Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN---4

Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN
BASICS OF OFDMA FRAME STRUCTURE
There are three types of OFDMA subcarriers: Data subcarriers for data transmission.
Pilot subcarriers for various estimation and synchronization purposes.
Null subcarriers for no transmission at all, used for guard bands and DC carriers.
Active subcarriers are divided into subsets of subcarriers called subchannels. The subcarriers forming one subchannel may be, but need not be, adjacent. Bandwidth and MAP allocations are done in subchannels.
The pilot allocation is performed differently in different subcarrier allocation modes. For DL Fully Used Subchannelization (FUSC), the pilot tones are allocated first and then the remaining subcarriers are divided into data subchannels. For DL Partially Used Subchannelization (PUSC) and all UL modes, the set of used subcarriers, that is, data and pilots, is first partitioned into subchannels, and then the pilot subcarriers are allocated from within each subchannel. In FUSC, there is one set of common pilot subcarriers, but in PUSC, each subchannel contains its own set of pilot subcarriers.

Figure 1: OFDMA frame structure (TDD, PUSC)
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In a DL, subchannels may be intended for different (groups of) receivers while in UL, Subscriber Stations (SS) may be assigned one or more subchannels and several transmitters may transmit simultaneously.
The subcarriers forming one subchannel may, but need not be, adjacent. Figure 1 shows the OFDM frame structure for Time Division Duplexing (TDD) mode. Each frame is divided into DL and UL subframes separated by Transmit/Receive and Receive/Transmit Transition (TTG and RTG, respectively) gaps. Each DL subframe starts with a preamble followed by the Frame Control Header (FCH), the DL-MAP, and a UL-MAP, respectively.
The FCH contains the DL Frame Prefix (DLFP) to specify the burst profile and the length of the DL-MAP immediately following the FCH. The DLFP is a data structure transmitted at the beginning of each frame and contains information regarding the current frame; it is mapped to the FCH.
According to the OFDMA specifications, a DL-MAP message, if transmitted in the current frame, shall be the first MAC PDU in the burst following the FCH. An UL-MAP message shall immediately follow either the DL-MAP message (if one is transmitted) or the DLFP. If Uplink Channel Descriptor (UCD) and Downlink Channel Descriptor (DCD) messages are transmitted in the frame, they shall immediately follow the DL-MAP and UL-MAP messages.
Simultaneous DL allocations can be broadcast, multicast, and unicast and they can also include an allocation for another BS rather than a serving BS. Simultaneous ULs can be data allocations and ranging or bandwidth requests.

Figure 2: Pilot distribution for FUSC
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