Not more than in a few years, most of the internet connections have shifted from LAN-enabled broadband to completely wireless Wi-Fi connections. In terms of other spectral connections, the world is gradually shifting to the high caliber and highly efficient wireless form of connectivity. And with a rise in 5G networks and possibly 6G networks, such efficiency is set to lead with leaps and bounds. However, there lie some challenges in the path of gaining better connectivity and improved spectral efficiency. Amid this the question arises, how do we improve the spectral efficiency of wireless networks as a whole?
Before answering this question, we must look at the inefficiencies of current spectrum usage. First of all, the spectrum is assigned in a fixed manner by national regulatory bodies, and their main principle is to avoid radio interference, which is achieved by dividing the spectrum into bands (e.g., frequency division) that are allocated to one or more services. These radio services include mobile, satellite, amateur radio, navigation, and others. A license gives an exclusive right to operate (transmit and receive wireless signals) in a specific frequency band, in a specific location or geographic area. But much of the licensed spectrum remains unused in practice at different times and/or locations. Those temporary spectrum slots (aka spectrum holes or white spaces) can be as high as 15–85 % of the licensed spectrum.
Clearly, to improve the overall spectral efficiency, unlicensed users can be allowed to access such spectrum holes. Thus, this fact suggests the need for opportunistic spectrum access without causing undue interference to licensed users. Such capability is the defining characteristic of cognitive radio (CR) nodes, which require algorithms and protocols for rapid spectrum sensing, coordination, and cooperation. In other words, CR nodes can recognize unused parts of the spectrum and adapt their communications to utilize them while minimizing the interference on licensed users. Consequently, CR improves the overall spectrum usage, by moving away from static assignments into more dynamic forms of spectrum access.
According to a research report, CR networks can be divided into the following three paradigms:
Interweave networks: These operate on an interference-free basis and hold true to the original premise of utilizing spectrum holes (e.g., spectrum slots or chunks which are vacant or underutilized within a given geographical area). As soon as a spectrum hole appears, interweave devices can begin a data transmission but must end their transmissions when the sensing algorithms indicate that PU devices are resuming. Such algorithms include matched filter, cylostationary, signal energy or eigenvaluesbased detection, waveform sensing, and beacon detection.
Underlay networks: In these, both PU and SU devices simultaneously transmit over the same spectrum slots. Thus, there is no need to detect spectrum holes. However, the interference temperature experienced by a PU receiver must be below a threshold. To reduce the interference temperature, the SU devices may reduce their transmit power, cancel interference, and implement non-transmitting regions (guard regions) around primary receivers. These regions can be enforced either through prior location information from a centralized controller using a geolocation database, GPS (Global Positioning System) data, or sensing pilot signals originating from the PU nodes.
Overlay networks: These also allow concurrent PU and SU transmissions. However, the difference from the underlay mode is that SU devices must have knowledge about the PU transmitted data sequence encoding methods. This information can be utilized in two different ways. First, it can be used to cancel the PU interference on SU receivers, using canceling techniques such as dirty paper coding (DPC) that precodes transmitted data to negate the effects of interference. Second, it can be used by SU nodes to cooperate with the primary network by relaying PU messages.
Note: In the context of CR, licensed spectrum users are called primary users (PUs) and unlicensed users are called secondary users (SUs) or CR nodes.
Advantages of Cognitive Radio
As noted by Worcester Polytechnic Institute, there are 5 significant advantages of Cognitive Radio.
Overcome radio spectrum scarcity cognitive-radio-icon: By sensing spectrum utilization (irrespective of channel allocation), cognitive radios can broadcast on the unused radio spectrum, while still avoiding interference with the operation of the primary licensee.
Avoid intentional radio jamming scenarios: By sensing channel availability and even predicting the jammer’s tactics, cognitive radios can evade jamming by dynamically and preemptively switching to higher quality channels.
Switch to power saving protocol: By switching to protocols that trade-off lower power consumption for lower bandwidth, cognitive radios conserve power when slower data rates suffice.
Improve satellite communications: By predicting rain fade and reconfiguring transmitters/receivers for optimum bandwidth, cognitive radios improve communication quality when and where the information is needed most.
Improves quality of service (QoS): By sensing environmental and inadvertent man-made radio interferences, cognitive radios can select frequency channels with a higher Signal to Noise Ratio (SNR).
Role of 5G in Cognitive Radio
5G and Cognitive Radio (CR) are the two emerging technologies to meet the heavy mobile data traffic of future wireless networks. The new era of communication will be dominated by 5G in the future. As the future mobile broadband will be largely driven by ultra-high-definition video and as the things around us will be always connected, 5G aims to provide higher capacity and network speed of 10Gbps. 5G equipment will also be available at a lower cost, lower battery consumption, and lower latency than 4G equipment. 5G platform can empower the growth of many industries ranging from entertainment, agriculture, IT, and manufacturing industries. The need for more capacity will demand more spectrums resulting in the integration of CR in 5G networks. The focus of CR is to enable much more efficient use of the spectrum though it adapts itself to provide the optimum communications channel.