About Ray Horak

Index of Professor Horak's CommWeb Tutorial Series (including Frame Relay topic).

"Wireless is the winner!" or so I proclaimed in this space two years ago. That predication certainly has come true in the LAN realm, as Wi-Fi (802.11) has gained in popularity.

Cellular telephony has achieved great success in the voice domain, and 2.5G and 3G systems are making early inroads into data networking. The missing link, so to speak, has been the Wireless Local Loop (WLL).

Local Multipoint Distribution Services (LMDS) and Multichannel Multipoint Distribution Services (MMDS) offered great promise a few years ago, but cost issues, technological limitations, competition from Digital Subscriber Line (DSL) and cable modems, and the downturn in the economy doomed them to failure while in their infancies. From the ashes of the WLL landscape has risen 802.16, aka WiMAX, and it looks to be a winner.

What Is WiMAX?

WiMAX (Worldwide Interoperability for Microwave Access) is a Broadband Wireless Access (BWA) solution based on standards recommendations from the Institute for Electrical and Electronics Engineers (IEEE) 802.16 working group and the European Telecommunications Standards Institute (ETSI). WiMAX is promoted by the WiMAX Forum a special interest group of approximately 300 members from the manufacturing, carrier, service provider, consulting and related communities.

In this column, we’ll use WiMAX and 802.16 interchangeably. Although they aren’t exactly the same, technically speaking, WiMAX is just easier to say and it looks better in print and on screen.

The IEEE set up the 802.16 group to standardize LMDS and MMDS, which were highly touted as WLL technologies that would allow competitive carriers and service providers to provision local loops quickly and at low cost. A key advantage of this approach was that of bypassing the ILEC (Incumbent Local Exchange Carrier) copper loops.

As WLL technologies matured in the late 1990s, the competitive landscape heated up and carriers spent incredible sums of money to acquire wireless spectrum — both LMDS and MMDS operate in licensed spectrum. However, it soon became clear that LMDS and MMDS were excessively costly and suffered from performance issues, especially given their Line-of-Sight (LOS) requirements.

About the same time, the economy stumbled. As a result, LMDS and MMDS were commercial failures. (Note: For more information on LMDS and MMDS, see the column on WLL.)

However, the IEEE 802.16 working group continued to develop the standards, which actually took several forms. Officially known as the WirelessMAN Air Interface for Broadband Wireless Access, the standard evolved over a number of years.

802.16, released in 2001, standardized LMDS. Focused on fixed wireless and designed to accommodate both point-to-point and point-to-multipoint topologies, 802.16 was tuned to frequencies in the 10-66GHx range and required LOS. This first standard got little attention.

802.16a, released in 2003, was based on MMDS and the European HiperMAN system. This extension operates in the 2-11GHz range, which includes both licensed and license-exempt bands. It is designed for both point-to-point and point-to-multipoint topologies, and usually requires LOS.

802.16d, aka 802.16-2004, the most recently released version of the standard, is a compilation and modification of previous versions and amendments 802.16a, b and c. Released in 2004 and operating in the 2-11GHz range, it was designed for point-to-point, point-to-multipoint and meshed topologies. 802.16-2004 operates best with LOS, but does not require it. This extension of the standard includes support for indoor CPE (Customer Premises Equipment).

802.16e, due to be finalized in October 2005, adds hand-off capability, thereby supporting portability and mobility. Operating in the 2-11GHz range, it is designed for point-to-multipoint applications and does not require LOS.

Tech Specs

WiMAX specifications primarily address fixed wireless, with the specifications for portability and mobility due out later this year. The standards address both Line-of-Sight (LOS) and Non-Line-of-Sight (NLOS) scenarios.

Where LOS can be achieved, WiMAX cell coverage can as much as 50 km (31 miles). Under NLOS conditions, the typical cell radius might be in the range of 8 km (5 miles). The fixed wireless standards provide for shared bandwidth up to about 70 Mbps per BS. The level of actual throughput depends on LOS, distance, air quality, interference and other factors that can affect signal quality. Mobile network deployments (802.16e) can be expected to provide up to 15 Mbps of shared bandwidth within a cell radius of up to three kilometers.

802.16 specifications include several multiplexing options. Time Division Duplex (TDD) supports half-duplex communications (one direction at a time) and Frequency Division Duplex (FDD) supports both half-duplex and full-duplex (simultaneous two-way transmission).

