How to choose an Application specific Signal Detection Threshold for TDA523x based ASK Mode Applications

Transcription

1 How to choose an Application specific Signal Detection Threshold for TDA523x based ASK Mode Applications The TDA523x has an integrated Signal Detector, a mechanism to distinguish in between useful data and noise. The most relevant part of the Signal Detector is the so-called Matched Data Filter. This digital filter compares the incoming signal stream with the expected waveform of Manchester coded bits (0, 1, space, mark) in the defined data-rate. The Matched Data Filter has a Signal Power Output, which is influenced by RF input power, and the shape and accuracy of the incoming bit-stream. In ASK-Mode, the decision between noise and data is done by comparing the signalpower of the filtered data signal against a user adjustable threshold. The chosen threshold of the Signal Detector influences Sensitivity, Dynamic Range and False Alarm Rate of the receiver system. This document describes how to determine optimal threshold setting of this unit in customer specific ASK-Mode Applications. At the end of this document there is also a Quick Guide, containing a short summary of required steps, and a simplified procedure. Version 1.4 tw 3/23/2006 Page 1/12

2 Equipment Required Arbitrary Signal Generator RF Waveform Generator Setup for reading and writing TDA523x registers Test environment similar to final use Anechoic or shielded chamber recommended Note: For the quick procedure Signal and Waveform Generator are not required Preparation Make sure that all SFR s of the TDA523x are set correctly. Use the available SW application tool for setting generation. TDA523x set to Run Mode Slave (SFR CMC0, 0x02 set to 0x22) System Properties and Definitions Message Error Rate (MER) indicates how many messages are received correctly. MER = 1 number of correctly received messages number of transmitted messages Sensitivity is the range of the signal strength at the antenna of the system, where the Message Error Rate is less or equal to 10%. Sensitivit y = signal strength MER 10 1 Dynamic Range is the range of the signal strength at the antenna of the system, where the Message Error Rate is lower than 0.1%. DynamicRan ge = signal strength MER 10 3 False Alarm Rate (FAR) is a quantum for how often the receiver mistakenly interprets noise present FAR = number of number of periods searching mistakenly wake ups for data on the channel Version 1.4 tw 3/23/2006 Page 2/12

3 Interdependency between False Alarm Rate and different Wake Up Criteria In Self-Polling Mode, while the TDA523x autonomously switches the receiver section between a low power sleep mode and active modes by following one of several available polling schemes, the TDA523x performs checks whether activity of interest was detected on one of the checked receive channels. If an activity of interest was detected, a so called Wake Up takes place and keeps the receiver in an active mode to allow the reception of the complete data. There are four different so called Wake-Up Criteria available for the decision. The criteria, which may be applied, possess different strength in supporting the Signal Detector in providing a low False Alarm Rate of the receiver system. A stronger Wake-Up Criteria improves the False Alarm Rate behavior. Four available Wake-Up Criteria are listed below, ordered from the strongest to the weakest: Pattern detection The data pattern received during the Wake-Up check has to match a predefined Manchester coded pattern. Equal Bits Detection The data pattern received during the Wake-Up check has to be either a Manchester coded sequence of ones or a Manchester coded sequence of zeros. Random Bit Detection The data pattern received during the Wake-Up check has to consist of a sequence of correctly Manchester coded bits. The length of the sequence is customizable. Valid Data Rate Detection The Wake-Up check is limited to test, if the data pattern received during the test allows the data clock recovery PLL to synchronize on it. Version 1.4 tw 3/23/2006 Page 3/12

