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. An amendment is almost finished to do the same for systems operating between 2 and 11 GHz. Additionally, the IEEE process stops short of providing conformance statements and test specifications. In order to ensure interoperability between vendors competing in the same market, the WiMAX technical working groups were created by the leaders in IEEE 802.16 technology. The working groups address these issues by developing system profiles and by producing PICS proforma, Test Suite Structure and Test Purposes specifications and Abstract Test Suite specifications according to the ISO/IEC 9464 series (equivalent to ITU-T x.290 series) of conformance testing standards.

Overview of IEEE 802.16
Task Group 1 of IEEE 802.16 developed a point-to-multipoint broadband wireless access standard for systems in the frequency range 10-66 GHz. The standard covers both the Media Access Control (MAC) and the physical (PHY) layers. Task groups a and b are jointly producing an amendment to extend the specification to cover both the licensed and unlicensed bands in the 2-11 GHz range.

A number of PHY considerations were taken into account for the target environment. At the 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. At the lower frequencies, line of sight is not required, giving other tradeoffs. Adaptive burst profiles (modulation and forward error correction (FEC)) are used to further increase the typical capacity of 802.16 systems with respect to older technology. The MAC was designed to accommodate different PHYs for the different environments. The single service provider PHYs are 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 OFDM PHYs are designed for TDD.

The MAC was designed specifically for the PMP wireless access environment. It is designed to seamlessly carry any higher layer or transport protocol 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 the very high bit rates (up to 268 mbps each way) of the truly broadband physical layer, while delivering ATM compatible Quality of Service (QoS) to ATM as well as non-ATM (MPLS, VoIP, etc.) service.

The basic downlink subframe structure is shown in the following Figure. The uplink is TDMA with each CPE bursting in an assigned slot using an individually assigned burst profile (modulation/FEC combination).

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 piece-meal 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.

As can be seen, by the time authentication, security, capability negotiation and a host of other features are added, the IEEE 802.16 standard becomes almost overwhelming.

The Interoperability Challenge
Plethora of Options
From the preceding overview, it is clear that the IEEE 802.16 Air Interface Specification is a very large specification. It was designed to cover the fixed broadband wireless access needs of a variety of different situations. There are allowances for different physical layers for different frequency bands and country-by-country frequency use restrictions. There are features that allow one to build 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. Do you build an IEEE 802.16 compliant system implementing every possible feature, even those features you know will never be used in systems for your target customers? Or, do you build a system with only the subset of features you need for your market, risking accusations of non-compliance and lack of interoperability?

The IEEE 802.16 working group started to address this issue by the inclusion of Chapter 12, "System Profiles" in the IEEE 802.16 specification. 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.

Unfortunately, this portion of the standard was not completed when the standard was released. Additionally, as new markets and new system scenarios emerge, the industry cannot operate on the schedule required to form a task group under IEEE 802.16 to create new profiles and take it through all the procedural steps required to officially publish an amendment to the standard. New profiles must be created by industry agreement in a more timely fashion, and then rolled back into amendments to the standard at the slower pace endemic to the formal process.

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. This continuing work will result in amendments to the standard, but they will still address requirements. 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 (ATS) specification, both described in ISO/IEC 9646-2 (ITU-T X.291), suit the purpose particularly well.

No Conformance Statements
A final issue facing developers of IEEE 802.16-compliant systems is that 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.

Conformance statement development is not an official part of the IEEE standards process. The IEEE process concentrates on the specification of requirements. This leads to the tendency to build systems that do much more than they need to and the likelihood of accusations that standards to not specify interoperable systems. In contrast, the ETSI process generally addresses this issue by the development of Implementation Conformance Statement (ICS) proforma documents, following the guidance of ISO/IEC 9649-7 (ITU-T X.296). The most common form of ICS is the Protocol ICS or PICS proforma.

For IEEE 802.16 equipment, a PICS proforma is a perfect means of describing the MAC protocol and PHY features that are required for various system profiles. An ICS proforma is also a necessary tool for vendors to specify their level of compliance and to specify which optional features have been implemented.

The Solution is WiMAX