Abstract:
A communication receiving end for receiving an inputted signal includes a signal amplifier for adjusting the inputted signal according to a first predetermined gain or a second predetermined gain to generate a first adjusted signal; an analog-to-digital converter (ADC), coupled to the signal amplifier, for converting the first adjusted signal; and a control unit, coupled to the ADC, for determining whether the ADC is saturated or not according to an output of the ADC. The first predetermined gain is associated with a first inputted signal power processed by the communication receiving end and a quantization noise of the ADC. The second predetermined gain is associated with a second inputted signal power processed by the communication receiving end and a full scale level of the ADC. The first inputted signal power is smaller than the second inputted signal power.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a communication system, especially to a communication receiving end of a wireless communication system and an auto gain control method thereof. 
         [0003]    2. Description of Related Art 
         [0004]      FIG. 1  illustrates a block diagram of a conventional signal receiving end. Because the signal power received at the receiving end is unknown, the receiving end has to rapidly complete the adjustment of the gain of the signal amplifier  110  so as not to interfere with the reception of the signal. The success rate of the cell search is subject to the speed at which the gain control unit  140  adjusts the gain. Only when the cell search is complete, the post-stage circuit  150  then processes the inputted signal that is already converted to a digital format. In a conventional gain adjusting process, the power estimation unit  130  calculates the power of the inputted signal, and then the gain control unit  140  compares the obtained signal power with a reference power to obtain a gain difference, which is subsequently fed back to the signal amplifier  110 . When the analog-to-digital converter (ADC)  120  becomes saturated (i.e., the gain of the signal amplifier  110  is too large), and no other information is available to obtain an appropriate gain, a converging process by a progressively decreasing approach is employed to obtain the appropriate gain—this mechanism is known as a close-loop control mechanism. However, in a time-division duplexing long-term evolution (TDD-LTE) system, the configurations of uplink (UL) and downlink (DL) are unknown before synchronization. The powers of the UL signal and the DL signal differ greatly, in a way that the signal passing the signal amplifier  110  may saturate the ADC  120  if the gain of the signal amplifier  110  is too large or that the signal passing the signal amplifier  110  becomes so small that the converted signal generated by the ADC  120  fails to precisely reflect the content of the signal if the gain of the signal amplifier  110  is too small. Further, in a frequency-division duplexing long term evolution (FDD-LTE) system, different numbers of resource blocks (RBs) may be used in different DL subframes, causing the power of a received signal to vary from subframe to subframe. For the above reasons, the conventional gain controller that employs the close-loop control mechanism has to adjust the gain back and forth for several times before the gain converges; this slow and repetitive procedure decreases the success rate of the cell search dramatically. 
       SUMMARY OF THE INVENTION 
       [0005]    In view of the issues of the prior art, an object of the present invention is to provide a communication receiving end and an auto gain control method thereof to increase the success rate of cell search, so as to make an improvement to the prior art. 
         [0006]    One of the multiple objects of this invention is to provide a communication receiving end and an auto gain control method thereof to reduce the required time for the cell search. 
         [0007]    The present invention discloses an auto gain control method of a communication receiving end. The communication receiving end comprising an ADC. The method comprises the steps of: adjusting an inputted signal according to a first predetermined gain; determining whether the adjusted inputted signal saturates the ADC; and adjusting the inputted signal according to a second predetermined gain. The first predetermined gain is associated with a first inputted signal power processed by the communication receiving end and a quantization noise of the ADC, the second predetermined gain is associated with a second inputted signal power processed by the communication receiving end and a full scale level of the ADC, and the first inputted signal power is smaller than the second inputted signal power. 
         [0008]    The present invention also discloses a communication receiving end for receiving an inputted signal, comprising a first signal amplifier, a first ADC, and a control unit. The first signal amplifier adjusts the inputted signal according to a first predetermined gain or a second predetermined gain to generate a first adjusted signal. The first ADC performs analog-to-digital conversion on the first adjusted signal. The control unit determines whether the first ADC becomes saturated according to an output of the first ADC. The first predetermined gain is associated with a first inputted signal power processed by the communication receiving end and a quantization noise of the first ADC. The second predetermined gain is associated with a second inputted signal power processed by the communication receiving end and a full scale level of the first ADC. The first inputted signal power is smaller than the second inputted signal power. 
         [0009]    The present invention further discloses a gain control method, applied to a gain control circuit of a communication receiving end. The gain control method comprises the steps of: in a first phase, utilizing a non-close-loop mechanism to perform gain control on the gain control circuit; and in a second phase subsequent to the first phase, utilizing a close-loop mechanism to perform gain control on the gain control circuit. 
         [0010]    The communication receiving end and the auto gain control method of this invention utilize the reference signal appearing regularly in LTE transmission signals as a reference for the cell search, and determine the high and low gains used in the automatic gain control according to the allowed maximum and minimum inputted signal powers at user equipment (UE) defined in the specification of the LTE communication system and the characteristics of the ADC in use. As opposed to the prior art, this invention provides a more stable automatic gain control mechanism. 
         [0011]    These and other objectives of the present invention no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiments with reference to the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates a block diagram of a conventional signal receiving end. 
