DIGITAL COMMUNICATION

Transcription

1 DEPARTMENT OF ELECTRICAL &ELECTRONICS ENGINEERING DIGITAL COMMUNICATION Spring 2010 Yrd. Doç. Dr. Burak Kelleci

2 OUTLINE Line Code Differential Encoding Regeneration, Decoding and Filtering Delta Modulation Delta-Sigma Modulation

3 LINE CODES Line code i the electrical repreentation of the binary tream. Line code often ued the terminology nonreturnto-zero (NRZ) and return-to-zero (RZ) Return-to-zero implie that the pule hape ued repreent the bit alway return to the 0 volt or the neutral level before the end of the bit. Nonreturn-to-zero indicate that the pule doe not necearily return to the neutral level before the end of the bit.

4 LINE CODES A good Line Code hould have the following propertie Timing Recovery for ynchronization The receiver hould be able to recover the timing from the tranmitted ignal. Long erie of one and zero could create timing problem. Low probability of bit error for the tranmitted power The line code mut how lowet probability of error for the given bandwidth and tranmitted power. Spectrum mut be uitable for the channel. In ome cae DC component can not be tranmitted through the channel and it hould be avoided. The line code mut ue the minimum channel bandwidth.

5 LINE CODES Unipolar Nonreturn-to-Zero (NRZ) Signaling: Symbol 1: tranmitting a pule amplitude A for the duration of the ymbol. Symbol 0: witching off the pule Thi line code i alo referred to a on-off ignaling. A diadvantage of thi ignaling i the wate of power due to the tranmitted DC

6 LINE CODES Polar Nonreturn-to-Zero(NRZ) Signaling: Symbol 1: tranmitting a pule amplitude +A Symbol 0: tranmitting a pule amplitude -A Thi code i eay to generate and i more powerefficient than it unipolar counterpart.

7 LINE CODES Unipolar Return-to-Zero (RZ) Signaling: Symbol 1: A rectangular pule of amplitude A and halfymbol width. Symbol 0: tranmitting no pule In the power pectrum of thi line code there i a delta function at f=0, ±1/T b, which can be ued for bit-timing recovery at the receiver. Thi line code require 3dB more power than it polar verion for the ame probability of ymbol error.

8 LINE CODES Bipolar Return-to-Zero (BRZ) Signaling: Symbol 1: Poitive and negative pule of equal amplitude (+A and A) are ued alternately Symbol 0: tranmitting no pule Thi ignal ha no DC component and relatively inignificant low-frequency component if ymbol 1 and ymbol 0 ha equal probabilitie.. Thi line code i alo called alternate mark inverion (AMI) ignaling.

9 LINE CODES Split-Phae (Mancheter Code): Symbol 1: a poitive pule of amplitude A followed by a negative pule of amplitude A with both pule being a half-ymbol wide. Symbol 0: the polaritie of thee two pule are revered. The mancheter code uppree the DC component and ha relatively inignificant low-frequency component regardle of the ignal tatitic.

10 LINE CODES

11 POWER SPECTRUM OF DIFFERENT LINE CODES Unipolar NRZ Polar NRZ R=1/T b

12 POWER SPECTRUM OF DIFFERENT LINE CODES Unipolar RZ Bipolar RZ R=1/T b

13 POWER SPECTRUM OF DIFFERENT LINE CODES Mancheter R=1/T b

14 DIFFERENTIAL ENCODING In thi method, the information i encoded at the ignal tranition. Since the information i coded at the tranition, thi encoding require a reference bit before tarting the encoding proce. The original binary information i recovered, by comparing the polarity of adjacent binary dk mk dk 1 ymbol. m k d k d k 1

15 REGENERATION In regenerator, the incoming noiy ignal i recovered and retranmitted again a a clean ignal. In a regenerative repeater three baic operation i performed. equalization: to compenate the effect of ditortion produced by the channel timing: ued to ample the ignal when the ignal-to-noie ratio i at it maximum. deciion making: the extracted ample i compared with a predetermined threhold. If threhold i exceeded, a clean new pule repreenting ymbol 1 i tranmitted. Otherwie, a clean pule repreenting ymbol 0 i tranmitted. Therefore, the accumulation of noie and ditortion i prevented.

16 DECODING AND FILTERING The received clean pule are regrouped into code word and mapped into a quantized PAM ignal. Decoding proce i baically linearly umming all the pule in the code word, with each pule being weighted by it place value (2 0, 2 1, 2 2, 2 3,,2 R-1 ) in the code where R i the number of bit per ample. The meage ignal i recovered by paing the decoder output through a low-pa filter whoe cutoff frequency i equal to the meage bandwidth W. If there i no error occurred on the tranmiion path, the recovered ignal include no noie except the quantization noie.

17 MULTIPLEXING In PCM, uually different meage ignal are multiplexed to utilize the channel efficiently. A the number of independent meage ource i increaed, the time interval that i allocated to each ource i reduced. Therefore, the pule width are reduced to accommodate the increaed number of channel. The narrow pule are difficult to produce and impairment on the channel interfere with the proper operation. Therefore, the number of independent meage ource i retricted depending on the channel and ytem characteritic.

