National Conference on Recent Trends in Engineering & Technology Performance evaluation of Power Spectral Density of different line coding technique Darshankumar C. Dalwadi1, Bhargav C. Goradiya2, Milendrakumar M. Solanki3, Mehfuza S.Holia4 1Asst. Prof., Electronics & Telecommunication dept., B.V.M. Engineering college, V.V.Nagar, 2Asso. Prof. & Head, Electronics & Telecommunication dept., B.V.M. Engineering College, V.V.Nagar, 3Asst. Prof., Electronics dept., B.V.M. Engineering College, V.V.Nagar, 4Asst. Prof., Electronics dept., B.V.M. Engineering College, V.V.Nagar Abstract—In this paper we have presented an effect of different the actual or implied integral clock cycle. The real question is line coding technique. We have plot power spectral density of that of sampling--the high or low state will be received different line coding technique like non return to zero, return to correctly provided the transmission line has stabilized for that zero and Manchester coding with respect to the different bit when the physical line level is sampled at the receiving frequencies. We have also shown the which line coding technique end. is best compare to all of them. We used the MATLAB in our simulation work. However, it is helpful to see NRZ transitions as happening on the trailing (falling) clock edge in order to compare NRZ- Level to other encoding methods, such as the mentioned I. INTRODUCTION Manchester code, which requires clock edge information (is The basic block diagram of digital communication system the XOR of the clock and NRZ, actually) and to see the involves sampler, quantizer and line coder. The sampler difference between NRZ-Mark and NRZ-Inverted. converts continuous time signal into discrete time signal. Return-to-zero (RZ) describes a line code used in Quantizer converts continuous valued signal into discrete telecommunications signals in which the signal drops (returns) valued signal. Line coder generates a bit pattern with respect to zero between each pulse. This takes place even if a number to the different technique. There a basic three types of line of consecutive 0's or 1's occur in the signal. The signal is self- coding technique: (i) On-off, (ii) Polar and (iii) Bipolar. All clocking. This means that a separate clock does not need to be these technique are with respect to non return to zero or return sent alongside the signal, but suffers from using twice the to zero method. bandwidth to achieve the same data-rate as compared to non- In telecommunication, a non-return-to-zero (NRZ) line code return-to-zero format. is a binary code in which 1's are represented by one significant The "zero" between each bit is a neutral or rest condition, condition (usually a positive voltage) and 0's are represented such as a zero amplitude in pulse amplitude modulation by some other significant condition (usually a negative (PAM), zero phase shift in phase-shift keying (PSK), or mid- voltage), with no other neutral or rest condition. The pulses frequency in frequency-shift keying (FSK). That "zero" have more energy than a RZ code. Unlike RZ, NRZ does not condition is typically halfway between the significant have a rest state. NRZ is not inherently a self-synchronizing condition representing a 1 bit and the other significant code, so some additional synchronization technique (for condition representing a 0 bit. example a run length limited constraint or a parallel Although return-to-zero (RZ) contains a provision for synchronization signal) must be used to avoid bit slip. synchronization, it still has a DC component resulting in For a given data signaling rate, i.e., bit rate, the NRZ code “baseline wander” during long strings of 0 or 1 bits, requires only half the bandwidth required by the Manchester just like the line code non-return-to-zero. code. When used to represent data in an asynchronous communication scheme, the absence of a neutral state requires other mechanisms for bit synchronization when a separate II. TYPES OF DIFFERENT LINE CODING TECHNIUQE clock signal is not available. NRZ-Level itself is not a synchronous system but rather an A. Unipolar Non Return to Zero encoding that can be used in either a synchronous or "One" is represented by one physical level (such as a DC asynchronous transmission environment, that is, with or bias on the transmission line). without an explicit clock signal involved. Because of this, it is "Zero" is represented by another level (usually a positive not strictly necessary to discuss how the NRZ-Level encoding voltage). acts "on a clock edge" or "during a clock cycle" since all transitions happen in the given amount