IEEE 802.22 Physical Layer / Air Interface
- an overview of the basics of the IEEE 802.22 WRAN physical layer / Air Interface, including the modulation techniques used.
The physical layers of the IEEE 802.22 system have been designed to enable a sufficient level of performance to be obtained along with the requirement to ensure the system is able to maintain its use of "free" or white space within the television spectrum. To achieve this the system requires a considerable degree of flexibility as well as the implementation of many cognitive radio techniques.
802.22 physical layer
In order to meet the requirements for the overall 802.22 system, the physical layer maintains a high degree of flexibility. This is built in to the basic specification of the system.
One of the first characteristics is the modulation scheme. An OFDM scheme has been adopted because the 802.22 WRAN system to provide resilience against multipath propagation and selective fading as well as a high level of spectrum efficiency and sufficient data throughput. To provide access for multiple users, OFDMA is used for both upstream and downstream data links.
Note on OFDM:
Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..
Click on the link for an OFDM tutorial
IEEE 802.22 allows a variety of modulation schemes to be used within the OFDMA signal: QPSK, 16-QAM and 64-QAM can all be selected with convolutional coding rates of 1/2, 3/4, and 2/3. The required modulation and error correction rates are chosen according to the prevailing conditions.
In order to meet the requirements for the individual users that may be experiencing very different signal conditions, it is necessary to dynamically adapt the modulation, bandwidth and coding on a per CPE basis.
In order to be able to obtain the required level of performance, it has been necessary to the IEEE 802.22 to adopt a system of what is termed "Channel Bonding." This is a scheme where the IEEE 802.22 system is able to utilise more than one channel at a time to provide the required throughput. Often it is possible to use adjacent channels because in many countries the regulatory authorities and frequency planners allow two or more empty channels between stations transmitting high power signals as this prevents interference on the TV signals. These multiple free channels allow the use of contiguous channel bonding. In practise the maximum number of channels that are bonded is likely to be limited to three as a result of the front-end bandwidth limitations.
To provide access for both upstream and downstream data, the form of duplex scheme that has been adopted is TDD. This has several advantages. First it only requires one channel to be used - FDD would not be viable because it would be more difficult to control two channels with sufficient transmit / receive spacing. Secondly the use of TDD enables dynamic change of the upstream and downstream capacity.
802.22 medium access control
The fact that the IEEE 802.22 standard is so flexible brings a number of new challenges to the practical implementation of the system. Accordingly the MAC has been designed to provide flexibility and to incorporate these new ideas.
In the first instance the initialisation and network entry needs to accommodate the elements of the spectrum usage flexibility. As there is not fixed channel for the system, and no pilot channel can be broadcast, any CPE when turning on and initialising needs to be able to find the signals. Accordingly, when initialising, any CPE first scans the available spectrum to look at channel occupancy. It will detect those channels free of television transmissions. In the remaining empty channels it will then scan for base station pilot signals and acquire any network information. Once it has acquired the correct network it can then proceed to connect to the network.
It is also necessary to have a defined format for the data. To enable the data to be suitably structured, the transmission is formatted into frames and superframes.
- Superframes: The superframe is built up from the smaller frames. The super-frame is used to provide overall synchronisation for the system, and in particular provides the initial network access / entry initialisation. At the beginning of each superframe, there is a preamble known as the Superframe Control Header, SCH. The SCH contains the information needed for any new CPEs that need to access the base station,
- Frames: The superframe is built up from frames. These frames are split into two elements: the upstream subframe (US) and the downstream subframe (DS). The boundary between these two subframes is variable and can be adapted to accommodate changes on the levels of upstream and downstream capacity required.
IEEE 802.22 equipment is designed to ensure that no undue interference is caused to existing television services. As a result the whole system has to be adaptive to ensure that the system avoids channels that are in use while still maintaining the required throughput. It even ahs to adapt to changes in radio propagation that may occur from time to time. As a result the cognitive radio and cognitive network technology has been incorporated to ensure this requirement is met.
By Ian Poole
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