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The Many Flavors of OFDMA

Start with a definition of Orthogonal Frequency Division Multiplexing (OFDM) : OFDM works by splitting the radio signal into multiple smaller sub-signals that are then transmitted simultaneously at different frequencies to the receiver. OFDM reduces the amount of crosstalk in signal transmissions.

When we talk about broadband wireless access, most of the action takes place at the Media Access Control (MAC) layer (layer 2) and the Physical (PHY) layer (layer 1 or the Air Interface).

There are numerous types of broadband wireless air interfaces including single carrier, Orthogonal Frequency Division Multiplexing (OFDM), and Orthogonal Frequency Division Multiple Access (OFDMA). Others are Wideband Code Division Multiple Access (WCDMA) a cellular 3G technology, and Universal Mobile Telecommunications System (UMTS) also cellular 3G.

With the advent of WiMAX, the terms OFDM and OFMDA, scalable OFDMA (sOFDMA), and Flarion's alternative version of OFDMA, Flash OFDM, have all become buzzwords—and subject to the standards process. Other terms such as Fast Fourier Transform (FFT), Time Division duplex (TDD), and Frequency Division Duplex (FDD) modes play a part in the various flavors of this modulation scheme.

The marketplace today seems to have decided that OFDM (or OFDMA) offers real advantages for broadband wireless transport. The WiMAX Forum has clearly focused on these technologies. Qualcomm's recent acquisition of Flarion only underscores industry confidence in this technology. The topics are, of course, very complex and in this article we will only be able to provide an overview. If one any particular aspect of this discussion is relevant to you, we recommend that you conduct further research.

Before we delve into the arcane minutia of what are essentially subtle differences in OFDMA, let's go over the history of the topics involved.

The background
We are all used to the term Wi-Fi, which generally refers to the 802.11a/b/g/n family of standards. However, 802.11 standards were written for indoor wireless networks. Many vendors built proprietary MAC and PHY systems that extended these capabilities to outdoor networks. Some of these systems used a single carrier. Several leveraged OFDM capabilities. Others chose WCDMA or UMTS approaches. But the idea was to create effective outdoor networks. This was and is a very fragmented marketplace.

Enter the 802.16 movement, which sought to define a proper metropolitan area network (MAN) standard for broadband wireless or WiMAX. This standard has evolved into two standards: One delivers fixed broadband wireless (802.16-2004) and another delivers mobile broadband wireless (802.16e). Interestingly, both support multiple PHY modes, none of which include WCDMA or UMTS.

802.16-2004 (fixed)
802.16e (mobile)
Single Carrier Single Carrier
OFDM 256 FFT OFDM 256 FFT
OFDMA 2048 FFT OFDM 2048 FFT
  sOFDMA 1024 FFT
  sOFDMA 512 FFT
  sOFDMA 128 FFT

The WiMAX Forum chose the OFDM 256 FFT mode for the first fixed WiMAX product profile. The first product profiles for mobile WiMAX have yet to be chosen as the standard is not yet ratified. However, it appears some version of OFDMA will get the nod, which brings us to why it makes sense to understand a bit about OFDMA.

There is a third flavor of OFDMA competitive to WiMAX called Flash OFDM that Flarion uses which is also very similar, but more on that later.

There could ultimately be WiMAX product profiles that have the same PHY mode for both fixed and mobile. For example, some vendors believe there will ultimately also be an OFDM 256 FFT mode for 802.16e. The rule is that three vendors must agree on the product profile for the Forum to define a product profile for interoperability testing. Many mobility proponents seem to prefer an OFDMA version. In any event, product profiles with different modes will not be interoperable. Also, profiles of modes at a given FFT size (512 for example) will not interoperate with the different fixed FFT size mode of 2048.

So what does OFDMA accomplish? In simplified terms, the OFDMA mode attempts to optimize mobile access by many simultaneous users through breaking a signal into sub-channels. Some camps believe OFDM can accomplish this as well as and cheaper than alternatives. Others believe OFDM is best suited for simple mobility or portability. Sub channelization was added to OFDM on the uplink and downlink technology but ultimately rejected by the IEEE 802.16 working group. Whatever the relative merits for mobility, the two modes are essentially very similar.