Wireless Communication Technology has seen an exponential growth over a last decade or so. All of us have personally seen that the number of mobile phone users has increased many folds and the quality of service (speed, efficiency, signal to interference, signal to noise ratio etc) has also improved. Needless to say that all this is possible through continual evolution in wireless communication technology. We all know that antenna system is a vital component of any wireless communication system. So as the wireless communication system evolve to cater to exponentially growing user base and to provide quality service, evolving with it is the Antenna System.

In this article we will see a progression to smart antennas starting from omnidirectional antennas in cellular communication, principle of smart antenna, need and advantages of smart antenna systems.

The cellular system designers have a single point objective of maximising capacity (number of users) over a fixed channel/bandwidth (as each service provided is provided with limited number of channels) and minimize interference. We model cellular system using hexagonal cells.

Consider the following cellular structure of seven cells reuse pattern. In this structure each hexagonal cell (with radius R) represent a geographical area with a base station in the middle equipped with omnidirectional antenna and operating at a fixed band of frequencies. The adjacent cells have different frequencies. Same frequencies are repeated in cells that are quite far away from each other so that there is no interference. In the following figure, cells designated 1 have same frequencies, cells designated 2 have same frequencies so on and so forth. This is a most simple design of cellular system which cater to significant capacity over limited bandwidth using frequency reuse technique. The antenna used here is omnidirectional - it is an antenna that is isotropic in one plane and directive in another.

Since the antenna here is omnidirectional only a small percentage of radiated energy reached the desired user and remaining is radiated in other directions. As the number of users increases, the interference increases and thereby cell capacity is restricted.

Another technique called cell splitting is used to increase the capacity where each hexagonal cell is further divided into small macrocells with each macrocell having its own omnidirectional antenna. The technique increase the capacity but upto some extent and it also has drawback of increased cost owing to installing new antenna systems in macrocells.

In order to further increase the capacity of the cells without increasing the bandwidth in each cell (as this is a limiting factor) it is required to use directive antennas instead of omnidirectional and the technique is called cell sectoring. The most common example is using 3 directive antennas in a cell, each covering a sector of 120degrees. This improves frequency reuse and provides more number of channels per geographic area.

Smart Antenna Systems that we are going to discuss next is actually an extension of cell sectoring technique. In smart antenna system the cell is sectored dynamically. If we start from very basics, in smart antenna system each cell sector has multiple beams and this is possible by using array antennas. Number of beams in a cell is a function of array geometry. Now we have multiple beams in a cell and our mobile user is moving inside the cell. Antenna system detects the movement of user by tracking the Signal of Interest (SOI) from the user and by applying Direction of Arrival Algorithms (DOA) it assigns the appropriate beam to the user, if necessary it switches beam depending upon user’s mobility as the objective is to maximise gain according to the user’s location. This is called Switched Beam System and is an example of smart antenna. However this antenna system is not smart enough, the actual smart antenna is Adaptive Array System which we are going to discuss next but before that let us understand the principle of Adaptive Array or Smart Antenna.

Switched Beam

Adaptive Array

Human hearing analogy is used to explain adaptive array system. Let us say there are two people in a room, one is listener (L) who is blind folded and other is speaker (S) who is mobile. While S speaks, L is able compute the direction from which the sound is coming. This is possible because there is always a time difference between the sound signals received by our two ears (SOI) and brain will process this information and deduce the direction of sound (DOA).

In adaptive array system, two antennas are used to capture the SOI from the user (similar to our ears), a DSP processor (similar to our brain) process the time difference between the two signals and decide on the DOA of the signal from the user. An Adaptive Algorithm running inside the DSP processor decides on the appropriate radiation pattern such that the antenna radiates in the desired direction of the user only. Adaptive Algorithm assigns weights to the antenna array elements such that it radiates only in the desired direction of the user. Weights are nothing but the amplitude and the phase of the signal assigned to each array element of the antenna. Below is a block diagram of a single beam former in adaptive array system or smart antenna system. The only difference in adaptive array and switched beam system is that in former case the beam is dynamically changing using adaptive algorithms but in latter case beam is fixed so the interference is more.

Single Beam former in Adaptive Array Antenna

I believe one thing is absolutely clear to the readers that in smart antenna system we use the same conventional antenna like array antenna etc and we make it smart by adding computational capability to it. As we know today that the computational capabilities of the processors, controllers have improved considerably and their costs have come down drastically this is a reason smart antennas are getting a lot of traction these days.

Advantages of using smart antenna systems are Increased Capacity of the channel, Improved Signal Range as smart antennas are more directional and More Security as it is difficult to tap the connection.

By - Sagar Juneja, Research Associate, Chitkara University H.P.

References:-

• Constantine A. Balanis, 'Antenna Theory Analysis and Design' (Book)