Communication Systems Chapter 6 Pulse Modulation

Communication Systems, chapter 6 Pulse Modulation Some parameter of pulse train is varied in accordance with the message signal Analog pulse modulation A periodic pulse train is used as the carrier wave Some characteristic feature of each pulse is varied (e.g. amplitude, duration, position) Information transmitted basically in analog form Digital pulse modulation Information transmitted in digital form as a sequence of coded pulses Communication Systems, chapter 6

Communication Systems, chapter 6 The Sampling Process An analog signal is converted into a corresponding sequence of samples that are usually spaced uniformly in time It is necessary that we choose sampling rate properly Sequence of samples uniquely defines the original analog signal Communication Systems, chapter 6

Communication Systems, chapter 6 The Sampling Process Communication Systems, chapter 6

Pulse-Amplitude Modulation The amplitudes of regularly spaced pulses are varied in proportion to the corresponding sample values of a continuous message signal Pulses can be of a rectangular form or some other appropriate shape PAM is somewhat similar to natural sampling Communication Systems, chapter 6

Pulse-Amplitude Modulation Two operations involved in the generation of the PAM signal : Instantaneous sampling of the message signal m(t) every Ts seconds, where the sampling rate fs = 1/Ts is chosen in accordance with the sampling theorem Lengthening the duration of each sample so obtained to some constant value T Communication Systems, chapter 6

Time-Division Multiplexing Enables the joint utilization of a common communication channel by a plurality of independent message sources without mutual interference among them Each input message if first restricted in bandwidth by a low-pass filter to remove the frequencies that are nonessential to an adequate signal representation Outputs are then applied to the commutator To take a narrow sample of each of the N input message at a rate fs To sequentially interleave these N samples inside the sampling interval Ts Communication Systems, chapter 6

Time-Division Multiplexing Pulse modulator (following the commutator) To transform the multiplexed signal into a form suitable for transmission over the common channel At the receiving end of the system Signal is applied to a pulse demodulator reverse operation of the pulse modulator Outputs are distributed to the appropriate low-pass reconstruction filters by means of a decommutator Operates in synchronism with the commutator Communication Systems, chapter 6

Time-Division Multiplexing Communication Systems, chapter 6

Pulse-Position Modulation The samples of the message signal are used to vary the duration of the individual pulses Known also as : pulse-width or pulse-length modulation May vary the time of occurrence of leading edge, trailing edge, or the both edges of the pulse Communication Systems, chapter 6

Pulse-Position Modulation Communication Systems, chapter 6

Pulse-Position Modulation Communication Systems, chapter 6

Pulse-Position Modulation Communication Systems, chapter 6

Bandwidth-Noise Trade-Off PPM and FM exhibit similar noise performance : Both have a figure of merit proportional to the square of the transmission bandwidth normalized with respect to the message bandwidth Both exhibit a threshold effect as the signal-to-noise ratio is reduced Communication Systems, chapter 6

The Quantization Process A continuous signal has a continuous range of amplitudes Infinite number of amplitude levels The original continuous signal may be approximated by a signal constructed of discrete amplitudes selected on a minimum error basis from available set Communication Systems, chapter 6

Communication Systems, chapter 6 Quantization noise Difference between the input signal and output signal is called quantization noise Communication Systems, chapter 6

Pulse-Code Modulation A message signal is represented by a sequence of coded pulses, which is accomplished by representing the signal in discrete form in both time and amplitude Basic operations in transmitter are : sampling, quantizing and encoding Basic operations in receiver are : regeneration, decoding and reconstruction Communication Systems, chapter 6

Pulse-Code Modulation Communication Systems, chapter 6

Pulse-Code Modulation Sampling : The incoming signal is sampled with a train of narrow rectangular pulses Low-pass filter is used before the sampler in order to exclude frequencies greater than W before sampling W = the highest frequency component Communication Systems, chapter 6

Pulse-Code Modulation Quantization : To provide a new representation of the signal that is discrete in both time and amplitude Works as presented in earlier slides It is preferable to use a variable separation between the representation levels Communication Systems, chapter 6

Pulse-Code Modulation Encoding : To translate the discrete set of sample values to a more appropriate form of signal Code word/character -> code element/symbol -> code There are several line codes that can be used for the electronical representation of binary symbols 1 and 0 On-Off signalling Nonreturn-to-zero (NRZ) signalling Return-to-zero (RZ) signalling Bipolar return-to-zero (BRZ) signalling Split-phase (Manchester code) Differential encoding Communication Systems, chapter 6

Pulse-Code Modulation (Encoding) Communication Systems, chapter 6

Pulse-Code Modulation Regeneration : Reconstruct the PCM signal with the regenerative repeaters located along transmission route to avoid the distortion and noise produced transmission through a channel The regenerated signal departs from the original The unavoidable presence of channel noise and interference causes the repeater to make wrong decisions occasionally If the spacing between received pulses deviates from its assigned value, a jitter is introduced into the regenerated pulse position, thereby causing distortion Communication Systems, chapter 6

Pulse-Code Modulation (Regeneration) Communication Systems, chapter 6

Pulse-Code Modulation Decoding : The first operation in the receiver is to regenerate the received pulses one last time These pulses are then regrouped into code words and decoded into a quantized PAM signal Communication Systems, chapter 6

Pulse-Code Modulation Filtering : The final operation in the receiver is to recover the message signal wave by passing the decoder output through a low-pass reconstruction filter Communication Systems, chapter 6

Pulse-Code Modulation Multiplexing : In PCM-applications it is natural to multiplex different message sources by time division Communication Systems, chapter 6

Pulse-Code Modulation Synchronization : To synchronize transmitter and receiver Local clocks to keep the same time at the transmitter and the receiver Communication Systems, chapter 6

Noise Considerations in PCM Systems Channel noise Anywhere between the transmitter output and the receiver input Always present Quantization noise In the transmitter and is carried all the way to the receiver output Signal dependent Communication Systems, chapter 6

Communication Systems, chapter 6 Delta Modulation An incoming sample is over sampled to purposely increase the correlation between adjacent samples of the signal Provides a staircase approximation to the over sampled version of the message signal Difference between the input and approximation is quantized into only two levels, namely ±Δ Communication Systems, chapter 6

Communication Systems, chapter 6 Delta Modulation Communication Systems, chapter 6

Communication Systems, chapter 6 Differential PCM Remove redundancy (not absolutely essential to the transmission of information) before encoding to obtain a more efficient coded signal As PCM suffers from quantization noise Communication Systems, chapter 6

Communication Systems, chapter 6 Differential PCM Communication Systems, chapter 6

Coding Speech at Low Bit Rates To remove redundancies from the speech signal as far as possible To assign the available bits to code the nonredundant parts of the speech signal in a perceptually efficient manner Adaptive DPCM adaptive quantization and prediction Adaptive subband coding The number bits used to encode each subband is varied dynamically Communication Systems, chapter 6