Pulse-position modulation (PPM) is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of 2M possible required time shifts. This is repeated every T seconds, such that the transmitted bit rate is M/T bits per second.
Pulse position modulation is one type of analog modulation which allows variation within the position of the pulses based on the sampled modulating signal’s amplitude is called PPM or Pulse Position Modulation. In this type of modulation, the amplitude & width of the pulses are kept stable & the position of the pulses only varied.
The PPM technique allows computers to transmit data by simply measuring the time taken to reach each data packet to the computer. So is frequently used within optical communication where there is small multi-pathway interference. This modulation totally transmits digital signals & cannot be utilized by analog systems. It transmits simple data which is not efficient while transferring files.
Fig: PPM Waveforms
Demodulation of PPM Signal
Effect of noise on pulse position modulation
Since in a PPM system the transmitted information is contained in the relative positions of the modulated pulses, the presence of additive noise affects the performance of such a system by falsifying the time at which the modulated pulses are judged to occur. Immunity to noise can be established by making the pulse build up too rapidly that the time interval during which noise can exert any perturbation is very short. Indeed, additive noise would have no effect on the pulse positions if the received pulses were perfectly rectangular, because the presence of noise introduces only vertical perturbations. However, the reception of perfectly rectangular pulses would require an infinite channel bandwidth, and the received pulses have a finite rise time, so the performance of the PPM receiver is affected by noise, which is to be expected.
As in a continuous wave (CW) modulation system, the noise performance of a PPM system may be described in terms of the output signal-to-noise ratio (SNR). Also, to find the noise improvement produced by PPM over baseband transmission of a message signal, we may use the figure of merit defined as the output signal-to-noise ratio of the PPM system divided by the channel signal-to-noise ratio. Assuming that the average power of the channel noise is small as compared to the peak pulse power, the figure of merit of the PPM system is proportional to the square of the transmission bandwidth of the (say, BT) normalized with respect to the message signal bandwidth (say, W). When, however, the input signal-to-noise ratio drops below a critical value, the system suffers a loss of the wanted message signal at the receiver output. That is a PPM system suffers from a threshold effect of its own.
As in a continuous wave (CW) modulation system, the noise performance of a PPM system may be described in terms of the output signal-to-noise ratio (SNR). Also, to find the noise improvement produced by PPM over baseband transmission of a message signal, we may use the figure of merit defined as the output signal-to-noise ratio of the PPM system divided by the channel signal-to-noise ratio. Assuming that the average power of the channel noise is small as compared to the peak pulse power, the figure of merit of the PPM system is proportional to the square of the transmission bandwidth of the (say, BT) normalized with respect to the message signal bandwidth (say, W). When, however, the input signal-to-noise ratio drops below a critical value, the system suffers a loss of the wanted message signal at the receiver output. That is a PPM system suffers from a threshold effect of its own.