Modulation is one of the most fundamental concepts in mobile communications and is applied multiple times during each transmission of a mobile signal. One of my embarrassing memories is when I had to explain the concept of modulation to an experienced telecom professional during a job interview just after finishing my Bachelors. Even though my explanation of ‘modulation’ was not completely incorrect, but I ended up throwing in a little too many telecom buzzwords in a single sentence and probably not in the right order. While my buzzwords did not exactly pay off that day, they did urge me to learn about modulation in a slightly different way. If you look up on the internet, you are likely to come across an analogy of a paper wrapped around a piece of rock to outline the concept of modulation. However, this post intends not to use any such analogy but instead focus on the real application of modulation in cellular transmission.
Mobile networks communicate wirelessly through electromagnetic waves that travel in the air between mobile operators’ antennas and our mobile phones. The 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 highly efficient manner.
What is modulation in mobile communications?
Modulation means modifying a signal which naturally raises a question as to 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 for achieving better data rates from the available frequency spectrum, analogue modulation is still used in the network for the transmission of 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 – The signal that contains the actual information data is called the modulating signal. The process of putting the information inside the carrier signal is called modulation, which requires some modifications to the carrier signal’s characteristics. 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.
Carrier signal– 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 like an empty but secure envelope where we can put information for successful delivery. Carrier signals operate at high frequencies, as shown in the visual representation below.
Modulation is the process of encoding the information signal (data) with the carrier signal to transmit the information securely, efficiently and effectively without interference. It involves making changes to the carrier signal’s characteristics such as amplitude, frequency and phase (angle) in line with the information signal. Modulation can be analogue and digital, and both forms of modulation are used in modern mobile-cellular networks.
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).
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 real 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 the amplitude, frequency, and phase (angle). The diagram below can give you a quick visual overview of where frequency, amplitude and phase fit in.
What digital modulation technique is used by LTE?
Digital modulation means transmitting digital data over an analogue signal. Even though the 4G LTE networks are digital, the 4G radio signals between the network and the mobile are analogue. Even though there are many digital modulation techniques such as Amplitude Shift Keying (ASK), and Frequency Shift Keying (FSK), 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” bit 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, then amplitude will go to the highest point whilst keeping all other characteristics (i.e. frequency and phase) the same. Below a simplified diagram to demonstrate this concept.
QAM is a combination of amplitude and phase modulation of the carrier signal. It is the most modern of the digital modulation techniques used in LTE and 5G and WiFi6 networks. There are various modulation orders in QAM, and the higher the order, the higher the efficiency, i.e. the signal can carry more data. The 4G LTE networks have adopted QAM because it is spectrally efficient and allows higher data rates in the given frequency spectrum. For technical accuracy, it is worth mentioning that the QAM technique can be used for analogue modulation also, but in this context, the modulation is digital.
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, it means that if an LTE radio unit was to employ 16-QAM for the transmission of digital data, each sub-carrier could carry 4 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 all the same steps but in reverse order.
The LTE Advanced networks employ 256-QAM, but the original LTE release in 2009 only supported 16 and 64-QAM. Higher QAM modulation order combined with larger bandwidths and superior antenna techniques allows the later versions of LTE to achieve much higher data rates than the earlier technologies. Other, less spectrally efficient modulation schemes like QPSK (Quadrature Phase Shift Keying) can also be used depending on the capacity and coverage requirements.