Wireless communication systems have evolved through several stages of multiple-access control. The foremost controllable resource has always been the frequency spectrum. Other resources such as time, code, and space were initially manipulated in a very precarious and, therefore, ineffective manner. The early systems operated in the simplex mode in the forward link. Halfduplex systems soon appeared, in which forward link and reverse link shared the same channel. Access control was performed on a push-to-talk basis with the access point still competing with the terminals for access. Double halfduplex systems, in which forward link uses one channel and reverse link another channel, granted the access point the privilege of not having to contend for access. The push-to-talk procedure was the access control mechanism used on the reverse channel. The full-duplex mode, or simply duplex mode, was then the last stage in this evolutionary cycle in which push-to-talk access control was no longer necessary.

Duplex communication can be implemented by means of frequency division, time division, code division, and space division methods.

Frequency Division Duplexing

In frequency division duplexing (FDD), forward and reverse channels use separate frequencies. Therefore, a duplex channel is in fact a set of two distinct carriers, which constitute the physical channels. Because forward and reverse channels are continuously on and share the same antenna, the use of a duplexer (a filter between transmitter and receiver) is necessary so that reverse and forward channels do not interfere with each other. In the same way, sharp filters with strong out-of-band rejection must be used to reduce adjacent-channel interference. Adjacent-channel interference is also minimized by allowing for a guard band within each channel. For a given continuous spectrum, insulation between forward and reverse channels is maximized if paired channels are separated by half of the spectrum. FDD is a well-known technology, widely used in wireless systems.

Time Division Duplexing

In time division duplexing (TDD), forward and reverse channels share the same frequency band but occupy this band in nonoverlapping periods of time (slots), also known as windows. Therefore, a duplex channel is in fact a set of two nonoverlapping windows within a given carrier, which constitutes the physical channels. Because transmission and reception alternate in time, this scheme does not require the use of a duplexer. The number of possible access points (windows) within the same frequency band is a function of the technology available. As can be inferred, TDD makes more efficient use of the spectrum as compared with FDD and is more flexible. Usually, but not necessarily, an equal number of windows is dedicated to the forward channels and to the reverse channels, with paired windows symmetrically placed in time for maximum insulation. On the other hand, asymmetrical window assignment is also possible and necessary in asymmetrical traffic operation conditions, and in this case special attention must be paid regarding interference issues.

Code Division Duplexing

In code division duplexing (CDD), forward and reverse channels simultaneously share the same frequency band but are discriminated by means of orthogonal codes. Therefore, a duplex channel is in fact a set of two orthogonal codes within a given carrier, which constitutes the physical channels. Practical implementation of such a scheme may render the circuitry very complex. Because transmission and reception occur simultaneously and continuously within the same band and because the transmitted signal is at a much greater power than the received signal, the level of interference may impair such a communication scheme. Some sort of interference cancellation mechanism is necessary to realize this scheme.

Space Division Duplexing

In space division duplexing (SDD), forward and reverse channels share the same frequency band but are discriminated in space. Therefore, a duplex channel is in fact a set of two distinct locations where signals share the same frequency band. In a line-of-sight condition, directional antennas provide for the required insulation of the signals and can be used in SDD communication. In a non-line-of-sight condition, smart antennas are necessary.

Brief Remarks on Duplexing Techniques

FDD is certainly the duplexing technique most commonly used in wireless networks; it has been employed in all the first-generation wireless systems, in most of the second-generation systems, and its deployment in higher generations is without question. TDD is used in some second-generation systems, as well as in higher generations. CDD alone does not seem to lend itself to easy implementation for sophisticated interference cancellation mechanisms may be required. SDD alone can be used in diverse system applications mainly to increase capacity. Combination of some of these techniques is a common practice.