Philadelphia University/Faculty of Engineering Communication and Electronics Engineering Digital Communication (650533) CH 3 Pulse Modulation Instructor: Eng. Nada Khatib Website: http://www.philadelphia.edu.jo/academics/nkhatib/ 1 Ch 3- lecture 2/5 Eng. Nada Khatib
Outline Introduction PCM Sampling Process Quantization Process Encoding Line codes Regeneration Noise in PCM TDM DPCM Delta Modulation 2 Ch 3- PCM Eng. Nada Khatib
Introduction Analog signal versus digital signal Almost every communication system in use today is a digital. Cell phone system (GSM, UMTS,LTE) GPS, etc
Why are digital communications becoming popular? Noise immunity of digital signals Information extraction is often easier from analog signal. Viability of regenerative repeater in Digital comm.
Why digital communication? The ability to detect and correct errors by using error detection and correction techniques. The ability to compress data The ability to securely send data by using encryption for example.
Disadvantage digital communication More bandwidth. Synchronization is more complicated.
PCM PCM = Pulse code Modulation It is a method of converting an analog signal into a digital binary signal (A/D conversion). An analog signal can be converted into binary digital signal by means of Sampling Quantization Encoding
Sampling Process
Sampling Process Ts= sampling interval fs 2fm Nyquist criterion Fs=2fm Nyquist rate Fs> 2fm oversampling Fs<2fm undersampling aliasing Aliasing = spectral folding = tail inversion
Sampling Process
Sampling Process Use low pass anti-aliasing filter prior to sampling process to combat the effects of aliasing in practice
Sampling Process the standard CCITT voice channel, 300 3400 Hz (a bandwidth of 3100 Hz) So for telephony the sampling rate is 8000 samples per second, or 8 khz. And the sampling period is therefore 1/8 khz (i.e., 125 μs).
Quantization The process of transformation the sample amplitudes of a signal into discrete amplitude taken from a finite set of possible amplitude (Lo, L1, L2,..). Number of representation levels is L
Quantization L V p p s
Quantization Noise Also known as Quantization error and round-off error. It is the difference between the input sampled signal and the output quantized signal e q m( nt ) m ( nt s q s ) The degradation of the signal due to quantization is limited to (± s/2) The maximum quantization error is ± s/2
Quantization Noise A useful figure of merit for the uniform quantizer is the mean-square quantization error. q 12 The output signal-to-quantization noise ratio of a uniform quantizer is S q R S m 2 2 ( t) q
Non uniform Quantization In telephonic communication, it is preferable to use a variable separation between the representation levels. Nonuniform quantization uses a variable step size.
Non uniform Quantization The use of a nouniform quantizer is equivalent to passing the baseband signal through a compressor and then applying the compressed signal to a unifrom quantizer.
Non uniform Quantization A particular form of compression law is used in practice is μ-law: which is defined by A-law: which is defined by
Non uniform Quantization
Non uniform Quantization The compressed samples must be restored to their original values at receiver by using an expander with a characteristic complements to that of the compressor. The combination of a compressor and an expander is called a compander.
Encoding Represent each quantized level by a group of n binary digit and transmit these n bits. L 2 n, The output SQNR of a uniform quantizer for in db is SQNR P 6n 10log(12 m 2 The output SN q R of a uniform quantizer for a full-load sinusoidal modulating signal in db is SN R 1.8 6n q m p p )
Bit Rate and bandwidth Bit rate (R b ) is the number of bits sent per second. R nf b s R b 1 T b The minimum required channel bandwidth is BW min 1 2 R b
Line codes: Unipolar NRZ signaling Polar NRZ signaling. Unipolar RZ signaling. Bipolar RZ signaling or AMI Split-phase or Manchester code
Differential encoding: Assuming reference bit 1 Waveform of differentially encoded data using unipolar NRZ signaling.
The basic elements of a PCM system.
Regenerative repeater. Block diagram of regenerative repeater.
Noise in PCM system Quantization Noise: Signal dependent Under the designer s control Channel Noise: The main effect of channel noise is o introduce bit error into the received signal. Bit error rate (BER) = P e R b
Block diagram of TDM system. f s 2 f max
E1 System
Conceptual diagram of multiplexingdemultiplexing.
European Digital Hierarchy
PDH E Line E-1 E-2 E-3 E-4 Rate (Mbps) 2.048 8.448 34.368 139.264 Voice Channels 30 120 480 1920
Differential Pulse code modulation (DPCM) Estimate (predict) the value of the kth sample m[k] from the knowledge of the previous sample values. Transmit the prediction error e[ nts] m[ nts] mˆ[ nts]
prediction filter a1, a2,.. a N = prediction coefficients = filter coefficients Block diagram of a linear prediction filter of order N.
DPCM Transmitter DPCM system Receiver DPCM system.
DPCM-Processing gain The processing gain (Gp) produced by the differential quantization scheme is defined by: Power of m(t) SNR improvement due to prediction G p P P m d Power of difference between m[nts] and its predicated value m
Delta Modulation (DM) The incoming signal is oversampled to increase the correlation between adjacent samples of the signal. Quantized into only two levels (encoded by 1-bit). DM provides a staircase approximation to the oversampled version of the message signal.
Delta Modulation (DM) Illustration of delta modulation
Delta Modulation (DM) Transmitter DM system
Delta Modulation (DM) Receiver DM system
Delta Modulation (DM) e q (nts) e(t) e q (t) Transmitter DM system
Delta Modulation (DM) e q (nts) Receiver DM system
Problems in DM The Granular noise lies in the range (-s,s). The granular noise power is s 2 /3 S
Problems in DM No overload occurs if Maximum slop of the original input signal must follow this condition: max dm( t) dt CVSD (continuous variable slope delta): Make the delta modulator adaptive in the sense that the step size is made to vary in accordance with the input signal. S f s