4964ch02.qxd_tb/lb 2/12/01 7:48 AM Page 55 CHAPTER 2 Formatting and Baseband Modulation Information From other source sources Message Channel symbols symbols Freq- X Source Channel Multi- Pulse Bandpass Multiple Format Encrypt uency M encode encode plex modulate modulate access spread T ui gi(t) si(t) Digital input C m i h (t) h Digital Digital c a Bit Synch- Channel baseband bandpass n stream ronization impulse n waveform waveform Digital response e output l mi ui z(T) r(t) Demod- Freq- R Source Channel Demulti- Multiple FormatDecrypt Detect ulate uency C decode decode plex access & Sample despread V Message Channel symbols symbols Optional Information To other sink destinations Essential 55 4964ch02.qxd_tb/lb 2/12/01 7:48 AM Page 56 The goal of the first essential signal processing step, formatting, is to insure that the message (or source signal) is compatible with digital processing. Transmit format- ting is a transformation from source information to digital symbols. (It is the re- verse transformation in the receive chain.) When data compression in addition to formatting is employed, the process is termed source coding. Some authors consider formatting a special case of source coding. We treat formatting (and base- band modulation) in this chapter, and treat source coding as a special case of the efficient description of source information in Chapter 13. In Figure 2.1, the highlighted block labeled “formatting” contains a list of topics that deal with transforming information to digital messages. The digital mes- sages are considered to be in the logical format of binary ones and zeros until they are transformed by the next essential step, called pulse modulation, into baseband (pulse) waveforms. Such waveforms can then be transmitted over a cable. In Figure 2.1, the highlighted block labeled “baseband signaling” contains a list of pulse mod- ulating waveforms that are described in this chapter. The term baseband refers to a signal whose spectrum extends from (or near) dc up to some finite value, usually less than a few megahertz. In Chapter 3, the subject of baseband signaling is contin- ued with emphasis on demodulation and detection. 2.1 BASEBAND SYSTEMS In Figure 1.2 we presented a block diagram of a typical digital communication sys- tem. A version of this functional diagram, focusing primarily on the formatting and transmission of baseband signals, is shown in Figure 2.2. Data already in a digital 56 Formatting and Baseband Modulation Chap. 2 4964ch02.qxd_tb/lb 2/12/017:48AMPage57 Formatting Source Coding Baseband Signaling Equalization Character coding Predictive coding PCM waveforms (line codes) Maximum-likelihood sequence Sampling Block coding Nonreturn-to-zero (NRZ) estimation (MLSE) Quantization Variable length coding Return-to-zero (RZ) Equalization with filters Pulse code modulation Synthesis/analysis coding Phase encoded Transversal or decision feedback (PCM) Lossless compression Multilevel binary Preset or Adaptive Lossy compression M-ary pulse modulation Symbol spaced or fractionally PAM, PPM, PDM spaced Bandpass Signaling Channel Coding Coherent Noncoherent Waveform Structured Sequences Phase shift keying (PSK) Differential phase shift keying (DPSK) Frequency shift keying (FSK) Frequency shift keying (FSK) M-ary signaling Amplitude shift keying (ASK) Amplitude shift keying (ASK) Antipodal Block Continuous phase modulation (CPM) Continuous phase modulation (CPM) Orthogonal Convolutional Hybrids Hybrids Trellis-coded modulation Turbo Synchronization Multiplexing/Multiple Access Spreading Encryption Frequency synchronization Frequency division (FDM/FDMA) Direct sequencing (DS) Block Phase synchronization Time division (TDM/TDMA) Frequency hopping (FH) Data stream Symbol synchronization Code division (CDM/CDMA) Time hopping (TH) Frame synchronization Space division (SDMA) Hybrids Network synchronization Polarization division (PDMA) Figure 2.1 Basic digital communication transformations 57 4964ch02.qxd_tb/lb 2/12/01 7:48 AM Page 58 Digital information Format Information Textual source information Pulse Transmit Analog modulate information Sample Quantize Encode Pulse Bit wave Channel stream forms Format Analog Low-pass Decode Demodulate/ information filter Receive detect Information Textual sink information Digital information Figure 2.2 Formatting and transmission of baseband signals. format would bypass the formatting function. Textual information is transformed into binary digits by use of a coder. Analog information is formatted using three separate processes: sampling, quantization, and coding. In all cases, the formatting step results in a sequence of binary digits. These digits are to be transmitted through a baseband channel, such as a pair of wires or a coaxial cable. However, no channel can be used for the transmission of binary digits without first transforming the digits to waveforms that are compati- ble with the channel. For baseband channels, compatible waveforms are pulses. In Figure 2.2, the conversion from a bit stream to a sequence of pulse wave- forms takes place in the block labeled pulse modulate. The output of the modula- tor is typically a sequence of pulses with characteristics that correspond to the digits being sent. After transmission through the channel, the pulse waveforms are recovered (demodulated) and detected to produce an estimate of the transmitted digits; the final step, (reverse) formatting, recovers an estimate of the source information. 