Patent Publication Number: US-9837702-B2

Title: Cognitive radio antenna assembly

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to communication systems, and particularly to a cognitive radio antenna assembly that includes an ultra-wide band sensing antenna and reconfigurable multiple-input multiple-output (MIMO) antennas and is operable in multiple bands between 700 MHz and 3 GHz. 
     2. Description of the Related Art 
     In modern wireless communications, the exponential growth of wireless services results in an increasing demand of the data rate requirements and reliability of data. These services can include high quality audio/video calls, online video streaming, video conferencing and online gaming, for example. These services can require wide bandwidth operation or covering operation across several frequency bands. This resulted in efforts to make efficient utilization of the available spectrum via sensing the available unused or underutilized bands. 
     Overcoming the inefficient and highly underutilized spectrum resources has led to the concept of cognitive radio (CR). CR systems are based on the structural design of software-defined radio (SDR) intended to enhance the spectrum utilization efficiency by interacting with the operating environment. A CR-based system should be aware of its environment by sensing the spectrum usage, and should also have the capability to switch over the operating points among different unoccupied frequency bands. CR-based systems may cover various features, including sensing spectrum of nearby devices switching between different frequency bands, and power level adjustment of transmitting antennas. 
     The front end of a CR can include two antennas, one being an ultra-wide band (UWB) sensing antenna and the other being a reconfigurable communication antenna. The UWB antenna can be used to sense the entire spectrum of interest, while the reconfigurable antenna can be used to dynamically change the basic radiating characteristic of the antenna system to utilize the available bandwidth. 
     Reconfigurable antennas are able to change their operating fundamental characteristics, i.e., resonance frequency, radiation pattern, polarization, and impedance bandwidth. A frequency reconfigurable antenna is a component of CR platforms. A feature of such an antenna is its switching across several frequency bands by activating different radiating parts of the same antenna. CR-based systems are capable of switching the frequency bands of single frequency reconfigurable antennas over different bands to efficiently and inclusively utilize the idle spectrum. 
     The high date rate requirement due to continuous escalation in wireless handheld device services can be accomplished by employing reconfigurable MIMO antenna systems. MIMO antenna systems are adopted to increase the wireless channel capacity and reliability of data requirements. A key feature of a MIMO antenna system is its ability to multiply data throughput with enhanced data reliability using the available bandwidth, which results in improved spectral efficiency. 
     To achieve the desired characteristics of reconfigurability and desired performance of MIMO antenna systems, several challenges need to be overcome to accomplish these tasks. These issues include the size of the antennas for low frequency bands, high isolation that is needed between closely spaced antennas, and control circuitry that is needed to be embedded within the given antenna size to achieve the desired reconfiguration. Moreover, the performance of the MIMO system degrades significantly for closely spaced antennas due to high mutual coupling. Additionally, a CR system requires an UWB sensing antenna to scan the wide frequency band. The design of the sensing antenna with the strict dimensions of a mobile terminal size can be a challenging job, as the sensing antenna is required to cover lower frequency bands as well. 
     Thus, a cognitive radio antenna assembly solving the aforementioned problems is desired. 
     SUMMARY OF THE INVENTION 
     The cognitive radio antenna assembly includes two boards, a main board that has an ultra-wideband antenna (UWB) and also serves as a ground plane for the reconfigurable antenna, and an elevated MIMO board having two planar inverted-F antennas (PIFAs) that are reconfigurable to selectively operate on different frequency bands. Each PIFA has a radiating patch having a slot bridged by PIN diodes and DC blocking capacitors on opposite sides of the slot. The resonant frequency of each PIFA is controlled by which diodes are switched on and off. The PIFA antennas are shorted to the ground plane (the UWB antenna) on the main board by shorting walls. The PIFA antennas are capable of resonating from the 700 MHz band through 3000 MHz, while the UWB senses the spectrum over the entire bandwidth. The antenna assembly is compact, being suitable for cellular phone and wireless applications in 4G networks. 
     These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of cognitive radio antenna assembly according to the present invention. 
         FIG. 2  is a bottom view of the main board of the cognitive radio antenna assembly of  FIG. 1 . 
         FIG. 3  is a top view of the upper or MIMO board of the cognitive radio antenna assembly of  FIG. 1 . 
         FIG. 4  is a bottom view of the upper or MIMO board of the cognitive radio antenna assembly of  FIG. 1 . 
         FIG. 5A  is a side view of the cognitive radio antenna assembly of  FIG. 1 . 
         FIG. 5B  is a front view of the cognitive radio antenna assembly of  FIG. 1 . 
         FIG. 6  is a plot showing the reflection coefficient curves of the cognitive radio antenna assembly of  FIG. 1  operating in Mode  1 . 
         FIG. 7  is a plot showing the reflection coefficient curves of the cognitive radio antenna assembly of  FIG. 1  operating in Mode  2 . 
         FIG. 8  is a plot showing the reflection coefficient curves of the cognitive radio antenna assembly of  FIG. 1  operating in Mode  3 . 
         FIG. 9  is a plot showing the reflection coefficient curves of the cognitive radio antenna assembly of  FIG. 1  operating in Mode  4 . 
         FIG. 10  is a plot showing the simulated mutual coupling curves of the reconfigurable MIMO antennas of the cognitive radio antenna assembly of  FIG. 1 . 
         FIG. 11  is a plot showing the measured mutual coupling curves of the reconfigurable MIMO antennas of the cognitive radio antenna assembly of  FIG. 1 . 
     