           [0013]      FIG. 2  illustrates the frame structure of the FDD-LTE communication system as well as the distribution of a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). 
           [0014]      FIG. 3  illustrates the frame structure of the TDD-LTE communication system as well as the distribution of an PSS and an SSS. 
           [0015]      FIG. 4  illustrates a functional block diagram of a signal receiving end of the LTE communication system according to an embodiment of this invention. 
           [0016]      FIG. 5  illustrates a flowchart of automatic gain control corresponding to  FIG. 4 . 
           [0017]      FIG. 6  illustrates another functional block diagram of a signal receiving end of the LTE communication system according to an embodiment of this invention. 
           [0018]      FIG. 7  illustrates a flowchart of automatic gain control corresponding to  FIG. 6 . 
           [0019]      FIG. 8  illustrates a relationship between the high gain H and the low gain L of the automatic gain control and a dynamic range of an ADC. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0020]    The following description is written by referring to terms of this technical field. If any term is defined in this specification, such term should be explained accordingly. In addition, the connection between objects or events in the below-described embodiments can be direct or indirect provided that these embodiments are practicable under such connection. Said “indirect” means that an intermediate object or a physical space exists between the objects, or an intermediate event or a time interval exists between the events. 
         [0021]    From the time-domain perspective, each frame of the frame structure of a long-term evolution (LTE) communication system has a duration of 10 ms, and includes 10 subframes each having a duration of 1 ms. Each subframe is further divided into 2 slots. As the lengths of the cyclic prefixes (CP) defined by the system differ, each slot has different numbers of Orthogonal Frequency Division Multiplexing (OFDM) symbols. According to the specification of the LTE communication system, a slot contains 7 OFDM symbols for a normal CP whereas a slot contains 6 OFDM symbols for an extend CP. 
         [0022]    The Orthogonal Frequency Division Multiple Access (OFDMA) technology is employed in the downlink of the LTE communication system, and there are several system bandwidths to choose from, such as 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. The channel bandwidth is decided by local system operators. Taking 20 MHz for example, there are a total number of 2048 subcarriers, in which 1200 close to the central frequency are data-carrying subcarriers. Because 12 successive subcarriers form a resource block (RB), the bandwidth of 20 MHz is equivalent to the total width of 100 resource blocks. A base station realizes multiplexing by distributing the 100 resource blocks to different user equipments (UEs). The length of time of one resource block is a slot; i.e., one resource block comprises the data carried by 12 successive subcarriers in one slot. 
         [0023]    In addition, in the FDD-LTE communication system, the uplink and the downlink transmit and receive signals at the same time at different radio frequencies. However, in the TDD-LTE communication system, as both the uplink and the downlink use the same radio frequencies, transmitting and receiving cannot be carried out at the same time; i.e., transmitting and receiving are carried out in different subframes. As a result, in the TDD-LTE communication system, the UE has to switch between uplink and downlink frequently (i.e., two successive subframes correspond respectively to uplink and downlink). Moreover, because the required transmitting powers for the uplink and the downlink are different, a more accurate control of the automatic gain in the LTE communication system becomes necessary to ensure correct signal transmitting and receiving. 
         [0024]      FIG. 2  illustrates the frame structure of the FDD-LTE communication system as well as the distribution of a primary synchronization signal (PSS) and a secondary synchronization signal (SSS). In  FIG. 2 , only one frame of a continuous signal is illustrated in the time domain and only 6 sections, each comprising 12 subcarriers, of a plurality of subcarriers are illustrated in the frequency domain. In fact, the signal extends in both the time domain and the frequency domain. The PSS and the SSS are periodic with a period of 5 subframes. As illustrated in the enlarged view of the 1 st  subcarrier section of the 0 th  subframe (the current example corresponding to a normal CP, i.e., one slot comprising 7 OFDM symbols), the PSS and the SSS are respectively located on the 6 th  and 5 th  OFDM symbols of the 1 st  slot. Prior to the PSS and the SSS, a periodic reference signal is presented. For a normal CP, the reference signal appears on the 0 th  and the 4 th  OFDM symbols of one slot; whereas for an extend CP, the reference signal appears on the 0 th  and the 3 rd  OFDM symbols of one slot; that is, in fact the reference signal appears every 3 to 4 OFDM symbols. 
         [0025]    Similarly, the PSS, the SSS and the reference signal in the TDD-LTE communication system are also periodic. As shown in  FIG. 3 , the PSS and the SSS appear every 5 subframes. As illustrated in the enlarged view of the 1 st  subcarrier section of the 0 th  and 1 st  subframes, the PSS and the SSS are separated by 2 OFDM symbols.  FIG. 3  also corresponds to normal CPs. Therefore, the reference signal appears on the 0 th  and 4 th  OFDM symbols of one slot; whereas for extend CP, the reference signal appears on the 0 th  and 3 rd  OFDM symbols of one slot. 
         [0026]    Based on the aforementioned regularity, a count period T can be set in the UE. In this count period T, the power Ps of the received signal is estimated at the UE. The gain is adjusted according a comparison between the power Ps and a predetermined reference power Pr, and whether the gain should be adjusted again is determined according to a change in the power Ps. In this way, the UE is able to complete the setting of the gain of the signal amplifier before the reception of the PSS and the SSS to ensure the synchronization signals can be correctly received. In addition to the aforementioned PSS, SSS and reference signal, for both the FDD-LTE communication system and the TDD-LTE communication system, the DL channel further includes signals of a physical downlink control channel (PDCCH) and a physical data share channel (PDSCH) in the physical layer. However, because the number of OFDM symbols occupied by the PDCCH signal is not constant and the number of resource blocks in the PDSCH signal can be any value, before performing automatic gain control in the LTE communication system, these uncertainties of the PDCCH signal and the PDSCH signal as well as the dramatic change in the signal power of the received signal caused by switching between the uplink and downlink configurations in the TDD-LTE communication system have to be overcome. 
