Baseband Data Transmission & Digital Modulation Techniques Chapter 4 Baseband Data Transmission & Digital Modulation Techniques

Chapter Overview Baseband transmission Line coding Intersymbol Interference (ISI) Nyquist Wave Digital Modulation techniques

Baseband Transmission The original band of frequencies of a signal before it is modulated for transmission at a higher frequency. A type of data transmission in which digital or analog data is sent over a single unmultiplexed channel, such as an Ethernet LAN. Baseband transmission use TDM to send simultaneous bits of data along the full bandwidth of the transmission channel.

Cont’d... Baseband Center Point Detection The detection of digital signals involve two processes: Reduction of each received voltage pulse (i.e. symbol) to a single numerical value. Comparison of this value with a reference voltage (or for multisymbol signaling, a set of reference voltages) to determine which symbol was transmitted.

Line Coding Pulse modulation applied to binary symbol, the resulting binary waveform is called PCM waveform. In telephony applications : line codes Pulse modulation is applied to nonbinary symbol, the resulting waveform called M-ary pulse modulation waveform.

Some basic nomenclature Information source: can be either analog or digital binary sequence: sequence of {1, 0} to describe information source Bit symbol: a symbol represents k bits (M=2k) Symbol alphabet: s0,s1,…,sM-1 (alphabet size M) symbol stream: sequence of symbols selected from the alphabet Data rate Bit rate Rb in bits/sec (bps). Symbol rate Rs in symbols/sec (sps) Bit interval Tb: duration of a bit (sec/bit) Symbol interval Ts: duration of a symbol (sec/symbol)

Important relation

Illustration and example

Cont’d... Line codes Converting standard logic level to a form more suitable to telephone line transmission. Four main groups Non return to zero (NRZ) Return to Zero (RZ) Phase encoded Multilevel binary

Cont’d... Non Return To Zero (NRZ) Subgroups : NRZ-L NRZ-M NRZ-S Used extensively in digital logic circuit Binary 1 one represented by one voltage level Binary 0 is represented by another voltage level. NRZ-M Used in magnetic tape recording. The 1 (mark) is represented by change in level The 0 (space) is represented by no change in level Differential encoding NRZ-S Complement of NRZ-M 1 is represented by no change in level 0 is represented by a change in level

Cont’d... Return To Zero (RZ) Subgroups: Unipolar RZ Bipolar RZ RZ-AMI 1 is represented by a half bit wide pulse. 0 is represented by the absence of pulse. Bipolar RZ 1 & 0 are represented by opposite level pulses that are one half bit wide. Pulse present in each bit interval. RZ-AMI 1 is represented by equal amplitude alternating pulses. 0 is represented by the absences of pulse.

Cont’d... Phase Encoded Multilevel Binary Subgroups: Bi-phase-level @ Manchester coding Bi-phase-mark Bi-phase-space Delay modulation @ Miller coding Multilevel Binary Used three levels to encode the binary data. Dicode and duo binary

Cont’d... Parameters : DC components Self-clocking Error detection Bandwidth Compression Differential encoding Noise immunity

Cont’d... ι ≥ log 2 (1/2p) bits PCM Word size Magnitude of the quantization distortion error |e| < pVpp |emax| = q/2 = Vpp 2(L-1) 2L L = number of quantization levels, large enough Number of bits/sample, ι 2ι = L ≥ (1/2p) levels ι ≥ log 2 (1/2p) bits

Cont’d... M-ary Pulse Modulation waveforms Three basic ways to modulate information on to a sequence of pulse PAM PPM PWM @ pulse duration modulation (PDM) M-ary pulse modulation When the information samples are first quantized, yielding symbols from an M-ary alphabet set, then modulated on to pulses, the resulting pulse modulation is digital.

Cont’d... M-ary PAM M-ary PPM M-ary PWM one of M allowable amplitude levels are assigned to each of the M possible symbol values. M-ary PPM modulation is effected by delaying or advancing a pulse occurrence. M-ary PWM modulation is effected by varying the pulse width by an amount that corresponds to the value of the symbols.

M-ary pulse modulation block-diagram

Illustration of PCM signals

Bandwidth efficiency How much date rate can be supported by the system with each unit frequency band The higher bandwidth efficiency, the better.

Intersymbol Interference (ISI) Tx – the information symbols characterized as impulse or voltage levels, modulate pulses that are then filtered to comply with some bandwidth constraint. Baseband system – the channel has distributed reactance that distorts the pulses. Bandpass system – characterized by fading channels that behave like undesirable filters manifesting signal distortion. Rx – equalizing filter is configured to compensate for the distortion caused by Tx & channel.

