Non-Coherent Amplitude Shift Keying Chapter 2
Chapter 2
2.1 Theory of Digital Modulation
A digital communication system is one which sends number of signals from one place to another in order to convey information.
Information can be presented as groups of (usually binary) digits. Such group is called a digital Word. It is usually convenient to send the digit serially (one after the other) and to put them together again as words at the receiving end.
Analog information, such as the voltage signal from a telephone, can be converted to digital from, sent over a digital communication channel, and recovered to analog form at the receiver.
In order to transmit a signal it is often modulated, and with digital signal the modulation is referred to as “keying”. The amplitude shift keying (ASK) is considered as the simplest way of shifting the frequency spectrum of a signal from base band to some other band of frequencies.
The use of a higher frequency range reduces antenna size. In the modulation process, the baseband signals constitute the modulating signal and the high-frequency carrier signal is a sinusiodal waveform. There are three basic ways of modulating a sine wave carrier. For binary digital modulation, they are called [ binary amplitude-shift keying (BASK), binary frequency-shift keying (BFSK) and binary phaseshift keying (BPSK)].
Modulation also leads to the possibility of frequency multiplexing. In a frequency-multiplexed system, individual signals are transmitted over adjacent, nonoverlapping frequency bands. They are therefore transmitted in parallel and simultaneously in time. If we operate at higher carrier frequencies, more bandwidth is available for frequency-multiplexing more signals.
Transmission of data across a noisy communications channel requires some manner of separating the valid data from the background noise. The most common way to accomplish this is to modulate the data at the transmission side and to demodulate the data on the reception side, the end result being that the data coming from the receiver are the same as the data being presented to the transmitter. The efficiency of the modulation/demodulation process determines the accuracy of the data coming from the receiver. Therefore, careful consideration must be given to the selection of an appropriate modulation-demodulation scheme.
Figure 21…Different digital modulation techniques
2.2 Amplitude Shift Keying [ASK]
Amplitude shift keying -ASK- as shown in figure 19 in the context of digital communications is a modulation process which imparts to a sinusoid two or more discrete amplitude levels (Also called on-off keying - OOK). These are related to the number of levels adopted by the digital message.
The digital data to be transmitted is the binary number . Two amplitudes are used to directly represent the data, either 0 or 1. In this case, the modulation is called binary amplitude shift keying or BASK
Figure 22…Binary Amplitude-Shift Keying (BASK)
A binary amplitude-shift keying (BASK) signal can be defined by
Equation 21…Characteristic of ASK
where A is a constant , fc is the carrier frequency, and Tb is the bit duration.
Figure 2-3 shows the BASK signal sequence generated by the binary sequence 0 1 0 1 0 0 1. The amplitude of a carrier is switched or keyed by the binary signal m(t). This is sometimes called on-off keying (OOK).
Figure 23…(a) Binary modulating signal (b) ASK signal.
There are sharp discontinuities shown at the transition points. These result in the signal having an unnecessarily wide bandwidth. Bandlimiting is generally introduced before transmission, in which case these discontinuities would be 'rounded off'. The band-limiting may be applied to the digital message, or the modulated signal itself.
It is a special case of amplitude modulation (AM). Amplitude modulation has the property of translating the spectrum of the modulation to the carrier frequency. The bandwidth of the signal remains unchanged.
The fact that AM simply shifts the signal spectrum is often used to convert the carrier frequency to a more suitable value without altering the modulation. This process is known variously as mixing, up-conversion or down-conversion. Some form of conversion will always be present when the channel carrier occupies a frequency range outside the modulation frequency range.
One of the disadvantages of ASK, compared with FSK and PSK, for example, is that it has not got a constant envelope. This makes its processing (eg, power amplification) more difficult, since linearity becomes an important factor. However, it does make for ease of demodulation with an envelope detector.
A block diagram of a basic non coherent ASK generator is shown in Figure 2-4.
