18-12-2012, 12:41 PM
Quadrature Amplitude Modulation
Quadrature Amplitude.ppt (Size: 2.48 MB / Downloads: 180)
Analog vs Digital
Information Theory vs Signal Analysis
Discrete Levels vs Analogous Representation
Sacrifice arbitrarily precise representation of signal
Gain arbitrary degree of reproducibility of given signal
KEY BENEFIT
Discrete information can be transmitted with arbitrarily low error rates EVEN ON A NOISY CHANNEL
Digital information content measured in units of bits, decimals, or nats
Shannon’s Channel Capacity
Channel capacity C (bits/sec) is the speed at which information can travel over a channel with an arbitrarily low error rate i.e. when a system is transmitting bits at or below C then for any BER e>0 there exists a code with block length n which will provide a BER < e.
Modulation
All channels consist of some continuous parameter
Must map discrete states onto continuous property
Must have a decision circuit to map the state of the modulated channel into a discrete state
As number of levels or states M the behavior of the digital system does not approach that of an analog system, due to the decision circuit
Number of Levels
Digital communications relies on a finite number of discrete levels
Minimum number of levels is two (binary code)
Shannon Capacity helps determine optimum number of levels for a given bandwidth, SNR, and BER
Modulation Scheme
A channel with lowpass frequency characteristics is called baseband. Digital information is transmitted directly
Ex. Pulse Amplitude Modulation (PAM)
A channel far removed from DC (like optical) is called a bandpass channel
Transmission on a bandpass channel requires modulation of a carrier
Amplitude Shift Keying (ASK)
Phase Shift Keying (PSK)
Frequency Shift Keying (FSK
Quadrature Amplitude Modulation (QAM)
Quadrature Amplitude Modulation
Amplitude and Phase of carrier are modulated
Discrete amplitudes and phases form a constellation
Can also think of QAM as a “complex” amplitude modulation scheme
Limitations
Almost always requires a highly stable local oscillator
In the optical domain this is very expensive
Possible (but difficult) to use differential phase keying
Performance limits still not reached for
Direct detection
Signal Dimensionality (DWDM)
Transmitter Power