Receiver Design for Ultrawideband PPM Communication Systems

1. Receiver Design for Ultrawideband PPM Communication Systems Vijay Ullal, Department of Electrical and Computer Engineering, Clemson University Faculty Advisor: Dr. Carl Baum 2004 Clemson SURE Program • Introduction • What is an ultrawideband system? • In an ultrawideband system, short pulses are transmitted across a wide range of frequencies. The FCC has allocated the frequency band from 3.1 to 10.6 GHz for UWB signals. • Why use ultrawideband? • UWB signals provide high data rates at short distances • Greater than 500 Mbps • Up to 10 meters • UWB Communication Scheme • Pulse position modulation with time hopping • Purpose of Research • To create various receiver designs, based on the communication scheme used, that provide low bit error rates for different signal to noise ratios. • To measure quantitatively the performance of these receiver designs. An Example Where m=3 Flexible Channel Model r(t) before added to n(t) aA a h(t) ab A s 0(t) a,b – propagation constants abA ab2 ab2A * ab3 ab3A ab4 = ab5 ab4A ab5A t t t 0 0 0 Multipath channel r(t) before added to n(t) aA a h(t) ab A s 1(t) a,b – propagation constants abA ab2 * ab2A ab3 = ab4 ab5 t t t 0 0 0 Multipath channel Weighted combining receiver does not perform significantly better than equal combining when a2/b2=0.2 UWB Communication Scheme Pulse Position Modulation Numerical Results Optimal Receiver Design s0(t) A • Means of all samples known • Values of channel propagation constants, a and b • Lowest probability of bit error for given signal to noise ratio • Problem: In reality, receiver does not know channel t 0 s1(t) A Equal Combining Design t 0 A “0” is represented by sending a pulse at time t=0, while a “1” is represented by sending the same pulse at a later time. vs. In weighted combining receiver, c was set equal to b. However, in reality, this is not possible since a receiver does not know the channel. In fact, no value of c performs best for all values of b. Time Hopping • Problem: Sample with small mean is weighted same as sample with large mean A Observations and Conclusions • As m increases, weighted receiver design has low bit error rates • But there is room for improvement • Would like to have low bit error rates for small value of m • In better receiver, weighted coefficient c changes over time t Weighted Combining Design 0 A2 A1 The time interval between when bits are sent is produced by a pseudo-random code that only the transmitter and receiver know. vs. • Assign greater weights to samples with larger means • For example, if m=5 • Z0 and Z5 are weighted by c0 = 1 • Z4 and Z9 are weighted by c4 Future Work • Make c parameter adaptive to channel conditions • Create more general channel models transmitted signal received signal AWGN channel impulse response As value of m increases, weighted combining receiver performs better Received sample is