计算机网络课件7-桂林电子科技大学(精品·公开课件).ppt

Code Division Multiple Access Each cell allocated frequency bandwidth Split in two Half for reverse, half for forward Direct-sequence spread spectrum (DSSS) (see chapter 9) Code Division Multiple Access Advantages Frequency diversity Frequency-dependent transmission impairments (noise bursts, selective fading) have less effect Multipath resistance DSSS overcomes multipath fading by frequency diversity Also, chipping codes used only exhibit low cross correlation and low autocorrelation Version of signal delayed more than one chip interval does not interfere with the dominant signal as much Privacy From spread spectrum (see chapter 9) Graceful degradation With FDMA or TDMA, fixed number of users can access system simultaneously With CDMA, as more users access the system simultaneously, noise level and hence error rate increases Gradually system degrades Code Division Multiple Access Self-jamming Unless all mobile users are perfectly synchronized, arriving transmissions from multiple users will not be perfectly aligned on chip boundaries Spreading sequences of different users not orthogonal Some cross correlation Distinct from either TDMA or FDMA In which, for reasonable time or frequency guardbands, respectively, received signals are orthogonal or nearly so Near-far problem Signals closer to receiver are received with less attenuation than signals farther away Given lack of complete orthogonality, transmissions from more remote mobile units may be more difficult to recover RAKE Receiver If multiple versions of signal arrive more than one chip interval apart, receiver can recover signal by correlating chip sequence with dominant incoming signal Remaining signals treated as noise Better performance if receiver attempts to recover signals from multiple paths and combine them, with suitable delays Original binary signal is spread by XOR operation with chipping code Spread sequence modulated for transmission over wireless channel Multipath effects generate multiple c