Communications via satellites have two unique characteristics: the ability to cover the globe with a flexibility that cannot be duplicated with terrestrial links, and the availability of large bandwidth for intercontinental communications. In satellite communication, the modulated carrier carrying information is transmitted through space as a radio wave to a radio receiver.
Modulation is the process of converting data or baseband signals into electrical signals optimized for transmission. Modulation, in general, is achieved by varying some characteristic of a periodic waveform, called the carrier signal, in accordance with another separate signal called the modulation signal that typically contains information to be transmitted.
Modulation is usually applied to electromagnetic signals: radio waves, lasers/optics and computer networks. For example, the modulation signal might be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal representing a sequence of binary digits, a bitstream from a computer. The carrier is higher in frequency than the modulation signal. The purpose of modulation is to impress the information on the carrier wave, which is used to carry the information to another location.
A sinusoid has just three features that can be used to distinguish it from other sinusoids-phase, frequency, and amplitude. For the purpose of radio transmission, modulation is defined as the process whereby the phase, frequency, or amplitude of a radio frequency (RF) carrier wave is varied in accordance with the information to be transmitted.
Modulation techniques are roughly divided into four types: Analog modulation, Digital modulation, Pulse modulation, and Spread spectrum method.
In analog modulation an analog modulation signal is impressed on the carrier. Examples are amplitude modulation (AM) in which the amplitude (strength) of the carrier wave is varied by the modulation signal, and frequency modulation (FM) in which the frequency of the carrier wave is varied by the modulation signal. These were the earliest types of modulation, and are used to transmit an audio signal representing sound, in AM and FM radio broadcasting.
FM has been largely used in satellite communications. It is particularly convenient when a single carrier per transponder is used and where the constant envelope of the FM signals allows the power amplifiers to operate at saturation, thus making maximum use of the available power
Modulation can be divided into single carrier modulation, by which the carrier occupies the entire bandwidth (i.e. amplitude, frequency, and phase), and a multicarrier scheme that modulates and transmits different data on multiple carriers. In addition, there is a pulse modulation technique used to change the pulse width and spread spectrum method that spreads the signal energy over a wide band.
Noise consists of all unwanted contributions whose power adds to the wanted carrier power. It reduces the ability of the receiver to reproduce correctly the information content of the received wanted carrier.
The origins of noise are as follows:
—the noise emitted by natural sources of radiation located within the antenna reception area;
—the noise generated by components in the receiving equipment.
Carriers from transmitters other than those which it is wished to receive are also classed as noise. This noise is described as interference.
Harmful noise power is that which occurs in the bandwidth B of the wanted modulated carrier. A popular noise model is that of white noise, for which the power spectral density N0 is constant in the frequency band involved. The equivalent noise power N (W) captured by a receiver with equivalent noise bandwidth BN, usually matched to B, is given by N = N0 * BN
Digital Communication systems
More recent systems use digital modulation, which impresses a digital signal consisting of a sequence of binary digits (bits), a bitstream, on the carrier. Like analog modulation, in digital modulation systems carrier parameters like phase, frequency or amplitude are varied with information signals.
In digital baseband modulation (line coding) used to transmit data in serial computer bus cables and wired LAN computer networks such as Ethernet, the voltage on the line is switched between two amplitudes (voltage levels) representing the two binary digits, 0 and 1, and the carrier (clock) frequency is combined with the data. In frequency-shift keying (FSK) modulation, used in computer buses and telemetry, the carrier signal is periodically shifted between two frequencies that represent the two binary digits.
Modern satellite communication has become predominantly digital communications because of the ever-growing demand for data communication, and because digital transmission offers data processing options and flexibilities not available with analog transmission.
A modulator is a device or circuit that performs modulation. A demodulator (sometimes detector) is a circuit that performs demodulation, the inverse of modulation. A modem (from modulator-demodulator) can perform both operations. The frequency band occupied by the modulation signal is called the baseband, while the higher frequency band occupied by the modulated carrier is called the passband.
While the transmitter, consists of a frequency up-conversion stage, a high-power amplifier, and antenna, the receiver portion is occupied by an antenna, a low-noise front-end amplifier, and a down-converter stage, typically to an intermediate frequency (IF).
Digital communication systems can have other optional signal processing steps. Source encoding, as defined here, removes information redundancy and performs analog-to-digital (A/D) conversion. Encryption prevents unauthorized users from understanding messages and from injecting false messages into the system. Channel coding can, for a given data rate, improve the error (PE) performance at the expense of power or bandwidth, reduce the system bandwidth requirement at the expense of power or PE performance, or reduce the power requirement at the expense of bandwidth or PE performance.
