Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN---5

SUBCARRIER ALLOCATION MODES
There are two main types of subcarrier permutations: distributed and adjacent. In general, distributed subcarrier permutations perform very well in mobile applications while adjacent subcarrier permutations can be properly used for fixed, portable, or low mobility environments. These options enable the system designers to trade mobility for throughput.
In the following section, various subcarrier allocation modes are identified and their main characteristics are summarized.
DL Distributed Subcarrier Permutations: Fully Used Subchannelization (FUSC)
This method uses all the subchannels and employs full-channel diversity by distributing the allocated subcarriers to subchannels using a permutation mechanism. This mechanism is designed to minimize the probability of hits (probably of using the same physical subcarriers in adjacent cells and sectors) between adjacent sectors/cells by reusing subcarriers while frequency diversity minimizes the performance degradation due to fast fading characteristics of mobile environments.
Table 3 summarizes the subcarrier allocation structure parameters. In DL FUSC, there are variable and fixed sets of pilots. The fixed sets are used in all OFDM symbols while the variable sets are divided into subsets that are used in odd and even symbols alternatively. This provides an appropriate tradeoff between allocated power and frequency diversity on pilots for channel estimation. Figure 2 shows the distribution of variable and fixed sets of pilots in the case of 2048 FFT. Pilot sets for other FFT sizes are subsets of those for the 2048 FFT.
Table 3: DL distributed subcarrier permutation (FUSC) Parameters Values
System bandwidth (MHz) 1.25 2.5 5 10 20
FFT size (N FFT) 128 N/A ** 512 1024 2048
Number of guard subcarriers 22 N/A 86 173 345
Number of used subcarriers 106 N/A 426 851 1703
Number of data subcarriers 96 N/A 384 768 1536
Number of pilot subcarriers (uses both variable and constant sets) 9* N/A 42 83 166
Number of subchannels 2 N/A 8 16 32
Subcarrier Permutation Uses Permutation Type 1 for Tone Distribution (Eq. 107 [20] )
* variable set only
** FFT size of 256 is not supported

Figure 3: DL PUSC cluster structure
click image for larger view
DL and UL Distributed Subcarrier Permutation: Partially Used Subchannelization (PUSC)
According to the OFDMA specification, all OFDMA DL and UL subframes shall start in DL and UL PUSC mode, respectively. In DL PUSC, subchannels are divided and assigned to three segments that can be allocated to sectors of the same cell. The method employs full-channel diversity by distributing the allocated subcarriers to subchannels. A permutation mechanism is designed to minimize the probability of hits between adjacent sectors/cells by reusing subcarriers, while frequency diversity minimizes the performance degradation due to fast fading characteristics of mobile environments.
Table 4 summarizes the parameters of DL PUSC subcarrier allocation. DL PUSC uses a cluster structure, as illustrated in Figure 3, which spans over two OFDM symbols (in time) of fourteen subcarriers, each with a total of four pilot subcarriers per cluster.
Table 5 summarizes the parameters of UL PUSC subcarrier allocation. UL PUSC uses a tile structure, as illustrated in Figure 4, that spans over three OFDM symbols (in time) of four subcarriers, each with total of four pilot subcarriers.
Note that because of the DL and UL, cluster and tile structures are composed of two and three OFDM symbols, respectively; the DL and UL subframe size and the granularity of the DL and UL allocations are also two or three OFDM symbols, respectively.
Table 4: DL distributed subcarrier permutation (PUSC) Parameters
Values
System bandwidth (MHz)
1.25
2.5
5
10
20
FFT size (N FFT)
128
N/A
512
1024
2048
Number of guard subcarriers
43
N/A
91
183
367
Number of clusters/subchannels
6/3
N/A
30/15
60/30
120/60
Number of used subcarriers
85
N/A
421
841
1681
Number of data subcarriers
72
N/A
360
720
1440
Number of pilot subcarriers
12
N/A
60
120
240
Subcarrier permutation
Uses Permutation Type 1 for Tone Distribution (Eq. 107 [20] )
Cluster renumbering
Activated
Optional DL Distributed Subcarrier Permutation: Fully Used Subchannelization (OFUSC)
This method employs full-channel diversity by distributing the allocated subcarriers to subchannels using a permutation mechanism designed to minimize the probability of hits between adjacent sectors/cells by reusing subcarriers, while frequency diversity minimizes the performance degradation due to fast fading characteristics of mobile environments.
Table 6 summarizes the parameters of OFUSC subcarrier allocation. In OFUSC, pilots are mapped as specified below, which is different from the assignment in the FUSC mode.
Compared to FUSC mode, the number of used subcarriers in this method is considerably larger (1681 vs. 1729). As a result, compliance with spectral mask requirements, without a change in the over-sampling factor, may be a challenge for this mode.
Table 5: UL distributed subcarrier permutation (PUSC) Parameters
Values
System bandwidth
1.25
2.5
5
10
20
FFT size (N FFT)
128
N/A
512
1024
2048
Number of guard subcarriers
31
N/A
103
183
367
Number of tiles
24
N/A
102
210
552
Number of subchannels
4
N/A
17
35
92
Number of subcarriers per tile
4
N/A
4
4
3
Number of used subcarriers
97
N/A
409
841
1681
Tile permutation
Uses Permutation Type 2 for Tile Distribution (Eq. 109 [20] )
Subcarrier permutation
Uses Permutation Type 3 for Subcarrier Distribution (Eq. 110 [20] )
Optional UL Distributed Subcarrier Permutation: Partially Used Subchannelization (OPUSC)
This method employs full-channel diversity by distributing the allocated subcarriers to subchannels using a permutation mechanism designed to minimize the probability of hits between adjacent sectors/cells by reusing subcarriers, while frequency diversity minimizes the performance degradation due to fast fading characteristics of mobile environments.

