As mentioned, the OFDMA PHY defines four scalable FFT sizes: 2048, 1024, 512, and 128. For convenience, 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. 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 differ-ent possible zones. For FUSC and PUSC, in the downlink, the pilot tones are allocated first; what remains are data subcarriers, which are divided into subchannels that are used
exclusively 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, but in PUSC uplink, each subchannel contains its own pilot subcarriers.
2.5.1 Downlink
Preamble
The first symbol of the downlink transmission is the preamble. There are three types of preamble carrier-sets, which are defined by allocation of different subcarriers for each one of them. The subcarriers are modulated using a boosted BPSK modulation with a specific pseudo-noise (PN) code. The preamble carrier-sets are defined using
P reambleCarrierSetn= n + 3 · k (2.1) where:
P reambleCarrierSetn n
k
specifies all subcarriers allocated to the specific preamble, is the number of the preamble carrier-set indexed 0–2, is a running index 0–283.
For the preamble symbol there are 86 guard band subcarriers on the left side and the right side of the spectrum. Each segment uses a preamble composed of a carrier-set out of the three available carrier-sets in the manner that segment i uses preamble carrier-set i, where i = 0, 1, 2. Therefore, each segment eventually modulates each third subcarrier.
In the case of segment 0, the DC carrier will be zeroed and the corresponding PN number will be discarded.
The 114 different PN series modulating the preamble carrier-set are defined in Table 309 of [1] for the 1024-FFT mode. The series modulated depends on the segment used and the IDcell parameter.
Symbol Structure for PUSC
The symbol is first divided into basic clusters and zero carriers are allocated. Pilots and data carriers are allocated within each cluster. Table 310 of [2] summarizes the parameters
Table 2.3: 1024-FFT OFDMA DL Carrier Allocation for PUSC
Parameter Value Comments
Number of DC subcarriers 1 Index 512
Number of guard subcarriers, left 92 Number of guard subcarriers, right 91 Number of used subcarriers, Nused 841
Renumbering sequence 6, 48, 37, 21, 31, 40, 42, Used to renumber 56, 32, 47, 30, 33, 54, 18, clusters before 10, 15, 50, 51, 58, 46, 23, allocation to 45, 16, 57, 39, 35, 7, 55, subchannels.
25, 59, 53, 11, 22, 38, 28, 19, 17, 3, 27, 12, 29, 26, 5, 41, 49, 44, 9, 8, 1, 13, 36, 14, 43, 2, 20, 24, 52,4, 34, 0
Number of subcarriers per cluster 14
Number of clusters 60
Number of data subcarriers in each 24 symbol per subchannel
Number of subchannels 30
Basic permutation sequence 6 3, 2, 0, 4, 5, 1 (for 6 subchannels)
Basic permutation sequence 4 3, 0, 2, 1 (for 4 subchannels)
of the symbol structure of different FFT sizes for PUSC mode. Here we only take the 1024-FFT OFDMA downlink carrier allocation for example, which is summarized in Table 2.3. Fig. 2.11 depicts the DL cluster structure.
Downlink Subchannels Subcarrier Allocation in PUSC
The subcarrier allocation to subchannels is performed using the following procedure:
1. Dividing the subcarriers into the number of clusters (Nclusters), where the physical clusters contain 14 adjacent subcarriers each (starting from carrier 0). The number of clusters varies with the FFT size.
2. Renumbering the physical clusters into logical clusters using the following formula:
Figure 2.11: DL cluster structure (from[10]).
RenumberingSequence(P hysicalCluster), first DL zone, or Use All SC indicator
= 0 in STC DL Zone IE, RenumberingSequence((P hysicalCluster)+ otherwise.
13 · DL P ermBase)modNclusters,
(2.2) In the first PUSC zone of the downlink (first downlink zone) and in a PUSC zone defined by STC DL ZONE IE() with “use all SC indicator = 0”, the default re-numbering sequence is used for logical cluster definition. For all other cases DL PermBase parameter in the STC DL Zone IE() or AAS DL IE() shall be used.
3. Allocating logical clusters to groups. The allocation algorithm varies with FFT size. For FFT size = 1024, dividing the clusters into six major groups. Group 0 includes clusters 0–11, group 1 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 used, then at least one group shall be allocated to it. By default group 0 is allocated to sector 0, group 2 to sector 1, and group 4 to sector 2.
4. Allocating subcarriers to subchannels in each major group, which is performed sep-arately for each OFDMA symbol by first allocating the pilot carriers within each cluster, and then partitioning all remaining data carriers into groups of contiguous subcarriers. Each subchannel consists of one subcarrier from each of these groups.
The number of groups is therefore equal to the number of subcarriers per
subchan-nel, and it is denoted Nsubcarriers. The number of the subcarriers in a group is equal to the number of subchannels, and it is denoted Nsubchannels. The number of data subcarriers is thus equal to Nsubcarriers· Nsubchannels. The parameters vary with FFT sizes. For FFT size = 1024, use the parameters from Table 2.3, with basic permuta-tion sequence 6 for even numbered major groups and basic permutapermuta-tion sequence 4 for odd numbered major groups, to partition the subcarriers into subchannels con-taining 24 data subcarriers in each symbol. The exact partitioning into subchannels is according to the permutation formula:
subcarrier(k, s) = Nsubchannels· nk
+{ps[nkmod Nsubchannels] + DL P ermBase} mod Nsubcahnnels (2.3)
is the subcarrier index of subcarrier k in subchannel s,
is the index number of a subchannel, from the set {0,..., Nsubcarriers− 1},
= (k + 13 · s) mod Nsubcarriers, where k is the subcarrier-in-subchannel index from the set {0,..., Nsubcarriers− 1},
is the number of subchannels (for PUSC use number of sub-channels in the currently partitioned major group),
is the series obtained by rotating basic permutation sequence cyclically to the left s times,
is an integer ranging from 0 to 31, which is set to the preamble IDCell in the first zone and determined by the DL-MAP for other zones.