Line-of-Sight (LOS) describes a direct, unobstructed signal path from transmitter to receiver. Clearly, so to speak, the tighter the focus of the transmitting antenna, the more direct and unobstructed the path, and the better the design of the receiving antenna, the stronger the received signal and the better the performance of the radio link.

That translates into better error performance, which supports higher transmission speeds. Non-Line-of-Sight (NLOS) describes a path that is at least partially obstructed. As a result, the signal reaches the receiving antenna after having been reflected, scattered, diffused and diffracted. Actually, the original signal is broken into components that propagate along multiple paths, which causes attenuation, delay spread and even polarization changes, all of which makes it extremely difficult for the receiving antenna to interpret the signal correctly. That results in poor error performance, which can effectively destroy the link, but which certainly forces a reduction in transmission rates.

NLOS technology included in WiMAX specifications addresses these issues, making WiMAX deployment much more flexible and even supporting indoor CPE. This is accomplished by a number of mechanisms, including the following:

• Orthogonal Frequency Division Multiplexing (OFDM) breaks the signal into a number of narrowband orthogonal (i.e., independent) subcarriers, across which the signal is sent in parallel fashion. The receiving antenna monitors all subcarriers, selecting those with the strongest and most cohesive signal.

• Sub-channelization is a WiMAX option for the uplink, i.e., the link from the remote terminal back to the Base Station (BS) at the headend of the network. Sub-channelization concentrates signal power into fewer OFDM subcarriers, thereby extending the reach of the system, mitigating the effects of physical obstructions in an NLOS environment and reducing CPE power consumption.

• Antenna design clearly has an impact on signal quality. In a fixed wireless scenario, WiMAX antennas are directional in nature, which reduces multipath fading and, thereby, improves signal strength and cohesiveness. The directional antennas at the customer premises may be adaptive in nature. These passive array pizza box antennas (so called because they are about the size and shape of a pizza box) possess beamforming properties that permit them to adjust their focus to maximize the strength of the incoming signal from the BS at the network headend. They also reduce co-channel interference, which enhances spectrum efficiency through increased spectrum re-use. These adjustments in focus are accomplished passively, as no mechanical reorientation is required.

• Space/Time Coding compensates for fading. Since copies of the same Radio frequency (RF) signal are transmitted through multiple transmit and receive antennas separated by physical space and are separated in time, intelligent antenna systems can compensate for multipath fading and realize diversity gain, i.e. increase in signal strength.

• Adaptive Modulation allows the system to dynamically adjust the signal modulation technique as signal quality varies. When the signal is strong, an intelligent WiMAX system can use the most robust modulation scheme, which yields the highest transmission rate. As the signal fades due to factors such as distance from the BS and Radio Frequency Interference (RFI) or Electromagnetic Interference (EMI), error performance drops and link stability is jeopardized, the system can shift to successively less robust modulation schemes to compensate for those conditions. While transmission rates suffer, the link is maintained. Note: The modulation schemes employed, from the most robust to the least, are specified as 256-state Quadrature Amplitude Modulation (256 QAM), 64 QAM, 16 QAM, Quadrature Phase Shift Keying (QPSK) and Binary Phase Shift Keying (BPSK).

• Error Control, including error detection and correction, is a critical element of any communications protocol. Wireless protocols often include Forward Error Correction (FEC) in order consideration of issues of poor Signal-to-Noise Ratio (SNR) over high-demand channels. WiMAX incorporates strong Reed Solomon FEC and other related mechanisms to improve throughput. While FEC inherently involves some degree of bit-level redundancy, it provides the receiver with enough data to reconstruct a large percentage of frames errored in transit. Automatic Repeat Request (ARQ) is employed to request retransmission of any remaining errored frames.

• Power Control information is sent by the BS to all CPE devices. Thereby, the remote terminals can dynamically adjust their transmission levels to conserve power and to minimize the likelihood of co-channel interference with other CPE in proximity.

Coverage Range, Transmission Rate and System Loading

The WiMAX Forum describes coverage range in terms of two scenarios. A standard BS includes only mandatory capabilities, including output power. A full-featured BS includes higher RF output power, transmit/receive diversity, sub-channelization, and ARQ. Table 1 assumes that the system runs in the 3.5GHz band, that each channel is 3.5 MHz wide and that the system is sectorized at 60 degrees:

*This assumes that a single subchannel is used to extend the range as far as possible.

