Saturday, October 17, 2009

Racetrack memory

Racetrack memory



IBM Racetrack Memory is an experimental non-volatile memory device under development at IBM's Almaden Research Center by a team led by Stuart Parkin.[1] In early 2008, a 3-bit version was successfully demonstrated.[2] If it is developed successfully, racetrack would offer storage density higher than comparable solid-state memory devices like Flash RAM and similar to conventional disk drives, but with much higher read/write performance. It is one of a number of new technologies vying to become a universal memory in the future.

Description

Racetrack Memory uses spin-coherent electric current to move the magnetic domains along a nanoscopic permalloy wire about 200 nm across and 100 nm thick. As current is passed through the wire, the domains pass by magnetic read/write heads positioned near the wire, which alter the domains to record patterns of bits. A Racetrack Memory device is made up of many such wires and read/write elements. In general operational concept, Racetrack Memory is similar to the earlier twistor memory or bubble memory of the 1960s and 70s. Both of these used electrical currents to "push" a magnetic pattern through a substrate. Dramatic improvements in magnetic detection capabilities, based on the development of spintronic magnetoresistive sensing materials and devices, allow the use of much smaller magnetic domains to provide far higher areal densities.

In production, it is expected that the wires can be scaled down to around 50 nm. There are two ways to arrange Racetrack Memory. The simplest is a series of flat wires arranged in a grid with read and write heads arranged nearby. A more widely studied arrangement uses U-shaped wires arranged vertically over a grid of read/write heads on an underlying substrate. This allows the wires to be much longer without increasing its 2D area, although the need to move individual domains further along the wires before they reach the read/write heads results in slower random access times. This does not present a real performance bottleneck; both arrangements offer about the same throughput. Thus the primary concern in terms of construction is practical; whether or not the 3D vertical arrangement is feasible to mass produce.
[edit] Comparison to other memory devices
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Current projections suggest that IBM Racetrack Memory will offer performance on the order of 20 to 32 ns to read or write a random bit. This compares to about 3,000,000 ns for a hard drive, or 6 to 40 ns for conventional DRAM. The authors of the primary work also discuss ways to improve the access times with the use of a "reservoir," improving to about 9.5 ns. Aggregate throughput, with or without the reservoir, is on the order of 250 to 670 Mbit/s for IBM Racetrack Memory, compared to 102400 for dual channel DDR2 DRAM, 1000 for high-performance hard drives, and much slower performance on the order of 30 to 100 Mbit/s for Flash devices. The only current technology that offers a clear performance benefit over IBM Racetrack Memory is SRAM, on the order of 2 ns, but is much more expensive and far lower density.[3]

Flash, in particular, is a highly asymmetrical device. Although read performance is fairly fast, especially compared to a hard drive, writing is much slower. Flash works by "trapping" electrons in the chip surface, and requires a burst of high voltage to remove this charge and reset the cell. In order to do this, charge is accumulated in a device known as a charge pump, which takes a relatively long time to charge up. In the case of "NOR" flash, which allows random bit-wise access like IBM Racetrack Memory, read times are on the order of 70 ns, while write times are much slower, about 2,500 ns. To address this concern, "NAND" flash allows reading and writing only in large blocks, but this means that the time to access any random bit is greatly increased, to about 1,000 ns. Additionally, the use of the burst of high voltage physically degrades the cell, so most flash devices allow on the order of 100,000 writes to any particular bit before their operation becomes unpredictable. Wear leveling and other techniques can spread this out, but only if the underlying data can be re-arranged.

The key determinant of the cost of any memory device is the physical size of the storage medium. The reason for this is due to the way memory devices are fabricated. In the case of solid-state devices like Flash or DRAM, a large "wafer" of silicon is processed into many individual devices, which are then cut apart and packaged. The cost of packaging is about $1 per device, so as the density increases and the number of bits per devices increases with it, the cost per bit falls by an equal amount. In the case of hard drives, data is stored on a number of rotating platters, and the cost of the device is strongly related to the number of platters. Increasing the density allows the number of platters to be reduced for any given amount of storage.

In most cases memory devices store one bit in any given location, so they are typically compared in terms of "cell size", a cell storing one bit. Cell size itself is given in units of F², where F is the design rule, representing usually the metal line width. Flash and racetrack both store multiple bits per cell, but the comparison can still be made. For instance, modern hard drives appear to be rapidly reaching their current theoretical limits around 650 nm²/bit,[4] which is defined primarily by our capability to read and write to tiny patches of the magnetic surface. DRAM has a cell size of about 6 F², SRAM is much worse at 120 F². NAND flash is currently the densest form of non-volatile memory in widespread use, with a cell size of about 4.5 F², but storing two bits per cell for an effective size of 2.25 F². NOR is slightly less dense, at an effective 4.75 F², accounting for 2-bit operation on a 9.5 F² cell size.[3]

IBM Racetrack Memory appears to scale to much smaller sizes than any current memory device. In the vertical orientation (U-shaped) about 128 bits are stored per cell, which itself can have a physical size of at least about 20 F². No other near-term solid-stage technology appears to be able to scale anywhere near these densities, representing a storage density about 100 times that of Flash.[3] The caveat here is that bits at different positions on the "track" would take different times (from ~10 ns to nearly a microsecond, or 10 ns/bit) to be accessed by the read/write sensor, because the "track" is moved at fixed rate (~100 m/s) past the read/write sensor.

