扩频通信1.ppt

Multiplexing Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c) Goal: multiple use of a shared medium Important: guard spaces needed! Frequency multiplex Separation of spectrum into smaller frequency bands Channel gets band of the spectrum for the whole time Advantages: no dynamic coordination needed works also for analog signals Disadvantages: waste of bandwidth if traffic distributed unevenly inflexible guard spaces Time multiplex Channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in themedium at any time throughput high even for many users Disadvantages: precise synchronization necessary Time and frequency multiplex A channel gets a certain frequency band for a certain amount of time (e.g. GSM) Advantages: better protection against tapping protection against frequency selective interference higher data rates compared tocode multiplex Precise coordinationrequired Code multiplex Each channel has unique code All channels use same spectrum at same time Advantages: bandwidth efficient no coordination and synchronization good protection against interference Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology 主要特点 ????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? ???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????? 2、可检性低(LPI---Low Probability of Intercept)，不容易被侦破，对各种窄带通信系统的干扰很小 。 3、抗多径衰落 多径分离与RAKE接收 主要特点 ???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????