Q&A: Mobile WiMAX

What is Mobile WiMAX?

Are there challenges faced by mobile WiMAX?

How are WiMAX and Mobile WiMAX related in the evolutionary history of WiMAX?

Does mobile WiMAX pose a challenge to existing wireless technologies?

What are the disadvantages of mobile WiMAX compared to 3G systems?

How do WiMAX and mobile WiMAX compare to existing wireless technologies in the areas of spectral efficiency and data performance?

How does mobile WiMAX technology compare to HSPA overall?

What may be some operator concerns regarding the deployment of mobile WiMAX?

What is Mobile WiMAX?

Mobile WiMAX, also known as IEEE 802.16e-2005, has emerged as a potential alternative to cellular technology for wide-area wireless networks. Based on OFDMA and approved by the International Telecommunications Union (ITU) as an IMT-2000 (3G technology) under the name OFDMA TDD WMAN (Wireless Metropolitan Area Network), mobile WiMAX has gained its greatest traction in developing countries as an alternative to wireline deployment.

Like GSM/UMTS, WiMAX is not a single technology; it is a family of interoperable technologies. Unlike GSM/UMTS, mobile WiMAX networks are not backward-compatible.

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, and Hybrid Automatic Repeat Request (HARQ). The principal difference from HSDPA is mobile WiMAX’s use of OFDMA. OFDM provides a potential implementation advantage for wide radio channels (for example, 20 MHz). In 5 to 10 MHz radio channels, there is no evidence indicating that mobile WiMAX will have any significant performance advantage on the downlink.

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Are there challenges faced by mobile WiMAX?

Although mobile WiMAX exploits significant radio innovations, it faces challenges like spectrum, economies of scale, and technology. Very few operators have access to 20 MHz of TDD spectrum for WiMAX that would permit them to provide widespread coverage. With more than 2.8 billion subscriptions already in place for GSM/HSPA technologies, there is a considerable amount of scope and scale for economics and value that mobile WiMAX can not replicate and even by 2012, the number of WiMAX subscriptions as a percentage of the overall wireless market is expected to be in the single digits according to most leading industry analysts. HSPA accounts for the majority of mobile broadband networks today and through the next five years.

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How are WiMAX and Mobile WiMAX related in the evolutionary history of WiMAX?

WiMAX is not a single technology; it is a family of interoperable technologies. The original specification, IEEE 802.16, was completed in 2001 and intended primarily for telecom backhaul applications in point-to-point line-of-sight configurations using spectrum above 10 GHz. This original version of IEEE 802.16 uses a radio interface based on a single-carrier waveform.

The next major step in the evolution of IEEE 802.16 occurred in 2004, with the release of the IEEE 802.16-2004 standard. It added multiple radio interfaces, including one based on OFDM-256 and one based on OFDMA. IEEE 802.16-2004 also supports point-to-multipoint communications, sub-10 GHz operation, and non-line-of-sight communications. Like the original version of the standard, operation is fixed, meaning that subscriber stations are typically immobile. Potential applications include wireless Internet Service Provider (ISP) service, local telephony bypass, as an alternative to cable modem or DSL service, and for cellular backhaul for connections from cellular base stations to operator infrastructure networks. Vendors can design equipment for either licensed or unlicensed bands.

Vendors are now delivering IEEE 802.16-2004-certified equipment. This standard does not compete directly with cellular-data and private Wi-Fi networks; thus, it can provide complementary services. In addition to operator-hosted access solutions, private entities such as municipal governments, universities, and corporations will be able to use this version of WiMAX in unlicensed bands (for example, 5.8 GHz) for local connectivity, though there has been little or no development in this area.

The IEEE has also completed a mobile-broadband standard—IEEE 802.16e-2005—that adds mobility capabilities including support for radio operation while mobile, handovers across base stations, and handovers across operators. Unlike IEEE 802.16-2004, which operates in both licensed and unlicensed bands, IEEE 802.16e-2005 (referred to as mobile WiMAX) makes the most sense in licensed bands. Operators are preparing to deploy mobile WiMAX networks in 2007 and 2008. Current WiMAX profiles emphasize TDD operation. Mobile WiMAX networks are not backward-compatible with IEEE 802.16-2004 networks.

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Does mobile WiMAX pose a challenge to existing wireless technologies?

WiMAX is trying to challenge existing wireless technologies, promising greater capabilities and greater efficiencies than alternative approaches such as HSPA. But as WiMAX, particularly mobile WiMAX, has come closer to reality, vendors have continued to enhance HSPA, and actual WiMAX advantages are no longer apparent. Any potential advantages certainly do not justify replacing 3G systems with WiMAX. Instead, WiMAX has gained the greatest traction in developing countries as an alternative to wireline deployment.

While GSM/UMTS/HSPA subscriptions today number in the billions, even by 2010 the number of WiMAX subscriptions is forecast to be low. In 2007, analyst Arthur D. Little summarized different forecasts for total WiMAX subscribers worldwide as between 20 million and 100 million by 2012, a very small fraction of global wireless subscribers. This is consistent with a June 2007 forecast by Senza Fili Consulting that projects 54 million WiMAX subscribers by 2012, with emerging markets driving growth.

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What are the disadvantages of mobile WiMAX compared to 3G systems?

One area where 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 reported in March 2007 that the radii of typical HSPA cells will be two to four times greater than typical mobile WiMAX cells for high-throughput operation. WiMAX cells can be made of comparable size to HSPA, but at the detriment of data rates no higher than HSPA and with no capital expenditure (CAPEX) advantage. Ericsson estimated in May 2007 that for the same power output, frequency, and capacity, mobile WiMAX requires 1.7 times more cell sites than HSPA.