2.2 FORMATTING TEXTUAL DATA (CHARACTER CODING) The original form of most communicated data (except for computer-to-computer transmissions) is either textual or analog. If the data consist of alphanumeric text, they will be character encoded with one of several standard formats; examples include the American Standard Code for Information Interchange (ASCII), the Extended Binary Coded Decimal Interchange Code (EBCDIC), Baudot, and Hollerith. The textual material is thereby transformed into a digital format. The ASCII format is shown in Figure 2.3; the EBCDIC format is shown in Figure 2.4. 58 Formatting and Baseband Modulation Chap. 2 4964ch02.qxd_tb/lb 2/12/017:48AMPage59 5 0 1 0 1 0 1 0 1 Bits 6 0 0 1 1 0 0 1 1 1 2347 0 0 0 0 1 1 1 1 0000 NULDLE SP 0 @ P ' p NUL Null, or all zeros DC1 Device control 1 1000 SOHDC1 ! 1 A Q a q SOH Start of heading DC2 Device control 2 STX Start of text DC3 Device control 3 0100 STXDC2 " 2 B R b r ETX End of text DC4 Device control 4 1100 ETXDC3 # 3 C S c s EOT End of transmission NAK Negative acknowledge 0010 EOTDC4 $ 4 D T d t ENQ Enquiry SYN Synchronous idle 1010 ENQNAK % 5 E U e u ACK Acknowledge ETB End of transmission BEL Bell, or alarm CAN Cancel 0110 ACKSYN & 6 F V f v BS Backspace EM End of medium 1110 BELETB ' 7 G W g w HT Horizontal tabulation SUB Substitute 0001 BSCAN ( 8 H X h x LF Line feed ESC Escape VT Vertical tabulation FS File separator 1001 HTEM )9IYiy FF Form feed GS Group separator 0101 LFSUB * : J Z j z CR Carriage return RS Record separator 1101 VTESC + ; K [ k { SO Shift out US Unit separator SI Shift in SP Space 0011 FFFS , < L\ l | DLE Data link escape DEL Delete 1011 CR GS –=M]m} 0111 SORS . > N > n~ 1111 SIUS / ? O – o DEL Figure 2.3 Seven-bit American standard code for information interchange (ASCII). 59 4964ch02.qxd_tb/lb 2/12/017:48AMPage60 60 50000000011111111PF Punch off HT Horizontal tab 6 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 Bits LC Lower case 7 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 DEL Delete 80101010101010101SP Space UC Upper case 1 234 RES Restore 0000 NULSOH STX ETX PF HT LC DEL SMM VT FF CR SO SI NL New line BS Backspace 0001 DLEDC1 DC2 DC3 RES NL BS IL CAN EM CC IFS IGS IRS IUS IL Idle 0010 DSSOS FS BYP LF EOB PRE SM ENQ ACK BEL PN Punch on EOT End of transmission 0011 SYN PN RS US EOT DC4 NAK SUB BYP Bypass 0100 SP c <(+! LF Line feed 0101 & !$*) ;¬ EOB End of block PRE Prefix (ESC) 0110 – / , % __ > ? RS Reader stop 0111 :#@'=" SM Start message 1000 abcdefghi DS Digit select SOS Start of significance 1001 j k lmnopqr IFS Interchange file 1010 s t uvwx yz separator IGS Interchange group 1011 separator 1100 ABCDEFGHI IRS Interchange record 1101 JKLMNOPQR separator IUS Interchange unit 1110 STUVWXYZ separator 1111 01 23456789 Others Same as ASCII Figure 2.4 EBCDIC character code set. 4964ch02.qxd_tb/lb 2/12/01 7:48 AM Page 61 The bit numbers signify the order of serial transmission, where bit number 1 is the first signaling element. Character coding, then, is the step that transforms text into binary digits (bits). Sometimes existing character codes are modified to meet spe- cialized needs. For example, the 7-bit ASCII code (Figure 2.3) can be modified to include an added bit for error detection purposes. (See Chapter 6.) On the other hand, sometimes the code is truncated to a 6-bit ASCII version, which provides ca- pability for only 64 characters instead of the 128 characters allowed by 7-bit ASCII. 2.3 MESSAGES, CHARACTERS, AND SYMBOLS Textual messages comprise a sequence of alphanumeric characters. When digitally transmitted, the characters are first encoded into a sequence of bits, called a bit stream or baseband signal. Groups of k bits can then be combined to form new digits, or symbols, from a finite symbol set or alphabet of M = 2k such symbols. A system using a symbol set size of M is referred to as an M-ary system. The value of k or M represents an important initial choice in the design of any digital communi- cation system. For k = 1, the system is termed binary, the size of the symbol set is M = 2, and the modulator uses one of the two different waveforms to represent the binary “one” and the other to represent the binary “zero.” For this special case, the symbol and the bit are the same.