September 21, 2005
A look at the various standards behind WiMax, fixed and mobile, and where WiMax might be heading in the future.
WiMax is big news today. People far and wide are talking about it -- although how many of them actually understand WiMax is less clear. We often hear terms like WiMax and 802.16 used together, even interchangeably. But what exactly is the relationship and the difference between the two? Also, how are the standards evolving?
The Institute of Electrical and Electronics Engineers (IEEE) is a standards body that develops and publishes standards for numerous networking elements, such as the IEEE 802.16 working group. The WiMax Forum is a non-profit organization formed to certify the compatibility and, more importantly, the interoperability of broadband wireless products (specifically those described by IEEE 802.16). The two groups work closely together. However, they are not the same. The Forum activities are an extension of the 802.16 working group in that its task is to develop the test procedures and certifications that demonstrate interoperability among WiMax vendors gear -- which is based on 802.16. So how did this all come about?Why 802.16?
Some might ask why we need a new standard. Why not just simply use the IEEE 802.11 (versions a/b/g/n, etc.) currently around? However, it is important to remember that the 802.11 standards (particularly a/b/g) were all originally written to support the wireless local area network (LAN), not a citywide metropolitan area network (MAN).
Wireless equipment providers have certainly extended the capability of 802.11 technology by writing proprietary media access controllers (MAC) to support specific physical layers (PHY) optimized for outdoor citywide delivery of wireless broadband. The problem, of course, is that this resulted in systems that had different capabilities -- and, more importantly, systems that do not work together.
Granted, the 802.11b specification is a consistent standard that has driven the Wi-Fi boom of recent years. Unfortunately, even with specific tweaks and improvements, that standard only poorly serves the wireless MAN. The various specifications were written to support specific physical layers or spectrum ranges. The 802.11a specification, for instance, uses the 5.2/5.3 GHz range, which is narrowly defined by FCC rules in regard to power, integrated antenna and radios and other aspects.
The broadband wireless access (BWA) industry, specifically the fixed broadband wireless industry, has desperately needed a consistent multi-spectrum defining interoperability standard for years.
When the cable industry accepted the data-over-cable service interface specification (DOCSIS) as a standard, prices dropped rapidly, customer counts rose and cable became the fastest-growing broadband player in the industry. The BWA industry need such a standard. Will Wi-MAX ultimately prove to be the standard for the fixed broadband wireless industry? That's up in the air at this point. The answer turns on whether or not you are talking licensed band or unlicensed band solutions.Standards & Benefits
Standards bodies have a number of purposes, many of which are technical. However, standards bodies also have economic purposes. Look at the European HiperLAN standard that was issued to compete with the US-centric 802.11 series of standards. Basically, European vendors had hoped to secure their markets against outside competition by drafting a competing but similar standard. The attempt failed miserably as Wi-Fi exploded.
Interestingly, todays WiMax standard incorporates Europes newer HiperMAN standard as well as the Korean WiBro, which is a version of what today is being called Mobile WiMax or 802.16e.An Overview
The IEEE 802.16 group is charged with defining an Air Interface Standard for fixed and mobile broadband wireless systems using a point-to-multipoint design and/or mesh technology. The original standard dealt with radios operating in spectrum between 10 GHz and 66 GHz. That has since been extended to include the 2-11 GHz range. A mobility standard is due in 2006. By all accounts, the spectrum range itself could easily expand down into the 900 MHz range and even the 700 MHz range or lower.
WiMax today shakes out into two standards. One is Fixed WiMax or 802.16-2004(d). The second, and the apparent darling of the Forum, is Mobile WiMax or 802.16e, which is in the process of being finalized. More about those later. For now, let's look at the progression of the standards.
The first version of the WiMax standard addressed spectrum ranges above 10 GHz (specifically 10 GHz to 66 GHz). Since line-of-sight (LOS) is a primary issue in this range, multipath was addressed in this first version with orthogonal frequency division multiplexing (OFDM) techniques. Thus it supports wide channels, defined as being greater than 10 MHz in size. This first standard basically addressed licensed-only service delivery (although there is license-free spectrum in this range).
The 802.16a update added support for spectrum ranges of 2 GHz to 11 GHz. It addressed both licensed and unlicensed ranges. It also incorporated non-line-of-sight (NLOS) capability. This version enhanced the medium access control (MAC) layer capabilities. It also improved quality of service (QOS) features. The European HiperMAN standard was supported and a total of three supported physical layers (PHY) were defined. Support for both time division duplexing (TDD) and frequency division duplexing (FDD) was incorporated -- providing for both half duplex and full duplex data transmission in cases where FDD is used. Transmission protocols such as Ethernet, ATM or IP are supported.
