Simulating Fading, Adding Noise and Predistortions, and Impairing the Signal
R&S
®
SMW200A
390User Manual 1175.6632.02 ─ 16
5.2.2 About the AWGN Generator
Provided the required options are installed, the R&S SMW allows you to superimpose
noise on the generated signal. The built-in internal noise generator generates an
AWGN signal (Additive White Gaussian Noise) with selectable bandwidth and adds it
to the digital baseband signal. The main characteristic of this kind of noise signal is the
Gaussian distribution of the noise power density and uniform frequency distribution.
Generation of the AWGN signal
The Gaussian noise is generated with a feedback shift register with subsequent proba-
bility transformation. The switching configuration is such that ideal statistical character-
istics are achieved:
●
I and Q paths are decorrelated from each other.
●
Small probabilities are ensured via the crest factor of 15 dB
●
The period of the noise signal depends on the selected system bandwidth. The
relationship of period P to system bandwidth B
sys
is approximated:
P ≈ (2
800
-1) / B
sys
–
This results in periods between 10
225
years with minimum bandwidth and
approximately 2 days with maximum bandwidth.
– For comparison, this corresponds to a period of approximately one month for
3GPP FDD with a bandwidth of 3.84 Mcps, and a period of 427 days for GSM
with 270.833 ksps.
●
Scalable low-pass filters are used to produce a noise level with both a broad
dynamic range and a broad bandwidth range (from 1 kHz to 60 MHz).
AWGN modes
The AWGN generator generates signal in one of the following different modes:
●
"Additive Noise": the generated noise signal superimposes the interference-free
useful signal
●
"Noise Only": a pure noise signal is generated and modulated to the carrier; the
connection to the baseband is interrupted.
Figure 5-1: Representation of an "AWGN > Mode > Noise Only" in the block diagram
●
"CW Interferer mode": a sinusoidal signal with an adjustable frequency offset and
carrier-to-interferer (C/I) power ratio is added to the baseband signal by a counter
instead of a shift register.
Signal and noise parameters
The Figure 5-2 illustrates the relation between the signal and noise parameters.
Adding Noise to the Signal
To Next Page
Loading...
