quality of service (QOS) capability of wimax

WiMAX radios will support very robust QOS capabilities up to and incorporating asynchronous transfer mode (ATM) quality. The MAC itself is configured to handle IP traffic, Ethernet and ATM natively. The MAC was designed to even support future transport protocols not yet invented. Links can be dynamically configured based on link conditions. Basically, this dynamic configuration technique smoothens the balancing act between raw capacity and quality on the fly. It should improve capacity or spectral efficiency a great deal. There are a lot of elements of wireless transmission which affect the quality of signal---needs also vary depending on the type of data. For example, VoIP can tolerate some errors, but must have low latencies (anything above 150 ms is a nonstarter) to operate. The packet sizes for VoIP are typically much smaller than for data. When networks must handle blended traffic, the polling mechanism that chooses which radio can transmit with either a smaller VoIP packet or a larger data packet is crucial to ensure that data traffic is not optimized at the expense of voice. Video transmission is similar. Conversely, data packets do not need especially low latencies, but cannot endure transmission errors.

WiMAX partly accomplishes this by assigning variable length Protocol Data Units (PDU)s, which is basically the data packet size in the Physical Layer, that can be combined in bursts to reduce signaling overhead in the PHY layer. This is called adaptive modulation and is a sharp contrast from the static modulation schemes of yesteryear. A similar technique is used for MAC signaling except they are called Service Data Units (SDU)s. Several other techniques are used for reducing signaling transmissions and to improve the polling or communications between radios. In the older 802.11b protocol for example, each radio and base station continues to signal and interact constantly with other radios---basically a carrier sense multiple access with collision detection (CSMA/CD) approach similar to Ethernet computer networks. This unfortunately results in packet collision, packet loss and a great deal of inefficient cross talk in a static mode.

WiMAX technology supports a variety of more efficient polling mechanisms that vendors and carriers can choose to use, including a defined contact cycle, grouping of radios into contact groups or even allowing customer radios to generate a brief signal indicating it needs a transmission cycle. All of these aspects, which are intended to solve multiple problems, also result in improved QOS capabilities. QOS is critical for delineating minimum bandwidth levels for VoIP sessions for example, as well as other leading edge IP services.

Both common duplexing schemes are supported in WiMAX---those being FDD and TDD.

The frequency division duplexing (FDD) requires two parallel channels for send and receive. This method is a well-understood holdover from cellular technology. The newer time division duplexing (TDD) allows for dynamic and symmetric transmission of data across a single channel. Where and when either should be used often depend on the frequency and the vendor’s emphasis on particular strengths. It is not unfair to suggest that TDD is more likely to be widely utilized by WiMAX product vendors. Suffice it to say that multiple duplexing support adds significant flexibility to WiMAX---capabilities not before supported by broadband wireless technology.

the actual throughput (data transfer rate) of WiMAX Technology

WiMAX supports very robust data throughput. The technology at theoretical maximums could support approximately 75 Mbps per channel (in a 20 MHz channel using 64QAM ¾ code rate). Real world performance will be considerably lower---perhaps maxing out around 45 Mbps/channel in some fixed broadband applications. Remember however, that service across this channel would be shared by multiple customers. Actual transmission capabilities on a per customer basis could vary widely depending on the carrier’s chosen customer base, which is actually an inherent strength because it can be defined by QOS in a deliberate fashion to offer different bandwidth capabilities to customers with different needs (and different budgets). Mobile WiMAX capabilities on a per customer basis will be lower in practical terms, but much better than competing 3G technologies. WiMAX is often cited to possess a spectral efficiency of 5 bps/Hz, which is very good in comparison to other broadband wireless technologies, especially 3G.

In practical terms, Sprint has stated that it intends to deliver service at 2 Mbps to 4 Mbps to its customers with Mobile WiMAX.

The modulation scheme, whether quaternary phase shift keying (QPSK), quadrature amplitude modulation (16QAM, 64 QAM etc.) and their attendant code rate variations deliver varying bandwidth capabilities by channel size. Like most things wireless, the devil as they say is in the details. The good news is that pretty much all of the news is good in this regard relative to other broadband wireless and wireline competitors of WiMAX. The OFDMA® technology actually supports multiple modulation schemes depending upon the users range from the cell with users at closer range receiving signal across more sub-channels at, for example, 64 QAM whereas a user at greater range would receive signal across fewer sub-channels (with higher gain or power per channel) using a lower bandwidth QPSK technique for example.

Many things affect transfer rate beyond simple radio capability---one major element being distance from the base station. The physics of radio cannot be avoided. Longer ranges result in lower bandwidth delivered. Also, the spectrum channel size (1.e. 20 MHz or other) that regulation defines as appropriate for different frequency bands will dictate bandwidth capabilities at least to some extent. Also, remember that the RF and physical environment play a strong role in throughput results. Essentially, the real world blunts theoretical performance.

