In order to create the OFDM symbol in the frequency domain, the modulated symbols are mapped on to the subchannels that have been allocated for the transmission of the data block.
Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers called subchannels. The minimum frequency-time resource unit of sub-channelization is one slot.
The number and exact distribution of the subcarriers that constitute a subchannel depend on the subcarrier permutation mode. The number of subchannels allocated for transmitting a data block depends on various parameters, such as the size of the data block, the modulation format, and the coding rate.
In the time and frequency domains, the contiguous set of subchannels allocated to a single user—or a group of users, in case of multicast—is referred to as the data region of the user(s) and is always transmitted using the same burst profile. In this context, a burst profile refers to the combination of the chosen modulation format, code rate, and type of FEC: convolutional codes, turbo codes, and block codes.
Two types of sub-carrier permutations: Diversity & Contiguous
The subcarriers that constitute a subchannel can either be adjacent to each other or distributed throughout the frequency band, depending on the
subcarrier permutation mode. A distributed subcarrier permutation provides better frequency diversity, whereas an adjacent subcarrier distribution is more desirable for beamforming.
Downlink Full Usase of Sub-Carrers (DL-FUSC)
In the case of DL FUSC, all the data subcarriers are used to create the various subchannels. Each subchannel is made up of 48 data subcarriers, which are distributed evenly throughout the entire frequency band, as depicted in Figure. In FUSC, the set of the pilot subcarriers is divided in to two constant sets and two variables sets. The variable sets allow the receiver to estimate the channel response more accurately across the entire frequency band.
==== Parameters of FUSC subcarrier permutation(1024 FFT) ====
Subcarriers per subchannel : 48
Number of subchannels : 16
Data Subcarriers used : 768
Fixed Pilot Subcarriers : 11
Variable Pilot Subcarriers : 71
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Downlink Partial Usase of Sub-Carrers (DL-PUSC)
DL PUSC is similar to FUSC except that all the subcarriers are first divided into six groups. Permutation of subcarriers to create subchannels is performed independently within each group, thus, in essence, logically separating each group from the others. In the case of PUSC, all the subcarriers except the null subcarrier are first arranged into clusters. Each cluster consists of 14 adjacent subcarriers over two OFDM symbols, as shown in Figure. In each cluster, the subcarriers are divided into 24 data subcarriers and 4 pilot subcarriers. The clusters are then renumbered using a pseudorandom numbering scheme, which in essence redistributes the logical identity of the clusters. After renumbering, the clusters are divided into six groups, with the first one-sixth of the clusters belonging to group 0, and so on. A subchannel is created using two clusters from the same group, as shown in Figure.
Frequency reuse is much simple in PUCS. E.g consider the BS with 3 segments, here it is possible to allocate all or only a subset of the six groups to a given transmitter. By allocating disjoint subsets of the six available groups to neighboring transmitters, it is possible to separate their signals in the subcarrier space, thus enabling a tighter frequency reuse at the cost of data rate.
==== Parameters of PUSC subcarrier permutation(1024 FFT) ====
Subcarriers per cluster: 14
Number of subchannel : 30
Data Subcarriers used : 720
Pilot Subcarriers : 120
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Uplink Partial Usage of Subcarriers (UL-PUSC)
In UL PUSC, the subcarriers are first divided into various tiles, as shown in Figure. Each tile consists of four subcarriers over three OFDM symbols. The subcarriers within a tile are divided into eight data subcarriers and four pilot subcarriers. The optional UL PUSC mode has a lower ratio of pilot subcarriers to data subcarriers, thus providing a higher effective data rate but poorer channel-tracking capability. The two UL PUSC modes allow the system designer a trade-off between higher data rate and more accurate channel tracking depending on the Doppler spread and coherence bandwidth of the channel. The tiles are then renumbered, using a pseudorandom numbering sequence, and divided into six groups. Each subchannel is created using six tiles from a single group. UL PUSC can be used with segmentation in order to allow the system to operate under tighter frequency reuse patterns.
Band Adaptive Modulation and Coding (B-AMC)
Unique to the band AMC permutation mode, all subcarriers constituting a subchannel are adjacent to each other. Although frequency diversity is lost to a large extent with this subcarrier permutation scheme, exploitation of multiuser diversity is easier. Multiuser diversity provides significant improvement in overall system capacity and throughput, since a subchannel at any given time is allocated to the user with the highest SNR/capacity in that subchannel.
In this subcarrier permutation, nine adjacent subcarriers with eight data subcarriers and one pilot subcarrier are used to form a bin, as shown in Figure. Four adjacent bins in the frequency domain constitute a band. An AMC subchannel consists of six contiguous bins from within the same band. Thus, an AMC subchannel can consist of one bin over six consecutive symbols, two consecutive bins over three consecutive symbols, or three consecutive bins over two consecutive symbols.
