Patent Publication Number: US-2015063504-A1

Title: Digital Receiver System Activated by RSSI Signal

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This application relates to a digital receiver, and more specifically, to turning off digital circuits within the digital receiver according to a received signal strength indicator (RSSI). 
     2. Description of the Prior Art 
     In typical receiver system for a digital communication system, there area radio frequency (RF) and analog unit, and a digital signal processing unit. Analog to digital converters (ADC) are used to convey transmitted information from analog domain to digital domain for digital processing and to connect these units in hardware. Within the RF and analog unit, a circuit called RSSI is often included to determine the received signal or interference level within a certain predetermined signal bandwidth. In addition to determining the signal or interference level, RSSI is also used for clear channel assessment and automatic gain control. 
     In a digital communication system, the transmitted signal must use certain data patterns, for example a predefined preamble, so that the receiver can sync and decode the transmitted signal. However, in the real world, due to noise, interference, and distortion, the transmitted data package can get lost. There are two scenarios resulting in a lost package. The first scenario is missed detection, which means that when the transmitted data package is presented at the receiver, the receiver cannot correctly receive and decode it. The second scenario can occur after a false alarm, which means that when there is no data package presented at the receiver, the digital processing unit mistakenly starts to process a nonexistent package. For example, the digital processing unit may receive an erroneous signal due to noise or the like and begin processing the erroneous signal. Since it takes a certain amount of time for the digital processing unit to determine that the erroneous signal is not a valid packet but a false alarm, if a real data package arrives when the receiver has not yet recovered from the false alarm, missed detection of the real data package may occur. 
     Please refer to  FIG. 1 , which illustrates a false alarm scenario  100 . In  FIG. 1 , packets  110  and  130  are being received sequentially from left to right. The first two packets  110  are received and processed normally. At time point  120 , a false alarm occurs. Shortly after the false alarm  120  occurs, packet  130  arrives at the receiver. However, detection of the packet  130  is missed because it arrived during a time window shown as T drop  where the digital processing unit is recovering from the false alarm  120  and is unable to correctly process the packet  130 . Therefore, it is very necessary to reduce or remove the above mentioned situation. 
     SUMMARY OF THE INVENTION 
     A digital receiver is proposed that comprises a radio frequency analog front end, a digital processing unit, a plurality of cascaded amplifier stages configured to receive output of the radio frequency analog front end, a first analog to digital converter configured to convert an analog signal output from the plurality of cascaded amplifier stages into a digital signal output to the digital processor, a first received signal strength indicator unit configured to receive outputs of each of the plurality of cascaded amplifier stages and output signal to the digital processing unit, a second received signal strength indicator unit configured to receive output of at least one amplifier stage in the plurality of cascaded amplifier stages, and a received signal strength indicator detection unit configured to activate and to deactivate the digital processing unit according to a comparison of output from the second received signal strength indicator unit to a predetermined threshold. The digital receiver may be further configured to deactivate the first received signal strength indicator unit when the digital processing unit is deactivated by the received signal strength indicator detection unit. 
     Another digital receiver is proposed that comprises a radio frequency analog front end, a digital processing unit, a plurality of cascaded amplifier stages configured to receive output of the radio frequency analog front end, a 1-bit analog to digital converter configured to convert an analog signal output from the plurality of cascaded amplifier stages into a digital signal output to the digital processor, a first received signal strength indicator unit configured to receive outputs of each of the plurality of cascaded amplifier stages and output signal to the digital processing unit, a second received signal strength indicator unit configured to receive output of at least one amplifier stage in the plurality of cascaded amplifier stages, and a received signal strength indicator detection unit configured to activate and deactivate the digital processing unit according to a comparison of output from the second received signal strength indicator unit to a predetermined threshold. The digital receiver may be further configured to deactivate the first received signal strength indicator unit when the digital processing unit is deactivated by the received signal strength indicator detection unit. 
     A method of operating a digital receiver comprising a radio frequency analog front end, a digital processing unit, a plurality of cascaded amplifiers configured to receive output of the radio frequency analog front end, a first analog to digital converter configured to convert an analog signal output from the plurality of cascaded amplifiers into a digital signal output to the digital processor, and a first received signal strength indicator unit configured to receive outputs of each of the plurality of cascaded amplifiers and output signal to the digital processing unit is proposed. The method comprises receiving output of at least one amplifier in the plurality of cascaded amplifiers to generate an received signal strength indicator signal, comparing the received signal strength indicator signal to a predetermined threshold to generate a comparison result, and activating the digital processing unit according to a the comparison result. The digital receiver may be further configured to deactivate the first received signal strength indicator unit when the digital processing unit is deactivated by the received signal strength indicator detection unit. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a missed packet due to a false alarm. 
         FIG. 2  illustrates an expanded RSSI curve according to one embodiment. 
         FIG. 3  is a functional block diagram of a digital receiver according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In a receiver system having an RF/analog front end, the RF signal may be down converted to intermediate frequency (IF) and then amplified. The IF amplifier comprises cascaded multi-stage amplifiers, and the regular RSSI unit can plot a curve corresponding to the strength of the signal according to the output signal of each IF amplifier. In order to have a high dynamic range of RSSI, a large number of stages are required, resulting in high power consumption. 