This standard update dealt mostly with updates in the 10 GHz to 66 GHz range. However, it also addressed issues such as performance evaluation, testing and detailed system profiling. This last was a crucial element of the WiMax toolkit. Because there are a great deal of options available with 802.16 in general, the system profile methodology evolved to define what would be mandatory features and what would be optional features. The intent was to guide vendors on mandatory elements that must be met to ensure interoperability. Optional elements such as different levels of security protocols incorporated allow vendors to differentiate their products by price, functionality and market niche.
All of the Fixed WiMax standards mentioned above have been rolled into 802.16-2004: it incorporates the original 802.16, 802.16a and 802.16c updates. This final standard supports numerous mandatory and optional elements. Vendors are already shipping their 802.16-2004 products to the Cetecom labs in Spain for interoperability testing.
The technology supports both TDD and FDD. Its theoretical effective data rate is around 70 Mbps, although real world performance will probably top out around 40 Mbps. It should be noted that while the technology supports at least three PHY layer Modulation schemes, the system profile chosen is OFDM 256-FFT. This is different from the OFDMA flexible FFT system used in 802.16e. Both standards, however, support the former PHY. This distinction is really a market choice. The Forum could have chosen to use OFDM 256-FFT instead of OFDMA. Market forces and in particular the WiBro standard may have precluded that.
Just some of the enhancements in this version are support for concatenation of both protocol data units (PDU) and service data units (SDU) which reduces the MAC overhead. The technology improves quality-of-service (QOS), particularly with very large SDUs. One clear improvement is support for multiple polling methodologies. The MAC facilitates polling individually or in groups. It can access allocated bandwidth to make requests, or signal that it needs polling. It can even piggyback polling requests over other traffic -- the upshot being that constant cross-talk is obviated with this system, reducing packet collisions and system overhead.
A standard still in flux, IEEE 802.16e conserves the technical updates of Fixed WiMax while adding robust support for mobile broadband. While not completely settled, the technology will likely be based upon the OFDMA technology developed by Runcom. This OFDMA technique supports 2K-FFT, 1K-FFT, 512-FFT and 128-FFT. Interestingly, both standards do support the 256-FFT chosen for 802.16-2004. Many of the mandatory elements for this standard have been agreed upon, and a lot of the remaining work centers around the optional elements.
The OFDMA system allows signals to be divided into many lower-speed sub-channels to increase resistance to multi-path interference. For example, if a 20 MHz channel is subdivided into 1000 sub-channels, each individual user would be allowed a dynamic number of sub-channels based on their distance and needs from the cell (i.e. 4, 64, 298, 312, 346, 610 and 944). If close in, a higher modulation methodology such as 64 quadrature amplitude modulation (QAM) can be used for higher bandwidth across more channels. If the user is farther away, the number of channels can be reduced with a resultant power increase per channel. Throughput slows a bit, but distant users are not dropped.
Koreans are always at the forefront of broadband adoption. They were ready to deploy a mobile wireless MAN and felt the standards as existed were good enough for that purpose, so WiBro was born. Product is already being shipped by several vendors in an OFDMA version using 1K-FFT in the 2.3 GHz band.
The WiMax Forum has chosen to incorporate this standard into its own testing. It is speculation but probably not far afield that this may have influenced the Forums decision to choose OFDMA for its Mobile WiMax standard (early indications were that the OFDM 256-FFT was being considered.) In any event, the potential of the Korean deployments seems to have strongly influenced WiMax proponents to leverage the technology as a primary 3G competitor.Conclusions
Certainly, these standards can seem very complex -- because they are. Their ultimate effect, if widely adopted, should be to reduce costs and speed proliferation of broadband wireless.
However, recent indications are that the WiMax Forum membership largely appears to be heading toward supporting the Mobile WiMax or 802.16e standard over the Fixed WiMax or 802.16-2004(d) iteration. Some companies such as Alvarion and Aperto are offering 802.16-2004 fixed products. One company, Motorola, freely admits that it expects its unlicensed band product, the Motorola Canopy system, to continue as a proprietary technology (without a Fixed WiMax upgrade path). Meanwhile, the company is busily working on Mobile 802.16e products for licensed spectrum.
Will other vendors act to protect their existing proprietary product lines? What this ultimately presages for the fixed wireless industry is unclear. One likely conclusion is that for many unlicensed band operators, especially in rural America, proprietary broadband wireless will likely remain the norm, at least for the foreseeable future.
Tim Sanders is founder of The Final Mile, a fixed wireless consulting group. His experience was gained running a multi-state fixed wireless ISP. He can be reached at www.thefinalmile.net or email@example.com