Difference between OFDM, OFDMA and SC-FDMA in 4G LTE

OFDM, OFDMA and SC-FDMA are technologies that enable radio communication between a mobile phone and the 4G LTE mobile network. OFDM stands for Orthogonal Frequency Division Multiplexing, OFDMA stands for Orthogonal Frequency Division Multiple Access, and SC-FDMA stands for Single Carrier Frequency Division Multiple Access.

OFDM is the primary transmission scheme in 4G LTE networks that employs multiple sub-carriers of 15 kHz each; OFDMA is the multi-user version of OFDM used in downlink communication; SC-FDMA is similar to OFDMA but is DFT-precoded to make it power-efficient for being used in uplink communication.

OFDM – Orthogonal Frequency Division Multiplexing and OFDMA – Orthogonal Frequency Division Multiple Access technologies are used in modern wireless networks, including 4G LTE, 5G New Radio (NR), WiMAX and Wi-Fi 6. OFDM is very robust and spectrally efficient, making it suitable for data networks that need to deliver high throughputs.

What do the terms OFDM, OFDMA and SC-FDMA mean in 4G LTE?

The transmission of mobile signals between cellular towers and mobile phones happens in the form of radio waves. One of the most critical aspects of this transmission is the use of specific frequencies at which the signals are sent and received. Mobile operators purchase licensed frequency spectrum from regulatory authorities like Ofcom in the UK or FCC in the US for nationwide cellular coverage. Regulatory authorities make sure that the frequencies are used in a controlled way so that mobile network operators do not interfere with each other. Mobile operators, therefore, use non-interfering channels of certain bandwidths (e.g. 20 MHz) operating at specific frequencies (e.g. 1.9 GHz) allocated to them by the regulators.

The communication between the mobile phone and the mobile networks happens in two directions: downlink and uplink. Downlink is when the cell tower (base station) transmits to the mobile phone, and uplink is when the phone transmits back to the network. The diagram below illustrates this basic concept.

Simplified view of downlink - base station to phone, and uplink - phone to base station
A simplified view of downlink – base station to phone, and uplink – phone to the base station

OFDM or Orthogonal Frequency Division Multiplexing is a multi-carrier transmission scheme in 4G LTE that splits the overall carrier bandwidth into smaller sub-carriers of 15 kHz each. OFDM is robust and has the ability to exploit both time and frequency domains. It also supports an optimum receiver complexity to work with MIMO antenna technology that enables spatial multiplexing in 4G LTE networks.

OFDMA or Orthogonal Frequency Division Multiple Access is the multi-user version of OFDM. As the underlying transmission scheme, OFDM is able to support one user at any given time interval. OFDMA can facilitate communication with multiple users simultaneously in the downlink direction.

SC-FDMA or Single Carrier Frequency Division Multiple Access is a special version of the OFDM-based multiple access technique that employs a single carrier, unlike OFDMA, which uses multiple carriers. SC-FDMA has a lower Peak-to-Average Power Ratio (PAPR) than OFDMA and is used in uplink communication because of its power efficiency, which ensures better battery life for mobile phones. In SC-FDMA, the symbols are precoded by a Discrete Fourier Transform (DFT) before applying Inverse Fourier Transform (IFT), which reduces the peak-to-average power ratio.

Why do 4G LTE networks use OFDM and OFDMA techniques?

4G LTE networks use OFDM and OFDMA to utilise the available bandwidth efficiently for high-speed data with minimal interference as OFDM suppresses interference by keeping all sub-carriers independent of each other. OFDM format does not require guard bands which ensures efficient spectrum usage.

Like 2G networks use FDMA & TDMA, 3G networks use CDMA, 4G LTE networks employ OFDMA (Orthogonal Frequency Division Multiple Access) and SC-FDMA (Single-Carrier Frequency Division Multiple Access) for the air interface. In LTE, OFDMA is used in the downlink for transmission from the mobile base station to the user device. On the other hand, the uplink transmission, i.e. the communication from the mobile device to the network, uses another access technology, SC-FDMA. SC-FDMA is more power-efficient than OFDMA and is a better choice for uplink to ensure better battery life for the mobile phone.