         [0027]      FIG. 4  illustrates a functional block diagram of a signal receiving end of the LTE communication system according to an embodiment of this invention, and  FIG. 5  is a flowchart of corresponding automatic gain control. The signal receiving end of the LTE communication system comprises a control unit  201 , a signal amplifier  210 , an ADC  220 , a power estimation unit  230 , a gain control unit  240  and a post-stage circuit  150 . In the beginning, the entire system is restarted; for example, the control unit  201  erases temporary data and resets counters and the signal power Ps of the power estimation unit  230  (step S 410 ). Then the control unit  201  controls the signal amplifier  210  to receive the inputted signal at a high gain H and causes the counter to start counting, and the power estimation unit  230  estimates the signal power Ps of the inputted signal (step S 420 ). Before the count value C of the counter reaches the count period T, the control unit  201  continuously monitors whether the inputted signal saturates the ADC  220 . An occurrence of signal clipping stands for the saturation of the ADC  220  (steps S 430  and S 440 ). If these two steps are both negative, the signal amplifier  210  continues receiving the inputted signal at the high gain H and the power estimation unit  230  continues estimating the signal power Ps of the inputted signal (step S 420 ). If in the above steps the ADC does not become saturated and the count value C of the counter reaches the count period T (step S 440  being affirmative), the gain control unit  240  sets the gain of the signal amplifier  210  by comparing the detected signal power Ps with the predetermined reference power Pr. As a result, the inputted signal is adjusted by the signal amplifier  210  to a state that is more suitable for the sampling process of the ADC  220  (step S 470 ). However, if the ADC  220  becomes saturated in the above steps (step S 430  being affirmative), the control unit  201  immediately switches to a low gain L to receive the inputted signal. More specifically, the control unit  201  immediately resets the counter and the signal power Ps (step S 450 ), controls the signal amplifier  210  to receive the inputted signal at the low gain L, and controls the counter to count from the beginning, and the power estimation unit  230  re-estimates the signal power Ps of the inputted signal (step S 455 ). Similarly, in the count period T, the signal amplifier  210  continues receiving the inputted signal at the low gain L and the power estimation unit  230  continues estimating the signal power Ps of the inputted signal. When the count value C of the counter reaches the count period T (step S 460  being affirmative), the gain control unit  240  sets the gain by comparing the detected signal power Ps with the predetermined reference power Pr (step S 470 ). In other words, a “non-close-loop” is employed in the steps S 410  to S 470  to carry out automatic gain control. The completion of the step S 470  means that the adjustment of the gain is completed, so the system is able to receive the inputted signal according to the ideal gain. However, if the power of the inputted signal changes afterwards, for example, switching from the uplink state to the downlink state in the TDD-LTE communication system, or vice versa, the gain needs to be determined again to prevent the old gain from saturating the ADC. Therefore, after the gain is set, the control unit  201  continues monitoring the signal power Ps further based on the calculation of the power estimation unit  230  (step S 480 ). When the change in two consecutive estimations of the signal power Ps exceeds a predetermined threshold (step S 490  being affirmative), the control unit  201  carries out the automatic gain control again (back to step S 410 ). In one embodiment, a close-loop mechanism is employed in the step S 480  to conduct the automatic gain control. 
         [0028]    In another embodiment where the UE comprises two sets of receiving circuits, as shown in  FIG. 6 , the signal receiving end of the LTE communication system of this invention further comprises a signal amplifier  215 , an ADC  225  and a power estimation unit  235 . Its corresponding flowchart of automatic gain control is shown in  FIG. 7 . After the system restarts (step S 410 ), the control unit  201  controls the signal amplifiers  210  and  215  to receive the inputted signal at the same time. One of the signal amplifiers  210  and  215  employs the high gain H whereas the other employs the low gain L. And the power estimation units  230  and  235  also start to estimate the signal power Ps (step S 420  and S 455 ). In other words, the estimations of signal power for both the high gain mode and the low gain mode can be conducted at the same time. In the following, it is assumed that the signal amplifier  215 , the ADC  225  and the power estimation unit  235  are in the high gain mode, whereas the signal amplifier  210 , the ADC  220  and the power estimation unit  230  are in the low gain mode. Before the count value reaches the count period (i.e., step S 440  being negative), the control unit  201  monitors whether the ADC  225  becomes saturated according to the output of the ADC  225  (step S 435 ). After the estimations of the signal power of both the high gain mode and the low gain mode are complete (step S 440  being affirmative), the gain is set according to whether the ADC  225  is saturated (step S 475 ). More specifically, if the ADC  225  is not saturated, in the step S 475 , the control unit  201  outputs the signal power Ps that the power estimation unit  235  estimates to the gain control unit  240  for determination, or else the control unit  201  outputs the signal power Ps that the power estimation unit  230  estimates to the gain control unit  240 . In other words, the “non-close-loop” is employed in steps S 410  to S 475  to conduct automatic gain control. Likewise, the signal power Ps is continuously monitored in steps S 480  and S 490 . To sum up, when the UE comprises two sets of receiving circuits, the converging process of automatic gain control is accelerated to one-half of the original time required. In one embodiment, a close-loop mechanism is employed in steps S 480  and S 490 . 
         [0029]    Setting the high gain H and the low gain L is associated with the characteristics of the ADC and the LTE communication system. In the specification of the LTE communication system, the maximum inputted signal power P max  allowed by the UE is −25 dBm, and the minimum inputted signal power P min  (a.k.a. lowest sensitivity of the cell search) is defined as received average power of resource elements that carry synchronization signal (SCH_RP), which is −127 dBm. Because the synchronization signal carries 62 subcarriers, the corresponding signal power is P min =−127+10×log 10 (62)=−109 dBm. The full scale level P of the ADC is: 
         [0000]        P= 10×log 10 ( 1/50)+30=13.01 dBm  (1)
 
