Fundamental to all wireless communications is modulation, the process of impressing the data to be transmitted on the radio carrier. Most wireless transmissions today are digital, and with the limited spectrum available, the type of modulation is more critical than it has ever been. The main goal of modulation today is to squeeze as much data into the least amount of spectrum possible.
That objective, known as spectral efficiency, measures how quickly data can be transmitted in an assigned bandwidth. Multiple techniques have emerged to achieve and improve spectral efficiency New Versions of Ethernet You Should Know About Amplitude Shift Keying ASK and Frequency Shift Keying FSK There are three basic ways to modulate a sine wave radio carrier: modifying the amplitude, frequency, or phase.
More sophisticated methods combine two or more of these variations to improve spectral efficiency. These basic modulation forms are still used today with digital signals Three basic digital modulation formats are still very popular with low-data-rate short-range wireless applications: amplitude shift keying aon-off keying band frequency shift keying c.
These waveforms are coherent as the binary state change occurs at carrier zero crossing points Figure shows a basic serial digital signal of binary zeros and ones to be transmitted and the corresponding AM and FM signals resulting from modulation.
There are two types of AM signals: on-off keying OOK and amplitude shift keying ASK. In Figure 1athe carrier amplitude is shifted between two amplitude levels to produce ASK. In Figure 1bthe binary signal turns the carrier off and on to create OOK. AM produces sidebands above and below the carrier equal to the highest frequency content of the modulating signal. The bandwidth required is two times the highest frequency content including any harmonics for binary pulse modulating signals.
Frequency shift keying FSK shifts the carrier between two different frequencies called the mark and space frequencies, or f m and f s Fig. The bandwidth produced is a function of the highest modulating frequency including harmonics and the modulation index, which is: Smaller values of m produce fewer sidebands. First, select data rates, carrier frequencies, and shift frequencies so there are no discontinuities in the sine carrier when changing from one binary state to another.
These discontinuities produce glitches that increase the harmonic content and the bandwidth. The idea is to synchronize the stop and start times of the binary data with when the sine carrier is transitioning in amplitude or frequency at the zero crossing points. This is called continuous phase or coherent operation. A second technique is to filter the binary data prior to modulation. This rounds the signal off, lengthening the rise and fall times and reducing the harmonic content.
Special Gaussian and raised cosine low pass filters are used for this purpose. The proper binary of BPSK requires the signal to be compared to a sine carrier of the same phase. The phase changes when the binary state switches so the signal is coherent. A simpler version is differential BPSK or DPSK, where the received bit phase is compared to the phase of the previous bit signal.
The two phases are added together to produce the final signal. Each unique pair of bits generates a carrier with a different phase Table Figure 3a illustrates QPSK with a phasor diagram where the phasor represents the carrier sine amplitude peak and its position indicates the phase.
A constellation diagram in Figure 3b shows the same information. QPSK is very spectrally efficient since each carrier phase represents two bits of data. The maximum data rate is directly proportional to the bandwidth and logarithmically proportional the SNR. Noise greatly diminishes the data rate for a given bit error rate BER.
Another key factor is the baud rate, or the number of modulation symbols transmitted per second. The term symbol in modulation refers to one specific state of a sine carrier signal. It can be an amplitude, a frequency, a phase, or some combination of them.
Basic binary transmission uses one bit per symbol. In ASK, a binary 0 is one amplitude and a binary 1 is another amplitude. In FSK, a binary 0 is one carrier frequency and a binary 1 is another frequency.
In each of these cases there is one bit per symbol. With one symbol per bit, the baud rate is the same as the bit rate.
However, if you transmit more bits per symbol, the baud rate is slower than the bit rate by a factor equal to the number of bits per symbol. For example, if 2 bits per symbol are transmitted, the baud rate is the bit rate divided by 2. QPSK can be referred to as 4-PSK because there are four amplitude-phase combinations. By using smaller phase shifts, more bits can be transmitted per symbol.
Some popular variations are 8-PSK and 16-PSK. This arrangement results in a transmission of 4 bits per symbol.
While Multiple Phase Shift Keying M-PSK is much more spectrally efficient, the greater the number of smaller phase shifts, the more difficult the signal is to demodulate in the presence of noise.
The benefit of M-PSK is that the constant carrier amplitude means that more efficient nonlinear power amplification can be used Quadrature Amplitude Modulation QAM The creation of symbols that are some combination of amplitude and phase can carry the concept of transmitting more bits per symbol further.