18 EXAMPLE T1 Carrier ytem i deigned to accommodate 24 voice channel which i limited to a band from 300 to 3100Hz. Since W i 3100Hz the Nyquit rate i 6200Hz. The voice ignal are ampled at 8KHz. -law with the contant =255 i ued and total 255 amplitude level are ued. Each frame ha alo an extra bit for ynchronization. Calculate the channel tranmiion rate.

19 EXAMPLE - SOLUTION Each frame conit of (24 x 8) + 1 = 193 bit The frame period i 1/8KHz=125 The duration for each bit i 125 /193 bit = The tranmiion rate i 1/0.647 =1.544 megabit/econd

20 DELTA MODULATION In Delta Modulation (DM), the meage ignal i overampled (at much higher rate than the Nyquit rate) to increae the correlation between adjacent ample. DM provide a taircae approximation to the overampled verion of the meage ignal. The difference between the input and approximation i quantized into only two level, ±.

21 DELTA MODULATION

22 DELTA MODULATION The error between the current ample of the meage ignal and previou approximated ignal i e nt m nt m nt T The error ignal i quantized and added to the previou approximated ignal to generate current output. e nt gn e nt q m The quantized error ignal i tranmitted with a rate of f =1/T, where f i the ampling frequency. nt m nt T e nt q q q q

23 DELTA MODULATION

24 DELTA MODULATION Delta Modulation tranmitter conit of comparator, quantizer and accumulator. The comparator compare two input and the quantizer conit of a hard-limiter. The quantizer output i applied to an accumulator, producing the reult m q n nt gn e it eq it i 1 n i 1

25 DELTA MODULATION In the receiver, poitive and negative pule are paed through an accumulator to generate the approximated meage ignal. The out-of-band quantization noie in the approximated meage ignal i rejected uing a low-pa filter.

26 DELTA MODULATION The input at the quantizer i e nt m nt m nt T q nt T where q(nt) i the quantization error. If we aume that the quantization error mall enough the error i firt backward difference of the input ignal. e nt m nt m nt T Firt backward difference can be een a the digital approximation of the derivative of the input ignal.

27 DELTA MODULATION Delta modulation ha two type of error Slope Overload Ditortion Granular Noie Slope overload Ditortion i due to limited lope of the delta modulation. Maximum lope of m(t) mut atify the following condition to prevent the lope overload ditortion. T S max dm t dt When the input ignal doe not change ignificantly, the DM output change between ±. Thi noie i inverely proportional to the tep ize.

28 DELTA MODULATION DM require high tep ize to reduce the lope overload ditortion and low tep ize to reduce the granular noie. To obtain optimum performance an adaptive DM modulator i needed.

29 EXAMPLE A linear delta modulator i deigned to operate on peech ignal limited to 3.4KHz, The pecification of the modulator are a follow Sampling rate=10 time of the Nyquit rate Step ize =100mV The modulator i teted with a 1KHz inuoidal ignal. Determine the maximum amplitude of thi tet ignal permiible to avoid lope overload.

30 EXAMPLE - SOLUTION The maximum lope of the ignal i 2 fa. Conequently, the maximum change during a ample period i approximately 2 AfT. To prevent lope overload, we require 100mV 2 AfT 2 A 0.092A 1 KHz / 68KHz T S max dm t dt or A 1.08V

31 DELTA SIGMA MODULATION The ignal at the quantizer input i the derivative of the meage ignal. Since the receiver mut accumulate the incoming ignal, it will alo accumulate the noie occurred during the tranmiion. Thi drawback can be mitigated by integrating meage ignal prior to applying it to the Delta Modulator. The integration prior the DM improve the lowfrequency repone, ince the low-frequency content i amplified by the integrator. Deign of the receiver alo i impler than DM receiver.

32 DELTA SIGMA MODULATION A delta modulation cheme that incorporate integration at it input i called delta-igma modulation (D- M). Since the integration performed before the delta modulation, thi cheme hould be called a igma-delta modulation. However, in the literature it i uually called a delta-igma modulation. Let conider the continuou-time form of the delta modulator, where accumulator i replaced by an integrator.

33 DELTA SIGMA MODULATION Since the integration of the meage ignal i cancelled by the differentiation in conventional delta modulator, the receiver conit of a lowpa filter.

34 DELTA SIGMA MODULATION To implify the deign, two integrator are combined in to a ingle integrator before the quantizer.

35 DELTA SIGMA MODULATION In a PCM ytem that ue 8KHz ampling rate with an 8-bit repreentation require 64KHz ymbol rate. On the other hand, delta modulation give the ame performance for 16KHz to 32KHz ampling frequency for deired voice quality. Therefore, DM provide bandwidth aving of 50% to 75% over PCM at the expene of a more complicated implementation. State-of-the-art technique ue correlated technique to reduce the bandwidth from 64kbp to 2.4kbp depending on the voice quality.