IBM Racetrack Memory is one of a number of new technologies aiming to replace Flash, and potentially offer a "universal" memory device applicable to a wide variety of roles. Other leading contenders include MRAM, PCRAM and FeRAM. Most of these technologies offer densities similar to Flash, in most cases worse, and their primary advantage is the lack of write endurance limits like those in Flash. Field-MRAM offers excellent performance as high as 3 ns access time, but requires a large 25 to 40 F² cell size. It might see use as a SRAM replacement, but not as a mass storage device. The highest densities from any of these devices is offered by PCRAM, which has a cell size of about 5.8 F², similar to Flash, as well as fairly good performance around 50 ns. Nevertheless, none of these can come close to competing with IBM Racetrack Memory in overall terms, especially density. For example, 50 ns allows about 5 bits to be operated in an IBM Racetrack Memory device, resulting in an effective cell size of 20/5=4 F², easily exceeding the performance-density product of PCM. On the other hand, without sacrificing bit density, the same 20 F² area can also fit 2.5 2-bit 8 F² alternative memory cells (such as RRAM or spin-torque transfer MRAM), each of which could individually operated much faster (~10 ns).
[edit] Development difficulties

One limitation of the early experimental devices was that the magnetic domains could only be pushed slowly through the wires, requiring current pulses on the orders of microseconds to move them successfully. This was unexpected, and led to performance roughly equal to hard drives, as much as 1000 times slower than predicted. Recent research at the University of Hamburg has traced this problem to microscopic imperfections in the crystal structure of the wires which led to the domains becoming "stuck" at these imperfections. Using an x-ray microscope to directly image the boundaries between the domains, their research found that domain walls would be moved by pulses as short as a few nanoseconds when these imperfections were absent. This corresponds to a macroscopic performance of about 110 m/s.[5]

The voltage required to drive the domains along the racetrack would be proportional to the length of the wire. The current density must be sufficiently high to push the domain walls (as in electromigration). For example, a permalloy racetrack of resistivity 5*10-7 ohm-m, that is 1 cm long to cover an entire chip array,[6] and uses a current density of 3*108 A/cm2, would require a driving voltage of 15 kV along the racetrack.










WIMAX - THE HIGH SPEED 3G TECHNOLOGY

WiMAX

WiMAX, meaning Worldwide Interoperability for Microwave Access, is a telecommunications technology that provides wireless transmission of data using a variety of transmission modes, from point-to-multipoint links to portable and fully mobile internet access. The technology provides up to 3 Mbit/s [1][2][3][4][5][6][7][8][9][10][11][12] broadband speed without the need for cables. The technology is based on the IEEE 802.16 standard (also called Broadband Wireless Access). The name "WiMAX" was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the 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".[13]

Definitions

The terms "WiMAX", "mobile WiMAX", "802.16d" and "802.16e" are frequently used incorrectly.[14] Correct definitions are the following:

  • 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.
  • 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, among other things and is therefore also known as "mobile WiMAX".

Fixed WiMAX is similar in some respects to WLAN with an OFDM-based physical layer. Mobile WiMAX is based on an OFDMA physical layer. It uses both frequency division multiplex and time division multiplex. Groups of sub-carriers represent individual data streams. Uplink and Downlink communications can also utilize Time Division techniques. Because of increased signal complexity, high-speed measurements are required. This is absolutely necessary for complicated OFDM and MIMO (multiple-input multiple-output) signal structures.

Uses

The bandwidth and range of WiMAX make it suitable for the following potential applications:

  • Connecting Wi-Fi hotspots to the Internet.
  • Providing a wireless alternative to cable and DSL for "last mile" broadband access.
  • Providing data and telecommunications services.
  • Providing a source of Internet connectivity as part of a business continuity plan. That is, if a business has both a fixed and a wireless Internet connection, especially from unrelated providers, they are unlikely to be affected by the same service outage.
  • Providing portable connectivity.

Broadband access

Companies are evaluating WiMAX for last mile connectivity. The resulting competition may bring lower pricing for both home and business customers or bring broadband access to places where it has been economically unavailable.

WiMAX access was used to assist with communications in Aceh, Indonesia, after the tsunami in December 2004. All communication infrastructure in the area, other than amateur radio, was destroyed, making the survivors unable to communicate with people outside the disaster area and vice versa. WiMAX provided broadband access that helped regenerate communication to and from Aceh.

In addition, WiMAX was donated by Intel Corporation to assist the FCC and FEMA in their communications efforts in the areas affected by Hurricane Katrina.[15] In practice, volunteers used mainly self-healing mesh, VoIP, and a satellite uplink combined with Wi-Fi on the local link.[16]

Subscriber units (Client Units)

WiMAX subscriber units are available in both indoor and outdoor versions from several manufacturers. Self-install indoor units are convenient, but radio losses mean that the subscriber must be significantly closer to the WiMAX base station than with professionally-installed external units. As such, indoor-installed units require a much higher infrastructure investment as well as operational cost (site lease, backhaul, maintenance) due to the high number of base stations required to cover a given area. Indoor units are comparable in size to a cable modem or DSL modem. Outdoor units are roughly the size of a laptop PC, and their installation is comparable to the installation of a residential satellite dish.

