Wireless communication systems are divided into three types according to modes and rules of information transmission / reception:
Simplex communication systems
In simplex communication systems, data can only be transmitted in one direction: from transmitter to receiver, roles of the transmitter / receiver are strictly assigned to devices and can not be changed with time. An example of such system is organization of on-air broadcasting: the repeater provides broadcasting on a certain location and does not receive data from subscribers; subscribers receive a signal from the repeater, without attempting to transmit data:
Figure 1 Simplex communication system scheme
Half-duplex communication systems
In half-duplex communication systems, data can be transmitted in two directions, but not simultaneously. Thus firstly Node 1 transmits data, Node 2 receives, then roles are changing and Node 2 transmits data, Node 1 receives. An example of such systems is the IEEE 802.11 standards family, where data transmission is carried out alternately in order to avoid collisions:
Figure 2 Half-duplex scheme
Duplex communication systems
In duplex communication systems, data can be transmitted in two directions simultaneously. The implementation of bi-directional data transmission is possible due to time or frequency division. An example of such system is GSM technology, where uplink and downlink channels are organized at different frequencies, so frequency duplex is used.
Figure 3 Duplex scheme
To review methods for organizing bi-directional communication, we assume that the connection is established between the base station sector and the subscriber. In this case, the channel from sector to subscriber is called a downlink, and from subscriber to sector - uplink.
Frequency division duplex (FDD)
In frequency duplex systems, the uplink and downlink channels are organized at different carrier frequencies F1 and F2. Such solution allows to reduce the interference of data transmission channels, since signals not intersecting in the frequency-domain are not coherent and will not interfere with each other:
Figure 4 Time-frequency diagram in the scheme with FDD
Frequency Half Duplex (Hybrid-FDD)
A kind of FDD systems is a system with a frequency half duplex, in which reception or transmission can be performed at the moment, but the uplink and downlink channels are divided in the frequency-domain. In compare with a FDD system, the throughput of Hybrid-FDD is twice lower because half the time the frequency resource is not used.
Figure 5 Time-frequency diagram in the scheme with Hybrid-FDD
Time division duplex (TDD)
In systems with a time duplex, a channel is divided in equal time intervals, each of which consists of two parts: the first part of each interval is allocated for the downlink channel, the second for the uplink. In this case, unlike FDD, there is no need to expand the operating frequency band. Moreover, the advantage of the time duplex is the possibility to redistribute dynamically the ratio of uplink and downlink channels rates by changing the ratio of first and second parts duration within the time interval:
Figure 6 Time-frequency diagram in the scheme with TDD