30-05-2012, 05:20 PM
hybrid modulation
[attachment=23610]
INTRODUCTION
We consider the classical problem of transmitting analog information over a band limited Gaussian channel. Traditional approach provides two primary alternatives: using analog transmission ,or sampling and quantizing the signal using digital transmission. Analog modulation technique such as Frequency Modulation or Phase Modulation (PM) provide signal-to-noise ratio improvement proportional to the square of the modulation index, and are thus able to trade off bandwidth for SNR.
We note that practical communication systems operate most of the time at an SNR which is much higher than the minimum required, because of the need to leave a margin for fading and other effects which may occasionally reduce the SNR of the analog signal will improve as the channel SNR increases. This property is inherent in analog modulation, but not in digital modulation.
Here we study the performance of a new technique which uses a combination of analog and digital modulation, and enjoys the advantages of both. This technique has been developed for a commercial point-to-multipoint communication system under the name “SIGNAL CODE MODULATION”. The technique is based on the idea of representing the analog signal by a digital component, and an analog component consisting of the quantization error. The combination of these digital and analog components provides an exact representation of the original signal.
SIGNAL CODE MODULATION
Signal code modulation is a method for transmitting analog information over a noisy channel. SCM provides an analog pipe through which any bandlimited waveform can pass, including truly analog information or the output of a digital modem. The operations that SCM performs on the payload signal are simple, as illustrated
Constellation Explanation
How does SCM cause this unusual constellation diagram?
This diagram represents a specific SCM mode, AAD, wherein the bandwidth expansion is b =1.5, and the input is an unmodulated carrier within the system pass band, although not necessarily at center frequency. It is a 16-QAM symbol carrying four bits, and the SCM conversion uses only a single bit per A dimension. The image is a superposition of many sampling points, each representing a single A or D point. The collection of all D points creates the familiar 16-QAM constellation.
When depicted as I vs. Q, the original analog carrier would produce a circle, a familiar Lissageau waveform.
PERFORMANCE COMPARISON
Before explaining why SCM is a nearly ideal analog communications method , it is necessary to define the ideal reference and compare it with existing alternatives. A communications link designer faced with an additive white Gaussian noise channel of bandwidth B and a limited signal-to noise ratio (SNR) might choose a digital link as a first choice.
Maximum Output SNR
Let us consider the best possible SNR that can be obtained by bandwidth expansion, when we wish to transmit a signal of bandwidth B through a Gaussian channel of bandwidth B.It can be derived using Shannon’s capacity theorem that the formula for capacity of a Gaussian Channel is given by
[attachment=23610]
INTRODUCTION
We consider the classical problem of transmitting analog information over a band limited Gaussian channel. Traditional approach provides two primary alternatives: using analog transmission ,or sampling and quantizing the signal using digital transmission. Analog modulation technique such as Frequency Modulation or Phase Modulation (PM) provide signal-to-noise ratio improvement proportional to the square of the modulation index, and are thus able to trade off bandwidth for SNR.
We note that practical communication systems operate most of the time at an SNR which is much higher than the minimum required, because of the need to leave a margin for fading and other effects which may occasionally reduce the SNR of the analog signal will improve as the channel SNR increases. This property is inherent in analog modulation, but not in digital modulation.
Here we study the performance of a new technique which uses a combination of analog and digital modulation, and enjoys the advantages of both. This technique has been developed for a commercial point-to-multipoint communication system under the name “SIGNAL CODE MODULATION”. The technique is based on the idea of representing the analog signal by a digital component, and an analog component consisting of the quantization error. The combination of these digital and analog components provides an exact representation of the original signal.
SIGNAL CODE MODULATION
Signal code modulation is a method for transmitting analog information over a noisy channel. SCM provides an analog pipe through which any bandlimited waveform can pass, including truly analog information or the output of a digital modem. The operations that SCM performs on the payload signal are simple, as illustrated
Constellation Explanation
How does SCM cause this unusual constellation diagram?
This diagram represents a specific SCM mode, AAD, wherein the bandwidth expansion is b =1.5, and the input is an unmodulated carrier within the system pass band, although not necessarily at center frequency. It is a 16-QAM symbol carrying four bits, and the SCM conversion uses only a single bit per A dimension. The image is a superposition of many sampling points, each representing a single A or D point. The collection of all D points creates the familiar 16-QAM constellation.
When depicted as I vs. Q, the original analog carrier would produce a circle, a familiar Lissageau waveform.
PERFORMANCE COMPARISON
Before explaining why SCM is a nearly ideal analog communications method , it is necessary to define the ideal reference and compare it with existing alternatives. A communications link designer faced with an additive white Gaussian noise channel of bandwidth B and a limited signal-to noise ratio (SNR) might choose a digital link as a first choice.
Maximum Output SNR
Let us consider the best possible SNR that can be obtained by bandwidth expansion, when we wish to transmit a signal of bandwidth B through a Gaussian channel of bandwidth B.It can be derived using Shannon’s capacity theorem that the formula for capacity of a Gaussian Channel is given by