wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

WiMAX in Industry 52-5
represents a single cell of network coverage. These cells are further connected through a core network
(CN). Transmissions can be PTP, point-to-multipoint (PMP0), or point-to-consecutive-point
(PTCP), where SSs can relay data (act as routers), supporting the nodes that do not have the LOS
connectivity with BSs [2]. It is important to note that BSs are equipped with wide-beam antennas,
where SSs are equipped with highly directional antennas pointing toward the BSs. This is why BSs
can provide superior data rates relative to the IEEE 802.11 WLANs, typically equipped with omnidirectional
antennas.
The SS-to-BS link is known as uplink, while BS-to-SS is typically referred to as the downlink.
52.3.1 MAC Layer/Data Link Layer
While the IEEE 802.11 Wi-Fi standard is based on the connectionless and contention-based access
(MAC) where the subscribers are connecting to a wireless AP (AP) on a random interrupt basis, 802.16
MAC uses a scheduling algorithm for which the subscriber station needs to compete with other subscribers
only once (for initial entry into the network). This entails that with Wi-Fi connection, subscribers
further away from the AP tend to be repeatedly interrupted by closer subscribers, greatly
reducing their throughput. This makes high data rate-dependent services such as Voice over Internet
Protocol (VoIP) or Internet Protocol Television (IPTV) difficult to maintain for larger number of
simultaneous users.
Unlike with Wi-Fi technology, the 802.16 MAC is based on a scheduling algorithm where subscriber
initially gets a time slot assigned by the BS. In addition to being stable under overload and oversubscription,
the 802.16 scheduling algorithm unlike 802.11, can also be more bandwidth efficient. The
scheduling algorithm can balance the time-slot assignments among the subscribers, hence controlling
the Quality of Service (QoS) parameters.
52.3.2 PHYsical Layer
The IEEE 802.16 standard (IEEE, 2004, 2005b) reference model comprises the data, control, and management
planes. The data plane protocol stack includes the PHY layer and the MAC layer. Multiple PHY
layers are supported, operating in the 2–66.GHz frequency spectrum, and support single- and multicarrier
air interfaces, depending on the particular operational environment [13].
The IEEE 802.16 PHY layer provides the following options [2,14]:
• Wireless MAN-SC (Metropolitan Area Network-Single Carrier) is a single-carrier option and is
intended for use in LOS environments.
• Wireless MAN-OFDM (Orthogonal Frequency-Division Multiple Access) with 256 subcarriers,
which is mandatory in the unlicensed bands. This mode features Time Division Multiplexing
(TDM) in the downlink direction and Time Division Multiple Access (TDMA) in the uplink
direction.
• Wireless MAN-OFDMA with 2048 carriers. This mode provides OFDMA access by separating
the users using FDD in the uplink direction in addition to TDMA.
While the original version of the IEEE 802.16 WiMAX standard, specified a PHY layer operating in
the 10–66.GHz range, 802.16a updated in 2004 (802.16-2004), added specifications for the 2–11.GHz
range. In 2005, IEEE 802.16-2004 was updated by 802.16e-2005, and introduced the scalable orthogonal
frequency-division multiple access (SOFDMA) as opposed to the orthogonal frequency-division
multiplexing (OFDM) with 256 subcarriers (of which 200 are used) in 802.16d. The following revisions
of IEEE 802.16 such as 802.16e introduced the multiple antenna support through MIMO, which
improved the coverage, power consumption, frequency reuse, and bandwidth efficiency, with the added
full mobility support. Finally, the WiMAX certification allowed vendors with 802.16d products to sell
their equipment as WiMAX certified.
© 2011 by Taylor and Francis Group, LLC