rfbasics

rfbasics - 2004 Matt Welsh Harvard University 1 CS263:...

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Unformatted text preview: 2004 Matt Welsh Harvard University 1 CS263: Wireless Communications and Sensor Networks Matt Welsh Lecture 2: RF Basics and Signal Encoding September 28, 2004 2004 Matt Welsh Harvard University 2 Today's Lecture Basics of wireless communications Analog and digital signals Bandwidth and channel capacity Signal encoding and modulation 2004 Matt Welsh Harvard University 3 What is RF? Radio Frequency is an electromagnetic signal with a frequency between 3 kHz and 300 Ghz RF signals carry analog or digital information Analog: Information content varies continuously over time Example: radio and TV stations Digital: Information content consists of discrete units (e.g., 0s and 1s) Example: Cell phones and wireless networks 2004 Matt Welsh Harvard University 4 Carrier Wave How do we send information in a radio signal? Carrier wave An RF signal usually a sinusoid that carries information Carrier is usually a much higher frequency than the information itself! Ex: 2.4 GHz 802.11b networks carry a lot less than 2.4 GBit/sec of data.... Rather, carry up to 11 MBit/sec of information Why use a carrier?? Easier to generate a sinusoid signal, and it will travel further. Carrier wave frequency The frequency of a radio transmission is the center frequency of the carrier Actual frequency of the carrier changes over time, e.g., with FM transmission 2004 Matt Welsh Harvard University 5 Signal Modulation How do we encode information in a carrier wave? An information signal must be modulated onto the carrier wave That is, we must modify the carrier wave in some way... Receiver must demodulate the carrier to get back the original signal Amplitude Modulation (AM) Modify the amplitude of the carrier with respect to the amplitude of the signal Frequency Modulation (FM) Modify the frequency of the carrier with respect to the amplitude of the signal 2004 Matt Welsh Harvard University 6 Signal Modulation Carrier Information signal Amplitude Modulation (AM) 2004 Matt Welsh Harvard University 7 Signal Modulation Carrier Information signal Frequency Modulation (FM) 2004 Matt Welsh Harvard University 8 Digital Modulation How do we modulate digital signals? Amplitude shift keying (ASK) 0 bit is the absence of the carrier (flat signal) 1 bit is the presence of the carrier with some fixed amplitude Frequency shift keying (FSK) 0 bit is carrier at frequency f ; 1 bit is carrier at frequency f 1 1 1 1 ASK FSK 2004 Matt Welsh Harvard University...
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rfbasics - 2004 Matt Welsh Harvard University 1 CS263:...

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