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History of OFDMA and How it Works

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).

Orthogonal Frequency Division Multiplexing (OFDM) is a technique for transmitting large amounts of digital data over a radio wave  The technology 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).

Broadband Wireless Air Interfaces

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. 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 of Wi-Fi and OFDMA

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
  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.

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.

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.

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 is Runcom.

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.

Conclusions about OFDMA

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.

Did You Know...
There are three types of OFDMA subcarriers: Data subcarriers for data transmission, Pilot subcarriers for various estimation and synchronization purposes, and Null subcarriers for no transmission at all, used for guard bands and DC carriers.
[Source: Intel Technology Journal]

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