     Please refer to  FIG. 2  that illustrates an expanded RSSI curve according to one embodiment.  FIG. 2  shows the RSSI in volts as the vertical axis, and power in a power ratio in decibels of the measured power referenced to one milli-watt (dBm) as the horizontal axis. The expected power region for RSSI detection extends from “A” to “B” but may be extended to the vertical line shown as Receiver Sensitivity Level  230 . Unfortunately, the regular RSSI curve  210  is shaped somewhat like a slash (“\”) during the linear region, and flat at the edges of the dynamic range. This means it is difficult to define a threshold direct current (DC) level for a comparator at a signal power level lower than the sensitivity level  230  because there is little change in voltage (from “C” to “D”) of the RSSI within this region and a wide range of power would correspond quite closely to the DC level. Choosing a DC level that falls within the linear region of the RSSI curve may solve this problem, but the linear region of the RSSI curve falls in the signal power level higher than the sensitivity level and it is not acceptable to sacrifice the sensitivity of a receiver. Additional amplifier stages can extend the linear region to a wanted power level, but it also means more power consumption when the receiver is not receiving packets. 
     To extend the useful dynamic range of the RSSI curve without the need for a larger number of RSSI stages, the use of a second RSSI curve  220  is proposed to activate and deactivate the digital unit to reduce false alarms while also reducing power consumption. Since only the second RSSI with lower stages needs to be turned on for detection, the regular RSSI can be turned off during detection and turned on only when the digital unit is needed to sample the RSSI value. 
     Preferred characteristics of the second RSSI curve  220  are low power consumption, have a sharp RSSI curve and the linear region must fall in the signal power level lower than the sensitivity level. This can be accomplished by adjusting the second RSSI curve  220  at the very low power end by using output from at least one last-stage amplifier as input to a second RSSI unit used for this purpose and power saving. The RSSI value is basically the multiplication of a full-wave rectifier output current and load resistor, adjusting the current or load resistor can change the shape of RSSI curve. In the regular RSSI curve  210 , the curve of the low power region (less than “B”) only occupies less than 5% of the curve (“C” to “D”). The second RSSI curve  220  could make the small DC range (“C” to “D”) extend to full range (“C” to zero volts) and discard the high power end, forming a much sharper curve. With proper design, the linear region can be farther lower than the sensitivity level. 
     Please refer to  FIG. 3 , which is a functional block diagram of a digital receiver  300  according to one embodiment. The digital receiver  300  may comprise a channel filter  310  which outputs a received analog signal to a chain of cascaded amplifiers  320 . The analog output of the chain of cascaded amplifiers  320  may be converted into data by a first 1-bit analog-to-digital converter (ADC) and sent to the digital unit  360  for processing. Output of each amplifier stage  320  in the chain is also sent to the first RSSI unit  340  which outputs the corresponding signals to a second ADC  350 . The ADC  350  then outputs corresponding signals to the digital unit  360 . 
     One of the differences between the digital receiver  300  and digital receivers in the prior art is that the digital receiver  300  also may include a second RSSI unit  370  coupled to receive output of at least one of the last amplifier stages  320  in the chain of amplifiers  320 . Because input to the second RSSI unit  370  is limited to coming from only the at least one of the last amplifier stages  320  in the chain, a different RSSI curve is generated than is generated by the first RSSI unit  340  which receives input from all of the amplifier stages  320 . When compared to the first RSSI unit  340 , the second RSSI unit  370  generates a sharper RSSI curve with the linear region falling in the signal power level lower than the sensitivity level as desired. An example of one possible sharper RSSI curve  220  is shown in  FIG. 2 . 
     Output of the second RSSI unit  370  is sent to the RSSI detection unit  380  where the output is compared to a predetermined threshold lower than the digital receiver&#39;s  300  sensitivity level. Only when the threshold is met or exceeded, the RSSI detection unit  380  causes the digital unit  360  to be turned on, otherwise when the threshold is not met or exceeded, the RSSI detection unit  380  causes the digital unit  360  to be turned off. 
     Because the RSSI curve generated by the second RSSI unit  370  has the linear portion of the RSSI curve lower than the sensitivity level, a suitable threshold indicating whether the received packet is a false alarm can be found experimentally that does not sacrifice the sensitivity of a receiver. Since the digital unit  360  is only turned on when a packet meets or exceeds the predefined threshold, the digital unit  360  does not waste the time Tdrop  recognizing and recovering from false alarms, and missed packets such as packet  130  shown in  FIG. 1  are no longer missed and can be processed normally. 
     These embodiments further are able to reduce power consumption in at least three ways. Firstly, because the digital unit  360  is only turned on when needed, power consumption is reduced by turning the digital unit  360  off when the RSSI detection unit  380  determines the digital unit  360  is not currently needed. Secondly, by limiting input to the second RSSI unit  370  to coming from only at least one of the last amplifiers  320  in the chain, no additional amplifier stages  320  to extend the dynamic range (and additional power consumption) are needed to avoid the false alarms. Thirdly, when the digital unit  360  is turned off, the first RSSI unit  340  can also be turned off because it is also not currently needed. 
     A high sensitivity, low dynamic range, and low power consumption second RSSI (or activation RSSI) unit  370  is presented. The dynamic range is located in the signal power level lower than the digital receiver&#39;s  300  sensitivity level, so that the digital unit  360  can be activated earlier and only when required, and prepare for receiving packet. The value of the output of the second RSSI unit  370  is compared with a predetermined threshold that corresponds to a specific power level. A full range constant voltage reference circuit may provide the predetermined threshold level, in other words, defines the turned on power level of digital units. Once the second RSSI unit  370  output value meets or exceeds the predetermined threshold, the output of the RSSI detection unit  380  pulls high and activates digital unit  360 . When output of the RSSI detection unit  380  is low so that the digital unit  360  is deactivated, the regular RSSI unit  340  may also be deactivated. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.