Radio Frequency - RF Spectrum

Radio signals are transmitted through electromagnetic waves, also called radio waves, in radio communication system. The radio waves have a wide frequency range starting from a few ten kilo Hertz (kHz) to several thousand Mega Hertz (MHz). This wide range frequencies is called the radio spectrum or RF spectrum.

The RF spectrum is classified according to the applications of the spectrum in different service areas. The classification of the RF spectrum is given in below table along with the associated applications in communication system.

16:10 | Posted in , | Read More »

Basic Concept of FDM and TDM


Why do we need Multiplexing technique?
In communication, under the simplest conditions, a medium can carry only one signal at any moment in time.For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth. This is where the concept of Multiplexing comes!
Multiplexing means “sharing a medium”. It is a form of data transmission in which one communication channel carries several transmissions at the same time. In simple words, the method of dividing a single channel into many channels so that a number of independent signals may be transmitted on it is known as Multiplexing.



Multiplexing techniques can be divided into two basic categories:
Frequency Division Multiplexing-FDM
Time Division Multiplexing-TDM
Frequency Division Multiplexing-FDM
In FDM the available bandwidth is divided into a number of smaller independent logical channels with each channel having a small bandwidth. It assigns “frequency ranges” to each “user” or “signal” on a medium. Thus, all signals are transmitted at the same time, each using different frequencies.The method of using a number of carrier frequencies, each of which is modulated by an independent signal is in fact frequency division multiplexing.




Time Division Multiplexing-TDM
In TDM, sharing is accomplished by dividing available “transmission time” on a medium/channel among users.
Each user of the channel is allotted a small time interval during which he transmits a message. Total time available in the channel is divided, and each user is allocated a time slice. In TDM, users send message sequentially one after another. Each user can use the full channel bandwidth during the period he has control over the channel.













20:13 | Posted in , , , | Read More »

Basics of Channel Coding

Environmental interference and physical defects in the communication medium can cause random bit errors during data transmission. Channel coding also known as Error coding is a method of detecting and correcting these errors to ensure that information is transferred from its source to its destination reliably.
The idea behind channel coding is, to make the data more secure and error free. Channel Coding is done to ensure that the signal transmitted is recovered with very low probability of error at the destination. You can think channel coding as a “Security guard” for data .
What happens If Channel coding is not used?
If data is transmitted without channel coding ,and an error occurs in data , the receiver will inform the transmitter that an error has occurred in data, therefore transmit it again.Transmitter will have to retransmit the whole data ! A question may arise in your mind that, incase of error, data will be retransmitted so, what’s the need of using channel coding?
The reason is, if transmitter retransmits the whole data definitely there will be wasteage of power,and delay in communication. Therefore to avoid these factors it is necessary to implement channel coding.
In channel coding extra bits are added along with actual data bits. These extra bits are termed as “Redundant bits” because they actually cause redundancy in data. They ensure minimal or no error.
Error coding uses mathematical formulas to encode data bits at the transmitter or source into longer bits for transmission. The "coded data " is then decoded at the destination to retrieve the information. These bits help the the destination or receiver to determine if the communication medium introduced errors, if yes correct them so that the data need not be retransmitted.
Error coding is used in satellite and deep space communications, network communications, cellular telephone networks, magnetic and optical data storage media, and almost any other form of digital data communication.

20:09 | Posted in , | Read More »

What is Bit Error Rate ?

When data is transmitted to its destination over a communication channel there is a possibility of errors being introduced into the the data. In digital communication performance of the system is assessed by a term called as bit error rate, BER. BER is also known by the term Probability of Error.

In simple words, it is actually the ratio of number of bits in error to the total number of bits transmitted.
BER = Bits in Error/Total Number of Bit transmitted
For instance, If 10 bits were transmitted, and 5 bits get in error,then :
BER=5/10
=0.2
In BER curve if a value is mentioned, say 0.01, it means ,out of 100 transmitted bits, 1 bit got error [1/100=0.01]
Example:
Let's consider an example, assume a transmitted bit sequence:
0 1 1 0 0 0 1 0 1 1
and the following received bit sequence:
0 0 1 0 1 0 1 0 0 1,
The number of bit errors (the underlined bits) is in this case 3. The BER is 3 incorrect bits divided by 10 transmittedbits, resulting in a BER of 0.3 or 30%.


