EE578 Assignment #3

Abdul-Aziz .M Al-Yami

October 25th 2010

Digital data, digital signal Equipment less complex and cheaper than digital-

to-analog modulation equipment (computers) Analog data, digital signal

Permits use of modern digital transmission schemes and robust switching equipment

Digital data, analog signal Some transmission media will only propagate analog signals -

optical fiber & unguided media (wireless) Analog data, analog signal

Analog data in electrical form can be transmitted easily/cheaply Done with voice transmission over voice-grade lines

What determines how successful a receiver will be in interpreting an incoming signal? Signal-to-noise ratio (or Eb/No) Data rate (R) Bandwidth (B) Encoding scheme (the topic of this chapter)

An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data

rate (but unless the power density remains the same then the increase in bandwidth will cause an increase in the BER)

Signal spectrum With lack of high-frequency components, less bandwidth

required With no dc component ac coupling via transformer can

be used (no direct physical connection) providing electrical isolation, RFI reduction, eliminate ground loops

Transfer function of a channel is worse near the band edges thus to reduce distortion, concentrate transmit power in the middle of the transmission bandwidth.

Clocking Ease of determining beginning and end of each bit position

(use clocks or encoding techniques)

Signal interference and noise immunity Performance in the presence of noise Encoding techniques that exhibit good BER

Cost and complexity The higher the signal rate to achieve a given data rate,

the greater the cost

Digital data to analog signal Amplitude-shift keying (ASK)

▪ Amplitude differences of carrier frequency

Frequency-shift keying (FSK)▪ Frequency differences near the carrier frequency

Phase-shift keying (PSK)▪ Phase of carrier signal shifted

Two-level PSK (BPSK) Uses two phases to represent binary digits

Where we can consider the above two functions to be multiplied by +1 and -1 for a binary 1 and binary 0 respectively

ts tfA c2cos tfA c2cos

1binary 0binary

tfA c2cos

tfA c2cos1binary 0binary

which equals

Differential PSK (DPSK) Phase shift with reference to previous bit

▪ Binary 0 – signal burst of same phase as previous signal burst

▪ Binary 1 – signal burst of opposite phase to previous signal burst

The term differential is used because the phase shift is with reference to the previous bit

Doesn’t require an accurate receiver oscillator matched with the transmitter for the phase information but obviously depends to the preceding phase (information bit) being received correctly.

Four-level PSK (QPSK - quadrature PSK) Each element represents more than one bit

ts

42cos

tfA c 11

4

32cos

tfA c

4

32cos

tfA c

42cos

tfA c

01

00

10

I stream (in-phase)

Q stream (quadrature data stream)

OQPSK has phase transitions between every half-bit time that never exceeds 90 degrees (π/2 radians)

Results in much less amplitude variation of the bandwidth-limited carrier

BER is the same as QPSK When amplified, QPSK results in significant

bandwidth expansion, whereas OQPSK has much less bandwidth expansion especially if the channel has non-linear components

Multilevel PSK Using multiple phase angles with each angle having

more than one amplitude, multiple signals elements can be achieved

▪ D = modulation rate, baud▪ R = data rate, bps (note the difference in baud and bps)▪ M = number of different signal elements = 2L

▪ L = number of bits per signal element If L = 4 bits in each signal element using M = 16 combinations of

amplitude and phase, then if the data rate is 9600 bps,the line signaling speed/modulation rate is 2400 baud

M

R

L

RD

2log

QAM is a combination of ASK and PSK Two different signals sent simultaneously on

the same carrier frequency tftdtftdts cc 2sin2cos 21

Program 3.2 (bpsk_fading)

0 5 10 15 20 25 30 35 4010

-4

10-3

10-2

10-1

100

Eb/N

0 [dB]

BE

R

BPSK performance

BPSK AWGN theory

BPSK AWGNBPSK Rayleigh theory

BPSK Rayleigh