With the potential of mobile WiMAX, there is an increasing focus on portable units. This includes handsets (similar to cellular smartphones), PC peripherals (PC Cards or USB dongles), and embedded devices in laptops, which are now available for Wi-Fi services. In addition, there is much emphasis from operators on consumer electronics devices such as Gaming consoles, MP3 players and similar devices. It is notable that WiMAX is more similar to Wi-Fi than to 3G cellular technologies.

Current certified devices can be found at the WiMAX Forum web site. This is not a complete list of devices available as certified modules are embedded into laptops, MIDs (Mobile internet devices), and private labeled devices.

Mobile handset applications

Sprint Nextel announced in mid-2006 that it would invest about US$ 5 billion in a WiMAX technology buildout over the next few years[17] ($5.29 billion in present-day terms[18]). Since that time Sprint has faced many setbacks, that have resulted in steep quarterly losses. On May 7, 2008, Sprint, Imagine, Google, Intel, Comcast, Bright House, and Time Warner announced a pooling of an average of 120 MHz of spectrum and merged with Clearwire to form a company which will take the name Clear. The new company hopes to benefit from combined services offerings and network resources as a springboard past its competitors. The cable companies will provide media services to other partners while gaining access to the wireless network as a Mobile virtual network operator. Google will contribute Android handset device development and applications and will receive revenue share for advertising and other services they provide. Sprint and Clearwire gain a majority stock ownership in the new venture and ability to access between the new Clear and Sprint 3G networks. Some details remain unclear including how soon and in what form announced multi-mode WiMAX and 3G EV-DO devices will be available. This raises questions that arise for availability of competitive chips that require licensing of Qualcomm's IPR.

Some analysts have questioned how the deal will work out: Although fixed-mobile convergence has been a recognized factor in the industry, prior attempts to form partnerships among wireless and cable companies have generally failed to lead to significant benefits to the participants. Other analysts point out that as wireless progresses to higher bandwidth, it inevitably competes more directly with cable and DSL, thrusting competitors into bed together. Also, as wireless broadband networks grow denser and usage habits shift, the need for increased backhaul and media service will accelerate, therefore the opportunity to leverage cable assets is expected to increase.

Backhaul/access network applications

WiMAX is a possible replacement candidate for cellular phone technologies such as GSM and CDMA, or can be used as an overlay to increase capacity. It has also been considered as a wireless backhaul technology for 2G, 3G, and 4G networks in both developed and poor nations.[19][20]

In North America, backhaul for urban cellular operations is typically provided via one or more copper wire line T1 connections, whereas remote cellular operations are sometimes backhauled via satellite. In most other regions, urban and rural backhaul is usually provided by microwave links. (The exception to this is where the network is operated by an incumbent with ready access to the copper network, in which case T1 lines may be used). WiMAX is a broadband platform and as such has much more substantial backhaul bandwidth requirements than legacy cellular applications. Therefore traditional copper wire line backhaul solutions are not appropriate. Consequently the use of wireless microwave backhaul is on the rise in North America and existing microwave backhaul links in all regions are being upgraded. [21] Capacities of between 34 Mbit/s and 1 Gbit/s are routinely being deployed with latencies in the order of 1ms. In many cases, operators are aggregating sites using wireless technology and then presenting traffic on to fiber networks where convenient.

Deploying WiMAX in rural areas with limited or no internet backbone will be challenging as additional methods and hardware will be required to procure sufficient bandwidth from the nearest sources — the difficulty being in proportion to the distance between the end-user and the nearest sufficient internet backbone.

Technical information

WiMAX is a term coined to describe standard, interoperable implementations of IEEE 802.16 wireless networks, similar to the way the term Wi-Fi is used for interoperable implementations of the IEEE 802.11 Wireless LAN standard. However, WiMAX is very different from Wi-Fi in the way it works.

MAC layer/data link layer

In Wi-Fi the media access controller (MAC) uses contention access — all subscriber stations that wish to pass data through a wireless access point (AP) are competing for the AP's attention on a random interrupt basis. This can cause subscriber stations distant from the AP to be repeatedly interrupted by closer stations, greatly reducing their throughput.

In contrast, the 802.16 MAC uses a scheduling algorithm for which the subscriber station needs to compete only once (for initial entry into the network). After that it is allocated an access slot by the base station. The time slot can enlarge and contract, but remains assigned to the subscriber station, which means that other subscribers cannot use it. In addition to being stable under overload and over-subscription, the 802.16 scheduling algorithm can also be more bandwidth efficient. The scheduling algorithm also allows the base station to control QoS parameters by balancing the time-slot assignments among the application needs of the subscriber stations.

Physical layer

The original version of the standard on which WiMAX is based (IEEE 802.16) specified a physical layer operating in the 10 to 66 GHz range. 802.16a, updated in 2004 to 802.16-2004, added specifications for the 2 to 11 GHz range. 802.16-2004 was updated by 802.16e-2005 in 2005 and uses scalable orthogonal frequency-division multiple access (SOFDMA) as opposed to the orthogonal frequency-division multiplexing (OFDM) version with 256 sub-carriers (of which 200 are used) in 802.16d. More advanced versions, including 802.16e, also bring multiple antenna support through MIMO. See: WiMAX MIMO. This brings potential benefits in terms of coverage, self installation, power consumption, frequency re-use and bandwidth efficiency. 802.16e also adds a capability for full mobility support. The WiMAX certification allows vendors with 802.16d products to sell their equipment as WiMAX certified, thus ensuring a level of interoperability with other certified products, as long as they fit the same profile.