Frequency spreading renders the signal less vulnerable to interference (both natural and intentional) and can be used to afford privacy to the communicators. Multiplexing and multiple access combine signals that might have different characteristics or originate from different, sources.
Digital Modulation types
The most fundamental digital modulation schemes are amplitude-shift keying (ASK), phase-shift keying (PSK), frequency-shift keying (FSK), and quadrature amplitude modulation (QAM) as shown below.
In QAM, an in-phase signal (or I, with one example being a cosine waveform) and a quadrature-phase signal (or Q, with an example being a sine wave) are amplitude modulated with a finite number of amplitudes and then summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK.
In the general M-ary signaling case, the processor accepts k source bits at a time and instructs the modulator to produce one of an available set of M = 2 **k waveform types for T seconds, the symbol period. A digital waveform is taken to mean a voltage or current waveform
representing a digital symbol. This k number of bits comprises the symbol that is represented by the particular phase, frequency, or amplitude.
In practice, M is usually a nonzero power of two (2,4,8,1.6..,). Binary modulation, where k = 1, is just a special case of M-ary modulation. Since theM symbols can be represented by k = log2M binary digits (bits), the data rate can be expressed as R = ( l / T ) log2M = k/T b/s. At various points along the signal route, noise corrupts the waveform s( t) so that its reception must be termed an estimate s(t).
Satellite Communication Requirements
With strong demand for faster data throughput, satellite communications use high-order modulation schemes to improve their spectral efficiency. However, satellite channel impairments such as large path losses, delays, and Doppler shifts pose severe challenges to the realization of a satellite network. The modulation techniques for satellite communications require not only faster data rates but also minimizing the impacts of the channel impairments.
In satellite transmission, RF power amplifiers often operate at their compression levels to maximize conversion efficiency. Operating at compression levels causes AM/AM and AM/PM distortion, as shown in Figure. For example, the I/Q constellation outer points have higher output power levels, and the compression is because of the saturated output power in the RF power amplifier. Thus, nonlinear amplifiers require a modulation scheme tolerant to distortion. Also, the higher output power creates more noise to the signal.
Nonconstant Envelope Digital Modulation Schemes
Quadrature amplitude modulation (QAM) is a nonconstant modulation that changes both phase and amplitude to increase spectral efficiency. Figure illustrates the constellation diagram of 16PSK and 16QAM. 16QAM increases the distance between the constellation points and has better resistance to signal impairments. However, 16QAM also increases the amplitude levels to three (rings) compared with 16PSK. RF power amplifiers require a wider linear range for nonconstant modulation schemes.
Satellite equipment must be capable of transmitting at a high power level while maintaining high output linearity. Also, the higher modulation schemes enable higher data throughput but are sensitive to signal impairments.
A more complicated digital modulation method that employs multiple carriers, orthogonal frequency division multiplexing (OFDM), is used in WiFi networks, digital radio stations and digital cable television transmission.
Constant Envelope Digital Modulation Schemes
The constant envelop class is generally considered as the most suitable for the satellite communications because it minimizes the effect of non-linear amplification in the high power amplifier like TWTA (Travelling Wave Tube Amplifier) or KTA (Klystron Tube Amplifier).
The constant envelope modulation schemes such as FSK and PSK are the most suitable for satellite communications because they minimize the effect of nonlinear amplification in the high-power amplifier.
Phase modulation, or phase-shift keying (PSK), is particularly well suited to satellite links. In fact, it has the advantage of a constant envelope and, in comparison with frequency-shift keying (FSK), it provides better spectral efficiency (number of bits/s transmitted per unit of radio-frequency bandwidth. In PSK system, envelope is constant but the phase changes discontinuously from symbol to symbol.
Depending on the number m of bits per symbol, different M-ary phase-shift keying modulations are considered:
—The simplest form is basic two-state modulation (M= 2), called ‘binary phase shift keying’ (BPSK) with standard direct mapping. When differential encoding is considered it is called ‘differentially encoded BPSK’ (DE-BPSK). It is of interest because it enables a simplified
—If two consecutive bits are grouped to define the symbol, a four-state modulation (M = 4) is defined, called ‘quadrature phase shift keying’ (QPSK) with direct mapping. Differentially encoded QPSK (DE-QPSK) could be envisioned, but it is not used in practice (except for the specific case of p/4 QPSK, as differential demodulation displays significant performance degradation compared to standard coherent demodulation when M is larger than 2.
