Chapter 4 part 2_a Digital Modulation Techniques

Chapter 4 (part 2a) Overview Digital Modulation techniques (part 2) Digital Modulation techniques (part 2) Bandpass data transmission Bandpass data transmission Amplitude Shift Keying (ASK) Amplitude Shift Keying (ASK) Phase Shift Keying (PSK) Phase Shift Keying (PSK) Frequency Shift Keying (FSK) Frequency Shift Keying (FSK) Quadrature Amplitude Modulation (QAM) Quadrature Amplitude Modulation (QAM)

Digital Modulation Techniques Digital modulation Digital modulation The process by which digital symbols are transformed into waveforms that are compatible with the characteristic of the channel. The process by which digital symbols are transformed into waveforms that are compatible with the characteristic of the channel. Bandpass modulation Bandpass modulation Process whereby the amplitude, frequency, or phase of an RF carrier, or a combination of them, is varied in accordance with the information to be transmitted. Process whereby the amplitude, frequency, or phase of an RF carrier, or a combination of them, is varied in accordance with the information to be transmitted.

Digital Bandpass Modulation A carrier signal has three parameters which can be used for impressing: A carrier signal has three parameters which can be used for impressing: Amplitude FrequencyPhase

Digital Bandpass Modulation If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK) If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK) If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK) If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)

Digital Bandpass Modulation If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK) If both the amplitude,V and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)

Amplitude Shift Keying (ASK) ASK demonstrates poor performance, as it is heavily affected by noise and interference. ASK demonstrates poor performance, as it is heavily affected by noise and interference. Used in radio telegraphy in the early 1900s Used in radio telegraphy in the early 1900s

Amplitude Shift Keying (ASK) M was chosen to be equal to 2, so it is corresponding to two waveform types. M was chosen to be equal to 2, so it is corresponding to two waveform types. Also know as Binary ASK signaling (also called on-off keying) Also know as Binary ASK signaling (also called on-off keying)

Frequency Shift Keying (FSK) Bandwidth occupancy of FSK is dependant on the spacing of the two symbols. A frequency spacing of 0.5 times the symbol period is typically used. Bandwidth occupancy of FSK is dependant on the spacing of the two symbols. A frequency spacing of 0.5 times the symbol period is typically used. FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states. FSK can be expanded to a M-ary scheme, employing multiple frequencies as different states.

Frequency Shift Keying (FSK) M was chosen to be equal to 3, corresponding to the 3 waveform types (3-ary). M was chosen to be equal to 3, corresponding to the 3 waveform types (3-ary). Emphasize the mutually perpendicular axes. Emphasize the mutually perpendicular axes. The signal set is characterized by Cartesian coordinates, such that each of the mutually perpendicular axes represents a sinusoid with a different frequency. The signal set is characterized by Cartesian coordinates, such that each of the mutually perpendicular axes represents a sinusoid with a different frequency. Such mutually perpendicular vectors are called orthogonal signals. Such mutually perpendicular vectors are called orthogonal signals.

Phase Shift Keying (PSK) Phase Shift Keying (PSK) demonstrates better performance than ASK and FSK. Phase Shift Keying (PSK) demonstrates better performance than ASK and FSK. PSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states. PSK can be expanded to a M-ary scheme, employing multiple phases and amplitudes as different states. Filtering can be employed to avoid spectral spreading. Filtering can be employed to avoid spectral spreading. Widely used in both military and commercial communications system. Widely used in both military and commercial communications system.

Phase Shift Keying (PSK) M was chosen as to be as 2, and it is called binary PSK (BPSK) M was chosen as to be as 2, and it is called binary PSK (BPSK) The modulating signal shifts the phase of the wave si(t) to one of two states, either zero or π (180º). The modulating signal shifts the phase of the wave si(t) to one of two states, either zero or π (180º). For the BPSK example, the vector picture illustrates the two 180º opposing vectors. For the BPSK example, the vector picture illustrates the two 180º opposing vectors. Signal sets that can be depicted with such opposing vectors are called antipodal signal sets. Signal sets that can be depicted with such opposing vectors are called antipodal signal sets.

Phase Shift Keying (PSK) Constellation of two-level PSK Constellation of two-level PSK

Phase Shift Keying (PSK)

4-PSK has more efficient usage of bandwidth than 2- PSK, because each signal unit has two bits. For the same bandwidth, the data bit rate doubles. 4-PSK has more efficient usage of bandwidth than 2- PSK, because each signal unit has two bits. For the same bandwidth, the data bit rate doubles.

Phase Shift Keying (PSK) Excellent performance of 2-PSK encourages us to go with 4-PSK, also called quadrature PSK (Q-PSK) Excellent performance of 2-PSK encourages us to go with 4-PSK, also called quadrature PSK (Q-PSK)

Phase Shift Keying (PSK) The idea can be extended to 8-PSK, 16-PSK, 32- PSK,…. The idea can be extended to 8-PSK, 16-PSK, 32- PSK,…. The limitation is the ability of equipment to distinguish small differences in signal’s phase. The limitation is the ability of equipment to distinguish small differences in signal’s phase. 8 PSK

BPSK Modulator Binary PSK (BPSK) modulation can be accomplished by simply multiplying the original signal d(t) (which is a binary random sequence) by the carrier signal, which is an analog sinusoidal oscillation. After multiplication a bandpass filter is required Binary PSK (BPSK) modulation can be accomplished by simply multiplying the original signal d(t) (which is a binary random sequence) by the carrier signal, which is an analog sinusoidal oscillation. After multiplication a bandpass filter is required

QPSK Modulator

Quadrature Amplitude Modulation (QAM) Combination of ASK and PSK which helps making a contrast between signal units. The number of amplitude shifts should be lower than the number of phase shifts due to noise susceptibility of ASK.

Quadrature Amplitude Modulation (QAM)

Summarize