Basic CDMA
concept
--CDMA assigns a unique code sequence (spreading code) to each user, and uses it to encode the signal of the bearing information. A receiver that the code sequence user is known to decode the received signal, and the original data is restored because the user code sequence is small with other user von sequences. Since the bandwidth of the code sequence is larger than the bandwidth of the signal of the carrier, the encoding process extends the spectrum of the signal, which is also referred to as spread spectrum modulation, and the resulting signal is also referred to as a spread spectrum signal. CDMA is usually characterized by spreading multiple access (SSMA). The expansion of the transmitted signal spectrum is given CDMA with multiple access capabilities. Therefore, it is important to understand the generation of spread spectrum signals and its performance. The spreading modulation technique must meet two basic requirements:
--1. The bandwidth of the transmitted signal must be much greater than the bandwidth of the information.
--2. The bandwidth of the generated radio frequency signal is independent of the transmitted information.
- Band width BT and information bandwidth BI ratio of information bandwidth BI is called a spread spectrum system processing gain GP GP = BT / BI
- The receiver uses the same spreading code with the received signal to restore the original information of the carrier.
- Since the spread spectrum signal extends the spectrum of the signal, it has a series of performances different from narrowband signals:
- ● Multiple access capabilities
- ● Ability to resist multi-diameter interference
- ● Has privacy performance
- ● The ability to prevent people is interference
- ● Performance with low load probability
- ● Ability to have narrowband interference
http://www.mc21st.com/techfield/systech/cdma/c2.htm