Most commercial interest is in the 802.16d and 802.16e standards, since the lower frequencies used in these variants suffer less from inherent signal attenuation and therefore give improved range and in-building penetration. Already today, a number of networks throughout the world are in commercial operation using certified WiMAX equipment compliant with the 802.16d standard.

Deployment

As a standard intended to satisfy needs of next-generation data networks (4G), 802.16e is distinguished by its dynamic burst algorithm modulation adaptive to the physical environment the RF signal travels through. Modulation is chosen to be spectrally more efficient (more bits per OFDM/SOFDMA symbol). That is, when the bursts have a high signal strength and a carrier to noise plus interference ratio (CINR), they can be more easily decoded using digital signal processing (DSP). In contrast, operating in less favorable environments for RF communication, the system automatically steps down to a more robust mode (burst profile) which means fewer bits per OFDM/SOFDMA symbol; with the advantage that power per bit is higher and therefore simpler accurate signal processing can be performed.

Burst profiles are used inverse (algorithmically dynamic) to low signal attenuation; meaning throughput between clients and the base station is determined largely by distance. Maximum distance is achieved by the use of the most robust burst setting; that is, the profile with the largest MAC frame allocation trade-off requiring more symbols (a larger portion of the MAC frame) to be allocated in transmitting a given amount of data than if the client was closer to the base station.

The client's MAC frame and their individual burst profiles are defined as well as the specific time allocation. However, even if this is done automatically then the practical deployment should avoid high interference and multipath environments. The reason for which is obviously that too much interference causes the network function poorly and can also misrepresent the capability of the network.

The system is complex to deploy as it is necessary to track not only the signal strength and CINR (as in systems like GSM) but also how the available frequencies will be dynamically assigned (resulting in dynamic changes to the available bandwidth.) This could lead to cluttered frequencies with slow response times or lost frames.

As a result the system has to be initially designed in consensus with the base station product team to accurately project frequency use, interference, and general product functionality.

Multi-Channel WiMAX Testing – Wave 2

Today’s radio devices use a SISO (single-input single-output) configuration: one transmitter and one receiver and information sent over a single data channel. This is a typical Wi-Fi setup. Multi-signal transmission and reception add another layer of complexity. The challenge at the receiver end is to decompose the mixed signal into two independent signals or streams.

Perhaps the greatest testing challenge involves synchronization: transmission of multiple signals requiring accurate synchronization of multiple channels in phase and sampling alignment. Since signal analyzers and generators must have precise alignment to make accurate and repeatable measurements, transitioning smoothly from single-channel to multi-channel testing is critical, making the choosing of instruments that provide a clear and easy upgrade path to MIMO configurations important.

Another key concern is keeping the cost of test per channel low while maintaining good performance, especially with respect to maintaining excellent channel isolation. This is important because measuring the channel characteristics is fundamental to verifying any MIMO device. Ideally, test equipment should have 16-bit or better amplitude resolution.

Bandwidth is another important consideration. For mobile WiMAX the sub-carrier spacing is fixed at 10.94 kHz. The standard allows for FFT sizes from 128 to 2048, which means that the maximum signal bandwidth will be in excess of 20 MHz – so test equipment needs to have at least 20 MHz of bandwidth. If working with WLAN, then 40 MHz of bandwidth is even better for the 802.11n standard.

Equally important is the instrument’s usability. Intuitive displays are crucial when debugging complex radios, especially when dealing with multiple signals. Going beyond the constellation diagram, users need to see how modulation quality behaves over time, and over sub-carriers. They also need to see how the radio responds to change in the channel, especial with different multi-path models. A number of key measurements for 802.11n MIMO signals are also central to WiMAX Wave 2 testing including EVM (error vector magnitude), per stream and composite, channel response, and sub-carrier flatness per stream.

Beam forming also presents many test challenges. Beam forming helps to increase receiver sensitivity to the desired signal, and to decrease the sensitivity to interference and noise. Test equipment for beam forming should be capable of finite phase and amplitude adjustment.

Integration with an IP based Network

The WiMAX Forum WiMAX Architecture

The WiMAX Forum has proposed an architecture that defines how a WiMAX network can be connected with an IP based core network, which is typically chosen by operators that serve as Internet Service Providers (ISP); Nevertheless the WiMAX BS provide seamless integration capabilities with other types of architectures as with packet switched Mobile Networks.

The WiMAX forum proposal defines a number of components, plus some of the interconnections (or reference points) between these, labeled R1 to R5 and R8:

  • SS/MS: the Subscriber Station/Mobile Station
  • ASN: the Access Service Network[22]
  • BS: Base station, part of the ASN
  • ASN-GW: the ASN Gateway, part of the ASN
  • CSN: the Connectivity Service Network
  • HA: Home Agent, part of the CSN
  • AAA: Authentication, Authorization and Accounting Server, part of the CSN
  • NAP: a Network Access Provider
  • NSP: a Network Service Provider

It is important to note that the functional architecture can be designed into various hardware configurations rather than fixed configurations. For example, the architecture is flexible enough to allow remote/mobile stations of varying scale and functionality and Base Stations of varying size - e.g. femto, pico, and mini BS as well as macros.

Comparison with Wi-Fi

Comparisons and confusion between WiMAX and Wi-Fi are frequent because both are related to wireless connectivity and Internet access.

