This is the first part of my project blog for the RF (Radio Frequency) Project 14 submission.
First, I would like to discuss the basic idea of my project and then cover the following topics-
- What is spectrum sensing and why is it needed?
- Types of printed antennas and understanding various parameters of antenna
- How to achieve wide bandwidth in antennas
I'm a research scholar from India. I primarily work on RF/Microwave antennas. However, apart from antennas, I'm also interested in building RF circuits and Software Defined Radios. Here in the first part of my blog I'm giving a detailed overview of my idea for the RF project 14 competition.
Spectrum sensing is important when you want to efficiently use the available spectrum or in case of cognitive radio. Here, I propose to design a system which can do spectrum sensing in a broadband. Firstly, by designing a broadband antenna, intergrating it with a NI USRP software defined radio and implementing a spectrum sensing technique in LabVIEW.
NI USRP is a SDR from National Intruments and LabVIEW is a GUI based programming software to control and automate hardware. LabVIEW is used to control the SDR and implement the spectrum sensing algorithm.
I will post more details about these in the next part of the blog. For the first part, I will discuss the basics of antennas and spectrum sensing.
What is spectrum sensing and why is it needed?
Spectrum sensing is detecting the presence of a 'user' (A transmitter) in a given frequency band. There are various methods/algorithms used to detect the presence of a user in a spectrum. These days lot of research is going on in spectrum sensing for cognitive radio, MIMO systems, etc. With the sub 6GHz spectrum becoming 'crowded' , now we are moving towards the mmWave spectrum so that we can cater the needs of more bandwidth, higher data rate. But the mmWave spectrum has got it's own pros and cons.
The pros of using the mmWave band are-
- Higher data rate
- More bandwidth
- Smaller antenna size
The cons of using mmWave band are -
- High propagation loss in medium (more attenuation) which leads to increase in number of transmitter to cater a large area (broadcast)
- High transmit power required to cover larger distance
- Different algorithms required for channel estimation in case of mmWave MIMO
Hence, in the upcoming 5G standards, there are two bands the sub 6GHz band and the mmWave bands.
The idea behind cognitive radio is that the spectrum can be utilized by two users at two different times. The licensed user is called as the primary user and the unlicensed user is the secondary user.
The primary user is authorized to use the band but sometimes the primary may not be transmitting anything, in that case the band is free and the secondary user (unlicensed user) can use the band for the time being.
This is the basic idea of cognitive radio.
In order to let the secondary user use the spectrum, the secondary user has to first detect the presence of the primary user in the band using spectrum sensing methods. If primary user is not detected in the spectrum, the secondary user can start it's transmission. This way, we can share the sub 6GHz spectrum among multiple users without going to the mmWave band.
Types of Printed Antennas
The first step is to design a broadband antenna. Various types of printed antennas can be found in literature which are used for different applications. A printed antenna is a metallic patch printed on a PCB substrate.
A simple rectangular microstrip patch antenna (RMSA) for 2.4GHz is shown in the pictures below. The length and width of the patch are calculated using certain equations which are beyond the scope of this first blog post.
Pic 1. Top view of RMSA Pic 2. Bottom view showing the ground plane and
coaxial probe feed
There are various other types of printed antennas available such as the printed dipole antenna, slot antennas, printed monopole, PIFA, etc.
Return loss or the S11 plot - The return loss of the antenna or S11 parameter gives the power being reflected from the input port of the antenna back to the transmitter. For efficient radiation, the reflected power from the antenna has to be as less as possible. The return loss is given by RL = 10log(Pr ) where, Pr = reflected power and Pin is input power.
For an antenna to radiate efficiently, a return loss of -10dB is considered.
But why -10dB?
Let's try to calculate from the equation above. RL = -10dB, consider Pin = 1mW.
Pr = 10^(RL/10)*Pin = 10^(-1)*1mW = 0.1mW which shows that the reflected power is 0.1mW meaning that 90% of the input power is going to the antenna and only 10% of the input power is being reflected back.
Hence, -10dB is used as the standard value by many authors. Some even consider -6dB as a good value.
The higher the value of return loss, much better is the antenna in radiating the power.
Given below is the S11 vs. frequency plot for the above RMSA which I have simulated using CST Microwave studio 2018 software.
Return Loss vs frequency
As it is seen from the above graph, the |S11| value is highest for 2.4GHz (curve marker 1), hence this antenna is suitable for 2.4 GHz applications. The -10dB bandwidth of the antenna can be calculated by taking the two points n x-axis from where the S11 curve crossed the -10dB points on the y-axis of the graph(curve markers 2 and 3). In this case the bandwidth is not more the 100 MHz. Beyond the markers, the return loss of the antenna is less than 10dB which means there will be high reflection. Hence, for applications that require a wider bandwidth a simple RMSA cannot be used.
Radiation pattern - The radiation pattern of the antenna gives the direction in which the power is radiated by the antenna.
Given below is the radiation pattern for the 2.4GHz RMSA -
It shows that the maximum power is radiated perpendicular to the patch which is also called as broadside radiation.
Ways to improve bandwidth of the antenna -
There are various ways to increase the bandwidth of an antenna, by cutting slots in the rectangular patch, by using partial or defected ground structure, or using slot antennas or using printed monopole antennas.
In my project, I'm going to design a printed monopole antenna which can give bandwidths as large as 1GHz and more.
In part 2 of the blog I will discuss the design of the printed monopole antenna, the calculations and simulation results.