Wimax standards and Security The Wimax

Other Features

The PHY layer also has other features, some of them are mandatory and the others are optional. These features empower the performance of the technol- ogy to provide for robust performance over a wide range of frequencies and under different channel conditions.

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  Adaptive antenna system (AAS): uses multiple antennas at both the receiver and the transmitter ends (MIMO system) to increase channel capacity by steering the antenna beams toward multiple users to achieve in-cell frequency reuse. MIMO system is also benefi- cial in increasing the signal-to-interference ratio through coherently combining multiple signals. Another benefit of AAS is the decre- ment of required power due to utilizing beams formed of adaptive antennas.
  
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  Adaptive modulation: 802.16-2004 allows for different modulation schemes in the down- and uplink communication, i.e., BPSK, QPSK, 16QAM, 64QAM, and 256QAM. The 802.16 standard defines differ- ent combinations of the aforementioned modulation schemes and coding rates, providing for a wide range of trade-offs of data rate and robustness depending on channel conditions. Although 802.11a/g standard uses similar modulation schemes as 802.16, there is one dif- ference between them, 802.16 uses Reed–Solomon block code with an inner convolution code or Turbo coding. The latter is left as an optional feature.
  
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  Space time coding: is an optional feature of 802.16 that can be used in the downlink communication to provide for space trans- mit diversity. Space time coding assumes that the base station is
  

using two transmit antennas and the subscriber station uses one transmit antenna.

2. 
   MAC Layer
   

The MAC layer supports the different PHY specifications by using time divi- sion multiplexing, where users are assigned time slots to access the channel. The uplink communication is based on time division multiple access (TDMA). TDMA facilitates different levels of QoS and bounded delay communication through a predetermined service level agreement. This can be achieved by allocating bandwidth based on a request/grant mechanism. The standard 802.16-2004 supports both TDD and FDD, full and half duplex.

Link layer control (LLC)
Convergence sublayer (CS)
Common part sublayer (CPS)
Privacy sublayer (PS)
Transmission convergence sublayer
QPSK	16QAM	64QAM	256QAM

802.16-2004 is designed to carry any present or future higher-layer protocol such as IP versions 4 and 6, packetized voice-over-IP (VoIP), Ethernet, ATM, and virtual LAN (VLAN) services. 802.16 accomplishes this by dividing its MAC layer into separate sublayers that handle different services as follows:
MAC
layer
PHY
layer

1.