Note that the transmission rate is stated in terms of throughput, which takes overhead into consideration. In a 3.5 MHz channel and at a maximum of 5 bits/Hz, the raw signaling rate is 17.5 Mbps. In consideration of overheads such as framing and error control, the maximum actual bidirectional data throughput is approximately 11 Mbps at “short range” and 8 Mbps at “long range,” as illustrated in Figure 1.

Also note that the transmission rate is symmetrical, i.e., the same for uplink (upstream) as for downlink (downstream) transmission. This is unlike the typical high-speed modem, cable modem, DSL and PON implementations, which are asymmetrical. The exception to symmetry is in the case of full-featured CPE at the cell edge, where uplink transmission rates are constrained by power limitations. This forces the invocation of less robust sub-channelization options and adaptive modulation options to extend the range as far as possible. In a point-to-multipoint configuration, WiMAX can serve a great number of customer premises, sharing bandwidth amongst them as illustrated in Figure 2.

Convergence

The specifications include convergence sublayers designed for mapping services to and from 802.16 connections. The ATM convergence sublayer is for ATM services and the packet convergence sublayer is for packet services such as IPv4, IPv6, Ethernet and Virtual LAN (VLAN). As 802.16 is connection-oriented, all services, including those inherently connectionless (e.g., SMTP and UDP) in nature, are mapped to a connection. This provides a mechanism for requesting bandwidth, establishing Quality of Service (QoS) and various traffic parameters, and otherwise taking actions associated with contractual terms of the specific service.

QoS is a distinctive feature of WiMax. As the link is under the control of the BS, downstream QoS is fairly straightforward. Upstream QoS is accomplished through a scheduling service that defines four polling schedules.

• Unsolicited Grant Service (UGS) is designed for services that periodically generate fixed units of data. Examples are ATM Constant Bit Rate (CBR) service for applications such as uncompressed voice and video, and Time Division Multiplexed (TDM) services such as T-1 and E1.

• Real-Time Polling Service is tailored for services that are dynamic in nature, but require periodic dedicated request opportunities to meet real-time demands. Examples are real-time compressed voice services such as Voice over Internet Protocol (VoIP), and streaming audio and video.

• Non-real-Time Polling Service is identical to Real-Time Polling Service, except for the fact that connections may use random transmit opportunities. Examples of such services include Internet access with a minimum guaranteed connection rate and ATM Guaranteed Frame Rate (GFR) connections.

• Best Effort Service provides neither throughput nor delay guarantees.

Security

The 802.16 security protocol is built on enhancements to the Privacy Key Management (PKM) developed for cable modem communications. The protocol uses X.509 digital certificates with RSA (Rivest-Shamir-Adleman ) encryption for authentication and key exchange. Traffic encryption mandates the use of Data Encryption Standard (DES) with 56-bit keys.

Strengths and Weaknesses

WiMAX offers some real advantages when compared to alternative broadband solutions, whether wired or wireless:

• Standards-Based specifications from the IEEE, 802.16 offer the advantage of broad support across the manufacturer, carrier and service provider communities. That ultimately promises to translate into large manufacturing runs, which translates into lower cost per unit. Also, 802.16 includes specifications for both fixed and mobile systems.

• Rapid and Low Cost Deployment and Reconfiguration, as compared to wired systems, is an advantage of all wireless systems. Trenching, pole placement, splicing, amplifier/repeater placement and related activities associated with wired transmission systems simply take time and cost money.

• Link Adaptation through adaptive modulation, time-space coding, adaptive antennas, sub-channelization and power control ensures that each link performs optimally, balancing speed and performance.

• Topologies include point-to-point and point-to-multipoint, with mesh extensions. That level of flexibility under the umbrella of a single standard is unusual.

• Survivability is a key advantage. Wireless systems just aren’t susceptible to catastrophic failures caused by backhoes, posthole diggers, train derailments and the like.

• Line-of-Sight (LOS) isn’t an absolute requirement. Although LOS is always desirable, WiMax is designed to tolerate an NLOS environment, which fact is a tremendous advantage.

