Spectrum efficiency and energy efficiency are critical issues in wireless communication. The cognitive radio (CR) concept addresses the spectrum efficiency problem, while RF energy-harvesting (RFEH) techniques tackle the energy efficiency problem. A secondary user (SU) in a CR system must possess RFEH capabilities to achieve spectrum and energy efficiency, resulting in a radio frequency energy-harvesting cognitive radio (CRRFEH) system. In the CRRFEH system, the SU can transmit data when the primary user (PU) is not using the channel, while the SU harvests energy when the PU occupies the channel. The application of reconfigurable dielectric resonator antennas (DRAs) and filters in CR and RFEH is discussed. Different switches and CR techniques are compared.
CR and RFEH represent crucial technologies for the future of wireless communication. With the continuous growth of wireless services such as video conferencing, online video streaming, high-quality audio and video calls and online gaming, the demand for high data rates, improved signal efficiency and greater coverage is increasing. The large amount of data traffic and high data rate requirements are significant challenges. Wireless spectrum is in short supply and not evenly allocated, resulting in inefficiency in spectrum use.
CR OVERVIEW
The fundamental ideas behind dynamic spectrum resource utilization have given rise to the concept of CR. CR is defined by the Federal Communication Commission (FCC) as “A radio that can change its transmitter parameters based on interaction with the environment in which it operates.” The main aim of CR technology is to provide more efficient use of the wireless spectrum.
System-based CR is aware of its spectrum usage and can move between multiple vacant frequency bands (see Figure 1). The CR system first senses the spectrum with the help of a sensing antenna and observes activity; then, it determines the best communication channel and directs the communications antenna to operate in the desired manner.
Figure 1 CR operation.
The process of developing cognition is included in the last phase. This is accomplished by learning from earlier channel activities. CR techniques enable the device to self-decide and self-configure. There are two types of users in CR technology, PUs (primary or licensed users) and SUs (secondary or unlicensed users). A PU can always access the spectrum, while an SU can access the spectrum only when a PU is inactive.
Figure 2 Spectrum interweave CR.
CR networks may be spectrum interweave (see Figure 2) or spectrum underlay (see Figure 3). Interweaving requires both a sensing antenna and a communicating antenna. The sensing antenna is ultra-wideband (UWB). Its main task is to sense the entire spectrum and obtain information on PU activity, identify holes in the spectrum not used by the PU and assign these holes to the SU. The communication antenna enables the SUs to use the spectrum holes. For spectrum interweave, it is a narrowband reconfigurable antenna.
Figure 3 Spectrum underlay CR.
For underlay CR, the PU sets a predetermined threshold for an acceptable interference level. This threshold ensures that the SU can access and use the spectrum concurrently with the PU without causing significant performance degradation. If the SU's communication on a particular frequency band impacts the PU's performance, the SU promptly ceases communication in that band. In this case, the communication antenna functions as a UWB antenna with a reconfigurable band notch for SU communication. This is required to facilitate selective communication and avoid interference with the PU. The tunable band notch prevents the SU from transmitting on frequencies that could disrupt the PU's operation. It enables precise control over the frequency bands used by the SU for communication, ensuring effective coexistence with the PU.1-3
CR. RFEH OVERVIEW AND CR INTEGRATION
The effectiveness of RFEH in wireless communication depends on factors such as the wavelength of the harvested energy signal and the distance between the harvesting device and the RF energy source. The integration of CR with RFEH offers the potential for efficient spectrum utilization and energy consumption solutions. With CRRFEH capabilities, wireless devices (SUs), can extract RF energy from RF signals and use this energy for data transmission. The RF signals originate from various sources, including cellular base stations, PUs and the ambient environment. These signals are then converted to DC for storage. SUs can then access this stored energy to transmit data.
In the CRRFEH system, SUs take advantage of idle periods when nearby PUs are not using the spectrum, or they are located far from the PUs. The system identifies these spectrum holes and actively searches for available bands that can be used to harvest RF energy. By leveraging these opportunities, the CRRFEH system makes efficient use of the RF energy resources available. Table I shows RFEH data from various RF sources. It is apparent that more RF energy is harvested when the distance between the harvesting device and the RF source is shorter.4-7
TABLE I RFEH MEASURED DATA
CR UWB ANTENNA
The CR UWB antenna can be either a microstrip patch antenna or a DRA. When compared to a microstrip patch, the DRA has several advantages. It possesses a wider impedance bandwidth, enabling better frequency coverage and it eliminates surface waves that typically occur on the ground plane, improving radiation efficiency. There are also a range of shape options, such as rectangular, cylindrical and hemispherical, providing flexibility in design. A notable advantage is the absence of conductor losses, resulting in high radiation efficiency. This characteristic makes DRAs suitable for various applications across microwave and optical frequency bands.8-10
RECONFIGURABLE ANTENNA (RA) TECHNIQUES
The classifications of RAs are shown in Figure 4. Frequency reconfigurability varies its operating frequency by shifting its passband. This is accomplished by tuning the antenna's reflection coefficient discretely or continuously. RF switching can provide discrete tuning, while electrical tuning elements such as varactor diodes can provide continuous tuning. Polarization reconfigurability varies the antenna’s polarization. Pattern reconfigurability modifies the antenna’s radiating pattern in terms of direction, gain and shape. A hybrid RA is the combination of all these approaches. For CR, frequency reconfigurability is the only type that applies.
Figure 4 Reconfigurable antenna types.
Many RA techniques for frequency reconfigurability are available such as electrical, optical, metamaterial, mechanical, and material (see Figure 5). The selection of the proper switching mechanism requires consideration of the application and the performance characteristics that are most important (see Figure 6).
Figure 5 RA techniques.
Figure 6 Switching mechanism selection.
With electrical RA techniques, characteristics are changed using electronic switching elements such as varactor diodes, PIN diodes, field-effect transistor transistors and/or radio frequency microelectromechanical switches (RF MEMS). These elements change the surface current distribution in the antenna radiating structure.
PIN diodes are commonly used as switching elements in wireless communication systems. A PIN diode has a high switching speed (on the order of 1 to 100 nsec) and a high-power handling capability but requires high current bias in the ON state. It is very reliable and low cost; hence it is a good choice for the RA application.11,12
A varactor diode’s frequency tuning property is achieved by adjusting its DC bias voltage to vary its capacitance. This can be done in a continuous fashion and is often used in conjunction with discrete switching techniques. However, it requires a source of high DC voltage.
RF MEMS switches are small mechanical switches placed on a substrate. They have high isolation, low power consumption and low insertion loss. However, switching speeds are low and they require high control voltages compared to PIN and varactor diodes.
For optical RA techniques, reconfiguration is accomplished with photoconductive switches. These switches are made of a semiconductor material (Si or Ge) and are actuated by the light from laser beams. Laser diodes must be appropriately integrated and controlled on the structure. The disadvantages of this mechanism are the expensive cost, slow (microsecond) tuning, power consumption of the laser diodes and misalignment of the laser beams on the switches. Compared to electrical switches, however, optical techniques have less interference and provide better isolation.
By physically altering the antenna radiating components, the electrical properties of antennas may be adjusted using mechanical techniques. This approach requires no biasing lines, switching instruments or even an optical fiber-to-laser diode connection. However, its effectiveness depends on how the device is physically reassembled. The flexibility and performance of this technique is limited compared to others.
