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21 Wireless Local Area Networks
Ethernet, and ATM. This lets service providers use WiMAX independently of the
transport technology they support.
The recent WiMAX standard, which adds full mesh networking capabilities,
enables WiMAX nodes to simultaneously operate in “subscriber” and “base sta-
tion” mode. This blurs the initial distinction and allows
for widespread adoption
of WiMAX based mesh networks and promises widespread WiMAX adoption.
Mobile WiMAX with OFDMA and scheduled MAC allows wireless mesh net-
works to be much more robust and reliable.
21.15.3 Spectrum Allocation for WiMAX
The IEEE 802.16 specifi cation applies across a wide swath of RF spectrum. There
is no uniform global licensed spectrum for WiMAX in the United States. The
biggest segment available is around 2.5 GHz, and is already assigned — primar-
ily to Sprint Nextel.
Elsewhere in the world, the most likely bands used will be
around 3.5 GHz, 2.3/2.5 GHz, or 5 GHz, with 2.3/2.5 GHz probably being most
important in Asia.
There is some prospect that some of a 700 MHz band might be made avail-
able
for WiMAX in the United States, but it is currently assigned to analog TV
and awaits the complete rollout of HD digital TV before it can become avail-
able, likely by 2009. There are several variants of 802.16, depending on local
regulatory conditions and thus of which spectrum is used.
Mobile WiMAX based on the 802.16e standard will most likely be in
2.3 GHz and 2.5 GHz frequencies — low enough to accommodate the NLOS
conditions between the base station and mobile devices.
The key technologies in
802.16e on PHY level are OFDMA and SOFDMA. OFDMA uses a multicarrier
modulation in which the carriers are divided among users to form subchannels
(see Figure 21.24). For each subchannel, the coding
and modulation are adapted
separately, allowing channel optimization on a smaller scale (rather than using
the same parameters for the whole channel). This technique optimizes the use of
spectrum resources and enhances indoor coverage by assigning a robust scheme to
vulnerable links. SOFDMA is an enhancement of
OFDMA that scale the number
of subcarriers in a channel with possible values of 128, 512, 1024, and 2048.
802.16e includes power-saving and sleep modes to extend the battery life
of mobile devices. 802.16e also supports hard and soft handoff to provide users
with seamless connections as they move across coverage areas of adjacent cells.
Other improvements for mobile devices include a real-time polling service to
provide QoS, HARQ scheme to
retransmit erroneous packets, and private key
management schemes to help with the distribution of encryption keys.
