DIGITAL TRANSMISSION AND CODING TECHNIQUES

CHAPTER

12 DIGITAL TRANSMISSION AND CODING TECHNIQUES

12.1

INTRODUCTION

Communications systems transmit signals by means of a number of coding techniques-electrical or optical. In this chapter, we review some of these techniques. Keep in mind that although in electrical transmission voltages may swing between a negative and a positive level, in optical transmission light may change between a nolight condition to some light intensity level; that is, there is negative voltage but no negative light. When light intensity is modulated between two amplitude values, the modulator is known as "amplitude modulator." Alternatively, the phase of light may be modulated between two (angular) values (phase-shift modulator), or the frequency of light between two values (frequency-shift modulator); see also section 6.14.

12.2

RETURN TO ZERO AND NON-RETURN TO ZERO

Figure 12.1 illustrates the return-to-zero (RZ) coding and the non-return to zero (NRZ). With either method, the signal alternates between a positive (+ V) and a negative (- V) voltage. Logic 1 represents the signal at positive voltage and logic 0 the signal at negative. However, in the NRZ method, transitions from logic 0 to logic 1, and vice versa, directly cross the zero voltage level, whereas in the RZ method, transitions stay temporarily on the zero voltage level. In optical communications, the terms RZ and NRZ are used differently. Because there is no negative light, NRZ means that a bit of logical value 1 (a pulse of light) changes its value (from light to no-light or vice versa) at the boundaries of the bit period. Conversely, RZ indicates that the pulse of light is narrower than the bit period. 167

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Part III

RZ

c=:>

Coding Optical Information

+V 0 V ---+---,+--,--+-,>J--r--'--'-'t-'--+-~----i---L1-..L...-;'--

- V -

Electonic regime

+V -

NRZ~

0V -

--+--+---+-----1:}----+-+-+-...l.L-- +-+------j-

-V-

I~~~~~iC

---- ---- -- ----- -- ---- ---- - - l- - - - - -- t -- -- - --f--- - - -~---- - - - + - - - - - - - f - - - +L -

RZ ~

oL +L NRZC=:>

OL -

- -+--+-- -+--B----!- 'l--+---'-'-------+--4--- +--

Figure 12.1 RZ and NRZ coding.

In an optical signal, a logic I on for about one-third of the bit period and is off for about two-thirds. A logic 0 remains off for two-thirds of the bit period.

12.3

UNIPOLAR AND BIPOLAR SIGNALS

A bipolar signal is a three-voltage-level signal that typically swings between a positive and a negative voltage (Figure 12.2). Bipolar signals may be RZ or NRZ. In a digital bipolar signal, the ones alternate between the two voltages, positive and negative. This results in a zero-de component on the transmission line. A unipolar signal is a two-level signal that typically swings between zero and a positive level. A unipolar signal is considered to be an on-off signal that may be applied to either electrical or optical signals. In electrical transmission, assuming that

Unipolar signal ,------1\.

~

Bipolar signal

q

+VOV -

o

+V 0V

-V Figure 12.2 Unipolar and bipolar coding.

o

Chapter 12

Digital Transmission and Coding Techniques

169

statistically there is an equal number of ones and zeros, then there is a de component that may reach half the peak. positive voltage. For transmission over long distances, this de component is undesirable. In optical transmission, a unipolar signal is also known as on-off keying (see Section 12.5). Another category is the multilevel signal. In this case, several voltage levels (e.g., 8) may be used, each level corresponding to one of eight codes. Although multilevel signals are attractive because of their inherent code compression properties, nevertheless they are not used for transmission in communications networks. In optical transmission, multilevel signals do not exist. However, the author has defined methods that use multiwavelength signals.

12.4

48/58, 88/108 CODING

The 4B/5B code translates 4 bits into one of 16 predetermined 5-bit codes. Thus, even if the original 4-bit code is 0000, it is translated to a 5-bit not-all-zero code. This method avoids having all zeros in any code. It may also be designed such that consecutive patterns avoid certain strings of ones. The 4B/5B implies that an initial 1 Gb/s bit rate, after the conversion, has been increased to 1.25 Gb/s because of the added bit. That is, there is 25% overhead bandwidth penalty. Similarly, the 8B/10B code translates 8 bits into one of 256 predetermined 10-bit codes. The bandwidth penalty is also 25%.

