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3.3 THE ‘WHY(S)’
WiMAX is an integrated suite of many innovative and advance techniques covering diverse areas like modulation, antenna diversity
and interference. Some of the key developments having direct bearing on commercial acceptance of system functionality are as follows.
Dynamic Burst Mode TDMA MAC
802.16 is optimized to deliver high, bursty data rates to subscriber stations. This means that IEEE 802.16 is uniquely positioned to extend broadband wireless beyond the limits of today’s systems, both in distance and in the ability to support applications.
Quality of Service
Voice capability is extremely important, especially in underserved international markets. For this reason the IEEE 802.16a standard includes quality of service features that enable services including voice and video that require a low-latency network.
The grant/request characteristics of the 802.16 MAC enable an operator to simultaneously provide premium guaranteed levels of service to businesses, such as T1-level service, and high-volume ‘best effort’ service to homes, similar to cable-level service, all within the same base station service area cell.
Adaptive modulation and coding occur subscriber by subscriber, burst by burst, uplink and downlink. Transmission adaptation, with the help of modulation depending on channel condition, provides high reliability to the system. It also keeps more users connected by virtue of its flexible channel widths and adaptive modulation. Because it uses channels narrower than the fixed 20 MHz channels used in 802.11, the 802.162004 standards can serve lower data-rate subscribers without wasting bandwidth. When subscribers encounter noisy conditions or low signal strength, the adaptive modulation scheme keeps them connected when they might otherwise be dropped.
Further, this feature imparts differential service provision to the system, making it economically more appealing to operators. Dynamic adaptive modulation allows the base station to trade off throughput for
range. For example, if the base station cannot establish a robust link to a distant subscriber using the highest order modulation scheme, 64 QAM, the modulation order is reduced to 16 QAM or QPSK (quadrature phase shift keying), which reduces throughput and increases effective range.
NLOS Support, Provides Wider Market and Lower Costs
The problems resulting from NLOS conditions are solved or mitigated by using multiple frequency allocation support from 2 to 11 GHz, OFDM and OFDMA for NLOS applications (licensed and licence-exempt spectrum), subchannelization, directional antennas, transmit and receive diversity, adaptive modulation, error correction techniques and power control (Figure 3.3).
Highly Efficient Spectrum Utilization, Provides High Efficiency
MAC, designed for efficient use of spectrum, incorporates techniques for efficient frequency reuse, deriving more efficient spectrum usage of the access system.
10 MHz, EV-DO with 6 channels, full site throughput. Full details: “Hard Numbers and Experts’ Insights on Migration to 4G Wireless Technology”
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UMTS EV-DO HSDPA EV-DO IEEE 16e
Rev A 802.16e MIMO
Figure 3.3 Spectrum efficiency
Flexible Channel Bandwidth, Provides Bandwidth on Demand and Scalability
As the distance between a subscriber and the base station (or AP) increases, or as the subscriber starts to move by walking or driving in a car, it becomes more of a challenge for that subscriber to transmit successfully back to the base station at a given power level. For power-sensitive platforms such as laptop computers or handheld devices, it is often not possible for them to transmit to the base station over long distances if the channel bandwidth is wide.
The IEEE 802.16-2004 and IEEE 802.16e standards have flexible channel bandwidths between 1.5 and 20 MHz to facilitate transmission over longer ranges and to different types of subscriber platforms. In addition, this flexibility of channel bandwidth is also crucial for cell planning, especially in the licensed spectrum. For scalability, an operator with 14 MHz of available spectrum, for example, may divide that spectrum into four sectors of 3.5 MHz to have multiple sectors (transmit/receive pairs) on the same base station. With a dedicated antenna, each sector has the potential to reach users with more throughput over longer ranges than an omnidirectional antenna can. Net-to-net, flexible channel bandwidth is imperative for cell planning.
Smart Antenna Support, Provides Better Throughput-to-range Relationship
Smart antennas are being used to increase the spectral density (i.e. the number of bits that can be communicated over a given channel in a given time) and the signal-to-noise ratio for both Wi-Fi and WiMAX solutions. Because of performance and technology, the 802.16-2004 standard supports several adaptive smart antenna types, including receive spatial diversity antennas, simple diversity antennas, beam-steering antennas and beam-forming antennas.