To fully comprehend OFDMA, we first need to understand Orthogonal Frequency Division Multiplexing (OFDM), the underlying LTE transmission scheme.

LTE networks use flexible bandwidths and support carriers of 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. In OFDM, a wideband frequency carrier (carrier with a wide bandwidth) is split into a large number of smaller narrowband carriers (carriers with a narrow bandwidth). These narrowband sub-carriers have a bandwidth of 15 kHz each and are tightly packed together in such a way that they overlap with each other.

The difference between OFDMA and the regular FDMA (Frequency Division Multiple Access) is that the latter requires a guard band between each carrier to avoid interference. On the other hand, OFDM consists of tightly packed sub-carriers that overlap with each other, but due to being orthogonal, they do not interfere.

The word orthogonal in OFDM suggests that the sub-carriers are independent of each other. It means sub-carriers are organised so that the highest point (peak) of each sub-carrier overlaps with the lowest point (zero) of neighbouring sub-carriers. As a result, there is no interference because all sub-carriers are independent of each other.

For example, if we were to take a 20 MHz frequency carrier (wideband carrier), it would be split into smaller chunks of 15 kHz sub-carriers (narrowband carriers). In a 20 MHz carrier, 2 MHz is reserved for interference control purposes by using guard bands (or other equivalents) between other carriers. The remaining 18 MHz is then divided into narrowband carriers of 15 kHz, which results in 1200 total sub-carriers.

Diagram showing how the individual sub-carriers are tightly packed together in OFDM

OFDM utilises the available carrier bandwidth in a highly efficient manner by removing the need for guard bands and using most of the bandwidth for the actual information content. Each 15 kHz sub-carrier is individually modulated using the most suitable digital modulation technique, Quadrature Amplitude Modulation (QAM) or Quadrature Phase Shift Keying (QPSK).

With the above approach, OFDM allows the available information content to be distributed to all the sub-carriers so that each sub-carrier has unique information content. OFDM achieves its objectives by modulating multiple sub-carriers individually instead of modulating the entire carrier. OFDM is therefore also called multi-carrier modulation.

OFDMA and SC-FDMA are both based on the OFDM waveform and utilise the sub-carriers created by OFDM in slightly different ways. OFDMA is for transmission from the radio base station transmitter to the mobile phone receiver. In contrast, SC-FDMA is for transmission from the mobile phone transmitter to the radio base station receivers.

In OFDM systems, once all the data has been digitally modulated with the sub-carriers using QAM or QPSK, it is sent towards the transmitter antennas. As there are many sub-carriers in one carrier (e.g. 1200 sub-carriers in a 20 MHz carrier), sending individual sub-carriers as radio waves is hardware-intensive. It requires a large number of oscillators at the transmitter and the receiver, which is not very practical. LTE networks use the Inverse Fast Fourier Transform (IFFT) transformation technique to address this challenge. IFFT applies sampling to the individual sub-carriers and transforms them from the frequency domain into the time domain. These steps occur even before the signal reaches the transmitter antennas for transmission. At this point, spatial multiplexing used by the LTE networks comes into action. Spatial multiplexing is enabled by the Multiple Input Multiple Output (MIMO) antenna technology.

OFDM vs OFDMA: The key difference between OFDM and OFDMA

In OFDM, the sub-carriers within a particular time interval (timeslot) can only be assigned to one specific user; OFDMA is the multi-user version of OFDM that allows dynamic allocation of sub-carriers to any user at any time by exploiting both time and frequency domains.

The diagram below provides a visual representation of the concept of sub-carrier allocation used in OFDM. In this diagram that shows only 12 sub-carriers for simplicity, each user has their dedicated timeslot, and all the available sub-carriers are assigned to them at different time intervals. In OFDM, the user allocation is not flexible in the frequency domain because all sub-carriers are assigned to all users at dedicated time intervals irrespective of how much bandwidth they need.

