Wireless Access Technologies and Signal Propagation

Wireless Access Technologies

FDMA (Frequency Division Multiple Access)

FDMA separates the spectrum into multiple voice channels by dividing the bandwidth into uniform frequency segments. FDMA is mostly used for analog transmission. This technology is not recommended for digital transmissions, even though it is capable of carrying digital information.

TDMA (Time Division Multiple Access)

TDMA technology compresses digital conversations and sends each one with the radio signal for only a third of the time. The compression of the voice signal is possible because digital information can be reduced in size using binary information (ones and zeros).

CDMA (Code Division Multiple Access)

CDMA technology, after scanning the information, transmits it through the entire available bandwidth. Multiple calls are overlaid on the channel, and each has a unique sequence code. Using CDMA technology, it is possible to compress 8 to 10 digital calls so they occupy the same space that a single call would take on an analog system.

Radio Wave Propagation Phenomena

Absorption

When electromagnetic waves pass through a material, they are usually weakened or attenuated. The amount of power lost will depend on the frequency and the material itself. For example, window glass is obviously transparent to light, while the glass used in sunglasses filters a portion of the intensity of light and blocks ultraviolet radiation.

Reflection

Like visible light, radio waves are reflected when they come into contact with materials that are appropriate for that; for radio waves, the main sources of reflection are metal and water surfaces. The rules of reflection are fairly simple: the angle at which a wave hits a surface is the same angle at which it is diverted.

Multipath

Multipath is the effect in which signals reach the receiver through different paths and, therefore, at different times; this phenomenon plays an important role in wireless networks. The surface of the water, with waves and ripples changing all the time, makes it virtually impossible to calculate the reflection precisely. In indoor environments such as offices, or outdoors in cities, there are elements or metal structures that cause multiple reflections.

Diffraction

Diffraction is the behavior of waves when hitting an object, giving the impression of bending. It is the ripple effect around corners, consisting of the bending and spreading of waves when they encounter an obstacle or pass through a crack.