22:13 | Posted in , , | Read More »

M-PSK Modulation

Uptill now, we have discussed the basic concepts of digital modulation techniques.Let’s move one step ahead!
In digital modulation two commonly used terms are Symbols n Modulation order(M) Symbol:
It is actually a combination of bits.For instance, So and S1 are two symbols.Where ,So=00,S1=11 etc
Modulation order-M:
It refers to the number of states used in modulation,or the number of symbols.If M=2 it means two distinct states are being used in modulation. States could be different frequencies, amplitudes, or phase angles.If M is represented in power of ‘2’, that shows the number of bits per symbol.
For M=4
There will be 4 symbols(S0, S1, S2, S3).4=2^2.Therefore ,each symbol will be represented by Two bits.S0=00,S1=01,S2=10,S3=11
Keeping in view the ‘M’ factor, let’s restate the modulations techniques as:
M-PSK,M-FSK,M-ASK, and M-QAM
M-PSK:
For M=2: It is called as BPSK,(where B stands for Binary).Number of symbols are two. S0 and S1.As 2=2^1, so each symbol is represented by a single bit, therefore we wil have, S0=0 and S1=1
For M=4: It is called as 4-PSK.Number of symbols are Four.S0,S1,S2,and S3.As 4= 2^2,so each symbol is a combination of two bits,so we will have,S0=00,s1=01,s2=10,s4=11
So on and so forth for M=8,16,32,64
As PSK is phase modulation ,therefore the number of states actually refer to distinct Phase angles.These phase angles are used to modulate the carrier.

For M=2 , we have 2 phases/states
 Total angle is 360 degree
360/M =180 deg
So.’S0’ will b represented by 0 degree and ‘S1’ will be represented by 180(phase difference of 180 between symbols).That is, if 0 bit appears in signal, it will modulate the carrier wave with 0 degree, and 1 bit will modulate the carrier with 180 deg phase.
For M=4, we have 4 distinct phases
360/4 =90
So, Symbols will be represented as:
S0=0 deg,S1=90 deg,S2=135 deg,S3=180 deg
As we have the following symbols ,S0=00,s1=01,s2=10,s4=11.So, if 00 bits appear in signal, they will modulate the carrier wave with 0 degree,01 will modulate the carrier with 90 deg phase.10 will modulate the carrier with 135 deg n 11 will modulate it with 180 deg.
For M=8,we have 8 phases
360/8=45 deg
So,So=0deg,S1=45 deg,S2=90 deg,S3=135 deg,S4=180 deg,S5=225 deg,S6=270deg deg,S7=360 deg
Each symbol is represented by three bits here,
S0=000,S1=001,S2=010,S3=011,S4=100,S5=101,S6=110,S7=111
So, 000 bits will indicate 0 degree phase shift ,001 will represent 90 deg phase.010 will modulate the carrier with 135 deg ,0 11 will refer to 180 deg, n so on .
Similarly, for M-FSK, M frequencies are used to modulate a carrier and for M-ASK , M amplitude levels will be used.

18:04 | Posted in , , | Read More »

Introduction to FDM, OFDM, OFDMA

Frequency Division Multiplexing (FDM)

In FDM system, signals from multiple users/transmitters are transmitted simultaneously (at the same time slot) over multiple frequencies. That is each user is assigned a different frequency for communication. Each frequency range is called as sub-carrier, n it is modulated separately by different data stream, that is there are different sources of data (users/transmitters) for each subcarrier frequency. A spacing (guard band) is placed between sub-carriers to avoid signal overlap.


