IEEE 802.22: The WRAN standard using white spaces in the TV band

Some time ago I wrote a post about the standard IEEE 802.16h [802.16h]. I was curious about the differences between IEEE 802.16h and the Wireless Regional Area Network (WRAN) standard IEEE 802.22 [802.22].

The standard 802.16h further extends the already complex specifications of 802.16 with coexistence mechanisms to allow the transmission in licensed bands. On the other hand, IEEE 802.22 pretends to be a lightweight standard with simple specifications focused on the task it is designed for: provide wireless network connectivity with up to 100km coverage (mostly targeted at rural and remote areas) in the VHF/UHF TV frequency spectrum. Hence the main difference between the two standards is the simplicity of [802.22] opposed to the generality of [802.16h], which supports different frequency bands, several mobility scenarios, a variety of access schemes, multiple antennas...

A comparison between the physical layer specifications of [802.22] versus the specifications of Wimax (subset of IEEE 802.16) follows:

Architecture

IEEE 802.22 specifies a hierarchical point-to-multipoint architecture. A central base station manages its own cell and all associated consumer nodes. The base station controls the medium access in its cell and transmits in the downstream direction to the the different nodes, which respond back to the base station in the upstream direction.

The coverage of IEEE 802.22 is considerably larger than the one of 802.16 because of the propagation characteristics of the VHF/UHF band. Note that in this band the physical size of a multiple antenna structure is of the order of meters, and hence, non-practical. Therefore multiple antenna specifications (such as beamforming or spatial multiplexing) are not supported in the 802.22.

Spectrum sensing

However, IEEE 802.22 requires two separate antennas at each particular user: one directional and one omni-directional. The directional antenna is pointed towards the base station and is used for communication purposes, while the omni-directional antenna is required for sensing purposes.

The base station asks the different nodes to perform sensing in certain television channels. It determines which user must measure which channels, and which is the desired probability of detection and false alarm (depending on the detection algorithms implemented at each consumer equipment, measurements can take different amount of time).

This monitoring can be both in-band and out-of-band. In-band sensing refers to the actual channel that is being used by the 802.22 network, while out-of-band sensing consist in monitoring the rest of the channels. Additionally, the nodes have the capability of performing coarse/fine two steps sensing. A coarse sensing estimation will be done at speeds of under 1ms per channel, while the fine sensing is slower (more than 25 ms per channel) and it can be used or not based on the outcome of the coarse monitoring.

Coexistence mechanisms

IEEE 802.22 introduces certain coexistence mechanisms for the case of having multiple secondary networks (based on 802.22 or of other nature) sharing the same spectrum within its coverage range (which can go up to 100 Km). For example, the standard allows different 802.22 networks to
synchronize their superframe structure.

References:
The content of this post is mainly based on the overview of the core technologies of the standard [802.22] presented here and here. Additionally, a good introductory paper to 802.22 is [C+06].

[802.22]	IEEE Std. 802.22.1 Base Standard (Sponsor Ballot Draft v2.0).
[802.16h]	IEEE Standard for Local and metropolitan area networks Part 16. Air Interface for Broadband Wireless Access Systems Amendment 2: Improved Coexistence Mechanisms for License-Exempt Operation. IEEE Std 802.16h-2010 (Amendment to IEEE Std 802.16-2009). July 30 2010.
[C+06]	Carlos Cordeiro, Kiran Challapali, Dagnachew Birru, Sai Shankar N IEEE 802.22: An Introduction to the First Wireless Standard based on Cognitive Radios. Journal of Communications, Vol 1, No 1 (2006), 38-47, Apr 2006.