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Orthogonal frequency division multiple access (OFDMA) is a version of OFDM (orthogonal frequency division multiplexing) modulation that happens to be optimized for multiple users, specifically for cell phones and other mobiles devices.
It is the most likely modulation scheme for cellular LTE (long-term evolution). In this incarnation, OFDMA goes by the name high speed OFDM Packet Access (HSOPA). Variants of OFDMA were chosen as the modulation scheme by the WiMAX Forum and were later standardized by the IEEE for IEEE 802.16-2004 (fixed) and 802.12e (mobile) WiMAX.
Its advantages over 3G modulation schemes such as CDMA (code division multiple access) wideband CDMA, and universal mobile telecommunications system (UMTS) include higher spectral efficiency and better resistance to fading. It also requires lower transmission power for low-data-rate users, has shorter delays that are constant instead of time varying, and a simpler methodology for collision avoidance.
OFDMA assigns subsets of subcarriers to individual users. Based on feedback about the channel conditions, the system can implement adaptive user-to-subcarrier assignment. As long as these subcarrier assignments are executed quickly, fast fading and narrow-band co-channel interference performance is improved compared to OFDM. This, in turn, improves system spectral efficiency.
Two perspectives
OFDMA obviously has both differences and similarities with other modulation schemes. For example, it could be seen as an alternative to combining OFDM with time division multiple access (TDMA) or time-domain statistical multiplexing. Instead of using a "pulsed" high-power carrier, low data-rate users can transmit continuously with low transmission power and this results in constant and shorter delay times.
On the other hand, OFDMA can also be viewed as a combination of frequency domain and time domain multiple access. From this perspective, spectrum is partitioned in the time-frequency space, and slots are assigned along the OFDM symbol index as well as OFDM sub-carrier index.
OFDMA evolution
The best way to understand the relationship between OFDMA and other technologies is through a short history. The IEEE 802.11 WLAN family of standards was conceived for indoor networks. Once analog cellular technology showed its market potential—and its technical inadequacies—engineers started designing proprietary MAC an PHY systems that extended Wi-Fi functionality to outdoor networks.
Indeed, most of the action in broadband access happens at ISO Layer 1 (the PHY layer) and Layer 2 (the media access control or MAC layer).
When work started on a broadband wireless MAN (metropolitan area network) standardization, it opened the door to investigating other modulation schemes and the value of OFDM and OFDMA became apparent. The WiMAX Forum was instrumental in evaluating the options and making recommendations to standards bodies.
This eventually evolved into the IEEE 802.16 standards. IEEE 802.16-2004 delivers fixed broadband wireless and IEEE 802.16e delivers mobile broadband wireless. Both support multiple PHY modes but none of the available options include the 3G modulations schemes of WCDMA or UMTS.
Along with OFDM and OFDMA, scalable OFDMA is also included in the standard.
A scalable 802.16 physical layer (sOFDMA) delivers optimum performance in channel bandwidths ranging from 1.25 MHz to 20 MHz with fixed subcarrier spacing for both fixed and portable/mobile usage models.
The architecture is based on a scalable sub-channelization structure with variable fast Fourier Transform (FFT) sizes according to the channel bandwidth. In addition to variable FFT sizes, the specification supports other features such as multiple-input-multiple-output (MIMO) antenna diversity.
The 802.16 options are noted in Table 1.
For most practical purposes, sOFDMA and OFDMA can be used interchangeably as they are so similar. Both support sub channelization as a key technology. Sub-channelization involves splitting channels into many sub-channels—theoretically into the thousands of sub-channels. Sub-channelization is a characteristic of all four variations: OFDM, OFDMA. sOFDMA and Flash OFDMA, a variant created by the Flarion Inc. Last year, Flarion was acquired by Qualcomm, a clear sign that variants of CDMA will not be the 4G modulation scheme.
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