The OFDMA PHY defines four selectable FFT sizes: 2048, 1024, 512, and 128. For con-venience, here we only take the 1024-FFT OFDMA subcarrier allocation for introduction.
The subcarriers are divided into three types: null (guard band and DC), pilot, and data.
Figure 2.5: Example of an OFDMA frame (with only mandatory zone) in TDD mode (from [3]).
Subtracting the guard tones from NF F T, one obtains the set of used subcarriers Nused. For both uplink and downlink, these used subcarriers are allocated to pilot subcarriers and data subcarriers. However, there is a difference between the different possible zones. For FUSC (full usage of subchannels)and PUSC, in the downlink, the pilot tones are allocated first;
what remains are data subcarriers, which are divided into subchannels that are used exclu-sively for data. Thus, in FUSC, there is one set of common pilot subcarriers, and in PUSC downlink, there is one set of common pilot subcarriers in each major group. In PUSC uplink, each subchannel contains its own pilot subcarriers.
2.4.1 Uplink
The contents of this subsection have been taken to a large extent from [2] and [3].
Data Mapping Rules
The UL mapping consists of two steps. In the first step, the OFDMA slots allocated to each burst are selected. In the second step, the allocated slots are mapped.
Step 1: Allocate OFDMA slots to bursts.
1) Segment the data into blocks sized to fit into one OFDMA slot.
2) Each slot shall span one or more subchannels in the subchannel axis and one or more OFDMA symbols in the time axis (see Figure 2.6 for an example). Map the slots such that the lowest numbered slot occupies the lowest numbered subchannel in the lowest numbered OFDMA symbol.
3) Continue the mapping such that the OFDMA symbol index is increased. When the edge of the UL zone is reached, continue the mapping from the lowest numbered OFDMA symbol in the next available subchannel.
4) An UL allocation is created by selecting an integer number of contiguous slots, accord-ing to the orderaccord-ing of steps 1 to 3. This results in the general burst structure shown by the gray area in Figure 2.6.
Step 2: Map OFDMA slots within the UL allocation.
1) Map the slots such that the lowest numbered slot occupies the lowest numbered sub-channel in the lowest numbered OFDMA symbol.
2) Continue the mapping such that the subchannel index is increased. When the last subchannel is reached, continue the mapping from the lowest numbered subchannel in the next OFDMA symbol that belongs to the UL allocation. The resulting order is shown by the arrows in Figure 2.6.
Figure 2.6: Example of mapping OFDMA slots to subchannels and symbols in the uplink (from [3]).
Figure 2.7: Description of an uplink tile (from [2]).
Carrier Allocations
The uplink supports 35 subchannels in the 1024-FFT PUSC permutation. Each transmission uses 48 data carriers as the minimal block of processing. Each new transmission for the uplink commences with the parameters as given in Table 2.2.
A slot in the uplink is composed of one subchannel in three OFDMA symbols. Within
Table 2.2: OFDMA Uplink Subcarrier Allocation [2], [3]
Parameter Value Notes
Number of DC subcarriers
1 Index 512 (counting from 0)
Nused 841 Number of all subcarriers used within a symbol Guard subcarriers: 92,91 Left, right
TilePermutation Used to allocate tiles to subchannels
11, 19, 12, 32, 33, 9, 30, 7, 4, 2, 13, 8, 17, 23,
4 Number of all subcarriers within a tile Tiles per subchannel 6
each slot, there are 48 data subcarriers and 24 pilot subcarriers. The subchannel is con-structed from six uplink tiles, each having four successive active subcarriers with the config-uration as illustrated in Figure 2.7.
The usable subcarriers in the allocated frequency band shall be divided into Ntilesphysical tiles with parameters from Table 2.2. The allocation of physical tiles to logical tiles in subchannels is performed according to:
T iles(s, n) = Nsubchannels·n+(P t[(s+n) mod Nsubchannels]+UL P ermBase)mod Nsubchannels
(2.1) where:
• T iles(s, n) is the physical tile index in the FFT with tiles being ordered consecutively from the most negative to the most positive used subcarrier (0 is the starting tile index),
• n = 0..5 is the tile index in a subchannel,
• P t is the tile permutation function,
• s is the subchannel number in the range 0..Nsubchannels− 1,
• UL P ermBase is an integer value in the range 0..69, which is assigned by a manage-ment entity, and
• Nsubchannels is the number of subchannels for the FFT size given in Table 2.2.
After mapping the physical tiles to logical tiles for each subchannel, the data subcarriers per slot are enumerated by the following process:
1) After allocating the pilot carriers within each tile, indexing of the data subcarriers within each slot is performed starting from the first symbol at the lowest indexed subcarrier of the lowest indexed tile and continuing in an ascending manner through the subcarriers in the same symbol, then going to the next symbol at the lowest indexed data subcarrier, and so on. Data subcarriers are indexed from 0 to 47.
