Apps2_cdma_intro - Stanford University Winter 2009-2010...

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Stanford University Winter 2009-2010 Signal Processing and Linear Systems I Applications 2: Introduction to Spread Spectrum Communications January 27, 2010 EE102A:Signal Processing and Linear Systems I; Win 09-10, Pauly 1 Cell Telephone Base Station Handsets Base station talks to all of the handsets in the cell. Simple solution, each link has a frequency. However, there are a limited number of frequencies. We would like to have multiple connections for each frequency. How do we do this? EE102A:Signal Processing and Linear Systems I; Win 09-10, Pauly 2
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Sharing a Channel If all of the handsets try to use the same frequency, the signals superimpose, and we can’t sort them out. C1 C2 C3 CR Simultaneous Conversations Received Signal Two methods (out of many) for sharing the channel are Time division multiplexing (TDMA) Code division multiplexing (CDMA) EE102A:Signal Processing and Linear Systems I; Win 09-10, Pauly 3 Time Division Multiplexing Each transmit bit is divided into several subintervals. Each handset only talks during its allotted times. C1 C2 C3 CR Simultaneous Conversations Received Signal Here there are four time slots. C1 only transmits during the Frst time slot, C2 the second time slot, etc. EE102A:Signal Processing and Linear Systems I; Win 09-10, Pauly 4
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There are a limited number of time slots, limiting the number of simultaneous users. We could add one more user, but the second would fail. The received signal bandwidth is increased by a factor of four in this case (the spectrum is spread by that factor). Higher data rates can be obtained by allocating one user several time slots. EE102A:Signal Processing and Linear Systems I; Win 09-10, Pauly 5 Identifying Individual Channels What we’d like is a ”tag” that allows each channel to be selected, while ignoring all of the others.
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Apps2_cdma_intro - Stanford University Winter 2009-2010...

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