Patent Publication Number: US-2019190765-A1

Title: Controller for detection of Bluetooth Low Energy Packets

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method detection of Bluetooth packets. In particular, the invention relates to detection of Bluetooth Low Energy (BLE) packets. 
     BACKGROUND OF THE INVENTION 
     In low power communications equipment, it is desired to reduce the power consumption requirements. For a battery powered network station, the current consumption governs battery life. For some communications protocols, such as the beacon frame of 802.11, it is possible to selectively power the station on during those times to save power. However, for a receiver operative on the Bluetooth protocol, the packets may asynchronously arrive, requiring that the network station be powered continuously. 
     It is desired to provide a method for reducing power consumption in a wireless Bluetooth receiver which may receive packets from remote stations, while ensuring that no such packets are missed. 
     OBJECTS OF THE INVENTION 
     A first object of the invention is a low power receiver for Bluetooth Low Energy (BLE) wireless packets, the BLE wireless packets having a Bluetooth preamble length of Tpre, the wireless receiver having a preamble detection time of Tpd, the low power receiver performing a series of variable length preamble detection cycles, each cycle of length Tcyc having a duration equal to or less than a shortest expected packet preamble to be detected, each Tcyc having an operative interval T1 for sampling a received energy level and comparing a previous value to a current value for an energy increase larger than a threshold, the low power receiver powering down during a subsequent T2 interval, the length of the T1 interval and T2 intervals being selected such that T1 is sufficient to allow detection of energy from a preamble followed by detection of the preamble itself, while reducing the consumed power during T2 intervals. 
     A second object of the invention is a controller for a receiver receiving Bluetooth wireless packets, the receiver providing samples of a baseband signal using an analog to digital converter, the controller operative over a series of cycles of T1 and T2 intervals, the controller powering the receiver on during each T1 interval and removing power from said receiver during each T2 interval, the controller sampling the baseband signal during T1 intervals to perform an automatic gain control (AGC) process and also determining whether an energy level increase occurred from a previous sample to a current sample, and asserting a packet detect and keeping power applied to the receiver when an energy level increase above a threshold occurs. 
     SUMMARY OF THE INVENTION 
     A receiver for Bluetooth Low Energy (BILE) packets has an analog front end (AFE) for amplification and conversion of received wireless signals to baseband, analog to digital converters to digitize the baseband signal, and an energy detector coupled to the analog to digital converter for detecting an energy rise in the baseband signal. The wireless receiver is powered on for a nominal interval T1 during which energy sampling occurs on the analog to digital outputs and then the receiver is powered down during a second interval T2, where T1+T2 has a cycle time Tcyc which is equal to, or shorter than, a preamble of the wireless packet to be detected, such that both energy detection and preamble detection may occur during the T1 interval. In an example embodiment, the wireless packets are sampled by an analog to digital converter for detection of energy increase from a previous sample to a current sample or over a history of samples to a current sample. In this manner, the receiver is able to detect a preamble in the shortened T1 interval and consume no power during the T2 interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a prior art Bluetooth Low Energy packet format. 
         FIG. 2  shows a block diagram for a Bluetooth receiver. 
         FIG. 3  shows a plot of waveforms for operation of an example energy detect controller of  FIG. 2 . 
         FIG. 4  shows a flowchart for a packet detection process operative on an energy detect controller. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a prior art Bluetooth Low Energy packet  100 , which has a preamble field  102  approximately 8 us long, which is followed by various fields of the packet  104 , including a 32 bit access address, variable length data part of the packet PDU, and a CRC for error checking and packet data validation. It is desired for the receiver to be powered on at periodic intervals to check for a preamble  106 , and if a preamble is present, remaining powered on to recover the remainder of the packet fields  104 , otherwise powering down until the next preamble detection period. 