—Higher-order modulation (M=8, 8-PSK;M= 16, 16-PSK; etc.) are obtained with m= 3, 4, etc. bits per symbol. As the order of the modulation increases, the spectral efficiency increases as the number of bits per symbol. On the other hand, higher-order modulations require more energy per bit (Eb) to get the same bit error rate (BER) at the output of the demodulator.
Figure illustrates the constellation diagrams of binary PSK (BPSK), quadrature PSK (QPSK), and 8PSK. They transmit 1, 2, and 3 bits per symbol, correspondingly. For higher-order PSK, the constellation points are closer to each other, and the system is more sensitive to channel impairments. For FSK, 4FSK (2 bits per symbol) has higher spectral efficiency than 2FSK’s but the smaller frequency deviation will cause a bad sensitivity in the receiver.
Resist Nonlinear Distortion with APSK
Satellite communications employ amplitude phase-shift keying (APSK) to resist nonlinear distortion. Figure illustrates a constellation diagram for APSK and QAM modulation schemes. The APSK’s states are in rings such that the amplitude compression is the same in a specific ring. The 16APSK constellation has only two amplitudes (rings), whereas 16QAM has three amplitudes. The 32APSK constellation has three amplitudes versus five in 32QAM. More amplitude levels make the rings closer together and more difficult to compensate for nonlinearities.
There are several variable parameters for APSK modulation such as the number of rings, number of symbols on a ring, and spacing between rings. A designer can also reach a balance between lower peak-to-average power ratio (PAPR) and better resistance to distortion.
For coherent detection, product integrators (or their equivalents) are used; for noncoherent detection, this practice is generally inadequate because the output of a product integrator is a function of the unknown phase angle. However, if we assume that phase varies slowly enough to be considered constant over two, period times (ZT), the relative phase difference between two successive waveforms is independent of phase angle.
In QPSK modulation, the voltages which modulate the two carriers in quadrature change simultaneously, and the carrier can be subjected to a phase change of 180. In a satellite link that includes filters, large phase shifts cause amplitude modulation of the carrier. The non-linearity of the channel transforms these amplitude variations into phase variations that degrade the performance of the demodulator. Several variants of QPSK modulation have been proposed to limit the amplitude of the phase shift.
Two digital modulation schemes of special interest for use on nonlinear bandlimited channels are called staggered (or offset) quadraphase PSK (SQPSK or OQPSK), and minimum shift keying (MSK). Both techniques retain low spectral sidelobe levels while allowing’ efficient detection performance.
With offset QPSK (OQPSK), also called staggered QPSK (SQPSK), the Ik and Qk modulating bit streams are offset by half a symbol duration, i.e. Ts=2 ¼ Tc, the duration of a bit. The phase of the carrier changes every bit period but only +- 90 or 0. This avoids the possible 180 phase shift associated with the simultaneous change in the two bits in the modulating dibit with QPSK. It results in a reduced envelope variation when the modulated carrier is filtered.
OQPSK uses rectangular pulse shapes, and MSK uses half-cycle sinusoid pulse shapes. Because of the sinusoidal pulse shaping in MSK, it can be viewed as continuous-phase FSK with a frequency deviation equal to one-half the bit rate.
For nonorthogonal schemes, such as MPSK signaling, one often uses a binary-to-M-ary code such that binary sequences corresponding to adjacent symbols (phase shifts) differ in only one bit position; one such code is the Gray code. When a M-ary symbol error occurs, it is more likely that only one of the k input bits will be in error. Ps = PE/k (for PE << 1) . For convenience, this discussion is restricted to BPSK (k=1, M = 2) modulation. For the binary case, the symbol error probability equals the bit error probability.
Modulation Schemes Analysis
The key feature of a digital communications system (DCS) is that it sends only a finite set of messages, in contrast to an analog communications system, which can send an infinite set of messages. In a DCS, the objective at the receiver is not to reproduce a waveform with precision; it is instead to determine from a noise-perturbed signal which of the finite set of waveforms had been sent by the transmitter. An important measure of system performance is the average number of erroneous decisions made or the probability of error (PE).
