Design and Analysis of a Beam Forming Network for WLAN Application

International Journal of Scientific Research Engineering & Technology (IJSRET) Volume 1 Issue6 pp 004-009 September 2012 www.ijsret.org ISSN 2278 - 0882

IJSRET @ 2012

Design and Analysis of a Beam Forming

Network for WLAN Application 1Ajay kumar yadav ,

2Vishal Upmanu ,

3 Satyendra kr. Yadav

Mewar University, Chittorgarh(Raj.)

[email protected]

ABSTRACT

In recent years, high-speed wireless communication is

in vogue. In wireless communication systems, multipath

fading, delay and interference occurred by reflection or

diffraction. In a high-speed wireless communication, it

becomes a necessity to separate desired signal from

delay or interference signal. Thus to overcome these

problems Smart antenna systems have been developed.

The arrays by themselves are not smart; it is the digital

signal processing that makes them smart. The process of

combining the signals and then focusing the radiation in

a particular direction is often referred to as digital beam

forming. Smart antennas are most often realized with

either switched-beam or fully adaptive array antennas. A

smart antenna system consists of an antenna array,

associated RF hardware, and a computer controller that

changes the array pattern in response to the radio

frequency environment, in order to improve the

performance of a communication or radar system. In the

switched beam system there is the beam forming

network which will form the multiple fixed beams and

cover certain angular are. Now the switched beam

system consists of a beam forming network, which

forms the multiple beams looking in the different

directions. Now there are two approaches to get the

multiple fixed beams, one is Butler Matrix Array and

another one is Blass Array This thesis presents the

Design,and Analysis of a beam forming network as a

key component of a switched beam smart antenna

system, operating at 5.3GHz for WLAN with a dielectric

substrate, FR4 of r =4.9 and h=1.6mm. Conception

details, simulation results and measurements are also

given for the components (microstrip antenna, hybrid

couplers, cross-coupler, phase shifter) used to implement

the matrix. In this dissertation, mathematical calculations

for all the components using MATLAB is done and then

every individual component is designed using the

commercial software SONNET. Then these entire

components have been combined on a single substrate

and simulated using SONNET.

I. INTRODUCTION Smart antenna [6] is one of the most promising

technologies that will enable a higher capacity in

wireless effectively reducing multipath and co-channel

interference. This is achieved by focusing the radiation

only in the desired direction and adjusting itself to

changing traffic conditions or signal environments.

Smart antennas [6] employ a set of radiating elements

arranged in the form of an array. The signals from these

elements are combined to form a movable or switchable

beam pattern that follows the desired user. In a Smart

antenna system the arrays by themselves are not smart, it

is the digital signal processing that makes them smart

[6]. The process of combining the signals and then

focusing the radiation in a particular direction is often

referred to as digital beam forming [7]. Smart antennas

are most often realized with either switched-beam or

fully adaptive array antennas. An array consists of two

or more antennas (the elements of the array) spatially

arranged and electrically interconnected to produce a

directional radiation pattern. A smart antenna system

consists of an antenna array, associated RF hardware,

and a computer controller that changes the array pattern

in response to the radio frequency environment, in order

Fig.1 Block diagram of 4x4 Butler matrix array

mailto:[email protected]


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to improve the performance of a communication or radar

system. Switched-beam antenna [8][9] systems are the

simplest form of smart antenna. By selecting among

several different fixed phase shifts in the array feed,

several fixed antenna patterns can be formed using the

same array. The appropriate pattern is selected for any

given set of conditions. An adaptive array controls its

own pattern dynamically, using feedback to vary the

phase and/or amplitude of the exciting current at each

element to optimize the received signal.

Smart or adaptive antennas [9] are being considered for

use in wireless communication systems. Smart antennas

can increase the coverage and capacity of a system. In

multipath channels they can increase the maximum data

rate and mitigate fading due to cancellation of multipath

components. Adaptive antennas can also be used for

direction finding, with applications including emergency

services and vehicular traffic monitoring. Smart

Antennas can be used to achieve different benefits. The

most important is higher network capacity, i.e. the

ability to serve more users per base station, thus

increasing revenues of network operators, and giving

customers less probability of blocked or dropped calls.

Also, the transmission quality can be improved by

increasing desired signal power and reducing

interference.

II. ANALYSIS AND DESIGN 90

0 HYBRID

Fig2. Geometry of 900 Hybrid

Quadrature hybrid [17] or 900 hybrid is a well known

device used for its ability to For Z0≤ 44 - 2r,

=

B-1-log(2B-1) +

[log(B-1) + 0.39-

] (1)

Generate signals 90 degree out of phase at its outputs.

Through Even-Odd mode analysis it can be shown that

the S matrix will have the following form,From the

above figure it is evident that the quadrature hybrid has

four ports, port 1 is the input port, port 2 is the output

port, port 3 is the coupled port and port 4 is the isolated

port. When power is applied to the port 1 it is equally

distributed in port 2 and port 3 and port 4 is isolated

since no power reaches it. There is a 900 phase

difference between port 2 and port 3. Basically the 900

hybrid is made by two main transmission lines shunt

connected by the two secondary branch lines. It has two

50Ω and two 35.4Ω transmission lines with length λ/4

[18]. So the perimeter of the square is approximately

equal to one wavelength.

