]      FIG. 2  is a block diagram illustrating an exemplary SC-FDMA transmitter for multiplexing data bits, CQI bits, and ACK/NAK bits in a sub-frame; 
           [0030]      FIG. 3  is a block diagram illustrating an exemplary SC-FDMA receiver, for de-multiplexing data bits, CQI bits, and ACK/NAK bits in a sub-frame; 
           [0031]      FIG. 4  is a block diagram illustrating an increase in transmission power in accordance with an exemplary embodiment of the present invention; 
           [0032]      FIG. 5  is a block diagram illustrating a decrease in transmission power in accordance with an exemplary embodiment of the present invention; 
           [0033]      FIG. 6  is a block diagram illustrating an increase in transmission power in accordance with another exemplary embodiment of the present invention; and 
           [0034]      FIG. 7  is a block diagram illustrating a decrease in transmission power in accordance with another exemplary embodiment of the present invention. 
           [0035]      FIG. 8  is a block diagram illustrating an exemplary SC-FDMA transmitter with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0036]    The invention now will be described more fully hereinafter with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
         [0037]    Additionally, although the invention assumes a SC-FDMA communication system, it may also be applied to other communication system, such as to all FDM systems in general and to OFDMA. OFDM, FDMA, DFT-spread OFDM, DFT-spread OFDMA, single-carrier OFDMA (SC OFDMA), and single-carrier OFDM in particular. 
         [0038]    The methods and apparatus in accordance with the exemplary embodiments of the present invention solve problems related to the need for adapting the data transmission power adjustment in PUSCH transmissions in order to maintain the desired data reception reliability when the resources used for data transmission differ due to the existence of control signals or reference signals. 
         [0039]    The exemplary embodiment of the invention considers that the UE autonomously adapts the PUSCH transmission power adjustment Δ TF  in a non-adaptive HARQ retransmission in order to address the variation in the data information MCS, relative to the one used in the initial transmission of the same TB. This MCS variation is due to the variation in the amount of UCI or SRS that is included in the initial transmission or the HARQ retransmission. Then, the following applies: 
         [0040]    (A) UCI or SRS are not included in the initial transmission but it is included in a HARQ retransmission. The UE autonomously increases the transmission power adjustment Δ TF  to compensate an increase in data MCS resulting from the reduction in the coding gain associated with the introduction of UCI or SRS (unless it is a power limited UE). The new, higher, data MCS is determined by the amount of inserted UCI or SRS. 
         [0041]    (i) To address possible power limitations from a UE, the serving Node B can configure a UE whether or not to increase the power during a HARQ retransmission. Moreover, a maximum level for the power increase may also be configured. For example, this maximum level may be the same as the one used by a transmission power control mechanism that may apply to any PUSCH transmission. 
         [0042]    (B) UCI or SRS is included in the initial transmission but it is not included in a HARQ retransmission. The UE autonomously decreases the transmission power adjustment Δ TF  to compensate for the decrease in data MCS resulting from the increase in the coding gain associated with the removal of UCI or SRS. The new MCS is determined by the amount of removed UCI or SRS REs. 
         [0043]    (i) The maximum level of power reduction may be the same as the one used by a transmission power control mechanism that may apply to the PUSCH transmission. 
         [0044]      FIG. 4  illustrates an exemplary embodiment of the invention for adapting the transmission power adjustment to compensate for the MCS increase when UCI is included in a HARQ retransmission. For simplicity, only the UCI is shown. However, the same concept may be applied for the SRS. In the initial transmission  410  that does not include UCI or SRS transmission, the UE uses “Transmission (Tx) Power  1 ”  412  for the PUSCH transmission power adjustment and uses Data MCS 1    414 . In a HARQ retransmission  420  for the same TB that includes UCI or SRS transmission, the UE uses “Tx Power  2 ”  422  for the PUSCH transmission power adjustment and uses Data MCS 2    424  where “Tx Power  2 ” is higher than “Tx Power  1 ” and MCS 2  is higher (in spectral efficiency) than MCS 1 . 