The physics of frequency range plays a powerful role in bandwidth capability. The higher the frequency, the greater the bandwidth delivery potential and the shorter range potential. Lower frequencies enjoy much greater range capability, but trade that off with much lower bandwidth potential. Fortunately, even with disclaimers centered on real world impediments, WiMAX throughput is excellent.

most factors will greatly affect range for WiMAX products

Many factors affect range for any broadband wireless product. Some factors include the terrain and density/height of tree cover. Hills and valleys can block or partially reflect signals. Bodies of water such as rivers and lakes are highly reflective of RF transmissions. Fortunately OFDM can often turn this to an advantage---but not always. The RF shadow of large buildings can create dead spots directly behind them, particularly if license-free spectrums are being used (with their attendant lower power allotments). How busy the RF environment of a city or town can greatly degrade signals---meaning that properly designed and well thought out networks are always desired. The physics of radio transmission dictate that the greater the range between the base station and customer radio, the lower the amount of bandwidth that can be delivered, even in an extremely well-designed network. The climate can affect radio performance---despite this there are ubiquitous wireless networks deployed today with great success in frozen Alaskan oil fields as well as lush South American and Asian climates.

No two cities are exactly alike in terms of the challenges and opportunities presented. In many respects, broadband wireless remains very much an art form. However, this is also true for the cellular carriers most of us use daily. It can be done quite well. Mobile broadband wireless will be more difficult. Achieving high quality of service (QOS) will be easier with fixed broadband wireless. Despite all of these challenges, current broadband wireless is very effectively serving customers even in the most challenging environments.

Range of WiMAX

The answer to this question probably generates more confusion than any other single aspect of WiMAX. It is common to see statements in the media describing WiMAX multipoint coverage extending 30 miles. In a strict technical sense (in some spectrum ranges) this is correct, with even greater ranges being possible in point to point links. In practice (and especially in the license-free bands) this is wildly overstated especially where non line of sight (NLOS) reception is concerned.

Due to a variety of factors explained in more detail in other FAQ answers, the average cell ranges for most WiMAX networks will likely boast 4-5 mile range (in NLOS capable frequencies) even through tree cover and building walls. Service ranges up to 10 miles (16 Kilometers) are very likely in line of sight (LOS) applications (once again depending upon frequency). Ranges beyond 10 miles are certainly possible, but for scalability purposes may not be desirable for heavily loaded networks. In most cases, additional cells are indicated to sustain high quality of service (QOS) capability. For the carrier class approach, especially in regards to mobility, cells larger than this seem unlikely in the near future.

The Real WiMAX Advantage

Smart antennas use, OFDMA and specifically the spectral efficiency of OFDMA are routinely offered as the advantages of a mobile WiMAX. The true competitive strengths of WiMAX may prove to be elsewhere.

Intelligent antennas are on the road map of every major standard not just WiMAX, including those from the 3GPP (WCDMA) and 3GPP2 (CDMA) and the emerging 802.20 standard. Existing mobile carriers are unlikely to deploy intelligent antennas with a new air interface initially for coverage or capacity since these carriers already possess substantial tower assets and existing mobile carriers generally scale capacity on new services and/or air interfaces gradually to meet initial and limited demand for those services. Thus WiMAX may have a time advantage in implementation of intelligent antennas but that gain will be neutralized by mobile carriers themselves.

OFDMA is also on the road map of virtually every major standard, including those mentioned above. 3GPP2, perhaps through the standardization of FLASH-OFDM, stands to be the first group to offer a mobile, deployment ready version of OFDMA. 3GPP will follow and 802.20 could ratify an OFDMA standard by late 2006.

Equally important are the benefits of OFDMA, especially when pertaining to spectral efficiency in a wide area, multi-cellular, mobile environment, remain unproven regarding the implementation which WiMAX purveyors and most others will use.

The advantage may exist from use of the air interface but the level of improvement, especially in a challenging mobile environment, may not be as great as many are expecting. Likewise, performance may vary substantially according to implementation and performance of the MAC level, which governs how the radio spectrum is employed and which is rarely considered in RF simulations. Ultimately more testing is required now that 802.16e has been ratified and vendors are working on implementing the standard.

So what are the advantages of mobile WiMAX at the current time? WiMAX is the first truly open mobile standard (802.16e). It is governed by the IEEE's fair licensing practices and participation in the group is open and democratic compared to other groups. This is in fact revolutionary as 3GPP and 3GPP2 are ultimately consortiums and its implications are wide. This open process should lead to greater innovation and hence a better performance when moving forward and also potentially lower intellectual property licensing fees and provide for a quicker rate of change compared to that of existing mobile technologies.

A lack of history within the mobile industry is also an advantage for WiMAX vendors. For the most part, and in contrast to CDMA/GSM/WCDMA vendors, key WiMAX equipment vendors lack a mobile product line to protect. They must push the envelope on technology and move forward as they cannot rely on a steady stream of existing GSM, WCDMA, or CDMA mobile contracts. In other words, WiMAX proponents benefit from disrupting the status quo and their survival may depend on it.

WiMAX is also the first major mobile standard to offer all IP as a standard feature set. 3GPP will get there in subsequent releases but it still employs a complicated and ultimately expensive core network. Major mobile carriers, who are often also wireline or even cable operators, will seek to consolidate their core networks under IP. Doing so offers cost advantages, the ability to offer multiple services over a single platform, reductions in operating and capital expenditures, rapid application development and often a competitive edge.

In summary, we need to look beyond RF performance criteria alone when evaluating the relative merits of mobile WiMAX vs. future incarnations of existing mobile technologies due to lack of real world data and the existence of other often ignored criteria.

GSM won the 2G mobile war despite offering inferior capacity figures due to strengths in other quadrants of the competitive matrix and we can not accurately gauge the performance that FLASH-OFDM, UMTS or 802.16e air interfaces will offer in 2008 or 2009 when mobile WiMAX gear is certified and ready to ship today.