12.5

ASK FORMAT

Amplitude-shift keying (ASK) is a technique that uses an electrical bit stream (the modulating frequency) to modulate the intensity of a light beam (the carrier) directly. The carrier has its maximum amplitude for bits having the value "I" and its minimum or zero amplitude for bits having the value "0." This, examined at the unipolar signal case (see Figures 12.1 and 12.2) is also known as on-off keying (OOK). There are two ASK variations: RZ, by which the signal returns to zero at every symbol (1, 0), and NRZ, by which it does not (see Figure 12.1, photonic regime). The ASK format can be used in coherent or in IM/DD systems. However, when a semiconductor laser is directly modulated, the signal phase also shifts..In IMIDD detection, phase shift is unimportant. Coherent detection, however, requires constant phase, and thus the amplitude is externally modulated by means of a titaniumdiffused LiNb0 3 waveguide in a Mach-Zehnder configuration (Figure 12.3) or a semiconductor directional coupler based on electroabsorption multiple quantum well (MQW) properties and structures (Figure 12.4).

170

Part III

Cod ing Optica l Information

Contact Mach-Zehnder external modulator Figure 12.3 In coherent detection the amplitude is externally modulated by mean s of a titanium-diffused LiNb0 3 MachZehnder waveguide.

Contact P-type Fiber

Fiber

MOW N-type

Semiconductor MOW external modulator

Figure 12.4 In coherent detection the amplitude is externally modul ated by means of an MQW directional coupler.

12.6

PSK FORMAT

Phase-shift keying (PSK) is a technique that modulates the phase of a light beam (the carrier) while the frequency and amplitude remain constant during all bits, thus achieving the appearance of a continuous light wave . For binary PSK, the phase is 0° or 180°. For multilevel PSK , the change may be in increments of, for example, 45° (8 levels). PSK is a coherent technique. PSK is implemented externally by passing the light beam through a device known as an electrorefraction modulator; when a voltage is applied to the beam, its refractive index changes. Such device s are made with electro-optic crystals with proper orientation, such as LiNb0 3 The phase difference is expressed by 0

(12.1)

Chapter 12

Digital Transmission and Coding Techniques

~,

171

Voltage

Waveguide

PSK external modulator Figure 12.5 An electrorefraction modul ator changes its refractive index when a voltage is applied to it. Thus the phase of a light beam passing through the device is changed , as well .

where the index change Bn is proportional to applied voltage V and L m is the length over which the index changes by the applied voltage (Figure 12.5).

12.7

FSK FORMAT

Frequency-shift keying (FSK) is a technique that modulates the frequen cy w of a light beam. The frequency of the lightwave changes by !J..f f + !J..f for logic "1," and f - !J..f for logic "0." FSK is a coherent two-state (on-off) PM technique. Typical frequency changes are about 1 GHz. The total bandwidth of an FSK signal is approximated to 2!J..f + 2B, where B is the bit rate and !J..f the frequency deviation. • When the deviation is large (!J..f » case is known as wideband FSK. • When the deviation is narrow (!J..f « case is known as narrowband FSK.

B) , the bandwidth approache s 2!J..f This B), the bandwidth approaches 2B . This

Both cases are distinguished by the frequency modulation index !iflB = f3FM. Clearly, the frequency modulation index is f3FM » 1 or f3FM « 1. The implementation of FSK devices is based on electroacoustic Bragg modulators or on distributed feedback (DFB) semiconductor lasers. Semiconductor laser s exhibit a frequency shift when the operating current changes. A small change in current (-1 rnA) would shift the frequency by -1 GHz . Because the current change is small, the intensity (amplitude) change is small , too . Thus, DFB semiconductor lasers make very good and fast coherent FSK sources with high modulation efficiency. Figure 12.6 summarizes all the shift-keying modulation methods.

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Part III

Binary data

Coding Optical Information

o

ASK (NRZ) ASK (RZ)

PSK

FSK

~ !~