Synchronization in Spread Spectrum Systems

Synchronization in Spread Spectrum Systems

Need for Synchronization

Synchronization is the process of ensuring that the locally produced carrier at the receiver is in frequency and phase synchronism with the carrier at the transmitter. For successful functioning in spread spectrum communication systems, the transmitted and received PN codes must be perfectly matched.

(i) Both the carrier frequency and the PN clock can drift with time.

(ii) If the transmitter and receiver are moving relative to one other, as in mobile and satellite spread spectrum systems, the carrier and PN clocks will experience Doppler frequency shift.

As a result, the receiver's PN sequence must be synchronized with the transmitter's.

Synchronization steps:

The synchronization of the locally generated spreading signal with the incoming spread spectrum signal normally takes two phases. They are:

1) Acquisition: Bringing the two spreading signals into coarse alignment with one another implies the acquisition stage.

2) Tracking: After acquiring the received spread-spectrum signal, the second phase, tracking, is used to fine-tune the alignment.

The feedback loop is used in both acquisition and tracking.

Acquisition:

There are three types of acquisition schemes. They are

1) Serial search acquisition

2) Parallel search acquisition

3) Sequential search acquisition

1. Serial search acquisition:

A) DS Spread spectrum systems:

In Direct Sequence spread spectrum systems, the serial search scheme is shown in Figure 1.

Between the transmitter and the receiver, there is always some initial timing ambiguity. Assume the transmitter contains N chips and each chip has a T_c duration. It's important to linger for T_d=NT_c to verify synchronism at each time instant if initial synchronization is to take place in the face of additive noise and other interference. In(coarse) time steps of 1/2 T_c, we search throughout the time uncertainty interval.

Figure 1 Direct Sequence spread spectrum systems – Serial Search Acquisition

The incoming PN signal and the locally produced PN signal are linked. The output signal is compared to a predefined threshold at specified search intervals of NT_c (search dwell time). The locally produced code signal is advanced in time by 1/2T_c seconds if the output is less than the threshold. The procedure of correlation is carried out once more. These actions continue until a signal is detected or a threshold is reached. After then, it's presumed that the PN code was obtained.

As a result, assuming the initial misalignment between the two codes was n chips, the entire acquisition time is equal to

T_acq = 2nNT_c seconds

B) FH spread spectrum systems

The serial search strategy for frequency hopping spread spectrum systems is shown in Figure 2.

A mixer is followed by a bandpass filter (BPF) and a square-law envelope detector in this non-coherent matched filter. The frequency hopper is controlled by the PN code generator. When the local hopping matches that of the received signal, the acquisition is finished.

Let f_i be the frequency of the transmitter's frequency synthesizer. Assume that f_j is the frequency of the signal generated by the frequency synthesizer in the receiver's acquisition circuit. If f_i ≠ f_j, the output of BPF will only create a little voltage less than the threshold. If f_i = f_j at a later time during the search, a huge voltage above the threshold will be created at the BPF output. This means that local hopping is matched with the received signal. 

                                 Figure 2 Frequency hopping serial search acquisition  

2. Parallel search acquisition

By running two or more correlators in parallel, the parallel search acquisition approach adds a level of parallelism to the process. They'll look for periods that don't overlap. The search time is lowered with this system, but the implementation is more complex and costly.

3. Sequential search acquisition

The dwell time at each delay in the search process is made variable using a correlator with a variable integration period whose (biased) output is compared to two thresholds in this method. As a result, the sequential search strategy produces a more efficient search by reducing the average search time.

Tracking

The initial search process is halted after the signal has been acquired, and fine synchronization and tracking may start. The PN code generator at the receiver is kept in sync with the incoming signal thanks to the tracking. Fine chip synchronization and carrier phase tracking are both included in tracking for coherent demodulation.

A) DS Spread spectrum system:

The Delay-locked loop (DLL) is a common tracking loop for a Direct sequence spectrum signal, as shown in Figure 3.

Figure 3: Delay-Locked Loop (DLL) for PN code tracking

The received DS spread spectrum signal is applied to two multipliers at the same time. The PN code for one of the multipliers is delayed by a fraction of the chip interval (δ). The identical PN code is sent to the other multiplier, but it is advanced by δ. Each multiplier's output is sent into a BPF centered on f₀.

Each BPF's output is envelope detected and then subtracted. The loop filter, that drives the voltage-controlled oscillator receives this difference signal. The PN code generator uses the VCO as a clock. If the synchronization isn't perfect, one correlator's filtered output will surpass the other. As a result, the VCO will be advanced or postponed properly. The two filtered correlator outputs will be equally displaced from the peak value at the equilibrium point. The output of the PN code generator will then be perfectly synchronized with the received signal supplied into the demodulator.

B) FH Spread spectrum system:

Figure 4 shows a common tracking method for FH spread spectrum communications.

The received signal and the receiver clock have a minor timing error, despite the initial acquisition is successful. The bandwidth of the BPF is on the order of 1/T_c, where T_c is the chip interval, and it is tuned to a single intermediate frequency. Its output is envelope detected before being multiplied by the clock signal to provide a three-level signal. The loop filter is activated by this.

Figure 4: Tracking loop for FH signals

Assume that the chip transitions from the locally generated sinusoidal waveform do not coincide with the incoming signal transitions. The loop filter's output will thus be either positive or negative, depending on whether the VCO is trailing or ahead of the input signal's time. The control signal for changing the VCO timing signal to drive the frequency synthesizer output to appropriate synchronization with the received signal will come from the error signal from the loop filter.