Need help on Direct Sequence Spread Spectrum

[smoky^2]

Homework Statement

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?

Homework Equations

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

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)

 

[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.

 

[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?

 

[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.

 

[rezeew]

and the receiver will know which channel to listen to based on the frequency synthesizer output. Additionally, the receiver can also use the Barker sequence as a channel decoder to retrieve the original message bits.

In Direct Sequence Spread Spectrum (DSSS), two users agree on a specific channel and time to communicate by using a technique called chipping. This involves mixing the message bits with a pseudo-random sequence, known by both parties, to create a chipped sequence. This chipped sequence is then used as input to the phase-shift keying (PSK) modulator to generate the transmitted signal.

To ensure that the receiver knows which channel and time to listen to, the pseudo-random sequence is also used in the frequency synthesizer. This allows the receiver to synchronize with the sender and determine the specific frequency at which the signal will be transmitted. The receiver can then use this information to tune in to the correct channel and time to receive the signal.

Moreover, the receiver can also use the same Barker sequence used for encoding as a channel decoder. By comparing the received chipped sequence with the Barker sequence, the receiver can retrieve the original message bits.

In summary, DSSS utilizes chipping and synchronization techniques to allow two users to communicate on a specific channel and at a specific time. The use of a pseudo-random sequence and Barker sequence helps to ensure secure and efficient communication between the two parties.