1. Primary users are the users who are licensed to operate in a particular spectrum band.
2. Spectrum holes are space available in a spectrum band that can be utilized by unlicensed users. Spectrum holes are created and removed dynamically in real-time.
Limitations of Spectrum Sensing Techniques
Spectrum Sensing is a best CR solution but it requires a large amount of sensing time. It also requires a complex algorithm to achieve reliability.
- Hardware Complexity
- High Cost
- Long sensing time as CR devices have to scan the frequencies of primary users one by one to detect availability of spectrum holes
- Requires prior knowledge of synchronization signature of all primary users in various spectrum bands
- This technique is susceptible to synchronization errors which can cause false detection of primary users
- Requires prior knowledge of waveform patterns of all primary users in the various spectrum bands
- Noise variance and uncertainty makes this technique unreliable as CR devices are unable to detect transmitted signal from primary users
- High computational complexity
Cooperative sensing technique needzzs to intelligently combine inputs from various CR devices and ascertain the availability of spectrum holes. The accuracy of the algorithm is critical here as incorrect detection can inversely affect the performance of primary users in spectrum bands. Designing such a ‘perfect algorithm’ is a challenge
Technique 2 - Spectrum Database
Federal Communications Commission (FCC) proposed a spectrum database concept to remove complexity of spectrum sensing technique and to use TV white space. All TV stations need to update their next week usage in database maintained by FCC. CR devices can seek free spectrum information from this database. CR devices will have knowledge about free spectrum for use and can negate the need for complex sensing which requires time and money.
Limitation of Spectrum Database Technique
A significant percentage of spectrum holes are created dynamically for a short duration of time. It is very difficult for a database to update dynamic and real-time activity in spectrum. This presents a massive opportunity loss for CR devices (especially in the IoT ecosystem where several devices need to transmit a small amount of information which can be effectively done on dynamically created spectrum holes).
A Consolidated Approach to Drive CR
Efficient use of spectrum is necessary to support the growing number of Massive IOT devices. Unused spectrum can be available in large portions (such as TV white space) or as spectrum holes. Depending on needs and in an opportunistic method both white space and spectrum holes need to be used to satisfy future spectrum requirement. A combined approach could prove ideal for CR solution to meet the massive spectrum demands.
An easy to access local database (maintained in network component) and spectrum sensing technique in CR device can give a better solution for Cognitive Radio. The database should contain below information.
- Primary user’s real-time usage (in term of frequency, time, space, transmission characteristics)
- History of primary user’s usage pattern in that region in term of frequency, time, power, transmission characteristics
A. Quick and accurate detection of unused spectrum
Any CR device can start searching spectrum holes which are indicated in the database. If database shows some real-time spectrum holes then CR devices can directly start using it otherwise from the historic information it can understand Primary User’s usage pattern in that region and start spectrum sensing to find out the spectrum holes. As the CR device knows about primary signal characteristics in a particular time from database, spectrum sensing can be less complex, accurate and less time consuming.
B. Optimized search and lower battery consumption
By combining information from the database, history of primary user’s usage patterns and signal characteristics of primary users, CR devices need not search the entire spectrum for availability but can rather zone in on a particular region. This process will save time and battery consumption.
The consolidated approach highlighted in this paper can help create less complex, cost effective solution. It can drastically reduce operator’s investment as CR use unlicensed spectrum. As the IoT phenomenon grows, tens of billions of devices will need to communicate with each other in real-time. The highlighted CR approach will help operators cater to massive spectrum requirements and help build a connected world.