Tech Topic 11: WiMAX Applications for Public Safety

In Tech Topic #10, the use of license-exempt radio systems by public safety organizations as an alternative approach to conventional land mobile radio systems was introduced. Specifically, the topic addressed the capabilities and some of the issues associated with the use of Wireless Fidelity, or Wi-Fi, systems. Many public safety organizations have used Wi-Fi systems to take advantage of the common access and minimal costs available in the license-exempt spectrum bands. One of the major issues related to Wi-Fi though is the limited range that results from the maximum power available coupled with the restrictions associated with the interference acceptance requirements. In a follow-on effort, the industry has attempted to resolve many of the issues of Wi-Fi by developing the next generation of wireless local area network¹ (WLAN) technologies. The most recent technology development is Worldwide Interoperability for Microwave Access, or better known as WiMAX. This topic addresses the basic concepts of WiMAX and suggests how the improvements over Wi-Fi make WiMAX a viable candidate for use by public safety agencies.

As noted in our last Tech Topic, the first wireless local area network (WLAN) technology called Wi-Fi was developed in 1997 under the auspices of the Wi-Fi Alliance to satisfy a demand for short range wireless access for local area networks. This technology was developed in response to the FCC spectrum allocations for license-exempt use under Part 15 of the Rules. In order to overcome the limitations of Wi-Fi, industry responded in 2001 by forming the WiMAX Forum to advocate the IEEE 802.16 standards for fixed and subsequently mobile applications². Interestingly, the Wi-Fi standards were developed for specific spectrum allocations as opposed to the WiMAX standards that were developed without any specific spectrum designation. So Wi-Fi was a set of standards developed pursuit to a detailed spectrum allocation, whereas WiMAX was a set of standards developed prior to any specific spectrum allocation and in fact was a standard in search of spectrum.

In general, broadband wireless applications have been expanding rapidly. Future public safety communications applications will demand mobile broadband service, migrating from the current dominant voice-only mode to multimedia applications. The use of common standards-based commercial technologies naturally supports interoperability and the sharing of commercial or municipal wireless network infrastructures at a lower cost for public safety systems. As indicated in the previous note, public safety agencies across the country have been able to leverage the widely successful private sector broadband wireless technology, Wi-Fi and Mesh Networks, as a complementary solution for disaster and municipal public safety communications. Several shortfalls to using this method for public safety communications were also addressed. Some Wi-Fi technical features have been greatly enhanced in a recently developed IEEE 802.11n standard³. These include higher data rates, new modulation schemes and access methods; channel bonding that simultaneously uses two separate non-overlapping channels to transmit data; antenna diversity; and spatial multiplexing of multiple independent data streams that are then transferred simultaneously within one channel of spectral bandwidth. Over-the-air data rates of up to 130 Mbps are attained by coupling two spatial streams of MIMO (Multiple Input Multiple Output) antenna architectures with a bandwidth of 20 MHz. MIMO is a technology that uses multiple antennas for multipath signal diversity to coherently resolve more information than possible using a single antenna. This results in increasing a receiver's ability to recover the data information from the source signal. If properly implemented, a single 40 MHz channel can provide greater than twice the usable channel data rate of two 802.11 legacy channels of 20 MHz - upwards of 300 Mbps over the 40 MHz of bandwidth.

Despite the recent IEEE 802.11n standard having significant technical advances, many of the fundamental shortfalls found in Wi-Fi for use in public safety communications remain. Such shortfalls may be resolved in WiMAX. Often WiMAX is compared to Wi-Fi because its technical attributes resemble those of the previously developed Wi-Fi. Both have common attributes of broadband access methods and capability of mesh architecture but Wi-Fi is intended for unlicensed use on a local area basis with limited range and transmission power. WiMAX thus far has been applied on a licensed basis for wide area applications. It should be noted at this point that operational range and data rates are inversely proportional so that longer operational ranges may be obtained but at the sacrifice of data rate.

WiMAX includes not only most of the enhanced technical features of IEEE 802.11n but also provides assured network access via embedded quality of service (QoS) measures and centralized network control. Thus, operations of the emerging WiMAX technology may be complementary to Wi-Fi in terms of coverage area and required data rates in certain circumstances of disaster and recovery.

