Technical Information

The IEEE 802.16 Air Interface Standard is truly a state-of-the-art specification for fixed broadband wireless access systems employing a point-to-multipoint (PMP) architecture. The initial version was developed with the goal of meeting the requirements of a vast array of deployment scenarios for BWA systems operating between 10 and 66 GHz. As a result, only a subset of the functionality is needed for typical deployments directed at specific markets. A revision to the base IEEE 802.16 standard targeting sub 11 GHz is near completion with a publishing target date of July 2004. This revision will include the amendments from Task Group c, Task Group a, and Task Group d.

The IEEE process stops short of providing conformance standards and test specifications. In order to ensure interoperability between vendors equipment, the WiMAX technical working groups have completed the work for 10 to 66 GHz and has started work for the sub 11 GHz part of the standard. The working groups develop a set of system profiles, Protocol Implementation Conformance Statement Proforma, Test Suite Structure & Test Purposes, and Abstract Test Suite specifications for 10 to 66 GHz and sub 11 GHz, all according to the ISO/IEC 9464 series (equivalent to ITU-T x.290 series) of conformance testing standards.

Overview of IEEE 802.16

The IEEE 802.16 Working Group has developed point-to-multipoint broadband wireless access standard for systems in the frequency ranges 10-66 GHz and sub 11 GHz. The standard covers both the Media Access Control (MAC) and the physical (PHY) layers.

A number of PHY considerations were taken into account for the target environment. At higher frequencies, line of sight is a must. This requirement eases the effect of multipath, allowing for wide channels, typically greater than 10 MHz in bandwidth. This gives IEEE 802.16 the ability to provide very high capacity links on both the uplink and the downlink. For sub 11 GHz non line of sight capability is a requirement. The original IEEE 802.16 MAC was enhanced to accommodate different PHYs and services, which address the needs of different environments. The standard is designed to accommodate either Time Division Duplexing (TDD) or Frequency Division Duplexing (FDD) deployments, allowing for both full and half-duplex terminals in the FDD case.

The MAC was designed specifically for the PMP wireless access environment. It supports higher layer or transport protocols such as ATM, Ethernet or Internet Protocol (IP), and is designed to easily accommodate future protocols that have not yet been developed. The MAC is designed for very high bit rates (up to 268 mbps each way) of the truly broadband physical layer, while delivering ATM compatible Quality of Service (QoS); UGS, rtPS, nrtPS, and Best Effort.

The frame structure allows terminals to be dynamically assigned uplink and downlink burst profiles according to their link conditions. This allows a trade-off between capacity and robustness in real-time, and provides roughly a two times increase in capacity on average when compared to non-adaptive systems, while maintaining appropriate link availability.

The 802.16 MAC uses a variable length Protocol Data Unit (PDU) along with a number of other concepts that greatly increase the efficiency of the standard. Multiple MAC PDUs may be concatenated into a single burst to save PHY overhead. Additionally, multiple Service Data Units (SDU) for the same service may be concatenated into a single MAC PDU, saving on MAC header overhead. Fragmentation allows very large SDUs to be sent across frame boundaries to guarantee the QoS of competing services. And, payload header suppression can be used to reduce the overhead caused by the redundant portions of SDU headers.

The MAC uses a self-correcting bandwidth request/grant scheme that eliminates the overhead and delay of acknowledgements, while simultaneously allowing better QoS handling than traditional acknowledged schemes. Terminals have a variety of options available to them for requesting bandwidth depending upon the QoS and traffic parameters of their services. They can be polled individually or in groups. They can steal bandwidth already allocated to make requests for more. They can signal the need to be polled, and they can piggyback requests for bandwidth.

The Interoperability Challenge

Plethora of Options
From the preceding overview, it is clear that the IEEE 802.16 Air Interface Specification is a very capable, while complex, specification. There are allowances for a number of physical layers for different frequency bands and region-by-region frequency regulatory rules. There are features that allow an IP centric system or an ATM centric system depending upon the needs of customers. The specification is designed to cover application to diverse markets from very high bandwidth businesses to SOHO and residential users.

Because of the wealth of options available, an implementer currently faces a tough decision. To address this issue WiMAX undertaken the development of System Profiles.

The purpose of these system profiles is to specify which features are mandatory or optional for the various MAC or PHY scenarios that are most likely to arise in the deployment of real systems. This allows vendors addressing the same market to build systems for that market that are interoperable while not requiring the implementation of absolutely every feature.

No Test Specifications
Another issue facing IEEE 802.16 developers is an artifact of the IEEE standards process concentrating primarily on requirements. The output of the IEEE 802.16 working group is a standard, that is to say, a requirement specification. The working group will continue to expand the standard to cover additional markets, through amendments and revisions to the base standard. In the past, there was no work item in IEEE 802.16 to address the creation of test specifications. Test specifications are necessary to:

• Ensure that equipment and systems claiming compliance to the standard or a profile have been sufficiently tested to demonstrate that compliance.
• Guarantee that equipment from multiple vendors has been tested the same way, to the same interpretation of the standard, increasing the interoperability of the equipment.
• Enable independent conformance testing, giving further credibility to the previous two items.

This test specification initiative is an area where ETSI has an official process and is typically more complete than the IEEE process. ETSI follows the guidelines of the ISO/IEC 9646 series (ITU-T X.29x series). The Test Suite Structure and Test Purposes (TSS&TP) document and the Abstract Test Suite (ATC) specification, both described in ISO/IEC 9646-2 (ITU-T X.291), suit the purpose particularly well.

No Conformance Statements
Having profiles is only part of the interoperability challenge. There must be a standard method of identifying which profiles a device or system complies with and which optional features are implemented so that system integrators can make educated decisions about specific features to provide to customers and to aid in the selection of equipment.

To address the IEEE short-coming, WiMAX initiated the development of a set of conformance documents, in accordance with ISO/IEC recommendations, to be submitted to the IEEE 802.16 Working Group. These include Protocol Implementation Conformance Statement (PICS), TSS&TP, and ATS. The 10 to 66 GHz documents are complete . For sub 11 GHz, the PICS is near completion while the TSS&TP and ATS have been initiated.  The System Profiles will be included in the IEEE 802.16 revision to be published in July 2004.

The Solution is WiMAX