When the receiver exploits knowledge of the carrier wave’s phase reference to detect the signals, the process is called coherent detection; when it does not have phase reference information, the process is called noncoherent. In ideal coherent detection, prototypes of the possible arriving signals are available at the receiver. These prototype waveforms exactly replicate the signal set in every respect, even RF phase. The receiver is then said to be phase-locked to the transmitter.
For coherent detection, product integrators (or their equivalents) are used During detection, the receiver multiplies and integrates (correlates) the incoming signal with each of its prototype replicas. Noncoherent modulation refers to systems designed to operate with no knowledge of phase; phase estimation processing is not required. Reduced complexity is the advantage over coherent systems, and increased PE is the trade-off.
The analysis of all coherent demodulation or detection schemes involves the concept of distance between an unknown received waveform and a set of known waveforms. Since any arbitrary waveform set, as well as noise, can be represented as a linear combination of orthonormal waveforms, we can use Euclidean-like distance in such an orthonormal space, as a decision criterion for the detection of any signal set in the presence of AWGN. The vector or point n is a random vector; hence, r is also a random vector. For example in binary modulation type, the detector’s task after receiving signal r is to decide which of the signal sl or s2 was actually transmitted. The decision stage must decide which signal was transmitted by measuring its location within the signal space.
The method is usually to decide upon the signal classification that yields the minimum PE, although other strategies are possible. The noise is modeled as a random process with zero mean and a Gaussian distribution. For the case where M equals two signal classes, with S I and s2 being equally likely and the noise being AWGN, the minimum-error decision rule turns out to be Choose the signal class such whose distance or norm from the received signal is minimum.
Symbol detection in a realizable system, even in the absence of noise, suffers from intersymbol interference, ISI; the tail of one pulse spills over into adjacent symbol intervals so as to interfere with correct detection: Nyquist showed that the theoretical minimum bandwidth needed to transmit x symbols per second (Symbols/s) without ISI is x / 2 Hz; this is a basic theoretical constraint, limiting the designer’s goal to expend as little bandwidth as possible. In practice, it typically requires x Hz bandwidth for the transmission of x symbols/s.
At large SNRs, it can be seen that there is approximately a 4-dB difference between the best (coherent PSK) and the worst (noncoherent FSK).In some cases, 4 dB is a small price to pay for the implementation simplicity gained in going from a coherent PSK to a noncoherent FSK; however, for some applications, even a 1-dB saving is worthwhile. There are other considerations besides PB and system complexity; for example, in some cases (such as randomly fading propagation conditions), a noncoherent system is more robust and desirable because there may be difficulty in establishing a coherent reference.
Notice also that the location of the MPSK points indicates that BPSK (M = 2) and QPSK (M = 4) require the same Eb/No. That is, for the same value of Eb/No QPSK has a bandwidth efficiency of 2 b/s/Hz, compared to 1 b/s/Hz for BPSK. This unique feature stems from the fact that QPSK is effectively a composite of two BPSK signals, transmitted on waveforms orthogonal to one another and having the same spectral occupancy. Each of the two orthogonal BPSK signals comprising QPSK yields half the bit rate and half the signal power of the QPSK signal; hence the required Eb/No for a given PB is identical for BPSK and QPSK.
In case of DPSK modulation, the carrier phase of the previous signaling interval is used as a phase reference for demodulation. Its use requires differential encoding of the message sequence at the transmitter since the information is carried by the difference in phase between two successive waveforms.
One way of viewing the difference between coherent PSK and DPSK is that the former compares the received signal with a clean reference wherein the latter however two noisy signals are compared with each other. Thus, we might say there is twice as much noise in DPSK as in PSK. Consequently, DPSK manifests a degradation of approximately 3 dB when compared with PSK; this number decreases rapidly with increasing signal-to-noise ratio. In general, the errors tend to propagate (to adjacent period times) due to the correlation between signaling waveforms. The trade-off for this performance loss is reduced system complexity.
(Energy per bit )Eb/No (Noise Density) = (Signal Power) S / N(Noise Power) X (Signal Bandwidth) W/R (Data Rate)
The dimensionless ratio Eb/No (required to achieve a specified Pa), is uniformly used for characterizing digital communications system performance. Therefore, required Eb /No can be considered a metric that characterizes the performance of one system versus another; the smaller the required Eb/No, the more efficient the system modulation and detection process.
System trade-offs are fundamental a to the digital communications designs. The goals of the designer are: (1) to maximize transmission bit rate R, (2) to minimize the probability of bit error PB, (3) to minimize required power, or relatedly, to minimize required bit energy per noise density Eb/No, (4) to minimize required system bandwidth W, (5) to maximize system utilization, that is, to provide reliable service for a maximum number of users, with minimum delay and maximum resistance to interference, and (6) to minimize system complexity, computational load, and system cost.