4 Special Function Registers Involved Several Special Function Registers (SFR s) have to be either read out or written. These SFR s are: Note: Most of the registers below are twice available. Once for channel A, starting with character A, once for channel B, starting with character B. Bit Name Register Addr. Bits R/W Description SLRXEN CMC0 0x02 1 W Slave Receiver Enable, 0 receiver is in Sleep Mode, 1 receiver is in Run Mode MSEL CMC0 0x02 0 W Operating Mode Select, 0 Slave Mode, 1 Self Polling Mode Chip Mode Control Register should be set to 0x22 to enable Run Mode Slave and allow required measurements. Bit Name Register Addr. Bits R/W Description ASKNP ASKNP 0xB0 5:0 R ASK Noise Power. Signal Power after Matched Data filter. Read out to get signal and noise power for threshold calculation The lower 5 bit of this register contains the actual signal power figure required for threshold calculation. The register is updated 16 times per data-bit-clock. Bit Name Register Addr. Bits R/W Description SDCNT SDTHR ASIGDET0 BSIGDET0 ASIGDET0 BSIGDET0 0x71 0x91 0x71 0x91 7:6 W Signal Detector Threshold Counter. Allows to ignore eventually occurring bit failures. 00b:disabled (default), 01b: ½ bit, 10b:1 bit, 11b:2 bit 5:0 W Signal Detector Threshold Level. Used to set the Signal Detector Threshold as calculated in this application note. This is the Signal Detection Threshold Register for the Run Mode. Use ASIGDET0 if used for Channel A, use BSIGDET0 if used for Channel B. Use of Signal Detector Threshold Counter is not recommended in normal applications; therefore SDCNT should be set to 00b to disable the counter. Use the result of the calculation described in this application note for SDTHR. Bit Name Register Addr. Bits R/W Description SDLORE SDSEL ASIGDETLO BSIGDETLO ASIGDETLO BSIGDETLO 0xB6 0xB7 0xB6 0xB7 7 W Source selection of ASK Noise Power status register. Use 0 for ASK 6 W Manual selection of SIGDET0/1 range This SFR is mainly used for FSK. But two bits are also relevant for ASK. SDLORE should bet set to 0 for ASK, and SDSEL should be also set to 0 in the majority of applications. Only if the calculated SIGDET values exceed the range of 63 decimal, set SDSEL to 1 and select a range value as below. Version 1.4 tw 3/23/2006 Page 4/12

5 Bit Name Register Addr. Bits R/W Description SDSELLO SDSEL ASIGDETSEL BSIGDETSEL ASIGDETSEL BSIGDETSEL 0xB8 0xB9 0xB8 0xB9 3:2 W Used for FSK only 1:0 W SIGDET0/1 range selection factor 00b:4, 01b:6, 10b:8(default), 11b:10 Set SDSEL to fit the SIGDET range. Usually this setting is not required, and should not be touched. In case the later described calculation result is a value higher 63 decimal, manual SIGDET range should be selected and SDSEL values should be increased step by step. Bit Name Register Addr. Bits R/W Description SDCNT SDTHR ASIGDET1 BSIGDET1 ASIGDET1 BSIGDET1 0x72 0x92 0x72 0x92 7:6 W Signal Detector Threshold Counter. Allows to ignore eventually occurring bit failures. 00b:disabled (default), 01b: ½ bit, 10b:1 bit, 11b:2 bit 5:0 W Signal Detector Threshold Level. Used to set the Signal Detector Threshold as calculated in this application note. This is the Signal Detection Threshold Register for Wake Up. Use ASIGDET1 if used for Channel A, use BSIGDET1 if used for Channel B. Usually SIGDET0 and SIGDET1 should be equal if ASK is used as well for wakeup and run mode. Version 1.4 tw 3/23/2006 Page 5/12

6 Signal Detector Threshold The diagram below shows the signal power output of the Matched Data Filter, and the influence of the chosen threshold setting. The black curve in the uppermost diagram shows the transmission of data at the transmitter side of the system. The high level represents valid Manchester coded data transmission in the correct bit-rate. During the low level time there is no data transmission from the transmitter. All other diagrams symbolize the signal power present at the output of the matched datafilter of the receiver system available from the SFR ASKNP (ASK Noise Power, Addr. 0xB0, bit 5:0). The shown values are influenced by the signal power at the antenna, but also by the waveform accuracy of the bit-stream. Low levels mean noise, distorted or not fitting bits, high levels represent good data and reliable transmission. This signal power is compared against a given threshold in the Signal Detector. The result of this comparison is used by the Signal Detector to decide between noise and valid data bits. In the drawing above, the three lower diagrams describe the influence of the selected Signal Detector s threshold on the receiver s system properties. A too low threshold causes increased False Alarm rate, and may also increase MER. A too high threshold decreases sensitivity and dynamic range. The middle diagram symbolizes an optimal threshold setting with minimal false alarms and message errors. Version 1.4 tw 3/23/2006 Page 6/12

7 Procedure to determine a practical Signal Detector threshold in customized receiver systems Generally the optimal threshold setting is as low as possible to have optimal sensitivity, but well above noise level to prevent incorrect wake ups. Step One Characterization The signal power is updated with a rate of 16*f data and can be read by the user via special function register ASKNP (the name ASK Noise Power was chosen, to illustrate the major usage of this register). The characterization requires a continuous ASK-modulated, Manchester coded pattern containing zeros and ones (PRBS9 recommended) to be applied to the antenna input of the receiver system and a power-level sweep for this input signal to be performed. The level sweep starts with a level below the system s noise floor and ends when a saturation of ASKNP register readouts can be observed. Typically such a sweep starts with a power-level of -130 dbm, extends to a level between dbm and applies a step size of 1 db. Typically 500 single measurements should be done for every power-level. An exemplary output of a characterization as described is shown in the diagram below: Example: Center frequency MHz Bit-rate 9600bps Results of a measurement as above are shown in the following graphic: ASKNP over Input Signal Power PRBS9 Pattern ASKNP Input Power [dbm] average standard-deviation Version 1.4 tw 3/23/2006 Page 7/12

