Advantages Of Adaptive Antenna

3.2.1 ADAPTIVE ANTENNA ARRAY

Multipath propagation, defined as the creation of multipath signal paths between the transmitter and the receiver due to the reflection of the transmitted signal by physical obstacles, is one of the major problems of mobile communications. It is well known that the delay spread and resulting inter-symbol interference (ISI) due to multiple signal paths arriving at the receiver at different times have a critical impact on communication link quality. On the other hand, co-channel interference is the major limiting factor on the capacity of wireless communication systems, resulting from the reuse of the available network resources (e.g., frequency and time) by a number of users.

An adaptive array is a system in which

More specifically, the benefits of adaptive antennas can be summarized as follows: Increased range/coverage: the array or beam forming gain is the average increase in signal power at the receiver due to a coherent combination of the signal received at all antenna elements. The adaptive antenna gain compared to a single element antenna can be increased by an amount equal to the number of array elements, e.g., an eight element array can provide a gain of eight (9 dB). Increased Capacity: One of the main reasons of the growing interest of adaptive antennas is the capacity increase. In densely populated areas, mobile systems are normally interference-limited; meaning that interference from other users is the main source of noise in the system. This means that the signal to interference ratio (SIR) is much larger than the signal to thermal noise ratio (SNR). Adaptive antennas will on average, increase the SIR. Experimental results report up to 10 dB increase in average SIR in urban areas. For UMTS networks, a fivefold capacity gain has been reported for

The main advantages of this method are its simplicity and it provides maximum output SNR if the noise is uncorrelated and there are no directional jammers. It is clear that this method is not reasonable to be used in mobile communication systems where there are many users sharing the same frequency (e.g. CDMA) so there are many unintentional jammers.

3.4 NULL STEERING BEAMFORMER

If there are Q users operating, and the weights are calculated for user i, then the desired weight vector is the solution of the following system of nonlinear equations:

▁(W_i^H ) ▁(a_i )=1

▁(W^H ) ▁a=0 ∀ k∈[1,Q] and k≠i (3.13)

The above system of linear equations can be solved perfectly if the number of users is less than or equal to the number of antenna elements. A null-steering beamformer allows the operator to remove the influence of one or more interferers or jammers by steering a null in the array gain pattern in the direction of the interfering source or sources.

CHAPTER SUMMARY

This chapter helps to build the foundation for selecting the type of antenna array and type of beamforming technique for required application. It discusses about: Types of antenna arrays and selection of antenna arrays Basic theory and advantages of antenna arrays over single antenna element Types of beamforming and their comparison

Advantages and disadvantages of beamforming as per their