Spreading Code Acquisition and Tracking

In spread spectrum, at the sender side, we spread the signal and at the receiver side we despread the signal. We need the timing information of the transmitted signal in order to despread the received signal and demodulate the despread signal. Therefore, the process of acquiring the timing information of the transmitted spread spectrum signal is essential to the implementation of any form of spread spectrum technique.

Usually the problem of timing acquisition is solved via a two-step approach:

1)   Initial code acquisition (coarse acquisition or coarse synchronization) which synchronizes the transmitter and receiver to within an uncertainty of T

2)   Code tracking which performs and maintains fine synchroniation between the transmitter and receiver.

Given the initial acquisition, code tracking is a relatively easy task and is usually accomplished by a delay lock loop (DLL). The tracking loop keeps on operating during the whole communication period. If the channel changes abruptly, the delay lock loop will lose track of the correct timing and initial acquisition will be re-performed. Sometimes, we perform initial code acquisition periodically no matter whether the tracking loop loses track or not. Compared to code tracking, initial code acquisition in a spread spectrum system is usually very difficult.

Initial Code Acquisition

The objective of initial code acquisition is to achieve a coarse synchronization between the receiver and the transmitted signal The receiver hypothesizes a phase of the spreading sequence and attempts to despread the received signal using the hypothesized phase. If the hypothesized phase matches the sequence in the received signal, the wide-band spread spectrum signal will be despread correctly to give a narrowband data signal. Then a bandpass filter, with a bandwidth similar to that of the narrowband data signal, can be employed to collect the power of the despread signal. Since the hypothesized phase matches the received signal, the BPF will collect all the power of the despread signal. In this case, the receiver decides a coarse synchronization has been achieved and activates the tracking loop to perform fine synchronization. On the other hand, if the hypothesized phase does not match the received signal, the despreader will give a wideband output and the BPF will only be able to collect a small portion of the power of the despread signal. Based on this, the receiver decides this hypothesized phase is incorrect and other phases should be tried.

Code Tracking

The purpose of code tracking is to perform and maintain fine synchronization. A code tracking loop starts its operation only after initial acquisition has been achieved. Hence, we can assume that we are off by small amounts in both frequency and code phase. A common fine synchronization strategy is to design a code tracking circuitry which can track the code phase in the presence of a small frequency error. After the correct code phase is acquired by the code tracking circuitry, a standard phase lock loop (PLL) can be employed to track the carrier frequency and phase. In this section, we give a brief introduction to a common technique for code tracking, namely, the early-late gate delay-lock loop (DLL)

We assume that the data signal is of constant envelope (e.g., BPSK) and the period of the spreading signal is equal to the symbol duration T . We choose the lowpass filter  to have bandwidth similar to that of the data signal

Since the data signal b(t) is of constant envelope, the samples of the signal x1(t) at time t = (nT + ) for any integer n will be:

 Between the samples, the signal x1(t) fluctuates about the constant value given in. Similarly,

The difference signal x2(t)-x1(t) is then passed through the loop filter which is basically designed to output the d.c. value of its input. As a result, the error signal

This error signal can be used to control a VCO which derives the local code generator.