18-07-2011, 11:02 AM
22ANALOG-DIGITAL-HYBRID-MODULATION-FOR-IMPROVED-EFFICIE.pdf (Size: 469.24 KB / Downloads: 101)
Abstract— This paper seeks to present ways to eliminate the
inherent quantization noise component in digital
communications, instead of conventionally making it
minimal. It deals with a new concept of signaling called
the Signal Code Modulation (SCM) Technique. The
primary analog signal is represented by: a sample which is
quantized and encoded digitally, and an analog
component, which is a function of the quantization
component of the digital sample. The advantages of such a
system are two sided offering advantages of both analog
and digital signaling. The presence of the analog residual
allows for the system performance to improve when excess
channel SNR is available. The digital component provides
increased SNR and makes it possible for coding to be
employed to achieve near error-free transmission.
Index Terms—SCM, Hybrid Modulation, Quantized residual
amplification.
1. INTRODUCTION
et 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.
However, the
SNR improvement
provided by these
techniques is much
lower than the ideal
performance as
shown by the
Shannon’s capacity
theorem [1].
On the other
hand, Digital
techniques of
transmission can
utilize errorcorrection
codes
that provide
performance close
to theoretical
prediction.
However, the major disadvantage of digital transmission
techniques is the inherent quantization error introduced,
which is imminent all the while the signal is relayed. This
error causes distortion in the original signal being relayed
and cannot be later recovered by any means possible.
If we quantize the sampled signal using QAM or any other
method, using a fixed number of bits, a fixed digital
distortion is introduced in the developmental stage itself.
This distortion is present regardless of the transmission
quality of the channel being used. Thus the original signal
can be considered to be permanently impaired.
Communications systems are normally constructed for
SNR much higher than the minimum that is required, so as to
leave a margin for fading and other effects which might
occasionally reduce the SNR [2].
So, it is essential to design a communications system
where the output SNR increases as the channel SNR
increases. While, as already stated, this technique is not
feasible through digital modulation, it is an inherent property
in analog modulation.
Here, we introduce the concept of Signal Code
Modulation (SCM) which utilizes both the analog, as well
as, digital modulation techniques. The primary analog input
signal is sampled at the appropriate rate and quantized. The
digital samples are denoted by symbols D. The resulting D
symbols are then transmitted using digital transmission
techniques (like QAM) optimized for that channel. Those D
symbols represent N bits per analog input sample.
The quantization residual, which is not left behind, is
transmitted over the noisy channel as an analog symbol A,
corresponding to the digital symbol D, as shown in the
figure 1. To produce
the quantization error
A, the quantized data
is converted back
into analog form and
subtracted from the
original analog input
signal. This symbol
A, for noise
immunity, is
amplified by a gain
of 2N (or any
proportional factor
that will optimize the
voltage swing of the
signal with that of
the channel).
The SCM receiver
performs the opposite operation by combining the D symbol
and its corresponding residual. This would not bring about
significant improvement if transmitted over a noisy channel
as noise could vary the symbol A and cause bit errors in the
Analog-Digital Hybrid Modulation
for improved efficiency over Broadband Wireless Systems
D symbols. However, the 2N amplitude gain of the analog
components provides a noise immunity of 22N to boost the
SNR and provide a near ideal scheme for error-free
transmission.