• Quality of Service (QoS), a distinctive feature of WiMax in the context of wireless systems, is critical for an integrated voice/data network.

• Bandwidth is considerable, at 70 Mbps (shared) per Base Station (BS).

WiMAX has its share of drawbacks, as well:

• Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI) are always issues with RF-based wireless systems.

• Shared Bandwidth places WiMAX at a disadvantage compared to DSL and PON, telco-provided broadband local loops that provide dedicated bandwidth.

• Competition from cable modems, DSL, PON, Wi-Fi (802.11) and 2.5/3G cellular systems is an issue. These relatively well-entrenched solutions will tend to discourage many commercial WiMAX deployments.

• Competition from IEEE 802.20 Working Group will also be an issue, as that group is chartered to develop similar standards for wireless access systems operating in licensed bands below 3.5 GHz. The technical goal is that of optimizing IP-based data transport, targeting peak data rates per user at over 1 Mbps and supporting vehicular traffic at speeds up to 250 km/hour (155 mph).

• Competition from the IEEE 802.22 Working Group is another issue. That group is developing a standard intended for wireless data over UHF and VHF spectrum currently used for broadcast TV. Also known as Wi-Fi TV, 802.22 targets the Radio Area Network (RAN), which it defines as having a range of up to 30 miles—the same as that of the 802.16 MAN.

• Competition extends to Europe and Asia, as well. ETSI chartered the Broadband Radio Access Networks (BRAN) project. HIPERACCESS is for frequencies above 11 GHz, and HIPERMAN below 11 GHz. ETSI and IEEE 802.16 cooperate to some extent. Korea’s Electronics and Telecommunications Research Institute (ETRI) developed WiBro for the 2.3 GHz band.

• Spectrum issues exist, even though WiMAX includes both licensed and license-exempt bands. WiMAX targeted spectrum includes license-exempt 2.4 GHz and 5.8 GHz bands, which invite interference. Sprint and Nextel control much of the licensed 2.5 GHz band in the U.S. That same band already is used for fixed wireless and cable transmission in Mexico.

Applications

Issues aside, the applications for WiMAX are numerous, including:

• Private campus networks

• T1 level service for large businesses

• Fractional T1 for medium and small businesses

• Rural or developing areas where broadband telco or cable access is available

• Urban areas where telco or cable upgrades are not easily, quickly or cost-effectively implemented

• Wi-Fi hotspot backhaul

• Disaster recovery and backup

Into the Future

802.16 is all about the future, as no commercial systems are yet in operation. There are several trials in process, however. AT&T is conducting trials with several customers in NJ. Miami University (Oxford, Ohio) is engaged in a trial with NCE Unified Solutions to cover students on campus and living off-campus within a three-to-five mile radius. This application certainly is an interesting broadband private network solution in a campus environment, and what is essentially a local loop extension of that campus network.

The WiMAX Forum is dedicated to promoting the development of interoperable products based on 802.16. Comprising approximately 300 members from the manufacturing, carrier, service provider, consulting and related communities, the WiMAX Forum certainly carries a lot of weight.

Carriers include Sprint and Nextel, which are scheduled to complete their merger in 3Q 2005. Sprint has announced plans for mobile systems based on 802.16e. BellSouth, British Telecom, France Telecom, SBC, Telefonica de Espana and Qwest are among the traditional carriers belonging to the WiMAX Forum. Chip manufacturers include Intel. LG Electronics, Nokia and Nortel have announced product plans. Motorola’s Canopy platform is intended to support VoIP and video.

A good deal of interest currently is focused on 802.16e, the mobile extension to WiMAX. Once that specification matures, WiMAX will be much more viable. Mobile WiMAX likely will be built into multimode chipsets that will enable wireless handheld devices to access multiple, disparate cellular networks based on various standards, Wi-Fi networks and WiMAX networks. “Cognitive radio,” aka “Software-Defined Radio” (SDR) devices, will be able to identify multiple standards-based networks, select the most appropriate one providing a signal of acceptable strength, and connect to that network. That sort of flexibility will provide a real boost to WiMAX sales.

All together, Research and Markets, a market research firm, projects worldwide WiMAX equipment sales to reach US$2.2 billion in 2009. Any way you look at it, wireless is bound to be a big winner into the next few decades, at least. WiMAX is bound to share in that success…at some level.