This method is called quadrature amplitude modulation QAM. For example, 8QAM uses four carrier phases plus two amplitude levels to transmit 3 bits per symbol. In this example, there are three amplitudes and 12 phase shifts.
While QAM is enormously efficient of spectrum, it is more difficult to demodulate in the presence of noise, which is mostly random amplitude variations. Linear power amplification is also required. QAM is very widely used in cable TV, Signal wireless local-area networks LANssatellites, and cellular telephone systems to produce maximum data rate in limited bandwidths Amplitude Phase Shift Keying APSK Amplitude phase shift keying APSKa variation of both M-PSK and QAM, was created in response to the need for an improved QAM.
Higher levels of QAM such as 16QAM and above have many different amplitude levels as well as phase shifts. These amplitude levels are more susceptible to noise.
Furthermore, these multiple levels require linear power amplifiers PAs that are less efficient than nonlinear e. The fewer the number of amplitude levels or the smaller the difference between the amplitude levels, the greater the chance to operate in the nonlinear region of the PA to boost power level. APSK uses fewer amplitude levels. This technique is widely used in satellites. Two close amplitude levels allow the amplifier to operate closer to the nonlinear region, improving efficiency as well as power output.
APSK is used primarily in satellites since it is a good fit with the popular traveling wave tube TWT PAs Orthogonal Frequency Division Multiplexing OFDM Orthogonal frequency division multiplexing OFDM combines modulation and multiplexing techniques to improve spectral efficiency. A transmission channel is divided into many smaller subchannels or subcarriers. The serial digital data to be transmitted is subdivided into parallel slower data rate channels.
These lower data rate signals are then used to modulate each subcarrier. The most common forms of modulation are BPSK, QPSK, and several levels of QAM. BPSK, QPSK, 16QAM, and 64QAM are defined with n. The complex modulation process is only produced by digital signal processing DSP techniques. An inverse fast Fourier transform IFFT generates the signal to be transmitted. An FFT process recovers the signal at the receiver. OFDM is very spectrally efficient. Because of the wide bandwidth it usually occupies and the large number of subcarriers, it also is less prone to signal loss due to fading, multipath reflections, and similar effects common in UHF and microwave radio signal propagation.
Currently, OFDM is the most popular form of digital modulation. It is used in Wi-Fi LANs, WiMAX broadband wireless, Long Term Evolution LTE 4G cellular systems, digital subscriber line DSL systems, and in most power-line communications PLC applications. Each type of modulation has a maximum theoretical spectral efficiency measure Table SNR is another important factor that influences spectral efficiency.
It also can be expressed as the carrier to noise power ratio CNR. The measure is the BER for a given CNR value. BER is the percentage of errors that occur in a given number of bits transmitted. As the noise becomes larger compared to the signal level, more errors occur. Some modulation methods are more immune to noise than others. Phase and frequency modulation BPSK, FSK, etc.
Note that for a given BER, a greater CNR is needed for the higher QAM levels Other Factors Affecting Spectral Efficiency While modulation plays a key role in the spectral efficiency you can expect, other aspects in wireless design influence it as well. For example, the use of forward error correction FEC techniques can greatly improve the BER. Such coding methods add extra bits so errors can be detected and corrected.
Such coding gain is common to almost all wireless systems today. Digital compression is another useful technique. The digital data to be sent is subjected to a compression algorithm that greatly reduces the amount of information. This allows digital signals to be reduced in content so they can be transmitted as shorter, slower data streams. For example, voice signals are compressed for digital cell phones and voice over Internet protocol VoIP phones. Music is compressed in MP3 or AAC files for faster transmission and less storage.
Video is compressed so high-resolution images can be transmitted faster or in bandwidth-limited systems. Another factor affecting spectral efficiency is the use of multiple-input multiple-output MIMOwhich is the use of multiple antennas and transceivers to transmit two or more bit streams.
A single high-rate stream is divided into two parallel streams and transmitted in the same bandwidth simultaneously. By coding the streams and their unique path characteristics, the receiver can identify and demodulate each stream and reassemble it into the original stream. MIMO, therefore, improves data rate, noise performance, and spectral efficiency. Today, most modern radios are software-defined radios SDR where functions like modulation and demodulation are handled in software.
DSP algorithms do the job previously assigned to modulator and demodulator circuits. The modulation process begins with the data to be transmitted being fed to a DSP device that generates two digital outputs, which are needed to define the amplitude and phase information required at the receiver to recover the data.
