Diversity methods

1. Diversity is a powerful communication receiver that provides wireless improvement at relatively low cost. It requires no training overhead since a training sequence is not required by the transmitter.
2. Diversity exploits the random nature of radio propagation by finding independent signal paths for communication. All diversity decisions are made by receiver and are unknown to transmitter.
3. If one radio path undergoes a deep fade, another independent path may have strong signal. By having more than one path to select from, both the instantaneous and average SNRs at the receiver may be improved by as much as 20db to 30db.
4. Type of diversity methods
   A. Microscopic diversity:- Small scale fading is characterized by deep and rapid amplitude fluctuations that occur when mobile moves over a distances of few wavelengths. These fades are caused by multiple reflections from the surroundings in the vicinity of mobile. In order to prevent rapid deep fades from occurring, microscopic diversity is used which exploits rapidly changing signal.
   B. Macroscopic diversity: Large scale fading is caused by shadowing due to variations in the nature of the surroundings. By selecting a base station which is not shadowed when others are, the mobile can improve the average signal to noise ratio on the forward link. This is called as macroscopic diversity.
   C. Space diversity:
• Space diversity also known as antenna diversity, is one of the most popular form of diversity used in wireless communication.
• In this there are multiple antennas placed at different spatial locations which results into different possibly independent received signals.
• Antenna space diversity is also used in base station design. At each cell site, multiple base station receiving antennas are used to provide diversity reception. Since the important scatterers are generally on the ground in the vicinity of the mobile, the base station antennas must be spaced far apart to achieve decorrelation.
• There are four types of space diversity methods
• Selection diversity
• Feedback diversity
• Maximal ratio combining
• Equal gain diversity

D. Polarization diversity:

• At base station narrow angle of incidence requires large antenna spacing, thus space diversity is less practical at base station. The comparative high cost using space diversity at base station introduced consideration of using orthogonal polarization to exploit polarization diversity.
• Measured horizontal and vertical polarization paths between a mobile and a base station reported to be uncorrelated. The decorrelation for the signals in each polarization is caused by multiple reflections in the channel between the mobile and base station antennas. After sufficient random reflections, the polarization state of the signal will be independent of the transmitted polarization. In practice there is some dependence of the received polarization on the transmitted polarization.
• Circular and linear polarized antennas have been used to characterize multipath inside the building. When path is obstructed, the polarization diversity reduces multipath delay spread without significantly decreasing received power.
• Polarization diversity has primarily been used for radio links which vary slowly with in time later it became more popular for portable users to improve link margin and capacity.

E. Frequency diversity:

• Frequency diversity transmits information on more than one carrier frequency. Frequencies separated by more than coherence bandwidth of the channel will not experience the same fading. Thus using different carrier frequencies information can be simultaneously sent over a same channel.
• In practice 1: N protection switching is used where on e frequency is idle but is available on stand-by basis to provide diversity to any of the N others carriers used on the same link each carrying independent traffic.
• The disadvantage of this technique is that it requires spare bandwidth also it requires that there be as many receivers as there are channels used in frequency diversity.

F. Time diversity:

• Time diversity transmits information at time spacings that exceed coherence time of the channel so that multiple repetition of the signal will be received with independent fading conditions.
• One modern implementation of time diversity is RAKE receivers for spread spectrum CDMA, where multipath channel provide redundancy in the transmitted message.