       FIG. 2  shows an example RF receiver  200 , having an antenna  202 , RF front end  204  with low noise amplifier  206 , quadrature mixers  220  and  228 , local oscillators  230  and  224  for conversation of received RF to baseband, low pass (or optionally band pass) filters  208  and  210 , variable gain amplifiers  212  and  214  for performing gain control, filters  216  and  218 , and analog to digital converters  242  and  246 , which are operative at a sufficiently low sample rate to detect an increase in received RF energy such as from a Bluetooth packet. Energy detect controller  254  generates the various signals for controlling the power distribution and signal examination for the various signals required for the energy detection to occur. Many other signals are required for operation as a Bluetooth receiver, but exemplar  FIG. 2  is restricted to only the signals required for the operation of the invention. Phase lock loop (PLL) power  250  is an enable signal to provide power to the various PLLs and other oscillators which may require a settling time Tpll, which is approximately 6 us. Shortly after the PLL and other clocks are settled, RF/ADC power  252  is enabled so that all of the remaining functions required for preamble detection may occur. 
       FIG. 3  shows example waveforms for the operation of the invention and controller  254  of  FIG. 2 . Sampling of the baseband RF is performed using A/D converters  242  and  246  of  FIG. 2  which are operative on the baseband signal stream  302 , which contains an additive mixture of RF from Bluetooth packets, noise, and interference from other stations in a continuous stream. The preamble detection is performed by cyclically sampling the baseband  302  signal with A/D converters  242  and  246  at a low rate during an operative T1 sample interval  304  followed by a T2 interval  305  where the receiver is powered off and no power is consumed. The T1  304  sample interval and T2  305  power down interval cyclically occur in a duration Tcyc  303 , where Tcyc is equal to, or shorter than, the Bluetooth packet preamble. In the case of Bluetooth Low Energy, the packet preamble interval is 8 us long as shown in  FIG. 1 . 
     The BLE receiver  200  operates at 10 dB Signal plus Interference to Noise (SINR) ratio or higher. Signal to Noise ratios down to 6 dB can be reliability detected by checking for Power-rise on the Rx 1 MHz Filter output. This would save the power in the digital baseband processor  240  but it wouldn&#39;t save much power in the LNA, Rx Mixer, LO Buffer, Rx ABB and the ADC of the analog front end  204 . One possible approach is for the RF receiver and ADC to turn ON and settle within 1 us and to employ fine grained duty-cycling. An example T1/T2 duty cycling when the receiver is listening for advertising frames is T1=2 us and T2=N us OFF, where N can be even as high as 10 us during listen and use the receiver effectively as an in-band (1 MHz) energy rise sensor. This first approach of duty-cycling the receiver on during T1 and off during T2 directly provides 2 to 4× savings in the listen power. 
     In one example of the invention, T1=2 us and T2=2 us. In this example, the worst case scenario is the preamble is coincident with T2, so the preamble energy is first detected 2 us into sampling, which leaves 6 us of preamble for the AGC to settle prior to decoding the address field  103  of  FIG. 1 . A second example case of T1=2 us and T2=4 us reduces the duty cycle and increases the power savings, but creates a worst case sampling scenario where the preamble energy is first detected 4 us into the preamble (where the preamble starts coincident with T2), leaving only 4 us for the AGC process to complete, which is not enough time for the AGC process to complete, so the AGC process will be operative into the access address field  103  before completing the AGC process. This second case can still be used for Bluetooth LE advertising frames, which have separate channels and the access code  103  used for advertising frames is robust. It is acceptable to not properly decode the access address  103  the first time that a scanning receiver receives and advertising frame because the slotting timeline between the master and slave is not yet established. Accordingly, the indication of a false access code correlation is not a problem during the initial advertising scan. In the preceding methods, a fine-grained power T1/T2 cycling of power to the RF front end  204 , ADCs  242  and  246 , and gain control  236  can be used for “power-rise” detection of the received signal energy, where “fine-grained” refers to sample times which are less than ½ or ¼ of a preamble symbol time of 8 us or bit time of 1 us. For these embodiments with the 8 us preamble  102 , the RF PLL and any clock oscillators with a startup time are maintained in a powered up state as shown in waveform  308 . An example range for T1 BLE scan values is 2 us-3 us. Example ranges for T2 are 2 us to 10 us. Typical values for the settling time for RF receiver are 0.5 us to 2 us, which is the advance turn-on time for the receiver prior to the T1 listen interval. In an example embodiment, controller  254  provides fine gain control of T1 and T2. 