The DSP produces two baseband streams that are sent to digital-to-analog converters DACs that produce the analog equivalents. These modulation signals feed the mixers along with the carrier. The resulting quadrature output signals from the mixers are summed to produce the signal to be transmitted. If the carrier signal is at the final transmission frequency, the composite signal is ready to be amplified and sent to the antenna.
This is called direct conversion. Alternately, the carrier signal may be at a lower intermediate frequency IF. The IF signal is upconverted to the final carrier frequency by another mixer before being applied to the transmitter PA. At the receiver, the signal from the antenna is amplified and downconverted to IF or directly to the original baseband signals.
The amplified signal from the antenna is applied to mixers along with the carrier signal. The mixers produce the original baseband analog signals, which are then digitized in a pair of analog-to-digital converters ADCs and sent to the DSP circuitry where demodulation algorithms recover the original digital data. There are three important points to consider. First, the modulation and demodulation processes use two signals in quadrature with one another.
The DSP calculations call for two quadrature signals if the phase and amplitude are to be preserved and captured during modulation or demodulation. Second, the DSP circuitry may be a conventional programmable DSP chip or may be implemented by fixed digital logic implementing the algorithm.
Fixed logic circuits are smaller and faster and are preferred for their low latency in the modulation or demodulation process. Third, the PA in the transmitter needs to be a linear amplifier if the modulation is QPSK or QAM to faithfully reproduce the amplitude and phase information.
For ASK, FSK, and BPSK, a more efficient non-linear amplifier may be used The Pursuit Of Greater Spectral Efficiency With spectrum being a finite entity, psk is always in short supply. The Federal Communications Commission FCC and other government bodies have assigned most of the electromagnetic frequency spectrum over the years, and most of that is actively used.
Shortages now exist in the cellular and land mobile radio sectors, inhibiting the expansion of services such as high data speeds as well as the addition of new subscribers. One approach to the problem is to improve the efficiency of usage by squeezing more users into the same or less spectrum and achieving higher data rates. Improved modulation and access methods can help.
One of the most crowded areas of spectrum is the land mobile radio LMR and private mobile radio PMR spectrum used by the federal government, state governments, and local public safety agencies like fire and police departments. Most radio systems signal handsets use FM analog modulation that occupies a 25-kHz channel. Recently the FCC has required all such radios to switch over to kHz channels. This conversion, known as narrowbanding, doubles the number available channels.
It also means that more radios can be added to the system. This conversion must take place before January 1, Otherwise, an agency or business could lose its license or be fined. This switchover will be expensive as new radio systems and handsets are required.
In the future, the FCC is expected to mandate a further change from the kHz channels to kHz channels, again doubling capacity without increasing the amount of spectrum assigned.
No date for that change has been assigned. The new equipment can use either analog or digital modulation. It is possible to put standard analog FM in a kHz channel by adjusting the modulation index and using other bandwidth-narrowing techniques.
However, analog FM in a kHz channel is unworkable, so a digital technique must be used. Digital methods digitize the voice signal and use compression techniques to produce a very low-rate serial digital signal that can be modulated into a narrow band. Such digital modulation techniques are expected to meet the narrowbanding goal and provide some additional performance advantages.
New modulation techniques and protocols—including P25, TETRA, DMR, dPMR, and NXDN—have been developed to meet this need. The most popular digital LMR technology, P25, is already in wide use in the U. Its frequency division multiple access FDMA method divides the assigned spectrum into kHz or kHz channels.
Phase I of the P25 project uses a four-symbol FSK 4FSK modulation. In Phase 2, a compatible QPSK modulation scheme is used to achieve a similar data rate in a kHz channel. A unique demodulator has been developed to detect either the 4FSK or QPSK signal to recover the digital voice. Only different modulators on the transmit end are needed to make the transition from Phase 1 to Phase 2. The most widespread digital LMR technology outside of the U. This ETSI standard is universally used in Europe as well as in Africa, Asia, and Latin America.
Its time division multiple access TDMA approach multiplexes four digital voice or data signals into a 25-kHz channel. A single channel is used to support a digital stream of four time slots for the digital data for each subscriber. This is equivalent to four independent signals in adjacent kHz channels.
Another ETSI standard, digital mobile radio DMRuses a 4FSK modulation scheme in a kHz channel. It can achieve a kHz channel equivalent in a kHz channel by using two-slot TDMA. A similar technology is dPMR, or digital private mobile radio standard.
