User:Brian Carbone/sandbox

IEEE 802.22 is a draft standard under development by the IEEE which creates the standard of Wireless Regional Access Networks(WRAN). This standard provides a cost-effective way to provide internet access to areas with a low population density or areas with other geographical restrictions which make more traditional networks infeasible. IEEE 802.22 does this by operating in the TV frequency spectrum which is currently underutilized due to the recent analog to digital TV transition. IEEE 802.22 takes into consideration that there still may be some primary users in this spectrum, such as emergency broadcasts, small-scale devices like wireless microphones, and some digital stations, and uses techniques to both protect these primary users while still providing a quality of service to the secondary users of the WRAN network. IEEE 802.22 specifies the Physical Layer and Medium access control layer of the network on top of which the rest of the traditional protocol stack will run on top of.

Network Overview
The main components of the WRAN network will be the customer premises equipment(CPE) and the base station(BS). Each BS will operate in a cellular network fashion cover an area of 30-100km depending on local conditions. The base station will provide a downstream link of up to 22mbps and an upstream link minimum of 384kbps. Both the BS and the CPEs will have GPS capabilities. CPE will consist of an antenna similar to that of a TV antenna, typically to be mounted on the roof of the premises, a GPS antenna, and a modem which may be similar to a standard set top box which will handle all of the network data and sensing.

Need for IEEE 802.22
A modern Fiber-optic network requires cables to be run to every location that needs access which is costly, time consuming, and in some locations impossible due to local geography. Many rural areas may have relatively few people over a relatively wide area who may not all require internet access, so it would not make sense to construct a fiber-optic network in these areas. IEEE 802.22 will be a relatively low cost deployment method in some of the more rural areas of the world. All residents within the cell area of a BS will be able to have access with little construction at their location.

Technology
IEEE 802.22 will use two techniques for avoiding interference with primary users. Primarily the BS and CPE equipment will be location aware via GPS and capable of checking against an index of primary users maintained by the FCC to avoid interference. The secondary technique involves spectrum sensing at a set interval to determine if a primary user is present. This sensing will be done at both the BS and CPE ends of the network. If primary user is found to be operating on a given channel this information will be used to determine what the current channel or power level to use for the WRAN network. If a primary user is found on the current channel of operation a channel switch must be done in a fast an efficient manner to avoid interference while still maintaining service to the network. This dynamic nature of the wireless network may be known as cognitive radio. IEEE 802.22 is the first standard to have a practical implementation of cognitive radio.

PHY layer
The TV spectrum exists in the range of 54-862mhz. The modulation scheme chosen by the standard is OFDMA using symbol rates of QPSK and 16-64QAM, which was chosen due to it's ability to facilitate the fast switching required by the nature of a WRAN. In the US, the standard TV channel bandwidth is 6mhz which can provide up to 22mbps; although, in other areas the standard channel bandwidth can be 7 or 8mhz which will allow for even greater performance. It is also possible that two or more immediately adjacent channels can be used simultaneously in a channel bonding mode to allow for even greater performance increases.

MAC layer
The MAC layer of IEEE 802.22 will be supported by the principles and ideas of cognitive radio. The proposed technique for the layer is to use a superframe which is composed of many smaller frames. Each superframe will have a header which will contain a preamble which expresses if the channel is safe to operate on and also describing at what intervals sensing must be done as well as the operating scheme. Each CPE will be performing full spectrum sensing as well as current channel of operation sensing for primary users. Full spectrum sensing can be preformed simultaneously with Rx /Tx and the information sensed is sent to the BS at a given interval and used to make optimizations in operation. In channel sensing requires that a “quiet window” is scheduled between the BS and CPEs so that accurate sensing can be done, this is proposed to be done for one frame out of the superframe.

Challenges Facing IEEE 802.22
As with many network standards in development there are some challenges that exist that are still in need of proper solutions for the standard to succeed. Television stations are typically easy to detect since the signal broadcasted is very strong and typically the presence and operating schedule of the station is well defined to the BS. Detecting small-scale primary users which also operating in the same spectrum, such as wireless microphones, can be very difficult. Wireless microphones operate at a very low transmit power typically 10-50mW, are often mobile by nature, and have no defined operational schedule. Research is being done to help detect these users quickly and efficiently.

Real-time internet applications are becoming more and more prevalent in modern computer networks. These real-time applications may have problems running on a WRAN due to the required sensing windows and possibility of a channel switch at any time. Research is being done to over come this issue.