Orthogonal Frequency Division Multiplexing - OFDM 4G LTE
Example: 5 users being served by a radio unit, one at a time, using Orthogonal Frequency Division Multiplexing – OFDM

Orthogonal Frequency Division Multiple Access (OFDMA) exploits both time and frequency domains flexibly and makes the allocation of sub-carriers and timeslots more dynamic. As shown in the simplified diagram below, the mobile phone users in LTE can be assigned a specific number of sub-carriers for a particular time duration depending on their data needs.

Orthogonal Frequency Division Multiple Access - OFDMA 4GLTE OFDMA vs OFDM
Example: 5 users being served by a 4G LTE cell (radio unit) for downloads (downlink) through Orthogonal Frequency Division Multiple Access – OFDMA

For example, suppose one user is watching a YouTube video, and the other is sending messages on WhatsApp. If they are served by the same 4G LTE radio unit, OFDMA can allocate more sub-carriers to the user watching the video because video consumes more bandwidth. The duration of the time intervals or timeslots can also be more or less depending on how long the user requires higher bandwidth.

OFDM and OFDMA – Multiplexing vs Multiple Access

While OFDM is a multiplexing technique, it is often referred to as multi-carrier modulation because it allows the digital modulation of multiple sub-carriers individually through QPSK or QAM. Further, when multi-user capability is applied to OFDM by utilising both the frequency and time domains, it enables multiple access.

Multiplexing is a technique that combines multiple signals into one and is used for sending various information-carrying signals through a shared medium such as a frequency carrier. Multiple-access is the multi-user version of multiplexing that allows multiple users to send and receive data through the frequency carrier.

The cellular base stations create mobile network coverage through electromagnetic radiations. These radiations use air as an interface to communicate with any cellular devices such as mobile phones. The air interface allows mobile phone users to access the mobile network through radio network technologies like FDMA, TDMA, CDMA, OFDMA, etc. These technologies enable mobile base stations to communicate with mobile devices and vice versa. Let’s look at the simplified network diagram below that shows the flow of signals between the base station and the mobile phone.

Mobile networks use multiple-access techniques to ensure that each radio unit within the base station can serve multiple mobile devices. For example, 2G GSM networks use a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA). Frequency channels called ARFCNs (Absolute Radio Frequency Channel Numbers) are created by splitting the available frequency band into channels of 200 kHz each. Each channel is assigned to a radio unit to serve multiple users at different time intervals by assigning each user a unique time slot. Multiplexing and multiple-access are interrelated, and it is vital to appreciate the subtle differences between them to avoid confusion.

Multiple-access is not the opposite of multiplexing but rather a technique that requires multiplexing as a prerequisite. The opposite of multiplexing is demultiplexing, which extracts the different signals from the combined signal. In addition, the intended recipient of multiplexing is a single user or terminal, whereas multiple-access recipients are multiple users.

Here are some helpful downloads

Thank you for reading this post. I hope it helped you in developing a better understanding of cellular networks. Sometimes, we need extra support, especially when preparing for a new job, studying a new topic, or buying a new phone. Whatever you are trying to do, here are some downloads that can help you:

Students & fresh graduates: If you are just starting, the complexity of the cellular industry can be a bit overwhelming. But don’t worry, I have created this FREE ebook so you can familiarise yourself with the basics like 3G, 4G etc. As a next step, check out the latest edition of the same ebook with more details on 4G & 5G networks with diagrams. You can then read Mobile Networks Made Easy, which explains the network nodes, e.g., BTS, MSC, GGSN etc.

Professionals: If you are an experienced professional but new to mobile communications, it may seem hard to compete with someone who has a decade of experience in the cellular industry. But not everyone who works in this industry is always up to date on the bigger picture and the challenges considering how quickly the industry evolves. The bigger picture comes from experience, which is why I’ve carefully put together a few slides to get you started in no time. So if you work in sales, marketing, product, project or any other area of business where you need a high-level view, Introduction to Mobile Communications can give you a quick start. Also, here are some templates to help you prepare your own slides on the product overview and product roadmap.

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