As technical as they may sound, OFDM and OFDMA are gaining popularity both in the mobile and fixed wireless industries. OFDM – Orthogonal Frequency Division Multiplexing and OFDMA – Orthogonal Frequency Division Multiple Access technologies are used in modern wireless networks, including 4G LTE, 5G NR, WiMAX and WiFi6. Before dividing into the ‘Orthogonal’ and ‘Frequency Division’ parts, let’s try to clarify what multiplexing and multiple access mean. With things moving quickly in the direction of 5G NR, it is becoming increasingly clear that 4G LTE is expected to co-exist with 5G for a long time. 4G LTE and 5G NR networks have technical similarities around multiplexing, multiple access, and modulation. This post aims to focus on multiplexing and multiple-access concepts to elucidate what these means for 4G LTE networks. The modulation part is covered in a separate post which you can access by clicking here.
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 and FCC in the US for nationwide mobile network coverage. Regulatory authorities make sure that the frequencies are used in a controlled way so that network operators do not interfere with others. Mobile operators, therefore, use non-interfering channels of certain bandwidths (e.g. 20 MHz) operating at specific frequencies allocated to them by the regulators.
What is the difference between multiple-access and multiplexing?
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 inter-related, and it is vital to appreciate the subtle differences between them to avoid confusion.
Multiplexing is the act of combining multiple signals into one. It is used for sending various signals carrying unique information through a shared medium such as a single frequency carrier. In mobile communications, multiple-access is the multi-user version of multiplexing that allows multiple users to send and receive data through a single frequency carrier.
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. The intended recipient of multiplexing is a single user or terminal, whereas multiple-access recipients are multiple users.
What multiple-access and multiplexing techniques 4G LTE networks use?
Unlike the third generation mobile networks (3G) that were CDMA-based, 4G LTE networks employ an access technology called Orthogonal Frequency Division Multiple Access or OFDMA. OFDMA is, however, only used in the downlink from the base station to the mobile device. The uplink in LTE, mobile device to the network, uses another access technology Single Carrier Frequency Division Multiple Access or 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 understand OFDMA, we first need to understand Orthogonal Frequency Division Multiplexing (OFDM), the multiplexing technique used in LTE networks. The carrier frequencies in 4G LTE networks can use different bandwidths, i.e. 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz. In OFDM, a carrier with a wide bandwidth (wideband carrier) is split into a large number of sub-carriers each with a narrow bandwidth (narrowband). If we were to take a wideband carrier, e.g. 20 MHz, it would be split into smaller chunks of narrowband carriers called sub-carriers. In a 20 MHZ carrier, 2 MHz is reserved for interference control purposes through the use of guard bands (or other equivalents) between other carriers. The remaining 18 MHz is then divided into narrowband carriers of 15 kHz each that gives 1200 sub-carriers. We have created a simplified diagram below showing 12 sub-carriers instead of 1200 to help visualise this concept.
These narrowband sub-carriers have a bandwidth of 15 kHz each and are tightly packed together. If you think about the regular Frequency Division Multiplexing, you may know that there is always a guard band between each channel to avoid interference, but with OFDM we have quite a contrast. In OFDM the sub-carriers are packed together so tightly that they are overlapping with each other. The word ‘orthogonal’ makes all the difference here. It basically means that each of these sub-carriers is organised so that its highest point (peak) is always when the overlapping neighbour sub-carriers are at their lowest point (zero). As a result, there is no interference because all sub-carriers are independent of each other. OFDM makes the most of the available carrier-bandwidth by removing the need for guard bands and utilising most of the bandwidth for the actual information content. Each of the 15 kHz subcarriers is individually modulated through a digital modulation technique, Quadrature Amplitude Modulation (QAM). This way, the available information content is distributed among all the sub-carriers such that each sub-carrier has unique information content. OFDM involves modulating multiple sub-carriers to achieve its objectives instead of modulating the entire carrier at once, and it is also referred to as multi-carrier modulation.
OFDMA or Orthogonal Frequency Division Multiple Access is the multi-user version of OFDM. In OFDMA, all sub-carriers can only be assigned to one specific user at any given time interval. OFDMA is highly dynamic and allows the same sub-carriers and time-slots to be used in a much more flexible way. With OFDMA, each sub-carriers can be allocated to any user at any time. 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 time-slot can also be more or less depending on how long the user requires higher bandwidth. The value OFDMA brings is that it makes the allocation of sub-carriers and time-slots a lot more dynamic. To help you visualise this concept; if the earlier diagram for OFDM was to use OFDMA, below is what it would look like.
Once all the data for a particular radio unit operating at a specific carrier frequency has been digitally modulated with the sub-carriers, it is sent towards the base station 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. To address this challenge, LTE networks use a transformation technique called Inverse Fast Fourier Transform (IFFT). IFFT applies sampling to the individual sub-carriers and transforms them from the frequency domain into the time-domain. These steps take place even before the signal reaches the base station antennas for transmission. This is where spatial multiplexing technique MIMO (Multiple Input Multiple Output) comes in, which you can learn more about in this post.