L6-PCM Detection

L6-PCM Detection - 1 Decision making in a PCM system In a...

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Unformatted text preview: 1 Decision making in a PCM system In a binary PCM transmission system, sharp-edge pulses of two well defined amplitude levels (either unipolar or polar) are transmitted once every signalling period T b . As this pulse train travels down the communication channel (cable, optical fibre, or radio link), it suffers from amplitude reduction due to the attenuation of the channel, and its sharp edges become dispersed in time because of bandlimitation of the channel. This weakened and dispersed signal is further perturbed by noise so that the signal arrived at the detection node in the receiver consists of the algebraic sum of the two components, i.e., (signal + noise). At the receiver, the first operation is the amplification and shaping of the weakened input signal by an equalising amplifier to an appropriate level and form so that a reliable decision on the transmitted pulse sequence may be made. Then the amplified sequence of pulse signal plus noise are sampled once every T b seconds, and a decision is made as to whether a binary " 1 " or " " is transmitted over a particular bit interval T b . The decision making process in PCM involves two key circuit operations: a decision function a timing function. A r 0 A r /2 -A r /2 T b A r <<A A 0 A/2 -A/2 T b Unipolar Polar Channel: (Attenuation & bandlimitation) Transmit Receive 2 Decision function and threshold detection Ignoring for the moment distortion due to bandlimitation of the transmission path, the signal will arrive at the receiver with noise and interference picked up during transmission superimposed on it. If, at the center of the time slot T b , the signal amplitude exceeds the preset threshold level V ref , a mark or " 1 " is decided; if it is below V ref , then a space or " " is detected. The threshold level for unipolar signalling will normally be set at mid-way the nominal signal level, i.e., V ref = V/2. With added noise, errors in detection occur when A " " is sent and the added noise exceeds V/2 , or A " 1 " is sent and the added noise, at the sampling instant, is 2 V < . Received signal Equalising amplifier Decision circuit V ref Timing unit Pulse generator Output Average clock frequency f b = 1/T b T b Signal + noise A typical PCM receiver. T b /2 T b /2 Nominal sampling instant. V 0 Nominal signal levels Threshold level V ref = V/2 Threshold decision with unipolar signaling. 3 Bit error performance The performance of a digital transmission link is usually expressed in terms of error probability (likelihood of errors or error rate) as a function of signal-to- noise ratio at the decision node. It is influenced by the kind of noise and interference present....
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L6-PCM Detection - 1 Decision making in a PCM system In a...

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