IEEE 802.16h - the cognitive Wimax
The proposed global framework is divided in two separated profiles. The first one provides uncoordinated coexistence mechanisms (WirelessMAN-UCP), i.e., without requiring much interaction among the different systems and hence adequate for heterogeneous systems, while the second provides coordinated coexistence mechanisms (WirelessMAN-CX), which addresses the required coordination of neighboring systems in order to reduce the interference generated to each other.
Uncoordinated coexistence mechanismIn an real scenario a Wimax network may coexist with licensed users (denoted in the amendment as specific spectrum users) and other unlicensed users (denoted as non specific spectrum users) sharing the same frequency band. In such case the interference that the Wimax system may cause to each of the users can be different. [802.16h] defines three possible levels of interference:
- Acceptable interference: This level of interference does not cause degradation in the receiver performance for a given choice of modulation and/or coding. This interference is admissible for both licensed and unlicensed users.
- Harmful interference: Strong interference that decreases the link performance in terms of modulation/coding. While this interference must be avoided in licensed links, some amount of communication is still possible and thus could be acceptable for unlicensed users.
- Destructive interference: The receiver is not capable of decoding the received signal for any available modulation at the transmitter. It must be avoided.
In order to achieve these acceptable interference levels, both to licensed users and unlicensed users, the standard provides a set of mechanisms. These include:
- Testing channels for other users.
- Discontinuing operations after detecting channel activity.
- Detecting other users.
- Scheduling for channel testing.
- Requesting and reporting measurements by different nodes.
- Selecting and advertising a new channel.
While all these procedures are described in detail in the standard, the timing and threshold parameters used are left open and must be specified by each regulatory administration. In the case of unlicensed users the proposed procedures are still valid with minor variations. For example, instead of performing a search for channels free of licensed users, the cognitive network looks for the best set of channels for operation either when certain unlicensed users are present.
The standard [802.16h], in the uncoordinated profile, permits distributed architectures for the radio resource management within the network formed by one 802.16 base station (BS) and its associated subordinated nodes. Each BS has a Distributed Radio Resource Management entity to execute the spectrum sharing policies of 802.16h and to build up a database for sharing information related to actual and intended future usage of radio spectrum. This database can be recovered from a master entity with the required information or from different devices (e.g. using the GPS, IP address, operator information, radio signature scheduling info...).
In order to avoid the regulation infringement the different interferers must be identified by their radio signature, which can be a short preamble, peak power, relative spectral density... Every transmitter will send the radio signature
during an interference-free slot. The time position of this slot (frame_number , sub-frame, time-shift) will be used for identification.
Once the environmental data is available the Radio Resource Management entity performs a real-time, adaptive scheduling, which can be done in terms of channel or even interference free regions within a MAC frame.
Coordinated coexistence mechanismWhen multiple secondary networks coexist in the same region, they can collaborate in order to coordinate their transmissions and build a neighbor relationship. In the standard [802.16h] the following three basic mechanisms for achieving coexistence are provided:
- MAC Frame Synchronization, including Tx and Rx intervals, for separating transmissions and enabling operation in synchronized zones.
- Dynamic Channel Selection (DCS) and Adaptive Channel Selection (ACS) for finding a less interfered or less used frequency (similar to the uncoordinated case).
- Separation of the remaining interference in the time domain, by using a Coexistence Frame, coordinated scheduling, and a fairness approach, thus allowing the usage of a frequency channel by more than one system.
Note the high degree of awareness and cooperation required for implementing these mechanisms. To this end the standard defines a Coexistence Control Channel based on a series of globally synchronized time-slots and used for inter-network coordination.
In this post I tried to summarize the main capabilities supported by the IEEE 802-16h amendment. In my opinion this standard on the one hand address the FCC rules for unlicensed access to unused television spectrum and on the other hand is flexible enough to allow inter system collaboration and future regulations, hence the title of the post: cognitive Wimax.
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. doi: 10.1109/IEEESTD.2010.5538195