WRAN which stands for Wireless Regional Area Network is designed and brought to existence, especially for rural areas, developing countries, places with low population density. IEEE 802.22 is the standard developed for Wireless Regional Area Network which uses white spaces of the Radio Frequency (RF) spectrum to provide the broadcast connection in the areas that are difficult to reach. White spaces are the unused part of a frequency spectrum. This is done with the help of cognitive radio (CR) which is designed for wireless systems to CPEs avoid interference and crowding.
WRAN is performed in unused channels in the Television (TV) spectrum to get finer propagation of signal at low frequencies. WRAN is the first system that was to regulate the effective use of unused broadbands of the TV spectrum with less interference all around the world.
Need of WRAN :
The prerequisites needed to use frequency spectrum are very high and the use of the entire spectrum is not efficient. A report taken from 2004 to 2005 proclaimed that only 5.2% of the entire spectrum is in use, which called the need to design a network that can efficiently use the remaining unused bands of a spectrum. The former systems were not enough to cover the entire geographical regions including the rural areas. But, it is a need to provide the service to people in that area because many countries are still in developing states and have many rural regions. The TV spectrum is selected specifically for WRAN due to its large vacant channels and the devices of WRAN need not be licensed. So, the cost is affordable.
Topology Used in WRAN :
IEEE 802.22 uses point to multipoint network topology, which means a single base station can provide service to multiple numbers of subscriber stations. The requirement for this setup is that one base station (BS) and any number of customer premise equipment (CPE) must be installed. The base station controls all Radio Frequency (RF) activities. The relationship between BS and CPE is a master-slave one. The functions involved are Radio Frequency (RF) sensing and receiving or transmitting signals. The RF sensing is performed by the base station in regular intervals and the reports are sent by CPE to the base station. Based on the report, BS decides which vacant channel of spectrum must be used to carry communication with CPEs. The CPEs are designed with two antennas. One antenna is used for communicating with the base station. Another omnidirectional antenna is used for collecting the reports and measurements.
Database interface model in WRAN :
Database interface model in WRAN
With the regulations provided, the WRAN interface to the CPEsdatabase must be directly connected between the base station and database service rather than the individual CPE itself. This is because the base station must find a vacant channel common to all the CPEsother than for individual CPEs.
Considerations in WRAN :
There are two layers which are needed to be considered carefully while designing the WRAN, PHY consideration and MAC consideration.
PHY consideration (Physical Consideration) :
- WRAN should have a high rate of performance. It must also be economically and technically efficient.
- CPEs must have the ability to withstand various conditions and they also should have the flexibility of jumping from one channel to another without any transmission errors.
- It must be highly flexible in terms of bandwidth adjustment, coding techniques, etc.
- Transmission Power Control (TPC) and frequency agility are two important factors while designing WRAN.
MAC consideration (Media Access Control) :
- The base station is the master which means any other CPEs should not initiate or carry any transmissions without the approval of the base station.
- CPE should be responsible for the identification of the correct vacant channel and should inform the base station indicating its presence.
- The cyclic spectrum sensing by the base station and feedback transfer from the CPEs must continue during transmission because if there is any need to change the channel occurs, it can be executed at any time.
- The base station sends a preamble signal to the CPU at the start of each frame to differentiate its message from TV broadcast messages.
Spectrum sensing :
The Spectrum sensing is performed by customer premise equipment (CSP). CSP observes and makes records of Radio Frequency (RF) to find the unused channels of the TV spectrum.
In-band spectrum sensing :
It finds the unused channels of the spectrum that are accessible for transmission for the users of cognitive radio. There are two types, Fine sensing, and Fast sensing.
Fine sensing :
- Fine sensing happens when the base station identifies the need for more accurate measurement.
- Sensing takes up to a time of 25 – 30 ms.
- In fine sensing, the CSP focus on the sense of the signals that may be a prime user of the spectrum(Television).
Fast sensing :
- The sensing task is performed fastly.
- The algorithm used is the energy detection algorithm.
- The time taken for sensing is 1ms.
Applications of WRAN :
The primary application of IEEE 802.22 is the availability of wireless broadband in distant areas, rural areas, or simply hard-to-go areas. WRAN gives access to the transmission of audio and video, voice, data, and support of Quality of service (QoS). In rural areas, the usage of lower frequency is effective due to the required conditions of the cognitive radio which experience low-frequency transmissions which are legally accepted for TV broadcasting and wireless microphones. From the spectrum, the unused channels are identified and then the TV signal is delivered using satellite. WRAN is widely used in startup companies and small businesses.
Advantages of WRAN :
- The users of the WRAN network are unlicensed which means IEEE 802.22 is cost-effective.
- The geographical range covered is higher than the other IEEE 802 standards.
- The spectrum is used efficiently by using unused channels.
- The service provided by the WRAN is wide like data, voice, video, etc.
- WRAN provides the rural areas with the benefit of broadcast services.
Disadvantages of WRAN :
- The propagation delay in IEEE 802.22 is more compared to other standards of IEEE 802.
- Self coexistence: The overlapping networks of 802.22
- Hidden Incumbent: The spectrum that can be sensed by the CPEs but not by the base station is called a Hidden Incumbent. The enhanced version of IEEE 802.22 standard solved the self coexistence and hidden incumbent problem using the graph coloring model and multiple broadcasting respectively.