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2.3.3 OFDM Parameters in WiMAX (Cont)
Table 2.3 OFDM Parameters Used in WiMAX
| Parameter | Fixed WiMAX OFDM-PHY | Mobile WiMAX Scalable OFDMA-PHY[a] | |||
| FFT size | 256 | 128 | 512 | 1,024 | 2,048 |
| Number of used data subcarriers[b] | 192 | 72 | 360 | 720 | 1,440 |
| Number of pilot subcarriers | 8 | 12 | 60 | 120 | 240 |
| Number of null/guardband subcarriers | 56 | 44 | 92 | 184 | 368 |
| Cyclic prefix or guard time (Tg/Tb) | 1/32, 1/16, 1/8, 1/4 | ||||
| Oversampling rate (Fs/BW) | Depends on bandwidth: 7/6 for 256 OFDM, 8/7 for multiples of 1.75MHz, and 28/25 for multiples of 1.25MHz, 1.5MHz, 2MHz, or 2.75MHz. | ||||
| Channel bandwidth (MHz) | 3.5 | 1.25 | 5 | 10 | 20 |
| Subcarrier frequency spacing (kHz) | 15.625 |
10.94 |
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| Useful symbol time (μs) | 64 |
91.4 |
|||
| Guard time assuming 12.5% (μs) | 8 |
11.4 |
|||
| OFDM symbol duration (μs) | 72 |
102.9 |
|||
| Number of OFDM symbols in 5 ms frame | 69 |
48.0 |
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- Boldfaced values correspond to those of the initial mobile WiMAX system profiles.
- The mobile WiMAX subcarrier distribution listed is for downlink PUSC (partial usage of subcarrier).
Mobile WiMAX OFDMA-PHY: In Mobile WiMAX, the FFT size is scalable from 128 to 2,048. Here, when the available bandwidth increases, the FFT size is also increased such that the subcarrier spacing is always 10.94kHz. This keeps the OFDM symbol duration, which is the basic resource unit, fixed and therefore makes scaling have minimal impact on higher layers. A scalable design also keeps the costs low. The subcarrier spacing of 10.94kHz was chosen as a good balance between satisfying the delay spread and Doppler spread requirements for operating in mixed fixed and mobile environments. This subcarrier spacing can support delay-spread values up to 20 μs and vehicular mobility up to 125 kmph when operating in 3.5GHz. A subcarrier spacing of 10.94kHz implies that 128, 512, 1,024, and 2,048 FFT are used when the channel bandwidth is 1.25MHz, 5MHz, 10MHz, and 20MHz, respectively. It should, however, be noted that mobile WiMAX may also include additional bandwidth profiles. For example, a profile compatible with WiBro will use an 8.75MHz channel bandwidth and 1,024 FFT. This obviously will require a different subcarrier spacing and hence will not have the same scalability properties.
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