  • WiMAX uses is a long range system, covering many kilometers, that uses licensed or unlicensed spectrum to deliver a point-to-point connection to the Internet.
  • Different 802.16 standards provide different types of access, from portable (similar to a cordless phone) to fixed (an alternative to wired access, where the end user's wireless termination point is fixed in location.)
  • Wi-Fi uses unlicensed spectrum to provide access to a network.
  • Wi-Fi is more popular in end user devices.
  • WiMAX and Wi-Fi have quite different quality of service (QoS) mechanisms.
  • WiMAX uses a mechanism based on connections between the base station and the user device. Each connection is based on specific scheduling algorithms.
  • Wi-Fi has a QoS mechanism similar to fixed Ethernet, where packets can receive different priorities based on their tags. For example VoIP traffic may be given priority over web browsing.
  • Wi-Fi runs on the Media Access Control's CSMA/CA protocol, which is connectionless and contention based, whereas WiMAX runs a connection-oriented MAC.
  • Both 802.11 and 802.16 define Peer-to-Peer (P2P) and ad hoc networks, where an end user communicates to users or servers on another Local Area Network (LAN) using its access point or base station.

Spectrum allocation issues

The 802.16 specification applies across a wide swath of the RF spectrum, and WiMAX could function on any frequency below 66 GHz,[23] (higher frequencies would decrease the range of a Base Station to a few hundred meters in an urban environment).

There is no uniform global licensed spectrum for WiMAX, although the WiMAX Forum has published three licensed spectrum profiles: 2.3 GHz, 2.5 GHz and 3.5 GHz, in an effort to decrease cost: economies of scale dictate that the more WiMAX embedded devices (such as mobile phones and WiMAX-embedded laptops) are produced, the lower the unit cost. (The two highest cost components of producing a mobile phone are the silicon and the extra radio needed for each band.) Similar economy of scale benefits apply to the production of Base Stations.

In the unlicensed band, 5.x GHz is the approved profile. Telecommunication companies are unlikely to use this spectrum widely other than for backhaul, since they do not own and control the spectrum.

In the USA, the biggest segment available is around 2.5 GHz,[24] and is already assigned, primarily to Sprint Nextel and Clearwire. Elsewhere in the world, the most-likely bands used will be the Forum approved ones, with 2.3 GHz probably being most important in Asia. Some countries in Asia like India and Indonesia will use a mix of 2.5 GHz, 3.3 GHz and other frequencies. Pakistan's Wateen Telecom uses 3.5 GHz.

Analog TV bands (700 MHz) may become available for WiMAX usage, but await the complete roll out of digital TV, and there will be other uses suggested for that spectrum. In the USA the FCC auction for this spectrum began in January 2008 and, as a result, the biggest share of the spectrum went to Verizon Wireless and the next biggest to AT&T.[25] Both of these companies have stated their intention of supporting LTE, a technology which competes directly with WiMAX. EU commissioner Viviane Reding has suggested re-allocation of 500–800 MHz spectrum for wireless communication, including WiMAX.[26]

WiMAX profiles define channel size, TDD/FDD and other necessary attributes in order to have inter-operating products. The current fixed profiles are defined for both TDD and FDD profiles. At this point, all of the mobile profiles are TDD only. The fixed profiles have channel sizes of 3.5 MHz, 5 MHz, 7 MHz and 10 MHz. The mobile profiles are 5 MHz, 8.75 MHz and 10 MHz. (Note: the 802.16 standard allows a far wider variety of channels, but only the above subsets are supported as WiMAX profiles.)

Since October 2007, the Radio communication Sector of the International Telecommunication Union (ITU-R) has decided to include WiMAX technology in the IMT-2000 set of standards.[27] This enables spectrum owners (specifically in the 2.5-2.69 GHz band at this stage) to use Mobile WiMAX equipment in any country that recognizes the IMT-2000.

Spectral efficiency

One of the significant advantages of advanced wireless systems such as WiMAX is spectral efficiency. For example, 802.16-2004 (fixed) has a spectral efficiency of 3.7 (bit/s)/Hertz, and other 3.5–4G wireless systems offer spectral efficiencies that are similar to within a few tenths of a percent. The notable advantage of WiMAX comes from combining SOFDMA with smart antenna technologies. This multiplies the effective spectral efficiency through multiple reuse and smart network deployment topologies. The direct use of frequency domain organization simplifies designs using MIMO-AAS compared to CDMA/WCDMA methods, resulting in more effective systems.

Limitations

A commonly-held misconception is that WiMAX will deliver 70 Mbit/s over 50 kilometers (~31 miles). In reality, WiMAX can either operate at higher bitrates or over longer distances but not both: operating at the maximum range of 50 km increases bit error rate and thus results in a much lower bitrate. Conversely, reducing the range (to <1 class="noprint Template-Fact" title="This claim needs references to reliable sources from April 2009" style="white-space: nowrap;">[citation needed]

Typically, fixed WiMAX networks have a higher-gain directional antenna installed near the client (customer) which results in greatly increased range and throughput. Mobile WiMAX networks are usually made of indoor "Customer-premises equipment" (CPE) such as desktop modems, laptops with integrated Mobile WiMAX or other Mobile WiMAX devices. Mobile WiMAX devices typically have omnidirectional antennae which are of lower-gain compared to directional antennas but are more portable. In current deployments, the throughput may reach 2 Mbit/s symmetric at 10 km with fixed WiMAX and a high gain antenna. It is also important to consider that a throughput of 2 Mbit/s can mean 2 Mbit/s, symmetric simultaneously, 1 Mbit/s symmetric or some asymmetric mix (e.g. 0.5 Mbit/s downlink and 1.5 Mbit/s uplink or 1.5 Mbit/s downlink and 0.5 Mbit/s uplink), each of which required slightly different network equipment and configurations. Higher-gain directional antennas can be used with a WiMAX network with range and throughput benefits but the obvious loss of practical mobility.

