More advanced MIMO techniques can increase performance well beyond the current limits of data rate and reach" (p. 238). It would also appear that a growing number of vendors are recognizing the inherent constraints involved in the previous MIMO configuration and are developing superior alternatives. In this regard, according to Hedayat and his associates (2007), "Many MIMO systems for WiMAX are being developed without beamforming, and although it helps in robustness and can add some capacity, MIMO does nothing for the uplink. The result will be an uplink limited system that either has very slow uplink and frequent coverage holes, or a system that requires many more base stations and cell sites for universal coverage" (p. 8). Navini is in the process of introducing a new standards compliant MIMO solution that it calls "Smart MIMO"; Smart MIMO applies adaptive beamforming to MIMO in order to provide additional benefits beyond what simple MIMO can provide (Hedayat et al.) in the case of MIMO-a, the Space Time Coded signals are both beamformed using the adaptive beamforming algorithms based on measurements taken on the uplink channel (Hedayat et al.). The signals in the MIMO -- a configuration are also managed in phase in order to assure their optimal reception at the mobile station (Hedayat et al.).
While the pundits continue to debate the pace at which the world will ultimately reach a truly ubiquitous computing environment, the hand-writing is on the wall for everyone else to see and it appears the time is ripe for WiMAX today. As Hedayat and colleagues emphasize, "The performance improvements promised by MIMO, and later beamforming in WiMAX deployment scenarios are essential components for the delivery of true broadband services. MIMO equipment is already being deployed in the Wi-Fi market for IEEE802.11n products and has demonstrated massive increases in capacity. Clearly the environment represented by WiMAX is quite different from that of Wi-Fi and interference management is a key concern for WiMAX" (p. 17). Interference management concerns, though, are not restricted to WiMAX but are equally relevant for other technologies in use and envisioned and there issues are discussed further below.
Current Scenario and Importance of Comparison to Other Technology.
Because resources are by definition scarce and the costs associated with implementing and maintaining a sophisticated WiMAX system are not small, it just makes good business sense to determine if the WiMAX approach is worthy of the capital investments involved or whether a "wait-and-see" approach might be more prudent. Nevertheless, the need is great today and experts predict that demand will continue to grow in the future. According to Chen, Ahmad and Hanzo (n.d.)., "The ever-increasing demand for mobile communication capacity has motivated the needs for new technologies, such as space division multiple access, to improve spectrum utilization" (p. 1). Not surprisingly, there have been a number of approaches developed and tested in recent years in an effort to satisfy this growing demand and generate a profit in the process, but some approaches have clearly been better suited to existing needs than others. For instance, in their study, "Universal Broadband Access: Going Wireless and Mobile," Hurel, Brouet, Le Gouriellec and Peruyero (2005) ask, "GSM/EDGE, UMTS/HSDPA/HSUPA, WiMAX, CDMA2000, UMTS-TDD-HCR, TD-SCDMA, WiFi, mobile broadcast! What is the best technology to select? Are there any bad technologies that we can forget?" (p. 1). It is easy to become confused in this alphabet of choices, but an increasing number of industry experts suggest that the technology of choice today is WiMAX IEEE 802.16e, and these issues are discussed further below.
WiMAX (IEEE 802.16e)
The evolution of the WiMAX system from its 80216a permutation to its current 802.16e approach has been enthusiastically received by the mobile telecommunications industry as a revolution in how cellular services are provided. For instance, in their white paper, Airspan (2007) reports that, "Multiple Antenna Systems in WiMAX," WiMAX, championed by the WiMAX Forum to promote conformance and interoperability of the IEEE 802.16 standard, has revolutionised the wireless wide area broadband communications. The latest version of the standard, IEEE 802.16e-2005, extends the earlier specifications in order to address the requirements...
3). According to Muquet, Biglieri, Goldsmith and Sari (n.d.), "WiMAX systems are based on the IEEE 802.16-2004 and IEEE 802.16e-2005 standards which define a physical (PHY) layer and the medium access control (MAC) layer for broadband wireless access systems operating at frequencies below 11 GHz. The first of these standards, published in 2004, addresses fixed services, and the second, published in 2005, is intended for mobile services" (p. 4). According to Airspan (2007), "The underlying WiMAX PHY is ideally suited to multipath operations in demanding mobile and fixed WiMAX deployment scenarios. One of the strengths of the WiMAX PHY is the ease with which it supports and cooperates with multiple antenna technologies" (p. 17).
