Overview of the IEEE 802.16e Standard
3.3 WirelessMAN-OFDMA TDD Downlink [3]
3.3.4 OFDMA Downlink Subcarrier Allocation
Here we only describe DL subcarrier allocation since our study of OFDMA PHY is only on DL. 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
Figure 3.8: Example of DL renumbering the allocated subchannels for segment 1 in PUSC (from [3]).
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 of the downlink, there is one set of common pilot subcarriers in each major group.
The downlink can be divided into a three segment structure and includes a preamble which begins the transmission. In this preamble, subcarriers are divided into three carrier-sets. There are three possible groups consisting of a carrier-set each, that may be used by any segment.
Preamble
The first symbol of the downlink transmission is the preamble. There are 3 types of preamble carriersets, 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 (3.2) where:
P reambleCarrierSetn specifies all subcarriers allocated to the specific preamble, n is the number of the preamble carrier-set indexed 0–2,
k is a running index 0–567.
For the preamble symbol there will be 172 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 following manner that segment i uses preamble carrier-set i, where i = 0, 1, 2. In the case of segment 0, the DC carrier will not be modulated at all and the appropriate PN will be discarded; therefore, DC carrier shall always be zeroed.
Therefore, each segment eventually modulates each third subcarrier.
The 114 different PN series modulating the preamble carrier-set are defined in Table 309 of [2] for the 2k 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
Table 3.2: 2048-FFT OFDMA DL Carrier Allocation Under PUSC
Parameter Value Comments
Number of DC subcarriers 1 Index 1024 (counting from 0) Number of guard subcarriers, left 184
Number of guard subcarriers, right 183
Number of used subcarriers, Nused 1681 Including all possible pilots and DC Number of subcarriers per cluster 14
Number of clusters 120
Renumbering sequence 1 Used to renumber clusters before allocation to subchannels, see [3]
Number of data subcarriers in each 4 symbol per subchannel
Number of subchannels 60
Basic permutation sequence 12 6,9,4,8,10,11,5,2,7,3,1,0 (for 12 subchannels)
Basic permutation sequence 8 7,4,0,2,1,5,3,6 (for 8 subchannels)
symbol structure of different FFT sizes for PUSC mode. Here we only take the 2048-FFT OFDMA downlink carrier allocation for example, and it is summarized in Table 3.2.
Fig. 3.9 depicts the cluster structure.
Downlink Subchannels Subcarrier Allocation in PUSC
The carrier 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 3.9: Cluster structure (from [3]).
LogicalCluster =
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,
(3.3) In the first PUSC zone of the downlink (first downlink zone) and in a PUSC zone de-fined 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 para-meter 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 sizes.
For FFT size = 2048, dividing the clusters into six major groups. Group 0 includes clusters 0–23, group 1 clusters 24–39, group 2 clusters 40–63, group 3 clusters 64–79, group 4 clusters 80–103, and group 5 clusters 104–119. 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 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
sepa-Table 3.3: 2048-FFT OFDMA DK Carrier Allocation Under FUSC
Parameter Value Comments
Number of DC subcarriers 1 Index 1024 (counting from 0) Number of guard subcarriers, left 173
Number of guard subcarriers, right 172
Number of used subcarriers, Nused 1703 Including all possible pilots and DC
Pilot sets 166 See Table 311 of [3]
Number of data subcarriers 1536
Number of data subcarriers per subchannel 48
Number of Subchannels 32
Basic permutation sequence 3,18,2,8,16,10,11,15,26,22,6,9,27,20,25,1,29, 7,21,5,28,31,23,17,4,24,0,13,12,19,14,30
and then taking all remaining data carriers within the symbol and using the same pro-cedure described in the next subsection (Symbol Structure for FUSC). The parameters vary with FFT sizes. For FFT size = 2048, use the parameters from Table 3.2, with basic permutation sequence 12 for even numbered major groups and basic permutation sequence 8 for odd numbered major groups, to partition the subcarriers into subchan-nels containing 24 data subcarriers in each symbol.