Like most wireless systems, available bandwidth is shared between users in a given radio sector, so performance could deteriorate in the case of many active users in a single sector. In practice, most users will have a range of 2-3 Mbit/s services and additional radio cards will be added to the base station to increase the number of users that may be served as required.

Because of these limitations, the general consensus is that WiMAX requires various granular and distributed network architectures to be incorporated within the IEEE 802.16 task groups. This includes wireless mesh, grids, network remote station repeaters which can extend networks and connect to backhaul.

Silicon implementations

A critical requirement for the success of a new technology is the availability of low-cost chipsets and silicon implementations.

Intel Corporation is a leader in promoting WiMAX, and has developed its own chipset. However, it is notable that most of the major semiconductor companies have not and most of the products come from specialist smaller or start-up suppliers. For the client-side these include Sequans, whose chips are in more than half of the WiMAX Forum Certified(tm) MIMO-based Mobile WiMAX client devices, GCT Semiconductor, ApaceWave, Altair Semiconductor, Comsys, Motorola with TI, NextWave Wireless, Wavesat, Coresonic and SySDSoft. Both Sequans and Wavesat manufacture products for both clients and network while Texas Instruments, DesignArt, and picoChip are focused on WiMAX chip sets for base stations.

Standards

The current WiMAX incarnation, Mobile WiMAX, is based upon IEEE Std 802.16e-2005,[28] approved in December 2005. It is a supplement to the IEEE Std 802.16-2004,[29] and so the actual standard is 802.16-2004 as amended by 802.16e-2005 — the specifications need to be read together to understand them.

IEEE Std 802.16-2004 addresses only fixed systems. It replaced IEEE Standards 802.16-2001, 802.16c-2002, and 802.16a-2003.

IEEE 802.16e-2005 improves upon IEEE 802.16-2004 by:

  • Adding support for mobility (soft and hard handover between base stations). This is seen as one of the most important aspects of 802.16e-2005, and is the very basis of 'Mobile WiMAX'.
  • Scaling of the Fast Fourier transform (FFT) to the channel bandwidth in order to keep the carrier spacing constant across different channel bandwidths (typically 1.25 MHz, 5 MHz, 10 MHz or 20 MHz). Constant carrier spacing results in a higher spectrum efficiency in wide channels, and a cost reduction in narrow channels. Also known as Scalable OFDMA (SOFDMA). Other bands not multiples of 1.25 MHz are defined in the standard, but because the allowed FFT subcarrier numbers are only 128, 512, 1024 and 2048, other frequency bands will not have exactly the same carrier spacing, which might not be optimal for implementations.
  • Advanced antenna diversity schemes, and hybrid automatic repeat-request (HARQ)
  • Adaptive Antenna Systems (AAS) and MIMO technology
  • Denser sub-channelization, thereby improving indoor penetration
  • Introducing Turbo Coding and Low-Density Parity Check (LDPC)
  • Introducing downlink sub-channelization, allowing administrators to trade coverage for capacity or vice versa
  • Fast Fourier transform algorithm
  • Adding an extra QoS class for VoIP applications.

802.16d vendors point out that fixed WiMAX offers the benefit of available commercial products and implementations optimized for fixed access. It is a popular standard among alternative service providers and operators in developing areas due to its low cost of deployment and advanced performance in a fixed environment. Fixed WiMAX is also seen as a potential standard for backhaul of wireless base stations such as cellular, or Wi-Fi.

SOFDMA (used in 802.16e-2005) and OFDM256 (802.16d) are not compatible thus most equipment will have to be replaced if an operator wants or needs to move to the later standard. However, some manufacturers are planning to provide a migration path for older equipment to SOFDMA compatibility which would ease the transition for those networks which have already made the OFDM256 investment. Intel provides a dual-mode 802.16-2004 802.16-2005 chipset for subscriber units.

Conformance testing

TTCN-3 test specification language is used for the purposes of specifying conformance tests for WiMAX implementations. The WiMAX test suite is being developed by a Specialist Task Force at ETSI (STF 252).[30]

Associations

WiMAX Forum

The WiMAX Forum is a non profit organization formed to promote the adoption of WiMAX compatible products and services.[31]

A major role for the organization is to certify the interoperability of WiMAX products.[32] Those that pass conformance and interoperability testing achieve the "WiMAX Forum Certified" designation, and can display this mark on their products and marketing materials. Some vendors claim that their equipment is "WiMAX-ready", "WiMAX-compliant", or "pre-WiMAX", if they are not officially WiMAX Forum Certified.

Another role of the WiMAX Forum is to promote the spread of knowledge about WiMAX. In order to do so, it has a certified training program that is currently offered in English and French. It also offers a series of member events and endorses some industry events.