This ETSI standard also uses a 4FSK modulation scheme, but the access is FDMA in kHz channels. LMR manufacturers Icom and Kenwood have developed NXDN, another standard for LMR. It is designed to operate in either or kHz channels using digital voice compression and a four-symbol FSK system.
A channel may be selected to carry voice or data. The access method is FDMA. NXDN and dPMR are similar, as they both use 4FSK and FDMA in kHz channels. The two methods are not compatible, though, as the data protocols and other features are not the same. Because all of these digital techniques are similar and operate in standard frequency ranges, Freescale Semiconductor was able to make a single-chip digital radio that includes the RF transceiver plus an ARM9 processor that can be programmed to handle any of the digital standards.
The MC system-on-a-chip SoC can form the basis of a handset radio for any one if not multiple protocols. Satellites are positioned in an orbit around the equator about 22,300 miles from earth. This is called the geostationary orbit, psk satellites in it rotate in synchronization with the earth so they appear fixed in place, making them a good signal relay platform from one place to another on earth. Satellites carry several transponders that pick up the weak uplink signal from earth and retransmit it on a different frequency.
These transponders are linear and have a fixed bandwidth, typically 36 MHz. Some of the newer satellites have 72-MHz channel transponders. With a fixed bandwidth, the data rate is somewhat fixed as determined by the modulation scheme and access methods. The question is how one deals with the need to increase the data rate in a remote satellite as required by the ever increasing demand for more traffic capacity.
The answer lies in simply creating and implementing a more spectrally efficient modulation method. That level of improvement comes from a revised version of Signal modulation covered earlier. One commonly used satellite transmission standard, DVB-S2, is a single carrier typically L-band, to MHz that can use QPSK, 8PSK, 16APSK, and 32APSK modulation with different forward error correction FEC schemes.
The most common application is video transmission. NS3 improves on DVB-S2 by offering 64APSK with multiple amplitude and phase symbols to improve efficiency. Also included is low density parity check LDPC coding. As a result, they can operate at a higher power level and achieve the higher data rate with a lower CNR than DVB-S2. NovelSat offers its NS modulator and NS demodulator units to upgrade satellite systems to NS3.
In most applications, NS3 provides a data rate boost over DVB-S2 for a given CNR Acknowledgment Special thanks to marketing director Debbie Greenstreet and technical marketing manager Zhihong Lin at Texas Instruments as well as David Furstenberg, chairman of NovelSat, for their help with this article.
I agree with Earl. Their might be an advantage in highly populated areas, but their is nothing that excites me here. My Fire Department wants the Digital version to eliminate "dead" spots", but if the RF signal is not their, it will not help. One signal is the message signal and contains the information to be modulated.
I AM an engineer and also a ham radio operator, and while it was slightly mentioned, noise resistance for the different modulation schemes is mostly inversely proportional to "spectrum efficiency". The more data sent in binary bandwidth the more impact any noise has on it, as in increasing the bit error rate. So the transmission power needs to be greater to keep an adequate signal to noise ratio.
In addition, the more data per bandwidth, the greater the requirement for amplifier linearity. Making things psk, more linearity reduces amplifier efficiency. The result is that the selection of binary types is not simple at all, but rather a compromise and collection of trade-offs. The other consideration is that very few of the modulation schemes described are inter-operable. That has been demonstrated to be a very serious problem in some cases.
PDF format This file type includes high resolution graphics and schematics when applicable Fundamental to all wireless communications is modulation, the process of impressing the data to be transmitted on the radio carrier. Most wireless transmissions today are digital, and with the limited spectrum available, the type of modulation is more critical than it has ever been The main goal of modulation today is to squeeze as much data into the least amount of spectrum possible.
Multiple techniques have emerged to achieve and improve spectral efficiency Related Print reprints Favorite EMAIL Tweet Discuss this Article Earl McCune not verified Lou, Thanks for this nearly-all-correct column. I do want to comment on the OFDM discussion. I am not sure where the "OFDM is spectrally efficient" claim originated from, but it is impossible for OFDM to approach the spectral efficiency of any one underlying QAM subcarrier. And in my 40 years of wireless hardware experience, I have never seen a modulation that was more expensive to implement in hardware.
This emperor has no clothes, as far as I can tell Art I agree with Earl. One signal is the message signal and contains the information to be modulated William K. PDF format This file type includes high resolution graphics and schematics when applicable. Sponsored How to Reduce PFC Harmonics and Improve THD Using Harmonic Injection - Bosheng Sun, TI Simulating Electromagnetic Interference — Is it Possible?