Symbol Structure for FUSC
The symbol structure is constructed using pilots, data, and zero subcarriers. The symbol is first allocated with the appropriate pilots and with zero subcarriers, and then all the remaining subcarriers are used as data subcarriers (which are divided into subchannels).
There are two variable pilot-sets and two constant pilot-sets. In FUSC, each segment uses both sets of variable/constant pilot-sets. We only summarize the parameters of 2048-FFT OFDMA in Table 3.3.
The Variable set of pilots embedded within the symbol of each segment shall obey the
following rule:
P ilotLocation = V ariableSet#x + 6 · (F USC SymbolNumber mod 2) (3.4)
where FUSC SymbolNumber counts the FUSC symbols used in the current zone starting from 0.
Downlink Subchannels Subcarrier Allocation
Each subchannel is composed of 48 subcarriers. The subchannel indices are formulated using a Reed-Solomon series, and is allocated out of the data subcarriers domain. The data subcarriers domain includes 48 × 32 = 1536 subcarriers.
After mapping all pilots, the remainder of the used subcarriers are used to define the data subchannels. To allocate the data subchannels, the remaining subcarriers are partitioned 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 subchannel, 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 exact partitioning into subchannels is according to the permutation formula:
subcarrier(k, s) =
Nsubchannels· nk+ {ps[nk mod Nsubchannels] + DL P ermBase} mod Nsubcahnnels (3.5)
where:
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,...,N − 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 major group),
ps[j] is the series obtained by rotating basic permutation sequence cyclically to the left s times,
DL PermBase 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 preamble, the user shall know the IDcell used for the data subchannels.
3.3.5 Modulation
Subcarrier Randomization
The PRBS generator, as known in Fig. 3.2, shall be used to produce a sequence wk. 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 shall be designated b10..b0, such that:
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 following AAS DL IE with-out Diversity Map support in the downlink. Five least significant bits of IDcell (as determined by the preamble) in the uplink. For downlink and uplink, b0 is MSB and b4 is LSB, respectively.
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 Diversity 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. The Gray-mapped modulations are the same as modulations in OFDM PHY.
Pilot Modulation
In all permutations except uplink PUSC and downlink TUSC1, each pilot shall be transmit-ted with a boosting of 2.5 dB over the average non-boostransmit-ted power of each data tone. The pilot subcarriers shall be modulated according to:
<{ck} = 83(12 − wk) · pk, ={ck} = 0. (3.6)
where pk is the pilot’s polarity for SDMA (stands for spatial division multiple access) allo-cations in AMC AAS zone, and p = 1 otherwise.
Preamble Pilot Modulation
The pilots in the downlink preamble shall be modulated according to:
√ 1
={P reambleP ilotModulation} = 0. (3.8)
3.3.6 Frequency and Timing Requirements
Timing Requirements
For any duplexing, all SSs shall acquire and adjust their timing such that all uplink OFDMA symbols arrive time coincident at the BS to a accuracy of ±25% of the minimum guard-interval or better. This translates into ±16 samples in the case of 2048-FFT OFDMA.
Frequency Requirements
At the BS, the transmitted center frequency, receive center frequency, and the symbol clock frequency shall be derived from the same reference oscillator. At the BS, the reference frequency accuracy shall be better than ±2 × 10−6.
At the SS, both the transmitted center frequency and the sampling frequency shall be derived from the same reference oscillator. Thereby, the SS uplink transmission shall be locked to the BS, so that its center frequency shall deviate no more than 2% of the subcarrier spacing, compared to the BS center frequency.
During the synchronization period, the SS shall acquire frequency synchronization within the specified tolerance before attempting any uplink transmission. During normal operation, the SS shall track the frequency changes by estimating the downlink frequency offset and shall defer any transmission if synchronization is lost. To determine the transmit frequency, the SS shall accumulate the frequency offset corrections transmitted by the BS (for example in the RNG-RSP message), and may add to the accumulated offset an estimated UL frequency offset based on the downlink signal.
Figure 3.10: Transmit spectral mask (from [2]).
Table 3.4: Transmit Sprctral Mask
Bandwidth (MHz) A B C D
10 9.5 10.9 19.5 29.5
20 4.75 5.45 9.75 14.75