TABLE OF CONTENTS The reader may recall from the previous chapter that the data in the received spread-spectrum signal cannot be efficiently recovered by a conventional wideband receiver. This was because the amount of noise power input to the detector overwhelms the useful information causing total erroneous output. Consequently, reliable data detection can only be carried out after the received spread-spectrum is converted to its original narrowband equivalent.
The prime task of the receiver is therefore to generate a local replica of the received spreading code to re-modulate or de-spread the incoming signal. When the local code phase is time synchronized to the received code to within a fraction of chip code phase offset, the received spread-spectrum signal collapses in bandwidth and reverts to its original narrowband form and the conventional detection techniques can be applied to recover the data.
The process of synchronizing the local code and the received code is commonly achieved in two stages: initially, the two code signals are aligned in phase to uncertainty less than one chip duration through a process called code acquisition or coarse synchronization. In other words, the acquisition is aligning the unknown phase of the received code with the known phase of the local code generated at the receiver. The acquisition process is random and the randomness is due to many sources such as initial uncertainty about the code phase offset; channel fading and interference; unknown carrier phase and frequency offset such as Doppler; and the presence of additive white Gaussian...