Cont’d... Equivalent system transfer function H(f) = Ht(f) Hc(f) Hr(f) Where Ht(f) – transmitting filter Hc(f) – filtering within the channel Hr(f) – equalizing filter

Cont’d... Due to effect of system filtering, the received pulses can overlap one another. Tail of pulse can smear into adjacent symbol intervals , thereby interfering with the detection process and degrading the error performance - Intersymbol interference (ISI) effects of filtering Channel-induced distortion

ISI measured by eye pattern

Cont’d... Nyquist filter is one whose frequency transfer function can be represented by a rectangular function convolved with any real even-symmetric frequency function Nyquist pulse is one whose shape can be represented by a sinc (t/T) function multiplied by another time function Most popular of Nyquist filter Raised-cosine filter Root-raised cosine filter

Cont’d... Pulse Shaping to reduce ISI Pulse that spread in time will degrade the system’s error performance due to increase ISI. Reduce the required system bandwidth. Compress the bandwidth of the data impulse to some reasonably small bandwidth greater than the Nyquist minimum – pulse shaping with Nyquist filter. Zero ISI is only when the sampling is performed at exactly the correct sampling time when the tails of pulses are large.

Cont’d... Raised-cosine filter One frequently used H(f) transfer function belonging to the Nyquist class (zero ISI at the sampling time). It can be expressed as

Amplitude response of raised-cosine filter with various roll-off factors

Impulse response of raised-cosine filter with various roll-off factors

Consecutive raised-cosine impulses, demonstrating zero-ISI property

Cont’d... Impulse response for the H(f) Minimum required bandwidth DSB bandwidth

Cont’d... Two types of error-performance degradation Due to a decrease in received signal power or an increase in noise or interference power, giving rise to a loss in signal-to-noise ratio, Eb/No Due to signal distortion such as ISI

Nyquist’s Wave Nyquist pulse Square-root Nyquist pulse Impulse response of the raised-cosine filter Faster transition Square-root Nyquist pulse Impulse response of a root-cosine filter Does not exhibit zero ISI

Digital Modulation Techniques The process by which digital symbols are transformed into waveforms that are compatible with the characteristic of the channel. Bandpass modulation The shaped pulses modulate a carrier wave Radio transmission – the carrier is converted to EM field for propagation to the desired destination

Cont’d... Carrier Signal If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK) If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)

Cont’d... If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK) If both the amplitude,V and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)

Cont’d...

Amplitude Shift Keying (ASK) A binary information signal directly modulates the amplitude of an analog carrier. Where vask (t) = amplitude shift keying wave vm(t) = digital information signal (volt) A/2 = unmodulated carrier amplitude (volt) ωc = analog carrier radian frequency (rad/s)

Frequency Shift Keying (FSK) Called as BFSK The phase shift in carrier frequency (∆f) is proportional to the amplitude of the binary input signal (vm(t)) and the direction of the shift is determined by the polarity Where vfsk(t) = binary FSK waveform Vc = peak analog carrier amplitude (volt) fc = analog carrier center frequency (Hz) ∆f = peak shift in analog carrier frequency (Hz) vm(t) = binary input signal (volt)

FSK Bandwidth

FSK Transmitter

Phase Shift Keying (PSK)

BPSK Transmitter

BPSK Receiver

BPSK Bit value 1 – sine wave Bit value 0 – inverted sine wave Very simple PSK Low spectral efficiency Robust , used in satellite system

Output phase vs time relationship for a BPSK modulator Binary input BPSK output Time 1

QPSK

QPSK 2 bits coded as one symbol Symbol determines shift of sine wave Needs less bandwidth compared to BPSK More complex

QPSK signal in the time domain The modulated signal is shown below for a short segment of a random binary data-stream. The two carrier waves are a cosine wave and a sine wave, as indicated by the signal-space analysis above. Here, the odd-numbered bits have been assigned to the in-phase component and the even-numbered bits to the quadrature component (taking the first bit as number 1). The total signal — the sum of the two components — is shown at the bottom. Jumps in phase can be seen as the PSK changes the phase on each component at the start of each bit-period. The topmost waveform alone matches the description given for BPSK above.

The binary data that is conveyed by this waveform is: 1 1 0 0 0 1 1 0. The odd bits, highlighted here, contribute to the in-phase component: 1 1 0 0 0 1 1 0 The even bits, highlighted here, contribute to the quadrature-phase component: 1 1 0 0 0 1 1 0

End of Chapter 4