4G LTE networks use the Orthogonal Frequency Division Multiple Access technique (OFDMA) for the air interface to enable wireless connectivity between the cell phone and the base station. OFDMA uses the multicarrier modulation scheme (MCM), which involves a large number of sub-carriers to transmit data by breaking it down into smaller chunks and then transmitting each chunk over a separate sub-carrier, that is modulated individually.
4G LTE networks employ OFDM, which is a multicarrier modulation scheme. OFDM first divides the carrier bandwidth (e.g. 20 MHz) into smaller sub-carriers (i.e. 15 kHz) through FDM and then applies digital modulation techniques QAM or QPSK to modulate the sub-carriers with the data being transmitted.
4G LTE uses multi-carrier modulation scheme OFDM
Orthogonal Frequency Division Multiplexing (OFDM) is the basic transmission scheme in 4G LTE networks. There are two critical steps involved in the operation of OFDM: (i) the overall available carrier bandwidth (e.g. 20 MHz) is divided into smaller 15 kHz subcarriers, and then (ii) a digital modulation technique is applied to each subcarrier individually. At this stage, all the work is being done on the digitally coded baseband signal, which is the signal that carries the actual information. For clarity, at this stage, the carrier frequency that mobile operators use as part of their frequency spectrum (e.g. 3.4 GHz) has no role to play. The radio frequency only becomes relevant when the information signal is ready for transmission.
Once the sub-carriers have the actual information (data), the next logical step is to transmit that through the radio base stations. But the signal is currently in a digital format, and in order for it to be transmitted through the radio base station, it needs to be analogue. Converting hundreds of sub-carriers from digital to analogue can be a costly exercise as it would require hundreds of digital-to-analogue converters (DAC). This is where IFFT (Inverse Fast Fourier Transform) saves the day. With IFFT, the sub-carriers that are in the frequency domain are converted into a single time-domain waveform. This OFDM signal then goes through a series of conversions and filtering before it can be transmitted through the antennas of the radio base station. That process includes digital-to-analogue conversion, channel filtering, up-conversion to the RF carrier frequency (e.g. 3.4 GHz) and amplification. Once that is done, the radio base station antennas can transmit the signal at licensed carrier frequencies. Finally, the process is reversed at the receiver end (e.g. your smartphone) to convert the signal back into the actual information.
QAM and QPSK are the digital modulation techniques in 4G LTE
The digital modulation techniques used by 4G LTE networks are Quadrature Amplitude Modulation (QAM) and Quadrature Phase Shift Keying (QPSK). Higher QAM modulation order, larger bandwidths and superior antenna techniques (MIMO) allow LTE to offer higher data rates. LTE Advanced networks use 256 QAM.
Digital modulation means transmitting digital data over an analogue signal. Like the earlier 2G and 3G networks, 4G LTE networks are digital, but the 4G radio signals between the network and the phone are analogue.
Even though there are many digital modulation techniques, such as Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), and Phase Shift Keying (PSK), the ones that are relevant for LTE are Quadrature Phase Shif Keying (QPSK) and Quadrature Amplitude Modulation (QAM).
The term “shift-keying” suggests that the digital information signal modulates an analogue signal by modifying its characteristics. Let’s clarify this “shift-keying” part through the example of Amplitude Shift Keying (ASK). As amplitude represents the height of a signal, the analogue signal’s height is modified using a special sequence in ASK. The sequence is determined by the binary values, i.e. 0 and 1 of the digital signal. For ASK, when the digital signal is at 0, the carrier signal’s amplitude will stay low, and when it moves to 1, the amplitude will go to the highest point whilst keeping all other characteristics (i.e. frequency and phase) the same. Below is a simplified diagram to demonstrate this concept.
Quadrature Amplitude Modulation (QAM) is a combination of amplitude and phase modulation of the carrier signal. It is a modern digital modulation technique used in 4G, 5G and Wi-Fi6 networks. 4G LTE adopted QAM because it is efficient and delivers higher data rates in the available spectrum.
For technical accuracy, it is worth mentioning that the QAM technique can also be used for analogue modulation, but in this context, the modulation is digital. There are various modulation orders in QAM, and the higher the order, the higher the efficiency, i.e. the signal can carry more data. Every interval in QAM is a unique combination of phase and amplitude, known as a symbol. The modulation order is represented in the multiples of 2, e.g., a modulation order of 16 in 16-QAM suggests 2 x 2 x 2 x 2 bits in total. Since 2 x 2 x 2 x 2 equals 24 , there are 4 bits in each symbol.
In simple terms, if an LTE radio unit was to employ 16-QAM for the transmission of digital data, each sub-carrier could carry four times as much data as it would have otherwise carried. So, the higher the modulation order, the higher the data rates due to higher spectral efficiency. Once the digital information has been modulated, it is transmitted through the base stations at carrier frequencies. At the receiver end, e.g. the mobile phone, another process, called demodulation, occurs. Demodulation is the opposite of modulation and involves the same steps but in reverse order.
What exactly is modulation and why is it used in 4G LTE?