Orthogonal Frequency Division Multiplexing (OFDM)



Like FDM, OFDM also uses multiple sub-carriers but the sub-carriers are closely spaced to each other without causing interference, removing guard bands between adjacent sub-carriers. This is possible because the frequencies (sub-carriers) are orthogonal. In OFDM multiple subcarriers are modulated by single source of data. If I am a transmitter, n using OFDM technique, my data will be modulated by multiple subcarrier frequencies , instead of single carrier frequency ,as it happens in normal modulation .if I assume that 3 subcarriers are being used for modulation so, they will modulate the data of just one Transmitter ,unlike FDM where 3 frequencies will b used to modulate three different sources of data (three transmitters).
In an OFDM system, a very high rate data stream is divided into multiple parallel low rate data streams. Each smaller data stream is then mapped to individual data sub-carrier and modulated using some sorts of PSK or QAM modulation . let’s assume that I have a high rate data stream, X=[abcdefghi] ,
I divide it into three low rate data streams [p=abc, q=def, r=ghi ].Assume that there are three sub-carrier frequencies: f1,f2 n f3
Each low data rate stream will be modulated individually by each sub-carrier,that is data stream ‘p’ will be modulated by carrier f1, data stream ‘q’ by f2 n similarly data stream ‘r’ by f3 and finally all are combined.If it were FDM technique ,the data stream X would have been modulated by a single carrier frequency instead of multiple carriers.Therefor OFDM is called as multi-carrier modulation

Orthogonal Frequency Division Multiple Access (OFDMA)


Like OFDM, OFDMA employs multiple closely spaced sub-carriers, but the sub-carriers are further divided into groups of sub-carriers. Each group is named a sub-channel, different colors in figure show a different sub channel. The sub-carriers that form a sub-channel need not be adjacent. Sub-channels actually used for multiple access (multi users communication). Each sub-channel consists of multiple carrier frequencies. For instance I have two sub-channels, S1 n S2. S1 has carrier frequencies f1 ,f2,f3, and S2 has carrier frequencies, f4,f5, n f6 . sub-channel 1 will b used to entertain one user/transmitter and sub-channel 2 will b used by another user/transmitter.

05:48 | Posted in , , , , | Read More »

Introduction to QAM modulation

In previous post we started about basic digital modulation techniques,ASK,PSK and FSK.Let’s look at another digital modulation technique called as QAM [Quadrature Amplitude Modulation],which is actually a combination of ASK and PSK .
QAM is the combination of Amplitude modulation and Phase modulation. More technically, QAM is a system of modulation in which data or signal is transferred by modulating the amplitude of two separate carrier waves, mostly sinusoidal, which are out of phase by 90 degrees (sine and cosine). The modulated carriers are then summed, and the resulting signal is a combination of both phase shift keying (PSK) and amplitude shift keying (ASK).
Keep in mind that,the term Quadrature refers to their phase difference of 90 degrees. As Sine and Cosine are 90 degree out of phase.
[Any two periodic waveforms whose phase difference is 1/4 th of their output period are said to have a Quadrature phase relationship].As Sine n Cosine have time period of 360 degrees so, 1/4 th of 360 degree is 90 degree! :)
In next post we will talk about different orders of ASK,PSK n QAM modulation

13:58 | Posted in , , | Read More »

Digital Modulation with introduction to ASK, FSK and PSK

Firstly, what do we mean by digital modulation? Typically the objective of a digital communication system is to transport digital data between two or more nodes. In radio communications this is usually achieved by adjusting a physical characteristic of a sinusoidal carrier, either the frequency, phase, amplitude. This is performed in real systems with a modulator at the transmitting end to impose the physical change to the carrier and a demodulator at the receiving end to detect the resultant modulation on reception. We begin our discussion by three basic types of digital modulations:
  • ASK [Amplitude Shift Keying]
  • FSK [Frequency Shift Keying]
  • PSK [Phase Shift Keying]
All of these techniques vary the parameter of a sinosoid to represent the information we wish to transmit.
A sinusoid has three parameters that can be varied, these are amplitude ,phase and frequency.

  • In FSK we change the frequency in response to information. One particular frequency for ‘1’ and another frequency for ‘0’.



  • In PSK, we change the phase of carrier signal to indicate information.



  • In ASK amplitude of carrier is changed in response to information n all else is kept fixed. Bit ‘1’ is transmitted by a carrier of one particular amplitude.To transmit ‘0’ we change the amplitude

23:56 | Posted in , , , , | Read More »

Labels

Recently Commented

Recently Added