Transmit Diversity of CDMA

Diversity technology means that after receiving two or more input signals with mutually uncorrelated fading at the same time, the system demodulates these signals and adds them up. Thus, the system can receive more useful signals and overcome fading.
A mobile communication channel is a multi-path fading channel and any transmitted signal reaches a receive end by means of multiple transmission paths, such as direct transmission, reflection, scatter, etc. Furthermore, with the moving of a mobile station, the signal amplitude, delay and phase on various transmission paths vary with time and place. Therefore, the levels of received signals are fluctuating and unstable and these multi-path signals, if overlaid, will lead to fading. The mid-value field strength of Rayleigh fading  has relatively gentle change and is called “Slow fading”. And it conforms to log normal distribution.
Diversity technology is an effective way to overcome overlaid fading. Because it can be selected in terms of frequency, time and space, diversity technology includes frequency diversity, time diversity and space diversity.
Time diversity
  • Block interleaving, error-correction
Frequency diversity
  • The CDMA signal energy is distributed on the whole 1.23MHZ bandwidth.
Space diversity
  • The introduction of twin receive antennas .
  • The RAKE receivers of the mobile station and the base station can combine the signals of different time delay.
  • During a handoff, the mobile station contacts multiple base stations and searches for the strongest frame.

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