| Wireless communication systems have evolved | | | | the same frequency band is a function of the |
| through several stages of multiple-access | | | | technology available. As can be inferred, TDD |
| control. The foremost controllable resource | | | | makes more efficient use of the spectrum as |
| has always been the frequency spectrum. Other | | | | compared with FDD and is more flexible. |
| resources such as time, code, and space were | | | | Usually, but not necessarily, an equal number |
| initially manipulated in a very precarious | | | | of windows is dedicated to the forward |
| and, therefore, ineffective manner. The early | | | | channels and to the reverse channels, with |
| systems operated in the simplex mode in the | | | | paired windows symmetrically placed in time |
| forward link. Halfduplex systems soon | | | | for maximum insulation. On the other hand, |
| appeared, in which forward link and reverse | | | | asymmetrical window assignment is also |
| link shared the same channel. Access control | | | | possible and necessary in asymmetrical |
| was performed on a push-to-talk basis with | | | | traffic operation conditions, and in this |
| the access point still competing with the | | | | case special attention must be paid regarding |
| terminals for access. Double halfduplex | | | | interference issues. |
| systems, in which forward link uses one | | | | |
| channel and reverse link another channel, | | | | Code Division Duplexing |
| granted the access point the privilege of not | | | | |
| having to contend for access. The | | | | In code division duplexing (CDD), forward and |
| push-to-talk procedure was the access control | | | | reverse channels simultaneously share the |
| mechanism used on the reverse channel. The | | | | same frequency band but are discriminated by |
| full-duplex mode, or simply duplex mode, was | | | | means of orthogonal codes. Therefore, a |
| then the last stage in this evolutionary | | | | duplex channel is in fact a set of two |
| cycle in which push-to-talk access control | | | | orthogonal codes within a given carrier, |
| was no longer necessary. | | | | which constitutes the physical channels. |
| | | | Practical implementation of such a scheme may |
| Duplex communication can be implemented by | | | | render the circuitry very complex. Because |
| means of frequency division, time division, | | | | transmission and reception occur |
| code division, and space division methods. | | | | simultaneously and continuously within the |
| | | | same band and because the transmitted signal |
| Frequency Division Duplexing | | | | is at a much greater power than the received |
| | | | signal, the level of interference may impair |
| In frequency division duplexing (FDD), | | | | such a communication scheme. Some sort of |
| forward and reverse channels use separate | | | | interference cancellation mechanism is |
| frequencies. Therefore, a duplex channel is | | | | necessary to realize this scheme. |
| in fact a set of two distinct carriers, which | | | | |
| constitute the physical channels. Because | | | | Space Division Duplexing |
| forward and reverse channels are continuously | | | | |
| on and share the same antenna, the use of a | | | | In space division duplexing (SDD), forward |
| duplexer (a filter between transmitter and | | | | and reverse channels share the same frequency |
| receiver) is necessary so that reverse and | | | | band but are discriminated in space. |
| forward channels do not interfere with each | | | | Therefore, a duplex channel is in fact a set |
| other. In the same way, sharp filters with | | | | of two distinct locations where signals share |
| strong out-of-band rejection must be used to | | | | the same frequency band. In a line-of-sight |
| reduce adjacent-channel interference. | | | | condition, directional antennas provide for |
| Adjacent-channel interference is also | | | | the required insulation of the signals and |
| minimized by allowing for a guard band within | | | | can be used in SDD communication. In a |
| each channel. For a given continuous | | | | non-line-of-sight condition, smart antennas |
| spectrum, insulation between forward and | | | | are necessary. |
| reverse channels is maximized if paired | | | | |
| channels are separated by half of the | | | | Brief Remarks on Duplexing Techniques |
| spectrum. FDD is a well-known technology, | | | | |
| widely used in wireless systems. | | | | FDD is certainly the duplexing technique most |
| | | | commonly used in wireless networks; it has |
| Time Division Duplexing | | | | been employed in all the first-generation |
| | | | wireless systems, in most of the |
| In time division duplexing (TDD), forward and | | | | second-generation systems, and its deployment |
| reverse channels share the same frequency | | | | in higher generations is without question. |
| band but occupy this band in nonoverlapping | | | | TDD is used in some second-generation |
| periods of time (slots), also known as | | | | systems, as well as in higher generations. |
| windows. Therefore, a duplex channel is in | | | | CDD alone does not seem to lend itself to |
| fact a set of two nonoverlapping windows | | | | easy implementation for sophisticated |
| within a given carrier, which constitutes the | | | | interference cancellation mechanisms may be |
| physical channels. Because transmission and | | | | required. SDD alone can be used in diverse |
| reception alternate in time, this scheme does | | | | system applications mainly to increase |
| not require the use of a duplexer. The number | | | | capacity. Combination of some of these |
| of possible access points (windows) within | | | | techniques is a common practice. |