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Application Note AN124A Jul 29, 1998

GSM Receiver SimulationBy Maurice L. Schiff and Stephen H. Kratzet

Introduction

Personal-computer (PC) based simulation tools are

available to design end-to-end communications, digital-

signal-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 variable-

gain amplifiers, operational-amplifier circuits (op-amps),active mixers, passive mixers, resistor-capacitor-inductor

(RCL) circuits, lowpass and highpass RCL filters, phase-

locked 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 signallevels, 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 intermediate-

frequency (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.

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

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 frequencyat 13 MHz.

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 FAR-

F5CH-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 orchip 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 gain-

control (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 four-

pole 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 chipsare 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:

The System dT = 244.140625e-12 sec.

After decimate by 16,the system dt = 3.906250e-9 sec.

The desired delay = 19.2308e-9 sec.

Sample delay = 19.2308e-9 /3.906250e-9

= 4.923077 samples

Now, split the samples into an integer, and afractional part:

Integer = 4Fraction = alpha = 0.923077 = 0.9231 - alpha = 0.076923 = 0.077

Use a linear system token, set theNo. Numerator Coeffs: = 6

Set coeff. 0 through 3 (the first 4 coefficents) = 0

(Integer)

Set coeff. 4) = 1 - alpha = 0.077Set coeff. 5) = alpha = 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.

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SystemView

0

0

20.e-6

20.e-6

40.e-6

40.e-6

60.e-6

60.e-6

-1.5

-500.e-3

500.e-3

1.5

Amplitude

Time in Seconds

Figure 5. An overlay of the In and Out plots.

(PN Seed = 22)

SystemView

0

0

20.e-6

20.e-6

40.e-6

40.e-6

60.e-6

60.e-6

-1.5

-500.e-3

500.e-3

1.5Amplitude

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.