2) The mapping of data onto the subcarriers follows the equation below. This equation calculates the subcarrier index (as assigned in item 1) to which the data constellation point is to be mapped:
Subcarrier(n, s) = (n + 13 · s) mod Nsubcarriers (2.2) where:
• Subcarrier(n, s) is the permutated subcarrier index corresponding to data sub-carrier n in subchannel s,
• n = 0..47 is a running index, indicating the data constellation point,
• s is the subchannel number, and
• Nsubcarriers is the number of subcarriers per slot.
Figure 2.8: PRBS generator for pilot modulation (from [2] and [3]).
Pilot Modulation
The PRBS (pseudo-random binary sequence) generator depicted in Figure 2.8 is used to produce a sequence, wk. The value of the pilot modulation, on subcarrier k, is derived from wk.
For the mandatory tile structure in the uplink, pilot subcarriers are inserted into each data burst in order to constitute the symbol and they are modulated according to their subcarrier location within the OFDMA symbol. The pilot subcarriers are modulated according to
<{ck} = 2(1
2 − wk), ={ck} = 0. (2.3)
2.4.2 Downlink
The contents of this subsection have been taken to a large extent from [2] and [3].
Data Mapping Rules
The downlink data mapping rules are as follows:
1. Segment the data after the modulation block into blocks sized to fit into one OFDMA
Figure 2.9: Example of mapping OFDMA slots to subchannels and symbols in the downlink in PUSC mode (from [3]).
slot.
2. Each slot shall span one subchannel in the subchannel axis and one or more OFDMA symbols in the time axis, as per the slot definition mentioned before. Map the slots such that the lowest numbered slot occupies the lowest numbered subchannel in the lowest numbered OFDMA symbol.
3. Continue the mapping such that the OFDMA subchannel index is increased. When the edge of the data region is reached, continue the mapping from the lowest numbered OFDMA subchannel in the next available symbol.
Figure 2.9 illustrates the order of OFDMA slots mapping to subchannels and OFDMA symbols.
Figure 2.10: Cluster structure (from [3]).
Subcarrier Allocations
The OFDMA symbol structure is constructed using pilots, data and zero subcarriers. The symbol is first divided into basic clusters and zero carriers are allocated. The pilot tones are allocated first; what remains are data subcarriers, which are divided into subchannels that are used exclusively for data. Pilots and data carriers are allocated within each cluster.
Figure 2.10 shows the cluster structure with subcarriers from left to right in order of increasing subcarrier index. For the purpose of determining PUSC pilot location, even and odd symbols are counted from the beginning of the current zone. The first symbol in the zone is even. The preamble is not counted as part of the first zone. Table 2.3 summarizes the parameters of the OFDMA PUSC symbol structure.
The allocation of subcarriers to subchannels is performed using the following procedure:
1) Divide the subcarriers into a number (Nclusters) of physical clusters containing 14 ad-jacent subcarriers each (starting from carrier 0).
2) Renumber the physical clusters into logical clusters using the following formula:
LogicalCluster =
13 · DL P ermBase)mod Nclusters¢
, otherwise.
(2.4) 3) Divide the clusters into six major groups. Group 0 includes clusters 0–11, group 1
Table 2.3: OFDMA Downlink Subcarrier Allocation under PUSC [2], [3]
Parameter Value Comments
Number of DC subcarriers
1 Index 512 (counting from 0) Number of guard
841 Number of all subcarriers used within a symbol, including all possible allocated
Renumbering sequence 1 Used to renumber clusters before allocation to subchannels:
clusters 12–19, group 2 clusters 20–31, group 3 clusters 32–39, group 4 clusters 40–51 and group 5 clusters 52–59. These groups may be allocated to segments. If a segment is being used, then at least one group shall be allocated to it. (By default group 0 is allocated to segment 0, group 2 to segment 1, and group 4 to segment 2) .
4) Allocate subcarriers to subchannel in each major group separately for each OFDMA symbol by first allocating the pilot subcarriers within each cluster and then taking all remaining data subcarriers within the symbol. The exact partitioning into subchannels is according to the equation below, called a permutation formula:
subcarrier(k, s) = (Nsubchannels · nk)
• subcarrier(k, s) is the subcarrier index of subcarrier k in subchannel s,
• s is the index number of a subchannel, from the set [0..Nsubchannels − 1],
• nk = (k + 13 · s)mod Nsubcarriers, where k is the subcarrier-in-subchannel index from the set [0..Nsubcarriers− 1],
• Nsubchannels is the number of subchannels (for PUSC use number of subchannels in the currently partitioned group),
• ps[j] is the series obtained by rotating basic permutation sequence cyclically to the left s times,
• Nsubcarriers is the number of data subcarriers allocated to a subchannel in each OFDMA symbol, and
• DL P ermBase is an integer from 0 to 31.
Pilot Modulation
Pilot subcarriers are inserted into each data burst in order to constitute the symbol. The PRBS generator depicted in Figure 2.8 is used to produce a sequence, wk.
Each pilot is transmitted with a boosting of 2.5 dB over the average non-boosted power of a data tone. The pilot subcarriers are modulated according to
<{ck} = 8 3
¡1
2− wk¢
, ={ck} = 0. (2.6)