# Homework Help: Need help on Direct Sequence Spread Spectrum

1. Mar 12, 2013

### smoky^2

1. The problem statement, all variables and given/known data

I need to explain DSSS in my applied mathematics project (duration is three minutes of my talk).

How do two users agree on which channel and the time to listen to so that they can exchange messages?

2. Relevant equations

Use Barker sequence as a channel encoder and PSK and the modulator

3. The attempt at a solution

The message bits are mixed (XORed) with pseudo random sequence (know by both communicating parties) to get the chipped sequence. The chipping sequence is used as input to the PSK to get the signal that is transmitted.

Its not quite clear to me how the receiver will know on which channel to listen to and at what time. Unlike in FHSS, the modulated signal is "mixed" with the output of the frequency synthesizer (the Pseudo-random sequence can be generated by the receiver, since they are in sync and check the corresponding frequencies at which the sender will be sending the signals)

2. Mar 13, 2013

### marcusl

In the CDMA 3G cellphone system (Verizon's, e.g.), there are conventional broadcast and paging channels. Once a cellphone connects to a basestation when it is powered on, it remains largely dormant (to conserve battery life) but wakes up periodically to listen for a page. If it hears itself being paged, it responds and the basestation tells it what channel to communicate on. Time synchronization is handled over one of these general channels as well. In fact, a great deal of handshaking goes on in order to bring a call up covering things such as the phone numbers on either end, billing info, channel quality and subscriber plan info which both affect the over-air data rates to be used, etc.

3. Mar 13, 2013

### smoky^2

What happens in situations where the basestation (or trusted third party) is offline when two nodes wants to communicate like in the case of wireless sensor networks?

4. Mar 14, 2013

### marcusl

I wasn't aware that wireless sensors use CDMA, and am unfamiliar with these systems generally, so someone else will need to assist you.