The Many Flavors of OFDMA---2

Much in common, among the differences
The OFDM modulation scheme offers many advantages for broadband wireless transport. It supports high data rates. The design not only obviates multipath interference (where reflected signals return slightly out of phase, creating interference at the receiver) it can actually utilize multipath to increase signal quality by processing the reflected packets to increase gain. This technique also improves non-line of sight delivery. It supports both TDD and FDD, the latter of which provides symmetrical data delivery.
The good news is that for most practical purposes, the terms sOFDMA and OFDMA can be used interchangeably as they are so similar. Both support sub channelization as a key technology. Flash OFDM is a bit different.
Sub channelization allows all four variations (OFDM, Flash OFDM, sOFDMA and OFDMA) to split channels up into sub channels, even into several thousand sub channels. Essentially, a user on an OFDMA network is assigned a number of sub channels across the band. A user close to the base station would normally be assigned a larger number of channels with a high modulation scheme such as 64 QAM (quadrature amplitude modulation) to deliver the most data throughput to that user. As the user moves farther away, the number of sub channels is re-assigned dynamically to fewer and fewer sub channels. However, the power allotted to each channel is raised. The modulation scheme could gradually shift from 16 QAM to Quaternary Phase Shift Keying (QPSK) (four channels) and even binary phase shift keying (BPSK) (two channels) at longer ranges. The data throughput drops as the channel capacity and modulation change, but the link maintains its strength.
Cell sizes must not expand or contract. Each user must have a strong link to their base station until handoff. The tradeoff is lower throughput at the edge.
In fixed wireless links, which typically use high gain directional antennas, this technique is less necessary. For mobile applications, especially with high speed handoffs, it is necessary. Customer devices typically feature omni-directional antennas which radiate in all directions but have lower gain than directional antennas. In this environment, sub channelization is necessary.
For more details on sub channelization, see the WiMAX Forum White Paper on Sub channelization [.pdf].
Another technique called scalability was developed, resulting in sOFDMA.
Two Flavors of WiMAX
Enter scalability. Because channels differ in size in different countries, the 802.16 standard supports all of the various channel sizes, ranging from 1.25 MHz to 20 MHz.
For a variety of complex technical reasons, Intel makes the argument that keeping the sub channel spacing fixed by changing the FFT size based on channel size or bandwidth provides better signal quality. One of the simpler arguments is that the Doppler shift of a moving body (amongst other aspects) affects signal quality if the sub channel spacing is not maintained at a fixed size. There is an excellenttutorial on Scalable OFDMA on Intel‘s website.
The OFDMA 2048 FFT version was conceived as a fixed FFT and is supported in both 802.16-2004 and 802.16e. One company in the forefront of this type of OFDMA technology isRuncom.
The scalable or sOFDMA versions encompass the 128 FFT, 512 FFT, and 1024 FFT as promoted by Intel and others. This last flavor of OFDMA can actually shift its FFT size based on channel and bandwidth, thus becoming scalable. So, for example, a user traveling through a cell might receive signal through 128 FFT or 512 FFT depending on factors such as channel size.
The Korean WiBro standard is basically a subset of the sOFDMA at 1024 FFT. For a variety of reasons, it appears most likely that the first product profile chosen by the Forum may be a sOFDMA one, though that is not certain as of press time for this article.
About Qualcomm and Flarion
Theacquisition of Flarion by Qualcomm is big news. Many look at it as a stamp of approval for OFDM and OFDMA. Qualcomm itself is directly involved in research into OFDM. Although Flarion‘s specific iteration of OFDMA, called Flash OFDM, is very similar to the other flavors we‘ve discussed in that it uses sub channelized OFDM (Flarion calls its sub channels "tones"), its technology has several unique features.
The biggest single difference is that Flarion has vertically integrated the MAC and PHY layer as well as the link and network layers much more closely together than in the other approaches.
One other major difference is that Flarion already has product in the field.
"We have been booking revenues since October of 2003," says Ronny Haraldsvik, vice president of marketing for Flarion. "We believe our vertically integrated MAC and PHY provide an easier path to multiple standards interoperability."
Haraldsvik adds that Flarion demonstrated handoffs to 802.11 systems in December, 2001. Flarion chose to ensure mobile capability from the beginning as well as to optimize throughput for small channel sizes (such as 1.25 MHz), particularly in low-frequency spectrum. Flarion‘s product line is optimized for frequencies ranging from below 450 MHz up to 1.9 GHz and 2.4 GHz. The company asserts its technology would work fine above those spectrum ranges, but it admits that the WiMAX sOFDMA and OFDM technologies may be better optimized today at 2.5 GHz and above.
"Higher frequency bands with larger channels such as 10 MHz and 20 MHz are not what we are interested in," says Haraldsvik. "On the flip side, we believe WiMAX will have to make serious engineering adjustments to work as well at lower frequency ranges."
It is Flarion‘s contention that spectrum does matter in regards to how its Flash OFDM system was designed. In particular it believes trade offs are necessary to optimize service based on spectrum and application.
For more information, see Flarion‘s white paper on mobile data communications [.pdf].
Conclusions
The physics of broadband wireless forces the designers of OFDMA to make choices, and those choices have tradeoffs. For every advantage engineered, there is always a price. The various flavors of OFDMA are about the demands of mobility and speed of handoffs, the size of the cell, spectrum range, channel sizes, and more.
Ultimately, each flavor has been optimized to meet the needs of a specific marketplace. Luckily, the technologies are flexible enough to allow growth into other segments.
In the long run, the marketplace will decide which products are made, and which products are sold.
Reprinted fromISP Planet.