The initial version of the WiMAX standard operated in the 10-66GHz frequency band and required line of sight towers; however, the 802.16a extension employs the lower frequency of 2-11GHz, which relaxed regulatory requirements and does not require a line of sight configuration as well as providing a 31-mile range compared with Wi-Fi's 200-300 yards, and 70 Mbps data transfer rates (Gabriel, 2003, p. 4). Nevertheless, because WiMAX is scheduled to operate in the 2.5, 3.5, or 5.8 GHz bands, the system may require more cells than 3G, an approach that generally has frequencies less than 2 GHz because of the higher frequencies involved (Understanding WiMAX). According to analysts at Intel, one of the primary developers and proponents of the WiMAX approach, "The main impact will be to operators planning to deploy in the unlicensed 5.8 GHz spectrum. However, the costs associated with licensed spectrum for 3G and 2.5/3.5 GHz spectrum may offset the cost for additional cell sites" (Understanding WiMAX, p. 6).
According to their white paper, the researchers at Intel advise, "The portable version of WiMAX, IEEE 802.16e utilizes Orthogonal Frequency Division Multiplexing Access (OFDM/OFDMA) where the spectrum is divided into many sub-carriers. Each sub-carrier then uses QPSK or QAM for modulation" (Understanding WiMAX, p. 4). Likewise, Sivaradje and Dananjayan report that, "OFDM allows many users to transmit in an allocated carrier. Each user is allocated several carriers in which to transmit their data. The transmission is generated in such a way that the carriers used are orthogonal to one another, thus allowing them to be packed together much closer than standard frequency division multiplexing. This leads to OFDM providing a high spectral efficiency" (p. 1).
As Muquet and his colleagues advise, "The IEEE 802.16e-2005 specifications actually define three different PHY layers: Single-carrier transmission, orthogonal frequency-division multiplexing (OFDM), and orthogonal frequency-division multiple access (OFDMA) (p. 4). According to Vasuki (1999), OFDM is a multicarrier transmission technique that is used in both wired and wireless communications; the author notes in the former instance, though, the use of the term "Discrete Multi-Tone" is considered more accurate. In this regard, Vasuki reports that, "The OFDM technique divides the frequency spectrum available into many closely spaced carriers, which are individually modulated by low-rate data streams. In this sense, OFDM is similar to FDMA (the bandwidth is divided into many channels, so that, in a multi-user environment, each channel is allocated to a user)" (p. 2). The fundamental difference between the two concerns the fact that the carriers selected in OFDM are much more closely spaced than in FDMA (1kHz in OFDM compared to approximately 30 kHz in FDMA); this difference serves to increase its spectral usage efficiency, with the orthogonality between the carriers being responsible for the close spacing of carriers (Vasuki).
According to Hedayat and his colleagues (2007), "There are two forms of MIMO supported by mobile WiMAX (based on the IEEE 802.16e-2005 standard), as part of the wave 2 certification profiles. Called MIMO Matrix a and Matrix B, the two types of MIMO have the potential to improve the performance of personal broadband systems in diverse ways (p. 7). In their study, "Design and Optimisation of an Antenna Array for WIMAX Base Stations," Mahler and Landstorfer (2005), report that they have developed an intelligent base station antenna with beam- and nullsteering for 360-degree coverage in order to increase the capacity and coverage in broadband data communication according to the IEEE 802.16e WiMAX standard.
Efficiency and Its Importance in WiMAX.
One of the more compelling advantages of the WiMAX technology is the increased efficiency it provides to existing infrastucture. As Wu (n.d.) emphasizes, "The WiMAX standard is set to bring the long-awaited spectral efficiency and throughput to meet users' needs for combined mobility, voice services and high data rates" (p. 3). As noted above, there are a number of alternatives available that could, alone or in combination, provide the short-term answer to the exponential growth in wireless telecommunications, but WiMAX represents more than just such a stop-gap solution. According to Wu, "[WiMAX] will enable access for more users due to its NLOS [non-line of sight] capability, lower deployment costs, wide-range capability and penetration into the mass consumer market. Needless…
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