2.4.1 Frequency Division Multiplexing, Time Division Multiplexing and Statistical Time Division Multiplexing
Multiplexing is a basic concept in communication technology.
It allows users to communicate using a shared channel, reducing costs and improving utilization .
Frequency Division Multiplexing FDM (Frequency Division Multiplexing)
- The entire bandwidth is divided into multiple parts. After users are allocated a certain frequency band, they occupy this frequency band throughout the communication process.
- All users of frequency division multiplexing occupy different bandwidth resources at the same time (please note that the "bandwidth" here is the frequency bandwidth rather than the data transmission rate).
Time Division Multiplexing TDM (Time Division Multiplexing)
- Time division multiplexing is to divide time into time division multiplexed frames (TDM frames) of equal length . Each time-division multiplexed user occupies a time slot with a fixed sequence number in each TDM frame .
- The time slot occupied by each user occurs periodically (the period is the length of the TDM frame).
- TDM signals are also called isochronous signals .
- All users of time division multiplexing occupy the same frequency bandwidth at different times .
Time division multiplexing may cause waste of line resources
When the time division multiplexing system is used to transmit computer data, the utilization rate of the allocated sub-channels by users is generally not high due to the burst nature of the computer data.
When a user has no data to send temporarily, the time slot allocated to the user in the time division multiplexing frame can only be in an idle state .
Statistical time division multiplexing STDM (Statistic TDM): an improved time division multiplexing
The STDM frame does not allocate time slots fixedly, but dynamically allocates time slots on demand . Therefore, statistical time division multiplexing can improve the utilization rate of the line.
2.4.2 Wavelength Division Multiplexing
Wavelength Division Multiplexing WDM (Wavelength Division Multiplexing) is the frequency division multiplexing of light . A single fiber is used to transmit multiple optical carrier signals simultaneously.
2.4.3 Code Division Multiplexing
- The commonly used term is Code Division Multiple Access (CDMA).
- Each user uses a different pattern that has been specially chosen so that they do not interfere with each other .
- The signal sent by this system has a strong anti-jamming capability , and its spectrum is similar to white noise, which is not easy to be detected by the enemy.
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, it sends its own m bit chip sequence .
- If bit 0 is sent, the one's complement of the chip sequence is sent .
- For example, the 8-bit chip sequence for station S is 00011011.
- When bit 1 is sent, the sequence 00011011 is sent,
- When bit 0 is sent, the sequence 11100100 is sent.
- Chip sequence for station S: (–1 –1 –1 +1 +1 –1 +1 +1)
Chip sequence realizes spread spectrum
- It is assumed that the data rate of the information to be sent by the S station is b bit/s. Since each bit needs to be converted into m-bit chips, the data rate actually sent by the S station is increased to mb bit/s, and the frequency bandwidth occupied by the S station is also increased to m times the original value.
- This communication method is a type of spread spectrum communication.
- Spread spectrum communications generally fall into two broad categories:
- One is Direct Sequence Spread Spectrum DSSS (Direct Sequence Spread Spectrum), which uses chip sequences as mentioned above.
- The other is Frequency Hopping Spread Spectrum FHSS (Frequency Hopping Spread Spectrum).
Important Features of CDMA
- The chip sequences allocated by each station must not only be different , but must also be orthogonal to each other .
- In practical systems a pseudorandom code sequence is used .
Orthogonal Relation of Chip Sequences
- Let vector S denote the chip vector of station S, and let T denote the chip vector of any other station.
- The chip sequences of two different stations are orthogonal, that is, the normalized inner product of the vectors S and T equals 0:
Another important property of orthogonal relations
- The normalized inner product of any chip vector and the chip vector itself is 1.
- The normalized inner product of a chip vector and the chip's complement vector is -1.
How CDMA Works