WiMAX has evolved as a wide area broadband access technology that delivers higher speed data over fixed wireless links for residential and enterprise use since 2004. Later, the mobility feature was included to accommodate broadband mobile multimedia (voice, data and video) applications⁵. The first is the original intent of WiMAX and is terrestrial fixed with operational spectrum in the range of 11 to 66 GHz. The other is a mobile application with operational spectrum below 6 GHz. Historically, WiMAX has evolved from a variety of microwave technologies: MDS (Multipoint Distribution Service) for distributing and transmitting TV signals, MMDS (Multichannel Multipoint Distribution Services) modified later for 2-way Internet access, and LMDS (Local Multipoint Distribution Services) operating over wide ranges of spectrum up to 66 GHz. Therefore, a set of WiMAX technologies inherited various access features such as point-to-point, and point-to-multipoint in fixed/nomadic modes. The fixed line of sight (LOS) operation claims to provide a range of 35 miles with data rates up to 70 Mbps, and the non line of sight (NLOS) claims to provide a typical range of several miles with data rates of 2 to 63 Mbps (depending on available bandwidths). Today, both applications of WiMAX technology are standardized, implemented in integrated circuit chipsets, and about to be deployed in the commercial market⁶.

The fixed wireless versions of WiMAX (IEEE 802.16-2004) have mostly been applied to broadband wireless backbone applications. Because this technology was developed for commercial licensed applications, the quality of service feature was well established, thus, it can support differentiated service levels. The fixed version of WiMAX operates with a TDM (Time Division Multiplexing) data stream on the downlink and TDMA (Time Division Multiple Access) on the uplink communications with a centralized scheduler controlling access. Mobile WiMAX on the other hand utilizes Scalable Orthogonal Frequency Division Multiple Access (S-OFDMA) with a scheduler controlling frequency selective access both on the uplink and downlink depending on five different service categories.

Mobile WiMAX (IEEE 802.16-2005) is the mobile extension version of WiMAX that has been developed as the industry standard. In 2007, Mobile WiMAX was approved by the International Telecommunication Union (ITU-R) as a new IMT-2000 standard⁷. The WiMAX Forum, serving as the industry lead, is developing specifications for conformance, interoperability and certification (set of profiles) that leverage the standards-based technical specifications defined by the standards bodies.

The following table summarizes some of the key attributes of the WiMAX technology standard.

WiMAX Standards
	IEEE 802.16	IEEE 802.16-2004	IEEE 802.16e
Standardization Date	2003	2005	2006
Spectrum Band	Licensed 10-66 GHz	Licensed and Unlicensed sub-11 GHz	Licensed, Sub-6 GHz
Channel Bandwidths		1.75, 3.5, 5, 7, 10, 20 MHz	1.25, 2.5, 5, 7, 10, 14, 20 MHz
Service	Fixed	Fixed/Nomadic	Fixed, Mobile
Air Interface	OFDM / OFDMA	OFDM / OFDMA	S-OFDMA
Range	LOS up to 5 km	LOS & near-LOS up to 30 km; Non-LOS up to 5 km	Non-LOS up to 10 km
Channel Capacity	Up to 134 Mbps	Up to 70 Mbps	Up to 35 Mbps
Duplexing Mode	TDD or FDD	TDD or FDD	TDD or FDD

The mobile version of WiMAX (IEEE 802.16e) is drawing considerable international attention because its data rate is far higher than current 3G wireless technologies. However, it will face competition in the near future with nascent advanced broadband wireless technologies such as the 3G-LTE (Long Term Evolution) technologies currently under development. 3G LTE is an advanced version of 3G UMTS (Universal Mobile Telecommunications System) being developed by the 3rd Generation Partnership Project (3GPP)⁸. Both have considerable promise for serving the public safety community with broadband capabilities.

Currently, mobile WiMAX can be implemented in three spectrum bands (2.3-2.4 GHz, 2.5-2.7 GHz, and 3.4-3.6 GHz) which have mostly common allocations worldwide. However, the technology is applicable in other bands as well (remember - a standard looking for some spectrum?). For example, it can be applied even in some unlicensed and public safety bands such as 2.4 GHz, 4.9 GHz and 5.8 GHz if needed. Due to such wide operational spectrum bands, the WiMAX Forum is developing feature sets - or profiles - to meet worldwide interoperability within each profile. Additional work to extend WiMAX into other spectrum bands such as 4.9 GHz is under way. Also, a profile for the newly available 700 MHz spectrum is expected soon.