The formulae for finding the length and width to height

ratio of the microstrip lines

CROSSOVER

In the butler matrix array [19][20] the signal path has to

physically crossover [18] while maintaining the high

isolation. This could be made by letting one of the

microstrip go over the other one, like a bridge, by adding

a substrate between them. But this gives extensive work

during the manufacturing and the signal is perhaps

nevertheless coupled or affected by the discontinuities.

Another possibility could be to use a multilayer substrate

with ground plane in the middle and the circuit elements

Fig.3 Geometry of Crossover

on the both sides. The major disadvantage is that it

cannot be manufactured without professional help and

that the vias are complex to be modeled in the

simulation. Now in this paper the crossover is

implemented by cascade of the two 900 hybrids with

slight modification on line widths. The geometry of the

crossover is given below.


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PHASE SHIFTER

The phase shifter [20] is implemented using microstrip

transmission lines. The length of the line corresponding

to 450 phase shift is given by the formula

Φ = (2π/λ) L ( 2)

Where L is in meters, φ is in radians. λ is the wavelength

in the microstrip line. The wavelength in the microstrip

transmission line is given by

λ= 0 /(

reff)0.5

(3)

Where λ0 is the free space wavelength and

reff is the

effective dielectric constant of the microstrip line. Since

the phase shift is implemented using simple transmission

line therefore it is linearly frequency dependent.

III. EXPERIMENTAL SETUP AND RESULTS

At first the important mathematical calculations for

finding the dimensions of all the individual components

is done using MATLAB and then the individual

components are designed and simulated using SONNET.

After getting good results all the individual components

are combined on a single substrate to implement the

Butler matrix and simulated using SONNET.

900 HYBRID

900 hybrid [23][24] is made by two main transmission

lines shunt connected by the two secondary branch lines.

It has two 50Ω and two 35.4Ω transmission lines with

length λ/4. So the perimeter of the square is

approximately equal to one wavelength. At first the

length and width of the shunt and series transmission

lines are calculated by using the equation. Now with the

calculated values the 900 hybrid is designed and

simulated by using commercial software SONNET. The

optimum design is given below

Fig.4 Optimum design of 900 Hybrid at 5.3GHz.

Simulation results of 900 hybrid

Fig.5 Current distribution at 5.3GHz

CROSSOVER

The crossover [24] is implemented by cascade of the two

900 hybrids with slight modification on line widths. The

crossover is implemented by 50Ω microstrip line.

Optimum design is given below

Fig 6. Optimum design of crossover at 5.3GHz

Fig 7. VSWR1, VSWR2, VSWR3, VSWR4 at 5.3 GHz.

PHASE SHIFTER

In this paper -450 phase shifter [2][23][24] is used. The

phase shifter is implemented using the microstrip lines

and the length of the microstrip lines are calculated by

the eq. Optimum design is given below.


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Fig 8 phase shifting = -49.210

IV. 4X4 BUTLER MATRIX ARRAY From the previous discussion it is evident that all the

individual components are designed and simulated. The

simulation results of the components are satisfactory.

Now all the individual components are combined on a

single substrate FR4 to implement the butler matrix

array [3]. Optimum design of butler matrix array is given

below

Fig 9. Optimum design of 4x4 butler matrix array at

5.3GHz

Fig 10 . Coupling levels S15, S16, S17, S18 at 5.3 GHz

Fig11 . Current distribution at 5.3 GHz

V. CONCLUSIONS This thesis presents the Design, Analysis and Gain

enhancement of a 4x4 planar Butler matrix array for

WLAN application. Initially all the necessary formulae

for dimensions of hybrid coupler, crossover, phase

shifter and radiating elements are evaluated using

MATLAB. Then all the components are designed and

simulated using commercial software SONNET. After

achieving proper simulation results of all individual

components, these components are then combined on a

single substrate and simulated using SONNET. Here a

high dielectric constant is considered for the design,

hence after studying the simulation results of individual

components and the whole structure it is evident that

high loss has occurred and after observing the radiation

pattern of the whole matrix when all the ports are excited

it is evident that properly four fixed overlapping beams

are not coming due to losses. To improve the radiation

pattern and the gain, a stacked antenna array is designed

with the butler matrix array. Basically another layer of

dielectric with the same dielectric constant is placed

above the whole matrix. Four rectangular patches of

modified width and length are placed on the second

layer. After simulation of the new structure of the butler

matrix array, better radiation pattern is achieved and the

gain is also satisfactory. Basically after seeing the

radiation pattern of the new structure it is evident that

four overlapping fixed beams are coming. But still losses

are there due to high dielectric constant. These designs

will cover 1200 cellular area, for larger coverage the

design should be extended to 8x8 matrix. Also to

increase the gain, different kinds of radiating elements

should be used.

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Beamforming Network for WLAN Application ,

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Documents

Design and Analysis of a Beam Forming Network for WLAN Application