22-06-2012, 05:25 PM
Analog-Digital Hybrid Modulation for improved efficiency over Broadband Wireless Systems
[attachment=25146]
INTRODUCTION
Let us consider the transmission of an analog signal over
a band-limited channel. This could be possible by two
conventional techniques: analog transmission, and digital
transmission, of which the latter uses sampling and
quantization principles. Analog Modulation techniques such
as Frequency and Phase Modulations provide significant
noise immunity as known and provide SNR improvement
proportional to the square root of modulation index, and are
thus able to trade off bandwidth for SNR.
THE SCM TECHNIQUE : AN ANALYTICAL APPROACH
Suppose we are given a bandlimited signal of bandwidth
B Hz, which needs to be transmitted over a channel of
bandwidth Bc with Gaussian noise of spectral density N0
watts per Hz. Let the transmitter have an average power of P
watts. We consider that the signal is sampled at the Nyquist
rate of 2B samples per second, to produce a sampled signal
x(n).
PERFORMANCE COMPARISON
SCM offers near ideal communications performance. To
show this is true, let us consider the role of a
communications link designer who has a noisy transmission
channel of bandwidth B and limited SNR. Let us choose a
digital link as a first and best choice. Here, the analog
samples are converted to digital with a resolution of b bits
per sample.
According to Shannon’s principle of the capacity of a noisy
transmission channel, by using an ideal error correction
coding technique the information can be passed error free at
a bit rate equal to channel capacity, given by equation (7).
Broadband Wireless Transmission
An SCM-based communications link is basically a
transparent, band-limited analog pipe with near-ideal
performance in noisy channels. Every analog signal could
potentially use SCM because it can outperform other
existing modulation schemes. However, SCM has a
compelling advantage for digital communications
applications as well.
For example, SCM can pass digital information by acting
as a repeater of a digital channel. This application provides a
wireless extension of cable modem digital information. As
illustrated in Figure 8, a cable modem termination system
(CMTS) transmits a 42 Mb/s 256-QAM signal in a 6 MHz
cable channel shared among the cable modems located at the
subscribers' premises. The return upstream path from the
cable modems is a 10 Mb/s 16-SQAM signal in a 3.2 MHz
cable channel. The signals are carried by a combination of
fiber and coax referred to as a hybrid fiber/coax (HFC)
network.
[attachment=25146]
INTRODUCTION
Let us consider the transmission of an analog signal over
a band-limited channel. This could be possible by two
conventional techniques: analog transmission, and digital
transmission, of which the latter uses sampling and
quantization principles. Analog Modulation techniques such
as Frequency and Phase Modulations provide significant
noise immunity as known and provide SNR improvement
proportional to the square root of modulation index, and are
thus able to trade off bandwidth for SNR.
THE SCM TECHNIQUE : AN ANALYTICAL APPROACH
Suppose we are given a bandlimited signal of bandwidth
B Hz, which needs to be transmitted over a channel of
bandwidth Bc with Gaussian noise of spectral density N0
watts per Hz. Let the transmitter have an average power of P
watts. We consider that the signal is sampled at the Nyquist
rate of 2B samples per second, to produce a sampled signal
x(n).
PERFORMANCE COMPARISON
SCM offers near ideal communications performance. To
show this is true, let us consider the role of a
communications link designer who has a noisy transmission
channel of bandwidth B and limited SNR. Let us choose a
digital link as a first and best choice. Here, the analog
samples are converted to digital with a resolution of b bits
per sample.
According to Shannon’s principle of the capacity of a noisy
transmission channel, by using an ideal error correction
coding technique the information can be passed error free at
a bit rate equal to channel capacity, given by equation (7).
Broadband Wireless Transmission
An SCM-based communications link is basically a
transparent, band-limited analog pipe with near-ideal
performance in noisy channels. Every analog signal could
potentially use SCM because it can outperform other
existing modulation schemes. However, SCM has a
compelling advantage for digital communications
applications as well.
For example, SCM can pass digital information by acting
as a repeater of a digital channel. This application provides a
wireless extension of cable modem digital information. As
illustrated in Figure 8, a cable modem termination system
(CMTS) transmits a 42 Mb/s 256-QAM signal in a 6 MHz
cable channel shared among the cable modems located at the
subscribers' premises. The return upstream path from the
cable modems is a 10 Mb/s 16-SQAM signal in a 3.2 MHz
cable channel. The signals are carried by a combination of
fiber and coax referred to as a hybrid fiber/coax (HFC)
network.