GSM Receiver Simulation

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Application Note AN124A Jul 29, 1998
GSM Receiver Simulation
By Maurice L. Schiff and Stephen H. Kratzet
Introduction
Personal-computer (PC) based simulation tools are
available to design end-to-end communications, digitalsignal-processing (DSP), and RF/analog systems, while
supporting linear and nonlinear, discrete and continuous
time, analog, digital, as well as mixed-mode (hybrid)
systems.
RF/analog libraries can include fixed and variablegain amplifiers, operational-amplifier circuits (op-amps),
active mixers, passive mixers, resistor-capacitor-inductor
(RCL) circuits, lowpass and highpass RCL filters, phaselocked loop (PLL) filters, LC tank, and quadrature
circuits. RF/analog library tokens may be used to create
complete transmitter/receiver systems, including the
propagated noise figure and intermodulation spurs.
Simulation Example
In this application example, the receiver is a Global
System for Mobile Communications (GSM) mobile unit.
The receiver architecture was supplied by CommQuest
Technologies, Inc. (Encinitas, CA) and is modeled after
the CommQuest CQT2030 RF integrated circuit (IC)
transceiver chip, one of a series of chips devoted to the
GSM system.
The objective of the simulation is to verify signal
levels, spurious products, and demodulation performance,
although the design example does not necessarily
represent CommQuest's recommended chip-set design.
The basic processing blocks (Fig. 1) include the
baseband Gaussian filter, the modulator/transmitter, the
Figure 1. A block diagram for a Global System for Mobile Communications (GSM) mobile unit.
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channel model, the RF section, the first intermediatefrequency (IF) section, the second IF section, and the
demodulator.
SystemView, for example, is a time-based simulator
which operates from a master system sample rate. As in
any computer simulation, the computations must be
carried out in discrete time. The rules of sampling theory
apply and must be taken into account. The system
sample rate is set at fs = 4.096 GHz. This value is
slightly greater than four times the RF frequency of
947.5 MHz. This sets the sum frequency out of the first
IF mixer to be to 1,824 MHz which is less than fs / 2,
which prevents aliasing of this signal term.
f+ = 947.5 MHz + 876.5 MHz = 1,824 MHz
In fact, any unrelated data rate below 4.096 GHz is
possible as long as proper consideration of aliasing is
taken into account.
The simulation is implemented by selecting the
desired functional elements, or tokens, which reside in
libraries. Each token element has an appropriate set of
parameters where the desired numbers are entered.
SystemView provides design hierarchy through a
mechanism designated a Metasystem. The processing
elements of this simulation were implemented as a set of
Metasystems.
The transmitter representing the base station is
straightforward. For the purposes of this simulation, the
transmitter is developed from individual component
parts. A complete mobile-transceiver architecture is
provided by the CommQuest chips.
A binary data source with rate R 270.833 kHz is
passed through a Gaussian lowpass filter with a BT = 0.3
setting. This highly compacts the occupied bandwidth of
the signal while introducing intersymbol interference.
Since BT = 0.3, the gaussian filter is set to a bandwidth
of 81.2499 e+3 Hz.
Modulator/Transmitter
In the modulator/transmitter section, the frequency
band covers 935 to 960 MHz. The mid-band frequency
of 947.5 MHz was chosen for the simulation. The
operation must shift the 947.5 MHz carrier by ±R/4 =
67.71 kHz. The voltage-controlled oscillator (VCO)
represents a Murata MQEOOI-902 modulator. The gain
of the part is 25 MHz/V. Therefore, the output of the
Gaussian filter is passed through a gain of G = 67.7l e3 /
25e6 = 2.71 e-3 (-25.7 dB).
The nominal output power of the VCO is -3 dBm.
The desired transmitter power is 5 W (37 dBm), which
is representative of a base station.
The power amplifier chosen is a MiniCircuits
(Brooklyn, NY) TIA-1000-4. It has a gain of 19 dB. A
pre-amp with a gain of 28 dB and an attenuator is used
to set an overall gain to 41.5 dB to provide the desired
output power of +37 dBm. The final element of the
transmitter is a lowpass filter used to eliminate spurious
harmonics of the power amplifier.
Channel
The channel block is comprised of two parts. First, a
gain (pad) token is used to reduce the 5-W transmit
power by the path loss of the link (including antenna
gains). The second element is the addition of thermal
KT noise which enters the receiver with the signal. It is
possible to add any variety of fading phenomena at this
point. It is also possible to add more transmitted signals
at different carrier frequencies to simulate the effects
adjacent-channel interference.
The receiver is a dual-conversion architecture with a
first IF frequency at 71 MHz and a second IF frequency
at 13 MHz.
270.833 e+3 x 0.3 = 81.2499 e+3
The design window for the Gaussian filter is shown
in Figure 2 and the resulting impulse plot is in Figure 3.