Figure 24…Transmitter Of ASK
While in figure 2-4 we can see a coherent ASK generator, the main difference between the coherent systems and non-coherent systems that the coherent systems needs carrier synchronization while this is not required in non coherent systems.
Figure 25…Coherent ASK transmitter
2.2.1 Bandwidth modification
As already indicated, the sharp discontinuities in the ASK waveform of Figure2-3 imply a wide bandwidth. A significant reduction can be accepted before errors at the receiver increase unacceptably. This can be brought about by bandlimiting (pulse shaping) the message before modulation, or bandlimiting the ASK signal itself after generation, Both these options are illustrated in Figure22, which shows one of the generators we will be modeling.
2.2.2 Using dual analog switch
The simplest method for binary ASK is to use a switch to gate the carrier on and off, driven by the data signal as shown in figure 2-4 earlier while we can use any mean of multiplication to achieve the goal which is modulating the carrier with the data signal.
2.2.3 Symmetry in ASK
Spectrum of an ASK signal can be determined from its baseband data stream if the ASK modulation process if seen as a multiplication or mixing of the baseband symbol stream with the carrier wave.
Consider a single frequency cos(wmt) from within the baseband spectrum and perform the mathematical multiplication with the carrier cos(wct) ...
2.2.4 Demodulation methods
For demodulation and detection purpose two techniques can be used.
· Non-coherent (Threshold) detection to detect presence or absence of a carrier signal.
· Coherent demodulation; multiplying the modulated signal by the carrier signal to recover the baseband data signal. This detection is better for a noisy environment.
2.2.4.1 Non-Coherent Detection
In the coherent detector, the exact reproduction of the carrier is needed (i.e. requires carrier or phase synchronization).
Figure 26…Non coherent Detection
The diagram in figure 2-6 is a non-coherent detector which does not require carrier or phase synchronization.
Both asynchronous and synchronous demodulation methods are used for the demodulation of ASK signals. It is apparent that the ASK signal has a well defined envelope. Thus it is amenable to demodulation by an envelope detector. A synchronous demodulator would also be appropriate. We note that: Envelope detection circuitry is simple. Synchronous demodulation requires a phase-Iocked local carrier and therefore carrier acquisition circuitry.
With band limiting of the transmitted ASK neither of these demodulation methods would recover the original binary sequence; instead, their outputs would be a band limited version. Thus further processing - by some sort of decision-making circuitry for example - would be necessary.
Thus demodulation is a two-stage process:
· Recovery of the bandlimited bit stream
· Regeneration of the binary bit stream
Envelope demodulation
Having a very definite envelope, an envelope detector can be used as the first step in recovering the original sequence. Further processing can be employed to regenerate the true binary waveform this is can be.
Figure 27…Envelope detector operation
2.2.4.2 Coherent ASK Detection
A coherent detector figure 2-7 operates by mixing the incoming data signal with a locally generated carrier reference and selecting the difference component from the mixer output.
Figure 28…Coherent detection of ASK
If the modulated data signal is a(t).cos(wct) and the reference carrier cos(wct + q) where q is the phase error between the source and reference carriers, the mixer output becomes:
a(t).cos(wct).cos(wct + q) = 0.5a(t).cos(q) + 0.5a(t).cos (2wct + q)
If q = 0 (reference carrier phase coherent) output is proportional to a(t)
Then coherent detection has better noise rejection
2.2.5 Coherent ASK Vs non-coherent ASK
In non-coherent detection, V is amplitude of the signal
Coherent ASK is more resistive to noise than non-coherent ASK, coherent ASK is more complicated than non-coherent ASK due to the need of a local carrier in coherent ASK receiver that is synchronized with the transmitted carrier (i.e. having the same frequency and phase as the transmitted carrier).
