Orthogonal frequency-division multiple access (OFDMA) is a multi-user version of the popular orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of

subcarriers A subcarrier is a sideband of a radio frequency carrier wave, which is modulated to send additional information. Examples include the provision of colour in a black and white television system or the provision of stereo in a monophonic radio broa ...

to individual users. This allows simultaneous low-data-rate transmission from several users.

Comparisons

OFDMA is often compared to the combination of OFDM with statistical time-division multiplexing. The advantages and disadvantages summarized below are further discussed in the Characteristics and principles of operation section. See also the list of OFDM key features.

Advantages

* Allows simultaneous low-data-rate transmission from several users. * Pulsed carrier can be avoided. * Lower maximal transmission power for low-data-rate users * Shorter delay and constant delay * Contention-based multiple access (collision avoidance) is simplified. * Further improves OFDM robustness to fading and interference * Combat narrow-band interference. * Flexibility of deployment across various frequency bands with little needed modification to the air interface * Averaging interferences from neighboring cells, using different basic carrier permutations between users in different cells * Interferences within the cell are averaged by using allocation with cyclic permutations * Enables single-frequency network coverage, where coverage problem exists and gives excellent coverage. * Offers frequency diversity by spreading the carriers all over the used spectrum. * Allows per-channel or per-subchannel power.

Disadvantages

* Higher sensitivity to frequency offsets and phase noise * Asynchronous data communication services such as web access are characterised by short communication bursts at high data rate. Few users in a base station cell are transferring data simultaneously at low constant data rate. * The complex OFDM electronics, including the FFT algorithm and

forward error correction In computing, telecommunication, information theory, and coding theory, forward error correction (FEC) or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels. The centra ...

, are constantly active, thus consuming power, independent of the data rate, while OFDM combined with data packet scheduling may allow FFT algorithm to hibernate at times. * The OFDM diversity gain and resistance to frequency-selective fading may partly be lost if very few sub-carriers are assigned to each user, and if the same carrier is used in every OFDM symbol. Adaptive sub-carrier assignment based on fast feedback information about the channel, or sub-carrier frequency hopping, is therefore desirable. * Dealing with co-channel interference from nearby cells is more complex in OFDM than in

CDMA Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communicatio ...

. It would require

dynamic channel allocation In radio resource management for wireless and cellular networks, channel allocation schemes allocate bandwidth and communication channels to base stations, access points and terminal equipment. The objective is to achieve maximum system spectral ...

with advanced coordination among adjacent base stations. * The fast channel feedback information and adaptive sub-carrier assignment is more complex than CDMA fast power control.

Characteristics and principles of operation

Based on feedback information about the channel conditions, adaptive user-to-subcarrier assignment can be achieved. If the assignment is done sufficiently fast, this further improves the OFDM robustness to fast

fading In wireless communications, fading is the variation of signal attenuation over variables like time, geographical position, and radio frequency. Fading is often modeled as a random process. In wireless systems, fading may either be due to mul ...

and narrow-band cochannel interference, and makes it possible to achieve even better system spectral efficiency. Different numbers of sub-carriers can be assigned to different users, in view to support differentiated

quality of service Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...

(QoS), i.e. to control the data rate and error probability individually for each user. OFDMA can be seen as an alternative to combining OFDM with time-division multiple access (TDMA) or time-domain

statistical multiplexing Statistical multiplexing is a type of digital communication link sharing, sometimes abbreviated as STDM. It is very similar to dynamic bandwidth allocation (DBA). In statistical multiplexing, a communication channel is divided into an arbitrary num ...

communication. Low-data-rate users can send continuously with low transmission power instead of using a "pulsed" high-power carrier. Constant delay, and shorter delay, can be achieved. OFDMA can also be described as a combination of frequency-domain and time-domain multiple access, where the resources are partitioned in the time–frequency space, and slots are assigned along the OFDM symbol index, as well as OFDM sub-carrier index. OFDMA is considered as highly suitable for broadband wireless networks, due to advantages including scalability and use of multiple antennas (

MIMO In radio, multiple-input and multiple-output (MIMO) () is a method for multiplying the capacity of a radio link using multiple transmission and receiving antennas to exploit multipath propagation. MIMO has become an essential element of wirel ...