         [0000]    The value 50 represents ADC input impedance, and the value 30 represents a conversion of dB to dBm. Assuming that the effective number of bits of the ADC is B, the quantization noise power of the ADC is: 
         [0000]        Q=P −(6.02× B+ 1.76)dBm  (2)
 
         [0000]    Referring to  FIG. 8  for better understanding how the high gain H and the low gain L are determined in this invention according to the dynamic range of the ADC, the maximum inputted signal power P max  and the lowest sensitivity of the cell search P min .
       1. The key to setting the low gain L is that, the power, after being amplified by the signal amplifier, should not exceed the signal saturation level S when the power of the inputted signal is P max . The signal saturation level S represents an allowed value for the ADC not to become saturated during operation, and a difference between the signal saturation level S and the ADC full scale level P should be more than the Peak to Average Power Ratio (PAPR) of the -received signal; that is, the signal saturation level S=P−PAPR. The low gain L can be decided as L=S−P max .   2. The key to setting the high gain H is that, the power, after being amplified by the signal amplifier, should fall within a tolerance error of estimating the signal power when the power of the inputted signal is P min . Assuming that the tolerance error of estimating the signal power is Δx dB, to avoid the signal power estimation error caused by the quantization noise, the minimum inputted signal, after being amplified by the gain at the frontend, should be higher than the quantization noise power by ΔQ.       
 
