Coded transmission for ultrasound Doppler detection using truncated long code

When applying the spread spectrum technology into the pulse-echo ultrasound system, the code selection is critical, as the duration time of the transmission pulse is normally short. It is difficult and as the mater of fact almost impossible in a nonlinear application like in the human body to have a good correlation property using a relatively short code excitation. A truncated long code idea is introduced in this paper, in which the coding time is extended to as long as the parameter estimation time window allows. The transmitted pulses are coded using different short codes from burst to burst and all the echoes are organized together coherently in the later estimation process to form a long code modulation. Very good correlation property can be achieved. The idea is applied to the ultrasound Doppler detection system for the blood velocity measurement in human body.