Diversity

What is Diversity ?

•Diversity is achieved by creating several independent paths between the transmitter and receiver 

•Each path fades independently, hence, there is a low chance they fade together

•Receiver combines the received signal for the several paths using some method

•Diversity is used in all mobile communication systems  

Types of Diversity

•Space diversity: multiple transmit and multiple receive antennas

–Multiple Tx: split power over several Tx antennas. More antennas = more power split

–Multiple Rx: collect signal by several Rx antennas. More antennas = more collected power

–Antennas separation about l/2 is required

–If directional antennas (typically) larger separation is required

•Polarization diversity:
–Transmit and/or receive with both vertical and horizontal polarization
–Scattering is independent for each polarization, giving independent paths
–Limited to 2 transmit and 2 receive diversity
–Tx polarization diversity: half power for each polarization

•Beam-forming:

–Transmit with antenna array

–Each antenna is fed with different phase

–Forms a directional beam towards the receiver, or group of receivers

–Antenna beam tracks the intended receiver

–Requires knowledge of the fading channel at Tx Optional is 4G mobile communication systems (WiMax and LTE)

•Frequency diversity:

–Transmit same signal with several frequencies

–Frequencies separated by > coherence bandwidth

–Also wideband signals achieve frequency diversity, like OFDM techniques over wideband

(WLAN, WiMax, LTE)

•Multipath diversity:

–In Direct-Sequence-Spread-Spectrum signals we can receive from multipath separately using Rake receiver

–Used in all CDMA systems (IS-95, CDMA2000, WCDMA) 

•Time diversity:

–Signal is re-transmitted (repeated) after > coherence time

Also achieved using coding and interleaving

–Reduces overall transmission data rates

–Coding and interleaving used in all mobile communication systems

–Also combined with repeat-diversity in what is called Hybrid Automatic Repeat Request (H-ARQ)