of time representing 13-14 May 2011 B.V.M. Engineering College, V.V.Nagar,Gujarat,India National Conference on Recent Trends in Engineering & Technology In clock language, "one" transitions or remains high on the coupled, and that a clock signal can be recovered from the trailing clock edge of the previous bit and "zero" transitions or encoded data. remains low on the trailing clock edge of the previous bit, or Manchester code is widely used (e.g. in Ethernet; see also just the opposite. This allows for long series without change, RFID). There are more complex codes, such as 8B/10B which makes synchronization difficult. One solution is to not encoding, that use less bandwidth to achieve the same data send bytes without transitions. Disadvantages of an on-off rate but may be less tolerant of frequency errors and jitter in keying are the waste of power due to the transmitted DC level the transmitter and receiver reference clocks. and the power spectrum of the transmitted signal does not approach zero at zero frequency. Figure 1 Unipolar Non Return to Zero B. Non Return to Zero Inverted Figure 3 Encoding of Manchester coding Non return to zero, inverted (NRZI) is a method of mapping a binary signal to a physical signal for transmission over some III. POWER SPECTRAL DENSITY OF DIFFERENT LINE CODING transmission media. The two level NRZI signal has a TECHNIQUE transition at a clock boundary if the bit being transmitted is a logical 1, and does not have a transition if the bit being transmitted is a logical 0. The Power spectral density of Non return to zero is given by "One" is represented by a transition of the physical level. y = v*t*(sin (w*(0.5*t)) / ((0.5*t)*w)) ^2 "Zero" has no transition. Where, v=1 Also, NRZI might take the opposite convention, as in t =.001 Universal Serial Bus (USB) signalling, when in Mode 1 f =-5000:1:5000 (transition when signalling zero and steady level when w= (2*pi)*f signalling one). The transition occurs on the leading edge of the clock for the given bit. This distinguishes NRZI from The power spectral density of Return to zero is given by, NRZ-Mark. y = 0.25*v*t*(sin (w*(0.25*t)) / ((0.25*t)*w)) ^2 However, even NRZI can have long series of zeros (or ones Where, v = 1 if transitioning on "zero"), so clock recovery can be difficult t = .001 unless some form of run length limited (RLL) coding is used f = -5000:1:5000 on top. Magnetic disk and tape storage devices generally use w = (2*pi)*f fixed-rate RLL codes, while USB uses bit stuffing, which is efficient, but results in a variable data rate: it takes slightly longer to send a long string of 1 bits over USB than it does to The power spectral density of Manchester cod is given by, send a long string of 0 bits. (USB inserts an additional 0 bit y = ((4*v*v)*(1-cos (w*t*0.5)) ^2)/ (t*w*w) after 6 consecutive 1 bits.) Where, v = 1 t =.001 f = -5000:1:5000 w = (2*pi)*f IV SIMULATION RESULTS Figure 2 NRZI encoding I. Power spectral density of Non Return to Zero Figure 4 shows the power spectral density of non return to C. Manchester coding zero. It is clearly seen from the figure at zero frequencies In telecommunication, Manchester code (also known as power is not zero. So, it is not a good line coding technique in Phase Encoding, or PE) is a line code in which the encoding of terms of ac coupling. each data bit has at least one transition and occupies the same time. It therefore has no DC component, and is self-clocking, which means that it may be inductively or capacitively 13-14 May 2011 B.V.M. Engineering College, V.V.Nagar,Gujarat,India National Conference on Recent Trends in Engineering & Technology Figure 5 shows the power spectral density of Return to zero. It is clearly seen from the figure at zero frequency power is not zero. So again it is not a good line coding technique in terms of ac coupling. III. Power spectral density of Manchester coding Figure 4 Power spectral density of Non Return to Zero II. Power spectral density of Return to Zero Figure 6 Power spectral density of Manchester coding Figure 6 shows the power spectral density of Manchester coding. It is clearly seen from the figure, at zero frequency power is zero. So, it is a good line coding technique compare to other techniques. CONCLUSION From this paper it is conclude that compare to all line coding techniques Manchester coding is best line coding technique. Because it is having zero power at zero frequency, it is having a transparency, less energy requirement and also it is easy to extract the timing information form that. So, that’s why Manchester coding is preferred compare to other line coding techniques.