On initialization, an SS must search for the downlink preamble. After finding the pream-ble, the user shall know the IDcell used for the data subchannels.
2.5.2 Uplink
The UL follows the DL model, therefore it also supports up to three segments. The UL supports 35 subchannels where each transmission uses 48 data subcarriers as the
minimal block of processing. Each new transmission for the uplink commences with the parameters as given in Table 2.4.
Symbol Structure for Subchannel (PUSC)
A slot in the uplink is composed of three OFDMA symbols and one subchannel. Within each slot, there are 48 data subcarriers and 24 fixed-location pilots. The subchannel is constructed from six uplink tiles, each tile has four successive active subcarriers and its configuration is illustrated in Fig. 2.12.
Partitioning of Subcarriers into Subchannels in the Uplink
The usable subcarriers in the allocated frequency band shall be divided into Ntilesphysical tiles as defined in Fig. 2.12 with parameters from Table 2.4. The allocation of physical tiles to logical tiles in subchannels is performed in the following manner:
T iles(s, n) = Nsubchannels·n+{Pt[(s+n) mod Nsubchannels]+UL P ermBase} mod Nsubcahnnels
(2.4) Table 2.4: 1024-FFT OFDMA UL Carrier Allocation for PUSC
Parameter Value Comments
Number of DC subcarriers 1 Index 512
Nused 841
Guard subcarriers: left, right 92,91
TilePermutation 11, 19, 12, 32, 33, 9, 30, 7, used to allocate 4, 2, 13, 8, 17, 23, 27, 5, tiles to subchannels 15, 34, 22, 14, 21, 1, 0, 24,
3, 26, 29, 31, 20, 25, 16, 10, 6, 28, 18
Nsubchannels 35
Ntiles 210
Tile per subchannel 6
Figure 2.12: Description of an UL tile (from [10]).
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),
is the tile index 0,...,5 in a subchannel, is the tile permutation,
is the number of subchannels,
is the subchannel number in the range {0,...,Nsubchannels− 1}, is an integer value in the range 0...69, which is assigned by a management entity.
After mapping the physical tiles in the FFT 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 shall be indexed from 0 to 47.
2. The mapping of data onto the subcarriers will follow (2.5), which calculates the subcarrier index (as assigned in item 1) to which the data constellation point is to be mapped:
Subcarriers(n, s) = (n + 13 · s) mod Nsubcarriers (2.5) where:
Subcarriers(n, s)
n
s
Nsubcarriers
is the permutated subcarrier index corresponding to data sub-carrier n is subchannel s,
is the running index 0,...,47, indicating the data constellation point,
is the subchannel number,
is the number of subcarriers per slot.
2.6 Modulation
Subcarrier Randomization
The pseudo random binary sequence (PRBS) generator, as shown in Fig. 2.13, shall be used to produce a sequence wk. The polynomial for the PRBS generator shall be X11+ X9+ 1. The value of the pilot modulation on subcarrier k shall be derived from wk.
The initialization vector of the PRBS generator for both uplink and downlink, desig-nated b10..b0, is defined as follows:
b0..b4 = five least significant bits of IDcell as indicated by the frame preamble in the first downlink zone and in the downlink AAS zone with Diversity Map support, DL PermBase following STC DL Zone IE() and 5 LSB of DL PermBase follow-ing AAS DL IE without Diversity Map support in the downlink. Five least signifi-cant bits of IDcell (as determined by the preamble) in the uplink. For downlink and uplink, b0 is MSB and b4 is LSB, respectively.
Figure 2.13: PRBS generator for pilot modulation (from [2]).
b5..b6 = set to the segment number + 1 as indicated by the frame preamble in the first downlink zone and in the downlink AAS zone with Diversity Map support, PRBS ID as indicated by the STC DL Zone IE or AAS DL IE without Diver-sity Map support in other downlink zone. 0b11 in the uplink. For downlink and uplink, b5 is MSB and b6 is LSB, respectively.
b7..b10 = 0b1111 (all ones) in the downlink and four LSB of the Frame Number in the uplink. For downlink and uplink, b7 is MSB and b10 is LSB, respectively.
Data Modulation
After the repetition block, the data bits are entered serially to the constellation mapper.
Gray-mapped QPSK and 16-QAM shall be supported, whereas the support of 64-QAM is optional.
Pilot Modulation
In all permutations except uplink PUSC and downlink TUSC1, each pilot shall be trans-mitted with a boosting of 2.5 dB over the average non-boosted power of each data tone.
The pilot subcarriers shall be modulated according to
<{ck} = 83(12 − wk) · pk, ={ck} = 0, (2.6) where pkis the pilot’s polarity for SDMA (spatial division multiple access) allocations in AMC AAS zone, and p = 1 otherwise.
Preamble Pilot Modulation
The pilots in the downlink preamble shall be modulated according to
<{P reambleP ilotModulation} = 4 ·√ 2 · (1
2 − wk), (2.7)
={P reambleP ilotModulation} = 0. (2.8)