# Dsss in digital communication, acquisition, tracking

I have some questions of direct sequence spread spectrum.

• Does dsss need a pulse shaped filter before it is transmitted?
• And is there any reference how to realize a dsss system in digital communication, especially PN code acquisition and tracking loop?
• What’s differences between a Code acquisition and time locked loop? It seems the purpose of both is to sample at accurate time.

Does dsss need a pulse shaped filter before it is transmitted?

Generally, every transmitter "needs" or physically has that, by physical necessity of limited bandwidths. You'll find DSSS systems with usual pulse shapes, but very often, these pulse shape filters are pretty close to rectangular, which is why when you look at a spectrum of a very wideband transmission and it looks nearly like a sinc, assuming there's DSSS going on there is correct. That's not a technical necessity! That's just historically the case.

And is there any reference how to realize a dsss system in digital communication, especially PN code acquisition and tracking loop?

Literally thousands, because 3G cellular systems (UMTS, CDMA2000) are DSSS systems. The basics of DSSS however are found in most digital communications textbooks. See, for example, Proakis.

Generally, DSSS always boils down to correlation. That's what it mathematically does.

What’s differences between a Code acquisition and time locked loop? It seems the purpose of both is to sample at accurate time.

One tracks the phase, the other finds the position of the sequence in time. Totally different jobs.

• So it doesn’t need a square root raised cosine filter used in BPSK , for instance. It’s just like a rectangular to transmit? It’s just bunch of one and zeros whose rate is faster than original bits after spreading by PN sequence. If I choose BPSK modulation in transmitter, what’s the difference between the spreading spectrum and normal BPSK modulation other than it has a faster rate. Can the synchronization and demodulation algorithm for bpsk link use in DSSS+bpsk system? Thank you. Jul 12, 2023 at 15:50
• RRC for BPSK is also just something you commonly see, not necessarily the optimal filter for any specific application. The channel doesn't care. If you spread BPSK with a bipodal direct sequence, you get a BPSK signal. Design of pulse shaping filters depends on your application and your channel, be it for DSSS or not. Jul 12, 2023 at 15:59
• "How to do synchronization for DSSS systems?" is a big question, and one which, again, 100% depends on your application. Synchronization for GPS is done differently than for IEEE802.11b, is done differently than for UMTS. You need to look at things on a system level before deciding which pulse shaping and which synchronization is appropriate. Jul 12, 2023 at 16:00
• Ok, many thanks. Jul 12, 2023 at 16:15