Radio Transmission Techniques

Radio Transmission Techniques

Based on the type of channels being utilized, mobile radio transmission systems may
be classied as the following three categories which is also shown in Fig. 1.3:
 Simplex System: Simplex systems utilize simplex channels i.e., the communication
is unidirectional. The rst user can communicate with the second
user. However, the second user cannot communicate with the rst user. One
example of such a system is a pager.
 Half Duplex System: Half duplex radio systems that use half duplex radio
channels allow for non-simultaneous bidirectional communication. The rst
user can communicate with the second user but the second user can communicate
to the rst user only after the rst user has nished his conversation.
At a time, the user can only transmit or receive information. A walkie-talkie
is an example of a half duplex system which uses `push to talk' and `release to
listen' type of switches.
 Full Duplex System: Full duplex systems allow two way simultaneous communications.
Both the users can communicate to each other simultaneously.
This can be done by providing two simultaneous but separate channels to both
the users. This is possible by one of the two following methods.
{ Frequency Division Duplexing (FDD): FDD supports two-way radio
communication by using two distinct radio channels. One frequency channel
is transmitted downstream from the BS to the MS (forward channel).

A second frequency is used in the upstream direction and supports transmission
from the MS to the BS (reverse channel). Because of the pairing of
frequencies, simultaneous transmission in both directions is possible. To
mitigate self-interference between upstream and downstream transmissions,
a minimum amount of frequency separation must be maintained
between the frequency pair, as shown in Fig. 1.4.
{ Time Division Duplexing (TDD): TDD uses a single frequency band
to transmit signals in both the downstream and upstream directions.
TDD operates by toggling transmission directions over a time interval.
This toggling takes place very rapidly and is imperceptible to the user.
A full duplex mobile system can further be subdivided into two category: a
single MS for a dedicated BS, and many MS for a single BS. Cordless telephone
systems are full duplex communication systems that use radio to connect to a
portable handset to a single dedicated BS, which is then connected to a dedicated
telephone line with a specic telephone number on the Public Switched
Telephone Network (PSTN). A mobile system, in general, on the other hand,
is the example of the second category of a full duplex mobile system where
many users connect among themselves via a single BS.