The designer usually seeks to achieve all these goals. However, goals (1) and( 2) are clearly in conflict with goals (3) and (4); they call for simultaneously maximizing R, while minimizing PB, Eb/No, and W. There are several constraints and theoretical limitations that necessitate the trading-off of any one requirement with each of the others. Some of the constraints are: the Nyquist theoretical minimum bandwidth requirement, the Shannon-Hartley capacity theorem, the Shannon limit, government regulations (for example, frequency allocations), technological limitations (for example, state-of-the-art components), and other system requirements (for example, satellite orbits).
Capacity relationship (Shannon-Hartley theorem)
It is possible to transmit information over such a channel at a rate R, where R < C, with an arbitrarily small error rate by using a sufficiently complicated coding scheme. For a rate R > C, it is not possible to find a code which can achieve an arbitrarily small error rate.
The ordinate R/W is a measure of how much data can be transmitted in a specified bandwidth within a given time therefore reflects how efficientlty the bandwidth resource is utilized. The abscissa is Eb/No in decibels. Notice that for MPSK modulation, R/W increases with increasing M; however, for MFSK modulation, R/W. decreases with increasing M.
For orthogonal signal sets, such as FSK modulation, M-ary signaling, compared to binary, can provide an improved Ps performance or a reduced Eb/No requirement, at the cost of an increased bandwidth requirement. For non-orthogonal signal sets, such as multiphase shift keyin(gM PSK) modulation, M-ary signaling, compared to binary, can provide a reduced bandwidth requirement, at the cost of a degraded-PB performance or an increased &,/No requirement.
Once modulation, coding scheme, and available Eb/No are chosen, system operation is characterized by a particular point in the error-rate plane. Possible trade-offs can be viewed as changes in the operating point on one of the curves, or as changes in the operating point from one curve to another curve of the family.
We are not as free to make trade-offs as we might like; Government regulations dictate the choice of frequencies, bandwidths, transmission power levels, and-in the case of satellites, orbit selection. The satellite orbit and geometry of coverage fixes the satellite antenna gain. Technological state-of-the-art constrains such items as satellite power transmission and earth station antenna gain. There may be other system requirements (for example, the need to operate under scintillation or interference conditions) that can influence the choice of modulation and coding.
Enhance data rate using OFDM
The orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier technique that possesses many unique advantages over single-carrier approaches. The technique has been adopted for many broadband wireless communication standards, such as 4G/5G, Wi-Fi, digital video broadcast for terrestrial and satellite communication systems.
OFDM uses many closely spaced orthogonal subcarrier signals to transmit data in parallel. That process provides better spectral efficiency than traditional digital modulation schemes, such as QAM and PSK, and robustness against channel linear distortion. Figure shows a single OFDM carrier (the left plot) and multiple subcarriers (the right plot). The peak of each subcarrier occurs at zero crossings of the others. The signal is orthogonal in the frequency domain, and each subcarrier does not interfere with the others. The subcarriers can apply different modulation formats and channel coding, depending on the noise and interference level of individual sub-bands that provide a robust communication link.
However, the OFDM signal has a higher PAPR than traditional modulation schemes, requiring a large back off to avoid the compression at a high output power level. Nonlinear effects generated by the high-power amplifier may introduce more distortions to a satellite system that causes a system failure. Therefore, characterizing the distortion performance of satellite RF components is essential for making a good system design.
Trellis Coded Modulation
Trellis coded modulation (TCM) combines modulation and encoding processes to achieve better efficiency without increasing the bandwidth. Bandwidth-constrained channels operate in the region R/W > 1, where R = data rate and W = bandwidth available.
For such channels, digital communication systems use bandwidth efficient multilevel phase modulation. For example, phase shift keying (PSK), phase amplitude modulation (PAM), or quadrature amplitude modulation (QAM).
When you apply TCM to a bandwidth-constrained channel, you see a performance gain without expanding the signal bandwidth. An increase in the number of signal phases from four to eight requires approximately 4dB in additional signal power to maintain the same error rate. Hence, if TCM is to provide a benefit, the performance gain of the rate 2/3 code must overcome this 4dB penalty. If the modulation is an integral part of the encoding process and is designed in conjunction with the code to increase the minimum Euclidian distance between the pairs of coded signals, the loss from the expansion of the signal set is easily overcome and significant coding gain is achieved with relatively simple codes.