Mobile networks communicate wirelessly through electromagnetic waves that travel in the air between mobile operators’ antennas and our mobile phones. Electromagnetic waves are also called radio waves, and the wireless interface is referred to as the air interface. Due to the laws of physics, these radio waves have certain characteristics that cellular networks utilise to transmit and receive mobile signals and use them in a network-efficient manner. 4G LTE networks have various utilities at their disposal to improve the achievable data rates, including carrier aggregation, MIMO antenna technology and digital modulation techniques. In digital data communications, a superior modulation technique allows a network to achieve higher efficiencies, resulting in higher data rates.
4G LTE delivers peak data rates of up to 3 Gbps with LTE Advanced Pro, requiring them to use the radio network resources efficiently to fully utilise the available bandwidth. In LTE, the spectrally efficient OFDM can employ QAM and QPSK modulations to deliver a higher number of bits per symbol.
Modulation is one of the most fundamental concepts in mobile communications and is applied multiple times during each transmission of a mobile signal. Modulation can be analogue and digital, and both forms of modulation are used in modern mobile-cellular networks.
Modulation is the process of encoding the information signal (data) with the carrier signal to transmit the data securely, efficiently and effectively without interference. It involves making changes to the carrier signal’s characteristics such as amplitude, frequency and phase in line with the data.
Modulation means modifying a signal that naturally raises a question about why someone would want to do that. The modification is done so that the data that we want to send (e.g. voice call, WhatsApp message, etc.) can be converted into a strong enough signal to be transmitted securely and in a way that fits the available network capacity. This is where the digital and analogue modulation techniques come in. While most modern mobile networks use a range of digital modulation techniques to achieve better data rates from the available frequency spectrum, analogue modulation is still used in the network to transmit radio waves into the air. Modulation involves mixing two separate signals, one of which is the actual data, and the other is the carrier signal that carries the data between the network and the phone. The data signal is also referred to as the modulating signal. Let’s look at the definitions and visual representations below.
Modulating signal or information signal or baseband signal
The signal that contains the actual information is called the modulating signal. The process of putting the data inside the carrier signal is called modulation, which requires some modifications to the carrier signal’s characteristics. The modulating signal or information signal is also known as a baseband signal. The information signal is usually at low frequencies in the order of a few Hertz (Hz) or Kilo Hertz (Hz). A simplified diagram below visualises an information signal.
Low frequency = lower number of cycles per second.
Carrier signal or carrier wave
A carrier wave or a carrier signal is a term that represents a frequency channel that carries essential information from one part of the network to another. This frequency channel is part of the overall frequency spectrum a mobile operator owns. The carrier signal is not the actual data and is just a carrier; it is essentially an empty but highly-secure envelope where we can put the information for successful delivery. As shown in the visual representation below, carrier signals operate at high frequencies.
Analogue modulation means modulating analogue data with an analogue signal. The most common example for analogue modulation can be radio stations that offer AM and FM channels. AM or Amplitude Modulation requires modification to the carrier signal’s height (amplitude) to accommodate the information signal. For the radio stations, that means modulating analogue data (audio/music etc.) with an analogue signal (radio wave). Once that is done, the modulated signal is transmitted through the radio stations’ transmitters. For FM channels, Frequency Modulation is applied, so instead of changing the height of the carrier signal, the frequency is modified to transmit the information. Phase modulation is the third form of analogue modulation, which requires alterations to the carrier signal phase. For clarity, there are also digital radio stations, e.g. Digital Audio Broadcasting (DAB) radio, that employs Orthogonal Frequency Division Multiplexing (OFDM) just like 4G LTE.
The carrier signal is a radio wave that operates at radio frequencies (RF), and this signal is always analogue. The data encoded within this analogue signal can be either digital or analogue. The fundamental difference between digital and analogue networks is this ‘digital data’ because the radio waves travel through the air the same way in all mobile networks. The key carrier signal characteristics that require modification are amplitude, frequency, and phase (angle). The diagram below can give you a quick visual overview of where frequency, wavelength, amplitude and phase fit in.
Conclusion
4G LTE networks are based on Orthogonal Frequency Division Multiplexing or OFDM, which supports multicarrier modulation. LTE networks use multiple narrowband subcarriers of 15 kHz each. The data content that is being transmitted over the LTE network is split into smaller chunks, and each chunk is communicated over a separate subcarrier. Each 15 kHz subcarrier is modulated individually in OFDM to deliver higher data rates. OFDM employs highly efficient digital modulation schemes Quadrature Amplitude Modulation (QAM) and Quadrature Phase Shift Keying (QPSK). The higher the order of modulation, the higher the data rates. The original LTE networks that were launched in 2009 supported 16-QAM and 64-QAM. LTE Advanced networks, however, use 256-QAM. Larger bandwidths, advanced antenna techniques (MIMO) and higher QAM modulation together allow 4G LTE networks to deliver up to 3 Gbps of peak download speeds with LTE Advanced Pro. Other, less spectrally efficient modulation schemes like QPSK (Quadrature Phase Shift Keying) can also be used in LTE, depending on the capacity and coverage requirements.
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.