WiMAX Spectrum Owners Alliance

WiSOA logo

WiSOA was the first global organization composed exclusively of owners of WiMAX spectrum with plans to deploy WiMAX technology in those bands. WiSOA focussed on the regulation, commercialisation, and deployment of WiMAX spectrum in the 2.3–2.5 GHz and the 3.4–3.5 GHz ranges. WiSOA merged with the Wireless Broadband Alliance in April 2008. [33]

Competing technologies

Speed vs. Mobility of wireless systems: Wi-Fi, HSPA, UMTS, GSM

Within the marketplace, WiMAX's main competition comes from existing, widely deployed wireless systems such as UMTS and CDMA2000, as well as a number of Internet-oriented systems such as HiperMAN, and of course long range mobile Wi-Fi and mesh networking.

3G cellular phone systems usually benefit from already having entrenched infrastructure, having been upgraded from earlier systems. Users can usually fall back to older systems when they move out of range of upgraded equipment, often relatively seamlessly.

The major cellular standards are being evolved to so-called 4G, high-bandwidth, low-latency, all-IP networks with voice services built on top. The worldwide move to 4G for GSM/UMTS and AMPS/TIA (including CDMA2000) is the 3GPP Long Term Evolution effort. A planned CDMA2000 replacement called Ultra Mobile Broadband has been discontinued. For 4G systems, existing air interfaces are being discarded in favor of OFDMA for the downlink and a variety of OFDM based techniques for the uplink, similar to WiMAX.

In some areas of the world, the wide availability of UMTS and a general desire for standardization has meant spectrum has not been allocated for WiMAX: in July 2005, the EU-wide frequency allocation for WiMAX was blocked.

Mobile Broadband Wireless Access

Mobile Broadband Wireless Access (MBWA) is a technology developed by IEEE 802.20 and is aimed at wireless mobile broadband for operations from 75 to 220 mph (120 to 350 km/h). The 802.20 standard committee was first to define many of the methods which were later funneled into Mobile WiMAX, including high speed dynamic modulation and similar scalable OFDMA capabilities. It apparently retains fast hand-off, Forward Error Correction (FEC) and cell edge enhancements.

The Working Group was temporarily suspended in mid-2006 by the IEEE-SA Standards Board because it had been the subject of a number of appeals. A preliminary investigation of one of these "revealed a lack of transparency, possible 'dominance,' and other irregularities in the Working Group".[34]

In September 2006, the IEEE-SA Standards Board approved a plan to enable the working group to continue under new conditions, and on 12 June 2008, the IEEE approved the new standard.

Qualcomm, a leading company behind 802.20, has dropped support for continued development in order to focus on LTE.[35]

Internet-oriented systems

Early WirelessMAN standards, the European standard HiperMAN and Korean standard WiBro have been harmonized as part of WiMAX and are no longer seen as competition but as complementary. All networks now being deployed in South Korea, the home of the WiBro standard, are now WiMAX.

As a short-range mobile Internet technology, such as in cafes and at transportation hubs like airports, the popular Wi-Fi 802.11b/g system is widely deployed, and provides enough coverage for some users to feel subscription to a WiMAX service is unnecessary.

Comparison

The following table should be treated with caution because it only shows peak rates which are potentially very misleading. In addition, the comparisons listed are not normalized by physical channel size (i.e., spectrum used to achieve the listed peak rates); this obfuscates spectral efficiency and net through-put capabilities of the different wireless technologies listed below.

v d e
Comparison of Mobile Internet Access methods
Standard ↓ Family ↓ Primary Use ↓ Radio Tech ↓ Downlink (Mbit/s) ↓ Uplink (Mbit/s) ↓ Notes ↓
LTE UMTS/4GSM General 4G OFDMA/MIMO/SC-FDMA 360 80 LTE-Advanced update to offer up to 1 Gbit/s fixed speeds.
WiMAX 802.16 Mobile Internet MIMO-SOFDMA 144 35 WiMAX update to offer up to 1 Gbit/s fixed speeds.
Flash-OFDM Flash-OFDM Mobile Internet
mobility up to 200mph (350km/h)
Flash-OFDM 5.3
10.6
15.9
1.8
3.6
5.4
Mobile range 18miles (30km)
extended range 34 miles (55km)
HIPERMAN HIPERMAN Mobile Internet OFDM 56.9 56.9
Wi-Fi 802.11
(11n)
Mobile Internet OFDM/MIMO 288.9
(Supports 600Mbps @ 40MHz channel width)

Antenna, RF front end enhancements and minor protocol timer tweaks have helped deploy long range P2P networks compromising on radial coverage, throughput and/or spectra efficiency (310km & 382km).

iBurst 802.20 Mobile Internet HC-SDMA/TDD/MIMO 95 36 Cell Radius: 3–12 km
Speed: 250kmph
Spectral Efficiency: 13 bits/s/Hz/cell
Spectrum Reuse Factor: "1"
EDGE Evolution GSM Mobile Internet TDMA/FDD 1.9 0.9 3GPP Release 7
UMTS W-CDMA
HSDPA+HSUPA
HSPA+
UMTS/3GSM General 3G CDMA/FDD

CDMA/FDD/MIMO
0.384
14.4
42
0.384
5.76
11.5
HSDPA widely deployed. Typical downlink rates today 2 Mbit/s, ~200 kbit/s uplink; HSPA+ downlink up to 42 Mbit/s.
UMTS-TDD UMTS/3GSM Mobile Internet CDMA/TDD 16 16 Reported speeds according to IPWireless using 16QAM modulation similar to HSDPA+HSUPA
1xRTT CDMA2000 Mobile phone CDMA 0.144 0.144 Succeeded by EV-DO
EV-DO 1x Rev. 0
EV-DO 1x Rev.A
EV-DO Rev.B
CDMA2000 Mobile Internet CDMA/FDD 2.45
3.1
4.9xN
0.15
1.8
1.8xN
Rev B note: N is the number of 1.25 MHz chunks of spectrum used. Not yet deployed.