Figure 4: UL PUSC tile structure
click image for larger view
Table 7 summarizes the parameters of UL OPUSC subcarrier allocation. UL OPUSC uses a tile structure, as illustrated in Figure 5, that spans over three OFDM symbols (in time) of three subcarriers each with one pilot subcarrier per tile.
Table 6: DL distributed subcarrier permutation (optional FUSC) Parameters
Values
System bandwidth
1.25
2.5
5
10
20
FFT size (N FFT)
128
N/A
512
1024
2048
Number of guard subcarriers
19
N/A
79
159
319
Number of used subcarriers
109
N/A
433
865
1729
Number of data subcarriers
96
N/A
384
768
1536
Number of pilot subcarriers (Npilots)
12
N/A
48
96
192
Number of data subcarriers per subchannel
48
N/A
48
48
48
Number of subchannels
2
N/A
8
16
32
Subcarrier permutation
Uses Permutation Type 3 for Tone Distribution (Eq. 108 [20] )
Pilot subcarrier index
9k+3m+1,
for k=0,1,……, Npilots and
m=[symbol index] mod 3
Optional DL and UL Adjacent Subcarrier Permutation: Advanced Modulation and Coding (AMC)
This method uses adjacent subcarriers to form subchannels. When used with fast feedback channels it can rapidly assign a modulation and coding combination per subchannel. The AMC subchannels enable the use of "water-pouring" types of algorithms, and it can be used effectively with an AAS option.
Table 8 summarizes the AMC subcarrier allocation parameters. In AMC, pilots are mapped as specified below.
Table 7: Optional UL distributed subcarrier permutation (OPUSC) Parameters
Values
System bandwidth
1.25
2.5
5
10
20
FFT size (N FFT)
128
N/A
512
1024
2048
Number of guard subcarriers
19
N/A
79
159
319
Number of used subcarriers
109
N/A
433
865
1729
Number of tiles
36
N/A
144
288
576
Number of tiles per subchannel
6
N/A
6
6
6
Number of data subcarriers per subchannel
48
N/A
48
48
48
Number of subchannels
6
N/A
24
48
96
Subcarrier permutation
Uses Permutation Type 4 for Tone Distribution (Eq. 111 [20] )
Table 8: UL/DL adjacent subcarrier permutation (optional AMC) Parameters
Values
System bandwidth
1.25
2.5
5
10
20
FFT size (N FFT)
128
N/A
512
1024
2048
Number of guard sub-carriers
19
N/A
79
159
319
Number of used sub-carriers (Nused)
109
N/A
433
865
1729
Number of pilots (Npilots)
12
N/A
48
96
192
Number of data sub-carriers
96
N/A
384
768
1536
Number of bands
3
N/A
12
24
48
Number of bins per band
4
N/A
4
4
4
Number of subcarriers per bin (8 data +1 pilot)
9
N/A
9
9
9
Number of subchannels
2
N/A
8
16
32
Sub-carrier permutation
None
Pilot subcarrier index
9k+3m+1,
for k=0,1,……, Npilots and
m=[symbol index] mod 3

Figure 5: UL OPUSC tile structure
click image for larger view

Figure 6: Multiple zones in Uplink and Downlink subframes
click image for larger view
Zone Switching
OFDMA PHY also supports multiple subcarrier allocation zones within the same frame to enable the possibility of support for and coexistence of different types of SS’s.
Figure 6 illustrates zone switching within the DL and UL subframes. The switching is performed using an information element included in DL-MAP and UL-MAP. DL and UL subframes both start in PUSC mode where groups of subchannels are assigned to different segments by the use of dedicated FCH messages. The PUSC subcarrier allocation zone can be switched to a different type of subcarrier allocation zone through a directive from the PUSC DL-MAP. Figure 6 shows the zone switching from the perspective of a PUSC segment. In the figure, the PUSC FCH/DL-MAP for a segment with IDCell X is followed with another possibly data PUSC zone for IDCell X. A PUSC zone for another sector/cell with IDCell Y (Y in general is different from X) is allocated next. An FUSC zone for IDCell Z is shown next in the figure. Note that IDCell Z may be the same as IDCell X which means that a PUSC to FUSC switching is scheduled within the segment for Frequency Reuse One operations. A switching to IDCell 0 can be planned for all network broadcast operations.
Optional PUSC, FUSC, and AMC zones in DL subframes and optional PUSC and AMC zones in UL subframes can be similarly scheduled. Allocation of AMC zones enables the simultaneous support of fixed, portable, and nomadic mobility users along with high mobility users (supported in PUSC/FUSC zones).