    
    
     Similar reference characters denote corresponding features consistently throughout the attached drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The cognitive radio antenna assembly includes two boards, a main board that has an ultra-wideband antenna (UWB) and also serves as a ground plane for the reconfigurable antenna, and an elevated MIMO board having two planar inverted-F antennas (PIFAs) that are reconfigurable to selectively operate on different frequency bands. Each PIFA has a radiating patch having a slot bridged by PIN diodes and DC blocking capacitors on opposite sides of the slot. The resonant frequency of each PIFA is controlled by which diodes are switched on and off. The PIFA antennas are shorted to the ground plane (the UWB antenna) on the main board by shorting walls. The PIFA antennas are capable of resonating from the 700 MHz band through 3000 MHz, while the UWB senses the spectrum over the entire bandwidth. The antenna assembly is compact, being suitable for cellular phone and wireless applications in 4G networks. 
     Referring to  FIGS. 1-5B , the cognitive radio antenna assembly  100  has a main board  102  and an upper or elevated MIMO board  106  raised above the main board  102  by spacers or standoffs. Each board  102 ,  106  is made from a flat sheet or panel of dielectric material that is clad with copper on both sides. The copper is etched or removed from the opposing faces of the boards  102 ,  106  to form the patterns shown in the drawings. The boards  102 ,  106  may be made from printed circuit boards. For example, the main board  102  may be made from printed circuit board having a dielectric constant ∈ r =4.4 and a thickness of 1.56 mm, and the upper or MIMO board  106  may be made from an FR-4 printed circuit board having a dielectric constant ∈ r =4.4 and a thickness of 0.8 mm. The height of the two-board assembly is about 5.8 mm. 
     The main board  102  may have dimensions of 65 mm×120 mm. The ultra-wideband antenna is a monopole antenna formed on the main board  102 . The sensing element  104  of the ultra-wideband antenna is formed on the bottom face of the main board  102 , as shown in  FIG. 2 . The sensing element  104  has a rectangular base measuring about 65 mm×54.72 mm and a trapezoidal portion extending from the base. The trapezoidal portion has a base leg of 65 mm, a parallel upper leg of 16 mm, and opposing diagonal legs of 39 mm. A 1.5 mm wide transmission line  103  extends from the upper leg of the trapezoidal portion to the edge of the main board  102  (a length of about 34.8 mm), terminating in a 3 mm wide terminal pad  105 . The center line of the transmission line  103  bisects the width of the main board  102  (about 32.5 mm from the longitudinal edge of the main board  102 . Two SMA connectors  116  are mounted on the upper two corners of the UWB sensing antenna  104 . As shown in  FIG. 1 , a rectangular ground plane  112  measuring 25 mm×40 mm is formed on the top face of the main board  102 . The ultra-wideband antenna is capable of sensing or receiving the entire spectrum from about 700 MHz to about 3 GHz. The sensing element  104  of the ultra-wideband antenna also serves as a ground plane or ground reference for the reconfigurable MIMO antenna on the upper or MIMO board  106 . 
     The upper or MIMO board  106  has two planar inverted-F antennas (PIFA)  108  formed thereon that are reconfigurable MIMO antennas.  FIG. 3  shows a top view of the upper or MIMO board  106  containing the two MIMO reconfigurable antennas, designated as left antenna  108   a  and right antenna  108   b  for clarity in Table 1, below. The upper or MIMO board  106  has dimensions of about 65 mm×30 mm. Each PIFA antenna  108   a ,  108   b  has a radiating patch having a slot bridged by PIN diodes  125   a ,  125   b ,  125   c , and  125   d , respectively, and DC blocking capacitors  124  on opposite sides of the slot Each patch has dimensions of about 28 mm×16 mm. Each slot is about 12 mm×6.3 mm. Each side of the slot has a 1.9 mm pad connected to the upper portion of the patch by a blocking capacitor  124  and connected to the lower portion of the patch by a PIN diode  125   a - 125   d . The PIN diodes have biasing circuitry  110  that includes a 1 μH RF choke in series with a 2.1 kΩ resistor, the passive components being designated  118  in the drawing. A voltage V cc  is applied at pads  120 , while a digital reference pad is shown at  122 . The two MIMO reconfigurable antennas  108  are similar in structure. 
       FIG. 4  shows the bottom face of the upper or MIMO board  106 . The bottom face of the MIMO board  106  includes radiating lines and coax feed-lines, and two feed points  126  for the two elements. The dimensions of the different radiating parts of the bottom layer of the PIFA are 12 mm, 3.4 mm, 1.7 mm, 16 mm, 1.7 mm, 8.6 mm, and 30 mm. 
       FIG. 5A  is a side view of the elevated PIFA, while  FIG. 5B  shows a front view of the MIMO reconfigurable antenna  108 . Both PIFAs are connected to the sensing element  104  of the UWB antenna through shorting walls  128  of width 1.7 mm extending between the edges of the upper or MIMO board  106  and the main board  102 . 
     Referring to  FIGS. 6-9 , the compact reconfigurable MIMO antennas system  100  can operate in four different modes depending on the state of the four PIN diodes  125   a - 125   b . The details of all modes are given in Table 1. The PIN diodes  125   a - 125   d  short the upper and lower portions of the PIFA patch antennas when they are turned ON (they are conducting), and leave the upper and lower portions open when they are OFF (they are not conducting) by adjusting the respective bias currents to the diodes  125   a - 125   d , thereby altering the electrical length of the PIFA patch antennas and their corresponding resonant frequencies. In mode  1 , the two resonating frequencies are 1093 MHz and 1900 MHz. The reflection coefficient curves  600  are shown in  FIG. 6  for both simulated and fabricated models. In mode  2 , both antennas were resonating at 770 MHz and 1640 MHz. The reflection coefficient curves  700  are shown in  FIG. 7 . Similarly, in mode  3 , the resonating frequencies are 994 MHz and 1500 MHz, while in mode  4 , the single resonating frequency achieved was 1740 MHz. The reflection coefficient curves  800  for mode  3  are shown in  FIG. 8  and the reflection coefficient curves  900  of mode  4  are shown in  FIG. 9 . The simulated coupling curves  1000  are shown in  FIG. 10  and the measured mutual coupling curves  1100  are shown in  FIG. 11 . Table 1 shows the switching state of the four PIN diodes  125   a - 125   d  in Modes  1  through  4 . Table 2 shows the resulting resonant frequencies in the four modes. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Diode Switching States in Mode 1 Through Mode 4 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                 Diode  
                 Diode  
                 Diode  
                 Diode  
               