     Ordinarily, AGC is performed prior to preamble detect. In an example embodiment, the AGC process is only operative during T1 when the RF is turned ON. By adjusting the AGC in several steps and oversampling each symbol, the AGC may complete during T1. For example, for an incoming stream of BLE symbols S 1 , S 2 , S 3 , S 4 , S 5 , S 6 , S 7 , each BLE symbol 1 us in duration may have 2 or 4 or 8 samples based on ADC sampling rate of 2 MSps or 4 MSps or 8 MSps, respectively. By oversampling each BLE sample to complete the AGC within a single 1 us symbol, a power rise can be detected, which then starts the AGC process and start the packet detect process of verifying the receipt of  106  preamble by the baseband processor  240 . 
     In another example of the invention, the RF receiver and ADCs are turned off during the T2 period, with the clocking sources such as PLL and crystal oscillator continuing to run. During T1, the AGC is enabled, with the AGC process searching for the power rise in input signal. This is illustrated in the waveform  330 , with ADC samples  314 -S 1 ,  314 -S 2 ,  314 -S 3 ,  314 -S 4 ,  314 -S 5 ,  314 -S 6  and  315 -S 7 . In one example embodiment, each current sample Sn is compared to an adjacent symbol Sn−1 in the series of samples for each T1 interval as shown in  332 , and in another example embodiment, the comparison is done between a current sample and Sn−2 in the samples of  334 . By comparison of signal increase with a single T1 period ( 314 -S 1  to  314 -S 2  or  314 -S 3 ,  314 -S 5  to  314 -S 6  or  314 -S 7 , for example, or across T1 periods ( 314 -S 7  to  316 -S 1  or  316 -S 2 , for example), and by using a high rate of sampling (faster than 1 us per sample, so that multiple samples are taken from a single 1 us Bluetooth symbol) for the case of 2 us T1 and 2 us T2 with AGC performed over 2.5 us of preamble (extending just beyond T1), that would leave 3.5 us of preamble (worst case) for preamble detection and achieve close to 2× reduction in listen mode power. 
     In another embodiment of the invention, a one or two symbol buffer is placed in the sample path of the receiver, which would provide the ability for the preamble detector to start on a delayed copy of the stream of digitized signals. For example, the use of a 1 us buffer in the A/D path which precedes the receiver part would result in the loss of only 2.5 us of preamble in the worst case. In this embodiment, the AGC finetune of the last sample period should be applied by digital multiplication of the signal samples to avoid the time delay of analog AGC and to ensure the samples are presented with uniform gain adjustment. The increase in complexity of this approach is only valuable for non-advertising Bluetooth frames, as Bluetooth advertising frames do not use the access address field  103  which is affected by late agc completion. 
       FIG. 4  shows an example flowchart for the packet detect controller  254  of  FIG. 2 . During a T1 interval of the timing diagrams of  FIG. 3 , the receiver is powered ON  402  and the AGC process  404  is operating, both setting the signal level to an optimum level, and simultaneously making measurements of energy level, as shown in the sample series  314 -S 1  etc,  316 -S 1  etc, and  318 -S 1  etc. and power rise sample measurements of  332  and  334 . Step  406  of examining power rise may be done concurrent with AGC  404  or separately, if an energy detect event occurs, a Bluetooth preamble detection process  406  occurs, examining the preamble for the 0xAA pattern, and continuing on to packet demodulation  418  if found, otherwise returning to the process step  402  at the end of T2. If no energy increase is detected in step  406 , the receiver is powered off  408  for the T2 duration  410 , and the cycle repeats at step  402 . Because of the short preamble detection interval, oscillators and Phase lock loop (PLL) clock sources are continuously enabled through T1 and T2 to allow them to be operative during the T1 interval. 
     The present examples are provided for illustrative purposes only, and are not intended to limit the invention to only the embodiments shown. High speed and high frequency are understood to refer to the same characteristic, and low speed and low frequency are similarly understood to refer to the same characteristic. The use of claims terms such as “order of magnitude” is meant to include the range from 0.1× to 10× the nominal value, whereas “approximately” is understood to include the range of one half to two times the nominal value. The scope of the invention is limited only by the claims which follow.