8 . Step Two Calculation Two numbers reflecting two major system parameters, signal-power and noise-power, are calculated from these measurements: Signal-power is the average of all measurements performed at a distinct power-level. Noise-power is the so-called standard deviation (sigma) of all measurements performed at a distinct power level. For our calculation the measurements at -130dBm to -125dBm are used (the measurements with higher levels are used to verify functionality of the receiver system). Threshold = Signal Power + ( λ Noise Power ) Signal Power average of measurements Noise Power standard-deviation of measurements (=sigma) Note: standard-deviation is calculated by (( x x ) ( 2 )... ( ) ) n 1 1 avg + x xavg + + xn x avg The factor λ in the rule above reflects the interdependence between the Signal-Detector s threshold and the strength of the additional Wake-Up criteria in place. To achieve a FAR of 10-5, the initial estimation for λ lies in the range of 2..3, depending on the Wake-Up criteria. Use λ =2.5 for Pattern Detection, λ =2.7 for Equal Bits Detection, λ =3 for Random Bit Detection and Valid Data Rate Detection. Use λ =2 only when FAR has low priority. Example: Using our example data, we calculated: Signal power = 11.8, noise power = 5.8, λ =2.7 (weak Wake Up criteria) Threshold = >> 27 decimal >> 0x1b Version 1.4 tw 3/23/2006 Page 8/12

9 ASKNP over Input Signal Power PRBS9 Pattern ASKNP Input Power[dBm] average standard-deviation average +3xsigma Threshold Step Three Verification The previous step yielded in an initial estimation for the Signal Detector threshold. The calculated value has now to be used for verified for practical use. Put special focus on: Sensitivity at center receive frequency Dynamic Range at center frequency False Alarm Rate Sensitivity over the complete receive frequency range Any system parameter, which is critical in a distinct application Example: The verification of our example showed a sensitivity of dBm, a dynamic range of dBm, and FAR was < The graphics below show sensitivity and dynamic range, and sensitivity over frequency offset. Version 1.4 tw 3/23/2006 Page 9/12

10 MER over Input Power 1,E+00 Message Error Rate 1,E-01 1,E-02 1,E Input Signal Power [dbm] Sensitivity over Frequency Range Sensitivity [dbm] ,72 433,77 433,82 433,87 433,92 433,97 434,02 434,07 434,12 Frequency [MHz] Step Four Optimization If all critical system properties have been verified and all targeted parameters were met, step four is obsolete and the determined Signal Detector s threshold can act as the basis for further system evaluation. The initial estimation for the Signal Detector s threshold may not always yield the fulfillment of all target parameters and thus an optimization of the threshold may be required. In this case, the current value of the threshold should be increased or decreased by one and the procedure should be continued with the previous step. The threshold value should be increased by one, if The assessment of the Dynamic Range shows a large number of Message Error Rate peaks of 10-3 or larger, although the power at the antenna of the system was substantially above the Sensitivity of the receiver system, when these Message Error peaks occurred. Version 1.4 tw 3/23/2006 Page 10/12

11 The False Alarm Rate target was not met. A loss of Sensitivity of the receiver system has to be expected in the case, that the Signal Detector s threshold is increased. The threshold value could be decreased by one, if All targeted system parameters where met in the preceding assessments. A gain in Sensitivity of the receiver system could possibly be achieved in the case, that the Signal Detector s threshold is decreased. Version 1.4 tw 3/23/2006 Page 11/12

12 Quick Guide Download TDA523x Config File via TDA523x Explorer, or configuration data via your application. Set TDA523x to Run Mode Slave by writing 0x22 to SFR CMC0 (address 0x02) Set RF Signal Generator to OFF. Start 500 readouts of SFR ASKNP (address 0xb0). Calculate average of readouts Signal Power, and standard deviation Noise Power Select a value for the factor λ between 1 and 3, depending on your wake up criteria and priority of False Alarm Rate. Typically use 2.5 to 3. For further information refer to the detailed part of this application note. Calculate Threshold = Signal Power + λ * Noise Power Load Threshold to SFR SIGDET.SDTHR (address 0x71, 0x91, 0x72, 0x92.5-0) Optimize Threshold. Increase by 1 if o Dynamic range shows Message Error peaks of 1E-3 or higher at high input signal levels. o False Alarm Rate target is not reached. Decrease by 1, if o All system parameters are met. A lower Threshold can mean higher Sensitivity and Dynamic Range. Version 1.4 tw 3/23/2006 Page 12/12