Baseband Modulation-Digital Communication Systems-Lecture Slides, Slides of Digital Communication Systems

Download Baseband Modulation-Digital Communication Systems-Lecture Slides and more Slides Digital Communication Systems in PDF only on Docsity! 1 1 Digital Communication Systems Dr. Shurjeel Wyne Lecture 4 Baseband Modulation (Line Codes) 2 Define baseband modulation? Symbol and bit duration Mapping of bits to symbols 2 3 Transmission of baseband signal Encode TransmitPulsemodulateSample Quantize Demodulate/ Detect Channel ReceiveLow-pass filter Decode Format/Analog-to-Digital Conversion Format source sink PCM Codeword (Sequence of bits) Pulses (waveforms) 4 Baseband transmission To transmit information through physical channels, PCM sequences (codewords) are transformed to pulses (waveforms). Each waveform carries a symbol from a symbol-set of size M. Each transmit symbol represents a grouping of k bits of the PCM words. Binary pulse modulation used for binary symbols (M=2). Waveforms called PCM waveforms or line codes M-ary pulse modulation used for non-binary symbols (M>2). Mk 2log= 5 9 Unipolar-RZ A one is represented by a half-bit-wide pulse and a zero is represented by the absence of a pulse. Bipolar-RZ Ones and zeros are represented by opposite-level pulse that are one-half bit wide. RZ-AMI (AMI for “alternate mark inversion”) Consecutive Ones are represented by equal-amplitude alternating pulses. Zeros are represented by the absence of pulses. PCM Waveform Types - RZ Return to Zero (RZ) - pulse lasts just half of a bit period RZ-AMI has limited error detection capability: what should Rx do if consecutive pulses received with same polarity? 10 Bi-Φ-L (bi-phase-level) or Manchester Coding Binary one is represented by a half-bit-interval wide pulse, positioned during first half of the bit interval; Binary zero is represented by a half-bit-interval wide pulse, positioned during second half of the bit interval. Bi-Φ-M (bi-phase-mark) A transition occurs at the beginning of every bit interval. Binary one is represented by a second transition one-half bit interval later. Binary zero is represented by no second transition. PCM Waveform Types – Phase Encoded 6 11 Bi-Φ-S (bi-phase-space) A transition occurs at the beginning of every bit interval. Binary one is represented by no second transition. Binary zero is represented by a second transition one-half bit interval later. Delay Modulation (DM) or Miller Coding Binary one is represented by a transition at the mid-point of the bit interval. Binary zero is represented by no transition, unless it is followed by another zero. In this case, a transition is placed at the end of the bit interval of the first zero. PCM Waveform Types – Phase Encoded… 12 These binary waveforms use three levels, instead of two, to encode the binary data Dicode-NRZ Pulse is full bit interval wide, a data transition: one-to-zero or zero-to- one, changes pulse polarity Without a data transition, the zero level is sent. Dicode-RZ Pulse is half-bit interval wide, a data transition: one-to-zero or zero- to-one changes polarity of transmitted pulse Without a data transition, the zero level is sent. PCM Waveform Types – Multilevel Binary 0 7 13 PCM waveform characteristics line code are designed to optimize one or more of the following goals: Synchronization capability DC component Spectral characteristics (power spectral density and bandwidth efficiency) Noise immunity Error detection capability Implementation cost and complexity 14 Synchronization: The receiver’s bit /symbol intervals must correspond exactly to bit/symbol intervals at transmitter in order to correctly interpret the received signals A self-synchronizing (self-clocking) digital signal includes the timing information in the data being transmitted. PCM waveform characteristics - Synchronization Capability Transmitter Receiver Assume synchronization error: Rx clock is faster than Tx clock 10 19 PCM waveform characteristics… Low probability of bit error Noise immunity: Some PCM waveforms are more immune to noise than others, e.g., for the same signal-to-noise ratio, NRZ waveforms provide a lower probability of error relative to unipolar RZ Error detection: Some PCM waveforms also provide limited error detection capability that contributes to a low probability of error, e.g. NRZ- AMI, a single error will violate AMI rule 20 BASEBAND TRANSMISSION M-ary pulse modulation Each transmitted waveform (pulse) carries a symbol from a symbol-set of size M where M>2 M-ary pulse modulation categories: M-ary pulse-amplitude modulation (PAM) M-ary pulse-width modulation (PWM) M-ary pulse-position modulation (PPM) 11 21 In M-ary PAM one of the M allowable amplitude levels are assigned to each of the M possible symbol values, each representing bits of PCM sequence . For the same data rate, M-ary PAM (M>2) requires less bandwidth than binary PCM. For the same average pulse power, binary PCM will have lower BER than M-ary PAM (M>2). Mk 2log= M-ary Pulse Amplitude Modulation (PAM) 22 Pulse Width Modulation (PWM) In PWM, the pulse width is varied by an amount that corresponds to the value of the digital message symbol. The pulse amplitude is held constant. Pulse Position Modulation (PPM) In PPM, the position (or time of occurrence) of each pulse from some reference time is varied by an amount that corresponds to the value of the digital message symbol. Both the pulse amplitude and width are held constant.. M-ary Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM) 12 23 Illustration of PAM, PWM and PPM (a) is input (information) signal 24 8-ary PAM (M=8) Symbol rate = 1/T = R/k symbols/s Comparison between Binary and M-ary Modulation Binary PAM (M=2) Symbol rate = Bit rate = R bits/s To maintain the same data rate, multilevel signaling requires less transmission bandwidth than binary signaling K = log28 = 3 bits per symbol We want to maintain the same data rate, R, in comparing different modulation schemes PCM sequence: 101 111 101 010 010 011 010 001