Difference between Modulation(Analog and Digital), multiplexing and multiple access

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Difference between Modulation(Analog and Digital), multiplexing and multiple access

Modulation, keying, Multiplexing and Multiple Access are the basic terms used for any type of network. Usually some of us get confused by these terms. In this post lets discuss about what exactly these terms means and how they are different from each other.

MODULATION:

Usually, the signal that we want to transmit, say a speech signal with 4000 Hz frequency, will require a very big antenna. For any signal the frequency f is related to wavelength L as

c = L * f ………………………… (i)

Where c is velocity of light. And antenna length is generally taken as L/2 which simply means for our case antenna length is 75000 m, obviously this size of antenna is too big to use on day to day basis. That is why we take our speech signal or the desired signal and take another high frequency signal known as carrier (carrier can be any signal but should have high frequency and in practice we use a simple continuous wave signal), now we alter one or more parameters of this career signal in accordance with our desired signal, this parameters can be any one or combination of parameters. The basic parameters are amplitude, frequency, and phase of the signal. The result of this alteration we get is known as modulated signal, the desired signal which we wanted to transmit is known as modulating signal also known as base band signal and modulated signal is also known as band pass signal. The whole process is known as MODULATION.

Two forms of modulation are generally distinguished, although they have many properties in common: If the modulating signal's amplitude varies continuously with time, it is said to be an analog signal and the modulation is referred to as analog. In the case where the modulating signal may vary its amplitude only between a finite number of values and the change may occur only at discrete moments in time, the modulating signal is said to be a digital signal and the modulation is referred to as digital or keying.

In most applications of modulation the carrier signal is a sine wave, which is completely characterized by its amplitude, its frequency, and its phase relative to some point in time. Modulating the carrier then amounts to varying one or more of these parameters in direct proportion to the amplitude of the modulating signal. In analog modulation systems, varying the amplitude, frequency, or phase of the carrier signal results in amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM), respectively. Since the frequency of a sine wave expressed in radians per second equals the derivative of its phase, frequency modulation and phase modulation are sometimes subsumed under the general term “angle modulation” or “exponential modulation.”

If the modulating signal is digital, the modulation is termed amplitude-shift keying (ASK), frequency-shift keying (FSK), or phase-shift keying (PSK), since in this case the discrete amplitudes of the digital signal can be said to shift the parameter of the carrier signal between a finite number of values. For a modulating signal with only two amplitudes, “binary” is sometimes added before these terms.
Digital modulating signals with more than two amplitudes are sometimes encoded into both the amplitude and phase of the carrier signal. For example, if the amplitude of the modulating signal can vary between four different values, each such value can be encoded as a combination of one of two amplitudes and one of two phases of the carrier signal. Quadrature amplitude modulation (QAM) is an example of such a technique.

In certain applications of modulation the carrier signal, rather than being a sine wave, consists of a sequence of electromagnetic pulses of constant amplitude and time duration, which occur at regular points in time. Changing one or the other of these parameters gives rise to three modulation schemes known as pulse-position modulation (PPM), pulse-duration modulation (PDM), and pulse-amplitude modulation (PAM), in which the time of occurrence of a pulse relative to its nominal occurrence, the time duration of a pulse, or its amplitude are determined by the amplitude of the modulating signal

MULTIPLEXING:

Basically there are two types of system, time domain and frequency domain. In time domain we transmit frames, and in frequency domain we transmit in accordance with frequency. Now if there is more than one source of signal and we want to transmit them together then we implement multiplexing. In multiplexing we mix the source signals (off course with some precautions) say if we want to mix them in time domain then our frame will contain some packets form source A and some packets from source B and so on depending upon the constraints of the channel and time frame. The signals that source are generating can either be modulated signals or we can even send our multiplexed signal to the modulator and then modulate the signal. At the receiving end be de-multiplex the signals. In multiplexing we do not provide a dedicated resource to a single source. I.e. we do not dedicate the complete time frame to a single source (in our case it is time frame). Multiplexing is also seen as you are travelling on a four lane road and suddenly it get narrower and turned to single lane, at this point the traffic police will allow one car from each lane to drive through that narrow single lane, this is what we called MULTIPLEXING.

MULTIPLE ACCESS:

As the name suggest, multiple access means multiple users can access the channel or link. Multiple access provides dedicated resources to the user (with a time constraint) in comparison to the multiplexing which does not provide any type of resources. There are many type of Multiple access schemes like FDMA frequency division multiple access, TDMA time division multiple access, CDMA code division multiple access, SDMA space division multiple access etc. take the example of FDMA, the whole frequency band is divided into small frequency bands called channels, now each channel is having certain capacity to take the traffic say a channel can accommodate single user at time, then the whole frequency bandwidth can be access by as many users as there are channels, mathematically if we are having a bandwidth of 200 KHz and channel bandwidth is 50 KHz then it means we can accommodate 4 users at a time by giving 50 KHz channel to each. This is so called multiple access, i.e. multiple users can access the bandwidth simultaneously and we do not require any additional hardware at the receiving end to separate the desired user from the other users as we do in Multiplexing. In reality the concept of Multiple Access is more complicated and In GSM each channel can accommodate 8 users at a time and each channel has 200 KHz bandwidth.