无线通信与网络chap6.ppt

Signal Encoding Techniques Chapter 6 Reasons for Choosing Encoding Techniques Digital data, digital signal Equipment less complex and expensive than digital-to-analog modulation equipment Analog data, digital signal Permits use of modern digital transmission and switching equipment Reasons for Choosing Encoding Techniques Digital data, analog signal Some transmission media will only propagate analog signals E.g., optical fiber and unguided media Analog data, analog signal Analog data in electrical form can be transmitted easily and cheaply Done with voice transmission over voice-grade lines Signal Encoding Criteria What determines how successful a receiver will be in interpreting an incoming signal? Signal-to-noise ratio Data rate Bandwidth 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 Factors Used to CompareEncoding Schemes Signal spectrum With lack of high-frequency components, less bandwidth required With no dc component, ac coupling via transformer possible Transfer function of a channel is worse near band edges Clocking Ease of determining beginning and end of each bit position Factors Used to CompareEncoding Schemes Signal interference and noise immunity Performance in the presence of noise Cost and complexity The higher the signal rate to achieve a given data rate, the greater the cost Basic Encoding Techniques Digital data to analog signal Amplitude-shift keying (ASK) Amplitude difference of carrier frequency Frequency-shift keying (FSK) Frequency difference near carrier frequency Phase-shift keying (PSK) Phase of carrier signal shifted Basic Encoding Techniques Amplitude-Shift Keying One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of carrier where the carrier signal is Acos(2πfct) Amplitude-Shift Keying Susceptible to sudden gain changes Inefficient modulation technique On