недостатки tdd fdd

Full duplex:   Full duplex, which is sometimes referred to simply as duplex is a scheme whereby transmissions may be sent in both directions simultaneously. However it sis till necessary for the transmissions to be separated in some way to enable the receivers to receive signals at the same time as transmissions are being made. There are two ways of achieving this. One is to use frequency separation (frequency division duplex, FDD, and the other is to use time, time division duplex, TDD.

Half duplex:   This is a duplex scheme whereby communication is possible in two directions, but communication is only possible in one direction at a time. If one transmitter is transmitting, the other one must wait until the first stops before transmitting. This form of communication is used for walkie-talkies, CB, etc. It may also be referred to as Simplex, in some circumstances although exact definitions can be contradictory at times.

Simplex:   Although the definition of simplex is not always clear the ANSI (American National Standards Institute) definition for a simplex transmission, is one that can only occur in one direction. One example of this may be a broadcast system. Occasionally simplex may refer to a half duplex scheme as described below.

There are a variety of different ways of controlling the two way passage of information using two transmitters. They range in complexity from the simplest systems requiring the least complex circuitry and providing more basic performance, to more complex systems providing higher levels of performance. However each scheme has its own advantages and disadvantages.

The different schemes for controlling the transmission range from simplex through half duplex to full duplex. Furthermore, schemes such as TDD and FDD need to be defined for the system depending upon its application and the traffic it is likely to carry. Many aspects of the performance will be governed by aspects such as whether FDD or TDD is used.

For cellular systems it is necessary that it is possible to talk or send data in both directions simultaneously, and this places a number of constraints on the schemes that may be used to control the transmission flow. As it is such a key element of the system, it is necessary to settle on the scheme that will be used from outset. As a result the duplex scheme to be used forms a very basic part of the overall specification for the cellular (or any radio communications system) that is to be used.

One of the key elements of any radio communications system is the way in which radio communications are maintained in both directions. Terms including simplex, duplex, frequency division duplex, FDD, and time division duplex, TDD, are all methods that can be used.

- an summary, tutorial about the forms of duplex transmission used for cellular technologies including FDD (Frequency Division Duplex) and TDD (Time Division Duplex).