What is Antenna Diversity?
To combat fading effect, an array of antennas is used. Diversity techniques have been recognized as an effective means which enhances the immunity of the communication system to the multipath fading. A diversity antenna system provides a number of receiving branches or ports from which the diversified signals are derived and fed to a receiver. The receiver then combines the incoming signals from the branches to produce a combined signal with improved quality in terms of signal strength or signal-to-noise ratio (S/N). The performance of a diversity antenna system primarily relies on the branch correlation and signal level difference between branches.
Why Diversity Antenna Systems Required?
Because we want to be able to make/receive calls with our mobile phones at every time and at every place.Unfortunately, the transmission quality of a wireless system is:
- Highly location/site-dependent: signal levels vary from place to place due to obstacles around the transmitters/receivers
- Highly time-dependent: signal levels vary in time due to a changing environment (e.g. moving cars)
In a typical cellular radio environment, the communication between the cell site and mobile is not by a direct radio path but via many paths : Multipath. The direct path between the transmitter and the receiver is obstructed by buildings and other objects.When various incoming radio waves arrive at the receiver antenna, they combine constructively or destructively, which leads to a rapid variation in signal strength.The signal fluctuations are known as ‘multipath fading’
Types of Diversity
- Time diversity: due to the time-dependence of the multipath environment, signals transmitted at different times will get un-correlated
- Frequency diversity: due to the frequency-dependence of the multipath environment, signals transmitted at different frequencies will get un-correlated
- Spatial diversity: due to the small-scale fading, signals received at different locations can get un-correlated
- Polarization diversity: due to different fading behavior for horizontal/vertical polarization, signals received with different polarizations can get un-correlated
MIMO: Multiple-Input Multiple-Output
Multiple-Input Multiple-Output (MIMO) is an antenna technology – Sometimes called smart antenna technology. MIMO uses multiple antennas to send multiple parallel signals (from transmitter). MIMO can be used to advance such applications as: Streaming video, music ,Video surveillance, Voice over Internet Protocol (VoIP), Video conferencing ,Interactive gaming, Mobile TV.
MIMO Antennas: S-Parameter Simulation
The antenna consists of two branches of folded monopole. By slightly tuning the length and width of the braches , the antenna can simultaneously achieve a flexible frequency ratio and a desired bandwidth.
Antenna Diversity Measurement
- Antenna Diversity always requires two far-zone patterns
- For each far-zone only one antenna can be fed
- Second antenna can be…
- Simply not present (mutual coupling between antennas not included!)
- Terminated by reference impedance (typically 50 Ohm)
- The two antennas are completely characterized by their far-zone patterns
- Reciprocity: far-zone patterns is valid for both receiving and transmitting
MIMO Antenna Diversity Result