Gain (dB)

1. Automatic Gain Control Response Delay and Acquisition in Direct-Sequence Packet CommunicationsStephanie Gramc, Clemson UniversityAdvisor: Dr. Noneaker 8 6 4 2 0 -2 0 2 4 6 8 10 12 14 s(t) Preamble Sequence a0 a3 aM-1 1 0 a1 a2 -1 0 1 2 3 4 5 6 7 time (T chips) c Matched Filter Introduction Results • 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 Noise-Free Matched Filter Output 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. Probability of Not Acquiring in AGC System with Idealized AGC Response for M=26, Q=65 Misses dominate failed acquisition at low SNR False alarms dominate failed acquisition at high SNR • 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 Probability • Alternate Representation • Chip-Matched Filter • Convolve incoming signal with one chip • Discrete time sequence matched filter • Sample chip-matched filter output at times t=kTc • Produces a sequence of approximately independent Gaussian random variables (sum represents matched filter output) SNR (db) Probability of Not Acquiring in AGC System with Response Delays for M=26, Q=65 Probability of Not Acquiring in AGC System with Response Delays for M=100, Q=250 Automatic Gain Control Research Focus • 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 • 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 P(nacq) P(nacq) More Realistic AGC Behavior Idealized AGC Behavior 12 A0= Gain levels for n(t) 12 Gain levels for n(t) 10 10 8 8 Gain (dB) 6 Gain (dB) A1= Steady state gain levels for s(t)+n(t) 6 4 Gain levels for s(t) + n(t) 4 2 SNR (db) 2 0 0 -120 -80 -40 0 40 80 120 Conclusions -119 -79 -39 1 41 81 time time tagc • Weaker signals  higher gain • Stronger signals  lower gain • Practical AGC has delayed response to signal-level change • Model response as linear with response delay tagc DS Packet Transmission Factor of Increase in Not Acquiring Packet for Different Delay Responses • As response delay time in AGC system increases, the probability of not acquiring a packet increases exponentially Preamble Sequence Data Acquisition Algorithm • 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 Factor of increase compared to ideal AGC with no delay • 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 QTc) • 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 Matched Filter Percent of preamble effected by delay n(t) t = kTs Probability of Not Acquiring a Packet Sequence-Matched Filter Future Plans Chip-Matched Filter IF Filter Acquisition Algorithm AGC s(t) r(t) • 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