5.3 OFDMA and Its Variant SOFDMA

5.3 OFDMA and Its Variant SOFDMA

5.3.1 Using the OFDM Principle for Multiple Access

The OFDM transmission mode was originally designed for a single signal transmission. Thus, in order to have multiple user transmissions, a multiple access scheme such as TDMA or FDMA has to be associated with OFDM. In fact, an OFDM signal can be made from many user signals, giving the OFDMA (Orthogonal Frequency Division Multiple Access) multiple access.

In OFDMA, the OFDMA subcarriers are divided into subsets of subcarriers. each subset representing a subchannel (see Figure 5.11). In the downlink, a subchannel may be intended for different receivers or groups of receivers; in the uplink, a transmitter may be assigned one or more subchannels. The subcarriers forming one subchannel may be adjacent or not. The standard [1] indicates that the OFDM symbol is divided into logical subchannels to support scalability, multiple access and advanced antenna array processing capabilities. The multiple access has a new dimension with OFDMA. A downlink or an uplink user will have a time and a subchannel allocation for each of its communications (see Figure 5.12). Different subchannel distributions and logical renumberings are defined in the 802.16 standard, as will be seen in the rest of this chapter. First, the SOFDMA concept is introduced.

Image from book
Figure 5.11: Illustration of the OFDMA principle. (Based on Reference [1].)
Image from book
Figure 5.12: Illustration of OFDMA multiple access

5.3.2 Scalable OFDMA (SOFDMA)

OFDMA multiple access is not the only specificity of OFDMA PHY. Another major difference is the fact that its OFDM transmission is scalable. Although this word does not appear in the standard, OFDMA PHY is said to have Scalable OFDMA (SOFDMA). The scalability is the change of the FFT size and then the number of subcarriers. The supported FFT sizes are 2048, 1024, 512 and 128. FFT size 256 (of the OFDM layer) is not included in the OFDMA layer. Only 1024 and 512 are mandatory for mobile WiMAX profiles.

The change in the number of subcarriers, for a fixed subcarrier spacing, provides for an adaptive occupied frequency bandwidth and, equivalently, an adaptive data rate, as shown in the following example. See the example shown in Table 5.3. In this example, the sampling factor is equal to 28/25, chosen according to the channel bandwidth. SOFDMA provides an additional resource allocation flexibility that can be used in the framework of radio resource management policy taking into account the dynamic spectrum demand, among others.

Table 5.3: Example of SOFDMA figures. (Inspired from Reference [10].)
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Numerical values

Subcarrier frequency spacing

10.95 kHz

Useful symbol duration (Td= 1/Δf)

91.4 μs

Guard time (TG= Td/8)

11.4 μs

OFDMA symbol duration (Ts= Td + TG)

102.9 μ

Number of OFDMA symbols in the 5 ms frame


FFT size (Nfft) or number of subcarriers



Channel occupied bandwidth



5.3.3 OFDMA in the OFDM PHYsical Layer: Subchannelisation

As a matter of fact, the OFDM PHY includes some OFDMA access. Subchannelisation was included in 802.16-2004 for the uplink and also for the downlink in amendment 802.16e. The principle is the following. The 192 useful data OFDM subcarriers of OFDM PHY are distributed in 16 subchannels made of 12 subcarriers each. Each subchannel is made of four groups of three adjacent subchannels each (see below).

A subchannelised transmission is a transmission on only part of the OFDM subcarrier space. The subchannelised transmission can take place on 1, 2, 4, 8 or 16 subchannels. A five-bit indexation shown in Table 5.4 indicates the number of subchannels and the subcarrier indices used for each subchannel index for the uplink. As shown in this table, one or more pilot subcarrier(s) (there are eight in total) are allocated only if two or more subchannels are allocated. The subcarriers other than the ones used for subchannelised transmission are nonactive (for the transmitter). The five-bit subchannel index is used in the uplink allocation message UL-MAP (see Chapter 9 for the UL-MAP).

Table 5.4: The number of subchannels and the subcarrier indices used for each (five bits) subchannel index. (Based on Reference [1].)
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Subchannell index

Pilot frequency index

Subchannel index (continued)

Subcarrier frequency indices





0b00001 0b00011

100: 98; 37: 35; 1: 3; 64: 66



97: 95, 34; 32, 4; 6, 67: 69




94: 92, 31: 29,7: 9, 70: 72




91: 89, 28: 26, 10: 12, 73: 75






87: 85, 50: 48, 14: 16,51: 53



84, 82, 47: 45, 17: 19, 54: 56





81: 79, 44: 42, 20: 22, 57: 59





78: 76, 41: 39, 23: 25, 60: 62

0b10000 (no subchannelisation


75: 73, 12: 10, 26:28,89:91





72: 70, 9: 7, 29: 31. 92: 94





69: 67, 6: 4, 32: 34, 95: 97





66: 64, 3: I. 35: 37, 98: 100





62: 60. 25: 23, 39: 41, 76: 78





59: 57, 22: 20, 42: 44, 79: 81





56: 54, 19: 17, 45: 47, 82: 84





53: 51, 16; 14, 48: 50, 85: 87

Subchannelised transmission in the uplink is an option for an SS. It can be used only if the BS signals its capability to decode such transmissions. The BS must not assign to any given SS two or more overlapping subchannelised allocations in the same time.

The standard [1] indicates that when subchannelisation is employed, the SS maintains the same transmitted power density unless the maximum power level is reached. Consequently, when the number of active subchannels allocated to an uplink user is reduced, the transmitted power is reduced proportionally, without additional power control messages. When the number of subchannels is increased the total transmitted power is also increased proportionally. The transmitted power level must not exceed the maximum levels dictated by signal integrity considerations and regulatory requirements. The subchannelisation can then represent transmitted power decreases and, equivalently, capacity gains.

The 802.16e amendment defined an optional downlink subchannelisation zone in the OFDM PHY downlink subframe. Uplink subchannels are partly reused.