Topics: Modulation, Quadrature amplitude modulation, Phase-shift keying Pages: 11 (2441 words) Published: August 11, 2012
Quadrature amplitude modulation (QAM) ([pic] /ˈkwɑːm/ or /ˈkæm/ or simply "Q-A-M") is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. The two carrier waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature components — hence the name of the scheme. The modulated waves are summed, and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK), or (in the analog case) of phase modulation (PM) and amplitude modulation. In the digital QAM case, a finite number of at least two phases and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as a modulation scheme for digitaltelecommunication systems. Spectral efficiencies of 6 bits/s/Hz can be achieved with QAM.[1]

QAM modulation is being used in optical fiber systems as bit rates increase – QAM16 and QAM64 can be optically emulated with a 3-path interferometer.[2]

|Contents |
|  [hide]  |
|1 Digital QAM |
|2 Analog QAM |
|2.1 Fourier analysis of QAM |
|3 Quantized QAM |
|3.1 Ideal structure |
|3.1.1 Transmitter |
|4 Quantized QAM performance |
|4.1 Rectangular QAM |
|4.1.1 Odd-k QAM |
|4.2 Non-rectangular QAM |
|5 Interference and noise |
|7 References |

Digital QAM

Like all modulation schemes, QAM conveys data by changing some aspect of a carrier signal, or the carrier wave, (usually a sinusoid) in response to a data signal. In the case of QAM, the amplitude of two waves, 90 degrees out-of-phase with each other (in quadrature) are changed (modulated or keyed) to represent the data signal. Amplitude modulating two carriers in quadrature can be equivalently viewed as both amplitude modulating and phase modulating a single carrier.

Phase modulation (analog PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the magnitude of the modulating signal is a constant, with only the phase varying. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), for these can be regarded as a special case of phase modulation.

Analog QAM

[pic]

[pic]

Analog QAM: measured PAL colour bar signal on a vector analyser screen.

When transmitting two signals by modulating them with QAM, the transmitted signal will be of the form:

[pic],

where I(t) and Q(t) are the modulating signals and f0 is the carrier frequency.

At the receiver, these two modulating signals can be demodulated using a coherent demodulator. Such a receiver multiplies the received signal separately with both a cosine and sine signal to produce the received estimates of I(t) and Q(t) respectively. Because of the orthogonality property of the carrier signals, it is possible to detect the modulating signals independently.

In the ideal case I(t) is demodulated by multiplying the transmitted signal with a cosine signal:

[pic]

Using standard trigonometric identities, we can write it as:

[pic]

Low-pass filtering ri(t) removes the high frequency terms (containing 4πf0t), leaving only the I(t) term. This filtered signal is unaffected by Q(t), showing that the in-phase component can be received independently of the...