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Last updated on 2024年5月30日

How can higher order modulation improve spectral efficiency in SATCOM?

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Satellite communications (SATCOM) are essential for many applications, such as broadcasting, navigation, and remote sensing. However, SATCOM also face challenges, such as limited bandwidth, high power consumption, and signal degradation. One way to overcome these challenges is to use higher order modulation (HOM) schemes, which can increase the spectral efficiency and data rate of SATCOM systems. In this article, we will explain what HOM is, how it works, and what are its benefits and trade-offs for SATCOM.

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Cmdr (Dr.?) Reji Kurien Thomas , FRSA, MLE?

I Empower Sectors as a Global Tech & Business Transformation Leader| Stephen Hawking Award 2024| Harvard Leader | UK…
John Burger

Systems Engineer & Program Manager

1 What is HOM?

Modulation is the process of changing the characteristics of a carrier wave, such as its amplitude, frequency, or phase, to encode information. The number of different states that a carrier wave can have is called the modulation order. For example, binary phase shift keying (BPSK) has two states, 0 and 1, corresponding to two phases of the carrier wave. HOM schemes use more than two states, such as quadrature phase shift keying (QPSK), which has four states, or 16-ary quadrature amplitude modulation (16-QAM), which has 16 states. HOM schemes can transmit more bits per symbol, thus increasing the data rate and spectral efficiency of the system.

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John Burger

Systems Engineer & Program Manager
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I would highlight the number of bits per modulation scheme and how that corresponds to the number of states. For instance, BPSK has 1 bit, which has two states (2 to the power of 1). QPSK has 2 bits, for four states (2 to the power of 2). 8PSK has 3 bits, eight states. 16-QAM (or 16PSK) has 4 bits, 16 states, etc. That helps illustrate the exponential increase in data (states) with increasing modulation scheme (bits), and directly addresses how those HOM schemes result in more bits per symbol.

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2 How does HOM work?

To use HOM schemes, the transmitter and the receiver need to have a common reference for the carrier wave, such as its frequency and phase. The transmitter encodes the data into symbols, which are mapped to different states of the carrier wave. The receiver demodulates the received signal by comparing it with the reference carrier wave and decoding the symbols into data. The mapping of symbols to states can be represented by a constellation diagram, which shows the amplitude and phase of each state on a complex plane. The more states there are, the more crowded the constellation diagram becomes, and the more difficult it is to distinguish between them.

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3 What are the benefits of HOM?

The main benefit of HOM is that it can increase the spectral efficiency and data rate of SATCOM systems. Spectral efficiency is the measure of how efficiently a given bandwidth is used to transmit data. Data rate is the amount of data that can be transmitted per unit of time. HOM schemes can achieve higher spectral efficiency and data rate than lower order modulation schemes by transmitting more bits per symbol. For example, QPSK can transmit twice as many bits per symbol as BPSK, and 16-QAM can transmit four times as many bits per symbol as QPSK. This means that HOM schemes can use the same bandwidth and transmit more data, or use less bandwidth and transmit the same data, compared to lower order modulation schemes.

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4 What are the trade-offs of HOM?

The main trade-off of HOM is that it requires higher signal-to-noise ratio (SNR) and higher power to maintain the same bit error rate (BER) as lower order modulation schemes. SNR is the ratio of the signal power to the noise power, and BER is the probability of a bit being transmitted or received incorrectly. HOM schemes are more sensitive to noise and interference, because the states of the carrier wave are closer together and more easily confused. Therefore, HOM schemes need higher SNR and higher power to ensure reliable communication. For example, 16-QAM needs about 6 dB more SNR and 4 dB more power than QPSK to achieve the same BER.

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John Burger

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I am not sure about quantifying SNR and Power as two different things. In most normal cases (equipment operating linearly), increasing the transmitter power by 4 dB would result in the signal being received on the other side with a 4 dB SNR increase, not 6 dB as is said in the last sentence. For the purposes of this high level article, the Power increase and resulting SNR increase should be 1-for-1.

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5 How to choose the best HOM scheme for SATCOM?

The choice of the best HOM scheme for SATCOM depends on several factors, such as the available bandwidth, the required data rate, the channel conditions, and the system design. Generally, the higher the modulation order, the higher the spectral efficiency and data rate, but also the higher the SNR and power requirements. Therefore, the optimal HOM scheme is the one that maximizes the spectral efficiency and data rate while satisfying the SNR and power constraints. For example, if the bandwidth is limited and the channel is noisy, a lower order modulation scheme may be preferable. If the bandwidth is abundant and the channel is clear, a higher order modulation scheme may be advantageous. The system design also affects the choice of the HOM scheme, as it determines the complexity and cost of the transmitter and receiver components, such as filters, amplifiers, mixers, and converters.

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John Burger

Systems Engineer & Program Manager
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I would add "antenna size" to the last sentence's list, as increasing the size of your antenna reflector will typically increase both your effective transmitted power (without actually increasing the transmitter power level) and your receive SNR. That is often a way to step up to a HOM without changing your ground equipment (amplifier and receiver) or satellite resources (bandwidth).

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Cmdr (Dr.?) Reji Kurien Thomas , FRSA, MLE?

I Empower Sectors as a Global Tech & Business Transformation Leader| Stephen Hawking Award 2024| Harvard Leader | UK House of Lord's Awardee | Fellow Royal Society | CyberSec | CCISO CISM CCNP-S CEH
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Higher-order modulation enables flexible resource allocation, allowing dynamic adjustment of modulation schemes based on the specific requirements of different users or applications. During the development of a multi-user SATCOM system for disaster response, we implemented adaptive modulation to allocate higher-order schemes like 64-QAM for high-priority data streams. This flexibility ensured optimal use of the available bandwidth, enhancing the system's overall efficiency & responsiveness during emergencies. Higher-order modulation complements advanced multiple access techniques such as OFDMA (Orthogonal Frequency-Division Multiple Access) & NOMA (Non-Orthogonal Multiple Access), which further improve spectral efficiency.

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How can higher order modulation improve spectral efficiency in SATCOM?

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1 What is HOM?

2 How does HOM work?

3 What are the benefits of HOM?

4 What are the trade-offs of HOM?

5 How to choose the best HOM scheme for SATCOM?

6 Here’s what else to consider

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