OFDM or OFDMA?

IEEE 802.16d (fixed service) uses Orthogonal Frequency Division Multiplexing (OFDM). IEEE 802.16e (mobile) uses Orthogonal Frequency Division Multiple Access (OFDMA). So, what’s the difference between the two, and why is there a difference?

OFDM is sometimes referred to as discrete multi-tone modulation because, instead of a single carrier being modulated, a large number of evenly spaced subcarriers are modulated using some m-ary of QAM. This is a spread-spectrum technique that increases the efficiency of data communications by increasing data throughput because there are more carriers to modulate. In addition, problems with multi-path signal cancellation and spectral interference are greatly reduced by selectively modulating the “clear” carriers or ignoring carriers with high bit-rate errors.

The OFDM spread-spectrum scheme is used for many broadly used applications, including digital TV broadcasting in Australia, Japan and Europe; digital audio broadcasting in Europe; Asynchronous Digital Subscriber Line (ADSL) modems and wireless networking worldwide (IEEE 802.11a/g).

OFDM allows only one user on the channel at any given time. To accommodate multiple users, a strictly OFDM system must employ Time Division Multiple Access (TDMA) (separate time frames) or Frequency Division Multiple Access (FDMA) (separate channels). Neither of these techniques is time or frequency efficient: TDMA is a time hog and FDMA is a bandwidth hog.

OFDMA is a multi-user OFDM that allows multiple access on the same channel (a channel being a group of evenly spaced subcarriers, as discussed above). WiMAX uses OFDMA, extended OFDM, to accommodate many users in the same channel at the same time.

OFDMA distributes subcarriers among users so all users can transmit and receive at the same time within a single channel on what are called subchannels. What’s more, subcarrier-group subchannels can be matched to each user to provide the best performance, meaning the least problems with fading and interference based on the location and propagation characteristics of each user.

The WiMAX forum established that, initially, OFDM-256 will be used for fixed-service 802.16d (2004). It is referred to as the OFDM 256 FFT Mode, which means there are 256 subcarriers available for use in a single channel. Multiple access on one channel is accomplished using TDMA. Alternatively, FDMA may be used.

On the other hand, OFDMA 128/512/1024/2048 FFT Modes have been proposed for IEEE 802.16e (mobile service). OFDMA 1024 FFT matches that of Korea’s WiBRO. OFDM 256 also is supported for compatibility with IEEE 802.16d (fixed, 2004). The final IEEE 802.16e standard is expected to be completed and published in December of this year.

The bottom line is that, most likely, the finalized selection for the OFDMA mode will be 1024 FFT, to be compatible with WiBRO. However, it will not be compatible with the OFDM 256 FFT Mode initially specified for WiMAX fixed service. Perhaps service providers will simply abandon 802.16d in favor of 802.16e for both fixed and mobile services. By Mark E. Hazen, EWT Editor


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