La Actuación y el Espectáculo en la Red

At the receiver errors occur when additive noise changes the level of the information data in the transmission channel, so that a wrong threshold decision is made at the sampling instant. The PIWCM code is recovered by determining the position of the pulse in each time frame relative to the clock interval. There are three sources of error for PIWCM:

1) Erasures: this occurs whenever noise destroys the pulse thus preventing detection. In PIWCM, this may result in change of mark and/or space, see Figure 4.9.a.

2) False alarm errors: in the interval between the start and arrival of the next rising or falling edge, the received output voltage may cross the threshold due to noise, see Figure 4.9.b.

3) Wrong slot errors: noise on the leading pulse or trailing by one time slot changes the width of mark, see Figure 4.9.c.

The effect of noise on the rising and falling edge (also known as wrong slot error) may be best explained in Figure 4.10.a where a frame may change by one or two time slots. A frame of combination (4,3), affected by wrong slot error, may be received as (4,4) or (4,5) etc., as shown in Figure 4.10.b, whilst false alarm and erasure errors may lead to a very different detected combination. For wrong slot errors the effect on the rising edge changes the length of the frame and may therefore extend or shorten the width of mark or the width of space. On the other hand, the falling edge does not influence the frame length but changes the mark-space ratio.

1 2 3

(b) Folse Alarm Z

2 3

(c) Wrong Slot

Figure 4.9 Error sources for PIWCM.

Combinations eg. (3,2) and (5,4) and those placed further away from the original frame due to very high noise levels, may not be considered, since they require errors on two edges and possibly an erasure or false alarm error in the same frame. The effect of the wrong slot changes for the code pattern (4, 3) may be summarised in Table 4.1.

<r I I I i i_

F,

Figure 4.10 Changes in the mark-space (4,3) ratio due to noise: (a) frame; (b) coding scheme).

detected change change high- low- decimal level

m, s of m, s of L word word value difference

3, 3 1 ,0 -1 0010 0010 34 -16 3 ,4 _1,1 0 0010 0011 35 -15 4 ,2 0 ,1 -1 0011 0001 49 -1 4 ,3 0 ,0 0 0011 0010 50 0 4 ,4 _{0 ,1} +1 0011 0011 51 +1 5 ,2 _1,1 0 0100 0001 65 +15 5 ,3 1 ,0 +1 0100 0010 66 +16

Table 4.1 Effect of noise on the mark-space combination.

A change of mark by one time slot results in a level difference of ±16 quantisation levels, whereas a change of space by one time slot results in a level difference of only ±1 quantisation level. Again, this phenomenon is according to the coding rules, ie. the division into high-word and low-word where the difference of bit changes in the high-word is obviously more significant.

Error sources for PICM are similar to those of DPPM, see Page 41. Differences between PICM and DPPM error occurrences are that the PICM pulse train has two pulses per frame and these pulses are also longer than the DPPM pulses assuming the same data rate. In other words, the error rate for PICM may be twice as much as for DPPM. Furthermore, when transmitting PICM over a sufficiently long period, a point is reached where due to occurrence of erasure or a false alarm error the pulses are eventually swapped around. As a consequence, the detection of PICM forces the demodulator to convert first the low-word and then the high-word which inevitably leads to an indistinguishable recovered message signal. The error sources of PICM are of three kinds and can be described as follows:

2) False alarm errors: violation by noise that causes the receiver to detect a pulse, see Figure 4.11.b.

3) Wrong slot errors: noise on the leading pulse edge produces a detection immediately preceding or following the containing pulse, see Figure 4.11.c.

(a) Erasure / K ! _ —^, ! > ! 1 i t ______________ i l _____ Error ! 1 r No / s Actu ninol >1 [I II H \ (b) False Alarm Error ! / i

_{/ |}

Ii i

;

Figure 4.11 Error sources for PICM.

4.7. Summary

Theoretical properties of PIWCM and PICM techniques have been introduced in this chapter. It has been shown that the frame length varies according to the amplitude value of the modulating signal and consequently the sampling frequency varies according to the frame length. The average transmission capacity of PIWCM/PICM may be doubled compared to the fixed transmission capacity of PCM, but at the expense of increased clock

frequency and transmission bandwidth. Mathematical representations for both the PIWCM and PICM codes have been derived both in time and frequency domains. The frequency spectrum of PICM shows very distinctive peaks at the slot frequency, whereas these peaks are cancelled out in the PIWCM spectrum. Hence optimum timing extraction can be accomplished with PICM (compared to PIWCM and PCM).

Three different error sources, namely erasure, false alarm and wrong slot errors, that might affect the performance of PIWCM and PICM were introduced. Although PICM is better suited for optical transmission than PIWCM and PCM, only a single erasure or wrong slot error may have a hazardous effect on the recovered message signal. This indicates the need for a high-power drive for the optical source and/or highly sophisticated detection and filtering devices in conjunction with error correction schemes at the receiver. With PIWCM, the problem of errors is substantially less, since the receiver always synchronises on the rising edge and a frame containing an error may then be regarded as worthless. On arrival of the next correct frame, the PIWCM receiver is ready to receive the next correct sequence. Thus requiring less advanced equipment for transmission and detection compared to PICM and PCM.

CHAPTER 5