Reuse: allows users to communicate using a shared channel, reducing costs and improving utilization
Divide the entire bandwidth into multiple parts. After allocating to a certain frequency band, users occupy this frequency band throughout the communication process
- All users of frequency division multiplexing occupy different bandwidth resources at the same time
Time Division Multiplexing (TDM)
Time-division multiplexing is to divide time into time-division multiplexing frames of equal length, and each time-division multiplexing user occupies a fixed time slot in each TDM frame
The time slots occupied by each user appear periodically (the period is the length of the TDM frame)
- All users of time division multiplexing occupy the same frequency bandwidth at different times
For example, as shown above
- Suppose user A occupies the first time slot, user B occupies the second time slot, user C occupies the third time slot, and user D occupies the fourth time slot
- Then it can be seen that these four users are repeatedly executing alternately, just like the CPU time slice
But time division multiplexing will cause waste of resources
- If user B has no resources to send, then this time slot can only be idle
From this we have another technology, statistical time division multiplexing
Statistical Time Division Multiplexing (STDM)
STDM frames are not fixed time slots, but dynamically allocated time slots as needed
As shown in FIG
- For example, in the first time slot, only A and B have data to send (a, b) in time division multiplexing, and C and D have no data to send, but still need to allocate time slots for C and D. waste
- However, in statistical time division multiplexing, no time slots are allocated for C and D, which greatly improves the utilization rate of resources
(STDM implementation technology is very complicated, usually only used in high-speed long-distance communication, not suitable for users using channels on average)
Wavelength Division Multiplexing (WDM)
Wavelength division multiplexing is optical frequency division multiplexing. The entire wavelength band is divided into several wavelength ranges, and each user occupies a wavelength range for transmission
- Use one fiber to transmit multiple optical carrier signals simultaneously
- Multiplexed dozens or more channels of optical carrier signals with close frequencies are called dense wavelength division multiplexing DWDM on a single fiber
As shown in the figure, 8 optical carriers with a transmission rate of 2.5Gbit / s. After being modulated by light, the wavelengths are converted to 1550 ~ 1557nm respectively. After these 8 optical carriers pass through the optical multiplexer, they are transmitted on one optical fiber. Therefore, the data transmission rate on one optical fiber reaches 8 × 2.5 Gb / s = 20Gb / s
Code Division Multiplexing (CDM)
Code division multiplexing is a multiplexing method that uses a group of code groups containing mutually orthogonal code words to carry multiple signals, and distinguishes each original signal by different codes
Chip sequence
Each bit time is divided into m short intervals, called chips
- Each station is assigned a unique sequence of m bit chips
- If bit 1 is sent, send its own m bit chip sequence
- If bit 0 is sent, the binary inverse of the chip sequence is sent.
Spread spectrum communication
Suppose the data rate of the S station to send information is b bit / s. Since each bit is converted into an m-bit chip sequence, the data rate sent by the S station is increased to mb bit / s, and the frequency band occupied by the S station The width is also increased to m times the original value. This communication method is called spread spectrum communication.
Code Division Multiple Access: CMDA
CDMA is that each sending end modulates the signal it sends with different, mutually orthogonal address codes. At the receiving end, the orthogonality of the pattern is used to select the corresponding signal from the mixed signal through address recognition (correlation detection).
- Through different spreading codes, multiple users can share channels on the same frequency at the same time. That is, each user uses the same frequency band at the same time.
- Each user uses a different code pattern that has been specially selected so that they do not interfere with each other.
- CDMA can improve call quality and reliability of data transmission, reduce interference, increase the capacity of the communication system (4 to 5 times that of the global mobile communication system GSM), and reduce the average transmission power of mobile phones.
Goodbye, CMDA
CDMA is widely used in 2G and 3G networks.
Entering the 4G era, some operators began to close CDMA networks
China Telecom has made it clear that from 2020, all 5G terminals will not be allowed to have CDMA frequency bands and standards, and at the same time, no VoLTE switch will be allowed.
This means that in the 5G era, CDMA will completely leave us
