Need help on Direct Sequence Spread Spectrum

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Discussion Overview

The discussion revolves around the concept of Direct Sequence Spread Spectrum (DSSS) in the context of applied mathematics, specifically focusing on how two users can agree on a communication channel and timing for message exchange. The conversation touches on theoretical aspects and practical applications, including comparisons with Frequency Hopping Spread Spectrum (FHSS) and the role of synchronization in communication systems.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes the process of mixing message bits with a pseudo-random sequence to create a chipped sequence for transmission using PSK, but expresses uncertainty about how the receiver determines the correct channel and timing for listening.
  • Another participant explains the operation of CDMA in 3G cellphone systems, detailing how cellphones connect to basestations and manage time synchronization and channel assignment through periodic listening and handshaking processes.
  • A different participant raises a question about communication in scenarios where the basestation is offline, particularly in wireless sensor networks, prompting further inquiry into alternative methods for node communication.
  • One participant admits a lack of familiarity with CDMA systems and suggests that others may need to provide assistance on the topic.

Areas of Agreement / Disagreement

The discussion includes multiple competing views regarding the synchronization and channel assignment in DSSS and CDMA systems. There is no consensus on how communication occurs in the absence of a basestation, and participants express varying levels of knowledge about the systems discussed.

Contextual Notes

The conversation highlights limitations in understanding the synchronization mechanisms in DSSS and CDMA, particularly in scenarios without a central coordinating entity. There are unresolved questions about the specifics of communication protocols in wireless sensor networks.

Who May Find This Useful

This discussion may be useful for students or professionals interested in communication systems, particularly those studying spread spectrum techniques and their applications in wireless networks.

smoky^2
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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)
 
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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.
 
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?
 
I wasn't aware that wireless sensors use CDMA, and am unfamiliar with these systems generally, so someone else will need to assist you.
 

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