Mobile WiMAX, or Institute of Electronic and Electrical Engineer’s (IEEE) 802.16e-2005, emerged as a potential alternative to cellular technology for wide-area wireless networks. Based on Orthogonal Frequency Division Multiple Access (OFDMA) and approved by the International Telecommunication Union (ITU) as an IMT-2000 (3G technology) under the name OFDMA Time Division Duplex (TDD) Wireless Metropolitan Area Network (WMAN), mobile WiMAX gained its greatest traction in developing countries as a fixed wireless alternative to wireline deployment.
WiMAX, or Worldwide Interoperability for Microwave Access, was a name created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the IEEE 802.16 technology standard. The forum describes WiMAX as a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL. WiMAX provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint links to portable and fully mobile internet access. It should be noted that most often, references to WiMAX deployments and ecosystem announcements include the fixed, portable and mobile WiMAX technologies.
In the United States, Clearwire, Sprint Nextel and others (Intel, Google, Comcast, Time Warner Cable, and Bright House Networks) created a joint venture to deploy a nationwide WiMAX network. In June 2012, this network was available in 80 cities across the U.S. and covered over 130 million people. Clearwire, however, has started deploying LTE, and indicates it will have 31 cities covered by the first half of 2013. WiMAX has gained the greatest traction in developing countries as an alternative to wireline deployment.
Mobile WiMAX employs many of the same mechanisms as HSPA to maximize throughput and spectral efficiency, including high-order modulation, efficient coding, adaptive modulation and coding as well as Hybrid Automatic Repeat Request (HARQ).The principal difference from HSPA is IEEE 802.16e-2005’s use of OFDMA. In 5 to 10 MHz radio channels, there is no evidence indicating that WiMAX will have any performance advantage compared with HSPA+. With respect to spectral efficiency, WiMAX is comparable to HSPA+. As for data performance, HSPA+ in Release 8—with a peak rate of 42 Mbps—essentially matches mobile WiMAX in 10 MHz in TDD 3:1 DL:UL using 2X2 MIMO with a peak rate of 46 Mbps.
Relative to LTE, WiMAX has the following technical disadvantages: 5 msec frames instead of 1 msec frames, Chase combining instead of incremental redundancy, coarser granularity for modulation and coding schemes and vertical coding instead of horizontal coding. One deployment consideration is that TDD requires network synchronization. It is not possible for one cell site to be transmitting and an adjacent cell site to be receiving at the same time. Different operators in the same band must either coordinate their networks or have guard bands to ensure that they don’t interfere with each other.
Although IEEE 802.16e exploits significant radio innovations similar to HSPA+ and LTE, it faces challenges such as economies of scale and technology maturity. Very few operators today have access to spectrum for WiMAX that would permit them to provide widespread coverage. This means that roaming with WiMAX is severely limited.
In reference to economies of scale, GSM-HSPA subscribers number in the billions. Even over the next five years, the number of WiMAX subscribers is likely to be quite low. Infonetics Research projects 132 million subscribers by 2016.
One specific area in which WiMAX has a technical disadvantage is cell size. In fact, 3G systems have a significant link budget advantage over mobile WiMAX because of soft-handoff diversity gain and an FDD duplexing advantage over TDD. Arthur D. Little reports that the radii of typical HSPA cells will be two to four times greater than typical mobile WiMAX cells for high-throughput operation. One vendor estimates that for the same power output, frequency, and capacity, mobile WiMAX requires 1.7 times more cell sites than HSPA. Given that many real world deployments of HSPA will occur at frequencies such as 850 MHz, and LTE at 700 MHz, WiMAX deployments at 2.5 GHz are at a significant disadvantage.
Current mobile WiMAX networks use 2X2 MIMO or 4X2 MIMO, TDD, and 10 MHz radio channels in a profile defined by the WiMAX Forum known as WiMAX Wave 2 or, more formally, as WiMAX System Profile 1.0. Beyond Release 1.0, the WiMAX Forum defined a profile called WiMAX Release 1.5. This profile includes various refinements intended to improve efficiency and performance and could be available for deployment in a similar timeframe as LTE.
Release 1.5 enhancements include Medium Access Control (MAC) overhead reductions for VoIP (persistent scheduling), handover optimizations, load balancing, location-based services support, Frequency Division Duplex (FDD) operation, 64 QAM in the uplink, downlink adaptive modulation and coding, closed-loop MIMO (FDD mode only), and uplink MIMO. There are no current Release 1.5 deployment plans.
A subsequent version, Mobile WiMAX 2.0, has been designed to address the performance requirements of ITU IMT-Advanced Project and is standardized in a new IEEE standard, IEEE 802.16m. It is uncertain and unclear whether 802.16m will ever be commercialized.
4G Americas considers WiMAX to be a niche technology that may be appropriate in some circumstances and for certain operators as a part of their network strategies. At best, the promises of mobile WiMAX are appealing but remain unproven in the real world. With so many WiMAX operators choosing LTE, the end of WiMAX market opportunity appears over already.