         [0045]      FIG. 5  illustrates an exemplary embodiment of the invention for adapting the transmission power adjustment to compensate for the MCS decrease when UCI is removed in a HARQ retransmission. For simplicity, only the UCI is shown. However, the same concept may be applied for the SRS. In the initial PUSCH transmission  510  that includes UCI or SRS transmission, the UE uses “Tx Power  1 ”  512  and Data MCS 1    514 . In a HARQ retransmission  520  for the same TB that does not includes UCI or SRS transmission, the UE uses “Tx Power  2 ”  522  for the PUSCH transmission power adjustment and uses Data MCS 2    524  where “Tx Power  2 ” is lower than “Tx Power  1 ” and MCS 2  is lower (in spectral efficiency) than MCS 1 . 
         [0046]    The adaptation of the transmission power adjustment for the setup in  FIG. 4  is as follows. For a data payload of X coded bits without any UCI or SRS transmission, the data information is transmitted using all sub-frame REs (except REs used for DM RS transmission) with MCS, and “Tx Power  1 ” for the transmission power adjustment. When REs equivalent to Y coded data bits (Y&lt;X) become unavailable for data transmission due to the inclusion of UCI or SRS transmission, the spectral efficiency of the data transmission increases and the transmission power adjustment needs to increase. Therefore, data MCS 2  when UCI or SRS is included in the PUSCH is higher than data MCS, and “Tx Power  2 ” is higher than “Tx Power  1 ”. 
         [0047]    In case UCI is not included in a HARQ retransmission, the number of Y coded bits is assumed to be fixed and equal to the one required to incorporate SRS transmission (corresponding to REs in one transmission symbol of the sub-frame in  FIG. 1 ). Then, MCS 2  is determined as having spectral efficiency SE 2 =X/(X−Y) SE 1 , where SE 1  is the spectral efficiency of MCS 1 , N RE =X for the initial transmission, and N RE =X−Y for the HARQ retransmission. 
         [0048]      FIG. 6  and  FIG. 7  illustrate exemplary embodiments that correspond to the extension of  FIG. 4  and  FIG. 5 , respectively, when UCI or SRS is included in both the initial transmission and in a HARQ retransmission. In  FIG. 6 , the REs required for UCI and SRS in the initial transmission  610  are less than the ones required in a HARQ retransmission  620  for the same TB. The UE uses “Tx Power  1 ”  612  for the PUSCH transmission power adjustment and Data MCS,  614  in the initial transmission and uses “Tx Power  2 ”  622  for the PUSCH transmission power adjustment and Data MCS 2    624  in the HARQ retransmission where “Tx Power  1 ” is higher than “Tx Power  2 ” and MCS 2  is higher than MCS 1 . 
         [0049]    In  FIG. 7 , the REs required for UCI and SRS in the initial transmission  710  are more than the ones required in a HARQ retransmission  720  for the same TB. The UE uses “Tx Power  1 ”  712  for the PUSCH transmission power adjustment and Data MCS 1    714  in the initial transmission and uses “Tx Power  2 ”  722  for the PUSCH transmission power adjustment and Data MCS 2    724  in the HARQ retransmission where “Tx Power  2 ” is lower than “Tx Power  1 ” and MCS 2  is lower than MCS 1 . 
         [0050]    If extreme data puncturing is required to accommodate UCI, data transmission may be entirely postponed. This may occur for power limited UEs having one of the lowest MCS when a PUSCH transmission is not adaptively scheduled and UCI or SRS needs to be included. In such cases, the PUCCH may be used for the UCI transmission, and the Node B scheduler may better utilize the PUSCH resources by allocating them to other UEs. 
         [0051]      FIG. 8  illustrates an exemplary embodiment of the present invention. Coded CQI bits and/or PMI bits  805  and coded data bits  810  are multiplexed through rate matching in an encoding/modulating unit  820 . If ACK/NAK bits are also multiplexed, coded data bits are punctured to accommodate ACK/NAK bits in puncturing unit  830 . The DFT of the combined data bits and UCI bits is then obtained in DFT unit  840 , the sub-carriers  850  for the assigned transmission BW are selected by control unit  855 , IFFT is performed in IFFT unit  860 , and the CP is inserted by CP unit  870 . Once the signal is ready to be transmitted, power control unit  880  selections the power amplification level as described above and controls power amplifier  885  to apply the selected power level to the transmitted signal  890 . For brevity, additional transmitter circuitry, such as digital-to-analog converter, analog filters, and transmitter antennas are not shown. Also, the encoding process for the data bits and the CQI and/or PMI bits, as well as the modulation process, are omitted for brevity. 
         [0052]    While the present invention has been shown and described with reference to certain exemplary 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 invention as defined by the appended claims.