Digital communication (Band pass signals) ASK, PSK, FSK, Demodulation, M-ary communication, M-ASK,PSK & FSK, PSD of ASK,PSK&FSK, And Signals orthogonality, more on quantization

Digital carrier systems In baseband digital systems, signals are transmitted without shift in frequencies of the signal. Because, baseband signals have high power at low bandwidth e.g.. Local telephone exchange. Baseband signals cannot be transmitted over radio link, because it would require large size antennas.

Digital carrier systems… Hence for such purpose, signal spectrum must be shifted to high frequency range. A spectrum shift to higher frequencies is also required to transmit several messages simultaneously by sharing the large bandwidth of the transmission medium for e.g. Frequency Division Multiplexing (FDM). One of the basic form of modulation is amplitude modulation (AM).

Digital carrier systems… Amplitude-Shift Keying (ASK) In AM, the carrier amplitude is varied in proportion to the modulating signal (i.e. the baseband signal). This is shown in Fig. 3.35. An un-modulated carrier cos ωct is shown in Fig.3.35a. The on-off baseband signal m(t) (the modulating signal) is shown in Fig.3.35b. When the carrier amplitude is varied in proportion to m(t), we have the modulated

Digital carrier systems… ASK… Carrier m(t)cos ωct , as shown in Fig. 3.35c. Note that modulated signal is still an on-off signal. This modulation scheme of transmitting binary data is known as on-off keying (OOK) or amplitude -shift keying (ASK).

Digital carrier systems… ASK…

Digital carrier systems… Phase-Shift Keying (PSK) If the baseband signal m(t) were polar (Fig.3.36a), the corresponding modulated signal m(t) cos ωct would appear as shown in Fig. 3.36b. In this case if p(t) is basic pulse, we are transmitting 1 by a pulse p(t) cos ωct and 0 by –p(t) cos ωct = p(t) cos (ωct + π).

Digital carrier systems… PSK, FSK The information resides in the phase of the pulse. For this reason this is known as phase-shift keying and the transmission is still polar. When the data is transmitted by varying the frequency, we have the case frequency-shift keying (FSK) as shown in Fig.3.36c. A 0 is transmitted by ωc0 and 1 by ωc1 Information about the transmitted data resides in the carrier frequency.

Digital carrier systems… Phase-Shift Keying (PSK) & Frequency-Shift Keying (FSK)

De-modulation of ASK,PSK,FSK De-modulation of digital-modulated signals is similar to that of analog-modulated signals. For example, ASK can be de-modulated coherently (synchronous) or non-coherently (envelop detection). In PSK signals cannot be demodulated non-coherently (envelope detection), because the envelope is same for both 1 and 0.

De-modulation of ASK,PSK,FSK… The coherent detection is similar to that used for analog signals. FSK can also be detected coherently by generating two references of frequencies ω0 and ω1 , and demodulating the received signal by two demodulators using the two carriers and then comparing the outputs of the two demodulators.

M-ARY communication Digital communication uses only a finite number of symbols for communication , the minimum number being two (binary case). M-ary communication means communication using M symbols. It is easy to show that the information transmitted by each symbol increases with M. for example when M=4(4-ary, or quaternary, case) we have four basic symbol or pulses

M-ARY communication… Available for communication as shown in Fig.3.33a. This type of signaling is called multi-amplitude signaling, which allows us to transmit each pair of binary digits by one 4-ary pulses or symbols as shown in Fig.3.33b. In general the information IM transmitted by an M-ary symbol is IM = log2 (M) binary digits or bits.

M-ary communication…

M-ARY communication… This means that as the rate of information increases by log2 (M) the transmitted power required also increases as M2. Example 16-ary IM = log2 (16) IM = 4bits per sec (transmits at a time)

M-ary Amplitude-shift keying (M-ASK)

M-ary Amplitude-shift keying (M-ASK)…

M-ary Phase-shift keying (M-PSK)

M-ary Phase-shift keying (M-PSK)

M-ary Frequency shift keying (M-FSK)

M-ary Frequency shift keying (M-FSK)

Power spectral density (PSD) of ASK,PSK and FSK Modulation causes a shift in the baseband signal spectrum. The ASK signal in Fig.3.35c is an on-off signal (using a full width or Non Return to Zero (NRZ)). The PSD of ASK signal is shown in Fig.3.37a. Note that it has a non-zero PSD (discrete component present) at ω = 0.

Power spectral density (PSD)… of ASK,PSK,FSK This will rule out the use of ac coupling during transmission, ac coupling helps in impedance matching. The other disadvantages are less immune to noise, bandwidth requirements are excessive, no error detection and correction capability, & requires high power. The PSK signal on the other hand is a polar signal. Its PSD is shown in Fig.3.37b.

Power spectral density (PSD)… of ASK,PSK and FSK Note that at ω =0; PSD=0 (no discrete component), otherwise PSD of PSK and ASK are same. FSK signal may be viewed as a some of two interleaved ASK signals as shown in Fig.3.37c. Hence the spectrum of FSK is the sum of two ASK. No discrete component appear in the spectrum.

PSD of ASK,PSK, and FSK…