Modulation

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   Document Title Security Level2013-09-15 HUAWEI Confidential Page1, Total13 1.1.1   BasicIn telecommunication, for information transfer, idea is to usehigh frequency signal and then modify the characteristics of this signal to communicate the information. In other words, what we do is superimpose (low frequency) information signalonto (high frequency) carrier signal. At receiving end, thisprocess is reversed.Have a look at example diagram of Analog modulation  below.Top one is a information signal. In 2nd waveform, amplitudecarrier signal is modified to reflect the value of informationsignal. In 3rd, frequency of carrier frequency is modified to sothe same.In similar way, we can modify the phase of carrier signal based on information signal. These techniques are known asamplitude modulation, frequency modulation, and phasemodulation respectively.   Document Title Security Level2013-09-15 HUAWEI Confidential Page2, Total13  As you may infer, the frequency difference betweeninformation signal and carrier signal need to be sufficient for receiver to decode the signals properly.Digital modulationIn digital modulation, the input/information signal is in onesand zeros. In a way, digital modulation is digital-to-analog conversion !There are numerous techniques:  Phase Shift Keying  Phase is given a value; in case of BPSK, two values (0 and 1)are separated by 180° phase; for QPSK, it would be 4 values(00, 01, 10, and 11) by 90° phase.QPSK can be viewed as two BPSK signals ! One way toimplement QPSK is split the actual data stream in even andodd bits. Then odd bits are given to modulate sine wave (likeBPSK) and even bits to cosine wave. Resulting wave form isQPSK.   Document Title Security Level2013-09-15 HUAWEI Confidential Page3, Total13 Below is an example of QPSK with information as 11 .In case of PSK, synchronisation is mandatory as absolutephases are given values. In DPSK (Differential PSK) ratherthan phase, phase change is used for indicating values.Other variants of PSK are 8PSK, 16PSK, Offset PSK etc.  Frequency Shift Keying  FSK uses frequency change to indicate information. Due to thenature of FSK, it is still used in amateur radio. Anotherpopular example is DTMF - used in today's keypad landlinephones.   Document Title Security Level2013-09-15 HUAWEI Confidential Page4, Total13 Modulation (QAM) - 2  Quadrature Amplitude Modulation (QAM)  [Under Modulation >>Digital modulation] In QAM, two amplitude modulated quadrature carriers aresummed - hence the name QAM. As carrier waves are 90° out of phase and amplitude modulated, QAM in a way (amplitudedifference between two carriers changes the phase;you really need to visualise this in your mind to grasp better) is a combination of ASK and PSK.Keep the amplitude same and we have PSK then. In fact, PSK may  be designed based on QAM. Constellation diagramConstellation diagrams useful tools for representingwaveforms. A sinusoidal signal/wave becomes acircle on constellation diagram. Refer two examplesfigures below for clarity:Above is for BPSK and below for 8PSK 
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