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. AN124a Jul 29, 1998 Page 1 of 4 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 AN124a Jul 29, 1998 Figure 3. Finished Gaussian Filter Showing Time Response Page 2 of 4 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: AN124a Jul 29, 1998 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 Page 4 of 4