The Mobile WiMAX air interface adopts Scalable OFDMA (S-OFDMA) that supports a wide range of channel bandwidths to flexibly accommodate the need for various spectrum allocations of OFDMA in a mobile non-line of sight environment. It uses various channel bandwidths of 1.25, 2.5, 5, 7, 10, 14 and 20 MHz to allow adaptable bandwidths flexible over licensed worldwide spectrum bands. It supports duplexing schemes of TDD (Time Divisional Duplex) where the uplink and downlink transmissions occur at different times and share the same frequencies, which has considerable throughput advantages by way of allocating flexible bandwidths for downlink and uplink. The mobile version of FDD (Frequency Divisional Duplex) where the uplink and downlink channels are located on separate frequencies is planned to be added in a later version of the mobile WiMAX profile to support local regulations that do not allow the TDD scheme.

The range of coverage is up to 30 miles with the downstream peak data rates up to 46 Mbps and upstream rates up to 7.1 Mbps. The access protocol supports multimedia applications on a single integrated platform with the provision of quality of service mechanism to enable differentiated services among multiple users.

Overall, mobile WiMAX technology is one of the emerging broadband access technologies that delivers higher speed data applications covering wider areas and has evolved beyond initial fixed wireless access for residential and enterprise applications. Public safety communication systems can leverage various WiMAX operational capabilities to benefit meeting PS fundamental requirements involving interoperability, reliability, ubiquity, flexibility, and security. In particular, WiMAX technology has some advantages for public safety use that are summarized below:

• Extended range over existing Wi-Fi systems.
• Applicability to dedicated public safety spectrum that is not shared with the public at large.
• Fixed and Mobile standards-based technologies consistent with potentially similar commercial mobile cellular systems developments.
• Higher data rates support full range of applications including voice, data, and video.

Some of the disadvantages of WiMAX include:

• Nascent technology that is currently on the leading edge of development.
• Equipment developments are new and must be consistent with potentially adaptable spectrum allocations/use.
• Current developments of the mobile WiMax standard are not consistent with the fixed standard making compatibility an issue

With this look at WiMAX - a next generation fixed and mobile technology - we have moved a bit away from our initial premise, which was public safety interoperability. In our next note, we'll come back to this basic theme by considering the current interoperability channel allocations that exist by FCC designation. In conclusion, I'd like to extend my sincere thanks to my colleague, Yoon Chang, for his assistance with this Tech Topic.

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¹ "The Wi-Fi Alliance is a global, non-profit industry association of more than 300 member companies devoted to promoting the growth of wireless Local Area Networks (WLANs). With the aim of enhancing the user experience for wireless portable, mobile, and home entertainment devices, the Wi-Fi Alliance's testing and certification programs help ensure the interoperability of WLAN products based on the IEEE 802.11 specification." See http://www.wi-fi.org/.

² "The WiMAX Forum® is an industry-led, not-for-profit organization formed to certify and promote the compatibility and interoperability of broadband wireless products based upon the harmonized IEEE 802.16/ETSI HiperMAN standard. A WiMAX Forum goal is to accelerate the introduction of these systems into the marketplace. WiMAX Forum Certified™ products are fully interoperable and support broadband fixed, portable and mobile services." See http://www.WiMAXforum.org/home/.

³ See http://grouper.ieee.org/groups/802/11/ for the IEEE listing of the wireless local area network protocols including the draft of 802.11n.

⁴ "802.16-2004 is often called 802.16d, since that was the working party that developed the standard. It is also frequently referred to as "fixed WiMAX" since it has no support for mobility." See http://en.wikipedia.org/wiki/WiMAX.

⁵ "802.16e-2005 is an amendment to 802.16-2004 and is often referred to in shortened form as 802.16e. It introduced support for mobility, amongst other things and is therefore also frequently called 'mobile WiMAX'". See http://en.wikipedia.org/wiki/WiMAX.

⁶ http://arstechnica.com/news.ars/post/20080507-sprint-clearwire-wimax-venture-sees-daylight.html?rel. and

⁷ See http://www.itu.int/home/imt.html and http://www.wimaxforum.org/technology/downloads/WiMAX_and_IMT_2000.pdf for additional information on IMT-2000 and the next generation mobile technologies as well as how WiMAX fits into next generation mobile services.

⁸ See http://www.3gpp.org/.