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