         [0000]    
       
         
           
             
               
                 
                   
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                 That is, the effective signal level E=Q+ΔQ. As a result, the lower limit of the high gain H can be determined to be H≧E−P min . Meanwhile, to ensure that the requirement of the tolerance error Δx is still met at a switching point from high gain to low gain, i.e., switching immediately from high gain to low gain as the received signal saturates the ADC in a high gain mode, the high gain H must satisfy S−H+L≧E. To sum up, the high gain H should be within the range: E−P min ≦H≦S−E+L. 
               
             
             3. To obtain a stable estimation of the received power, an appropriate count period T must be selected. As shown in  FIGS. 2 and 3 , for whether the FDD-LTE communication system or the TDD-LTE communication system, each subframe comprises a total number of 4 OFDM symbols for transmitting the reference signal, which are respectively on the 0 th , 4 th , 7 th  and 11 th  OFDM symbols for a normal CP and on the 0 th , 3 rd , 6 th  and 9 th  OFDM symbols for an extend CP. The counting interval of the counter should cover at least one reference symbol to avoid a serious error in the estimation of the received power when the resource block does not transmit data. In other words, the count period T should be equal to or greater than 4 OFDM symbols for a normal CP and be equal to or greater than 3 OFDM symbols for an extend CP. According to the flow of  FIG. 4 , the gain can be determined in at most 2 count periods (2T). To be specific, for the TDD-LTE communication system (as shown in  FIG. 3 ), there are 13 OFDM symbols, which cover more than three count periods (3T), from the beginning of a frame to the arriving of the synchronization signals, so the system is ensured to become stable within these 3 gain adjusting opportunities. Alternatively, according to the flow of  FIG. 5  where the UE comprises 2 sets of receiving circuits, ideally the gain can be determined within one count period T; 
             4. In normal counting, the count value C of the counter can be expressed as: C=mod(C+1, T); 
             5. To determine the reference power Pr, the transmission density of the resource blocks in one subframe has to be taken into account. In light of a situation where in one count period T there are only reference signals while the resource blocks do not transmit data at all, a margin should be saved for the reference power Pr to avoid saturation. As a result, the reference power Pr can be expressed as: 
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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                           r 
                         
                         = 
                         
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                 where N sym  is the number of OFDM symbols in one count period T, N RB  is the number of resource blocks in one subframe, 12 represents the number of subcarriers in one resource block, and 2 represents the number of subcarriers occupied by the reference signal in one resource block; 
               
             
             6. The estimation of the signal power Ps can be the Received Signal Strength Indicator (RSSI) provided by the radio frequency circuit, which can be represented at a digital terminal as: 
           
         
       
     
         [0000]    
       
         
           
             
               
                 
                   
                     
                       P 
                       s 
                     
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                 where x(t) is a digital inputted signal and T is the count period. 
               
             
           
         
       
     
         [0039]    Taking an ADC having an effective number of 12 bits as an example, the high gain H and the low gain L can be obtained by the following steps:
       1. defining a dynamic range of the ADC with P=13.01 dBm and Q=−60.99 dBm;   2. setting PAPR to be 12 dB, then S=13.01−12=1.01 dBm, and L=13−12−(−25)=26 dB;   3. setting the tolerance of the signal power to be 1 dB, ΔQ=5.86 dB, E=−60.99+5.86=−55.13 dBm, then the range of H being 53.87≦H≦82.14 and setting H=60 dB;   4. calculating the switching point from high gain to low gain: 13.01−12−60=−58.99 dBm.       
 
         [0044]    Since people of ordinary skill in the art can appreciate the implementation detail and the modification thereto of the present method invention of  FIG. 5  and  FIG. 7  through the disclosure of the device invention of  FIGS. 4 and 6 , repeated and redundant description is thus omitted. Please note that there is no step sequence limitation for the method inventions as long as the execution of each step is applicable. Furthermore, the shape, size, and ratio of any element and the step sequence of any flow chart in the disclosed figures are exemplary for understanding, not for limiting the scope of this invention. 
         [0045]    The aforementioned descriptions represent merely the preferred embodiments of the present invention, without any intention to limit the scope of the present invention thereto. Various equivalent changes, alterations, or modifications based on the claims of the present invention are all consequently viewed as being embraced by the scope of the present invention.