Notes: All speeds are theoretical maximums and will vary by a number of factors, including the use of external antennae, distance from the tower and the ground speed (e.g. communications on a train may be poorer than when standing still). Usually the bandwidth is shared between several terminals. The performance of each technology is determined by a number of constraints, including the spectral efficiency of the technology, the cell sizes used, and the amount of spectrum available. For more information, see Comparison of wireless data standards.

LTE is expected to be ratified at the end of 2008, with commercial implementations becoming viable within the next two years.

Future development

Mobile WiMAX based upon 802.16e-2005 has been accepted as IP-OFDMA for inclusion as the sixth wireless link system under IMT-2000. This can hasten acceptance by regulatory authorities and operators for use in cellular spectrum. WiMAX II, 802.16m will be proposed for IMT-Advanced 4G.

The goal for the long-term evolution of both WiMAX and LTE is to achieve 100 Mbit/s mobile and 1 Gbit/s fixed-nomadic bandwidth as set by ITU for 4G NGMN (Next Generation Mobile Network) systems through the adaptive use of MIMO-AAS and smart, granular network topologies. 3GPP LTE and WiMAX-m are concentrating much effort on MIMO-AAS, mobile multi-hop relay networking and related developments needed to deliver 10X and higher co-channel reuse multiples.

Since the evolution of core air-link technologies has approached the practical limits imposed by Shannon's Theorem, the evolution of wireless technology has embarked on pursuit of the 3X to 10X+ greater bandwidth and network efficiency by advances in the spatial and smart wireless broadband networking technologies.

Interference

A field test conducted by SUIRG (Satellite Users Interference Reduction Group) with support from the U.S. Navy, the Global VSAT Forum, and several member organizations yielded results showing interference at 12 km when using the same channels for both the WiMAX systems and satellites in C-band.[36] The WiMAX Forum has yet to respond.

Current deployments

Networks

The WiMAX Forum now claims there are over 455 WiMAX networks deployed in over 135 countries.

By territory

This section gives details of regulatory decisions in various parts of the world. For information on deployments around the world see the List of deployed WiMAX networks

Africa

In South Africa Telecoms Regulator ICASA has only issued four licences for commercial WiMAX services: to wireless broadband solutions provider iBurst, state-owned signal distributor Sentech, second network operator Neotel, [Amatole Telecommunication Services] (under serviced area license holder in S.A.) and Telkom, all on the 3.5 GHz band. See the List of deployed WiMAX networks for details.

Americas

See the List of deployed WiMAX networks for details.

Asia

See the List of deployed WiMAX networks for details.

Europe

Commission Decision of 2008-05-21 on the harmonisation of the 3400-3800 MHz frequency band for terrestrial systems capable of providing electronic communications services in the Community.[37]

It includes:

  • Pursuant to Article 4(2) of Decision 676/2002/EC (of the European Parliament and of the Council of 7 March 2002 on a regulatory framework for radio spectrum policy in the European Community - Radio Spectrum Decision -),[38] the Commission gave a mandate dated 4 January 2006 to the European Conference of Postal and Telecommunications Administrations (hereinafter the “CEPT”) to identify the conditions relating to the provision of harmonised radio frequency bands in the EU for Broadband Wireless Access (BWA) applications.
  • In response to that Mandate, the CEPT issued a report (CEPT Report 15) on BWA, which concludes that the deployment of fixed, nomadic and mobile networks is technically feasible within the 3400-3800 MHz frequency band under the technical conditions described in the European Conference of Postal and Telecommunications Administrations Decision ECC/DEC/(07)02 and Recommendation ECC/REC/(04)05.
  • No later than six months after entry into force of this Decision, Member States shall designate and make available, on a non-exclusive basis, the 3400-3600 MHz band for terrestrial electronic communications networks.
  • By 1 January 2012 Member States shall designate and subsequently make available, on a non-exclusive basis, the 3600-3800 MHz band for terrestrial electronic communications networks.
  • The designation of the 3400-3800 MHz band for fixed, nomadic and mobile applications is an important element addressing the convergence of the mobile, fixed and broadcasting sectors and reflecting technical innovation. Member States shall allow the use of the 3400-3800 MHz band in for fixed, nomadic and mobile electronic communications networks.
  • This Decision is addressed to the Member States.

Germany

German Federal Network Agency has begun assigning frequencies for wireless Internet access in the band 3400 to 3600 MHz (in some places up to 4000 MHz).[39]

United Kingdom

The UK telecoms industry is waiting for OFCOM the UK’s telecoms regulator, to launch the tender process for the 2.6 GHz spectrum range[40] for a number of services which can include WiMAX, including mobile services based on the 802.16e standard. This is currently delayed indefinitely due to the digital Britain report.

Indonesia

  • The Indonesian government announced on January 22, 2009 two ministry decrees and three regulations releasing spectrum at 2.3GHz and 3.3GHz for wireless broadband access across all regions of Indonesia. This means Indonesia will be using 2.3-GHz band for the Wimax 16.e standard while 3.3-GHz will be used for the 16.d standard.[41]