Wimax standards and Security The Wimax

MPDU Transmission

At the MAC layer, MAC protocol data unit (MPDU) transmission is sup- ported. The following issues support the MPDU transmission.

1. 
   Duplex Techniques
   

Several duplexing techniques are supported by the MAC protocol. The choice of duplexing technique may affect certain physical layer (PHY) parameters as well as impact the features that can be supported.
In an frequency division duplex (FDD) system, the uplink and downlink channels are located on separate frequencies and the downlink data can be transmitted in bursts. A fixed duration frame is used for both uplink and downlink transmissions. This facilitates the use of different modulation types. It also allows simultaneous use of both full-duplex SSs and, option- ally, half-duplex SSs. If half-duplex SSs are used, the bandwidth con- troller shall not allocate uplink bandwidth for a half-duplex SS at the same time that it is expected to receive data on the downlink chan- nel, including allowance for the propagation delay, SS transmit/receive transition gap (SSTTG), and SS receive/transmit transition gap (SSRTG). The fact that the uplink and downlink channels utilize a fixed duration frame simplifies the bandwidth allocation algorithms. A full-duplex SS is capable of continuously listening to the downlink channel, while a half-duplex SS can listen to the downlink channel only when it is not transmitting in the uplink channel.
In the case of time division duplex (TDD), the uplink and downlink trans-
missions occur at different times and usually share the same frequency. A TDD frame has a fixed duration and contains one downlink and one uplink subframe. The frame is divided into an integer number of physical slots (PSs), which help to partition the bandwidth easily. The TDD framing is adaptive in that the bandwidth allocated to the downlink versus the uplink can vary. The split between uplink and downlink is a system parameter and is controlled at higher layers within the system.
The DL-MAP message defines the usage of the downlink intervals for a burst mode PHY. The uplink bandwidth allocation map (UL-MAP) defines

the uplink usage in terms of the offset of the burst relative to the allocation start time.

2. 
   Uplink Timing and Allocations
   

Uplink timing is referenced from the beginning of the downlink subframe. The allocation start time in the UL-MAP is referenced from the start of the downlink subframe and may be such that the UL-MAP references some point in the current or a future frame. The SS shall always adjust its concept of uplink timing based upon the timing adjustments sent in the ranging response (RNG-RSP) messages.
For the single carrier (SC) and single carrier access (SCa) PHY layers, the UL-MAP uses units of minislots. The size of the minislot is specified as a function of PSs and is carried in the upper link channel descriptor (UCD) for each uplink channel. For the orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) PHY layers, the UL-MAP uses units of symbols and subchannels.
Through the request IE, the BS specifies an uplink interval in which requests may be made for bandwidth and for uplink data transmission. The character of this IE changes depending on the type of CID used in the IE. If broadcast or multicast, this is an invitation for SSs to contend for requests. If unicast, this is an invitation for a particular SS to request bandwidth. Unicasts may be used as part of a QoS scheduling scheme. For any uplink allocation, the SS may optionally decide to use the allocation for data or requests (or requests pig- gybacked in data). PDUs transmitted in this interval shall use the bandwidth request header format. For bandwidth request contention opportunities, the BS shall allocate a grant that is an integer multiple of the value of “Band- width request opportunity size,’’ which shall be published in each UCD transmission.
Timing information in the DL-MAP and UL-MAP is relative. The following
time instants are used as a reference for timing information: (1) DL-MAP: The start of the first symbol (including the preamble if present) of the frame in which the message was transmitted. (2) UL-MAP: The start of the first symbol (including the preamble if present) of the frame in which the message was transmitted plus the value of the allocation start time. Information in the DL-MAP pertains to the current frame (the frame in which the message was received). Information carried in the UL-MAP pertains to a time interval starting at the allocation start time measured from the beginning of the current frame and ending after the last specified allocation. This timing holds for both the TDD and FDD variants of operation.

3.