RF Quadrature Transceiver / RF Quadrature Receiver - Datasheet
Quadrature Amplitude Modulation Forrest Sedgwick UC Berkeley EECS Dept. EE290F October 2003.
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Transcript of Quadrature Amplitude Modulation Forrest Sedgwick UC Berkeley EECS Dept. EE290F October 2003.
Quadrature Amplitude Modulation
Forrest Sedgwick
UC Berkeley EECS Dept.
EE290F
October 2003
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.
WN
PWC
0
1logAssumes noise is thermal – Gaussian and White
www-gap.dcs.st-and.ac.uk/~history/ Mathematicians/Shannon.html
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
0
1logN
Err b
W
Cr
Limits on Communication Channels
Two types of communication channels
r<<1 – Power Limited High dimensionality
signaling schemes Binary
r>>1 – Bandwidth Limited Low dimensionality Multilevel
0
1logN
Err b
Proakis and Salehi, pp. 738
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)
Amplitude Shift Keying (ASK)
Amplitude of carrier wave is modulated Equivalent BER vs SNR to baseband PAM
Proakis and Salehi, pp. 306
Angle Modulation (PSK and FSK) Frequency is time derivative of phase, PSK
and FSK are somewhat equivalent
Proakis and Salehi, pp. 332
PSK: Digital Angle Modulation Usually in digital communications PSK is chosen over
FSK Easier to create multilevel codes Possibility of using differential phase shift keying (DPSK)
Uses phase shifts relative to previous bit Eliminates need for local oscillator at receiver Use Gray Code to minimize effect of errors
Proakis and Salehi, pp. 631
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
Proakis and Salehi, pp. 653
Constellations
Different constellations require different SNR for a given BER
(d) is lowest power by about 1dB (for given BER)
(a) and (b) are rectangular Rectangular constellations
offer very simple modulation/ demodulation schemes
ASK two quadrature carriers - same frequency but 90 out of phase
Mix quadrature carriers for output
Proakis and Salehi, pp. 653
QAM vs ASK (multilevel)
QAM has a tremendous advantage in noise performance Energy in every bit (including zero) Substantially more complex (coherent detection vs photodiode)
Proakis and Salehi, pp. 565 Proakis and Salehi, pp. 495
QAM vs PSK
4-QAM and 4-PSK have same power penalty
For k>4, k-QAM is an improvement over k-PSK
Proakis and Salehi, pp. 639
Applications of QAM
Used in bandwidth-limited applications Modems: telephones have 3kHz bandwidth, excellent SNR
(20dB) => M-ary QAM Cellular Telephones: Bandwidth is at a premium, very
expensive (However, POWER is also at a premium...)
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
References
John G. Proakis, Masoud Salehi, Communications Systems Engineering, Prentice Hall 1994