Tuesday, August 3, 2010

Frequency Allocation in Mobile WiMAX

In orthogonal frequency division multiple access (OFDMA), the frequency band is divided into subcarriers. These subcarriers are grouped into sets, called subchannels in WiMAX, which are allocated to users. We will consider in our numerical applications the case of an Fast Fourier Transform (FFT) size of 1024 subcarriers, with an intercarrier spacing of 7.8 kHz.
Add a note hereTwo allocation modes are possible: distributed and adjacent.

Add a note hereDistributed Allocation
Add a note hereIn the distributed subcarriers allocation, full channel diversity is obtained by distributing the allocated subcarriers to subchannels using a permutation mechanism. This mechanism is designed to introduce frequency diversity, thus minimizing the performance degradation due to fast fading which is characteristic of mobile environments. In addition to that, WiMAX standards specify two different distributed allocation modes: the fully used subchannelization (FUSC) mode where all subcarriers are used to form subchannels in each cell, and the partially used subchannelization (PUSC) mode where the frequency band is divided into three segments.
Add a note hereFor illustration, with an FFT size of 1024 and after reserving the pilot and guard subcarriers, a FUSC allocation will correspond to 16 subchannels of 48 data subcarriers each, whereas a PUSC allocation will correspond to 30 subchannels, each containing 24 data subcarriers. Note that assigning subcarriers to subchannels in PUSC is a bit complicated, as it employs two permutations:
§  Add a note hereAn outer permutation divides the subcarriers into six major groups of clusters using a specific renumbering sequence.
§  Add a note hereAn inner permutation operates separately on each major group, distributing subcarriers to subchannels within the group and is based on the FUSC permutation with distinct parameters for the odd and even major groups.
Add a note hereThis is illustrated in Figure 1, where two groups are assigned to one segment corresponding to a sector of the cell. Note that a segment can also be allocated to a cell in an omni-directional setting.

Figure 1: Construction of groups and segments in the partially used subchannelization allocation mode.
Add a note here

Add a note hereAdjacent Allocation
Add a note hereThis method uses adjacent subcarriers to form subchannels. It corresponds to the WiMAX AAS (Advanced Array Systems) mode, designed to support Multiple-Input Multiple-Outpt (MIMO) techniques and adaptive modulation. Note that, to achieve a frequency diversity, mobiles using adjacent allocation may hop rapidly between different subchannels during their communication times.

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