Figure 2. Gaussian Filter Parameter Entry Window
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Jul 29, 1998
Figure 3. Finished Gaussian Filter
Showing Time Response
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RF Section
The first section of the receiver is the RF which
covers the 935-to-960 MHz band. The first element of the
receiver, after the antenna, is the Murata
DFY2R902CR947BGH duplexer. This part effectively
acts as bandpass filter with the specification given. The
next element is the HP MGA 87563 low-noise amplifier
(LNA).
All intercept points up to fourth order as well as the
1-dB compression point, noise figure, and linear gain,
can be specified. The parameters listed are with respect
to the output of the amplifier. The RF filter is a Fujitsu
Compound Semiconductor, Inc. (San Jose, CA FARF5CH-947M50-L2EM surface acoustic-wave (SAW)
bandpass filter. This was implemented as a 319-tap
finite-infinite-response (FIR) bandpass filter.
The
attenuator pads are used to simulate the filter loss and
add the appropriate noise.
First IF Section
The first CommQuest CQT2030 or chip component is
the first IF mixer. The mixer local oscillator (LO) can be
tune over the 25-MHz wide 864-to-889 MHz range.
The specific value of this device is 876.5 MHz, which is
required to product the 71 MHz first IF frequency. The
LO leakage values can be specified, along with the
intercept points and other parameters.
The first IF filter is an off-chip Sawtek, Inc.
(Orlando, FL) 854252-1 SAN filter. For simplicity, a
three-pole Butterworth filter was used. After the SAW
filter, the highest frequency is 71 MHz (as opposed to
947.5 MHz), making possible to decimate the filter
output to much lower sampling rate. In this case the
decimation rate of four is used. This decimation
decreases the simulation time. The output of this filter
(after decimation) is entered into an automatic gaincontrol (AGC) amplifier/mixer with parameters.
Second IF Section
The second IF LO is a set 58 MHz, and produces a
13-MHz second IF frequency. This second IF frequency
signal is passed through a ceramic filter model by a fourpole Bessel and an AGC amplifier with nominal gain of
60 dB. The output of this section corresponds to the
output of the CQT2030.
Demodulator
The CommQuest CQT2020 and CQT2010 chips
are designed for optimum demodulation and final voice
recovery. For this simulation, the effort is focused totally
on recovery of the digital data. A simple quadrature
frequency-modulation (FM) detector operating directly
on the 13-MHz IF signal was chosen for this purpose.
This supports a relative comparison of the effects of
different RF components. A delay line is used to shift the
13 MHz carrier 90 degrees. The linear system token
H[z] (Figure 4) is set as follows:
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The System dT =
After decimate by 16,
the system dt =
The desired delay =
244.140625e-12 sec.
3.906250e-9 sec.
19.2308e-9 sec.
Sample delay =
3.906250e-9
19.2308e-9 /
= 4.923077 samples
Now, split the samples into an integer, and a
fractional part:
Integer =
Fraction = alpha =
1 - alpha =
4
0.923077 = 0.923
0.076923 = 0.077
Use a linear system token, set the
No. Numerator Coeffs: =
6
Set coeff. 0 through 3 (the first 4 coefficents) = 0
(Integer)
Set coeff. 4) = 1 - alpha =
Set coeff. 5) = alpha =
0.077
0.923
Figure 4. The linear system design window.
The output of the quadrature mixer is amplified and
filtered to recover the original data. In this example,
SystemView was used to simulate a complete GSM
system, from bits in to bits out (Figures 4 and 5). The
emphasis was on the receiver design as well as
implementation. The parameters used were taken from
commercially available components. All of the real-world
effects, such as thermal noise and intermodulation
products due to nonlinearities, are accurately taken into
account.
Page 3 of 4
SystemView
0
A
m
p
l
i
t
u
d
e
20.e-6
40.e-6
60.e-6
20.e-6
40.e-6
60.e-6
1.5
500.e-3
-500.e-3
-1.5
0
Time in Seconds
Figure 5. An overlay of the In and Out plots.
(PN Seed = 22)
SystemView
0
A
m
p
l
i
t
u
d
e
20.e-6
40.e-6
60.e-6
20.e-6
40.e-6
60.e-6
1.5
500.e-3
-500.e-3
-1.5
0
Time in Seconds
Figure 6. An overlay of the In_sampled and the
Out_sampled plots, with sample points enabled.
(PN Seed = 22)
For more information on SystemView simulation
software please contact:
ELANIX, Inc.
5655 Lindero Canyon Road, Suite 721
Westlake Village CA 91362
Tel: (818) 597-1414
Fax: (818) 597-1427
Visit our web home page (www.elanix.com) to download
an evaluation version of the software that can run this
simulation as well as other user entered designs.
AN124a
Jul 29, 1998
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