communication from one node to the other application_number US 201314081512 A description referring to fig1 the echo profile probe is a message 100 used to determine the channel impulse response the message 100 consists of a preamble 101 and a time domain data payload 104 the preamble contains information for gain control 102 and for timing and frequency offset adjustments 103 the payload contains the echo profile sequence consisting of 1024 pseudo random bpsk modulated bits this sequence is generated using an m sequence of degree 10 given by the generator polynomial 2201 8 with an initial seed of 3ff 16 in the receiver the received echo profile sequence rx is cross correlated to a local copy of the same sequence lseq as shown in equation 1 corr_out u03c3 i 0 1024 rx t u2212 i l seq i equation 1 referring to fig2 the time distance between the earliest significant correlation peak 200 and the latest significant correlation peak 201 is the significant delay spread in the channel response this delay spread can be used to determine the minimum cyclic prefix that would maintain the orthogonality of the ofdm symbol and eliminate isi the time distance between peaks is determined by further processing of the output of the correlation corr_out from equation 1 an example of the processing steps are described below step 1 the peak value of corr_out is determined this is found using the following pseudo code and recorded as max_peak the time where the max_peak occurs is recorded as peak_time step 2 look for the first instance of a peak that exceeds a threshold x db below the peak and record this time as start_profile x assumes an initial value determined by system analysis for a particular system and is a function of the echo amplitude.