-friendliness), and ability to take advantage of channel frequency selectivity. In spectrum sensing

cognitive radio A cognitive radio (CR) is a radio that can be programmed and configured dynamically to use the best channels in its vicinity to avoid user interference and congestion. Such a radio automatically detects available channels, then accordingly change ...

, OFDMA is a possible approach to filling free

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...

bands adaptively. Timo A. Weiss and Friedrich K. Jondral of the University of Karlsruhe proposed a spectrum pooling system in which free bands sensed by nodes were immediately filled by OFDMA subbands.

Usage

OFDMA is used in: * The mobility mode of the IEEE 802.16 Wireless MAN standard, commonly referred to as WiMAX * The

wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office building ...

(WLAN) standard IEEE 802.11ax (marketed as Wi-Fi 6 / Wi-Fi 6E) * The IEEE 802.20 mobile Wireless MAN standard, commonly referred to as MBWA * MoCA 2.0 * The downlink of the

3GPP The 3rd Generation Partnership Project (3GPP) is an umbrella term for a number of standards organizations which develop protocols for mobile telecommunications. Its best known work is the development and maintenance of: * GSM and related 2G and ...

Long-Term Evolution (LTE) fourth-generation mobile broadband standard. The radio interface was formerly named ''High Speed OFDM Packet Access'' (HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA). * The downlink and the uplink of the 3GPP 5G New Radio (5G NR) fifth-generation mobile network standard. 5G NR is the successor to LTE. * The Qualcomm Flarion Technologies Mobile Flash-OFDM * The now defunct

Qualcomm Qualcomm Incorporated () is an American multinational corporation headquartered in San Diego, California, and Delaware General Corporation Law, incorporated in Delaware. It creates semiconductors, software and services related to wireless techn ...

3GPP2 The 3rd Generation Partnership Project 2 (3GPP2) was a collaboration between telecommunications associations to make a globally applicable third generation (3G) mobile phone system specification within the scope of the ITU's IMT-2000 project. ...

Ultra Mobile Broadband Evolution-Data Optimized (EV-DO, EVDO, etc.) is a telecommunications standard for the wireless transmission of data through radio signals, typically for broadband Internet access. EV-DO is an evolution of the CDMA2000 (IS-2000) standard which s ...

(UMB) project, intended as a successor of

CDMA2000 CDMA2000 (also known as C2K or IMT Multi‑Carrier (IMT‑MC)) is a family of 3G mobile technology standards for sending voice, data, and signaling data between mobile phones and cell sites. It is developed by 3GPP2 as a backwards-compatib ...

but replaced by LTE OFDMA is also a candidate access method for the IEEE 802.22 ''Wireless Regional Area Networks'' (WRAN), a

technology which uses white spaces in

television Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...

spectrum, and the proposed access method for

DECT Digital Enhanced Cordless Telecommunications (DECT) is a cordless telephony standard maintained by ETSI. It originated in Europe, where it is the common standard, replacing earlier standards, such as CT1 and CT2. Since the DECT-2020 standard ...

-5G specification which aims to fulfill IMT-2020 requirements for high-throughput mobile broadband (eMMB) and ultra-reliable low-latency (URLLC) applications. OFDMA subcarriers

References

External links

Orthogonal Frequency Division Multiple Access: is it the multiple access system of the future?
S. Srikanth, V. Kumaran, C. Manikandan et al., AU-KBC Research Center, Anna University, India.
Short Introduction to OFDM
– Tutorial written by Prof. Debbah, head of the Alcatel-Lucent Chair on flexible radio. {{Channel access methods Multiplexing Quantized radio modulation modes