8-PSK is a fixed envelops modulation system with greater efficiency in bandwidth. The first TCM application to satellite transmission occurred with the 8-PSK trellis codes. Using a 72 M Hz transponder bandwidth, transmission at up to 155.52 Mbits/s have been realized. TCM with 8-PSK provides high bit rate which is essential in future for the high bit rate application like images, TV and HDTV services over the satellite transmission . There are various forms of TCM like PTCM (pragmatic) and PPTCM (Punctured PTCM).
Trellis coded 16-Phase Shift Keying (PSK) and 16-Quadrature Amplitude Modulation (QAM) modulation systems are used for satellite communications. But the fact is that when the modulation level increases, the constant envelop M-ary PSK modulation systems are inferior to the QAM systems. On the other hand, QAM suffers more distortion in the non-linear satellite communications channels . QAM is suitable for geostationary orbit satellite channel with only Gaussian impairments because of being amplitude and phase modulated signal, QAM is more sensitive to the effects of interference and fading than MPSK.
Wavelet Packet Modulation (WPM)
WPM is a multicarrier modulation system like OFDM using Discrete Wavelet Transform (DWT). DWT is a transformation technique which is a presentation of the composite signal in time and frequency domain. So in WPM, packets structure is divided into time and frequency domain. So when any interference is realize, in TDMA or FDMA system all packets are degraded but in case of WPM, packets are keep away from the interference with the help of providing the appropriate packet structure .
Both WPM and OFDM are multicarrier modulation system but the difference is OFDM uses FFT to combine the transmission where WPM use DWT and Bit Error Rate of Wavelet Packet Modulation is much better than the OFDM. Similarity between these modulation systems is High Peak to Average Power Ratio (PAPR). For getting better performance of OFDM, single carrier OFDM (SC-OFDM) is proposed where decreasing PARP was the main goal. It is seen that the PARP is also high in WPM so SC-OFDM can be used to improve efficiency of the WPM. SC-WPM also can be used by exploiting the principal of SC-OFDM.
Some experiment shows that the Wavelet Packet Modulation is the effective modulation systems for satellite communications and with lower PARP, SC- Wavelet Packet Modulation (WPM) would enable the broadband satellite communications. PARP performance of SC-WPM is superior to WPM and OFDM. The Bit Error Rate performance of the WPM is better than OFDM.
Multi-Level Gaussian Frequency Shift Keying (MGFSK)
For specific reasons, MGFSK modulation systems uses in the satellite communications. It also exploiting the technique of narrow band FM which has constant envelop throughout the signal. MGFSK is suitable for satellite communications where the transponders are in saturations and it is also useful for the transmitter where output amplifier is also saturated. As compared to the 8PSK bandwidth efficiency (3 bit/s/Hz), MGFSK providing bandwidth efficiency is 6 bit/s/Hz . BW efficiency of MGFSK is very similar to the 64 QAM but 64 QAM is not feasible to use in satellite communication because it requires highly linear and well-equalised satellite channels. The key applications of MGFSK are in those satellites which dedicated for ISP traffic, news gathering satellites and some specific military applications.
Error Vector Magnitude (EVM) measurements provide a simple and quantitative figure-of-merit for a digital modulation signal. EVM is the Root Mean Square (RMS) of the error vectors computed and expressed as a percentage of the EVM Normalization Reference. The error vector originates from the detected point of the I/Q reference signal vector to the I/Q measured-signal vector.
n = symbol index
N = number of symbols
Ierr = I Ref – I Meas
Qerr = Q ref – Q Meas
The errors may result from phase noise of local oscillators, noise from power amplifiers, I/Q modulator impairments, and many other sources. EVM is a major indicator that evaluates the modulation quality of vector signal generators. For example, a high-performance signal generator can provide EVM as low as 0.4% for a 160 MHz 802.11ac signal. Low EVM performance provides more test margin for receiver sensitivity and component characteristics tests.
EVM (dB) = 20 log10 (EVM (%))
Most communications systems optimize efficiencies in system designs, including spectral, power, and cost. The selection of modulation schemes for satellite communications depends on the communication channels, hardware limitations, and data throughput requirements. Also, both custom APSK and OFDM modulation schemes bring in test challenges – generating and analyzing custom, proprietary modulation schemes. Next post, we will discuss how to simplify the custom signal generation and analysis.