Mobile WiMAX contains some aspects that may limit its performance, particularly in scenarios where a sector contains a large number of mobile users. The performance of the MAC layer is inefficient when scheduling large numbers of users, and some aspects—such as power control of the mobile station—are provided using MAC signaling messages rather than the fast power control used in WCDMA and other technologies.

Although mobile WiMAX exploits significant radio innovations, it faces challenges like spectrum, economies of scale, and technology. Very few operators have access to spectrum for WiMAX that would permit them to provide widespread coverage.

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How do WiMAX and mobile WiMAX compare to existing wireless technologies in the areas of spectral efficiency and data performance?

In the area of spectral efficiency, WiMAX is comparable to HSPA+. Mobile WiMAX also experiences gains in spectral efficiency as various optimizations, like MRxD and MIMO, are applied. WiMAX Wave 2 includes 2X2 MIMO. Enhancements to WiMAX will come from potentially new profiles, as well as a new version of the standard IEE 802.16m which likely will match LTE and UMB spectral efficiency.

However, HSPA+ with MIMO is spectrally more efficient than WiMAX with MIMO. This is because HSPA MIMO supports closed-loop operation with precode weighting and multicode word MIMO, which enables the use of SIC receivers. Other reasons are that HSPA supports incremental-redundancy HARQ, while the initial WiMAX profiles support only Chase combining HARQ, and that WiMAX has larger control overhead in the downlink than HSPA, because the uplink in WiMAX is fully scheduled. OFDMA technology requires scheduling to avoid two mobile devices transmitting on the same tones simultaneously. An uplink MAP zone in the downlink channel does this scheduling.

Conversely, HSUPA can use autonomous transmission on the uplink. Hence, there is no downlink overhead required to schedule the uplink. This leads to a disadvantage for HSUPA in the uplink when compared to WiMAX, because the HSUPA uplink is not orthogonal. But autonomous transmission does provide the advantage of lower downlink control overhead for HSPA relative to WiMAX. It also helps to mitigate other-cell interference, which may become a problem when WiMAX is deployed.

LTE also has higher spectral efficiency than WiMAX, because it includes incremental redundancy and supports closed-loop operation with precoder weighting as well as multicode word MIMO, thus enabling the use of SIC receivers.

Regarding data performance, HSPA+ in Release 8—with a peak rate of 42 Mbps—exceeds mobile WiMAX in 10 MHz in TDD 2:1 using 2X2 MIMO of 40 Mbps. The sometimes quoted peak rate of 63.4 Mbps for mobile WiMAX in 10 MHz assumes no bandwidth applied to the uplink.

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How does mobile WiMAX technology compare to HSPA overall?

Both WiMAX and HSPA networks are based on OFDM/OFDMA technology. IEEE 802.16e-2005 employs many of the same mechanisms as HSPA to maximize throughput and spectral efficiency, including high-order modulation, efficient coding, adaptive modulation and coding, and Hybrid Automatic Repeat Request (HARQ). The principal difference from HSDPA is IEEE 802.16e-2005’s use of OFDMA. OFDM provides a potential implementation advantage for wide radio channels (for example, 10 to 20 MHz). In 5 to 10 MHz radio channels, there is no evidence indicating that IEEE 802.16e-2005 will have any significant performance advantage on the downlink.

OFDM systems—including IEEE 802.16e-2005—exhibit greater orthogonality on the uplink, so IEEE 802.16e-2005 may have slightly greater uplink spectral efficiency than even HSUPA. IEEE 802.16e-2005 achieves its greatest spectral efficiency in a 1/1 reuse pattern, where each sector uses the same radio channel. However, this may introduce greater levels of other-cell interference that may in turn introduce problems, because these signals would not be orthogonal. Another deployment option for IEEE 802.16e-2005 is 1/3, where each cell site uses the same frequency band but each sector uses one of three radio channels. The 1/3 configuration is not as spectrally efficient as 1/1, but it improves both cell throughput and higher user data rates at the cell edge. A final option is 1/1 reuse with interference mitigation techniques that emulate 1/3 reuse only for cell edge users.

From a technology standpoint, mobile WiMAX on paper may be slightly more capable than today’s available versions of HSPA. But by the time it becomes available, mobile WiMAX will actually have to compete against evolved HSPA systems that will offer both similar capabilities and enhanced performance. And by then, LTE will not be that far from deployment. Telstra in Australia has already announced potential deployment of HSPA+ at the end of 2008. LTE is expected to have commercial trials in 2008 with equipment availability in late 2009.

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What may be some operator concerns regarding the deployment of mobile WiMAX?

Today’s cellular networks can finance the deployment of data capabilities through a successful voice business. Building new networks for broadband wireless mandates substantial capacity per subscriber. Consumers who download 1 gigabyte of data each month represent a ten times greater load on the network than a 1,000-minute a month voice user.

If the future is in multimedia services such as movie downloads, it is important to recognize that downloading a single high-definition movie—even with advanced compression—consumes approximately 2 gigabytes. It is not clear how easily the available revenue per subscriber will be able to finance large-scale deployment of network capacity. Despite numerous attempts, no terrestrial wireless-data-only network has ever succeeded as a business. Although there is discussion of providing voice services over WiMAX using VoIP, and announcements regarding the development of devices to enable this service, mobile-voice users demand ubiquitous coverage—including indoor coverage. Matching the cellular footprint with WiMAX will require significant operator investments.

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