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EE578 Assignment #3 Abdul-Aziz.M Al-Yami October 25 th 2010.

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Presentation on theme: "EE578 Assignment #3 Abdul-Aziz.M Al-Yami October 25 th 2010."— Presentation transcript:

1 EE578 Assignment #3 Abdul-Aziz.M Al-Yami October 25 th 2010

2 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

3 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

4 What determines how successful a receiver will be in interpreting an incoming signal? Signal-to-noise ratio (or E b /N o ) 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)

5 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)

6 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

7 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


9 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 which equals

10 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 Doesnt 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.

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

12 I stream (in-phase) Q stream (quadrature data stream)

13 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

14 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 = 2 L 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

15 QAM is a combination of ASK and PSK Two different signals sent simultaneously on the same carrier frequency


17 Program 3.2 (bpsk_fading)

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