               
                   
                   
                 1-LA- 
                 2-LA- 
                 3-RA- 
                 4-RA- 
               
               
                   
                 S. No. 
                 LD 125a 
                 RD 125b 
                 LD 125c 
                 RD 125d 
               
               
                   
                   
               
               
                   
                 Mode-1 
                 OFF 
                 OFF 
                 OFF 
                 OFF 
               
               
                   
                 Mode-2 
                 ON 
                 OFF 
                 OFF 
                 ON 
               
               
                   
                 Mode-3 
                 OFF 
                 ON 
                 ON 
                 OFF 
               
               
                   
                 Mode-4 
                 ON 
                 ON 
                 ON 
                 ON 
               
               
                   
                   
               
               
                   
                 LA = Left Antenna (108a) 
               
               
                   
                 RA = Right Antenna (108b) 
               
               
                   
                 LD = Left Diode 125a or 125c 
               
               
                   
                 RD = Right Diode 125b or 125d 
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Resonant Frequencies of PIFA Antennas 
               
            
           
           
               
               
               
            
               
                 S. No.  
                 Band 1 
                 Band 2 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Mode-1 
                 1093 
                 1900 
               
               
                 Mode-2 
                 770 
                 1640 
               
               
                 Mode-3 
                 994 
                 1500 
               
               
                 Mode-4 
                 1740 
                 — 
               
               
                   
               
            
           
         
       
     
     It will be seen that the antenna assembly  10  has a compact form factor, measuring 65×120 mm 2  and 5.8 mm high, rendering the assembly suitable for smart phones and LTE mobile handsets, as well as other compact wireless devices. The frequency range of the antenna assembly  10 , including an ultra-wideband antenna for sensing the spectrum for available frequencies and reconfigurable multiband MIMO transmit and receive antennas to support communications on any available frequency, makes it suitable for a cognitive radio platform for 4G devices. The planar structure of the antennas and operating characteristics of the antennas and control circuitry are easily integrated with other microwave or digital ICs and other low profile microwave components so that the assembly  10  can be easily accommodated within wireless handheld devices in wireless bands between 700 MHz and 3 GHz. Research for the above was funded by the National Plan for Science, Technology and Innovation (MAARIFAH), located in King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, award number 12-ELE3001-04. 
     It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.