2.3 Frequency Shift Keying [ FSK ]
Frequency-shift keying (FSK) is a method of transmitting digital signals. The two binary states, logic 0 (low) and 1 (high), are each represented by an analog waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is represented by a wave at a different frequency. A modem converts the binary data from a computer to FSK for transmission over telephone lines, cables, optical fiber, or wireless media. The modem also converts incoming FSK signals to digital low and high states, which the computer can "understand."
Figure 29…Frequency shift keying
FSK is simplified a form of Frequency Modulation (FM). For good noise performance and high bandwidth operation, FSK is the modulation technique of choice. In true FM, an analog signal is represented with a linear frequency deviation from center. FSK is a binary form of frequency modulation which uses hard shifts between deviant frequencies to represent the data originally impressed on the carrier. The magnitude of frequency shift is directly related to the magnitude of the modulation source voltage.The modulation source is allowed two states: “on” and “off”. When the modulation source is “off ” , the carrier frequency is shifted down from the center frequency. When the modulation source is “on”, the carrier frequency is shifted up from the center frequency. The amount that the carrier frequency is shifted is referred to as the frequency deviation.
2.3.1 Modulation Process
In FSK, the instantaneous frequency of the carrier is switched between 2 or more levels according to the baseband digital data, for example at logic 0 frequency a wave of Acos wct is sent while when logic 1 data come, the system will produce a wave of Acos n wct (where n = integer)
Figure 210…Generated waveform of FSK
Also in FSK systems we can use two different methods of modulation, either we use non-Coherent scheme or a coherent scheme.
A non-coherent FSK transmitter is shown in figure 2-11 where the simply data source is connected directly to a voltage controlled oscillator that can give two different frequencies, one sent at logic 1 and the other is sent at logic 0 inorder to generate and FSK signal.
Figure 211…Non-coherent FSK transmitter
Unlike ASK, a carrier is always present with FSK modulation. This affords the designer several benefits. First, the carrier will load the receiver at all times providing greatly increased noise immunity. Secondly, the strength (or amplitude) of the carrier can be used to determine the quality of the incoming signal.
As illustrated in figure 2-12 the coherent FSK system is more complex than non-coherent one, but as said earlier the coherent systems have more immunity to nose but they are more expensive, the same idea of non-coherent FSK is applied here.
Figure 212…Coherent FSK transmitter
2.3.2 Demodulation process
Demodulation of FSK depends on the modulation used either coherent or non-coherent and below we can see the both receiver diagrams.
2.3.2.1 Non-Coherent FSK receiver
Figure 2-13 shown a non-coherent FSK receiver where the received signal enters the system through two band pass filters so that we can separate each frequency of the FSK signal either fc+fd or fc-fd after getting each frequency separated we pass the signal top an envelope detector that will reconstruct the level of the data (the peak voltage of the signal) the resultant is passed to a summer that will give us the final value of the voltage the value will be compared with a reference voltage so that we can regenerate the original data.
Figure 213…Non-coherent FSK receiver
2.3.2.2 Coherent FSK receiver
Coherent FSK method uses a local carrier generator so that we can reconstruct the data sent, this can be achieved by multiplying the entering signal by a carrier signal as shown in figure 2-14.
Figure 214…Coherent FSK receiver
A drawback of the continuous carrier is that the transmitter is always drawing power and generating an output. Therefore, the transmitter will ultimately require a higher supply current than ASK-based systems. In addition, the output power cannot be legally increased in countries
FSK is a Non Return to Zero modulation method. This means that the non-modulated condition is between the “off” and “on” condition. In other words, the carrier should never be at the center frequency when modulation is present. The benefit here is noise immunity. Hysteresis can be applied to the detector, eliminating the effect of spurious frequency modulation generated from sources other than the data stream.
Since FSK relies on frequency change, and not amplitude change, to indicate data states, an FSK receiver is inherently immune to amplitude noise. This is of great importance in bands which are extremely crowded and have a high potential for near-band interference. This increased noise immunity suggests a potential for higher data rates.
FSK modulation for the transmission of data has many features and limitations to consider.