1. Gain (dB) Benefits –Resistance to jamming –Resistance to detection –Sharing of channels among multiple users Applications –Cellular code-division multiple-access networks –Tactical military radio networks –Wireless local area networks with high data rate Acquisition of a DS packet transmission –Timing uncertainty of arriving signal –Receiver must achieve synchronization to demodulate data –Can be limiting factor in communication system performance Model delay caused by Automatic Gain Control (AGC) system’s delay in responding to change in signal power Analyze the effect of the AGC delay on acquisition performance Matched filter input is convolved with local copy of preamble Ideally, matched filter output peaks when last chip of incoming (matched) preamble sequence is received Use acquisition threshold to detect end of arriving preamble sequence Alternate Representation –Chip-Matched Filter Convolve incoming signal with one chip –Discrete time sequence matched filter Sample chip-matched filter output at times t=kT c Produces a sequence of approximately independent Gaussian random variables (sum represents matched filter output) AGC automatically adjusts the gain (increase in power) of the received signal based on the strength of the input signal Designed to keep average power constant into subsequent electronics Determine an acquisition threshold (η) Declare hit if matched filter output crosses η Enter verification mode to check if synchronization has occurred –If verified, enter data-detection mode and start demodulating data –If verification fails, return to acquisition mode (takes time QT c ) Probability of a miss –Acquisition fails because matched-filter output does not exceed acquisition threshold when the end of the preamble is received Probability of a false alarm –Acquisition fails because algorithm is in verification mode when the end of the preamble is received and acquisition threshold is exceeded P(not acquiring) = P(miss) + P(false alarm) Preamble sequence –Not modulated –Known a priori at receiver –Values of +1, -1, +j, -j s(t): Transmitted signal where n(t): Additive White Gaussian Noise (AWGN) r(t): Received signal Misses dominate failed acquisition at low SNR False alarms dominate failed acquisition at high SNR Weaker signals  higher gain Stronger signals  lower gain Practical AGC has delayed response to signal-level change Model response as linear with response delay t agc Automatic Gain Control Response Delay and Acquisition in Direct-Sequence Packet Communications Stephanie Gramc, Clemson University Advisor: Dr. Noneaker IntroductionIntroduction Direct-sequence (DS) spread-spectrum modulation uses bandwidths much wider than the minimum required for simple point-to-point communication at the same data rate. Research Focus DS Packet Transmission a M-1 a3a3 a2a2 a1a1 a0a0 Preamble SequenceData 0 1 01234567 s(t) Preamble Sequence time (T c chips) t = kT s s(t) r(t) Chip- Matched Filter Sequence- Matched Filter n(t) AGC Acquisition Algorithm Preamble Matched Filter IF Filter Matched Filter -2 0 2 4 6 8 02468101214 Noise-Free Matched Filter Output Automatic Gain Control Gain levels for s(t) + n(t) Idealized AGC Behavior Gain levels for n(t ) 0 2 4 6 8 10 12 -119-79-3914181 t agc time A 0 = Gain levels for n(t) 0 2 4 6 8 10 12 -120-80-4004080120 A 1 = Steady state gain levels for s(t)+n(t) Gain (dB) time More Realistic AGC Behavior Acquisition Algorithm Probability of Not Acquiring a Packet ResultsResults P(nacq) Probability of Not Acquiring in AGC System with Response Delays for M=26, Q=65 SNR (db) Probability of Not Acquiring in AGC System with Idealized AGC Response for M=26, Q=65 Probability SNR (db) P(nacq ) Probability of Not Acquiring in AGC System with Response Delays for M=100, Q=250 Future Plans ConclusionsConclusions Factor of Increase in Not Acquiring Packet for Different Delay Responses Factor of increase compared to ideal AGC with no delay Percent of preamble effected by delay As response delay time in AGC system increases, the probability of not acquiring a packet increases exponentially Model the delay of the AGC with more accuracy –Realistic gain adjustment will not be a linear line Account for response delay in AGC system to determine the acquisition threshold