I. Introduction multiplex
Multiplexing (multiplexing):
- Link / network resources (such as bandwidth) is divided into "resource sheet"
- The resources allocated to each group piece "calls" (calls)
- Each call is assigned to an exclusive communication resource sheet
- Resource film possible "Idle" (idle) (no sharing)
Second, the typical multiplexing method
- Frequency division multiplexing (frequency division multiplexing-FDM)
- Time division multiplexed (time division multiplexing-TDM)
- Wavelength division multiplexing (wavelength division multiplexing-WDM)
- Code division multiplexing (code division multiplexing-CDM)
1. Frequency Division Multiplexing FDM
Frequency division multiplexing different user occupied bandwidth (here, the "bandwidth" is the frequency bandwidth (Unit: Hz) instead of the data transmission rate).
After the user is assigned to a certain frequency band during communication occupies throughout this channel.
Applications: cable television network.
Example: The following four colors represent four user, four users share the channel, is divided into four frequency bands, isolated from each other, without disturbing each other.
2. The time-division multiplexing TDM
Time division multiplexing the time is divided into a section division multiplex frame length and the like (TDM frame), each user occupies a fixed number of time slots in each TDM frame.
Each user is a periodically occurring time slot occupied (which is the length of the TDM frame period).
All users occupy the same time-division multiplexing is the bandwidth at different times.
For example: In the figure, ABCD are four users, each user transmits only its own data in the corresponding time slot.
3. The wavelength division multiplexed WDM
Optical wavelength division multiplexing is frequency division multiplexing.
4. The code division multiplexer CDM
- Widely used in wireless uplink shared (e.g., a cellular network, satellite communication, etc.).
- Each user is assigned a unique chip sequence of m bit (chipping sequence), where "0" as "-1", "1" by "+1", respectively.
- Since the frequency of each user is neither divided nor split time each user uses the same frequency carrier, each chip sequence using the coded data.
- Encoded signal = raw sequence data chips ×
The transmission bit 1 (+1), then sends its chip sequence of m bit code
as the transmission bit 0 (-1), m bit code is sent anti-chip sequence of the chip sequence
- Because the same frequency for each user, so that each message sent by the user necessarily mutually superposed channels, in order to allow interference between information, each user must be satisfied chip sequence orthogonal to each other .
A user chip sequence is multiplied by themselves only obtained when 1, multiplied by the other chip sequences is 0
0:00 of a user chip sequence is multiplied only with their inverted, with other anti-chip sequence It is multiplied by code 0
Order D { I } the original data sequence, each user is superimposed vector
decoding: inner product chip sequence encoding a signal, according to the results obtained, determines whether the user information transfer channel at the moment.
